Explain the process of digestion of food in mouth, stomach and small intestine in human body. Or (a) List the three events that occur during the process of photosynthesis. Explain the role of stomata in this process. (b) Describe an experiment to show that “sunlight is essential for photosynthesis.”

Digestion in Human Body

1. Mouth: Food is crushed by teeth and wetted with saliva. Salivary glands secrete salivary amylase, an enzyme that breaks down starch (complex molecule) into simple sugar.
2. Stomach: Gastric glands in the stomach wall release hydrochloric acid (creates acidic medium), pepsin (protein-digesting enzyme), and mucus (protects stomach lining). The muscular walls mix food with these juices.
3. Small Intestine: This is the site of complete digestion of carbohydrates, proteins, and fats.

• Liver: Secretes bile juice to make the food alkaline and emulsify fats (break large globules into smaller ones).
• Pancreas: Secretes pancreatic juice containing trypsin (for proteins) and lipase (for emulsified fats).
• Intestinal Juice: Final enzymes convert proteins to amino acids, complex carbohydrates to glucose, and fats to fatty acids and glycerol.
• Absorption: Villi increase surface area for absorption into blood vessels.

OR

(a) Photosynthesis and Stomata

Three events of photosynthesis:

(i) Absorption of light energy by chlorophyll.

(ii) Conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen.

(iii) Reduction of carbon dioxide to carbohydrates.

Role of Stomata: Stomata are tiny pores on the leaf surface. They allow massive gaseous exchange (intake of $CO_2$ and release of $O_2$). The opening and closing of these pores are regulated by guard cells, which swell when water flows into them (opening the pore) and shrink when water leaves (closing the pore).

(b) Experiment: Sunlight is Essential for Photosynthesis

1. Destarching: Keep a potted plant in a dark room for 3 days to use up all starch.
2. Experimental Setup: Cover a part of one leaf with black paper or a light-proof strip. Leave the rest of the leaf exposed.
3. Exposure: Place the plant in sunlight for about 6 hours.
4. Starch Test: Pluck the leaf, remove the paper, and boil it in alcohol (using a water bath) to remove chlorophyll. Rinse and add dilute iodine solution.
5. Observation: Only the part of the leaf that was exposed to sunlight turns blue-black (presence of starch). The covered part remains pale (absence of starch).
6. Conclusion: Sunlight is necessary for the production of starch (photosynthesis).