Light Reactions and Photosystems: Understanding the Photosynthesis Process

Photosynthesis is the process in which plants are able to transform energy from the sun into chemical energy in the form of carbohydrates. In flowering plants, it is the green portions that actually carry out the photosynthesis process. What actually happens is that carbon dioxide enters the leaves, while roots absorb water from the ground. The carbon dioxide and water then diffuse into the chloroplasts.

There are two sets of reactions that occur during the photosynthesis process. The first type of reaction is a light reaction. These reactions occur in the thylakoid membrane. What happens is that chlorophyll takes in the solar energy and energizes the electrons, this in turn, produces ATP. The other type of reaction is the Calvin cycle. These reactions occur in the stroma. During the reaction, carbon dioxide is absorbed and ATP and NADPH are used to reduce carbon dioxide into a carbohydrate.

Light reactions are an essential part of the photosynthesis process. During the process pigments of the plant are used to absorb energy from the sun. Solar energy is categorized based on its wavelength and its energy content. Shorter wavelengths contain more energy, while longer wavelengths contain more energy. Cells that are responsible for this photosynthesis process have both chlorophyll and carotenoids. Chlorophyll is best at absorbing blue, violet, and red wavelengths from the solar energy. However, they tend to reflect green wavelengths which is why leaves often appear green. Carotenoids usually absorb light that is violet and green, but they reflect yellow and orange light.

Electrons also play a very important role in the process of photosynthesis. Their two photo systems that are used. In photosystem one, it is the absorption of energy from the sun that energizes electrons. In this system, electrons are taken in by another electron acceptor molecule. In photosystem two, the absorption of energy from the Sun also energizes electrons, however electrons escape to an electron acceptor molecule. The electron transport chain is what helps these electrons move. It is a series of carriers that move will electrons along giving off energy. Additionally, it is used in the production of ATP. This is a very complex series of events that involve the thylakoid and hydrogen ions. When the hydrogen ions pass through the ATP synthesis complex they release energy, which in turn generates ATP. These hydrogen ions can generate a large amount of potential energy is perfect for the creation of ATP.

Source:

Benitz, Bill. Lecture: Biology. September 7, 2009