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Transport across Plasma Membranes

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TRANSPORT ACROSS PLASMA MEMBRANES By Johnny McKinstry A phospholipid bilayer makes a good barrier, particularly against the movement of water-soluble molecules. The water contents of the cell are stopped from escaping. However, some exchange between the cell and its environment is essential. Diffusion & Facilitated Diffusion If you open a bottle of perfume in a room, it is not long before molecules of its scent have spread to all parts of the room. This will happen, even in still air, by the process of diffusion. Diffusion is the movement of molecules from a region of their higher concentration to a region of their lower concentration. The molecules move down a concentration gradient. It happens because of the natural kinetic energy possessed by molecules or ions, which make them move about at random. As a result of diffusion, molecules tend to reach an equilibrium situation where they are equally spread within a given volume of space. Some substances have molecules or ions that are able to pass through cell membranes by diffusion. The rate at which a substance diffuses across a cell membrane depends on a number of factors, including: > the steepness of the concentration gradient, that is the difference in the concentration of the substance on the two sides of the surface. ...read more.


Water Potential & Solute Potential It is useful to be able to measure the tendency of water molecules to move from one place to another. This tendency is known as water potential. The symbol for water potential is the Greek letter psi. Water always moves from a region of high water potential to a region of lower water potential. It therefore moves down a water potential gradient. Equilibrium is reached when the water potential in one region is the same as in the other. There will be no net movement of water molecules. We can now define osmosis as the movement of water molecules from a region of higher water potential to one of lower water potential through a partially permeable membrane. By convention the water potential of pure water is set at zero. Since solutes make water potential lower, they make the water potential of solutions less than zero, thus negative. The more solute, the more negative (lower) the water potential becomes. The amount that the solute molecules lower the water potential of a solution by, is called the solute potential. Solute Potential is therefore always negative. Osmosis in Plant Cells Pressure potential is especially important in plant cells. Unlike animal cells, plant cells are surrounded by cell walls that are very strong and rigid. ...read more.


Active transport is important in re-absorption in the kidneys where certain useful molecules and ions have to be reabsorbed into the blood after filtration into the kidney tubules. In plants, active transport is used to load sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant, and to load inorganic ions from the soil into root hairs. Bulk Transport So far we have been looking at ways in which individual molecules or ions cross membranes. Mechanisms also exist for the bulk transport of large quantities of materials into cells (endocytosis) or out of cells (exocytosis). Endocytosis involves the engulfing of the material by the plasma membrane to form a small sac, or 'endocytotic vacuole'. It takes two forms: > Phagocytosis or 'cell eating' - this is the bulk uptake of solid material. Cells specialising in this are called phagocytes. The process is called phagocytosis and the vacuoles phagocytic vacuoles. An example is the engulfing of bacteria by certain white blood - this is the bulk uptake of liquid. > Exocytosis is the reverse of endocytosis and is the process by which materials are removed from cells. It happens, for example, in the secretion of digestive enzymes from cells of the pancreas. Secretory vesicles carry the enzymes to the cell surface and release their contents. Plant cells use exocytosis to get their cell wall building materials to the outside of the plasma membrane. ...read more.

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