2.5 Transport across Cell Membranes

Question 1

What does the fluid-mosaic model suggest?

Answer 1

• Molecules are free to move laterally in phospholipid bilayer
• Intrinsic/extrinsic proteins, glycolipids, glycoproteins and cholesterol present

Question 2

How are phospholipids arranged?

Answer 2

• Phospholipids form a bilayer
• Hydrophobic fatty acid tails repelled from water so face inwards
• Hydrophilic phosphate heads attracted to water so face outwards

Question 3

What is the role of cholesterol?

Answer 3

• Restricts movement of phospholipids
• So decreases fluidity

Question 4

Simple diffusion

Answer 4

• Passive movement of small lipid-soluble substances across phospholipid bilayer
• From an area of higher to lower concentration / down concentration gradient

Question 5

Facilitated diffusion

Answer 5

• Passive movement of larger water-soluble substances through specific channel/carrier proteins
• From an area of higher to lower concentration / down concentration gradient

Question 6

Osmosis

Answer 6

• Passive movement of water across a partially permeable membrane
• From an area of higher to lower water potential / down water potential gradient

Question 7

Active transport

Answer 7

• Active movement of substances through specific carrier proteins
• Using energy from ATP
• From an area of lower to higher concentration / against concentration gradient

Question 8

Co-transport

Answer 8

• Movement of two different substances via a co-transporter protein
• Movement of one substance against its concentration gradient and the other down its concentration gradient

Question 9

How do channel proteins work?

Answer 9

• Hydrophilic pore filled with water
• Facilitate diffusion of water-soluble substances

Question 10

How do carrier proteins work?

Answer 10

• Complementary substance attaches to binding site
• Protein changes shape, releasing substance on other side

Question 11

How is glucose absorbed from the lumen, into the epithelial cell then blood?

Answer 11

• Na+ is actively transported from the epithelial cell into the blood by the Na+/K+ pump
• Establishing a concentration gradient of Na+
• Na+ diffuses into the epithelial cell down its concentration gradient with glucose against its concentration gradient
• Via a co-transporter protein
• Glucose diffuses down its concentration gradient inti the blood by facilitated diffusion

Question 12

What factors affect rate of movement across cell membranes?

Answer 12

• Surface area
• Number of channel/carrier proteins
• Concentration gradient
• Water potential gradient
• Amount of mitochondria (ATP)