Forestry/Photosynthesis

Main article: Forestry

Photosynthesis Overview

Photosynthesis is a process in which a plant (including trees) uses energy from sunlight to produce sugar then uses it to make adenosine triphosphate (ATP). Photosynthesis occurs in the chloroplasts, a feature found only in plant cells.

The process of photosynthesis is done mostly in the leaves of a plant. Water from the roots travels to the leaves through passages called xylem. Water, carbon dioxide, and sunlight work together to produce sugar and oxygen.

A diagram of a plant cell.

Electron Transport Chain (Light-Dependent Reactions)

The electron transport chain (ETC) takes place in the thylakoid membrane. It begins with light hitting a photosystem, which is essentially a bundle of light-recepting molecules. At the core is chlorophyll, which, when hit by a photon of light, loses an electron. This electron is later replaced by splitting a molecule of water into 2 protons, 2 electrons, and an oxygen molecule. Using the energy from the excited electron, specialized channel proteins pump protons across the thylakoid membrane from the outside fluid, called the stroma, into the inside of the thylakoid, which resembles a hollow discus. A gradient is then formed , where there are too many protons in the thylakoid, and they want to come out. They come out through a special protein channel, called ATP synthase. As they are entering the stroma, the protons release potential energy, which is harnessed by the synthase molecule to make ATP.

Meanwhile, the electron that started this whole cycle, after it loses energy, gets stuck to a molecule called NADP+, adding on another hydorgen molecule, to form NADPH. This, and the ATP that is formed, move into the Calvin Cycle, and the oxygen is excreted as waste.

The Calvin Cycle (Light-Independent Reactions)

This process takes place on the interior of the thylakoid. In the Calvin Cycle, the carbon dioxide molecule is combined via carbon fixation with a molecule of ribulose bisphosphate (RuBP) to form the molecule 3PG. 3PG is then altered to form the molecule G3P, also called PGAL. Since there are 6 G3P formed, one G3P is sent to other locations within the plant cell to be converted into glucose (which needs 2 G3P molecules). The other 5 G3P molecules are converted, with the aid of an ATP molecule, back into ribulose bisphosphate. The new RuBP molecule is then ready to fix the next carbon dioxide molecule. The enzyme used to fix carbon dioxide is Ribulose Bisphosphate Carboxylase, better known as Rubisco. Rubisco is the most abundant enzyme on the planet.

The ETC is a part of the light-dependent reaction because it cannot occur without light. The light energy allows for the excitation of electrons, which results in the electron transport chain being able to function. Similarly, the Calvin Cycle is a light-independent reaction because no light is needed to run it. However, if there is no light, the ETC would soon stop, leading to the Calvin Cycle stopping as well. Therefore, light is still needed for photosynthesis to continually occur.