ANSWERS TO PG 26-28

Section A

1. a) Draw a graph of percentage of burst red blood cells against concentration of salt solution from Table 1.

b)(i) about 0.435 g/100cm³ (read from graph)

(ii) 0.55 g/100cm³(where the graph intersects the x- axis) No bursting of red blood cells occurs for this concentration because it is isotonic to the concentration of RBC.

c)(i) The RBC will shrink/crinkle.

(ii) At concentration more than 0.55 g/100cm³ , water molecules will diffuse out from the RBC by osmosis. As a result, RBC will shrink due to loss of water.

2. a)(i) Solution X is hypotonic to the cell sap of potato cells. Water molecules from solution X move into the vacuoles of the potato cells by osmosis. The enlarged vacuole will push against the cytoplasm, causing the cells to inflate. This causes the potato strip to lengthen.

(ii) Solution Y is isotonic to the cell sap of potato cells. The rate of movement of water molecules in and out of the cells is the same. Therefore, there is no change in length of the strip.

(iii) Solution Z is hypertonic to the cell sap of potato cells. Water molecules out from the vacuoles of the potato cells by osmosis. The cells shrinks and plasmolysis takes place.

(b)(i) Hard

(ii) Soft

(c) The use of excessive fertilizers will increase the osmotic concentration in the soil water, causing water molecules to move out from the root hairs. The plant will wilt and die.

Section B

3 a) Simple diffusion - Movement of molecules in gas or liquid from a region of high concentration to a region of lower concentration.

Facilitated diffusion - Movement of big molecules along a concentration gradient with the help of protein carriers across the plasma membrane.

Osmosis - Movement of water molecules from a region of less concentrated solution to a region of more concentrated solution across a semi-permeable membrane.

Active transport - Movement of particles across the plasma membrane against the concentration gradient with the help of protein carriers and the presence of energy from ATP.

b)

Active transport	Osmosis
Active transport needs energy	Osmosis does not need energy
Active transport involves the movements of molecules or ions against a concentration gradient.	Osmosis transport involves the movements of water molecules along a concentration gradient.
Active transport takes places through the plasma membrane of a living cell.	Osmosis takes places through a semi-permeable membrane.
Active transport needs protein carriers	Osmosis does not need protein carriers

4.(a) Plasma membrane is selectively permeable. It permits lipid-soluble molecules such as glycerol, vitamins A, D, E and K to move across. Small, uncharged molecules such as water move freely across. Large molecules such as glucose and amino acids move across the plasma membrane with the aid of carrier proteins. Larger molecules such as starch cannot move across the plasma membrane.

(b) Plasma membrane consists of phospholipids bilayer and proteins. Phospholipid molecule consists of a polar head which is hydrophilic and a pair of non-polar fatty acid tails which is hydrophobic. Two types of proteins are pore proteins and transport proteins.

Plasma membrane is semi-permeable which allows certain substances to move in and out freely. Small, uncharged molecules such as oxygen and carbon dioxide move freely through the phospholipids bilayer through simple diffusion.

Water molecules which are attracted to the hydrophilic heads of the phospholipids move across through osmosis.

Lipid-soluble molecules such as fatty acids and ethanol dissolve in the lipid bilayer and move across through simple diffusion.

Large, water-soluble molecules such as glucose and amino acids require the aid of transport proteins to move them across the plasma membrane through facilitated diffusion or active transport.

Ions such as K+ and Na+ are transported across the plasma membrane through facilitated diffusion or active transport with the help of transport proteins.

c) Vegetables soak in salt solution which is hypertonic to the cell sap of vegetable cells. Harmful insecticides or fungicides which had been sprayed on the vegetables earlier diffuse out of the cells to the salt solution. Water from the cell sap in the vacuole also diffuses out the salt solution through osmosis. The vegetables become flaccid. This action cleans the vegetables of harmful insecticides but causes the vegetables to be flaccid and soft.