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|Eukaryotic Cells||Prokaryotic Cells||Cell Fractionation|
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 = "before
carrier bag" i.e. without a
Eukaryote = "good carrier bag" i.e. with a nucleus
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 glycolysis (part of respiration) and other 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 and ribosomes from the nucleus. The interior is called the nucleoplasm, which is full of chromatin- a DNA/protein complex in a 1:2 ratio 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 (8Ám long), and is where aerobic respiration takes place in all eukaryotic cells. Mitochondria are surrounded by a double membrane: the outer membrane is simple and quite permeable, 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 mitochondrial matrix, and contains small circular strands of DNA. The inner membrane is studded with stalked particles, which are the site of ATP synthesis.
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 other photosynthetic pigments
arranged in photosystems, together with stalked particles, and is
the site of photosynthesis and ATP synthesis. Chloroplasts also contain
starch grains, ribosomes and circular DNA.
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.
Endoplasmic Reticulum (SER). Series of membrane channels involved in
synthesising and transporting materials, mainly lipids, needed by the
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.
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.
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.
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
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
This is a pair of short microtubules involved in cell division. Before
each division the centriole replicates itself and the two centrioles move
to opposite ends of the cell, where they initiate the spindle that
organises and separates the chromosomes (see module 2).
(Cilium and Flagellum). This is a long flexible tail present in some
cells and used for motility. It is an extension of the cytoplasm,
surrounded by the cell membrane, and is full of microtubules and motor
proteins so is capable of complex swimming movements. There are two kinds:
flagella (no relation of the bacterial flagellum) are longer than
the cell, and there are usually only one or two of them, while cilia
are identical in structure, but are much smaller and there are usually
very many of them.
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.
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.
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 can also contain hemicellulose,
pectin, lignin and other polysaccharides. They are built up in three
layers called the primary cell wall, the secondary cell wall and the
middle lamella. There are often channels through plant cell walls called plasmodesmata,
which link the cytoplasms of adjacent cells. Fungal cell walls are made of
chitin (poly-glucosamine). Animal cells do not have a cell wall,
though they do have a layer of carbohydrate outside the cell membrane
called the cell coat, or glycocalyx.
Cytoplasm. Contains all the enzymes needed for all metabolic reactions, since there are no organelles
Ribosomes. The smaller (70 S) type.
Nucleoid (or 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. Can also be associated with the nucleoid.
Cell Wall. Made of murein (not cellulose), which is a glycoprotein (i.e. a protein/carbohydrate complex, also called peptidoglycan). 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, like the glycocalyx of eukaryotes. 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 the only known example of a rotating motor in nature.
|small cells (< 5 mm)||
larger cells (> 10 mm)
|always unicellular||often multicellular|
|no nucleus or any membrane-bound organelles, such as mitochondria||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|
|motility by rigid rotating flagellum made of flagellin||motility by flexible waving undulipodium, made of tubulin|
|cell division is by binary fission||cell division is by mitosis or meiosis|
|reproduction is always asexual||reproduction is asexual or sexual|
|huge variety of metabolic pathways||common metabolic pathways|
Prokaryotic cells are far older and more diverse than eukaryotic cells. Prokaryotic cells have probably been around for 3.5 billion years, while eukaryotic cells arose only about 1 billion years ago. It is thought that eukaryotic cell organelles like nuclei, 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.
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.
organelles reproduce by binary fission, like bacteria.
organelles are very like some bacteria that are alive today.
This means separating different parts and organelles of a cell, so that they can be studied in detail. All the processes of cell metabolism (such as respiration or photosynthesis) have been studied in this way. The most common method of fractionating cells is to use differential centrifugation:
Cut tissue (e.g. liver, heart, leaf, etc) in ice-cold isotonic
buffer. Cold to stop
enzyme reactions, isotonic to stop osmosis, and buffer to stop pH
Grind tissue in a blender to break open cells.
3. Filter. This removes insoluble tissue (e.g. fat, connective tissue, plant cell walls, etc). This filtrate is not called a cell-free extract, and is capable of carrying out most of the normal cell reactions.
4. Centrifuge filtrate at low speed
(1 000 x g for 10 min)
5. Centrifuge supernatant at medium speed
(10 000 x g for 30 min)
6. Centrifuge supernatant at high speed
(100 000 x g for 1 hour)
7. Centrifuge supernatant at very high speed
(300 000 x g for 3 hours)
Supernatant is now organelle-free cytoplasm
8. Supernatant is now organelle-free cytoplasm
A more sophisticated separation can be performed by density gradient centrifugation. In this, the cell-free extract is centrifuged in a dense solution (such as sucrose or caesium chloride). The fractions don't pellet, but instead separate out into layers with the densest fractions near the bottom of the tube. The desired layer can then be pipetted off. This is the technique used in the Meselson-Stahl experiment (module 2) and it is also used to separate the two types of ribosomes. The terms 70S and 80S refer to their positions in a density gradient
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Last updated 23/05/2004