R2101 – Respiration

Respiration is the process in which the sugars the plant has got (through photosynthesis) are used to make energy to live. [We respire just as plants do – we breathe in oxygen to burn the food we have eaten to provide energy]

Respiration continues day and night, in all of the living cells of the plant. It continues after fruits are harvested or flowers are cut. It is important that the plant produces enough sugars during the day through photosynthesis to be able to keep it going through the night.

Respiration can occur in the presence of oxygen – aerobic respiration – this is favourable and the preferred way. Respiration can also occur without oxygen – anaerobic respiration – for short periods of time if there is no alternative.

The equations to learn are:

Aerobic respiration

Note that sometimes the equation is written specifying glucose as the sugar. Glucose is a carbohydrate, created through photosynthesis. The amount of energy this equation creates is 2830 kJ (not needed for the exam).

Anaerobic respiration

Anaerobic respiration only produces 118 kJ of energy, far less than aerobic respiration which is one of the reasons why it is unfavourable. The other is that it produces ethanol which can be toxic to the plant. It occurs in waterlogged or compacted soil.

Factors affecting respiration

• Oxygen – a lack of oxygen leads to anaerobic respiration which uses up the sugar stores of the plant quickly without providing much energy. The ethanol produced is also toxic to the plant.
• Temperature – the equations above rely on enzymes in the plant to carry them out. Enzymes work slowly at low temperatures, speeding up at higher temperatures until they reach an optimum temperature. After that, they get damaged and don’t work so well. The optimum is different for different plants.

Reasons to control respiration

• During propagation, it might be helpful to provide warmth to raise the respiration rate and for the plant to therefore have energy to grow.
• when storing seeds or food crops, it might be good to slow down respiration so that the food does not spoil and it stays fresh for longer. This can be achieved by chilling the produce and also by controlling the gases in the atmosphere.