It was hard for me to choose the right topic from section B. The fifth option was the one I found best. The goal is to explain the cell membrane structure and, mentioning examples, to discuss the mechanisms by which materials move into and out of animal cells.
The cell membrane, also called plasma membrane, is a very complex structure. Its task is to regulate a constant traffic of materials into and out of the cell. The main part of the cell membrane is a huge amount of phospholipid molecules, ordered into a bilayer. Each phospholipid has a hydrophilic, “water-loving” head and a couple of hydrophobic, “water-fearing” tails. They are always turned so that their heads point outward the bilayer and their tails point inward. There are also peripheral and integral proteins in the bilayer. The difference between them is that peripheral proteins are individual molecules attached to the inner or outer membrane surface and integral are embedded into the membrane. The function of both is mechanically to carry different substances into or out of the cell. Most of them have either carbohydrate or glycolipid chains on their surface; some chains are even attached directly to the phospholipids, but always on the outer side of the membrane only. Sometimes cholesterol molecules may appear between the phospholipids. There are more types of cell material transport: simple diffusion, carrier-facilitated diffusion and active transport. Simple diffusion is a tendency of a substance to move from a place of higher concentration to one with lower concentration (to equalize the concentrations). The problem is, that the semipermeable cell membrane does not let all substances to move through its pores and that is why mostly water (but also other, for example urea and ethanol) molecules, which are small enough to pass, use this type of transport. As a consequence, osmosis occurs here. Animal cells lack the cell wall and therefore can easily expand or contract. There are three basic osmotic situations: 1.) cell in hypotonic solution- the concentration outside the cell is less then inside and so water tends to rush into the cell, it swells and may burst 2.) cell in isotonic solution- concentrations are equal and no water movements occur 3.) cell in hypertonic solution- due to the higher concentration outside the cell, water tends to leave the cell and it shrinks. Bigger molecules, for example the negative Cl ion, mostly use the carrier-facilitated diffusion.
They link to carrier (integral) proteins that change their own shape in order to move the molecule through the semipermeable membrane. This very important feature, called specificity, is in great importance and causes that only one kind of polar molecule (or group of structurally related ones) can pass. Because by both simple and carrier-facilitated diffusions the movements are always along the concentration gradient (from a region with a higher concentration to one with lower), no energy is needed for these kinds of transport. The movement that requires energy is the active transport, mostly used by bigger molecules and ions moving against concentration gradients (from places with lower concentration to ones with higher). During the active transport, a set of proteins in the plasma membrane breaks down energy in ATP and uses it for binding the cargo, changing shape and releasing it outside or inside the cell. Some carriers move single cargo in a single direction and therefore are called uniports. Others move two cargoes simultaneously in the same direction – those are the symports. They make the co-transport (coupled transport), used for example by sugars and amino acids, possible. That is very important, because neither the sugar nor the amino acid can be transported alone. The third group, the antiports (for example the K-Na-ATPase pump), are most complex – they move two cargoes simultaneously in opposite ways.
Transporting materials into and out of cells, the plasma membrane and its components play a great role. They are the very important not only for cell nutrition but also excretion, protection and communication between cells.