2.1 Cell theory

2.1

Cell theory

KEY IDEAS

In this topic, you will learn that: ✚ cells are the basic unit of life on Earth Cells: the building blocks of life For any object to be classified as living, it must have at least one cell. Cells are the smallest structural and functional units of life. Robert Hooke, an English natural philosopher of the 17th century, studied cork under a microscope (Figure 1). He identified the small square structures in the cork, and named them after the rooms (‘cells’) where monks lived in monasteries. ✚ there are two types of cells: prokaryotic and eukaryotic cells. FIGURE 1 Robert Hooke’s microscope Some living organisms are unicellular – they exist as a single cell. Other organisms are multicellular – they are composed of many cells, sometimes even trillions. Cell theory Before the invention of the microscope, biologists believed in a theory known as spontaneous generation. People believed that maggots appeared spontaneously from rotting meat and tadpoles appeared from a new puddle of water. Eventually this theory was disproved with experiments that showed, for example, that maggots only appeared in meat after flies had laid their eggs. In the 19th century, Louis Pasteur disproved the theory of spontaneous generation. His work demonstrated that all microorganisms arise from pre-existing ones and that if the microorganisms are isolated and destroyed in a sterile environment, no new microorganisms will grow. This became the foundation for cell theory and revolutionised the field of medicine with the development of antiseptic techniques. Cell theory specifies that: • all living organisms consist of at least one cell • all cells arise from pre-existing cells unicellular consisting of a single cell multicellular consisting of many cells cell theory the theory that describes cells as the basic component of all living organismsDRAFT ONLY - NOT FOR SALE • cells are the smallest structural unit of life (Figure 2). This theory is based on both biochemical and microscopic observations and experimentation.

Cell structure

plasma membrane

Although cells can be structurally and functionally different, all cells have: • a plasma membrane that separates the internal contents from the external environment • cytosol , which is a fluid made of water, enzymes, ions and salts • deoxyribonucleic acid (DNA) , which carries the genetic information required for DNA replication, cell division and protein production • ribosomes for protein production.

the boundary of all cells that separates the cytosol from the external environment and controls the entry and exit of substances

cytosol

the internal fluid component of a cell, excluding organelles Cell classification FIGURE 2 Cells are the building blocks of life. These cells are from onion epithelial tissue. deoxyribonucleic acid (DNA) the genetic material that carries cellular instructions There are two type of cells – prokaryotic cells and eukaryotic cells .

ribosome

Prokaryotes an organelle in cells where proteins are Prokaryotes are extremely small, unicellular organisms that consist of a single prokaryotic made cell. Bacteria, cyanobacteria (photosynthetic bacteria) and archaea are all types of prokaryotes. Prokaryotic cells do not have a nucleus or other membrane-bound structures. All of the chemical processes required to sustain life occur within the cytosol of a single cell. Prokaryotes have the following generalised structures (Figure 3). • Genetic material is located within the cytosol in a central region of the cell called the nucleoid as a single circular chromosome . • Ribosomes are scattered throughout the cytosol for protein production. • A plasma membrane allows entry and exit of substances. • A cell wall provides structural support, shape and protection. Although all prokaryotes have these common features, there is a lot of variation between species and groups. Some prokaryotes die in the presence of oxygen, whereas others will only survive if oxygen is present. Some prokaryotes are heterotrophic, which means they feed on other organisms; other prokaryotes are autotrophic and rely on chemosynthesis or photosynthesis for their energy.

prokaryotic cell

a cell that does not contain a nucleus or other membranebound organelles

eukaryotic cell

a cell that contains a nucleus and other membrane-bound organelles

prokaryote

a single-cell organism made of a prokaryotic cell Cell wall

nucleoid

a region of a prokaryotic cell where Plasma membrane DNA is located

chromosome

Ribosomes a coiled, condensed structure of DNA and associated histone proteins Flagellum

photosynthesis

the process by which light energy is converted into chemical energy (glucose) in the chloroplasts of plant cells

Nuclear material (circular chromosome) FIGURE 3 The generalised structure of a prokaryotic cell DRAFT ONLY - NOT FOR SALE

Bacteria

Bacteria are the most ancient, abundant and diverse organisms on Earth (Figure 4). They consist of a cell wall made of carbohydrates and proteins that provides strength, support and protection. Many bacteria have a sticky capsule surrounding the cell wall that enables them to adhere easily to surfaces.

flagellum

Flagellum enables movement. Single chromosomal loop located within the nucleoid of the cell. FIGURE 4 Detailed structural components of a bacterial cell

Cell wall provides support and protection and prevents dehydration. Different types of pili allow attachment, movement and exchange of genetic information. Capsule is sticky to adhere to surfaces. Ribosomes produce proteins. Cellular extensions of bacteria Bacteria often have specialised structures sticking out from the cell surface that allow them to move around, attach to surfaces and exchange genetic material. These include the following. • Flagella are tail-like extensions from the cell surface that allow some bacteria to move. Often only a single flagellum is shown in images of bacteria, but flagella are regularly found in clusters of 1–4 at one end or both ends of the cell, or spread out to cover the entire cell surface. Flagella work together in a coordinated whip-like motion that enables directional and controlled movement. • Pili are hollow cellular extensions that allow different prokaryotic cells to join with each other to exchange small circular sections of DNA known as plasmids. Advantageous mutations such as resistance to antibiotics can be transferred to other bacteria through pili. • Fimbriae are a specific type of pili that are finer, more numerous and less specialised. They give a bacterial cell a hairy appearance and help it attach to surfaces. Even if flagella are not present, when a bacterial cell lands on a surface, fimbriae secrete a sticky gluelike substance that adheres to the surface, allowing individual bacteria to attach to form a colony. Eukaryotes Protists, fungi, plants and animals are all classified as eukaryotes. Some species are unicellular and others are multicellular. All eukaryotes contain a nucleus and other membrane-bound organelles. These organelles are like small rooms in a house that each have a specific function. Each organelle has a unique environment that allows the cell to concentrate energy a tail-like extension of the bacterial plasma membrane that allows movement; plural flagella pilus a small extension of the bacterial plasma membrane used to exchange genetic material (plasmids); plural pili plasmid a small circular piece of extra DNA found in bacterial cells fimbria a fine extension of the bacterial plasma membrane that allows the cell to adhere to surfaces; plural fimbriae eukaryote an organism consisting of eukaryotic cells, such as animals, plants, fungi and protists organelle a compartment within the cytoplasm of eukaryotic cell where specialised functions are carried outDRAFT ONLY - NOT FOR SALE cytoplasm and resources where they are needed. The internal content of eukaryotic cells is called the the contents of eukaryotic cells, cytoplasm because it contains both the cytosol and fluid contained in the organelles. The excluding the nucleus cytoplasm does not include the nucleus, which contains the genetic material of the cell.

Each group of eukaryotic cells has a unique set of organelles (Figure 5). For example, plant cells have chloroplasts and a cell wall, whereas animal cells do not. Protists are unicellular and often have extensions for movement, much like those of bacterial cells. The cells within a single organism also vary slightly. Multicellular organisms are larger, and need many different cell types, each with a different structure and chemicals for carrying out specialised tasks.

a b

Cytoplasm

Plasma membrane

Cell wall Chloroplast Plasmodesmata Starch granules

Cell walls of neighbouring cells

FIGURE 5 A generalised structure of a a eukaryotic animal cell and b a eukaryotic plant cell Endosymbiotic theory Study tip According to endosymbiotic theory, eukaryotic cells evolved when a prokaryotic bacterial cell All eukaryotic entered (‘endo’) and became incorporated into a larger cell. This allowed the large cell and cells contain mitochondria for the small cell to share energy and resources, resulting in the smaller prokaryotic cell evolving cellular respiration. into an organelle of the larger cell. Chloroplasts are only Most scientists accept this theory as the process that formed mitochondria and located within cells that obtain nutrition chloroplasts within eukaryotic cells. For example, it is thought that aerobic bacteria were via photosynthesis. consumed by ancestral eukaryotic cells and eventually became the mitochondria of these cells, enabling eukaryotes to carry out cellular respiration within an organelle (Figure 6). Other eukaryotic cells engulfed photosynthetic bacteria, which eventually became specialised chloroplasts, forming photosynthetic eukaryotes that can photosynthesise, such as plant cells and some protists. Anaerobic eukaryotic cell Early aerobic eukaryotic cell Nucleus Endoplasmic reticulum Vacuole Nucleolus Nucleoplasm Nucleoplasm Nuclear Nuclear membrane membrane Mitochondrion Golgi body Cell walls of

Golgi body membrane Nucleus Chromatin Plasma membrane Mitochondrion Lysosome Ribosomes Rough endoplasmic reticulum Smooth endoplasmic reticulum Pinocytic vesicle Nucleolus Nuclear membrane Nucleus Cilium Golgi body Centrioles DRAFT ONLY - NOT FOR SALE Aerobic proteobacterium Mitochondria

FIGURE 6 The formation of mitochondria according to the endosymbiotic theory of eukaryotic cell evolution

Comparison of prokaryotic and eukaryotic cells

Eukaryotic cells are larger than prokaryotic cells and they contain cytoplasm rather than cytosol (Figure 7). This is because cytoplasm includes the fluid contained in organelles. Prokaryotic cells have free DNA in the cytosol, whereas the DNA of eukaryotic cells is enclosed by a double-layered membrane to form a nucleus. The inclusion of these membranebound organelles allows a eukaryotic cell to grow larger than a typical prokaryotic cell. Prokaryotic cells have ribosomes that are free within the cytosol, whereas eukaryotic cells also have ribosomes fixed on the surface of the rough endoplasmic reticulum as well as others that are free within the cytoplasm. FIGURE 7 A comparison of eukaryotic and prokaryotic cell structure. Note that some eukaryotic cells, such as plant cells, have cell walls. TABLE 1 Characteristics of prokaryotic and eukaryotic cells Feature Prokaryotic Eukaryotic Bacteria Protists Fungi Plants Animals Nucleus Absent Present Present Present Present Mitochondria Absent Present Present Present Present Chloroplasts Absent Present in some forms Absent Present Absent

Mode of nutrition Heterotrophic or autotrophic (chemosynthesis or photosynthesis) Photosynthesis or heterotrophic or combination of both

Heterotrophic by absorption Photosynthesis Heterotrophic by ingestion Multicellularity Absent Absent in many groups

Present except in yeasts

Present Present Cytosol Plasma membrane Cell wall DNA Ribosomes Capsule Prokaryotic cellEukaryotic cell Nucleus Ribosomes Golgi apparatus Endoplasmic reticulum Plasma membrane Mitochondrion Cytoplasm Lysosome Study tip Chloroplasts are not found in every cell of a plant. A leaf cell may contain thousands of chloroplasts, whereas flower petal cells contain none.DRAFT ONLY - NOT FOR SALE

CASE STUDY 2.1

Human red blood cells

Human red blood cells (Figure 8) are produced in bone marrow and move through the body in blood vessels, transporting oxygen that is bound to a protein called haemoglobin. Red blood cells are an unusual type of eukaryotic cell. They contain no nucleus and are therefore unable to divide. This means they need to be continually produced to replace old red blood cells that are removed and broken down by the liver after approximately 100 days. Red blood cells also do not have organelles associated with making proteins, such as endoplasmic reticulum or the Golgi apparatus. The haemoglobin protein is made in bone marrow when a red blood cell is produced. Red blood cells have no mitochondria because they do not require energy for active cellular functions, as the binding of oxygen is a passive process.

Describe and explain

1 Explain why cells are described as the basic structural features of life. 2 Describe the differences between cytoplasm and cytosol.

3 Explain how you could identify the cell in Figure 9 as eukaryotic. 4 Describe three components that are required by all cells for them to function correctly. Apply, analyse and compare 5 Apply your understanding of cell theory to explain why a virus is classified as non-living. 6 Contrast prokaryotic and eukaryotic cells. Design and discuss 7 Create a diagram of a generalised cell and label all the components possessed by all cells. 8 Read Case study 2.1 a Explain how red blood cells differ from typical eukaryotic cells. CHECK YOUR LEARNING 2.1 Red blood cells have no mitochondria because they do not require energy for active cellular functions, as the binding of oxygen is a passive process. FIGURE 8 Human red blood cells DRAFT ONLY - NOT FOR SALE b Suggest why red blood cells are still classified as eukaryotic cells, and not prokaryotic cells.

FIGURE 9 An amoeba is a microscopic unicellular organism.