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2.5 Biological Membranes

Plasma membranes are partially permeable as they allow some but not all substances to pass
through them.
Very small molecules diffuse through the plasma membrane
Some substances dissolve in the lipid layer to pass through
Larger substances pass through protein channels or are carried by carrier proteins…

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Protein channels and carriers covering the long axon allow the transport of ions to bring
the conduction of electrical impulses along their length
They have a myelin sheath of flattened cells around them several times to give more
membrane layers and to insulate the electrical impulses
Root hair cell membranes…

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Passive processes only use the kinetic energy of the molecules, not ATP.

When molecules move down their concentration gradient they are still moving randomly but
remain evenly dispersed which is called net diffusion. They have reach equilibrium.
Concentration gradient is maintained by the respiring cells using the O 2 in…

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Pure water has the highest water potential of 0kPa
Solute molecules lower the water potential
Water molecules move from a high water potential to a low water potential
When water potential is equal on both sides there will be no net movement of osmosis
Water with solutes has negative water…

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Carrier proteins
ATP allows some carrier proteins to change their conformation to carry the molecule from one
side of a gradient to another

Bulk transport
Pinocytosis ­ cells ingesting liquids
Phagocytosis ­ cells ingesting solid matter (e.g. WBC ingesting a bacteria)

1. A membrane bound vesicle, containing substance…

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Factors affecting membranes
Increased temperature
Molecules have more kinetic energy and therefore move faster
Membrane fluidity increases as phospholipids have more kinetic energy
Permeability increases
The tertiary structure of the proteins may be compromised both on the plasma membrane
and the cytoskeleton threads beneath the membrane surface which may…


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