(a)Describe briefly the process involved in the conversion of food to chyme in the stomach of a mammal. (b)Complete the table below by stating one enzyme, d...

Answer Details

(a) Conversion of food to chyme in the stomach

When food (the bolus) reaches the stomach, the gastric glands in the stomach wall secrete gastric juice, which contains hydrochloric acid, the enzyme pepsin(ogen) and rennin. The hydrochloric acid provides an acidic medium, kills harmful bacteria and activates pepsinogen into active pepsin. Pepsin digests proteins into shorter chains (peptones and polypeptides), while rennin curdles the soluble milk protein (caseinogen) into insoluble casein. At the same time the muscular wall of the stomach contracts rhythmically (churning / peristalsis), thoroughly mixing the food with the gastric juice. The food is gradually turned into a semi-liquid, acidic, partly digested paste called chyme, which is then released in small amounts into the small intestine.

(b) Completed table

(c) Experiment to show that carbon dioxide is necessary for photosynthesis

1. Destarch a potted plant by keeping it in the dark for about 48 hours, then test one leaf to confirm no starch is present.
2. Enclose one leaf (still attached) in a flask containing a little potassium hydroxide (KOH) or soda lime, which absorbs all the carbon dioxide inside; seal the neck with a split cork and vaseline so it is air-tight.
3. Set up a control: enclose another leaf in a flask containing sodium hydrogen carbonate solution (which supplies carbon dioxide).
4. Expose the whole plant to bright sunlight for several hours.
5. Remove both leaves and test each for starch (boil in water, decolourise in alcohol, then add iodine solution).

Result: the leaf deprived of carbon dioxide shows no starch (iodine stays brown), while the control leaf supplied with carbon dioxide turns blue-black, showing starch. Conclusion: carbon dioxide is necessary for photosynthesis.