(a) Give reasons for the following :

(i) Alveoli in lungs are richly supplied with blood capillaries.

(ii) Respiratory pigment in the blood takes up oxygen and not carbon dioxide.

(iii) During anaerobic respiration, a 3-carbon molecule is formed as an end product instead of $\mathrm{CO}_2$ in human beings.

OR

(b) (i) Name the movements that occur all along the gut in human digestive system. How do they help in digestion ?

(ii) Where is bile juice stored in human body ? List two roles of bile juice.

(a)

(i) Alveoli are richly supplied with blood capillaries to facilitate the efficient exchange of gases. The thin walls of the alveoli and the extensive network of capillaries maximize the surface area, allowing oxygen to diffuse rapidly from the alveolar air into the blood, and carbon dioxide to diffuse from the blood into the alveoli to be exhaled.

(ii) Respiratory pigment (hemoglobin) takes up oxygen and not carbon dioxide because carbon dioxide is highly soluble in water and is therefore mostly transported in the dissolved form in our blood plasma. Oxygen, having low solubility in water, requires hemoglobin which has a very high affinity for oxygen to transport it efficiently to all body tissues.

(iii) During anaerobic respiration in human muscle cells, there is a temporary lack of oxygen (especially during vigorous exercise). To meet the immediate energy demand, the 6-carbon glucose is first broken down into 3-carbon pyruvate, which is then converted into lactic acid (a 3-carbon molecule) instead of being completely broken down into $\mathrm{CO}_2$ and water.

OR

(b)

(i) The movements that occur all along the gut are called peristaltic movements.

• How they help in digestion: The muscles in the walls of the alimentary canal contract and relax rhythmically. This pushes the food forward in a highly regulated manner along the digestive tract, ensuring that the food is properly mixed with digestive juices and processed in each specialized part of the gut.

(ii) Bile juice is stored in the gallbladder.

• Two roles of bile juice:

1. Neutralization/Alkalinity: It makes the acidic food coming from the stomach alkaline so that the pancreatic enzymes can act on it.
2. Emulsification of fats: Bile salts break down large fat globules in the small intestine into much smaller globules, which increases the surface area and efficiency of lipase action.