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Cell Membrane Structure and Function

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Cell membrane structure and function The total thickness of the membrane is about 7.0 nm. It is made of three layers, as seen on the light microscope, two dark and a lighter one separating them. The two dark layers are caused by the deposition of heavy metal on both sides of the membrane during the staining process, and the light region between is lipid. Beyond the plasma membrane of animal cells, there is a glycoprotein coat of different thicknesses. Cells that touch each other are separated by this intercellular material. When materials flow in and out of cells they pass through the glycoprotein coat. In phospholipids, the long hydrocarbon chains which project from the glycerol part of the molecule are insoluble in water whereas the glycerol end is soluble because it contains polar groups. When such a lipid is allowed to spread on a surface of pure water, the water soluble ends of the lipid molecules are drawn into the water and the insoluble hydrocarbon chains, if the molecules are sufficiently packed together, point directly away from the surface of the water. This creates a single layer of lipid molecules with hydrocarbon chains at right angles to the surface, called a monolayer. The lipid component of the plasma membrane cannot be a monolayer because this is possible only when there is a water surface in contact with air. ...read more.


Facilitated diffusion: A diffusing molecule combines with a carrier protein which transports it across the membrane and deposits it on the other side. The diffusion occurs down a concentration gradient and no metabolic energy is required. This is the main way by which glucose and amino acids are taken up into cells. Because their molecules re polar, they cannot diffuse through the lipid bilayer and they are too large to pass through the channel proteins. Carrier proteins are likened to enzymes because the relationship between the protein and the transported molecule is specific and the mode of attachment is like that of a substrate and an enzyme. Carrier proteins are susceptible to poisons, and several different molecules may compete for the transport by the same carrier. Osmosis: The membrane is permeable to water molecules but impermeable to larger molecules, this means that the membrane is partially permeable. Osmosis is the net movement of water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane. Water potential: the potential energy of the water molecules, where there are fewer water molecules, the water potential is lower than when there are more water molecules. The steeper the water potential gradient, the greater the tendency for water to diffuse in a determined direction. The3 symbol used for the water potential and other energy potentials in cells is ?. ...read more.


The calcium pump: The concentration of calcium ions inside the cell is generally much higher than outside. This unequal distribution of calcium ions is maintained by a calcium pump, which actively expels calcium ions across the plasma membrane. If the calcium pump stops working, calcium cells diffuse rapidly into the cell down the steep concentration gradient. If the pump is momentarily stopped, it is a way of transmitting signals to the cells. Exocytosis: A vesicle containing the material to be expelled moves towards the surface of the cell and fuses with the plasma membrane. The vesicle opens outside and the contents leave the cell. The vesicle membrane becomes part of the surface membrane. Endocytosis is another active process by which substances are taken into a cell by infoldings in the surface membrane. An example is neutrophils. There are two different types of endocytosis, which differ in the size of the vesicles. Phagocytosis involves relatively large particles being taken up into large vesicles. The plasma membrane invaginates to form a phagocytic vesicle which encloses the particles. The vesicle then fuses with a lysosome which digests the particles. This is called intracellular digestion. The soluble products of the digestion process are then absorbed into the surrounding cytoplasm. Any indigestible material may be removed by the vesicle moving to the surface of the cell and fusing with the plasma membrane. Pinocytosis means cell drinking. Tiny channels pinch off small vesicles which pinch off even smaller ones. These pinocycotic vesicles mean that liquids can be brought into the cell and distributed within it. ...read more.

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