THE 3 STAGES OF PHOTOSYNTHESIS AND THEIR CHARACTERISTICS - BIOLOGY - 2025

The stages of photosynthesis can be divided based on the amount of sunlight the plant receives. Photosynthesis is the process by which plants and algae feed. This process consists of the transformation of light into energy, necessary for survival.

Unlike humans who need external agents such as animals or plants to survive, plants can create their own food through photosynthesis. This is known as autotrophic nutrition.

The word photosynthesis is made up of two words: photo and synthesis. Photo means light and synthesis mix. Therefore, this process is literally turning light into food. Organisms that are capable of synthesizing substances to create food, as well as plants, algae, and some bacteria, are referred to as autotrophs.

Photosynthesis requires light, carbon dioxide, and water to perform. Carbon dioxide from the air enters the leaves of the plant through pores found in them. On the other hand, the water is absorbed by the roots and moves until it reaches the leaves and the light is absorbed by the pigments of the leaves.

During these phases, the elements of photosynthesis, water and carbon dioxide, enter the plant and the products of photosynthesis, oxygen and sugar, leave the plant.

Phases / stages of photosynthesis

First, the energy of light is absorbed by proteins found in chlorophyll. Chlorophyll is a pigment that is present in the tissues of green plants; photosynthesis usually occurs in leaves, specifically in the tissue called mesophyll.

Each cell of the mesophilic tissue contains organisms called chloroplasts. These organisms are designed to carry out photosynthesis. Structures called thylakoids, which contain chlorophyll, are grouped in each chloroplast.

This pigment absorbs light, therefore it is mainly responsible for the first interaction between the plant and light.

In the leaves there are small pores called stomata. They are responsible for allowing carbon dioxide to spread within the mesophilic tissue and for oxygen to escape into the atmosphere. Thus, photosynthesis occurs in two stages: the light phase and the dark phase.

- Light phase

Light phase and dark phase. Maulucioni, from Wikimedia Commons

These reactions only happen when light is present and occur in the thylakoid membrane of chloroplasts. In this phase, the energy that comes from sunlight is transformed into chemical energy. This energy will be used like gasoline to be able to assemble the glucose molecules.

The transformation to chemical energy happens through two chemical compounds: ATP, or molecule that stores energy, and NADPH, which carries reduced electrons. It is during this process that the water molecules are converted into the oxygen that we find in the environment.

Solar energy is converted into chemical energy in a complex of proteins called the photosystem. There are two photosystems, both found within the chloroplast. Each photosystem has multiple proteins that contain a mixture of molecules and pigments such as chlorophyll and carotenoids so that the absorption of sunlight is possible.

In turn, the pigments of the photosystems act as a vehicle to channel energy, as they move it to the reaction centers. When light attracts a pigment, it transfers energy to a nearby pigment. This nearby pigment can also transmit that energy to some other nearby pigment and the process is repeated successively.

These light phases begin in photosystem II. Here, light energy is used to divide the water.

This process releases electrons, hydrogen and oxygen. Electrons charged with energy are transported to photosystem I, where ATP is released. In oxygenic photosynthesis the first donor electron is water and the oxygen created will be the waste. Several donor electrons are used in anoxygenic photosynthesis.

In the light phase, light energy is captured and stored temporarily in the chemical molecules of ATP and NADPH. ATP will be broken down to release energy and NADPH will donate its electrons to convert carbon dioxide molecules into sugars.

- Dark phase

In the dark phase, carbon dioxide from the atmosphere is captured to be modified when hydrogen is added to the reaction.

Thus, this mixture will form carbohydrates that will be used by the plant as food. It is called dark phase because light is not directly necessary for it to take place. But despite the fact that light is not necessary for these reactions to take place, this process requires ATP and NADPH that are created in the light phase.

This phase occurs in the stroma of chloroplasts. Carbon dioxide enters the interior of the leaves through the stromata of the chloroplast. Carbon atoms are used to build sugars. This process is carried out thanks to the ATP and NADPH formed in the previous reaction.

Dark phase reactions

First, a carbon dioxide molecule is combined with a carbon receptor molecule called RuBP, resulting in an unstable 6-carbon compound.

Immediately this compound is divided into two carbon molecules that receive energy from ATP and produce two molecules called BPGA.

Then one electron from NADPH is combined with each of the BPGA molecules to form two G3P ​​molecules.

These G3P molecules will be used to create glucose. Some G3P molecules will also be used to replenish and restore RuBP, necessary for the cycle to continue.

Importance of photosynthesis

Photosynthesis is important because it produces food for plants and oxygen. Without photosynthesis, it would not be possible to consume many fruits and vegetables necessary for the human diet. Also, many animals consumed by humans could not survive without feeding on plants.

On the other hand, the oxygen produced by plants is necessary for all life on Earth, including humans, to survive. Photosynthesis is also responsible for keeping the levels of oxygen and carbon dioxide in the atmosphere stable. Without photosynthesis, life on Earth would not be possible.

References

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