2.3 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 glycolipidsi

Describe the arrangement of a cell membrane

• phospholipids form a bilayer

• Proteins (intrinsic and extrinsic)

• Glycolipids

• Glycoproteins

• Cholesterol

Structure of cell membrane: phospholipids

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

Structure of cell membrane: proteins

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

• Extrinsic/ peripheral proteins, on surface of membrane

Structure of cell membrane: glycolipids

• lipids with polysaccharide chains attached

• Found on exterior surface

Structure of cell membrane: glycoproteins

• proteins with polysaccharide chains attached

• Found on exterior surface

Structure of cell membrane: cholesterol

• sometime present

• Bonds to phospholipid hydrophobic fatty acid tails

Explain the arrangement of phospholipids in a cell membrane

• bilayer, with water present on either side of

• Hydrophobic fatty acid tails repelled from water so point away from water/ to inferior

• Hydrophillic phosphate heads attracted to water so points to water/ exterior

Explain the role of cholesterol in cell membranes

• restricts movement of other molecules making up the membrane

• So decreases fluidity and permeability, increasing rigidity

How are cell membranes adapted for other functions

• phospholipid bilayer is fluid so membrane can bend for vesicle formation/ phagocytosis

• Glycoproteins/ glycolipids act as receptors/ antigens which are involved in cell signalling/ recognition

How does movement occur across membranes by simple diffusion

• Lipid soluble (non-polar) or very small substances move from an area of high conc to low conc

• Down a concentration gradient

• Across phospholipid bilayer

• Passive - doesn’t require energy from ATP/ respiration (only kinetic)

What limitations are imposed by the nature of the phospholipid bilayer

• restricts movement of water soluble (polar) and larger substances

• Due to hydrophobic fatty acid tails in interior of bilayer

How does movement across the membranes occur by facilitated diffusion

• water soluble (polar) and slightly larger substances move down a concentration gradient

• Through specific channel/ carrier proteins

• Passive - doesn’t require energy from ATP/ respiration (only kinetic)

Describe the role of carrier proteins in facilitated diffusion

• shape/ charge of protein determines which substances move

• Carrier proteins facilitate diffusion of slightly larger substances

• Complementary substance attaches to binding site

• Protein changes shape to transport substance

Describe the role of channel proteins in facilitated diffusion

• shape/ charge of protein determines which substances move

• Channel proteins facilitate diffusion of water-soluble substances

• Hydrophillic pore filled with water, may be gates so can open/ close

Describe how movement across membranes occurs 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 - doesn’t require energy from ATP (only kinetic energy)

What is water potential

How likely water molecules are to move out of a solution (Pure distilled water has the max potential)

How does movement across membranes occur 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 hydrolysis of ATP in active transport

1. Complementary substance binds to specific carrier proteins

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

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

4. Pi released, so protein returns to original shape

How does movement across membranes occur by co-transport

• two different substances bind to and move simultaneously via a co-transporter protein (Type of carrier)

• Movement of one substance against its concentration 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)

• Na is actively transported from epithelium cells to blood by Na/K pump

• Establishing a conc gradient of Na (higher in lumen than epithelial cells

• Na enters epithelial cells down its concentration gradient with glucose against its concentration gradient

• Via a co-transporter protein

• glucose moves down a conc gradient into blood via facilitated diffusion

Co-transport - why is sodium considered as indirect/ secondary active transport

• as it relies on a concentration gradient established by active transport

How does surface area affect the rate of movement across cell membranes

Increase in surface area, increases rate of movement

How does number of channel/ carrier proteins affect the rate of movement across cell membranes

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

How does concentration gradient affect the rate of movement across cell membranes

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

• Facilitated increases until the number of carrier/ channel proteins become a limiting factor as all in use/ saturated

How does water potential affect the rate of movement across cell membranes

• increasing water potential gradient increases rate of osmosis

Explain the adaptions of some specialised cells in relation to the rate of transport across their internal and external membranes

• membrane folded which increases surface area

• More channel/ carrier proteins for facilities diffusion

• Large number of mitochondria which makes more ATP by aerobic respiration for active transport