transport across cell membranes Flashcards
Describe what is meant by the fluid-mosaic model of membrane structure
- Molecules within membrane can move laterally (fluid) e.g. phospholipids
- Mixture of phospholipids, proteins, glycoproteins and glycolipids
describe the structure of a cell membrane
- Phospholipid bilayer
- Phosphate heads are hydrophilic so attracted to water – orientate to the aqueous environment either side of the membrane
- Fatty acid tails are hydrophobic so repelled by water – orientate to the inside/interior of the membrane
- Embedded proteins (intrinsic or extrinsic)
- Channel and carrier proteins (intrinsic)
- Glycolipids (lipids and attached polysaccharide chain) and glycoproteins (proteins with polysaccharide chain attached)
- Cholesterol (binds to phospholipid hydrophobic fatty acid tails
Explain, using the fluid-mosaic model, how molecules can enter/leave a cell. bilayer
Phospholipid bilayer
- Allows movement of non-polar small/lipid-soluble molecules e.g. oxygen or water,
- down a concentration gradient (simple diffusion)
- Restricts the movement of larger/polar molecules
Explain, using the fluid-mosaic model, how molecules can enter/leave a cell. channel proteins
Channel proteins and carrier proteins
- Allows movement of water-soluble/polar molecules / ions
down a concentration gradient
facilitated diffusion
Explain, using the fluid-mosaic model, how molecules can enter/leave a cell. carrier proteins
Carrier proteins
- Allows the movement of molecules
against a concentration gradient
using ATP (active transport)
Explain how features of the plasma membrane adapt it for its other functions
Phospholipid bilayer
- Maintains a different environment on each side of the cell or compartmentalisation of cell
- Phospholipid bilayer is fluid
- Can bend to take up different shapes for phagocytosis / to form vesicles
- Surface proteins / extrinsic / glycoproteins / glycolipids
- Cell recognition / act as antigens / receptors
- Cholesterol
- Regulates fluidity / increases stability
Describe the role of cholesterol in membranes
- Makes the membrane more rigid / stable / less flexible,
by restricting lateral movement of molecules making up membrane
e.g. phospholipids (binds to fatty acid tails causing them to pack more closely together)
Describe the movement across membranes by simple diffusion and factors affecting rate
- Net movement of small, non-polar molecules across a selectively permeable membrane, down a concentration gradient
- Passive / no ATP / energy required
- Factors affecting rate – surface area, concentration gradient, thickness of surface / diffusion distance
Describe the movement across membranes by facilitated diffusion and factors affecting rate
- Net movement of larger/polar molecules e.g. glucose, across a selectively permeable membrane, down a concentration gradient
- Through a channel/carrier protein
- Passive /no ATP/energy required
- Factors affecting rate – surface area, concentration gradients (until the number of proteins is the limiting factor as all are in use / saturated), number of channel/carrier proteins
Describe the role of carrier/channel proteins in facilitated diffusion
- Carrier proteins transport large molecules, the protein changes shape when molecule attaches
- Channel proteins transport charged/polar molecules through its pore (some are gated so can open/close e.g. Voltage-gated sodium ion channels)
- Different carrier and channel proteins facilitate the diffusion of different specific molecules
Describe the movement across membranes by osmosis and factors affecting rate
- Net movement of water molecules across a selectively permeable membrane down a water potential gradient
- Passive
- Factors affecting rate – surface area, water potential gradient, thickness of exchange surface / diffusion distance
what is water potential
- Water potential is the likelihood (potential) of water molecules to diffuse out of or into a solution;
what has the highest water potential
pure water
how is water potential lowered
by adding solutes
Describe the movement across membranes by active transport and factors affecting rate
- Net movement of molecules/ions against a concentration gradient
- Using carrier proteins
- Using energy from the hydrolysis of ATP to change the shape of the tertiary structure and push the substances though
- Factors affecting rate – pH/temp (tertiary structure of carrier protein), speed of carrier protein, number of carrier proteins, rate of respiration (ATP production)
