Analysing Cells
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Cell Transport
Gas Exchange
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There are two major types of cell. eukaryotic cells, which have membrane-bound organelles, like a nucleus and mitochondrion; and prokaryotic which do not have these organelles.

Eukaryotic Cells

Below is a diagram of a typical eukaryotic cell. As you can see, it is much more complicated than the models you are presented with at lower levels of study.

diagram of a eukaryotic cell

Now that we have had a look at the different organelles that are found in animal and plant cells, we will go on to look at these organelles in more detail. An organelle is a body within a cell that has a partiular function. It is to the cell, what an organ is to your body.

NameDiagramDescription
Nucleus diagram of a nucleus
  • The nucleus has a nucleuolus, this is where ribosomes are made.
  • It is surrounded by the nuclear membrane, which attaches to the endoplasmic reticulum.
  • Genetic material is stored in the nucleus in the form of DNA.
    		•
    Mitochndrion
    See respiration for more information.
    Endoplasmic reticulum diagram of rough endoplasmic reticulum
  • There are two types of endoplasmic reticulum (ER), rough ER and smooth ER
  • Rough endoplasmic reticulum (RER) is covered in ribosomes that synthesize proteins that are then transported by the RER to be used elsewhere in the body.
  • Smooth endoplasmic reticulum (SER) has more tubular sacs, and is not covered by ribosomes (hence, it looks smooth). It contains enzymes important for synthesizing fats, lipids, etc.
    		•
    Golgi Body diagram of the golgi body
  • Also known as the golgi apparatus, this organelle recieves substances from the ER. It then modifies them before secreting them out of the cell via vesicles, these are smaller sacs that separate from the golgi body.
    		•
    Chloroplasts
    See photosynthesis for more information.

    Prokaryotic Cells

    For more information about the structure of bacteria see bacteria.

    This is a cell with no membrane-bound organelles. The best example of such a cell is a bacterium, the specific functions carried out by organelles are performed by other parts of a prokaryotic cell. Below is a theory which explains why bacteria do not have membrane-bound organelles.

    Endosymbiosis

    Pronounced [en - do - sim - by - oh - sis], and meaning in Ancient Greek: living together inside, this is a theory which explains the origins of some organelles in eukaryotic cells.

    Organelles like chloroplasts and mitochondria both have a membrane, contain their own genetic material, and can produce food. And this theory (now generally accepted by biologists) says that these were once bacteria that lived independently.

    However, many millions of years ago, in the early stages of life on earth; the mitochondria (as bacteria) developed a symbiotic relationship with other cells. A symbiotic relationship is where two organisms depend on each other for survival with each providing something that helps the other survive. Such a relationship exists in your digestive system where bifidobacterium baceria live and help with digestion.

    Over millions of years of this relationship developed and evolved, until the two cells are so dependent on each other that they cannot survive without each other.

    Updated: 09 May 2012