4.1 - 4.5 Transport across cell membranes

Question 1

Describe what is meant by the fluid-mosaic model

Answer 1

• Refers to the structure of cell membranes
• Fluid = Phospholipids are constantly moving and flexible
• Mosaic = Interspersed with many other molecules

Question 2

What are the components of a cell membrane?

Answer 2

• Phospholipid bilayer
• Cholesterol
• Proteins
• Glycoproteins
• Glycolipids

Question 3

Describe the structure and function of the phospholipid bilayer in a cell membrane

Answer 3

Structure:
- Hydrophobic tails point in, hydrophilic heads point out
Function:
- Hydrophobic centre of bilayer prevents movement of water soluble substances
- Allows movement of lipid soluble substances
- Makes membrane flexible and self sealing

Question 4

Describe the structure and function of the proteins in a cell membrane

Answer 4

• Surface proteins = Give mechanical support or (with glycolipids) act as as cell receptors for molecules e.g hormones
• Protein channels = Water-filled tubes allowing water soluble ions to diffuse across the membrane
• Carrier proteins = Bind to ions or molecules (e.g glucose or amino acids) then change shape in order to move them across the membrane

Question 5

Describe the function of the cholesterol in a cell membrane

Answer 5

• Reduce lateral movement of other molecules including phospholipids
• Make the membrane less fluid at higher temps
• Prevent leakage of water and dissolved ions from the cell

Question 6

Describe the structure and function of the glycolipids in a cell membrane

Answer 6

Structure:
- A carbohydrate covalently bonded with a lipid
- Extends from phospholipid bilayer into area around cell
Function:
- Act as recognition sites
- Help maintain membrane stability
- Help cells attach to one another to form tissues

Question 7

Describe the structure and function of the glycoproteins in a cell membrane

Answer 7

Structure:
- Carbohydrate chains attached to extrinsic proteins on the outer surface of the membrane
Function:
- Act as recognition sites
- Help cells attach to one another to form tissues
- Allow cells to recognise one another

Question 8

For what reasons may a molecule not freely diffuse across the phospholipid bilayer?

Answer 8

• Not soluble in lipids
• Too large to pass through protein channels
• Of the same charge as the protein channels
• Are polar (so cannot pass non-polar phospholipid tails)

Question 9

What factors affect the permeability of a cell membrane?

Answer 9

• Temperature
• pH
• Solvent

Question 10

Define diffusion

Answer 10

The net movement of molecules or ions from a region where they are more highly concentrated to a region where their concentration is lower until evenly distributed. This is a passive process so does not require energy.

Question 11

Define facilitated diffusion

Answer 11

The net movement of molecules or ions from a region where they are more highly concentrated to a region where their concentration is lower until evenly distributed - with the help of a transport molecule. This is a passive process so does not require energy

Question 12

What are the two transport molecules used in facilitated diffusion and how do they work?

Answer 12

Carrier proteins:

1. A large molecule attaches to a carrier protein in the membrane
2. The protein changes shape
3. The molecule is released on the opposite side of the membrane

Channel proteins:

• Form pores in the membrane for charged particles to diffuse through
• Different channel particles facilitate the diffusion of different charged particles

Question 13

What factors affect the rate of simple diffusion?

Answer 13

• Concentration gradient
• Surface area
• Diffusion distance

Question 14

What factors affect the rate of facilitated diffusion?

Answer 14

• Concentration gradient
• Number of channel/carrier proteins

Question 15

Define osmosis

Answer 15

The movement of water molecules across a partially permeable membrane from an area of higher water potential to an area of low water potential, no energy required

Question 16

List 3 factors that affect the rate of osmosis

Answer 16

• Water potential gradient
• Surface area
• Osmosis distance

Question 17

Define active transport

Answer 17

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

Question 18

How is active transport different to passive forms of transport?

Answer 18

• Uses ATP
• Against the concentration gradient
• Carrier proteins act as pumps
• Very selective process

Question 19

How is ATP used in active transport?

Answer 19

• ATP binds to protein
• This causes it to split into ADP and a phosphate molecule
• As a result the protein changes shape and opens to the opposite side of the membrane
• Phosphate molecule is released from the protein so it goes back to its original shape
• Phosphate recombines with ADP to form ATP during respiration

Question 20

How is the small intestine adapted for absorption?

Answer 20

• Muscular villi (move contents around to maintain concentration gradient)
• Carrier proteins
• Good blood supply (maintain concentration gradient)
• One cell thick walls (smaller diffusion distance)
• Villi and micro-villi (larger surface area)

Question 21

Describe the absorption of glucose in the ileum

Answer 21

1. Sodium ions ACTIVELY TRANSPORTED (by the sodium-potassium pump) out of the epithelial cells and into the blood
2. There is now a lower concentration of sodium ions in the epithelial cell compared to the ileum
3. Sodium ions move into the epithelial cell (down the concentration gradient) bringing glucose or amino acids (against the concentration gradient), using a CO-TRANSPORT PROTEIN. The movement of the sodium ions powers the movement of the glucose or amino acids so this is INDIRECT ACTIVE TRANSPORT.
4. There is now a higher concentration of amino acids/glucose in the epithelial cell than in the blood so they can now move into the blood via FACILITATED DIFFUSION.