Animal Cells

Here is a typical animal cell. Cells are specialised to do different jobs. They are usually arranged into tissues. A tissue contains many cells all of the same kind and all doing the same jobs. You must know about a variety of animal cells for your GCSE exam, so here are some examples.

This is a fairly typical animal cell. Like all other animal cells it has a nucleus, cytoplasm and a cell membrane. Liver cells have many jobs. They remove toxic chemicals from the blood, they store glycogen, they remove excess amino-acids from the blood and make urea which is excreted by the kidneys; they generate heat to warm your blood on a cold day; they store iron, and so on.

Like all other animal and plant cells they contain mitochondria (singular mitochondrion). These are cell organelles. Their job is to obtain energy from glucose by tissue respiration. The energy produced by mitochondria is released into the cell in the form of ATP (Adenosine Tri-Phosphate).

Here is a diagram of a sperm. It is a single cell and just like other cells it has a nucleus, cytoplasm and a nucleus. It is specialised to carry genetic information. It has very little cytoplasm. It has one very large mitochondrion. It has a ling tail. The tail gets ATP energy from the mitochondrion. The tail lashes about like a whip driving the sperm forwards (swimming). The acrosome contains enzymes which help it break into an egg to fertilize the egg.

This is a motor neurone (nerve cell). It is very long. The cell body at the left side of the diagram is found in the spinal cord or brain. The motor end plates are found on the surface of a muscle. Electrical messages (nerve impulses) are carried along the axon from your brain to one of your muscles. When the impulse gets to a muscle, it makes the muscle contract.

You can see that the axon (nerve fibre) is covered in "Schwann Cells" which contain a lot of fat and insulate each axon from all the others. I have not bothered to draw the nuclei of the Schwann Cells in place, but they all have one.

This is a white blood cell: there are usually a few thousand white blood cells per cubic millimetre of blood. Their jobs are to produce antibodies and to engulf bacteria, i.e. they fight disease. When you have an infection, the number of white blood cells in your blood will rise. They can change their shape and this helps them to squeeze themselves through cracks in your capillary walls. White blood cells can escape from the blood into your tissues to fight infections.

This is a red blood cell. There are approximately five million red blood cells per cubic millimetre of blood, slightly more in men and less in women. The number of red blood cells is higher in people who live at high altitudes. Red blood cells have a perfect shape for absorbing and releasing oxygen. They are full of an iron containing protein called haemoglobin. This protein has a very high affinity for oxygen. When blood passes through your lungs it becomes saturated with oxygen.

Haemoglobin is an amazing chemical because if you warm it slightly and add a bit of acid, it will give up all its oxygen. Well, when your blood goes through your muscles, the heat from exercise and carbon dioxide force the heamoglobin to give up its oxygen.

Red blood cells do not have nuclei. Developing red blood cells in your bone marrow do have nuclei, but the nuclei disintigrate when they are fully developed: this is because the nuclei are not needed any more.

I hope that you have learnt that all animal cells have a cell membrane, cytoplasm and a nucleus (red blood cells excepted) and that cells are specialised to do different jobs. A biologist would say that "their structures are related to their functions".

Nucleus

The nucleus of a cell contains a very special chemical called DNA. This stands for Deoxyribo Nucleic Acid: you do NOT need to remember the full name for your GCSE exam though you will have to know it for "A" Level biology. It is actually acidic, hence the name. It is found in the nucleus, hence the name. I contains a sugar called deoxyribose, hence the name.

DNA is very special because it contains information to control what the cytoplasm does. Another similar chemical called RNA is used to pass the information from the nucleus to the cytoplasm. RNA stand for Ribo Nucleic Acid: it contains a sugar called ribose. Again you do not need to remember the full name for GCSE.

You and I have inherited feature from our parents: we are similar but not identical to our siblings. I have three younger siblings, two are sisters and one is a brother. Please don't e-mail me to tell me that you are identical to one of your siblings: if you are one of a pair of identical twins you are a special case. You will already know that you and your sibling are identical because you were made from the same sperma and egg.

The bits of information in the DNA molecules are called "genes". Have a look at my genetics page if you want to know more about inheritance.

Cytoplasm

The cytoplasm is the part of the cell which does all the work. Different cells do different jobs, however whatever the job is it will require energy in the form of a chemical called ATP. This energy containing chemical is produced by tiny organelles in the cell called mitochondria.

Muscle cells contain two very special chemicals called actin and myosin. These are made of protein and they can join together to form actinomyosin. When a nerve impulse simulates a muscle it makes the actinomyosin contract.

The cells in your salivary glands make mucin (a very slippery protein) and amylase (an enzyme). The nuclei of the secretory cells in your salivary gland contain the information about how to make these two proteins in their DNA.

Cell Membranes

The cell membrane controls what comes into and goes out of the cell. Cell membranes are semi-permeable. That means that they allow some things to pass through and prevent other things from passing through. Membranes are NOT fully permeable, NOR are they completely impermeable. 

Definition
Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
The definition contains three important statements:

1. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
   
2. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.
   
3. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration.

It does not matter too much which order you put these statements in. Nor does it matter if you write the definition as one sentence or three sentences. All that matters in your exam is that you make all three points when you explain what osmosis is.

Explanation
First the definition of osmosis:

1. Semi-permeable membranes are very thin layers of material (cell membranes are semi-permeable) which allow some things to pass through them but prevent other things from passing through.
   
   Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through.
   
2. A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water: there is a high concentration of water.
   
   Some teachers use the definition which starts "Osmosis is the passage of water from a dilute solution to a......" this means exactly the same as the definition I have given.
   
3. A region of low concentration of water is a concentrated solution of something like sucrose. In this case there is much less water.
   
   So you could use the definition "Osmosis is the passage of water from a dilute solution through a semi-permeable membrane to a more concentrated solution.

Now to explain osmosis:
When you put an animal or plant cell into a liquid containing water one of three things will happen.

1. If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis.
   
   Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.
   
2. If the medium is exactly the same water concentration as the cell there will be no net movement of water across the cell membrane.
   
   Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
   
3. If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis.
   
   Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.

The Consequences of Osmosis
Firstly what happens to plant cells:
Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.
When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight.

And now for the animal cells:
When animal cells are placed in sugar solutions things may be rather different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst: they do not become turgid because there is no cell wall to support the cell membrane. In concentrated solutions, water is sucked out of the cell by osmosis and the cell shrinks. In either case there is a problem. So animal cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why we have kidneys. The exact amount of water and salt removed from our blood by our kidneys is under the control of a part of the brain called the hypothalamus. The process of regulating the amounts of water and mineral salts in the blood is called osmoregulation. My insulin page will tell you more about other homeostatic mechanisms.
Animals which live on dry land must conserve water; so must animals which live in the sea (the sea is very salty!), but animals which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis.