Organelles in Eukaryotic Cells

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Organelles in Eukaryotic Cells

Plant and animal cells are both eukaryotes. They contain several types of organelle within the cytoplasm and these organelles each have a distinctive shape and size.

Many organelles are present in all eukaryotic cells, although the number that a specific cell contains depends on its function.  Each organelle has a particular job allocated to it, so by having various organelles in each cell there is division of labour and the cell can perform its function efficiently. For example, cells needing lots of energy contain many mitochondria and cells that synthesise very actively need a very extensive endoplasmic reticulum and many golgi bodies.  

The nucleus is the most obvious structure in a eukaryotic cell; it is usually spherical and about 10µm across. One of its major functions is to act as a control centre for the activities of the cell.  Virtually all eukaryotic cells have a nucleus (red blood cells lose theirs and so are enucleate for most of their life-span). It is surrounded by the nuclear envelope, which is a double membrane consisting of two lipid bilayers. The nuclear envelope contains a large number of perforations called nuclear pores. These are about 100 nm across and control the exchange of materials between the nucleus and the cytoplasm. Contained within the nuclear envelope is the nucleolus and DNA.

DNA is a complex molecule and carries the information which the cell needs to divide and carry out its cellular processes. The DNA is the same in most cells of a particular individual, but depending on the specific cell type, some genes may be turned on or off. When a cell is dividing, the DNA and surrounding protein condense into chromosomes. The chromosomes are moved to opposite ends of the cell so that, when the cell splits, each daughter cell receives the correct amount of DNA.

The most prominent structure in the nucleus is the nucleolus and this is observed as a dark staining structure. The nucleolus contains copies of the genes that code for ribosomal RNA, this is then combined with ribosomal proteins to produce ribosomes. The ribosomes move out of the nucleus to positions on the rough endoplasmic reticulum or float freely in the cytoplasm, where they are critical in protein synthesis.

Ribosomes are tiny, dense organelles and are about 20nm in diameter.They are present in great numbers in most eukaryotic cells. Ribosomes consist of two subunits, one smaller than the other and are made up of about two thirds RNA and one third protein. Ribosomes use chemical instructions from the nucleus to assemble amino acids and synthesise the new proteins that the cell needs and also those which are produced to send out of the cell. They do this by the process of translation.

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Most ribosomes are attached to the surface of the rough endoplasmic reticulum. The endoplasmic reticulum is a series of interconnecting flattened tubular tunnels, which are continuous with the outer membrane of the nucleus. It runs through the cytoplasm of all eukaryotic cells. The ER of a cell often takes up more than a tenth of the total cell volume. There are two types of endoplasmic reticulum- rough and smooth. Smooth ER contains no ribosomes so appears smooth, rough ER appears rough due to the presence of ribosomes on the membrane. Smooth ER and rough ER also have different functions but ...

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