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.
The energy stored in big molecules (such as carbohydrates) created via photosynthesis is derived in part through the light energy in photons. In order to tap into this energy, light must be absorbed by plants and other photosynthetic organisms.
As you know, visible light ranges in wavelength with colour:
Between 400-700 nm, light passes through several colours from violet to red. Pigments absorb some wavelengths more than others, just like anything else we see as coloured. For example, something appears yellow if it absorbs other colours like blue (500 nm) and red (700 nm) but reflects yellow (600 nm).
The two main classes of pigments in photosynthesis are chlorophyll of which there are multiple types (a, b, c, etc.) and carotenoids of which there are also multiple. The former are, surprise! green, while the latter are yellow, orange or red.
Their absorption spectra are different. Chlorophyll b for example, absorbs blue light excellently, as well as some orange light. Carotenoids only absorb blue light, with some towards the violet end of the spectrum as well as towards the green wavelengths.
Plants can make use of these multiple pigments to maximise their light absorption potential. Together, these pigments offer a range of 400-530 nm and 650-700 nm which is a total of 180 nm accessible wavelength values, out of 300 nm of visible light. That’s 60% of wavelengths. These are available as light for photosynthesis.
This information was discovered by looking at the action spectra of pigments, together with their absorption spectra. A tight correlation was found.
The photosynthetic rate (bold curve) covers all areas of wavelength corresponding to the absorption of three different pigments (faint curves). This confirms that the light absorbing property of the pigments is linked to the ability to carry out photosynthesis.
If a plant were to only have chlorophyll b (faint dark green curve), imagine what the action spectrum would look like! It would follow the chlorophyll b curve and overall provide a much narrower range of ability to photosynthesis compared with all three together. This is why having a variety of pigments is crucial.
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.