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Photosynthesis is a process whereby light energy is converted to chemical energy and that chemical energy is stored in the bonds of carbohydrates. While this process mainly takes place in plants, it also occurs in some bacteria and algae. Because photosynthetic organisms can produce their own food, they are called autotrophs. In the process of photosynthesis in plants, algae and cyanobacteria, carbon dioxide and water are used to produce carbohydrates and relase oxygen as a by product of the process.  Photosynthesis is necessary for the existence of all aerobic life on Earth. Namely, photosynthesis helps maintain normal levels of oxygen in the atmosphere and is the source of energy for nearly all life on earth. The role of photosynthesis in energy production is both direct, through primary production, and indirect, as the ultimate source of the energy in food. Photosynthesis is performed in the chloroplasts, specifically using chlorophylls, the green pigments involved in photosynthesis.

Photosynthesis takes place in green parts of plants. A typical leaf is made up of the upper and lower epidermis, the mesophyll, the vascular bundle(s) (veins), and the stomata. Because the upper and lower epidermal cells do not contain chloroplasts, photosynthesis does not occur there. Instead, the primary function of these cells is the protection of the rest of the leaf. The stomata are pores that can primarily be found in the lower epidermis. Their function is to facilitate gas exchange, letting in CO2 and out O2. The vascular bundles or veins in a leaf form part of the plant's transportation system.  They move water and nutrients to different parts of the plant as needed. The mesophyll cells of a plant have chloroplasts and therefore, this is where photosynthesis takes place.

A chloroplast is made up of the following parts: the outer and inner membranes, intermembrane space, stroma, and thylakoids stacked ingrana (singular granum). The chlorophyll molecules are located within the thycaloid membranes of chloroplasts.

The sunlight that reaches the Earth is made up of many different wavelengths of light. A wavelength of light corresponds to its colour and therefore, the mixture of wavelengths found in sunlight also includes those that we perceive as colours. The process of photosynthesis requires visible radiation. While various colours of light can be used for photosynthesis, not all the colours of visible light are equally good at helping the photosynthesis take place.

The green colour of chlorophyll comes from the fact that the clorophyll absorbs red and blue light and reflects green light. It is the energy of the red and blue light that is captured and used in the process of photosynthesis. (See Figure 3). The green light that makes the plant appear green is not absorbed by the plant, rather it is relfeced by the pant. Therefore, it cannot be used in photosynthesis

The overall chemical reaction of photosynthesis is:

6CO2 + 6H2O (+ light energy) = C6H12O6 + 6O2

Photosynthesis, therefore, is the source of the O2 that we breathe. The latter fact is an important consideration in a debate on deforestation1.

Photosynthesis takes place in two parts. Namely, there are light reactions and light-independent reactions or dark reactions. Importantly, the light reactions capture and use light energy to produce high energy chemicals. These chemicals in turn are used to power light-independent reactions whereby carbohydrates are made.

1 Only new plantations make a contribution to atmospheric oxygen. Old forests that have reached a steady state absorb as much oxygen as they produce.

The light reaction

The light reaction (Figure 5) takes place in the thylakoid membrane. Because the light reaction converts light energy to chemical energy, it must, therefore, take place in the light. The light reaction involves chlorophylls and a number of other pigments, such as beta-carotene. They are organised in clusters and situated in the thylakoid membrane.  While clorophylls can exist in different forms, the most important ones are chlorophyll a and b shown in Figure 4.

As differently-coloured pigments are involved in the light reaction, each of these pigments can capture a slightly different colour of light.  This light energy is then passed on to the central chlorophyll molecule to power photosynthesis. The main part of a chlorophyll molecule is a porphyrin ring. This consists of several rings of carbon and nitrogen atoms joined together with a magnesium ion in the centre of this main part.

1. As photons are absorbed by pigment molecules in the antenna complexes of Photosystem II, excited electrons from the reaction centre are picked up by the primary electron acceptor of the Photosystem II electron transport chain. During this process, Photosystem II splits molecules of H2O into 1/2 O2, 2H+, and 2 electrons. These electrons continuously replace the electrons being lost by the chlorophyll A molecules in the reaction centres of the Photosystem II complexes.

H2O -> 1/2 O2 + 2H+

2. During this process, ATP is generated by the Photosystem II electron transport chain and chemiosmosis. According to the chemiosmosis theory, as the electrons are transported down the electron transport chain, some of the energy released is used to pump protons across the thylakoid membrane from the stroma of the chloroplast to the thylakoid interior space producing a proton gradient or proton motive force. As the accumulating protons in the thylakoid interior space pass back across the thylakoid membrane to the stroma through ATP synthetase complexes, this proton motive force is used to generate ATP from ADP and Pi.

3. Meanwhile, photons are also being absorbed by pigment molecules in the antenna complex of Photosystem I and excited electrons from the reaction centre are picked up by the primary electron acceptor of the Photosystem I electron transport chain. The electrons being lost by the chlorophyll a molecules in the reaction centres of Photosystem I are replaced by the electrons traveling down the Photosystem II electron transport chain. The electrons transported down the Photosystem I electron transport chain combine with 2H+ from the surrounding medium and NADP+ to produce NADPH + H+.

The light-independent (dark) reaction

The light-independent (dark) reaction (Figure 6) occurs in the stroma of the chloroplast. During the dark reaction CO2 is converted to carbohydrates. While light is not directly necessary for this reaction, it is the products of the light reaction (ATP and another chemical called NADPH) that are needed. In the light-independent reaction CO2 and energy from ATP are used to form glycose. This process is called the Calvin cycle. In fact, the first product of photosynthesis is a three-carbon compound called glyceraldehyde 3-phosphate. Almost immediately, two such compoundsjoin to form aglucose molecule. The glucose molcule can then be transported to other cells, or packaged for storage as insoluble polysaccharides (e.g. starch).

3 CO2 + 6 H+ → C3H6O3-phosphate + 3 H2O

Factors that affect photosynthesis

There are four main factors that affect the rate of photosynthesis. In addition to these these four factors, several other factors also impact the process. The main factors, however, are: (1) light intensity and (2) wavelength, (3) carbon dioxide concentration, and (4) temperature.

Light intensity

When the temperature is constant, the rate of photosynthesis varies with light intensity. Intially, as light intensity increases, the rate of photosynthesis also increases. However, at a higher level of light intensity this correlative relationship ceases and the rate of photosynthesis reaches a plateau

Wave length

Variations in the colour of light have impact the rate of photosynthesis. While the entire spectrum of light reaches the plant at the same time, there are certain colours that bring about higher amounts of photosynthesis than others. Because of chlorophyll each plant is of individual colouring. The clorophyll is created of four kinds of pigments: Chlorophyll A, Chlorophyll B, Xanthophyll, and Carotene. Some leaves contain more of certain colour pigments than of others, and as a consequence some leaves can appear yellow-green while others appear bright green, blue-green or even orange or red. With regard to photosynthesis, however, such pigmentation does not matter As was exhibited in Figure 3, the colours that influence photosynthesis most are blue and red. Whereas, yellow light is the least helpful in the process of photosynthesis. It is important to bear in mind that, when testing the rate of photosynthesis in the case of various colours, the leaf used in such an experiment should not be exposed to natural light. It is important to use an absolutely dark room andany light that is not a part of the experiment itself should be carefully screened from the experimentation area. White light, also a part of the spectrum of light, should be used as a control element in the experiment.

Temperature

The rate of photosynthesis increases as the temperature is increased over a limited range. However, this relationship holds true only at high irradiance. At low intensity, increasing the temperature has little influence on the rate of photosynthesis.

Carbon dioxide concentration

The level of carbon dioxide in the atmosphere is relatively constant. Therefore, CO2 concentration does not limit the rate of photosynthesis. Nevertheless, higher concentrations of CO2 increase the rate of photosynthesis (the rate at which sugars are made by the light-independent reactions). Thus, in industrial greenhouses, this factor is used to encourage photosynthesis.