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What are the building blocks of membranes?
Phospholipids
Globular Proteins
Cholesterol (only in animal cells)
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Phospholipids:
Have a hydrophilic phosphate head and two fatty acid (hydrocarbon) hydrophobic tails which makes them amphipathic
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In water,
the heads attract to water, tails attract to each other.
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Dawson Danielli Model
Phospholipids are in the middle with proteins on the outside and inside (sandwich)
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Fluid-Mosaic Model
Proteins are embedded in the phospholipid.
With fluorescent antibody tagging, they proved that proteins moved (mixing human and mouse membrane)
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Membrane Proteins:
Some have hydrophobic and hydrophilic parts.
Some are integral (inside) and some are peripheral (outside)
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What are the functions of membrane proteins?
Sites for hormone binding
Enzymatic action
Cell adhesion to form junctions between groups of cells in tissues and organs
Cell communication
Channels for passive transport
Pumps for active transport
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Glycoprotein:
Like antennas
are composed of carbohydrate chains attached to peripheral proteins. They play a role in recognition of like cells and are involved in immune responses.
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Non-Polar substances like Vitamins, Oxygen and Carbon dioxide
can make their way through the phospholipid bilayer, but polar and charged molecules can’t.
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Integral Proteins:
- Are integrated throughout the entire membrane. Very hard to move.
- channels, transporters, links
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Peripheral Proteins:
Small, and outside. Help anchor the membrane. Attachment sites of the cytoskeleton. Can attach and remove itself. Can activate and deactivate itself.
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Channel proteins:
Hot dog bun, passive transport
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Cholesterol:
Our membranes do not correspond exactly to any of the three states of matter. The hyrdrophobic hydrocarbon tails usually behave as a liquid, but the hydrophilic phosphate heads act more like a solid. The fluidity needs to be controlled somehow. If it was too liquid, there would be limited control. If it was too solid, movement would be restricted. Cholesterol disrupts both compounds and stops them from becoming too liquid or too solid. It also helps the formation of vesicles during endocytosis.
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Proteins that serve as hormone-binding sites have specific shapes (of hormones) exposed to the exterior. The attachment between the protein and the hormone causes a change in the shape of the protein, which results in a message being relayed to the interior of the cell.
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