What is Photosynthesis? 

All living organisms ultimately depend on energy derived from the sun. Photosynthesis is a biological process in plants and other organisms such as cyanobacteria. This process involves the conversion of solar energy into chemical energy. The chemical energy is utilised in cellular respiration to generate ATP which is used for metabolic activities. This process is anabolic and endergonic, synthesising carbohydrates with the help of carbon dioxide, water, sunlight and light-harvesting pigments. This process involves the transfer of energy in solar radiation and through metabolic pathways converted into utilisable chemical energy.

Photosynthesis’s significance is as follows: Globally, 90% of the photosynthetic activity is carried out by algae in oceans and fresh waters.  Atmospheric oxygen is contributed mainly by this process, food for living organisms is produced directly or indirectly by this process. Photosynthesis also provides vast energy reserves to humankind in the form of fuel-rich substances- coal, oil, peat, wood, and dung.

Diagram of Photosynthesis Process

Light reaction

Dark reaction

Photosynthesis occurs in two phases:

• Light reaction: It is a photochemical phase that takes place in the light. This process occurs in the grana of chloroplasts, i.e. in the thylakoid membranes. These membranes contain some integral membrane proteins which catalyse the light-dependent reactions. During this process, NAD+ is reduced to NADPH, ATP is generated, and water molecules are split into hydrogen and oxygen. In this process, oxygen is liberated through stomatal pores.
• Dark reaction: A biochemical phase occurs in the presence or absence of light. This metabolic pathway occurs in the stroma of the chloroplast. During this process, products of the light reactions, i.e. ATP and NADPH, are utilised in the fixation of carbon dioxide molecules to a glucose molecule. 

In a typical photosynthesis process, 18 molecules of ATPs are required to synthesise a glucose molecule from 6 molecules of carbon dioxide.

What are the requirements of Photosynthesis?

The mechanism of photosynthesis occurs in two phases: Light reaction and Dark reaction.

Requirements for Light reaction: 

• Light: The optimum wavelengths in the visible light region are – the blue range (425 – 450nm) and the red range (600 – 700nm).
• Photosystem I and II: These are protein complexes composed of a reaction centre and light-harvesting complex both are involved in trapping light and generating electrons.
• The water-splitting protein complex is associated with the splitting of water molecules.
• Set of integral protein complexes- Photosystem I and II, cytochrome b6f complex and ATP synthase, which are involved in generating ATP and NADPH

The entire process of light reaction occurs in the thylakoid membrane of chloroplasts.

Requirements for Dark reaction: 

• The key enzyme involved in the dark reaction is RuBisCO (Ribulose 1-5 bisphosphate carboxylase oxygenase). Other enzymes are also required for phosphorylation, reduction, isomerisation and transfer of functional groups.

What are the factors affecting Photosynthesis?

Factors which are mainly affecting the rate of photosynthesis are as follows.

• Light intensity: As light intensity increases, the rate of light-dependent reactions also increases. The more the photons fall on the leaf, the more chlorophyll pigments are ionised, and more ATP and NADPH are generated. As the light intensity increases, the rate of photosynthesis remains constant, which is due to limitations by some other factors.
• Carbon dioxide: The concentration of carbon dioxide is directly proportional to the rate of photosynthesis. However, the rate of photosynthesis remains constant when the rate of carbon dioxide fixation is reached maximum levels.
• The light-independent reaction (Dark reaction) is dependent on the temperature. When the photosynthetic enzymes are at optimum temperature, the overall rate of photosynthesis also increases. When the temperature rises above the optimum values, the photosynthesis rate decreases due to enzymes’ denaturation.

The rate of photosynthesis is also regulated by limiting factors. According to the law of limiting factors, the rate of photosynthesis will be limited by the factor in the shortest supply (limiting factor). For example,  due to variations of light intensity in the day, light is the limiting factor of photosynthesis, even though there is a higher concentration of carbon dioxide.  

Why is light essential for Photosynthesis?

Photosynthesis is the process in which solar energy is converted into chemical energy. Using the energy of photons, energy-rich molecules, i.e. carbohydrates, are synthesised from carbon dioxide and water. The photons in the sunlight are absorbed by chlorophyll pigments which results in the excitation of electrons in the photosystem I and II, these excited electrons are passed into an electron transport chain which subsequently results in the generation of ATPs and NADPH. For photosynthesis to occur, the light should be composed of optimum wavelengths, i.e. visible light region – blue range (425 – 450nm) and red range (600 – 700nm).

The importance of light in photosynthesis is experimentally proven with the help of a destarched plant. In this experiment, the leaf was covered with a black strip, and the leaf part did not have access to sunlight. Photosynthesis did not occur in the leaf part, which is further confirmed by the iodine test. 

Light is Essential for Photosynthesis Experiment

Experiment Title – Light is essential for photosynthesis

Experiment Description – Photosynthesis is the process whereby plants synthesise carbohydrates using CO₂, water, sunlight and chlorophyll. Learn how light is an essential requirement for photosynthesis.

Aim of Experiment – To show that light is essential for photosynthesis

Material Required – Destarched potted plant, thick black paper strips, paper clips, alcohol, iodine Solution, beaker (250 ml), burner, tripod stand, wire gauze, boiling tube, forceps and Petri dish.

Procedure – 

• Take a destarched plant. Using strips of thick black paper and paper clips, cover a part of an intact leaf of the plant. You may cover several leaves of the same plant with black paper strips and clips. Place the set-up in bright sunlight for about two hours.
• Take about 150 ml of water in a beaker and boil it.
• Pluck the experimental leaves from the potted plant and remove the black paper strips from them. Keep these leaves in boiling water for some time till the leaves become soft. Stop heating the water. Remove the beaker from the tripod stand. Allow it to cool to about 60°C.
• Transfer the leaves to a boiling tube containing alcohol.
• Place the boiling tube (containing experimental leaves in alcohol) in the beaker containing hot water at about 60°C. Keep the boiling tube in the beaker till the leaves become colourless.
• Take some iodine solution in a petri dish.
• Wash the leaves in water and dip them in iodine solution in the petri dish.
• After about five minutes, remove the leaves from the iodine solution, wash them with water and observe the colours of the exposed part and unexposed (covered with black paper) part of the leaves.

Precautions – 

• Use a healthy potted plant.
• Since alcohol is highly inflammable, it should not be heated directly on a flame.
• The plant needs to be completely destarched to obtain satisfactory results.
• Be careful while clipping the leaves to avoid tearing or breaking.

FAQs on Light is Essential for Photosynthesis Experiment