Respiration in Plants

Plants also respire aerobically to obtain energy for metabolism. They derive most energy from cellular respiration.

During cellular respiration, energy is obtained by breaking down glucose. The energy released is stored in ATP molecules.

Although plants cannot photosynthesise without sunlight, respiration continues because plants need energy constantly to sustain vital living processes.

Gaseous exchange between plant cells and the environment occurs by diffusion mainly through the stomata and lenticels. Respiratory gases enter and leave plants via stomata in the epidermis of the leaves and the stems of herbaceous plants. Lenticels are raised pores found on the stems and roots of plants.

When stomata are open, they connect the air spaces within the leaves to the atmosphere.

Oxygen from the atmosphere diffuses into the air spaces and then dissolves in the film of water around the mesophyll cells.

Oxygen is then used in aerobic respiration.

The concentration of oxygen in the cells becomes lower than the concentration of oxygen in the air spaces.

The difference in concentration gradient allows oxygen to diffuse continuously from the air spaces into the cells.

During the day, the carbon dioxide which is produced during aerobic respiration is used in photosynthesis.

The excess carbon dioxide diffuses into the air spaces and then through the stomata into the atmosphere.

Aerobic respiration is usually carried out by all plants throughout the day and night.

However, under certain conditions, plants can carry out anaerobic respiration for short periods.

For example, in flood. 

Anaerobic respiration also occurs during initial stages of seed germination.

Respiration and photosynthesis are interdependent. Photosynthesis produces the raw materials required by respiration and respiration produces the raw materials required by photosynthesis.

As light intensity increases during the day, the rate of photosynthesis also increases. Eventually a point reached at which all carbon dioxide produced during respiration is used in photosynthesis. At this point, there is no net gain or net loss in the sugar produced. The plant has reached a compensation point.

The compensation point is the light intensity at which the rate of carbon dioxide production during respiration is equal to the carbon dioxide consumption during photosynthesis.

As light intensity continues to increase during the day, the rate of photosynthesis exceeds the rate of respiration. The carbon dioxide produced during respiration is no longer sufficient fro the plants. Plants must take in carbon dioxide from the atmosphere to supplement the need for a higher concentration of carbon dioxide during photosynthesis. At the same time plants release the excess oxygen into the atmosphere.