Introduction

Photosynthesis is a metabolic process which makes stuff using light. How? How can you make anything from light? And why? Living things are made of complex organic molecules such as carbohydrates and proteins, as opposed to simple inorganic molecules such as carbon dioxide and water.

The vast majority of plants on Earth today undergo photosynthesis via a specific route (C3) which is slightly different to two other potential routes (C4 and CAM). The general balanced reaction for photosynthesis is:

H2O + CO2 + energy –> C6H12O6 + O2

…where water, carbon dioxide and energy are the starting materials, and glucose and oxygen the products. Here, glucose is the key product because it is the complex organic molecule made from simple inorganic reactants. The “energy”, as you may have noticed, is where the light comes in.

Photosynthesis is the process by which most plants as well as other organisms e.g. photosynthetic bacteria obtain their energy (glucose) ultimately in the form of ATP upon respiration. So photosynthesis produces the glucose, and the glucose is the substrate for respiration which produces ATP.

All living things undergo respiration to produce ATP from substrates including glucose, but only some (notably plants) undergo photosynthesis to produce the glucose themselves.

So where do other organisms get their respiration substrates – “food” – from? Well, most do directly from the plants by eating them, indirectly from other organisms who ate the plants (herbivores) or even more indirectly from carnivores. Fungi, for example, do neither – they simply digest any organic compounds from their environment, the soil.

That is why plants are considered autotrophs (they make their own “food” via photosynthesis), while humans amongst others are considered heterotrophs (they must obtain their “food” indirectly from other organisms which photosynthesise)…