Prokaryote = "before carrier bag" i.e without a nucleus
Eukaryote = "good carrier bag" i.e 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.
Summary of the Differences between
Prokaryotic and Eukaryotic Cells.
Prokaryotic Cells
- Small cells (< 5 microm.)
- Always unicellular
- No nucleus or any membrane bound organelles, such as mitochondria
- DNA is circular, without proteins
- Ribosomes are small (70S)
- No cytoskeleton
- Motility by rigid rotating flagellum (made of flagellin)
- Cell division is by binary fission
- Reproduction is always asexual
- Huge variety of metabolic pathways
Eukaryotic Cells.
- Larger cells (> 10 micro.m)
- Often multicellular
- Always have nucleus and other membrane bound organelles
- DNA is linear and associated with proteins to form chromatin
- Ribosomes are large (80S)
- Always has a cytoskeleton
- Motility by flexible waving cilia and flagellae (made of tubulin)
- Cell division is by mitosis or meiosis
- Reproduction is asexual or sexual
- Common metabolic pathways
MORE ON PROKARYOTIC AND EUKARYOTIC CELLS
- Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles
- Ribosomes. The smaller (70S) type.
- Nuclear Body. 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 loops of DNA, used to exchange DNA between bacterial cells. Used in genetic engineering, they often contain genes giving resistance to antibiotics.
- 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. Can also be associated with the nucleoid.
- Cell wall. Made of murein (not cellulose), which is glycoprotein (i.e a protein/carbohydrate complex, also called peptidoglycan). There are two kinds of cell wall, which can be distinguish by Gram's stain:
- Gram +ve bacteria have a thick cell wall, stain purple, may have spores and are sensitive to penicillin and lysosome (an antibacterial enzyme found in tears and saliva)
- Gram -ve bacteria have a thin cell wall with an outer lipid layer, have no spores and stain pink-these are thought to be more highly evolved.
- Capsule. A thick polysaccharide layer outside the cell wall. Used for sticking cells together, as a food reserve, as protection against desiccation and chemicals, and as protection against phagocytosis. Found only in some Gram +ve bacteria, if a capsule is present, then flagellae are not.
- Flagellum. A rigid rotating helical-shaped tail used for propulsion. The motor is embedded in the cell membrane and is driven by a H+ (hydrogen) gradient across the membrane. They always rotate clockwise - the only known example of a rotating motor in nature-rather like a propeller on a ship, it has to pass through the 'hull' of the cell via a waterproof seal.
- Cell membrane (or plasma membrane). This is a thin, flexible layer round the outside of all cells made of phopholipids and proteins. It separates the content of the cell from the outside environment, and controls the entry and exit of materials. The membrane is examined in detail later.
- Cytoplasm (or Cytosol). This is the solution within the cell membrane. It contains enzymes for glycolysis (the first stage of respiration) and other metabolic reactions together with sugars, salts, amino acids, nucleotides and everything else needed for the cell to function. This is where the first stage of respiration (=glysolysis) takes place.
- 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 mRNA and ribosomes 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 the nucleus, involved in making ribosomes and 'processing' m-RNA (i.e removing introns)
- 80S 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 nuclues. Ribosomes can be free in the cytoplasm, or (more commonly) are attached to the rough endoplasmic reticulum. They are often found in groups called polysomes. NB All eukaryotic ribosomes are of the larger, 80S, type.
- Endoplasmic Reticulum (ER). This collection of membrane channels forms an important transport 'highway' within the cell, allowing molecules to move from one place to another. It is attached to, and formed from, the outer membrane of the nucleus, and plays an important part in protein synthesis. It comes in two distinct forms:
- Rough Endoplasmic Reticulum (RER). this is studded with numerous 80S ribosomes which give it its rough endoplamic appearance. The ribosomes synthesise proteins which are processed in the SER (e.g by modifying the polypeptide chain, or adding carbohydrates), before being exported from the cell via the Golgi Body.
- Smooth Endoplasmic Reticulum (SER). Similar to the RER, but without the ribosomes. Series of membrane channels involved in the syntheses and transport of materials, mainly lipids and glycoproteins, needed by the cell.
- Golgi Body (or Apparatus). Another series of flattened membrane vesicles, formed from the endoplasmic reticulum. It's job is to transport proteins from the RER to the cell membrane for export. Parts of the SER 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 a dilute solution. most cells 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 cyotplasm (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 turgid. Some unicellular protoctists have feeding vacuoles for digesting food, or contractile vacuoles for expelling water (osmoregulation)
- Lysosomes. these are small membrane-bound vesicles 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. Responsible for cell death - 'autolysis'
- Cytoskeleton. this is a network 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 inpositon. There are two types of protein fibres (microfilaments and microtubules); each has corresponding protein that can carry a 'cargo' such as an organelle, chromosome or other cytoskeleton fibres along the fibre. They are responsible for chromosome movement in mitosis and the subsequent division of the cell, cytoplasmic streaming or cycloses (in plants only), cilia and flagella movements and muscle contraction.
- Centriole. This is a set of short microtubules ('9+2') involved in cell division. before each division the centriole replicates itself and the two centrioles move to opposite ends of the cell, forming the spindle that organises and separates the chromosomes.
- Cilia and Flagellae (or Undulipodia) these are long flexible 'tails' present in some cells and used for movement. They are surrounded by the cell membrane, and are full of microtubules and motor proteins, so they are capable of complex swimming movements. There are two kinds:
- Flagellae (no relation of the bacterial flagellum) are longer than the cell, and there are usually only one or two of them, whilst
- cilia, though identical in structure are much smaller and there are usually very many of them
- Microvilli These are small finger-like extensions of the cell membrane found in some animal cells (e.g the epithelial cells of the gut and kidney), where they increase the surface area for absorption. They are just visible under the light microscope as the brush border.
- 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). (A wickerwork basket is a good analogy) Plant cell walls are made mainly of cellulose, but also contain pectin, lignin and other polysaccharides too. It is built up in layers with the middle lamella separating the cell walls called plasmodesmata, which link the cytoplasm of adjacent cells.
- fungal cell walls are made of chitin
- animal cells do not have a cell wall
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