Biology AS Chapter 4 - Transport Across Cell Membranes

Question 1

What are the 3 functions of phospholipids in the cell surface membrane?

Answer 1

• Allow lipid soluble substances to enter and leave the cell
• Prevent water soluble substances entering and leaving the cell
• Make the membrane flexible and self sealing

Question 2

What is the different between protein channels and carrier proteins?

Answer 2

Protein channels form water filled tubes to allow water soluble ions to diffuse across the membrane. Carrier proteins bind to ions and then change shape to move these molecules across the membrane

Question 3

What are the 6 functions of proteins in the membrane?

Answer 3

• Provide structural support
• Act as channels to transport water soluble substances across the membrane
• Allow active transport through carrier proteins
• Form cell surface receptors for identifying cells
• Help cells adhere together
• Act as receptors

Question 4

What are the 3 functions of cholesterol in the membrane?

Answer 4

• Reduce lateral movement of other molecules, including phospholipids
• Make the membrane less fluid at high temperatures
• Prevent leakage of water and dissolved ions from the cell as they are very hydrophobic

Question 5

What are the 3 functions of glycolipids in the membrane?

Answer 5

• Act as recognition sites
• Help maintain the stability of the membrane
• Help cells attach to one another to form tissues

Question 6

What are the 3 functions of glycoproteins in the membrane?

Answer 6

• Act as recognition sites
• Help cells attach to one another and so form tissues
• Allows cells to recognise one another, for example, lymphocytes can recognise an organism own cells

Question 7

What are the 4 reasons why most molecules cannot freely diffuse across the membrane?

Answer 7

• Not soluble in lipids so cannot pass through the phospholipid bilayer
• Too large to pass through the channels in the membrane
• Of the same charge as the charge on the protein channels and so are repelled
• Electrically charged (polar) so cannot pass through non-polar hydrophobic tails in the phospholipid bilayer

Question 8

Why is the fluid mosaic model named this way?

Answer 8

Fluid: individual phospholipid molecules can move relative to one another, meaning the membrane is flexible
Mosaic: the proteins embedded vary in shape, size, and pattern

Question 9

Define diffusion

Answer 9

The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until they are evenly distributed

Question 10

Define facilitated diffusion

Answer 10

The movement of molecules from an area of high concentration to low concentration, facilitated by transmembrane channels and carriers in the membrane. No ATP is required

Question 11

Define osmosis

Answer 11

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 12

What is a solute?

Answer 12

Any substance dissolved in a solvent

Question 13

What is water potential measured in?

Answer 13

kPa

Question 14

Define active transport

Answer 14

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

Question 15

What are the 4 ways active transport differs from passive forms of transport?

Answer 15

• Metabolic energy in the form of ATP is required
• Substances are moved against a concentration gradient
• Carrier proteins are involved
• The process is very selective

Question 16

Describe the process of the direct active transport of a single molecule/ion

Answer 16

1. The carrier proteins span the plasma membrane and the molecule/ion binds to the receptor sites on the carrier protein
2. On the inside of the cell/organelle, ATP bind to the protein, causing it to be hydrolysed into ADP and a phosphate molecule. As a result, the protein molecule changes shape and opens to the opposite side of the membrane
3. The molecule/ion is then released to the other side of the membrane
4. The phosphate molecule is released from the protein which causes it to revert back to its original shape, ready for the process to be repeated. ATP is then re-synthesised

Question 17

How do microvilli increase the rate of transport across membranes?

Answer 17

The provide a larger surface area for the insertion of carrier proteins

Question 18

Give a detailed explanation using an example of the role of diffusion in absorption in the ileum

Answer 18

As carbohydrates and proteins are being digested continuously, there is usually a greater concentration of glucose and amino acids in the ileum than the blood. Glucose therefore moves down the concentration gradient by facilitated diffusion. Given that the blood is constantly being circulated, the glucose absorbed is continuously being removed by cells as they use it up during respiration. This helps to maintain the concentration gradient between the inside of the ileum and the blood and so the rate of movement by facilitated diffusion across epithelial cell surface membranes is increased

Question 19

How are glucose and amino acids absorbed from the small intestine when there is not a diffusion gradient?

Answer 19

Co-transport

Question 20

Describe the process by which the sodium-potassium pump allows glucose and amino acids to pass into the blood (co-transport)

Answer 20

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

Question 21

What are 3 causes of diarrhoea?

Answer 21

• damage to the epithelial cells lining the intestine
• loss of microvilli due to toxins
• excessive secretion of water due to toxins

Question 22

Why is drinking water to treat diarrhoeal diseases ineffective?

Answer 22

• water is not being absorbed from the intestine
• drinking water does not replace the electrolytes (ions) which are being lost from the intestinal cells

Question 23

What does a rehydration solution need to contain and why?

Answer 23

1. Water - to rehydrate the tissues
2. Sodium ions - to replace those lost from the epithelium of the intestine and optimise the use of sodium-glucose carrier proteins
3. Glucose - to stimulate the uptake os sodium ions from the intestine and provide energy
4. Potassium ions - to replace those lost and stimulate appetite
5. Other electrolytes - such as chloride ions and citrate ions to help prevent an imbalance