C6 — Cell Membrane And Transport

Question 1

Diffusion definition

Answer 1

Diffusion is the net movement of a substance (solute molecules) from a region of higher concentration to a region of lower concentration, that is, down a concentration gradient.

Question 2

Osmosis definition

Answer 2

Osmosis is the net movement of freely moving water molecules from a region of less negative water potential to a region of more negative water potential through a partially permeable membrane.

Question 3

Water potential definition

Answer 3

Water potential, (trident) symbol, is a measure of the tendency for water to move from one region to another.

Question 4

Solute potential definition

Answer 4

Solute potential, (trident)s, is the measure of the ability of a solute to make the water potential more negative.

Question 5

Pressure potential definition

Answer 5

Pressure potential, (trident)p, is the measure of the pressure exerted by the cell wall on its contents. It is not applicable for animal cells as they lack cell walls.

Question 6

Incipient plasmolysis definition

Answer 6

For a population of plant cells, incipient plasmolysis is defined as the point where 50% of the plant cells are plasmolysed.

Question 7

Active transport definition

Answer 7

Active transport refers to the movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient. The energy required is provided in the form of ATP.

Question 8

Explain why the cell surface membrane model is called fluid mosaic. [3]

Answer 8

Fluidity
- It is composed of 2 layers consisting mainly of phospholipids ;
- Phospholipids, cholesterol and glycolipids / proteins are free to move within the membrane between or within layers ;

Mosaic
- Presence of proteins that are embedded in a scattered manner, asymmetrical on the 2 lipid layers

Question 9

Explain why transmembrane proteins are necessary for glucose, potassium ions and calcium ions to pass through cell surface membrane. [3]

Answer 9

• Glucose molecules are polar and large and potassium and calcium ions are charged ;
• The lipid bilayer consists of hydrocarbon fatty acid chains which is hydrophobic ;
• Thus, they cannot pass through the hydrophobic core of the lipid layer ;
• ref to specificity transport proteins ;

Question 10

Describe how a change in the ATP-sensitive potassium channels will cause the calcium channels to open. [2]

Answer 10

• High levels of ATP causes the ATP sensitive potassium channels in the cellular membrane to close, preventing potassium ions from leaving the cell ;
• Hence, increases the positive charges inside the cell and voltage-gated calcium channels are activated to transport calcium ions into the cell ;

Question 11

Suggest why there are no channels for insulin release across the membrane. [2]

Answer 11

• Insulin is a too large a protein to pass through the channels in the membrane ;
• If insulin channel exists, the opening of the channel could result in leakage of other smaller molecules or ions ;
• ref to the idea that insulin is released in regulated amounts and in response to high glucose concentration, hence insulin release via exocytosis will be more suitable ;

Question 12

Describe how the release of insulin from a beta cell in the pancreas occurs. [3]

Answer 12

• Insulin are enclosed in secretory vesicles which bud off from the trans face of the GA ;
• These vesicles migrates to and fuse with the cell membrane with the aid of transport proteins ;
• Thus, releasing insulin by exocytosis into the extracellular fluid ;