Transport Across Membranes

Question 1

Describe the basic structure of the cell-surface membrane

Answer 1

Double layer of phospholipids (phospholipid bilayer)

Question 2

Why is the phospholipid bilayer described as the ‘fluid mosaic model’?

Answer 2

The bilayer is ‘fluid’ because the phospholipids are constantly moving. Proteins are scattered through the bilayer, like tiles in a mosaic.

Question 3

The centre is ______ so the membrane doesn’t allow ______ substances (e.g.___) through, acting as a barrier.

Answer 3

The centre is hydrophobic so the membrane doesn’t allow water-soluble substances (ions) through, acting as a barrier.

Question 4

Cholesterol is a type of _____

Answer 4

Lipid

Question 5

Why is cholesterol an important component of the phospholipid bilayer?

Answer 5

• They fit between phospholipids.
• They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together.
• This restricts movement of phospholipids, making membrane more rigid.
• Cholesterol helps to maintain the shape of animal cells.

Question 6

Function of glycolipids? x3

Answer 6

• Cell-surface receptors for specific chemicals found in the external environment.
• Recognition sites. And help cells stick to one another and so form tissues.
• Maintain membrane stability.

Question 7

Function of glycoproteins? x3

Answer 7

• Cell-surface receptors for specific chemicals found in the external environment.
• Recognition sites. And help cells stick to one another and so form tissues.
• Allow cells to recognise one another.

Question 8

Increasing the temperature, increases the membrane permeability. T/F?

Answer 8

True!

Question 9

Define ‘diffusion’

Answer 9

‘The net movement of molecules from an area of higher concentration to an area of lower concentration’.

Question 10

Diffusion is a _____ process

Answer 10

Passive process

Question 11

3x criterium for molecules to pass through the phospholipid bilayer via simple diffusion?

Answer 11

• Small
• Non-polar
• Lipid soluble

Question 12

Two types of diffusion?

Answer 12

Simple and facilitated diffusion

Question 13

Mechanism of action of carrier proteins? 3 steps

Answer 13

1. A large molecule attaches to carrier protein in membrane.
2. Protein changes shape.
3. Releases contents on opposite end of membrane.

Question 14

Describe channel proteins

Answer 14

Form pores in membrane for charged particles to diffuse through. Different channel proteins cater for different molecules.

Question 15

State Fick’s Law

Answer 15

Rate of diffusion is proportional to

surface area x conc. gradient
/ diffusion distance

Question 16

How can the rate of diffusion be increased without altering surface area/conc. gradient/diffusion distance?

Answer 16

More/higher concentration of carrier proteins/channel proteins

Question 17

Define ‘osmosis’

Answer 17

‘Osmosis is the net movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane.’

Question 18

What is the solution called if it has an equal water potential to that of the cell?

Answer 18

Isotonic

Question 19

What is the solution called if it has a higher water potential to that of the cell? Net movement of water?

Answer 19

Net movement of water: into cell

Hypotonic

Question 20

What is the solution called if it has a lower water potential to that of the cell? Net movement of water?

Answer 20

Net movement of water: out of cell

Hypertonic

Question 21

Water potential is measured in ___

Highest water potential = ____ (pure water)

Answer 21

kPa

0kPa (100% H2O)

Question 22

Define ‘active transport’

Answer 22

‘Active transport is the movement of molecules through a cell membrane from an area of a low concentration to an area of higher concentration using ATP and carrier proteins.’

Question 23

How do co-transporter molecules work?

Answer 23

• They bind two molecules at the same time.
• The positive concentration gradient of one of the molecules is used to move the other molecule against its concentration gradient.

Question 24

Outline how amino acids/glucose are absorbed into the bloodstream across the ileum of the small intestine

Answer 24

1. Sodium ions are actively transported out of epithelial cells into blood via a sodium-potassium to maintain the sodium low concentration
2. This causes sodium ions to diffuse into epithelial cell from lumen down its concentration gradient via a co-transporter protein.
3. The co-transporter carrier glucose into cell with sodium. As a result the concentration of glucose inside the cell increases.
4. Glucose diffuses into blood out of cell down its concentration gradient through protein channels by facilitated diffusion.