2.3 transport across cell membranes

describe the fluid mosaic model of membrane structures

molecules are free to move laterally in phospholipid bilayer

many components - phospholipids, proteins, glycoproteins and glycol ions

describe the arrangement of the components of a cell membrane (5)

phospholipids form a bilayer - fatty acid tails face inwards, phosphate heads face outwards

intrinsic/integral proteins span bilayer e.g. channel and carrier proteins

extrinsic/peripheral proteins on surface of membrane

glycolipids found on exterior surface

glycoproteins found on exterior surface

cholesterol bonds to phospholipid hydrophobic fatty acids tails

diagram of cell membrane

explain the arrangement of phospholipids in a cell membrane

bilayer with water present on each 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

role of cholesterol

restricts movement of other molecules making up membrane

decreases fluidity / increases rigidity

suggest how cell membranes are adapated 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

explain limitations imposed by the nature of phospholipid bilayer

restricts movement of water soluble (polar) and larger substances

due to hydrophobic fatty acid tails in interior of bilayer

what are the way substances can move in the cell surface membrane

• diffusion - small substances down a conc gradient

• osmosis - water, down a water potential gradient

• active transport - against a conc gradient via protein carrier using ATP

• facilitated diffusion - down a conc gradient via channel protein

• co transport -2 different substances using carrier protein

describe how movement across membranes occurs by simple diffusion

lipid soluble (non-polar) or very small substances

move from an area of higher concentration to an area of lower concentration down a concentration gradient

across phospholipid bilayer

passive - doesn’t require ATP

describe movement across membranes by facilitated diffusion

water soluble (polar) / slightly larger substances

move down a concentration gradient across

through specific channel/carrier proteins extrinsic

passive

what feature of a protein determines which substances move

shape/charge of protein

explain role of channel proteins

facilitate diffusion of water soluble substances

hydrophobic pore filled with water

may be gated - can open/close

explain the role of carrier proteins

facilities diffusion of slightly larger substances

complementary substances attaches to binding site

protein changes shape to transport substance

describe movement across membranes by osmosis

water diffuses/moves

from an area of high to low water potential / down a water potential gradient

through a partially permeable membrane

passive

describe movement across membranes by active transport

substances move from an 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

1. complementary substance binds to specific carrier protein

2. ATP binds, hydrolysed into ADP + Pi releasing energy

3. carrier protein changes shape releasing substance on side of higher concentration

4. Pi released - protein returns to original shape

describe movement across membranes by co transport

two different substances bind to and move simultaneously via a co transporter protein

movement of one substance against its concentration gradient is often couple with the movement of another down its concentration gradient

example of co transport

absorption of sodium ions and glucose by cells lining the mammalian ileum

co transport of sodium ions and glucose

1. Na+ actively transported from epithelial cells to blood, establishing a concentration gradient of Na+

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 concentration gradient into blood via facilitated diffusion

factors that affect rate of movement across cell membranes (5)

• increasing surface area of membrane increases rate of movement across membranes

• increasing number of channel/carrier proteins increase rate of facilitated diffusion/ active transport

• increasing concentration gradient increases rate of simple / facilitated diffusion and osmosis

• increasing concentration gradient increases rate of facilitated diffusion until number of channel/carrier proteins becomes a limiting factor as all in use

• increasing water potential gradient increases rate of osmosis

explain adaptations of some specialised cells in relation to rate of transport

membrane folded e.g. microvilli in ileum increases surface area

more protein channels/carriers for facilitated diffusion

large number of mitochondria - makes more ATP by aerobic respiration for active transport