Transport across membranes

Question 1

How are the phospholipids arranged in the bilayer?

Answer 1

The hydrophilic heads of phospholipids point outwards attracted to the water. The hydrophobic fatty acids tails point inwards repelled by the water.

Question 2

What is the function of phospholipids in the membrane? (2)

Answer 2

• Allows lipid soluble substances to enter and leave the cell
• Prevents water soluble substances entering and leaving the cell

Question 3

What do channel proteins allow through the membrane? (2)

Answer 3

• Specific charged ions
• Small molecules such as water

Question 4

How do substances move through channel proteins?

Answer 4

Facilitated diffusion

Question 5

What do carrier proteins transport across membrane?

Answer 5

• polar molecules
• large molecules e.g. amino acids and glucose

Question 6

How are molecules transported across carrier proteins?

Answer 6

Facilitated diffusion and active transport

Question 7

What is the role of receptor proteins?

Answer 7

Specific receptors for complementary molecules e.g. hormones such as insulin

Question 8

What is the role of glycoproteins?

Answer 8

• act as recognition sites
• help cells attach to each other to form tissues
• allow cells to recognise each other

Question 9

What is the function of cholesterol in the cell membrane/

Answer 9

• reduce movement of other molecules
• make the membrane less fluid at higher temperatures
• prevent leakage of water nd dissolved ions

Question 10

Define diffusion

Answer 10

net movement of molecules from an area of higher to lower concentration across a partially permeable membrane

Question 11

Fick’s Law

Answer 11

rate of diffusion = surface area x concentration gradient / diffusion distance

Question 12

What is facilitated diffusion?

Answer 12

Transports polar molecules, ions and large molecules.

Question 13

How do water soluble, charged and large molecules pass?

Answer 13

They can’t pass between the hydrophobic fatty acid tails of the phospholipid bilayer. The tails are non- polar and repel polar molecules so they move through channel and carrier proteins.

Question 14

Describe how glucose is absorbed from the ileum into the blood

Answer 14

• Na+ are actively transported out of the epithelial cell into the blood by the sodium potassium pump
• This creates a concentration gradient of Na+ between lumen of ilium and epithelial cell
• Na+ and glucose enter by facilitated diffusion using co-transporter protein
• Na+ diffuses into the cell down its concentration gradient
• Glucose moves into the cell against its concentration gradient
• Glucose moves into the blood by facilitated diffusion