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Transport across plasma membranes.

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Transport across plasma membranes The cell surface membrane is approximately 7.5mm thick and is a bi-molecular phospholipid bilayer with inwardly directed hydrophobic (substances which repel water molecules) tails. It is also a fluid structure. A partially permeable membrane is one which allows some substances through but not others. There are a number of different ways in which substances are transported across plasma membranes. The first being diffusion, which occurs across the cell surface membrane. This is a passive process (requires no energy) by which substances move from a region of high concentration to a region of low concentration, of the same substance. The rate of diffusion depends on a number of factors: * The concentration gradient * The distance between the areas * The size of the molecules that are diffusing Particles of gas or solute can also diffuse through a membrane, as long as the membrane has pores that are larger than the particles. Every substance diffuses down its own concentration gradient. ...read more.


Hypotonic solutions have a higher concentration of water molecules compared to the inside of a cell so there is a net movement of water molecules into the cell. This causes the cell to swell; a cell that is full of water but has not burst is said to be turgid. An example of osmosis taking place is in the vacuole of a plant cell. Facilitated diffusion is a passive process, as it does not need energy. (There is no requirement for ATP as there is no energy consumption.) It uses a carrier protein to transfer a molecule across a membrane along its electrochemical gradient. There are different transport proteins for different molecules or ions. Transport proteins have a highly specific tertiary structure, which gives the molecule a distinctive shape. Facilitated diffusion makes diffusion easier because it is a more rapid exchange due to the channels produced by the carrier proteins. The carrier proteins bind with the molecules causing it to change shape allowing the molecules to be released on the other side of the membrane. ...read more.


A lysosome with lytic enzymes fuses with the phagocytic vessel, and then the lysosome releases enzymes which break down nutrient particles/bacterium. The soluble products are absorbed into the cytoplasm and the insoluble material is removed as the vesicle regains. Macromolecules and larger particles, such as bacteria, are taken into cells by endocytosis. Large molecules, such as proteins, are transported out of cells by exocytosis. Endocytosis includes phagocytosis and pinocytosis. Phagocytosis is when cells obtain particles that are too large to be absorbed by diffusion or active transport. Pinocytosis is similar to phagocytosis, but pinocytosis takes in small droplets of the external solution, forming vesicles. There are different processes for transporting any substance between cells, some which require energy and some which don't. Cells are complex factories and they need constantly to import raw materials and get rid of waste. Molecules and ions with different sizes and electrical charges enter and leave all the time. Some of the exchange of materials occurs as a result of passive processes such as diffusion and osmosis, and some occur as a result of processes which require energy such as facilitated diffusion and active transport. Rebecca Stuckey 12 GBB 27th September 2002 ...read more.

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