Describe the structural compartmentation of mammalian cells

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DESCRIBE THE STRUCTURAL COMPARTMENTATION OF MAMMALIAN CELLS
AND THE DIFFERING FUNCTIONS OF THESE COMPARTMENTS
All mammalian cells are eukaryotic, and whilst the eukaryotic type of cell is not exclusive to mammals, mammalian cells differ from other eukaryotic cells because of the organelles that are or are not present. For instance some plant cells have chloroplasts which are not present in mammalian cells, but both plant cells and mammalian cells are eukaryotic in nature. The term eukaryotic refers to the cell having specific membrane bound organelles, which are not present in prokaryotic cells. The defining feature of a eukaryotic cell is usually its membrane bound nucleus (the exception being the red blood cell) [1]. Because of the sheer number of organelles in eukaryotic cells I will not be describing each independently and fully in this essay. Instead I will first provide an overview of the organelles involved in protein synthesis so as to give a logical order and clearer picture of each independent organelles specific function, and then move on to some of the most important organelles with a more independent function. The membrane that bounds the organelles into specific space is called a phospholipid bilayer. As shown in Fig 1, the bilayer is permeated with different types of protein, glycolipid etc. These allow the transmembrane movement of molecules that would otherwise be unable to permeate the phospholipid bilayer. This is necessary for the correct flow of molecules from one side of the membrane to the other, so the organelle is not starved of vital nutrients or unwanted electrochemical gradient is made, for example.

The nucleus usually the largest organelle in a mammalian cell, and like almost all other organelles is encased within a phospholipid bilayer. The phospholipid bilayer, as can be seen in Fig. 1 has different channels and routes which different molecules can diffuse or be transported through. Unlike most other organelles, which have a single phospholipid bilayer, the nucleus is double layered. At points this double membrane touches, forming nuclear pore complexes. This double membrane is necessary due to the need for macromolecular exchange through these pores that a normal phospholipid bilayer inhibits [2]. The outer bilayer of the nuclear envelope is partially made up of endoplasmic reticulum, an organelle I will come onto later [6]. The nucleus is where the genetic code for the production of different proteins is stored, therefore controlling the enzymes present and thus regulating the activity of the cell. These genes are encoded in a long series of different combinations of the 4 DNA bases; adenine, thymine, guanine and cytosine. The long series of bases are attached to each other by hydrogen bonds, and each end has a pentose phosphate backbone that twists round into the double helix structure discovered by Crick and Watson in the late 60’s. Within the nucleus is where chromatin is found. Chromatin is DNA strands wound around histones [3]. Many chromatin fibres further condensed make up chromosomes. It is these chromosomes that hold the genetic code for that particular cell, and is the site for one of the first steps of protein synthesis. This is known as transcription.

The nucleolus is a non-membrane bound structure in the nucleus of eukaryotic cells. It is made up of proteins and nucleic acid. The nucleolus is responsible for ribonucleoprotein maturation, where ribosomal RNA is transcribed and combined to proteins to form nearly complete ribosomes [9].

The ribosome is made up of large complexes of RNA and protein. They are the most abundant RNA-protein complex in the cell. A ribosome is composed of two parts; the small subunit, and the large subunit. The function of the ribosome as a whole is to translate the genetic information in the form of RNA (gathered via transcription) into a chain of amino acids. This chain will later become a protein. The large ribosomal subunit is...
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