Q.How are photosynthesis and respiration related to each other?

Photosynthesis and respiration are closely linked, complementary processes built around opposite chemical transformations.

• Photosynthesis is the process by which green plants use light energy to synthesise organic compounds (carbohydrates) from carbon dioxide and water, releasing oxygen as a by-product; its overall equation is 6CO2 + 12H2O, in light, giving C6H12O6 + 6H2O + 6O2.
• Photosynthesis is the primary source of all food on earth, and it is also the source of the oxygen that respiration in most living organisms depends upon. …

Green plants are autotrophs precisely because photosynthesis allows them to synthesise their own organic food from simple raw materials — carbon dioxide and water — using light energy trapped by chlorophyll. The overall reaction, as established through the work of van Niel and others, is 6CO2 + 12H2O, in the presence of light, yielding C6H12O6 (glucose) + 6H2O + 6O2. Two of the major outcomes of this process are, first, that it is the primary source of essentially all food on earth, and second, that it releases oxygen into the atmosphere as a by-product of the splitting of water.

Respiration is, in essence, the reverse of this transformation. Where photosynthesis builds glucose from CO2 and water using light energy and releases oxygen, respiration breaks glucose back down, consuming the oxygen that photosynthesis released, and in the process regenerates carbon dioxide and water while releasing usable chemical energy for the organism's activities. …

Method 1: Step-by-step

1. Write the overall photosynthesis equation: 6CO2 + 12H2O, in the presence of light, gives C6H12O6 + 6H2O + 6O2.
2. Note its two key outputs: organic food (glucose) and oxygen.
3. Recall respiration breaks down organic food using O2, releasing CO2 and H2O plus usable energy — essentially the reverse transformation. …