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Summary Biology book Cambridge A & AS levels chapter 4 Cell membranes and transport
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Biology book Cambridge A & AS levels chapter 4 Cell membranes and transport.
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Chapter 44.1
Membranes enable cells to receive hormone messages and in chloroplasts they contain
light-absorbing pigments needed for photosynthesis.
4.2
Fluid mosaic model
Phospholipids and proteins can move by diffusion. Phospholipids move sideways. Some
protein molecules also move. Others remain fixed.
Membrane=bilayer of phospholipid molecules that move by diffusion. Non-polar tails
(inwards) and polar heads (outwards into aqueous medium). Fluidity:
- More unsaturated tailsmore fluid membrane, the tails are bentso fit together
more loosely.
- Tail length: longerless fluid membrane
Temperature decreasesless fluidity, organisms who can’t regulate body temperature
increase the proportion of unsaturated fatty acids.
Protein: in outer layer, inner layer, or the whole membrane. They have hydrophobic and
hydrophilic regions and stay in the membrane: hydrophobic regions next to hydrophobic
tails and repelled by water from either side of the membrane. Hydrophilic regions repelled
by the hydrophobic interior of the membraneface the aqueous environment or line
hydrophilic pores
Many proteins and lipids have short, branching carbohydrate chains, facing outside the
membrane and form glycoproteins and glycolipids.
4.3
Lipids: phospholipids, cholesterol, and glycolipids
Phospholipids: form a bilayer ‘two layers’, the basic structure of a membrane.
Mixed with water, they form:
- Ball-like structures called micelles or
- Sheet-like structures called bilayers
Membranes act as barrier to most water-soluble substances (cuz tails)sugars, amino acids
and proteins can’t leak out the cell and unwanted water-soluble substances can’t enter.
Cholesterol: mechanical stability. They fit between phospholipid molecules, reducing
fluidity cell breaks less quickly and won’t burst; it strengthens.
Animal cells: as much cholesterol as phospholipid. Plant cells: cholesterol less common and
not absent in prokaryotes. Compounds similar serve the same function in these organisms.
-Low temp: phospholipids packed closer, but cholesterol prevents this, maintaining the
correct fluiditycells survive colder temperatures.
Glycolipids, glycoproteins: all proteins and many lipid molecules on the outer surface have
short carbohydrate chains attached glycoproteins / glycolipids. Functions:
- Receptor molecules: binds with substances at the cell surface. Different cells have
different receptors. Signaling receptors: part of signaling system coordinating
activities of cells. They recognize messenger molecules when it binds series of
chemical reactions started.
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