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Membranes and their functions.

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Membrane Membranes form boundaries both around the cell (the plasma membrane) and around distinct sub cellular compartments (e.g. nucleus, mitochondria, lysosomes, etc.). They act as selectively permeable barriers allowing the inside environment of the cell or the organelle to differ from that outside. Membranes are involved in signaling processes; they contain specified receptors for external stimuli and are involved in both chemical and electrical signal generation. All membranes contain two basic components: lipids (mainly phospholipids) and proteins. Some membranes also contain carbohydrate. The decomposition of lipid, protein, and carbohydrates vary from one membrane to another. The cell membrane's main function is to serve as a boundary between the cell and its environment. It consists of a bimolecular phospholipid layer with inwardly directed hydrophobic tails and a variety of protein molecules with an irregular arrangement. Some proteins occur on the surface of the phospholipid layer and are called peripheral proteins. These normally act as antigens due to the glycolipids that extend from the protein e.g. the human ABO blood system. The glycolipids also help to make the membrane more stable. The glycoproteins also allows recognition of substances such as hormones or neurotransmitters as proteins have very specified shapes and is thus ideal as receptor cells for chemical signaling. It also means that they can recognize foreign antigens and attack them by the immune system. Those that extend into the layer are called integral proteins and usually act as enzymes e.g. ...read more.


Proteins destined to be secreted are synthesized on ribosomes bound to the RER membrane and are then transported in membrane bound vesicles to the gogi apparatus when they are sorted and packaged up into secretary vesicles. Endocytosis is the uptake of macromolecules from the extracellular space into the cell across the plasma membrane via the formation of an intracellular vesicle pinching off from the plasma membrane. Phagocytosis is the uptake of large particles (bacteria and cell debris). The particle binds to receptors on the surface of the phogocytic cell and the plasma membrane then engulfs the particle and ingests it via the formation of a large endocytic vesicle. Pinpcytosis is the nonspecific uptake of extracellular fluid via small endocytic vesicles that pinch off from the plasma membrane. Two membranes, the inner and outer membranes, bound the nucleus. These two membranes fuse together at the nuclear ribosomes and it controls cell division. Chloroplasts also have inner and outer membranes. In addition there is an extensive internal membrane system made up of thylakoid vesicles (interconnected vesicles flattened to form discs) stacked upon each other to form grana. Within these is the green pigment chlorophyll along with enzymes that trap light energy and convert it into chemical energy in the form of ATP. The stroma is the site of CO fixation. Chloroplasts are therefore able to produce sugar stored as starch through CO and water using light energy. ...read more.


Lysosomes which are found only in animal cells, have a single boundary membrane. The internal pH of these organelles is mildly acidic (pH 4-5) and is maintained by integral membrane proteins which pump H ions into them. Lysosomes are mainly used to digest material which the cell consumes from the environment e.g. white blood cells and bacteria, to digest worn out organelles in a process called autophagy, and to release their enzymes outside the cell (exocytosis) in other to break down other cells. Peroxisomes are organelles that have a single boundary membrane and contain enzymes that degrade fatty acids and amino acids. A byproduct of these reactions is toxic hydrogen peroxide. The presence of large amounts of the enzyme catalase in the peroxisomes rapidly converts the toxic hydrogen peroxide to H and O . It is therefore important in metabolizing cells like those of the liver. A fluid filled sac bounded by a single membrane may be termed a vacuole. A mature plant cell usually contains one or more vacuoles. These are used to nutrients. Like lysosomes in animal cells, vacuoles have an acidic pH maintained by H pumps in the membrane and contain a variety of degradation enzymes. Vacuoles in plants are mainly used to store sugar and amino acids which may act as a food store, the anthocyanins (color pigments within the vacuole) may color petals or fruits to attract insects or animals, a temporary store for organic wastes such as tannins, and as a support for plant cells. In animal cells, vacuoles are much smaller but may occur in large numbers e.g. food vacuoles. ...read more.

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