2.3 Transport Across Cell Membranes Flashcards
describe the fluid-mosaic model of membrane structure
-molecules free to move laterally in phospholipid bilayer
-many components - phospholipids, proteins,
glycoproteins and glycolipids
describe the arrangement of the components of a cell membrane
-phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards
-proteins
○ intrinsic / integral proteins span bilayer eg. channel and carrier proteins
○ extrinsic / peripheral proteins on surface of membrane
-glycolipids (lipids with polysaccharide chains attached) found on exterior surface
-glycoproteins (proteins with polysaccharide chains attached) found on exterior surface
-cholesterol (sometimes present) bonds to phospholipid hydrophobic fatty acid tails
explain the arrangement of phospholipids in a cell membrane
-bilayer, with water present on either side
-hydrophobic fatty acid tails repelled from water so point away from water / to interior
-hydrophilic phosphate heads attracted to water so point to water
explain the role of cholesterol (sometimes present) in cell membranes
-restricts movement of other molecules making up membrane
-so decreases fluidity (and permeability) / increases rigidity
suggest how cell membranes are adapted for other functions
-phospholipid bilayer is fluid → membrane can bend for vesicle formation/ phagocytosis
-glycoproteins / glycolipids act as receptors / antigens → involved in cell signalling / recognition
describe how movement across membranes occurs by simple diffusion
-lipid-soluble (non-polar) or very small substances eg. O2, steroid hormones
-move from an area of higher concentration to an area of lower conc, down a conc. gradient
-across phospholipid bilayer
-passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
explain the limitations imposed by the nature of the phospholipid bilayer
-restricts movement of water soluble (polar) & larger substances eg. Na+ / glucose
-due to hydrophobic fatty acid tails in interior of bilayer
describe how movement across membranes occurs by facilitated diffusion
-water-soluble / polar / charged (or slightly larger) substances eg. glucose, amino acids
-move down a concentration gradient
-through specific channel / carrier proteins
-passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
explain the role of carrier and channel proteins in facilitated diffusion
-shape / charge of protein determines which substances move
-channel proteins facilitate diffusion of water-soluble substances
○ hydrophilic pore filled with water
○ may be gated - can open / close
-carrier proteins facilitate diffusion of (slightly larger) substances
○ complementary substance attaches to binding site
○ protein changes shape to transport substance
describe how movement across membranes occurs by osmosis
-water diffuses / moves
-from an area of high to low water potential (ψ) / down a water potential gradient
-through a partially permeable membrane
-passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)
what is water potential
-a measure of how likely water molecules are to move out of a solution
water potential of pure water
-has the maximum possible ψ (0 kPA)
how does increasing solute concentration affect water potential
-decreases ψ
describe how movement across membranes occurs by active transport
-substances move from area of lower to higher concentration / against a concentration gradient
-requiring hydrolysis of ATP and specific carrier proteins
describe the role of carrier proteins and the importance of the hydrolysis of ATP in active transport
- complementary substance binds to specific carrier protein
- ATP binds, hydrolysed into ADP + Pi, releasing energy
- carrier protein changes shape, releasing substance on side of higher concentration
- Pi released → protein returns to original shape
describe how movement across membranes occurs by co-transport
-two different substances bind to and move simultaneously via a co-transporter protein (type of carrier protein)
-movement of one substance against its concentration gradient is often coupled with the movement of another down its concentration gradient
describe an example that illustrates co-transport (absorption of sodium ions and glucose (or amino acids) by cells lining the ileum)
1-Na+ actively transported from epithelial cells to blood (by Na+/K+ pump)
● establishing a conc.gradient of Na+ (higher in lumen than epithelial cell)
2 - Na+ enters epithelial cell down its concentration gradient with glucose against its concentration gradient
● via a co-transporter protein
3 - glucose moves down a conc. gradient into blood via facilitated diffusion
what can the movement of sodium in co transport be considered
-indirect / secondary active transport
-as it is reliant on a concentration gradient established by active transport
describe how surface area affects the rate of movement across cell membranes
-increasing surface area of membrane increases rate of movement
describe how number of channel or carrier proteins affect the rate of movement across cell membranes
-increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport
describe how differences in gradients of concentration affect the rate of movement across cell membranes
-increasing concentration gradient increases rate of simple diffusion
-increasing concentration gradient increases rate of facilitated diffusion
○ until number of channel / carrier proteins becomes a limiting factor as all in use / saturated
describe how differences in water potential affect the rate of movement across cell membranes
-increasing water potential gradient increases rate of osmosis
explain the adaptations of some specialised cells in relation to the rate of transport across their internal and external membranes
-cell membrane folded eg. microvilli in ileum → increase in surface area
-more protein channels / carriers → for facilitated diffusion (or active transport - carrier proteins only)
-large number of mitochondria → make more ATP by aerobic respiration for active transport
