DESCRIBE THE STRUCTURAL COMPARTMENTATION OF MAMMALIAN CELLS AND THE DIFFERING FUNCTIONS OF THESE COMPARTMENTS

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        11/10/2003

DESCRIBE THE STRUCTURAL COMPARTMENTATION OF MAMMALIAN CELLS AND THE DIFFERING FUNCTIONS OF THESE COMPARTMENTS

Mammalian cells are eukaryotic and therefore have organelles that are common to all eukaryotic cells. Each of these organelles has a basic function to carry out in the cell.

The nucleus, usually the largest organelle in animal cells, is separated from the cytoplasm in the cell by two nuclear membranes comprised of phospholipid bilayers and many different proteins. The inner membrane is the one which defines the nucleus’ shape. At certain points around the nuclear membranes, the two membranes join to form nuclear pores, which are important for the movement of material in and out of the nucleus that could otherwise not pass through the phospholipid bilayer around the structure. Examples of these include lengths of tRNA or mRNA which are too large to pass through the membrane but are necessary for functions elsewhere in the cell.

The nucleus has many important functions in the cell, the most important being the storage of DNA which codes for all the proteins made by the cell. The way in which this is stored differs depending on whether the cell is a growing cell or a ‘resting’ cell, i.e. one in which minimal synthesis of DNA and RNA takes place. The heterochromatin- a threadlike mass of DNA and associated proteins found when a cell is not reproducing- breaks down during the early stages of cell division.

Inside the nucleus there is also a nucleolus, which shows dark on an electron microscope picture, which is the site of ribosome formation.

Ribosomes are the sites of protein synthesis, where RNA is translated into protein. Protein synthesis is extremely important to cells, and so large numbers of ribosomes are found throughout cells (often numbering in the hundreds or thousands). Ribosomes exist floating freely in the cytoplasm, and also bound to the endoplasmic reticulum (ER).

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Figure 1 Rough endoplasmic reticulum in a mammalian cell

ER bound to ribosomes is called rough ER because the ribosomes appear as black dots on the ER in electron microscope photos, giving the ER a rough texture. These organelles are quite small, made up of 50 proteins and several long RNAs intricately bound together. Ribosomes have no membrane. Ribosomes disassemble into two subunits when not actively synthesizing protein.

As well as rough endoplasmic reticulum, there exists in a cell smooth endoplasmic reticulum, which does not have ribosomes bound to the outer membrane. The smooth ER is ...

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This essay covers the structure and function of mammalian cell organelles in a fair level of detail, certainly adequate for A' Level. The information provided about each organelle is well-organised and accurate. However, the essay could be improved significantly by focusing on a unifying theme - compartmentalisation. The writer does refer briefly to this concept at the start but it gets lost as the essay proceeds. The functions of the cell often need to be kept separate from other activities, eg. the hydrolytic enzymes of lysosomes must not spill out into the cytoplasm. On the other hand, organelles frequently work together. Ribosomes make protein which the Golgi modifies and packages into lysosomes or secretory vesicles. Overall, then, the essay is a good starting point with regard to organelle structure and function, but would have benefited from a discussion of why compartmentalisation was created, and how compartments work together.