Photosynthesis

Human beings consume foods like cereals, vegetables, pulses, fruits, meat, nuts, seeds, etc. to gain energy for their survival. Have you ever wondered how plants survive, what do they eat and from where they get their food?

What is Photosynthesis?

Plants do need food for their survival and they would not wait or depend on others to provide them with food. There is a particular mechanism in plants with which they prepare their food. This mechanism is known as Photosynthesis.

Photosynthesis
Diagram of photosynthesis

Photosynthesis is a biological process used by plants to prepare food with the help of sunlight water and carbon dioxide. The name photosynthesis is derived from the Greek word ”Photo” meaning light and “Synthesis” meaning connecting together. This means combining together with the help of light energy.

This process is also used by algae and several bacteria to convert solar energy into chemical energy. Oxygen is liberated as a by-product, and light is considered as the major factor to complete the process of photosynthesis. This process occurs usually when plants use light energy to convert carbon dioxide and water into glucose and oxygen gas. Leaves are made up of small cells which have a tiny structure known as chloroplasts. Each chloroplast contains a green colored pigment  called chlorophyll. Light energy is absorbed by chlorophyll molecules whereas carbon dioxide (CO2) and oxygen (O2) enter through the tiny pores of stomata located in the epidermis of leaves.

Oxygen is considered one of the most important by-products of this process on which most of the living organism depend upon. Glucose/Sugar is a form of carbohydrates that is processed during the process of photosynthesis. It is commonly used by green plants in the form of an energy source to produce leaves, fruits, flowers, and seeds. The glucose molecules then combine with each other to develop more complex carbohydrates like cellulose and starch. The cellulose is considered as the structural material that is used in plant cell walls. The overall reaction of photosynthesis process is:

        6CO2    +  6H2O  —-> C6H12O6  + 6O2

Where does Photosynthesis occur?

Photosynthesis occurs mainly in leaves of specialized cell structures known as chloroplasts.

  • A leaf comprises a petiole, epidermis and a lamina.
  • The Lamina is used for absorption of sunlight and carbon dioxide during the process.
  • This process that occurs, takes place in chloroplasts that contain a green colored pigment called chlorophyll which is mainly responsible for green colored leaves.
  • During the process of photosynthesis, chlorophyll absorbs the light energy from the sun to split water molecules into hydrogen and oxygen.
  • The hydrogen from water molecules and carbon dioxide absorbed from the air are used in the production of glucose and the oxygen is liberated into the atmosphere through the leaves.

Glucose is a source of food for plants which provide energy for the growth and development, while the rest is stored in the roots, leaves, or fruits for their later use.

Pigments are other fundamental cellular components of photosynthesis. They are the molecules which impart color and they absorb light at some specific wavelength and reflect back the unabsorbed light. All green plants mainly contain chlorophyll a, chlorophyll b, and carotenoids which are present in the thylakoids of chloroplasts and help them in capturing the light energy. Chlorophyll a is the main pigment.

Structure of chlorophyll

Structure of chlorophyll

Stages of Photosynthesis

Stages of Photosynthesis

Photosynthesis comprises two phases.

Light-dependent reaction (or) Light reaction.    

Light Reaction

Photosynthesis begins with the light reaction which is carried out only during the day in the presence of sunlight. In plants, the light-dependent reaction takes place in the thylakoid membranes of chloroplasts. The Grana, membrane-bound sacs like structures present inside the thylakoids functions by gathering light and is called as photosystems. These photosystems have large complexes of pigment and proteins molecules present within the plant cells which plays the primary role during the process of light reactions.  There are two types of photosystems: photosystem I and photosystem II.

Under the light-dependent reactions, the light energy is converted to ATP and NADPH which are used in the second phase of photosynthesis. During the light reactions, ATP and NADPH are generated by two electron transport chains, water is used and oxygen is produced.

The chemical equation in the light reaction can be reduced to:

2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP

Light-independent reaction (or) Dark reaction.

Calvin cycle

Dark reaction is also called carbon-fixing reaction. It is a light-independent process in which sugar molecules are formed from the water and carbon dioxide molecules. The dark reaction occurs in the stroma of the chloroplast where they utilize the NADPH and ATP products of the light reaction. Plants capture the carbon dioxide from the atmosphere through stomata and proceed to the Calvin cycle. In the Calvin cycle, the ATP and NADPH formed during light reaction drive the reaction and convert 6 molecules of carbon dioxide into one sugar molecule or glucose.

The chemical equation for the dark reaction can be reduced to:

3CO2 + 6 NADPH + 5H2O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate

Importance of Photosynthesis

  • It is essential for the existence of life on planet earth both as a source of food, shelter, and energy.
  • This process is responsible for supplying all of the earth’s organic compounds and the energy required to sustain life on the planet.
  • It is also responsible for maintaining and balancing the atmosphere’s oxygen and carbon dioxide levels and protects our planet from global warming

Factors affecting Photosynthesis

There are several factors that are required at an optimum level for a constant rate of photosynthesis.

  • Light Intensity: Increased light intensity results in the high rate of photosynthesis and low light intensity would be considered as a low rate of photosynthesis.
  • The concentration of CO2: Higher concentration of carbon dioxide helps in increasing the rate of photosynthesis. Usually,0.03 to 0.04 percent concentration of carbon dioxide is adequately for photosynthesis.
  • Temperature: For an efficient execution of the process, it is important to have an optimum temperature that ranges between 25° to 35° C.
  • Water: As water is an important factor in this process, lack of water can lead to a problem of intake of carbon dioxide. The scarcity of water leads to refusal of stomatal opening to retain the amount of water they have stored inside.
  • Polluted Atmosphere: All the pollutants and gases when settles on leaves surface, it blocks the pores of stomata which makes it difficult to take in carbon dioxide.

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Further Reading


Practise This Question

In which of the following wavelengths, photosystem-I is inactive?