Cells

Chapter Three

Cells

The cell is the basic unit of life. All organisms are made up of cells (or in some cases, a single cell). Most cells are very small; most are invisible without using a microscope. Cells are covered by a cell membrane and come in many different shapes. The contents of a cell are called the protoplasm.

The components of animal cells are centrioles, cilia and flagella, endoplasmic reticulum, golgi apparatus, lysosomes, microfilaments, microtubules, mitochondria, nucleus, peroxisomes, plasma membrane and ribosomes. • Prokaryotes = Lack a nucleus, Have a single chromosome, Reproduce by binary fission, Include bacteria • Eukaryotes = Contain a nucleus & membrane bound organelles, Asexually reproduce cells by mitosis, include Plants & animals Somali National University

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Cell membrane is the thin layer of protein and fat that surrounds the cell. The cell membrane is semipermeable, allowing some substances to pass into the cell and blocking others.

Centrosome - (also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center and radiating tubules. Centrosome is where microtubules are made.

During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell. The centriole is the dense center of the centrosome.

Cytoplasm - the jellylike material outside the cell nucleus in which the organelles are located.

Golgi body - (also called the Golgi apparatus or golgi complex) a flattened, layered, sac-like organelle that is located near the nucleus. It produces the membranes that surround the lysosomes.

The Golgi body packages proteins and carbohydrates into membrane-bound vesicles for "export" from the cell.

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Lysosome - (also called cell vesicles) round organelles surrounded by a membrane and containing digestive enzymes. This is where the digestion of cell nutrients takes place.

Mitochondrion - Mitochondria are of various shapes and sizes and are numerous in the cytoplasm of all eukaryotic cells. Mitochondria are the power generators of the cell. They are capable of selfreplication as they possess their own DNA. The main function of mitochondria is to produce energy through metabolism. In the mitochondria sugar is finally burnt during cellular respiration.

The energy released in this process is stored as high-energy chemicals called adenosine triphosphate (ATP). The energy is used by the body cells for synthesis of new chemical compounds.

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Nuclear membrane - the membrane that surrounds the nucleus.

Nucleolus - an organelle within the nucleus - it is where ribosomal RNA is produced. Some cells have more than one nucleolus.

Nucleus - spherical body containing many organelles, including the nucleolus. The nucleus controls many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes).

The nucleus is surrounded by the nuclear membrane.

Nucleopore - is the tiny hole in the nuclear membrane, allows the movement of nucleic acids and proteins in/out of the cell.

Ribosome - small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis.

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Endoplasmic reticulum (ER) They are found in all eukarytoic cells and are structurally continuous with the nucleus of the cell. The ER is a complex network of tubes. The lumen is filled with fluid. There are two types of endoplasmic reticulum - smooth ER and rough ER. Rough endoplasmic reticulum - They are tubes with rough surface as the ribosomes are attached to its surface. Rough ER is covered with ribosomes that give it a rough appearance. The rough endoplasmic reticulum manufactures membranes and secretory proteins. In certain leukocytes (white blood cells), the rough ER produces antibodies. In pancreatic cells, the rough ER produces insulin. The rough and smooth ER are usually interconnected and the proteins and membranes made by the rough ER move into the smooth ER to be transferred to other locations.

Smooth endoplasmic reticulum - They are tubes with a smooth surface as they lack ribosomes. The smooth ER has a wide range of functions including carbohydrate and lipid synthesis. The space within the ER is called the ER lumen. Smooth ER transports materials through the cell.

In liver cells the smooth ER produces enzymes that help to detoxify certain compounds.

In muscles the smooth ER assists in the contraction of muscle cells, and in brain cells it synthesizes male and female hormones.

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• Vacuole - fluid-filled, membrane-surrounded cavities inside a cell. The vacuole fills with food being digested and waste material that is on its way out of the cell.

• Peroxisomes are found in liver and kidney cells.

Peroxisomes have enzymes that are responsible to get rid of the toxic peroxides from the cell.

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Plant cells Plant cells are similar to animal cells in that they are both eukaryotic cells and have similar organelles. Plant cells are generally larger than animal cells. While animal cells come in various sizes and tend to have irregular shapes, plant cells are more similar in size and are typically rectangular or cube shaped. A plant cell also contains structures not found in an animal cell.

Some of these include a cell wall, a large vacuole, and plastids. Animal cells also contain structures such as centrioles, lysosomes, and cilia and flagella that are not typically found in plant cells.

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Plant Cell: Structures and Organelles • The following are examples of structures and organelles that can be found in typical plant cells:

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Cell (Plasma) Membrane - a thin, semi-permeable membrane that surrounds the cytoplasm of a cell, enclosing its contents.

Cell Wall - outer covering of the cell that protects the plant cell and gives it shape.

Chloroplast - the sites of photosynthesis in a plant cell. They contain chlorophyll, a green pigment that absorbs energy from sunlight.

Cytoplasm - gel-like substance within the cell membrane containing water, enzymes, salts, organelles, and various organic molecules.

Cytoskeleton - a network of fibers throughout the cytoplasm that helps the cell maintain its shape and gives support to the cell.

Endoplasmic Reticulum (ER) - extensive network of membranes composed of both regions with ribosomes (rough ER) and regions without ribosomes (smooth ER).

Golgi complex - responsible for manufacturing, storing and shipping certain cellular products.

Microtubules - hollow rods that function primarily to help support and shape the cell.

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Mitochondria - this organelle generates energy for the cell.

Amyloplast - an organelle in some plant cells that stores starch. Amyloplasts are found in starchy plants like tubers and fruits.

Nucleus - membrane bound structure that contains the cell's hereditary information.

Nucleolus - structure within the nucleus that helps in the synthesis of ribosomes.

Nucleopore - tiny hole within the nuclear membrane that allows nucleic acids and proteins to move into and out of the nucleus.

Centrosome - (also called the "microtubule organizing center") a small body located near the nucleus - it has a dense center and radiating tubules. The centrosomes are where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell. Unlike the centrosomes in animal cells, plant cell centrosomes do not have centrioles.

Peroxisomes - tiny structures bound by a single membrane that contains enzymes that produce hydrogen peroxide as a by-product. These structures are involved in plant processes such as photorespiration.

Ribosomes - consisting of RNA and proteins, ribosomes are responsible for protein assembly.

Vacuole - structure in a plant cell that provides support and participates in a variety of cellular functions including storage, detoxification, protection, and growth. When a plant cell matures, it typically contains one large liquidfilled vacuole.

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Plant Cell Types As a plant matures, its cells become specialized in order to perform certain functions necessary for survival. Some plant cells synthesize and store organic products, while others help to transport nutrients throughout the plant. Some examples of specialized plant cell types include: Parenchyma Cells - although not highly specialized, these cells synthesize and store organic products in the plant. Most of the plant's metabolism takes place in these cells.

Collenchyma Cells - Have a support function in plants, particularly in young plants. These cells help to support plants while not restraining growth due to their lack o fsecondary cell walls and the absence of a hardening agent in their primary cell walls.

Sclerenchyma Cells - provide a support function in plants, but unlike collenchyma cells, they have a hardening agent and are much more rigid.

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Cell divisions Cell division is the process by which a parent cell divides into two or more daughter cells.

Depending on the type of cell, there are two ways cells divide mitosis and meiosis. Each of these methods of cell division has special characteristics. One of the key differences in mitosis is a single cell divides into two cells that are replicas of each other and have the same number of chromosomes. This type of cell division is good for basic growth, repair, and maintenance. In meiosis a cell divides into two cells that have half the number of chromosomes.

Reducing the number of chromosomes by half is important for sexual reproduction and provides for genetic diversity Mitosis Cell Division Mitosis is how somatic or non-reproductive cells divide. Somatic cells make up most of your body's tissues and organs, including skin, muscles, lungs, gut, and hair cells. Reproductive cells (like eggs) are not somatic cells.

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In mitosis, the important thing to remember is that the daughter cells each have the same chromosomes and DNA as the parent cell. The daughter cells from mitosis are called diploid cells. Diploid cells have two complete sets of chromosomes. Since the daughter cells have exact copies of their parent cell's DNA, no genetic diversity is created through mitosis in normal healthy cells.

Mitosis consists of four basic phases and they are:- Prophase, metaphase, anaphase, and telophase; the steps follow one another without interruption. The entire four-stage division process averages about one hour in duration, and the period between cell divisions, called interphase or interkinesis.

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Prophase • Prophase is the first stage of mitosis. Chromatin condenses and becomes visible in the light microscope as chromosomes. (Remember that chromatin/DNA replicate during Interphase), the nuclear envelope dissolves; nucleolus disappear, centrioles (if present) divide and migrate, kinetochores and kinetochore fibers form, and the spindle forms.

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Metaphase

• Metaphase follows Prophase. The chromosomes (which at this point consist of chromatids held together by a centromere) migrate to the equator of the spindle, where the spindles attach to the kinetochore fibers.

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Anaphase • Anaphase begins with the separation of the centromeres, and the pulling of chromosomes (we call them chromosomes after the centromeres are separated) to opposite poles of the spindle.

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Telophase • Telophase is when the chromosomes reach the poles of their respective spindles, the nuclear envelope reforms, chromosomes uncoil into chromatin form, and the nucleolus (which had disappeared during Prophase) reform. Where there was one cell there are now two smaller cells each with exactly the same genetic information. These cells may then develop into different adult forms via the processes of development.

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Cytokinesis • Cytokinesis is the process of splitting the daughter cells apart. Whereas mitosis is the division of the nucleus, cytokinesis is the splitting of the cytoplasm and allocation of the golgi, plastids and cytoplasm into each new cell.

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Meiosis Meiosis is a type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. This process is required to produce egg and sperm cells for sexual reproduction. During reproduction, when the sperm and egg unite to form a single cell, the number of chromosomes is restored in the offspring.

Meiosis I, the first meiotic division, begins with prophase I. During prophase I, the complex of DNA and protein known as chromatin condenses to form chromosomes. The pairs of replicated chromosomes are known as sister chromatids, and they remain joined at a central point called the centromere. A large structure called the meiotic spindle also forms from long proteins called microtubules on each side, or pole, of the cell.

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. Between prophase I and metaphase I, the pairs of homologous chromosome form tetrads. Within the tetrad, any pair of chromatid arms can overlap and fuse in a process called crossing-over or recombination. Recombination is a process that breaks, recombines and rejoins sections of DNA to produce new combinations of genes.

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In metaphase I • In metaphase I, the homologous pairs of chromosomes align on either side of the equatorial plate.

Microtubules attached to kinetochore

Centromere (with kinetochore)

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Anaphase I

• In anaphase I, the spindle fibers contract and pull the homologous pairs, each with two chromatids, away from each other and toward each pole of the cell.

Sister chromatids remain attached

Homologous chromosomes separate

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Telophase I • During telophase I, the chromosomes are enclosed in nuclei. The cell now undergoes a process called cytokinesis that divides the cytoplasm of the original cell into two daughter cells. Each daughter cell is haploid and has only one set of chromosomes, or half the total number of chromosomes of the original cell.

Cleavage furrow

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• Meiosis II is a mitotic division of each of the haploid cells produced in meiosis I • During prophase II, the chromosomes condense, and a new set of spindle fibers forms. The chromosomes begin moving toward the equator of the cell.

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Metaphase II • During metaphase II, the centromeres of the paired chromatids align along the equatorial plate in both cells.

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Anaphase II • In anaphase II, the chromosomes separate at the centromeres. The spindle fibers pull the separated chromosomes toward each pole of the cell.

Sister chromatids separate

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TELOPHASE II AND CYTOKINESIS • during telophase II, the chromosomes are enclosed in nuclear membranes. Cytokinesis follows, dividing the cytoplasm of the two cells. At the conclusion of meiosis, there are four haploid daughter cells that go on to develop into either sperm or egg cells.

Haploid daughter cells forming

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Steps in meiosis I MEIOSIS I: Homologous chromosomes separate MEIOSIS II: Sister chromatids separate INTERPHASE

Centrosomes (with centriole pairs)

Nuclear envelope

PROPHASE I

Sites of crossing over

METAPHASE I Microtubules attached to kinetochore

ANAPHASE I

Metaphase plate

Sister chromatids remain attached

Spindle

Chromatin

Sister chromatids

Tetrad

Centromere (with kinetochore)

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Homologous chromosomes separate

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MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS

PROPHASE II

ANAPHASE II

METAPHASE II

TELOPHASE II AND CYTOKINESIS

Cleavage furrow

Sister chromatids separate

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Haploid daughter cells forming

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Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of 33 chromosomes Somali National University