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Transport across cell membranes - A-Level Biology notes
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Transport across cell membranes - A-Level Biology notes
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Chapter 4: Transport across cell membranes4.1 Structure of cell-surface membranes
Plasma membranes
- Membranes around and within all cells
- It forms a boundary between the cytoplasm and the environment
- It controls what exit and enters the cell
Phospholipids
- Hydrophilic heads point to the outside of the cell surface membranes as they attract water
- Hydrophobic tails point to the centre as they repel water
- They allow lipid-soluble substances to exit and enter the cell
- They prevent water soluble substances entering and leaving the cell
- They make membranes flexible and self-sealing
Proteins
- There are embedded in the phospholipid bilayer in 2 ways:
o Some are on the surface of the membrane which act as mechanical support or work with glycolipids
o Some go completely across the membrane and become either protein channels or carrier proteins. Protein
channels are water-filled tubes for water soluble ions to diffuse across. Carrier proteins bind to ions and
change shape to transport the molecules across by active transport
- They provide structural support
Cholesterol
- They form cell-surface receptors to identify cells
- Add strength
- Homophobic meaning it prevents water loss and loss
Permeability of cell-surface membrane
of dissolved ions
- Controls movement of molecules in and out of cell
- It pulls together fatty acid tails of the bilayer which
- Most molecules can diffuse easily because:
limits the movement of phospholipids without
o They’re too big
making it rigid
o Not soluble in lipids therefore can’t pass through
the bilayer
Glycolipids (carbohydrate + lipid)
o They have the same charge as the protein
- Act as a recognition site
channels therefore they will be repelled
- Help maintain stability of membrane
o They’re a polar molecule as the hydrophobic tails
- Help cells attached to one another to form tissues
are nonpolar
Glycoproteins (carbohydrate + protein)
Fluid-mosaic model of the cell-surface membrane - Act as a recognition site
- Fluid: the individual phospholipids can move - Help cells attached to one another to form tissues
therefore the structure is flexible - Allows cell recognition ex. lymphocytes can recognise
- Mosaic: because proteins in the bilayer vary in shape, an organism’s own cell
size and pattern
4.2 Diffusion: the net movement of molecules or ions from high to low concentration until evenly distributed
Facilitated diffusion: Simple diffusion:
- Charged ions, polar molecules and large ones diffused by o Passive diffusion: natural energy
facilitated diffusion o Active diffusion: added energy like ATP
- Passive process - Particles are always in random motion
- Down a concentration gradient - Particles are always bouncing off things
- Two types: - Down a concentration gradient
o Protein channels: - Let small, non-polar molecules through
Water soluble molecules only
Closes and opens to control what enters and leaves
Ions bind with the protein so it changes shape and opens to the other side
Selective
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