All living things are made of cells, and cells are the smallest units that can be alive. Life on Earth is classified into five kingdoms, and they each have their own characteristic kind of cell. However the biggest division is between the cells of the prokaryote kingdom (the bacteria) and those of the other four kingdoms (animals, plants, fungi and protoctista), which are all eukaryotic cells. Prokaryotic cells are smaller and simpler than eukaryotic cells, and do not have a nucleus.
- Prokaryote = without a nucleus
- Eukaryote = with a nucleus
We'll examine these two kinds of cell in detail, based on structures seen in electron micrographs (photos taken with an electron microscope). These show the individual organelles inside a cell.
- Cytoplasm (or Cytosol). This is the solution within the cell membrane. It contains enzymes for metabolic reactions together with sugars, salts, amino acids, nucleotides and everything else needed for the cell to function.
- Nucleus. This is the largest organelle. Surrounded by a nuclear envelope, which is a double membrane with nuclear pores - large holes containing proteins that control the exit of substances such as RNA from the nucleus. The interior is called the nucleoplasm, which is full of chromatin- a DNA/protein complex containing the genes. During cell division the chromatin becomes condensed into discrete observable chromosomes. The nucleolus is a dark region of chromatin, involved in making ribosomes.
- Mitochondrion (pl. Mitochondria). This is a sausage-shaped organelle (8µm long), and is where aerobic respiration takes place in all eukaryotic cells. Mitochondria are surrounded by a double membrane: the outer membrane is simple, while the inner membrane is highly folded into cristae, which give it a large surface area. The space enclosed by the inner membrane is called the matrix, and contains small circular strands of DNA. The inner membrane is studded with stalked particles, which are the site of ATP synthesis.
- Chloroplast. Bigger and fatter than mitochondria, chloroplasts are where photosynthesis takes place, so are only found in photosynthetic organisms (plants and algae). Like mitochondria they are enclosed by a double membrane, but chloroplasts also have a third membrane called the thylakoid membrane. The thylakoid membrane is folded into thylakoid disks, which are then stacked into piles called grana. The space between the inner membrane and the thylakoid is called the stroma. The thylakoid membrane contains chlorophyll and stalked particles, and is the site of photosynthesis and ATP synthesis. Chloroplasts also contain starch grains, ribosomes and circular DNA.
- Ribosomes. These are the smallest and most numerous of the cell organelles, and are the sites of protein synthesis. They are composed of protein and RNA, and are manufactured in the nucleolus of the nucleus. Ribosomes are either found free in the cytoplasm, where they make proteins for the cell's own use, or they are found attached to the rough endoplasmic reticulum, where they make proteins for export from the cell. They are often found in groups called polysomes. All eukaryotic ribosomes are of the larger, "80S", type.
- Smooth Endoplasmic Reticulum (SER). Series of membrane channels involved in synthesising and transporting materials, mainly lipids, needed by the cell.
- Rough Endoplasmic Reticulum (RER). Similar to the SER, but studded with numerous ribosomes, which give it its rough appearance. The ribosomes synthesise proteins, which are processed in the RER (e.g. by enzymatically modifying the polypeptide chain, or adding carbohydrates), before being exported from the cell via the Golgi Body.
- Golgi Body (or Golgi Apparatus). Another series of flattened membrane vesicles, formed from the endoplasmic reticulum. Its job is to transport proteins from the RER to the cell membrane for export. Parts of the RER containing proteins fuse with one side of the Golgi body membranes, while at the other side small vesicles bud off and move towards the cell membrane, where they fuse, releasing their contents by exocytosis.
- Vacuoles. These are membrane-bound sacs containing water or dilute solutions of salts and other solutes. Most cells can have small vacuoles that are formed as required, but plant cells usually have one very large permanent vacuole that fills most of the cell, so that the cytoplasm (and everything else) forms a thin layer round the outside. Plant cell vacuoles are filled with cell sap, and are very important in keeping the cell rigid, or turgid. Some unicellular protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water.
- Lysosomes. These are small membrane-bound vesicles formed from the RER containing a cocktail of digestive enzymes. They are used to break down unwanted chemicals, toxins, organelles or even whole cells, so that the materials may be recycled. They can also fuse with a feeding vacuole to digest its contents.
- Cytoskeleton. This is a network of protein fibres extending throughout all eukaryotic cells, used for support, transport and motility. The cytoskeleton is attached to the cell membrane and gives the cell its shape, as well as holding all the organelles in position. There are three types of protein fibres (microfilaments, intermediate filaments and microtubules), and each has a corresponding motor protein that can move along the fibre carrying a cargo such as organelles, chromosomes or other cytoskeleton fibres. These motor proteins are responsible for such actions as: chromosome movement in mitosis, cytoplasm cleavage in cell division, cytoplasmic streaming in plant cells, cilia and flagella movements, cell crawling and even muscle contraction in animals.
- Centriole. This is a pair of short microtubules involved in cell division.
- Cilium and Flagellum. These are flexible tails present in some cells and used for motility. They are an extension of the cytoplasm, surrounded by the cell membrane, and are full of microtubules and motor proteins so are capable of complex swimming movements. There are two kinds: flagellacilia (pl.) are identical in structure, but are much smaller and there are usually very many of them. (pl.) (no relation of the bacterial flagellum) are longer than the cell, and there are usually only one or two of them, while
- Microvilli. These are small finger-like extensions of the cell membrane found in certain cells such as in the epithelial cells of the intestine and kidney, where they increase the surface area for absorption of materials. They are just visible under the light microscope as a brush border.
- Cell Membrane (or Plasma Membrane). This is a thin, flexible layer round the outside of all cells made of phospholipids and proteins. It separates the contents of the cell from the outside environment, and controls the entry and exit of materials. The membrane is examined in detail later.
- Cell Wall. This is a thick layer outside the cell membrane used to give a cell strength and rigidity. Cell walls consist of a network of fibres, which give strength but are freely permeable to solutes (unlike membranes). Plant cell walls are made mainly of cellulose, but can also contain hemicellulose, pectin, lignin and other polysaccharides. There are often channels through plant cell walls called plasmodesmata, which link the cytoplasms of adjacent cells. Fungal cell walls are made of chitin. Animal cells do not have a cell wall.
- Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles
- Ribosomes. The smaller (70 S) type.
- Nuclear Zone. The region of the cytoplasm that contains DNA. It is not surrounded by a nuclear membrane.
- DNA. Always circular, and not associated with any proteins to form chromatin.
- Plasmid. Small circles of DNA, used to exchange DNA between bacterial cells, and very useful for genetic engineering.
- Cell membrane. made of phospholipids and proteins, like eukaryotic membranes.
- Mesosome. A tightly-folded region of the cell membrane containing all the membrane-bound proteins required for respiration and photosynthesis.
- Cell Wall. Made of murein, which is a glycoprotein (i.e. a protein/carbohydrate complex). There are two kinds of cell wall, which can be distinguished by a Gram stain: Gram positive bacteria have a thick cell wall and stain purple, while Gram negative bacteria have a thin cell wall with an outer lipid layer and stain pink.
- Capsule (or Slime Layer). A thick polysaccharide layer outside of the cell wall. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis.
- Flagellum. A rigid rotating helical-shaped tail used for propulsion. The motor is embedded in the cell membrane and is driven by a H+ gradient across the membrane. Clockwise rotation drives the cell forwards, while anticlockwise rotation causes a chaotic spin. This is an example of a rotating motor in nature.
Summary of the Differences Between Prokaryotic and Eukaryotic Cells
| small cells (< 5 mm)
|| larger cells (> 10 mm)
| always unicellular
|| often multicellular
| no nucleus or any membrane-bound organelles
|| always have nucleus and other membrane-bound organelles
| DNA is circular, without proteins
|| DNA is linear and associated with proteins to form chromatin
| ribosomes are small (70S)
|| ribosomes are large (80S)
| no cytoskeleton
|| always has a cytoskeleton
| cell division is by binary fission
|| cell division is by mitosis or meiosis
| reproduction is always asexual
|| reproduction is asexual or sexual
Prokaryotic cells are far older and more diverse than eukaryotic cells. Prokaryotic cells have probably been around for 3.5 billion years - 2.5 billion years longer than eukaryotic cells. It is thought that eukaryotic cell organelles like mitochondria and chloroplasts are derived from prokaryotic cells that became incorporated inside larger prokaryotic cells. This idea is called endosymbiosis, and is supported by these observations:
- organelles contain circular DNA, like bacteria cells.
- organelles contain 70S ribosomes, like bacteria cells.
- organelles have double membranes, as though a single-membrane cell had been engulfed and surrounded by a larger cell.