College Level Anatomy and Physiology by AudioLearn

COLLEGE LEVEL Anatomy &Physi ology

Anatomy & Physiology

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TABLE OF CONTENTS Preface .................................................................................................................................... 1 Chapter One: Cell Anatomy and Physiology ............................................................................. 5 The Cell Membrane ..................................................................................................................... 5 Cellular Organelles and their Function...................................................................................... 10 DNA, the Nucleus, and RNA ...................................................................................................... 15 Protein Synthesis ....................................................................................................................... 20 Cellular Differentiation .............................................................................................................. 20 Key Takeaways .......................................................................................................................... 22 Quiz............................................................................................................................................ 23 Chapter Two: Body Tissues .................................................................................................... 26 Tissue Organization ................................................................................................................... 26 Types of Tissues ......................................................................................................................... 28 Epithelial Tissues.................................................................................................................... 29 Connective Tissue .................................................................................................................. 35 Muscle Tissue ........................................................................................................................ 39 Nerve Tissue .......................................................................................................................... 41 Key Takeaways .......................................................................................................................... 44 Quiz............................................................................................................................................ 45 Chapter Three: Integumentary System .................................................................................. 48 Layers of the Skin ...................................................................................................................... 48 The Epidermis ........................................................................................................................ 49

The Dermis ............................................................................................................................. 52 The Hypodermis..................................................................................................................... 53 Skin Pigmentation ..................................................................................................................... 54 Accessory Skin Structures ......................................................................................................... 54 The Hair.................................................................................................................................. 55 The Nails ................................................................................................................................ 57 Sweat Glands ......................................................................................................................... 58 Sebaceous Glands .................................................................................................................. 59 Functions of the Skin ................................................................................................................. 59 Disorders of the Skin ................................................................................................................. 61 Skin Coloration Diseases ........................................................................................................ 61 Skin Cancer ............................................................................................................................ 62 Benign Skin conditions........................................................................................................... 63 Key Takeaways .......................................................................................................................... 66 Quiz............................................................................................................................................ 67 Chapter Four: Skeletal System ............................................................................................... 70 Bone Structure and Function .................................................................................................... 70 Types of Bones .......................................................................................................................... 70 Bone Anatomy ........................................................................................................................... 71 Bony Markings ........................................................................................................................... 72 Bony Tissue ................................................................................................................................ 74 Spongy Bone versus Compact Bone .......................................................................................... 76 Bone Growth ............................................................................................................................. 76

Bones and Nutrition .................................................................................................................. 78 Hormones affecting Bone ......................................................................................................... 78 Axial Skeleton ............................................................................................................................ 79 The Cranium .............................................................................................................................. 80 Facial Bones ............................................................................................................................... 83 Inside the Skull .......................................................................................................................... 85 The Hyoid Bone ......................................................................................................................... 86 Vertebral Column ...................................................................................................................... 86 Vertebrae .................................................................................................................................. 87 Coccyx and Sacrum.................................................................................................................... 88 The Intervertebral Discs ............................................................................................................ 88 Ribcage and Sternum ................................................................................................................ 89 Appendicular Skeleton .............................................................................................................. 90 The Scapula and Clavicle ........................................................................................................... 91 Upper Limb ................................................................................................................................ 92 The Pelvic Girdle ........................................................................................................................ 95 The Lower Limb ......................................................................................................................... 96 Joints.......................................................................................................................................... 98 Synovial Joints ......................................................................................................................... 100 Key Takeaways ........................................................................................................................ 102 Quiz.......................................................................................................................................... 103 Chapter Five: Muscles and the Muscular System ................................................................. 106 Types of Muscle Tissue............................................................................................................ 106

Skeletal Muscle Fibers ............................................................................................................. 108 The Neuromuscular Junction and Muscle Contraction........................................................... 109 Muscle Structure ..................................................................................................................... 111 Fascicle Arrangements ............................................................................................................ 112 Muscles of the Head and Neck................................................................................................ 113 Muscles that Move the Head .................................................................................................. 115 Muscles of the Posterior Neck and Back ................................................................................. 115 Muscles of the Trunk ............................................................................................................... 116 Muscles of the Upper Extremity ............................................................................................. 119 Muscles of the Pectoral Girdle ................................................................................................ 119 Muscles of the Lower Extremity.............................................................................................. 124 Key Takeaways ........................................................................................................................ 127 Quiz.......................................................................................................................................... 128 Chapter Six: Central Nervous System ................................................................................... 131 Brain Cell Types and Function ................................................................................................. 131 Types of Neurons .................................................................................................................... 133 Types of Glial Cells ................................................................................................................... 134 Myelin ...................................................................................................................................... 135 Action Potential ....................................................................................................................... 136 Synapses .................................................................................................................................. 139 Neurotransmitters................................................................................................................... 140 Brain Structure and Function .................................................................................................. 141 The Cerebrum ...................................................................................................................... 141

The Diencephalon ................................................................................................................ 143 The Brainstem ...................................................................................................................... 144 The Cerebellum ................................................................................................................... 145 The Ventricles ...................................................................................................................... 145 Blood Supply to the Brain........................................................................................................ 147 Protective Coverings in the Brain and Spinal Cord ................................................................. 148 Spinal Cord Structure and Function ........................................................................................ 148 Key Takeaways ........................................................................................................................ 151 Quiz.......................................................................................................................................... 152 Chapter Seven: Peripheral Nervous System ......................................................................... 155 Basics of the Nervous System ................................................................................................. 155 Glial Cells of the PNS ............................................................................................................... 156 Ganglia ..................................................................................................................................... 157 Peripheral Nerves .................................................................................................................... 158 Cranial Nerves ......................................................................................................................... 158 Spinal Nerves ........................................................................................................................... 161 Sensory Receptors ................................................................................................................... 163 Smell .................................................................................................................................... 166 Hearing ................................................................................................................................ 167 Balance ................................................................................................................................ 169 Touch ................................................................................................................................... 169 Vision ................................................................................................................................... 170 Cranial versus Somatic Nerves ................................................................................................ 173

Generating Motor Responses ................................................................................................. 174 Autonomic Nervous System .................................................................................................... 174 Key Takeaways ........................................................................................................................ 177 Quiz.......................................................................................................................................... 178 Chapter Eight: Endocrine System ......................................................................................... 181 Hormones ................................................................................................................................ 181 Hormone Types ....................................................................................................................... 183 Hormone Pathways and Actions ............................................................................................. 184 Regulation of Hormone Secretion........................................................................................... 186 Pituitary Gland and Hypothalamus ......................................................................................... 188 Posterior Pituitary Gland ......................................................................................................... 189 Anterior Pituitary Gland .......................................................................................................... 189 Adrenal Glands ........................................................................................................................ 191 Thyroid Gland .......................................................................................................................... 193 Parathyroid Glands .................................................................................................................. 194 Pineal Gland ............................................................................................................................ 195 Endocrine Pancreas ................................................................................................................. 195 Secondary Endocrine Organs .................................................................................................. 197 Key Takeaways ........................................................................................................................ 199 Quiz.......................................................................................................................................... 200 Chapter Nine: Heart Anatomy and Physiology ..................................................................... 203 Basic Heart Anatomy ............................................................................................................... 203 Right Atrium......................................................................................................................... 207

Right Ventricle ..................................................................................................................... 207 Left Atrium ........................................................................................................................... 208 Left Ventricle ....................................................................................................................... 208 Electrical Activity of the Heart ................................................................................................ 209 Conduction System of the Heart ............................................................................................. 210 The Electrocardiogram ............................................................................................................ 213 Cardiac Cycle ........................................................................................................................... 214 Cardiac Physiology................................................................................................................... 215 Heart Rate ............................................................................................................................... 216 Stroke Volume ......................................................................................................................... 217 Coronary Arteries .................................................................................................................... 218 Key Takeaways ........................................................................................................................ 220 Quiz.......................................................................................................................................... 221 Chapter Ten: Blood and Blood Vessel Anatomy and Physiology........................................... 224 Blood Components .................................................................................................................. 224 Plasma Components................................................................................................................ 225 Formed Elements in Blood ...................................................................................................... 226 Erythrocytes ......................................................................................................................... 227 Leukocytes ........................................................................................................................... 229 Platelets ............................................................................................................................... 230 Blood Clotting Process............................................................................................................. 231 Blood Typing ............................................................................................................................ 232 Arterial Blood Pressure ........................................................................................................... 232

Regulation of the Cardiovascular System ............................................................................... 233 Nervous System Impact on the Cardiovascular System .......................................................... 234 Endocrine System Impact on the Cardiovascular System ....................................................... 235 Autoregulation of the Cardiovascular System ........................................................................ 235 Pulmonary Circulation ............................................................................................................. 236 Systemic Arteries ..................................................................................................................... 236 Ascending Aorta and Aortic Arch Branches ............................................................................ 238 Thoracic and Abdominal Aorta Branches ................................................................................ 238 Upper Limb Artery ................................................................................................................... 239 Arteries serving the Lower Limbs ............................................................................................ 241 Veins: Anatomy and Physiology .............................................................................................. 242 Upper Body Veins .................................................................................................................... 242 Vein Drainage in the Lower Body ............................................................................................ 245 Hepatic Portal System ............................................................................................................. 245 Key Takeaways ........................................................................................................................ 246 Quiz.......................................................................................................................................... 247 Chapter Eleven: Lymphatic and Immune System ................................................................. 250 Anatomy and Physiology of the Lymphatic System ................................................................ 250 The Immune System ................................................................................................................ 251 Lymphocyte Function .............................................................................................................. 252 Lymphoid Organs .................................................................................................................... 253 The Barrier Mechanisms ......................................................................................................... 256 Innate Immune Responses ...................................................................................................... 257

Natural Killer Cells ................................................................................................................... 258 Recognizing a Pathogen .......................................................................................................... 258 Soluble Factors ........................................................................................................................ 258 Complement System ............................................................................................................... 259 Inflammation in the Immune System...................................................................................... 259 The Adaptive Immune System ................................................................................................ 260 What is an Antigen? ................................................................................................................ 261 T Cell Development and Maturation ....................................................................................... 263 Antibodies and B Cells ............................................................................................................. 264 T Cell-related Antigens ............................................................................................................ 267 Hypersensitivity in the Immune System ................................................................................. 267 Autoimmune Diseases............................................................................................................. 268 Key Takeaways ........................................................................................................................ 269 Quiz.......................................................................................................................................... 270 Chapter Twelve: Respiratory System ................................................................................... 273 Respiratory System Anatomy .................................................................................................. 273 The Nose .............................................................................................................................. 273 Pharynx ................................................................................................................................ 275 Larynx................................................................................................................................... 276 Trachea ................................................................................................................................ 278 Bronchial Tree...................................................................................................................... 278 Alveoli .................................................................................................................................. 278 Lung Anatomy ......................................................................................................................... 280

Pulmonary Ventilation ......................................................................................................... 281 Respiratory Volumes ........................................................................................................... 283 Control of Ventilation .......................................................................................................... 284 Gas Exchange ....................................................................................................................... 285 Ventilation and Perfusion .................................................................................................... 286 Oxygen Transport ................................................................................................................ 287 Carbon Dioxide Transport.................................................................................................... 289 Key Takeaways ........................................................................................................................ 290 Quiz.......................................................................................................................................... 291 Chapter Thirteen: Digestive System ..................................................................................... 294 Structure and Basic Function of the Digestive System ........................................................... 294 Alimentary Canal Histology ..................................................................................................... 295 Nervous System in the GI Tract ............................................................................................... 296 Circulation in the GI Tract ....................................................................................................... 297 The Peritoneum ....................................................................................................................... 297 Process of Digestion ................................................................................................................ 297 Regulation of Digestion ........................................................................................................... 298 Mouth Anatomy and Physiology ............................................................................................. 299 Tongue ................................................................................................................................. 300 Saliva .................................................................................................................................... 301 Teeth .................................................................................................................................... 301 The Pharynx ......................................................................................................................... 302 Esophageal Anatomy ............................................................................................................... 303

Stomach Anatomy and Physiology .......................................................................................... 304 Small Intestine ......................................................................................................................... 308 Large Intestine ......................................................................................................................... 311 Exocrine Pancreas ................................................................................................................... 313 Liver ......................................................................................................................................... 314 Gallbladder .............................................................................................................................. 316 Key Takeaways ........................................................................................................................ 317 Quiz.......................................................................................................................................... 318 Chapter Fourteen: Metabolism and Human Nutrition ......................................................... 321 The Process of Chemical Digestion ......................................................................................... 321 Carbohydrate Digestion .......................................................................................................... 322 Protein Digestion ..................................................................................................................... 323 Lipid Digestion ......................................................................................................................... 323 Nucleic Acid Digestion ............................................................................................................. 324 Nutrient Absorption ................................................................................................................ 324 Metabolism ............................................................................................................................. 326 Catabolic Reactions ............................................................................................................. 326 Anabolic Reactions .............................................................................................................. 327 Hormones that Regulate Metabolism ..................................................................................... 327 Oxidation-Reduction Reactions............................................................................................... 328 Carbohydrate Metabolism ...................................................................................................... 328 Gluconeogenesis.................................................................................................................. 332 Lipid Metabolism ..................................................................................................................... 333

Lipogenesis .......................................................................................................................... 334 Protein Metabolism................................................................................................................. 334 Basic Human Metabolism ....................................................................................................... 335 Nutrition and Metabolism ....................................................................................................... 337 Key Takeaways ........................................................................................................................ 339 Quiz.......................................................................................................................................... 340 Chapter Fifteen: Urinary System .......................................................................................... 344 Urinary Tract Anatomy ............................................................................................................ 344 Ureters ................................................................................................................................. 344 Bladder................................................................................................................................. 345 Micturition Reflex ................................................................................................................ 346 Urethra ................................................................................................................................ 347 Gross Kidney Anatomy ............................................................................................................ 348 Microscopic Kidney Anatomy .............................................................................................. 351 Juxtaglomerular Apparatus ................................................................................................. 353 Proximal Convoluted Tubule (PCT) ...................................................................................... 354 Loop of Henle....................................................................................................................... 354 Distal Convoluted Tubule (DCT)........................................................................................... 354 Collecting Ducts ................................................................................................................... 354 Kidney Physiology .................................................................................................................... 355 Secretion and Reabsorption ................................................................................................ 356 Regulation of Kidney Blood Flow ......................................................................................... 358 Other Kidney Functions ....................................................................................................... 359

Urine Composition .................................................................................................................. 360 Key Takeaways ........................................................................................................................ 362 Quiz.......................................................................................................................................... 363 Chapter Sixteen: Fluids, Electrolytes, and the Acid-Base System.......................................... 367 Fluid Compartments ................................................................................................................ 367 Water and Electrolyte Balance ................................................................................................ 370 Antidiuretic Hormone (ADH) ................................................................................................... 372 Electrolyte Balance .................................................................................................................. 372 Acid Base Physiology ............................................................................................................... 375 The Lungs and Acid-Base Balance ........................................................................................... 376 The Kidneys and Acid-Base Balance ........................................................................................ 377 Acid-Base Disorders................................................................................................................. 378 Key Takeaways ........................................................................................................................ 380 Quiz.......................................................................................................................................... 381 Chapter Seventeen: Male Reproductive System .................................................................. 385 Male Reproductive Anatomy .................................................................................................. 385 The Scrotum......................................................................................................................... 385 Testes ................................................................................................................................... 386 Structure of Formed Sperm ................................................................................................. 389 Sperm Maturation ............................................................................................................... 390 Duct System ......................................................................................................................... 390 Male Reproductive Physiology ................................................................................................ 394 Male Reproductive Development ........................................................................................... 394

Puberty .................................................................................................................................... 395 Key Takeaways ........................................................................................................................ 397 Quiz.......................................................................................................................................... 398 Chapter Eighteen: Female Reproductive System.................................................................. 401 Female Reproductive Anatomy ............................................................................................... 401 The Ovarian Cycle and Oogenesis ....................................................................................... 404 Folliculogenesis.................................................................................................................... 406 Hormones and the Ovarian Cycle ........................................................................................ 407 Fallopian Tubes .................................................................................................................... 409 Cervix and Uterus ................................................................................................................ 409 Menstrual Cycle Physiology .................................................................................................... 410 Breast Anatomy and Physiology .......................................................................................... 411 Female Reproductive Development .................................................................................... 413 Key Takeaways ........................................................................................................................ 414 Quiz.......................................................................................................................................... 415 Chapter Nineteen: Developmental Anatomy and Physiology............................................... 419 Genetics and Development ..................................................................................................... 419 Inheritance Patterns ................................................................................................................ 420 Fertilization.............................................................................................................................. 423 The Zygote ............................................................................................................................... 425 Embryologic Development ...................................................................................................... 426 Pre-Implantation Phase ........................................................................................................... 426 Implantation ............................................................................................................................ 428

Embryogenesis ........................................................................................................................ 430 Placental Development ........................................................................................................... 430 Organ Formation ..................................................................................................................... 432 Fetal Development .................................................................................................................. 433 Key Takeaways ........................................................................................................................ 435 Quiz.......................................................................................................................................... 436 Course Summary ................................................................................................................. 440 Course Questions and Answers ........................................................................................... 444

PREFACE This course is intended to appeal to the undergraduate interested in the complete story on human anatomy and physiology. Through pictures and words, you will study the entirety of the human body, from the cellular/microscopic levels, to the developmental levels, and finally to the macroscopic/anatomic levels of the human body. After talking about cells in general, each body system is discussed in turn so that, by the end of the course, you will feel comfortable regarding your understanding of the form and function of the human body. Chapter one covers the basics of cellular anatomy and physiology. This includes a study of the cell membrane, the cytoplasm of the cell, and the different organelles in the cell. The nucleus is one of the more important organelles of the cell, containing the genetic information in the form of chromosomes and directing cellular functions. Also covered in this chapter is protein synthesis and the differentiation of cells. The topic of chapter two is the formation of body tissues and tissue types. While cells have many similarities, there are differences in structure and function that create different types of tissues. How cells make tissues is covered in this chapter as well as the structure and function of different tissue types, including epithelial, connective, muscle, and nerve tissue. The focus of chapter three in the course is the integumentary system, which is basically the skin. The skin is considered the largest organ of the body, covering most of the external surface of the body. There are several layers, which have different functions. In the dermis of the skin are many different accessory skin structures, which are microscopic in nature. The skin serves several different functions in the body, which are discussed in the last part of this chapter. The skeletal or bone system is the topic of chapter four. The discussion starts with the anatomy and physiology of bone cells and bone tissue. Then the axial skeleton (the skull, spine, and ribcage) is covered in detail as well as the appendicular skeleton (mainly the extremities). The functions of joints and ligaments are discussed as they are important aspects of the skeletal system.

The anatomy and physiology of the muscular system are the topics of chapter five. It begins with a discussion of muscles and tendons as well as the different types of muscle tissue. The muscles of the head, neck, trunk, upper extremities, and lower extremities are also discussed. Chapter six examines the structure and function of the central nervous system. There are different types of brain cells—some of which conduct electricity and others that protect and support the electrically-active nerve cells. The gross anatomy and basic functions of the brain are covered in this chapter as well as the gross anatomy and basic functions of the spinal cord, which is also technically a part of the central nervous system, or CNS. Chapter seven covers the structure and function of the peripheral nervous system. These are the nerves located outside of the brain and spinal cord. The major somatic sensory and motor nerves are covered, including the way the major senses are picked up by the body. The cranial nerves, that do not come from the spinal cord, have unique function, which are discussed. The structure and function of the autonomic nervous system are also covered in this chapter. The focus of chapter eight is the endocrine system. This involves the hormones and their interactions with the various body systems. There are numerous “endocrine” organs both within the brain and outside in the rest of the body. The hypothalamus, the pituitary gland, and the pineal gland are all located near or within the brain. The other endocrine glands covered are the adrenal gland, the thyroid gland, the parathyroid glands, and the endocrine portion of the pancreas. The reproductive hormones are covered in a later chapter on the reproductive system. The anatomy and physiology of the heart are discussed in chapter nine. The heart plays a central role in the cardiovascular system, being the major pump that allows blood to flow through the rest of the body. The cells of the heart (cardiac muscle cells) have a unique electrical activity and synchronize the activity of the chambers of the heart. The coronary arteries are the major arteries supplying the heart; they are important because damage to any of these arteries can potentially cause a heart attack. The discussion of the cardiovascular system continues in chapter ten. There are arteries, veins, and capillaries that allow for blood flow and gas exchange in the tissues after oxygenated blood

is pumped out of the heart and before deoxygenated blood reenters it. The components of blood and blood typing are also an important aspect of the cardiovascular system and are topics in this chapter. The lymphatic and immune system are covered together in chapter eleven in the course. The lymphatic system includes the vessels of the lymphatic system that filter blood and pathogens, making these structures also important to the immune system. The thymus gland is a part of the immune system as well as the spleen. Both of these are discussed in this chapter. The immune system is broadly divided into the innate and adaptive immune system. The cells of the immune system and the physiology of these aspects of the immune system are explained in detail in this chapter. The structure and function of the respiratory system are the main topics of chapter twelve. The respiratory system is broadly divided into the upper and lower respiratory tract. The lower respiratory tract is primarily made up of the lungs and the bronchial tree. The anatomy and physiology of the gas exchanging structures of the lungs (the alveoli) finish up this chapter. The topic of chapter thirteen is the human digestive system. It entails everything in the digestive process from the mouth to the anus as well as several other structures involved in digestion, including the liver, gallbladder, and the exocrine portion of the pancreas. The different anatomical structures included as part of the digestive system are covered in detail, including how they function to take food and turn it into nutrients used by the entire body. Chapter fourteen is a continuation of the digestive system; however, it involves the microscopic and molecular aspects of metabolic processes. Carbohydrate, protein, and lipid metabolism are discussed as well as the important energy-producing process of glucose metabolism. The overall physiology of human nutrition and nutritional needs are also examined as part of this chapter. The topic of chapter fifteen is the urinary system. The urinary tract begins with the kidneys with urine traveling through the ureters, bladder, and urethra. The anatomy and physiology of the kidneys are discussed in this chapter as well as the microscopic anatomy of these structures. Closely connected to the anatomy is the unique physiology of the kidneys. After

the kidneys form urine, this fluid is composed of several smaller molecules, which are discussed in the last part of this chapter. Chapter sixteen covers fluids, electrolytes, and the acid-base system. This includes a discussion of the various fluid compartments in the body as well as how water and electrolytes are balanced by the somatic cells and by the renal system. Acid-base physiology depends on activities of both the lungs and the kidneys, which are examined in this chapter; there are several acid-base disorders that require compensation by both of these systems—also covered in the last part of the chapter. The male reproductive system is the topic of chapter seventeen. It includes coverage of the anatomy of the male reproductive system as well as the physiology (including the male reproductive hormones) involved in this system. Finally, the embryological development of the male reproductive system is discussed. Chapter eighteen examines the female reproductive system. First, the anatomy of the female reproductive system is covered, including the internal and external anatomic structures. The physiology of the menstrual cycle and female hormones are included in this chapter as well as the anatomy and physiology of the female breast. Finally, the embryological development of the female reproductive system is discussed in this chapter. The focus of chapter nineteen is developmental anatomy and physiology. This includes a discussion of genetics and how it applies to human phenotypes. Fertilization is more completely covered than it will be in the previous reproductive system chapters and the development of the embryo is explained in detail. As the embryo becomes a fetus at eight weeks gestation, the development continues; this is covered as the final topic of this chapter.

CHAPTER ONE: CELL ANATOMY AND PHYSIOLOGY This chapter includes a discussion of the cell membrane, the cytoplasm of the cell, and the different organelles that can be found in the cell. The nucleus is one of the more important organelles of the cell, containing the genetic information in the form of chromosomes and directing cellular functions. Also covered in this chapter is protein synthesis and the differentiation of cells.

THE CELL MEMBRANE The cell membrane is primarily composed of lipids, and surrounds the cells of the human body. It defines the cell border and allows the cell to interact in a controlled way with the environment around the cell. Cells need to take in nutrients, eliminate waste and avoid the uptake of toxic or other unwanted substances into the cell. Cell-cell communication is also important and is a function of the lipid membrane. Lipid provide a semi-permeable barrier or membrane between the cell and its environment. It also needs to contain proteins, which help in transport across membranes and cell communication. In addition, there are carbohydrates on the proteins and lipids, allowing for cells to recognize one another. The basic structure of the plasma membrane is referred to as the fluid mosaic model. This means that the phospholipid structure of the lipids (which involves a phosphorus-containing water-loving end and a lipid water-aversive tail) is the main structure in the membrane, with cholesterol and proteins imbedded in and floating throughout the structure. This is a dynamic situation in which the membrane does not look the same from one second to the next. Figure 1 in your guide shows a phospholipid and how it forms a membrane in the cell:

Besides phospholipids, cholesterol, proteins, and carbohydrates are part of the plasma/cell membrane. Let’s look at these in turn in order to understand what the cell membrane looks like: •

The phospholipid is made from a glycerol linkage, two fatty acid tails (which are waterhating or hydrophobic), and a phosphate head group (which is water-loving or hydrophilic). They are arranged in a phospholipid bilayer as is seen in figure 1.

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Cholesterol, which is formed by the fusion of four carbon rings, is lipid that forms part of the core of the membrane. It is less abundant than phospholipids, which make up the majority of the membrane.

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Plasma proteins, which can be loosely attached to the outside or inside of the plasma membrane or can be imbedded partly or completely within the membrane.

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Carbohydrates can be present alone and attached to the outer surface of the cell membrane or are attached to proteins (making glycoproteins) or lipids (forming glycolipids).

Different cells have different proportions of the previously mentioned cell membrane components. Proteins make up 50 percent of the mass of the cell membrane, while lipids of all types make up about 40 percent of the membrane. Ten percent of the membrane by weight comes from carbohydrates. 6

The phospholipid bilayer in the cell membrane formed by these interactions makes a good barrier between the exterior and interior of the cell. It means that polar or positively-charged substances of all sizes have a difficult time crossing the hydrophobic center of the membrane. Phospholipids form a micelle in water, which is a single-layered sphere that keeps out water and other positively-charged substances. Those that are bigger form a bilayer membrane, the largest of which is the cell membrane. There are multiple smaller membranes that form liposomes and other organelles (small cellular organs) inside the cell. Figure 2 shows what a liposome looks like:

There are two types of proteins in the cell membrane: integral and peripheral proteins. Integral proteins are “integrated” into the cell membrane and have at least one hydrophobic segment that can be aligned and in contact with the hydrophobic core of the cell membrane. Some will stick out of either side of the membrane with hydrophilic ends. Trans-membrane proteins extend completely through the plasma membrane. Some integral proteins form channels that allow water-loving ions, water, and other hydrophilic substances to pass through the membrane—usually in a controlled fashion. Figure 3 shows an open and closed membrane channel:

A peripheral membrane protein is found on the inside or outside surface of the cell membrane. They can be attached to phospholipids or to integral proteins in the membrane. They do not have hydrophobic portions that are imbedded in the cell membrane. Instead they are loosely attached to the cell. Carbohydrates are less abundant in the cell membrane. They can be bound to lipids (forming glycolipids) or proteins (forming glycoproteins). These can be branched or straight and consist of up to 60 monosaccharide units that stick out of or inside the cell membrane. These are the main cellular markers that are important in the immune system. They allow the body’s immune cells to recognize self-cells (belonging to the self) and non-self-cells (that do not belong to the cell). The structure of the fatty acid tails of the phospholipids is important in determining how the membrane acts—how fluid it is. Saturated fatty acids have no double bonds (are completely saturated with hydrogen atoms), which makes them straight molecules. Unsaturated fatty acids have fewer hydrogen atoms on them and are kinked. They behave differently in the cell membrane. Saturated fatty acid phospholipids make dense cell membranes, while unsaturated fatty acid phospholipids are less tightly packed and more fluid. Most cell membranes have a mixture of saturated and unsaturated fatty acids.

The transport of substances through the membrane is complex. The membrane transport system is what allows different molecules to enter the cell. There are two types of transport across a membrane: passive transport and active transport. Passive transport does not require energy to function. Molecules will pass from a high to a low concentration in an effort to equalize the concentration on either side of the membrane. Passive transport can involve simple diffusion/passive diffusion, osmosis, or facilitated diffusion. Simple diffusion involves no transporter protein and no energy. Carbon dioxide passes via simple diffusion as does water. The transport stops when the concentration is equalized. Osmosis is the movement of water across the cell membrane. Facilitated diffusion is similar to simple diffusion but requires a transporter protein in order to allow the substance to pass through. These are specific to a certain molecule (although some will transport multiple compounds at the same time). No energy is required for this. Active transport requires both a transporter protein and a continuous supply of ATP or other form of cellular energy. We will talk more about ATP (adenosine triphosphate) but keep in mind it is an energy molecule in the cell. This type of transport can cause molecules to pass from a lower concentration to a higher concentration. A permease or transporter protein will transport amino acids, glucose, organic acids, and inorganic ions (like phosphate, potassium, and sulfate) using energy supplied by the cell. The Na-K (sodium-potassium) pump is an example of an active transport mechanism. Figure 4 is an example of the sodium-potassium ion pump:

There is primary active transport that involves the hydrolysis of ATP to provide energy to directly transport the molecule. The molecule goes from a lower to a higher concentration. In secondary electron transport, energy is released when a molecule goes from a higher to a lower concentration, while the energy released goes into transporting a different molecule across the membrane from a lower to higher concentration.

CELLULAR ORGANELLES AND THEIR FUNCTION The fluid inside the cell is called the cytoplasm. In human cells, cytoplasm is everything between the cell membrane and the nuclear envelope (which encloses the nucleus inside the cell). The cytoplasm consists of a gel-like substance known as cytosol. Cytosol contains numerous ions, different types of small molecules, and large molecules (known as macromolecules). The cytoplasm also contains a cytoskeleton, which is a network of fibers that interlace to form the shape of the cell. The cytoskeleton also keeps the cell structures (organelles) in place within the cell so they can function and interact better with one another. All of the organelles

(tiny organs in the cell) are suspended within the cytoplasm by the cytoskeleton. Figure 5 shows what the cytoskeleton of the cell looks like and how it suspends organelles:

The organelles in the cell have different functions within the cell. There are multiple major organelles and even more minor organelles. Let’s take a look at the different organelles in the cell and what they do: •

Endoplasmic reticulum—this is a continuous but stacked membrane that performs two major functions. The rough endoplasmic reticulum contains ribosomes that are involved in the translation and folding of proteins, while the smooth endoplasmic reticulum helps in the making of lipids in the cell. Figure 6 shows the endoplasmic reticulum in the cell:

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Golgi apparatus—this is a single membrane compartment that helps the cell sort and modify proteins. The proteins come to the Golgi apparatus after being built, but before they go to their destination. This organelle helps in post-translational modification of proteins; it checks them for errors and discards those that are incorrect. The good proteins are packaged and sent for delivery.

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Mitochondrion—this is an organelle that is a double-membrane compartment that is primarily involved in energy production. Later, you will see how this is done biochemically. It has some of its own DNA passed on by the person’s mother. Mitochondria are known as "the powerhouses of the cell." Figure 7 shows what the mitochondrion looks like:

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Nucleus—this is one of the major organelles in the human cell. It is a double-membrane compartment that contains nearly all of the genetic material of the cell and is involved in the maintenance of DNA and in RNA transcription. We will talk more about the nucleus later in this chapter.

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Vacuole—this is a single membrane compartmented organelle that is involved in both storage and homeostasis is the cell. They often store waste products and allow them to exit the cell by fusing with the cell membrane, extruding the waste to the outside of the cell.

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Autophagosome—this is a double-membrane structure that collects different aspects of the material inside the cytoplasm and digests it or degrades it so that the molecules can be broken down and the digested material can be recycled.

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Lysosome—this is a single-membrane structure that is involved in the storage of certain molecules within the cytoplasm. It is also an organelle that degrades certain large macromolecules within the cell. There are some diseases (called lysosomal storage diseases) that affect the ability of the lysosome to degrade certain substances, causing them to build up.

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Centriole—this is made from microtubules (which is a protein involved in the cytoskeleton). It provides an anchor for the cytoskeleton in the cell.

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Cilium—this is made from microtubules as well and is on the outside of the cell. It is found in certain cells of the human body and allows for movement of external medium and other substances located on the outside of the cell. Cells of the bronchial tree, for example, have cilia.

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Myofibril—this will be discussed when muscles are discussed. It is made from bundled filaments and allows for muscle contraction in the cell to occur.

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Nucleolus—this is a structure located inside the nucleus that is involved in the production of ribosomes. It is made from proteins, RNA (ribonucleic acid), and DNA (deoxyribonucleic acid).

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Peroxisome—this is a single-membrane compartment that contains hydrogen peroxide and is involved in the breakdown of hydrogen peroxide in the cell.

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Proteasome—this is a large protein complex that uses proteolysis (protein breakdown) to degrade any damaged or unneeded proteins in the cell.

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Ribosome—this is made from RNA and protein. It is involved in the translation of RNA, which is how proteins are made.

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Vesicle—this is a single-membrane structure that takes up certain molecules and transports the substances to different parts of the cell or outside of the cell.

Figure 8 in the guide shows what the interior of the cell is supposed to look like:

DNA, THE NUCLEUS, AND RNA The nucleus is a double-walled structure inside the cell that contains nearly all of the genetic material in the cell in the form of DNA (deoxyribonucleic acid). DNA is a double-stranded, coiled molecule made up of four different nucleotide bases that together form the instructions to make all of the proteins and enzymes inside the cell and determine what the human or other organism looks like. All of the DNA inside each cell of the body is the same but, through the process of differentiation, the DNA that is “active” inside each cell is unique to the cell type. Figure 9 gives the basic structure of the DNA molecule:

The two DNA strands linked together are called polynucleotides as they are made from simpler “monomeric” molecules known as nucleotides. Each nucleotide is made from one of four nucleobases or nucleosides (called guanine, cytosine, thymine, or adenine), as well as the sugar called deoxyribose, and a phosphate group. Covalent bonds connect the different nucleotides in a specific order that makes up the “alphabet” of the DNA molecule. Hydrogen bonds (which are much weaker) connect the base pairs in the center of the ladder-shaped molecule. Adenine (A) always pairs with thymine (T), while cytosine (C) always pairs with guanine (G). Both strands of nucleotides store the same information. The hydrogen bonds between the base pairs is weak but together they result in a molecule that resists cleavage. When the two strands separate in cell division, a matching strand is created to match with the cleaved strands, creating two identical DNA strands that go to two different daughter cells when the cell divides. The enzymes that assist with this process are DNA replicase, polymerase, helicase, and ligase. More than 98 percent of DNA is “non-coding,” meaning they are DNA sequences that do not specifically code for a known protein. The process of replication, as just described, involves several enzymes and is what makes two DNA molecules out of a single DNA strand. Figure 10 describes the process of DNA replication:

DNA that is coded is referred to as genes. Genes are segments of DNA that are transcribed into ribonucleic acid (RNA), which sends messages via “messenger RNA” or “mRNA” to make certain proteins outside of the nucleus. The nucleobases (or just bases) that make up a gene have their message consisting of a three-base code called a codon. Each combination of three bases codes for a certain amino acid, which is the building block molecule of proteins. Together, the three-base codes are read and translated into a specific sequence of amino acids that make up a specific protein. The process of transcription occurs in the nucleus. It involves slightly separating the DNA strands at a specific gene site and making a “matching” single stranded mRNA molecule that carries the DNA message outside of the nucleus. The enzyme that participates in the transcription process is called RNA polymerase. Figure 11 shows what the process of transcription looks like in the nucleus:

Later, we will discuss the process of translation, which is when the mRNA molecule’s message gets turned into an amino acid sequence to make a protein. Inside the human cell, the DNA is organized into 23 pairs of chromosomes for a total of 46 chromosomes. Twenty-two of these chromosomes are numbered with one coming from the genetic mother and one coming from the genetic father of the individual. There are two unmatched chromosomes in males, the XY chromosome pair (with the Y chromosome coming from the father and the X chromosome coming from the mother). In females, there are two X chromosomes, one from each parent. Figure 12 shows the human karyotype (DNA collection) in male and female humans:

In translation, the mRNA molecule goes to the ribosomes, where protein synthesis takes place. The codons involve 3-letter combinations for a total of 64 possible codons. This is plenty, considering there are just 20 standard amino acids. It means that some amino acids match to more than one codon and that there are three “stop codons” that end the coding region (the TAA, TGA, and TAG codons). Ribonucleic acid or RNA is similar to DNA but is single-stranded and contains slightly different bases. The sugar molecule as part of the RNA strand is ribose instead of deoxyribose as is seen in DNA. The bases include uracil, which replaces thymine in the molecule. There are several different types of RNA, which perform different functions. These include the following: •

Messenger RNA (mRNA)—this is the RNA that is transcribed by DNA and goes on to the ribosome, where it passes the DNA message that makes up the protein.

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Transfer RNA (tRNA)—this is RNA that attaches to both an amino acid and the mRNA molecule, helping to make the protein strand in the ribosome.

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Ribosomal RNA (rRNA)—this is RNA that makes up the structural component of the ribosome, which is the protein-making machinery of the cell.

There is also RNA that acts as an enzyme, in which case it is called a ribozyme. It helps to speed a chemical reaction in the cell. RNA also plays important roles in other cellular processes,

including cell differentiation, cell division, and cell aging. Defects in RNA function have been implicated in several human diseases, although the exact defects and diseases have yet to be completely discovered.

PROTEIN SYNTHESIS As mentioned, protein synthesis happens in the ribosome in most cases. Protein synthesis is occuring all the time and involves taking the DNA message in the gene, transcribing it onto a mRNA molecule, and then taking the mRNA molecule out to the ribosome. There, the message on the mRNA molecule is coupled with multiple molecules of transfer RNA. The amino acid sequence is put together by the tRNA molecule and enzymes, so that a naked strand of protein is made. The naked strand is the “primary structure” of the protein. It has not undergone any modification. After it is made, it undergoes post-translational modification. This involves changing the primary structure of the protein strand so that it can make bonds between different amino acids and can fold into a three-dimensional shape. The 20 standard amino acids can be expanded by modifying an existing functional group on an amino acid or introducing a new one such as phosphate. Phosphorylation is a common event in controlling the activity of enzymes and is the most common post-translational modification that happens to the protein strand. The ends of the protein chain can be connected to one another and segments of protein can be cleaved in order to activate the protein from the proprotein—the protein precursor molecule. Other things that can happen in post-translational modification include acetylation, hydrogen bonding, and disulfide bonding.

CELLULAR DIFFERENTIATION Cellular differentiation involves the process of taking a less specialized cell and turning it into a more specialized cell type. While all cells have the same DNA, the cells undergo a differentiation process to make a multicellular organism. Differentiation occurs as soon as the

single-celled human zygote begins to divide. After several cell divisions, differentiation starts to occur. When differentiated daughter cells divide themselves, they divide into cells that are either identical to the cells they divide from or differentiate further until fully differentiated cells are created. Stem cells are not differentiated and that make fully-differentiated daughter cells in the adult human. The most commonly-known stem cells are those that make the different types of white blood cells or red blood cells in the body. These are called hematopoietic stem cells. A differentiated cell can have a different size, a different shape, modified metabolic activity, and varying external markers on them when compared to the stem cell. These changes are largely due to alterations in the gene expression of the differentiated cell. The DNA sequence does not change but certain genes are turned on or turned off in the differentiated cell to make the cell different from its precursor. A cell that can turn into many different cell types is called a pluripotent stem cell, while a cell that can differentiate into all other types of cells is called a totipotent stem cell. Only the zygote and very early embryonic cells are truly totipotent. After a few differentiations, the resulting cells lose this ability and become less and less pluripotent.

KEY TAKEAWAYS •

The cell membrane surrounds and defines the cell; it is important in cell-cell communication, cell identification, and controlling what goes into and outside of the cell.

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The cell membrane is made up of a phospholipid bilayer, proteins, carbohydrates, and cholesterol.

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There are numerous major and minor organelles that function in the body to perform all of the cell’s functions; they are suspended in the cell by the cytoskeleton.

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The DNA molecule makes up the genetic material of the cell.

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Through the processes of transcription and translation, the DNA message goes on to create protein molecules.

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Through cell differentiation, cells that are less differentiated go on to become increasingly differentiated by turning on or turning off certain genes in the cell.

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