Light Dependent Reactions: Definition, Examples, Diagram, Process, Explanation

What Are Light-Dependent Reactions?

Light-dependent reactions are the first step of photosynthesis, capturing light energy and then transforming it into chemical energy in the shape of ATP and NADPH. Light-dependent reactions take place in the thylakoid membranes of the chloroplast.

Chloroplasts are plant cell and algae organelles specialized to perform the process of photosynthesis. An embedded thylakoid is stacked into grana, and around it—the stroma—which is the fluid surrounding the thylakoids.

The main constituents involved in light-dependent reactions are found in thylakoid membranes: photosystems, electron transport chains, and the enzyme ATP synthase. These membranes create a structure where chemical processes can be run efficiently.

Components Involved In Light-Dependent Reactions

The components of light-dependent reactions are:

Role Of Chlorophyll

The role of chlorophyll in the light dependent reaction includes:

• Chlorophyll is the main pigment in photosystems, and it absorbs light energy, especially in the blue and red wavelengths.

• It can excite electrons and hence transfer the light energy into chemical energy.

• Other auxiliary pigments extend the range of absorption of light and hence protect chlorophyll from damage while enhancing the efficiency of photosynthesis.

Steps Of Light-Dependent Reactions

Following are the steps of the light-dependent reactions:

Light Absorption

• A photon is absorbed by chlorophyll.

• Excitement of electrons in the photosystems.

• This electron is then transferred to primary electron acceptors.

Photolysis of Water

• The light energy of PSII causes water molecules to split, producing oxygen and protons with electrons.

• The reaction is: 2H₂O → 4H⁺ + 4e⁻ + O₂

Electron Transport Chain (ETC)

• The energy released from high-energy electron transport across the ETC pumps protons into the thylakoid lumen, developing a proton gradient.

ATP Formation (Photophosphorylation)

• Protons driven by the gradient drive the ATP synthase to convert ADP and inorganic phosphate into ATP.

Formation of NADPH

• Electrons arrive at PSI, get re-energised.

• Electrons reduce NADP+ into NADPH. Another essential energy carrier.

Products Of Light-Dependent Reactions

The products of light dependent reactions and their formation site in included in the table below:

Factors Affecting Light-Dependent Reactions

The factors affecting light-dependent reactions are:

Light Intensity

• Higher intensity elevates the rate of light-dependent reactions.

• Too much light may result in photoinhibition.

Wavelength Of Light

• Wavelengths of red and blue light are the most effective.

• Greenlight has the lowest effect due to its reflection by chlorophyll.

Water Availability

• It is essential for photolysis.

• Limited water can slow down or stop the reactions.

Temperature

• Optimal temperatures enhance enzyme activities.

• Extreme temperatures can denature enzymes.

Importance Of Light-Dependent Reactions

The importance of light dependent reactions is:

• The reactions help in the generation of ATP and NADPH which are crucial for the Calvin cycle.

• The reactions release oxygen as a product which is vital for aerobic respiration.

• It maintains the energy flow and carbon cycle balance in the ecosystems.

Light-Dependent Reactions NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Stages of Light dependent reactions

• Products formed in Light dependent reactions

• Factors affecting Light dependent reactions

Practice Questions for NEET

Q1. Which of the following is correct for photosynthesis?

1. Biological oxidation process

2. Photochemical catabolic process

3. Photo-oxidative metabolism

4. Biological photo metabolism

Correct answer: 4) Biological photo metabolism

Explanation:

Photosynthesis is the vital biological process through which green plants, algae, and certain bacteria transduce light energy to chemical energy, primarily in chloroplasts. This conversion yields glucose.

The fundamental equation of photosynthesis is:

6 CO₂ + 6 H₂O + light energy ↦ GLUCOSE + 6 O₂

Key components of this reaction are:

1. Carbon dioxide (CO₂) - Obtained from the atmosphere via stomata.
2. Water (H₂O) - Absorbed from the soil by roots.
3. Light energy - Captured by chlorophyll pigments in leaves.
4. Glucose - Utilized for energy and cellular expansion.
5. Oxygen (O₂) - Released into the atmosphere as a byproduct.

This process is essential for life on Earth, as it generates oxygen and organic compounds necessary for the survival of most organisms.

Hence, the correct answer is Option (4) Biological photo metabolism.

Q2. Oxygen is not produced during photosynthesis by

1. Cycas

2. Nostoc

3. Green sulphur bacteria

4. Chara

Correct answer: 3) Green sulphur bacteria

Explanation:

Photosynthesis -

Enzyme-regulated anabolic process of manufacture of organic compounds inside the chlorophyll-containing cells.

Since Charan Cycas come under the plant kingdom, they perform oxygenic photosynthesis. Green sulfur bacteria utilize H2S instead of H2O and perform anoxygenic photosynthesis or do not evolve oxygen.

Hence, the correct answer is Option (3) Green sulfur bacteria.

Q3. Greatest proportion of photosynthesis in the world is carried out by:

1. trees in the rain forests of the world

2. trees in the temperate forests of the world

3. algae in oceans

4. irrigated crop fields

Correct answer: 3) Algae in oceans

Explanation:

Algae are the largest oxygen-producers in the world through photosynthesis, in the oceans, and these organisms are very significant in the production of oxygen. Estimates have suggested that marine plants, particularly unicellular photosynthetic algae, produce about 70% to 80% of the oxygen produced in the atmosphere. These kinds of organisms, such as Prochlorococcus, play an important role in the aquatic ecosystem and, because of photosynthesis, play a crucial role in affecting the balance of oxygen in the environment.

Hence, the correct answer is Option (3) algae in oceans.

Also Read:

• MCQs on Photosynthesis

• Photosynthesis in Higher Plants

• Difference between Photosystem 1 and 2