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Cell Membrane and Transport - Coggle Diagram

- - - - The rate at which a substance diffuses across a membrane depends on a number of factors, including the following:
        
        Temperature. At high temperatures, molecules and ions have much more kinetic energy than at low temperatures. They move around faster, and thus diffusion takes place faster
        
        The surface area across which diffusion is taking place. The greater the surface area, the more molecules or ions can cross it at any one moment, and therefore the faster diffusion can occur.
        
        The ‘steepness’ of the concentration gradient that is, the difference in the concentration of the substance on the two sides of the surface.
        
        The nature of the molecules or ions. Large molecules require more energy to get them moving than small ones do, so large molecules tend to diffuse more slowly than small molecules
      - Diffusion can be defined as the net movement, as a result of random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration. The molecules or ions move down a concentration gradient. The random movement is caused by the natural kinetic energy of the molecules or ions.
    - - If the concentration of particular ions, such as potassium and chloride, inside cells is measured, it is often found that they are 10–20 times more concentrated inside than outside. In other words, a concentration gradient exists, with a lower concentration outside and a higher concentration inside the cell. The ions inside the cell originally came from the external solution, therefore diffusion cannot be responsible for this gradient because, as we have seen, ions diffuse from high concentration to low concentration. The ions must therefore accumulate against a concentration gradient. The process responsible is called active transport. It is achieved by carrier proteins, each of which is specific for a particular type of molecule or ion. However, unlike facilitated diffusion, active transport requires energy, because movement occurs up a concentration gradient. The energy is supplied by the molecule ATP which is produced during respiration inside the cell. The energy is used to make the carrier protein change its shape, transferring the molecules or ions across the membrane in the process. Active transport is important in reabsorption in the kidneys, where certain useful molecules and ions have to be reabsorbed into the blood after filtration into the kidney tubules, for example.
    - - Facilitated diffusion is the diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas that allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them. ‘Facilitated’ means made easy or made possible, and this is what the proteins do. There are two types of protein involved, namely channel proteins and carrier proteins. Each is highly specific, allowing only one type of molecule or ion to pass through it.
    - - There are mechanisms for the bulk transport of large quantities of materials into cells (endocytosis), or out of cells (exocytosis). Large molecules such as proteins or polysaccharides, parts of cells or even whole cells may be transported across the membrane. This requires energy, so it is a form of active transport. .
        
        Endocytosis involves the engulfing of the material by the cell surface membrane to form a small sac, or ‘endocytic vacuole’. It takes two forms.
        
        Phagocytosis or ‘cell eating’ – this is the bulk uptake of solid material. Cells specialising in this are called phagocytes
        
        Pinocytosis or ‘cell drinking’ – this is the bulk uptake of liquid. The vacuoles (vesicles) formed are often extremely small, in which case the process is called micropinocytosis.
        
        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 from the Golgi body 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 cell surface membrane.
    - - It is the movement fo water from a dilute solution to a concentrated solution, through a partially permeable membrane.
        
        In terms of water potential, Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, as a result of their random motion.
        
        The contribution of the concentration of the solution to water potential is called solute potential
        
        The contribution of pressure to the water potential of a solution is called pressure potential.