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Basics of photosynthesis

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Photosynthesis overview

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Structure of chloroplast

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Photosynthesis in higher plants

“Photosynthesis is a process used by plants in which energy from sunlight is used to convert carbon dioxide and water into molecules needed for growth. These molecules include sugars, enzymes and chlorophyll. Light energy is absorbed by the green chemical chlorophyll.”


All animals and human beings are dependent on plants for food and these plants synthesize the food via physio – chemical process called Photosynthesis. This process is important because:

It is the primary source of food.

It results in the release of oxygen in atmosphere.

Early experiment for Photosynthesis
In the year 1770, Joseph Priestly performed several experiments that revealed the role of air in growth of green plants. Following figure shows the experiment carried out by Priestly:

Priestly observed that the candle burning in closed space, i.e. a bell jar, extinguishes after some time. As shown in figure (a) and (b) mouse is also fainted after some time. This concluded that both candle and mouse require air, but somehow damaged it. But when the mint plant was placed in the jar, (c) and (d) the candle was burning after some time and mouse also stayed alive. After this experiment, Priestly hypothesized that:
“Plants restore to the air whatever breathing animals and burning candles remove.”
Using the similar set up used by Priestly, Jan Ingenhousz (1730 – 1799) carried out an experiment to show the importance of sunlight to plants, which somehow purifies the air fouled by breathing animals or burning candles. He took aquatic plants into observation and showed that in the presence of bright sunlight, small bubbles were formed around the green parts, while there were not bubbles during night. Later, he concluded that the bubbles were of oxygen and only the green parts are able to release oxygen.
In the year 1854, Julius Von Sachs provided evidence of the production of glucose during the growth of plants. This glucose is stored as starch and later, he concluded that the green substances are located in special bodies within plant cells. He also concluded that glucose is made in the green part of the plant and is stored as Starch.
T.W. Engelmann (1843 – 1909) also carried out an interesting experiment. He used prism and split the light in several components and illuminated green algae, called Cladophora, placed in the suspension of aerobic bacteria. These bacteria helped in detecting the site of evolution of oxygen. During the experiment, he observed that the bacteria accumulated mainly in the region of red and blue light of the split spectrum. This was the first time when photosynthesis was described and it resembled the absorption of spectra of chlorophyll.
By the middle 19 th century, the were features about the photosynthesis were known and following empirical equation was introduced that represented the entire process of photosynthesis:

Then, Cornelius van Niel (1897 – 1985) added that photosynthesis is a light dependent reaction in which hydrogen form an oxidisable compound and reduces carbon dioxide to carbohydrates. The entire reaction is represented as follows:

He also concluded that O 2 evolved from green plants comes from H ₂ O and not from CO ₂. This was later proved via radio isotopic techniques. The correct equation that represented the entire photosynthesis process is:

Where, C ₆ H ₁₂ O ₆ is glucose and O 2 is released from water.
Where Photosynthesis does takes place?
Photosynthesis includes series of chemical reactions which are carried out in chloroplast,i.e.The specialized structures found on cells of plants. In these series of reaction, water and carbon dioxide are converted into glucose and in this reaction energy from sunlight is used. Because, it is an endothermic reaction, the entire process requires input of energy. Photosynthesis is also classified as oxidation – reduction reaction as it includes loss of electrons by water and gain of electrons by carbon dioxide.
The process of photosynthesis takes place in Mesophyll Cells and the carbon dioxide required by the process enters the process via stomata, i.e. the small holes present on the outer layer of leaves. The water required for the process is transported via roots through the vascular tissues.
The chloroplast contains membranous system (shown in image below) consisting of the stroma lamellae, grana and the fluid stroma. The membrane system traps the light energy and helps in synthesizing ATP and NADPH. Following diagram shows the electron micrograph of a section of chloroplast:

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Biochemistry of photosynthesis