(a) Describe briefly the role of the stomach in digestion. (b) (i) Name three parts of plants in which food can be stored. (ii) Give one example in each cas...

(a) Role of the stomach in digestion

The stomach is a muscular, J-shaped organ that stores food temporarily and continues digestion. Its walls contain gastric glands that secrete gastric juice. When food arrives, the muscular walls churn and mix it with the gastric juice to form a semi-liquid mass called chyme. The gastric juice contains:

• Hydrochloric acid, which provides the acidic medium needed for the enzymes to act, kills bacteria in the food, and activates pepsin.
• Pepsin, an enzyme that begins the digestion of proteins, breaking them down into peptones and polypeptides.
• Rennin (in young mammals), which curdles the soluble milk protein caseinogen into insoluble casein so it can be acted upon by pepsin.

The stomach also absorbs a little water, alcohol and simple substances, and releases the chyme gradually into the duodenum. Mucus secreted by the wall protects the stomach lining from being digested by its own acid and enzyme.

(b)(i) Three parts of plants in which food can be stored, with examples

(Leaves, as in onion bulb scales, are another storage part.)

(c) Regulation of blood sugar level in mammals

The level of glucose in the blood is kept fairly constant (about 90 mg per 100 cm³) by two hormones from the islets of Langerhans in the pancreas, working with the liver.

• When blood sugar rises (for example after a meal), the pancreas secretes insulin. Insulin causes the liver and muscles to convert the excess glucose into glycogen for storage, and increases the oxidation of glucose by cells. This lowers the blood sugar back to normal.
• When blood sugar falls (for example during fasting or exercise), the pancreas secretes glucagon. Glucagon causes the liver to convert stored glycogen back into glucose which is released into the blood, raising the level to normal. (The hormone adrenaline also raises blood sugar in the same way.)