2 Cells: 4 Transport Across Cell Membranes

Question 1

Why is the structure of the cell-surface membrane described as a fluid-mosaic model?

Answer 1

Fluid because the individual phospholipid molecules can move relative to one another which gives the membrane a flexible structure that is constantly changing shape.
Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern - much like the tiles of a mosaic.

Question 2

What is the arrangement of phospholipids in the cell-surface membrane?

Answer 2

The hydrophilic heads point to the outside, attracted by water.
The hydrophobic tails point into the centre, repelled by water.
Lipid-soluble material moves through the membrane via the phospholipid portion.

Question 3

What are the functions of phospholipids in the cell-surface membrane?

Answer 3

Allows lipid-soluble materials to enter and leave the cell.
Prevents water-soluble substances entering and leaving.
Makes the membrane flexible and self-sealing.

Question 4

What does cholesterol do in the cell-surface membrane?

Answer 4

Adds strength to the membrane.
Very hydrophobic so helps in preventing loss of water and dissolved ions from the cell.
Pulls together the fatty acid tails, restricting their movement and that of other molecules, without making the membrane too rigid.

Question 5

What are the functions of cholesterol in the cell-surface membrane?

Answer 5

Restricts lateral movement of phospholipids and other molecules.
Makes the membrane less fluid at high temperatures.
Prevents leakage of water and dissolved ions from the cell.

Question 6

What are the two arrangements of proteins in the cell-surface membrane?

Answer 6

Peripheral (attached to the surface).
Integral (piercing through the bilayer).

Question 7

What do the peripheral proteins do in the cell-surface membrane?

Answer 7

Gives mechanical support or acts as cell receptors for molecules.

Question 8

What do the integral proteins do in the cell-surface membrane?

Answer 8

Channel proteins allow water-soluble ions to diffuse across the membrane.
Carrier proteins bind to ions or molecules, then change shape to move these molecules across the membrane.

Question 9

What are the functions of proteins in the cell-surface membrane?

Answer 9

Provides structural support.
Acts as channels transporting water soluble substances across the membrane.
Allows active transport across the membrane through carrier proteins.
Forms cell-surface receptors for identifying cells.
Helps cells adhere together.
Acts as receptors, e.g., for hormones.

Question 10

What is the arrangement of glycolipids in the cell-surface membrane?

Answer 10

The carbohydrate portion extends from the membrane.

Question 11

What are the functions of glycolipids in the cell-surface membrane?

Answer 11

Acts as recognition sites.
Helps maintain the stability of the membrane.
Helps cells to attach to one another and so form tissues.

Question 12

What is the arrangement of glycoproteins in the cell-surface membrane?

Answer 12

Carbohydrate chains are attached to proteins on the outer surface of the cell-surface membrane.
Acts as cell-surface receptors for hormones and neurotransmitters.

Question 13

What are the functions of glycoproteins in the cell-surface membrane?

Answer 13

Acts as recognition sites.
Helps cells to attach to one another and so form tissues.
Allows cells to recognise one another.

Question 14

What is the permeability of the cell-surface membrane?

Answer 14

Most molecules don’t freely diffuse across the membrane because they are:
- not soluble in lipids and therefore cannot pass through the phospholipid bilayer
- too large to pass through the protein channels
- of the same charge as the protein channels so are repelled
- polar and therefore have difficulty passing through the non-polar hydrophobic tails.

Question 15

What is facilitated diffusion?

Answer 15

The movement of charged ions and polar molecules is sped up by diffusion through carrier proteins or channel proteins in the membrane.

Question 16

What is the role of channel proteins in facilitated diffusion?

Answer 16

They form water-filled pores in the membrane for charged particles to diffuse through.
Channels are selective, each opening in the presence of a specific water-soluble ion.
Aquaporins allow the facilitate diffusion of water.

Question 17

What is the role of carrier proteins in facilitated diffusion?

Answer 17

They move large molecules across the membrane.
When a molecule that is specific to the carrier protein is present, it attaches to the protein.
Then the protein changes shape which releases the molecule to the opposite side of the membrane.

Question 18

What is osmosis?

Answer 18

The diffusion of water molecules across a partially permeable membrane down the water potential gradient.

Question 19

What happens to cells in a hypertonic solution?

Answer 19

If the concentration of solute outside the body cells is too high, the water will leave the cells and cause them to shrivel.
Plasmolysis happens in plant cells (irreversible), crenation in animal cells (reversible).

Question 20

What happens to cells in a hypotonic solution?

Answer 20

If the concentration of a solute inside the body cells is higher than outside, the water will move into the cells, causing them to bulk up.
Lysis happens in animal cells (irreversible) whilst plant cells become turgid (reversible).

Question 21

What happens to cells in an isotonic solution?

Answer 21

If the concentration of solute inside and outside the cell is the same, the cell will maintain its normal osmotic pressure.

Question 22

What is the process of active transport involving carrier proteins?

Answer 22

1. The carrier proteins bind to the molecule/ion.
2. The molecule/ion binds to the receptor sites on the carrier protein.
3. On the inside of the cell, ATP binds to the protein, causing it to split into ADP and a phosphate molecule. As a result, the carrier protein changes shape and releases the molecule/ion to the other side.
4. The phosphate molecule is released from the protein which causes it to revert to its original shape, ready for the process to be repeated.
   The phosphate molecule then recombines with the ADP to form ATP during respiration.

Question 23

What is the importance of the hydrolysis of ATP in active transport?

Answer 23

ATP is used to directly move molecules

Question 24

How do sodium ions and glucose molecules get absorbed by cells lining the mammalian ileum?

Answer 24

1. Na+ ions leave epithelial cells and enter blood by active transport, using ATP, through a carrier protein. (sodium-potassium pump.)
2. Na+ ion concentration is lower in cells than in lumen.
3. Na+ ions enter cells by facilitated diffusion through a co-transport protein.
4. Glucose is absorbed with Na+ ions against their concentration gradient by co-transport.
5. The concentration of glucose inside the cells increases.
6. Glucose molecules leave the cells into the blood by facilitated diffusion down their concentration gradient, through a channel protein.

Question 24

How may cells be adapted for rapid transport across their membranes?

Answer 24

An increase in surface area.
An increase in the number of channel and carrier proteins in their membranes.