All living organisms require energy to live. That energy is usually stored in the form of chemical energy in complex carbon compounds that organisms get by consuming food. But some organisms have the ability to convert light energy into chemical energy.

Photosynthesis is a process in which organisms convert light energy from the Sun into chemical energy. It is carried out by plants, algae and some bacteria.

Plants convert water and carbon dioxide into carbohydrates and oxygen by the process of photosynthesis. In this way they provide energy for the rest of the ecosystem they are part of.

\(\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}\)

Water from the soil is necessary because it contains hydrogen that is needed for the reduction reaction in which carbon dioxide is converted into glucose. The hydrogen needed for making the glucose molecules comes from photolysis of water, a reaction in which, with the presence of light, hydrogen is released from water in the form of hydrogen ions and electrons. If we write this like a chemical equation, you can see that the extra product of this reaction is molecular oxygen that the plant diffuses out of its cells.

That excess oxygen is very important in ecosystems because other organisms then use it in the process of cellular respiration.

As you already know light is needed for photosynthesis, but not the whole spectrum of sunlight is used in this process. Pigments are chemicals that absorb light. The most important pigments for photosynthesis are called chlorophylls. These pigments, found in cells of photosynthetic organisms, absorb red and blue light waves from the sun, but reflect the green part of the spectrum. Because of that they appear green to us. In plants, there are more pigments than just chlorophyll, and there are also more types of chlorophyll that each absorb a slightly different part of the light spectrum (a different wavelength of light) and thus allow the organism to convert energy more efficiently. Chlorophyll in plants is found inside a specialized structure – the thylakoid membranes inside an organelle called chloroplast.

There are two key parts of photosynthesis: Light Dependent and Light Independent (Calvin Cycle).

• * Please note that light dependent reactions REQUIRE light input. Light independent reactions don’t necessarily have to happen in the darkness, they just DON’T require light.
• Light dependent reactions (which take place in the thylakoid membrane of chlorophyll) require light to perform photolysis (the splitting of water molecules into high-energy electrons and oxygen).
• Those high-energy electrons are passed through pigments and the Electron Transport Chain, forming ATP and NADH along the way (which are the inputs for the light independent reactions).
• The light independent reactions (AKA Calvin Cycle) take place in the stroma.
• The Calvin Cycle uses Carbon Dioxide, ATP, and NADH to form glucose.
• There are three main stages:
  1. Fixation: Carbon dioxide is incorporated into an organic molecule, RuBP, via the enzyme RuBisCO.
  2. Reduction: The six-carbon compound is converted into G3P (glyceraldehyde-3-phosphate) using ATP and NADPH.
  3. Regeneration: RuBP is regenerated from G3P to continue the cycle.

Written by Tamara Bračko