Crossing cell membranes

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  • Created on: 11-05-14 15:48
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Passive processes
The net movement of particles from an area of high concentration to an area of low
The net movement of water from an area of high water potential to an area of low water potential
through a selectively permeable membrane.
Facilitated diffusion:
When particles move through channel proteins. This is passive as channel proteins don't need
ATP to function (unlike carrier proteins). They allow polar molecules through, such as calcium
Active processes
Active transport:
When the concentration gradient is the opposite way round to what you want (e.g. root hair cell in
soil has a greater nutrient concentration than the soil itself). Carrier proteins are used to actively
transport specific molecules that are complementary to its shape. They require ATP to change
shape, and let the molecule in:
Cotransport is when two
substances are transported at
the same time (e.g. a sodium
potassium pump in neurones)
Bulk transport (exocytosis and endocytosis):
Exocytosis is the bulk movement of materials of the cell. For example, a pancreas cell can
produce a lot of insulin molecules which it doesn't need in the cell, so the Golgi apparatus
pinches them off into a vesicle. The vesicle heads towards the cells membrane, fuses with it and
releases the insulin molecules into the blood vessels.
Endocytosis is the bulk absorption of molecules into a cell. It is done by a vesicle fusing with the
membrane and releasing its contents into the cytoplasm. An example of this is phagocytosis. A
receptor of the phagocyte binds to one on a bacterium, then engulfs it. The membrane pinched
off to form a phagosome. Lysosomes then fuse with the membrane of the phagosome, releasing
lysins (digestive enzymes) into it, breaking the pathogen down. The cell absorbs any nutrients
from the phagosome it needs, then egests the waste.

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How things actually get through a cell membrane:
Cell membranes are partially permeable, which means water can get through by
Big molecules like glucose get in through carrier proteins.
Charged ions (e.g. calcium) enter via the channel proteins, which can change their
charge to let them through.
Small molecules such as oxygen and carbon dioxide can simply diffuse through the
membrane following the concentration gradient.
Viruses get in cells by endocytosis.
Cells that make things (e.g.…read more


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