BY1: Cell Membrane & Transport

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Cell Membrane

-The cell membrane is mainly phospholipid and protein.

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-Hydrophobic tails point inwards, and hydrophillic heads point outwards.
-This allows lipid-soluble molecules to enter and leave, but not water-soluble.
......................................................... (A bit like that, but each head has two fatty acid tails). 

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There are two sets of proteins embedded in the plasma membrane:
-Extrinsic, which occur on the surface; giving structural support and cell recognition. 
-Intrinsic, which extend both layers; act as carriers for facillitated diffusion/active transport.

The model is referred to as the "fluid-mosaic" mode because the phospholipid can move in respect to one another, and the proteins embedded are different sizes and shapes.

Cholesterol is also found in the membrane, fitting in between the phospholipid molecules; increasing rigidity and stability.
Glycolipids are involved in cell-to-cell recognition.
Glycoproteins stick out of some membranes.

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Membrane As A Barrier

Lipid-soluble move via the phospholipid part, which also prevents water-soluble molecules; the latter moves through transport proteins. The membrane is selectively permeable to water, and lipid-soluble molecules move through faster.

Small, uncharged molecules can pass through (i.e. O2, and CO2).

Lipid-soluble (i.e. glycerol). 

The hydrophobic core impedes the transport of ions and polar molecules.

Charged particles (ions) and large molecules (glucose), cannot diffuse across due to being insoluble in lipid; so use transport proteins (facilitated diffusion).

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Passive transport - the movement of molecules from a region of high concentration to a low concentration down a concentration gradient; until reaching equal concentration.
-The net movement will be from high to low until equilibrium is reached.
-Calculated as thus: Surface Area x Concentration Difference / Length Of Diffusion Path

Rate is affected by:
-Concentration gradient: Greater difference = Quicker rate
-Distance of diffusion path: Shorter length = Quicker rate
-Surface area of membrane: Larger SA = Quicker rate
-Thickness of membrane: Thinner = Quicker rate
-Temperature: Higher = Quicker due to increase in kinetic energy

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Facilitated Diffusion

-Allows faster movement of non-lipid-soluble molecules, also passive.
Proteins used:
-Channel: Pores lined with polar groups for ions to pass through (i.e. Na+); specific.
-Carrier: Larger molecules (i.e. sugars), a molecule binds and changes shape for release.

            CHANNEL                                CARRIER

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Active Transport

-An energy requiring process (ATP), moving against/up a concentration gradient.
-Ions/molecules can move in the opposite direction of diffusion, using carrier proteins.
-Processes where it is used: Protein synthesis, muscle use, nerves, and salt uptake.
-Happens as such: Ion combines with protein, ATP transfers a phosphate group to the protein and changes its shape, the molecule moves in, and then released.
-In place of cyanide, will not take place; as it is an ATP dependant process.


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-The passage of water from a region of higher to lower water potential.
-Water potential is the pressure created by water molecules and is measure in kPa.
-Pure water has a WP of 0, so solute added makes it LOWER.
-Solute reduces the number of molecules free to move about of water, so being negative.


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Osmosis In Plant Cells

-Water that enters causes the vacuole to swell, pushing the cytoplasm outwards.
-If WP is lower than inside cell = hypertonic, water flows out; plasmolysis -becomes flaccid.
-If WP is higher than inside cell = hypotonic, water flows in; becomes turgid.
-If WP is same, then isotonic and no net movement.

-Water Potential = Solute Potential + Pressure Potential (From cell wall opposing force).
-Solute lowers WP.
-Hydrostatic pressure that is built up is the pressure potential. 

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Osmosis In Animal Cells

-Pure water causes haemolysis, where water moves in and the cell bursts.
-Isotonic causes no change.
-Hypertonic causes water to leave, and cells shrink. 

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Bulk Transport

-Phagocytosis is the process by which the cell can obtain solid materials, lysosome fuses with vesicle formed and enzymes digest it.
-Pinocytosis is the entry of liquid, same as above; but smaller vesicles.

-The substances leaving the cell after being transported.  

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