transport across cell membranes

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 (phospholipid bilayer)

● Passive - doesn’t require energy from ATP / respiration (only kinetic energy of substances)

Water potential is a measure of how likely water molecules are to move out of a solution - pure (distilled) water

has the maximum possible ψ (0 kPA). Increasing solute concentration decreases ψ.

Hypertonic solution

When the water potential of a solution is more negative than the the cytoplasm of the cell

water moves out of the cell by osmosis

both animal and plant cells will shrink and shrivel

Hypotonic

solution

When the water potential of a solution is more positive (closer to zero) than the cytoplasm of the cell

water moves into the cell by osmosis

animal cells will lyse (burst)

plant cells will become turgid

Isotonic

When the water potential of the surrounding solution is the same as the water potential inside the cell

no net movement in water

cells would remain the same mass

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

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 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

how is glucose moving against the conc gradient to the co transporter protein without atp?

• Na⁺ moving down its gradient from lumen releases energy

• That energy is used to pull glucose INTO the epithelial cell against its gradient

Describe how surface area, number of channel or carrier proteins and

differences in gradients of concentration or water potential affect the rate of

movement across cell membranes

● Increasing surface area of membrane increases rate of movement

● Increasing number of channel / carrier proteins increases rate of facilitated diffusion / active transport

● 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

● 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

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