4. Transport Across Cell Membranes

Question 1

Describe the structure of the cell surface membrane.

Answer 1

Phospholipids form a bilayer, allowing lipid-soluble substances to move through. The hydrophilic heads of both phospholipid layers point to the outside of the membrane, attracted by water. The hydrophobic tails of both layers point to the centre of the membrane, repelled by water on both sides.
Some proteins occur within the bilayer surface to give support or act as cell receptors.
Other proteins e.g. carrier proteins/protein channels, span the phospholipid bilayer from one side to the other.
Cholesterol molecules occur within the phospholipid bilayer, adding strength to the membrane.
Glycolipids carbohydrate section extends from the phospholipid bilayer into the aqueous environment outside the cell.
Glycoproteins have carbohydrate chains attached to extrinsic proteins on the outer surface of the cell membrane.

Question 2

Describe the role of carrier proteins in facilitated diffusion

Answer 2

Carrier proteins bind to specific molecules, e.g glucose, causing it to change shape so a molecule is released inside the membrane. Molecules move form a region where they are highly concentrated to one of lower concentration, using only the kinetic energy of the molecules themselves.

Question 3

Describe the role of protein channels in facilitated diffusion

Answer 3

Protein channels form water filled hydrophilic tubes across the membrane, allowing water –soluble ions to pass through. The channels are selective, only opening in the presence of a specific ion, so controls entry and exit of ions. The ions bind to the protein, causing it to change shape so it closes to one side of the membrane and open to the other.

Question 4

Suggest why this model is known as fluid mosaic.

Answer 4

The model is known as fluid because the individual phospholipid molecules can move relative to another, giving the membrane a flexible structure, constantly changing in shape.
Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern, like mosaic tiles.

Question 5

Explain the function of proteins in a plasma membrane

Answer 5

Provide structural support,
Act as channels transporting water soluble substances
Allow active transport of ions through carrier proteins
Form cell-surface receptors
Help cells adhere together.

Question 6

Explain the function of cholesterol in a plasma membrane

Answer 6

Reduces lateral movement of other molecules and phospholipids
Make membrane less fluid at high temperatures
Prevent leakage of water and dissolved ions
Adds strength to the membranes

Question 7

Explain the function of glycoproteins in a plasma membrane

Answer 7

Act as recognition sites
Help cells attach to one another and form tissues
Allow cells to recognise each other, e.g. lymphocytes can recognise another organisms cells.

Question 8

Explain the function of glycolipids in a plasma membrane

Answer 8

Acts as recognition sites
Help maintain stability of membrane
Help cells attach to one another and form tissues

Question 9

State the overall function of the cell-surface membrane

Answer 9

To control movement of substances in and out of the cell.

Question 10

From your knowledge of cell membranes, suggest properties a drug should possess if it is to enter a cell rapidly.

Answer 10

• Lipid-soluble
• Small in size
• Possess no overall charge

Question 11

Define ‘diffusion’

Answer 11

The net movement of molecules or ions from a region where they’re highly concentrated to a region where their concentration is lower until evenly distributed.

Question 12

State three factors that affect the rate of diffusion

Answer 12

• concentration gradient
• thickness of exchange surface
• temperature
• area over which diffusion takes place

Question 13

Contrast facilitated diffusion and diffusion

Answer 13

Facilitated diffusion only occurs at channels on the membrane where there are special carrier protein molecules.

Question 14

Explain why facilitated diffusion is a passive process

Answer 14

There is no ATP from respiration used in the process. The only energy used is the inbuilt kinetic energy of the molecules themselves.

Question 15

Glucose molecules are transported into cells through pores in the proteins spanning the phospholipid bilayer. Explain why glucose does not pass easily through the phospholipid bilayer.

Answer 15

Only lipid-soluble substances diffuse across the phospholipid bilayer easily. Glucose is a water-soluble substance; so is transported very slowly.

Question 16

List two changes to the structure of the cell-surface membrane that would increase the rate at which glucose is transported into the cell.

Answer 16

(BY FACILITATED DIFFUSION)…

• Increase its surface area with microvilli
• Have more proteins with pores spanning the phospholipid bilayer

Question 17

Define ‘osmosis’

Answer 17

The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.

Question 18

What is the water potential of pure water

Answer 18

Zero.

All other values are negative- the more negative, the lower the water potential

Question 19

Give the symbol and unit for measuring water potential.

Answer 19

Psi- Ψ

Measured in kiloPascals- kPa

Question 20

If the net movement of water is entering a red blood cell, what is its water potential and state of the cell?

Answer 20

The water potential is higher/less negative.

The cell will swell and burst

Question 21

If the net movement of water is leaving a red blood cell, what is its water potential and state of the cell?

Answer 21

The water potential is lower/more negative.

The cell will shrink/crenation

Question 22

If water enters a plant cell, what is its water potential, and what is the condition of the cell and protoplast?

Answer 22

The water potential is higher/less negative.

The protoplast will swell, pushing against the cell wall and the cell will become turgid.

Question 23

What is the condition of a plant cell if the water potential is equal?

Answer 23

Incipient plasmolysis

Question 24

If water leaves a plant cell, what is its water potential, and what is the condition of the cell and protoplast?

Answer 24

The water potential is lower/more negative.

The protoplast will shrink, and the cell will become plasmolysed.

Question 25

Explain what is meant by a selectively permeable membrane

Answer 25

A membrane that is permeable to small water molecules, but not to larger molecules.

Question 26

Define active transport

Answer 26

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration, against a concentration gradient, using ATP and carrier proteins.

Question 27

How is ATP used in active transport?

Answer 27

• Directly moves molecules

- Individually moves molecules by co-transport.

Question 28

How does active transport differ from passive forms of transport?

Answer 28

• ATP is needed (metabolic energy)
• Substances moved against a concentration gradient
• Process highly selective

Question 29

Describe the process of active transport

Answer 29

• Carrier proteins span the plasma membrane and bind at receptor sites to the molecule/ion to be transported.
• Once inside the cell/organelle, ATP binds to the protein, splitting it into ADP and a phosphate molecule, resulting in the protein molecule changing shape, and opens to the opposite side of the membrane, where the molecule/ion is released.
• The phosphate molecule is released from the protein which causes the protein to revert to its original shape, ready to the process to repeat. The phosphate molecule recombines with ADP to form ATP during respiration.

Question 30

Name a specific example of co-transport where a molecule/ion is being actively removed from a cell, whilst another molecule/ion is being actively taken into it.

Answer 30

Sodium-potassium pump

Question 31

State one similarity and one difference between active transport and facilitated diffusion

Answer 31

Both use carrier proteins in the plasma membrane.

Active transport requires ATP, diffusion does not.

Question 32

Why is the presence of mitochondria typical of cells that carry out active transport

Answer 32

Active transport requires ATP. Mitochondria supply ATP in cells and therefore are numerous in cells carrying out active transport.

Question 33

Why is it important that glucose reabsorption happens by active transport rather than diffusion?

Answer 33

Diffusion, at best, can only reabsorb 50% of glucose lost from the blood. The other 50% will be lost from the body.
Active transport absorbs all the glucose, so none is lost from the body.

Question 34

What is the role of diffusion in glucose absorption?

Answer 34

As carbohydrates and proteins are digested continuously, there’s a greater concentration of glucose and amino acids within the ileum than in the blood- creating a concentration gradient for glucose to move via facilitated diffusion. The concentration gradient is maintained as the glucose absorbed in blood is constantly removed from cells in respiration.

Question 35

How is glucose absorbed by active transport/co-transport?

Answer 35

• Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein- carrier-molecule found in epithelial cell membrane.
• This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside inside the epithelial cells.
• Sodium ions diffuse into epithelial cells down this concentration gradient, through a co-transport protein carrier in the cell membrane. As the sodium ions diffuse in this second carrier protein, they carry either glucose or amino acid into the epithelial cell with them.
• The glucose/amino acids pass into blood plasma by facilitated diffusion using another carrier protein

Question 36

Why is co-transport of glucose an indirect form of active transport?

Answer 36

While sodium ions move down their concentration gradient, glucose molecules move against their concentration gradient. It is the sodium ion concentration gradient, not ATP directly, that powers the movement of glucose into cells.

Question 37

State 3 ways in which the rate of movement across cells can be increased

Answer 37

• Increasing the concentration gradient
• Increasing the surface area
• Increasing the density of protein channels

Question 38

Why is the term ‘co-transport’ used when describing the movement of glucose into cells.

Answer 38

Because glucose molecules and sodium ions move into cells coupled together.