2B: Transport across cell membranes

Question 1

Describe the phospholipid bilayer.

Answer 1

• A partially permeable barrier regulating entry/ exit of particles
• Has binding sites
• Confers antigenic properties to the cell
• Site of certain reactions
• Compartmentalises different cell reactions in organelles

Question 2

Describe Cholesterol

Answer 2

• Fatty substance
• Hydrophobic so pulls together the phospholipid tails
• Adds strength + rigidity to the membrane (particularly at high temps)
• Reduces the movement of other molecules in the membranes

Question 3

Describe Glycolipids

Answer 3

• Carbohydrate bound to lipid
• Lipid portion embedded in phospholipid bilayer, carbohydrate portion extends outside the cells
• Act as recognition and binding cell

Question 4

Describe Intrinsic proteins

Answer 4

• Span the membrane + act as a channel and carrier proteins

- Made only of proteins

Question 5

Describe Extrinsic proteins

Answer 5

• On the surface of the membrane (internal/external)
• Act as receptors or enzymes
• May have carbohydrates bonded to them

Question 6

What is diffusion?

Answer 6

• The net movement of molecules or ions from a region of high concentration to a lower concentration until they are evenly distributed.
• Passive
• Only small non- polar molecules can diffuse across the cell membrane e.g Oxygen, CO2

Question 7

Describe facilitated diffusion

Answer 7

• Passive
• Relies on kinetic energy of diffusing molecules
• Through transmembrane channels and carriers.
• Has to happen at specific points
• Occurs down a concentration gradient

Question 8

Describe protein channels

Answer 8

• Water filled hydrophilic channels
• Allows specific water soluble ions to pass through
• Channels are selective
• Channel will remain closed unless specific ion is present

Question 9

Describe carrier proteins

Answer 9

• Molecule has to be specific to the protein
• Molecule binds to protein, which causes it to change shape when it is released
• Passive, only uses kinetic energy of the molecule

Question 10

What is Osmosis?

Answer 10

The passage of water from a region where there is a higher water potential to a region with a lower water potential through a selectively permeable membrane

Question 11

Describe water potential

Answer 11

• Osmosis can be quantified using water potential
• Pure water has a water potential of 0
• The addition of solutes reduces the water potential (to a - value)
• Water potential cannot be above 0
• It always moves from high - low
• Measured in terms of pressure (e.g kPa)

Question 12

Define Isotonic

Answer 12

Water in/ water out is balanced

Question 13

Define Hypotonic

Answer 13

Water potential is higher outside - cell is turgid

Question 14

Define Hypertonic

Answer 14

Water potential is higher inside - cell is shriveled

Question 15

What happens to a plant cell in a hypotonic environment?

Answer 15

The cell will become turgid, as more water enters the cell

Question 16

What happens to a plant cell in a hypertonic environment?

Answer 16

The cell will become plasmolyzed, as more water leaves the cell

Question 17

What happens to an animal cell in a hypotonic environment?

Answer 17

The cell will go through lysis (it will burst), as more water enters the cell

Question 18

What happens to an animal cell in a hypertonic environment?

Answer 18

The cell will go through crenation (it will shrivel)

Question 19

Define active transport

Answer 19

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

Question 20

Describe the sodium/ potassium pump & what type of transport is involved?

Answer 20

The Na+/ K+ pump involves active transport

• Found in all animal membranes (3Na+/ 2K+)
• Continually uses ATP (ATP cycle) to actively pump Na+ out of the cell and K+ into the cell
• This creates conc. gradients across the membrane

Question 21

Describe co-transport?

Answer 21

• Carrier proteins can sometimes transport 2 particles at once
• Both must be present for transport to take place
• Often both an active + passive process as one particle is moved with the conc. gradient and the other has moved against it.
• Substances can be moved in the same or opposite direction

Question 22

Give an example of co-transport

Answer 22

The absorption of sodium ions and glucose by cells lining the mammalian ileum.

Question 23

Define antiport

Answer 23

Antiport is a type of transport in which there are two molecules which move at once through the membrane in opposite directions.

Question 24

Define symport

Answer 24

Symport is the type of transport in which two compounds can move simultaneously across a cell membrane in the same direction.

Question 25

Describe the co- transport of glucose and sodium in the ileum (5 points)

Answer 25

1. Na+ actively pumped out of the epithelial cell &K+ pumped in (active transport so needs ATP) - against conc. gradient
2. Low conc of Na+ inside epithelial cell
3. So Na+ moves out of the lumen and into the epithelial cell - carrying glucose with it
4. = High glucose levels in cell - conc. gradient between cell and the blood.
5. So glucose moves out of cell and into blood capillary