Transport Across Cell Membranes

Question 1

What is diffusion (simple)?

Answer 1

The net movement of molecules or ions from an area of high concentration to an area of lower concentration until equilibrium reached. (This is a passive process so it relies on kinetic energy, not ATP.)

Question 2

What is facilitated diffusion?

Answer 2

When charged ions/polar molecules move across the cell surface membrane, relying on intrinsic proteins. It is a passive process and occurs down a concentration gradient.

Question 3

What is a channel protein?

Answer 3

A protein that is a hydrophilic water-filled channel across the membrane which allows specific water soluble ions to pass. It is selective for a specific ion that bonds and causes a conformational change in shape.

Question 4

What is an aquaporin?

Answer 4

A channel protein for water only.

Question 5

What is a carrier protein?

Answer 5

An intrinsic protein which a specific molecule binds to , causing a change in shape which releases that molecule inside.

Question 6

What is a co-transport protein?

Answer 6

A carrier protein that allows more than 1 molecule to move, e.g. glucose-sodium co-transport protein.

Question 7

What does the rate of diffusion depend on?

Answer 7

• Concentration gradient
• Thickness of exchange surface
• Surface Area
• Temperature
• Stirring/moving
• Size of molecule
• Increase in channel/carrier proteins (facilitated diffusion)

Question 8

What is a partially permeable membrane?

Answer 8

A membrane which only allows some substances through.

Question 9

What is osmosis?

Answer 9

The movement of water from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

Question 10

What is water potential?

Answer 10

Pressure on cell membrane caused by water molecule collisions.

Question 11

How does the amount of solutes dissolved affect water potential?

Answer 11

It makes it more negative.

Question 12

What does isotonic mean?

Answer 12

When the concentration of solutes is equal in and outside of the cell.

Question 13

What does hypertonic mean?

Answer 13

Where the concentration of solutes in the solution is greater outside the cell than inside. (e.g. high sugar concentration)

Question 14

What does hypotonic mean?

Answer 14

Where the concentration of solutes in the solution is greater inside the cell than outside. (e.g. pure water)

Question 15

What happens when an animal cell is placed in a hypertonic solution?

Answer 15

The cell shrinks and shrivels.

Question 16

What happens when an animal cell or a plant cell is placed in an isotonic solution?

Answer 16

There is no change.

Question 17

What happens when an animal cell is placed in a hypotonic solution?

Answer 17

The cell swells and bursts. (can be described as being haemolysed if the cell has a haem group, e.g. erythrocyte [red blood cell])

Question 18

What happens when a plant cell is placed in a hypertonic solution?

Answer 18

The cell is plasmolysed (the cell membrane pulls away from the cell wall).

Question 19

What happens when a plant cell is placed in a hypotonic solution?

Answer 19

The cell becomes turgid.

Question 20

What factors influence the movement of water into and out of cells?

Answer 20

• The concentration of solutes.
• The amount of water present in the cytoplasm and the exterior environment.
• The presence of aquaporins membranes.
• (Plants) The pressure exerted on cell contents by cell wall, which is rigid and resists expansion, and therefore uptake of water.

Question 21

What is active transport?

Answer 21

The movement of molecules or ions from a region of lower concentration to a region of higher concentration (against a concentration gradient) using ATP and carrier proteins.

Question 22

Describe active transport.

Answer 22

• Molecule or ion binds to specific receptor site on carrier protein
• ATP binds to protein causing ATP -> ADP + Pi
• Protein changes shape + molecule/ion released inside
• Phosphate (Pi) released from protein, causing protein to return to original shape. Pi recombines ADP -> ATP in respiration

Question 23

Why are membrane described by the ‘fluid mosaic model’?

Answer 23

‘Fluid’ refers to the lateral, fluid movement of phospholipids and ‘mosaic’ refers to the different proteins embedded in the phospholipid bilayer.

Question 24

What do phospholipids allow the membrane to be?

Answer 24

Flexible and self sealing.

Question 25

What are the two types of proteins embedded in the phospholipid bilayer?

Answer 25

1). Extrinsic / peripheral proteins - don’t span the entire bilayer, provide mechanical support, and/or are in conjunction with glycoproteins as cell receptors + cell recognition.
2). Intrinsic / integral proteins - span the entirety of the phospholipid bilayer, e.g. channel proteins + carrier proteins for transport.

Question 26

What is a glycoprotein and what are its roles (csm)?

Answer 26

It is a carbohydrate attached to a protein.
- Recognition sites
- Allows cell attachment to form tissue
- Present antigens
- Hormone + neurotransmitter receptors

Question 27

What is a glycolipid and what are its roles (csm)?

Answer 27

It is a carbohydrate attached to a phospholipid.
- Acts as a cell-surface receptors for specific chemicals
- Helps maintain stability of the membrane
- Helps cell to attach to one another and form tissue

Question 28

What is the role of cholesterol (in eukaryotes, csm)?

Answer 28

• Strengthen membranes ~ sits between fatty acid tails and makes the barrier more complete
• Hydrophobic ~ helps prevent loss of water and dissolved ions from the cell (maintains osmotic balance)
• Limits movement of phospholipids