How Does Photosynthesis Use Solar Energy?

Photosynthesis is the process plants use to convert carbon dioxide into sugars which provide the energy necessary for cellular activity and the growth of the organism. It is a very complex process, and understanding how it works can give you a better idea of how life on Earth functions.

The process begins with the absorption of sunlight by the leaves of the plant. The chemical makeup of the leaves enables them to absorb more sunlight than other things, such as water or soil. This enables the plant to grow vigorously, providing more surface area for the absorption of sunlight. After the leaves absorb the sunlight, they convert it into sugar using various chlorophyll-containing proteins, also known as enzymes. This encourages other organisms in the area, such as bacteria, to feed on the available nutrients, resulting in an ‘eco-system’ similar to what is seen in nature.

How Does Photosynthesis Work?

When you observe photosynthesis in action, you will see that a plant uses two processes to convert carbon dioxide into glucose, a simple sugar. The first process is called the ‘light dependent reaction’ which happens when photons of light are available. Photosynthesis utilises light-harvesting devices called photosynthetic complexes in this step. These complexes contain several proteins which act as the ‘reaction centre’ around which the rest of the molecule revolves. They are most effective at harvesting sunlight in the range of 400nm to 750nm. The second process, called the ‘chemical reaction’ or the ‘dark reaction’, takes place regardless of whether or not light is present.

When light is absent, the plant uses the pigment chlorophyll, which is also present in the reaction centre, to capture photons from the surrounding environment. These chlorophyll-containing proteins are known as LHCPs (light-harvesting chlorophyll-proteins). Under low light conditions, this enables the plant to continue to grow.

What Are The Components Of A Plant’s Photosynthetic Complex?

The photosynthetic complexes in plants contain several proteins and cofactors which work together to process the glucose. There is usually a light-harvesting complex and a reaction centre, and many plants have a third component known as an oxygen-evolving complex. These components are listed below.

Light-Harvesting Complex

The light-harvesting complex is responsible for absorbing light and guiding it towards the reaction centre. It does this by acting as a conduit, transmitting the light energy to the rest of the photosynthetic complexes as well as providing additional cofactors which are necessary for photosynthesis. A typical plant is equipped with a myriad of light-harvesting complexes which range in size from 75 to over 300nm in diameter. This enables them to absorb a variety of light frequencies, ranging from blue to red, and in different wavelengths. The light-harvesting complex’s main function is to collect the solar energy and convert it into an electrochemical gradient which enables a plant to manufacture ATP (Adenosine Triphosphate) from the food sources available in the area. Most plants also contain smaller subunits known as LHC-I and LHC-II which help form the light-harvesting complex. In smaller species of plant, such as cyanobacteria (blue-green algae), these complexes are known as Bchls.

Reaction Centre

The reaction centre is the part of the photosynthetic complexes where the actual chemical reaction takes place. In this complex, the glucose from the light-dependent reaction is converted into pyruvic acid using a ferrous iron (Fe2+) and oxidised nicotinamide adenine dinucleotide phosphate (NADPH, also known as ‘reduced nicotinamide adenine dinucleotide phosphate’).

Within the reaction centre, a special pair of flavin molecules (FAD (flavin adenine dinucleotide) or NADP (nicotinamide adenine dinucleotide phosphate)) are associated with two hemes (ferrous iron molecules) known as the FAD heme and the NADP hemes. These molecules serve as a bridge between light and the rest of the photosynthetic complexes. During the daylight hours, the electron transport chain within the reaction centre is constantly turning over, with the NADPH molecules serving as the electron donor. This enables the plant to synthesise sugars from carbon dioxide as well as to continue to grow. The light-dependent reaction is most effective in the range of 400nm to 550nm and the chemical reaction takes place in the range of 600nm to 750nm.

Oxygen-Evolving Complex

The oxygen-evolving complex is found in some plants, such as legumes, sugar cane, and oak trees, and performs the same function as the reaction centre. It is responsible for converting glucose into oxygen as well as releasing additional hydrogen ions (H+). The oxygen released during this process serves as a respiratory gas which allows these organisms to gain energy from the food they consume and grow.

Other Components Of A Plant’s Photosynthetic Complex

Other components of a plant’s photosynthetic complex include a protective coating around the reaction centre known as the ‘peripheral light-harvesting complex’ or the ‘Pchlide coat’ and a starch sheath around the outside of the photosynthetic complexes. The peripheral light-harvesting complex is most effective at protecting the reaction centre from damage caused by high-energy radiation. This range of protection is from 100 to 300nm and is found in all higher plants. The outer coating of starch, known as the ‘amyloplast’, enables the plant to store food in its cells for later use. The overall length of a plant’s photosynthetic complexes is about 2 to 5 μm.

The length of a plant’s PSII (Photosystem II) is about 1.9μm in Cicer arietinum (chickpea) and 2.3μm in Arabidopsis thaliana (thale cress).

The distance between the centres of adjacent photosynthetic complexes is known as the ‘interconnecting length’ and it varies from 200 to over 300nm in some higher plants. This is because some species grow in highly branched fashion, resulting in a greater surface area for the capture of sunlight. When this distance is increased, it enables the plant to absorb more sunlight and produce more energy.

Now that you know a bit more about how photosynthesis works, you can better understand some of the ways in which plants adapt to their environment. For example, you will know that blue light is essential for photosynthesis and that longer wavelengths (red light) are more harmful because they cause the pigments in the plant to open up. In contrast, shorter wavelengths (green light) encourage the synthesis of new cells and tissues in the plant. This means that short-wavelength green light can be used to trigger plant growth. All of this information can be useful in devising an environment for your own plants.

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