(a) Describe briefly the process involved in the breakdown of glucose in the cell of living organisms to produce energy (AT P) (b) State two conditions unde...

(a) Breakdown of glucose in the cell to produce energy (ATP)

The release of energy from glucose is called cellular respiration. It takes place in two main stages:

• Glycolysis (in the cytoplasm): a molecule of glucose is broken down through a series of enzyme-controlled steps into two molecules of pyruvic acid, producing a small amount of ATP. This stage does not require oxygen.
• Aerobic breakdown (in the mitochondria): in the presence of oxygen, pyruvic acid is completely broken down through the Krebs cycle and the electron transport chain into carbon dioxide and water, releasing a large amount of ATP.

The overall aerobic reaction is:

\[ C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)} \]

In the absence of oxygen (anaerobic respiration), the pyruvic acid is only partly broken down, giving lactic acid (in animals) or ethanol and carbon dioxide (in plants and yeast), with the release of only a little energy.

(b) Two conditions under which glycogen can be converted to glucose

1. When the blood glucose level falls (e.g. during hunger/fasting or strenuous exercise), the hormone glucagon (or adrenaline) stimulates the liver to convert glycogen to glucose.
2. When the body needs extra energy quickly, such as during vigorous muscular activity.

(c) Experiment to demonstrate that oxygen is released by green plants during photosynthesis

• Place some water plants (e.g. Elodea or Hydrilla) in a beaker of pond water and cover them with a short-stemmed glass funnel, mouth downwards.
• Fill a test tube completely with water and invert it over the funnel stem so that no air enters.
• Set up a second identical apparatus in the dark as a control, and place the first in bright sunlight.

Observation: in the apparatus kept in sunlight, bubbles of gas rise from the plant and collect in the test tube, gradually displacing the water. When enough gas has collected, a glowing splint inserted into it is rekindled (relights), showing that the gas is oxygen. No gas collects in the apparatus kept in the dark.

Conclusion: green plants release oxygen during photosynthesis, and only in the presence of light.