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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).
Back to photosynthesis itself now! We know that photosynthesis requires light, however the twist is that the process is split into two: the light-independent and light-dependent reactions. So some parts of photosynthesis don’t actually require light. The very first stages of photosynthesis are the ones which require light, and once those have been accomplished, the subsequent reactions may proceed regardless.
Overview of the Light-dependent and Light-independent Reactions
The LD reactions take place on the thylakoid membranes within chloroplasts, whereas the LI reactions take place in the surrounding space called the stroma.
The LD reactions produce protons, electrons and oxygen, while the LI reactions produce triose phosphate which ultimately is converted to glucose and other organic molecules. So the overall purpose of the LD reactions is to convert light energy into chemical energy, while the overall purpose of the LI reactions is to convert the LD products into useful molecules like glucose.