Topic 2: transport across cell membranes

Question 1

State the components found in cell membranes (cell-surface and organelle membranes):

Answer 1

1. Phopholipids
2. Proteins
3. Glycoproteins
4. Glycolipids
5. Cholesterol

Question 2

Describe and explain the role of phospholipids

Answer 2

Phospholipids align as a bilayer due to the hydrophilic heads being attracted to water and the hydrophobic tails being repelled by water.

Question 3

Describe and explain the role of proteins

Answer 3

Proteins are embedded across the cell surface membrane. Integral proteins are protein carriers and channels for transport.

Question 4

Describe and explain the role of glycoproteins

Answer 4

Glycoproteins are carbohydrates attached to peripheral proteins and are involved in cell recognition and as receptors.

Question 5

Describe and explain the role of glycolipids

Answer 5

Glycolipids are lipids attached to peripheral proteins and are involved in cell recognition and as receptors.

Question 6

Describe and explain the role of cholesterol

Answer 6

Cholesterol is present in some membranes too and this will restrict the lateral movement of other molecules in the membrane. This is useful as it makes the membrane less fluid at high temperatures and prevents water and dissolved ions from leaking out of the cell.

Question 7

Cell membranes are partially permeable. List which molecules can: Pass through the membrane by simple diffusion:

Answer 7

Lipid-soluble substances
Very small molecules

Question 8

Cell membranes are partially permeable. List which molecules Cannot pass through the membrane by simple diffusion:

Answer 8

Water-soluble substances
Large molecules

Question 9

Define and describe simple diffusion

Answer 9

The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process does not require ATP, and therefore it is a passive process.

Question 10

define and describe facilitated diffusion:

Answer 10

This is a passive process (it does not require ATP) but it differs from simple diffusion as proteins are used to transport molecules. The movement of ions and polar molecules, which cannot simply diffuse, can be transported across membranes by facilitated diffusion using protein channels and carrier proteins.

Question 11

Name the two types of proteins that are
involved in facilitated diffusion and describe
how they work:

Answer 11

1.Protein channels form tubes filled with water and this enables water-soluble ions to pass through the membrane. This is still selective, as the channel proteins only open in the presence of certain ions when they bind to the protein.

2.Carrier proteins will bind with a molecule, such as glucose, which causes a change in the shape of the protein. This shape change enables the molecule to be released to the other side of the membrane.

Question 12

define and describe osmosis:

Answer 12

Osmosis is the movement of water from an area of higher water potential to an area of lower water potential across a partially permeable membrane.

Question 13

Define
Hypertonic solution:

Answer 13

The water potential of a solution is more negative than the cell

Question 14

Define isotonic solution

Answer 14

The water potential is the same in the solution and the cell within the solution

Question 15

Define hypotonic solution

Answer 15

the water potential of a solution is more positive than the cell

Question 16

Describe what would happen to an animal and plant cell in a hypotonic solution and explain the difference.

Answer 16

A lot of water will move into the cell by osmosis. As animal cells do not have a cell wall, the pressure will cause the cell to burst ( lyse).

Question 17

Explain why cells do not gain or lose mass
in an isotonic solution.

Answer 17

There is no net movement of water in/out of the cell. No osmosis as there is no difference in water potential.

Question 18

Define active transport:

Answer 18

The movement of molecules and ions from an area of lower concentration to an area of higher concentration (against the concentration gradient) using ATP and carrier proteins.

Question 19

describe the process of active transport:

Answer 19

Certain molecules can bind to the receptor site on carrier proteins. ATP will bind to the protein on the inside of the membrane and is hydrolysed into ADP and Pi. This causes the protein to change shape and open towards the inside of the membrane.

This causes the molecule to be released on the other side of the membrane. The Pi molecule is then released from the protein, and this results in the protein reverting to its original shape. This is how ATP and carrier proteins are used in active transport

Question 20

Which two molecules are absorbed by co-transport in the ileum?

Answer 20

1. Glucose
2. Amino acids

Question 21

Describe the process of co-transport:

Answer 21

1.Sodium ions are actively transported out of the epithelial cell into the blood in the capillary.

2.This reduces the sodium ion concentration in the epithelial cell.

3.Sodium ions can then diffuse from the lumen down their
concentration gradient into the epithelial cell.

4.The protein the sodium ions diffuse through is a co-transporter
protein, so either glucose or amino acids also attach and are transported into the epithelial cell against their concentration gradient.

5.Glucose then moves by facilitated diffusion from the epithelial cell to the blood.

Question 22

Explain how the following factors affect the rate of movement across a membrane: surface area

Answer 22

The larger the surface area, the more space there is for transport and therefore this increases the rate of movement.

Question 23

Explain how the following factors affect the rate of movement across a membrane: no. of channel or carrier proteins

Answer 23

If there are more channel/carrier proteins embedded within a membrane, then this will increases the surface area and the rate of movement will increase.

Question 24

Explain how the following factors affect the rate of movement across a membrane: difference in conc gradient or water potential gradient

Answer 24

If the concentration gradient is steeper then the rate of movement will increase because there are more molecules in the concentrated area and therefore more molecules to move across.

Question 25

Give two examples of cells that are adapted for rapid transport across their internal or external membranes:

Answer 25

1. Microvilli
   2.Chloroplasts (thylakoid membrane has a large
   surface area and many embedded proteins)