Photosynthesis in Higher Plants: Overview, Topics, MCQs, Books, Tips

Introduction

Early experiments on photosynthesis, like those by Joseph Priestley and Jan Ingenhousz, proved that plants release oxygen and use sunlight for this process, which was a starting point for understanding photosynthesis. Various processes are involved in photosynthesis in higher plants, but the principle remains the same. Sunlight is used for synthesising organic compounds by this physicochemical process. Oxygen is released as a by-product in the process.

The process of photosynthesis uses light energy to synthesise organic compounds in green plants. It is an anabolic process regulated by enzymes. Life on Earth depends on photosynthesis as it is the primary source of food, and it is responsible for releasing oxygen into the atmosphere. The process of photosynthesis requires chlorophyll. It can only be found on the green parts of leaves and in the presence of light.

It can be represented by the equation: 6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂

Structure Of Chloroplast

Chloroplasts are found in the mesophyll cells of the leaves, where photosynthesis occurs. Photosynthesis involves the four biological pigments in plants:

• Chlorophyll a

• Chlorophyll b

• Xanthophylls

• Carotenoids

All these pigments participate in the process of photosynthesis. Hence, they are also called photosynthetic pigments. While chlorophyll traps the light energy for photosynthesis, xanthophylls and carotenoids act as accessory pigments.

The Process of Photosynthesis in Higher Plants

Photosynthesis in higher plants is the process by which green plants convert sunlight into chemical energy using chlorophyll, water, and carbon dioxide. It mainly takes place in the chloroplasts of leaf cells and is vital for plant growth and oxygen production. Photosynthesis in higher plants involves the following processes:

• Light Reaction

• Dark Reaction

Light Reaction (Light-Dependent Phase)

• The light-dependent reaction occurs in the presence of light.

• Light wavelengths are absorbed by pigments and ATP is produced.

• Absorption of light, splitting of water, oxygen release, and formation of ATP and NADPH are all involved in the process.

• A pigment molecule attached to a protein in the light-harvesting complex is found in both types of photosystems, where it helps capture light energy for photosynthesis.

• Antennae containing accessory pigments are attached to each photosystem's reaction centre, which consists of chlorophyll.

• It is P-700 for PS-I, as chlorophyll a has an absorption peak of 700 nm, while it is P-680 for PS-II, as chlorophyll a has an absorption peak of 680 nm.

Photophosphorylation

Photophosphorylation is the process by which light energy is used to produce ATP during photosynthesis. It occurs in the thylakoid membranes of chloroplasts.

The two types of photophosphorylation are:

• Non-cyclic Photophosphorylation

• Cyclic Photophosphorylation

The table shows the difference between cyclic and non-cyclic photophosphorylation:

Dark Reaction

When there is no light, this process takes place in the chloroplast stroma. The process involves the following cycles:

Calvin Cycle (C3 Cycle)

The steps of the Calvin cycle include:

• In a carbon-fixation process, ribulose-1, 5-bisphosphate combines with carbon dioxide to form 3-phosphoglyceric acid, a 3-carbon compound. In this process, a protein enzyme called RuBisCO is involved.

• Glyceraldehyde-3-phosphate is formed by the reduction of one molecule of carbon dioxide with two molecules of ATP and NADPH.

• A series of reactions occur to form glucose from glyceraldehyde-3-phosphate molecules, and RuBP regenerates to continue the cycle.

C4 Cycle (Hatch and Slack Pathway)

• The pathway follows a cycle, where mesophyll cells and bundle sheath cells contain enzymes involved in the C4 pathway.

• Through this pathway, plants produce a chemical compound containing four carbons from atmospheric carbon dioxide.

• Carbon dioxide is taken up by phosphoenolpyruvate located in the mesophyll cells. Phosphoenolpyruvate carboxylase is responsible for the reaction.

• In the resulting mesophyll cells, aspartic acid and malic acid are formed and transported to the bundle sheath cells. Three-carbon molecules are released, and carbon dioxide is released as a result of the breakdown of C4 acids.

• They are then converted into phosphoenolpyruvate in the mesophyll cells at the end of the cycle.

• The cycle is completed when carbon dioxide enters the bundle sheath cells.

The difference between the C3 pathway and C4 pathway is given below-

CAM Pathway (Crassulacean Acid Metabolism)

CAM is a special photosynthetic adaptation seen in some desert plants where stomata open at night to reduce water loss. Carbon dioxide is fixed into organic acids at night and used during the day for photosynthesis, helping the plant conserve water.

• CAM plants are adapted to arid conditions and open their stomata at night to minimise water loss, unlike C3 and C4 plants, which open their stomata during the day.

• In the CAM pathway, CO₂ is fixed at night into a 4-carbon acid (like malic acid), which is stored in vacuoles and later used during the day for photosynthesis.

• The C4 and CAM pathways have the process such as the carbon fixation into a 4-carbon compound, but are in different times, C4 separates processes by organellar space, and CAM separates the CO₂ fixation by time.

• In the day, CAM plants close their stomata and use the stored CO₂ for the Calvin cycle, and allow photosynthesis to occur without any water loss.

• Examples of such plants- cacti, pineapple, and Agave.

Photorespiration

Photorespiration is a light-dependent process that occurs when the enzyme RuBisCO fixes oxygen instead of carbon dioxide. It leads to the loss of energy and carbon for the plant and typically happens under high light intensity and low carbon dioxide levels.

• Photorespiration is a phenomenon in plants where, instead of using carbon dioxide for photosynthesis, the plant uses oxygen.

• It occurs when there is more oxygen and less carbon dioxide, typically in hot and dry conditions.

• The plant will, therefore, emit carbon dioxide but cannot produce any energy or sugar from the process; hence, the whole process is inefficient and wasteful for the plant.

Factors Affecting Photosynthesis

Several factors influence the rate of photosynthesis, including light intensity, carbon dioxide concentration, temperature, and availability of water. These factors determine how efficiently a plant can produce food and grow. The following list the factors affecting photosynthesis in higher plants:

Importance of Photosynthesis

Photosynthesis is essential for life on Earth. There are several ecological implications, but photosynthesis also derives some practical benefits for the living world.

• It has a vital role in the carbon cycle, CO2 is converted into organic compounds, leading to a decrease in atmospheric CO2.

• It maintains the levels of CO2 and O2 in the ecosystems.

• It serves as the basic source of energy for all trophic levels.

• In agriculture, improved crop yields and artificial photosynthesis are being researched as a means of sustainable energy production.

Photosynthesis in Higher Plants NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Stages of photosynthesis

• Factors affecting Photosynthesis

Practice Questions for NEET

Q1. Which among the following has the greatest energy in its quantum?

1. Red light

2. Blue light

3. Green light

4. Yellow light

Correct answer: 2) Blue light

Explanation:

According to the equation E = hc/λ, energy is inversely proportional to wavelength. Blue light has the greatest energy in its quantum compared to longer wavelengths because it has a shorter wavelength (450–495 nm). According to the wave-particle duality of light, energy is inversely proportional to wavelength. This means that shorter wavelengths like blue light carry more energy per photon than longer wavelengths, such as red or infrared light.

Hence, the correct answer is Option (2) Blue light.

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

Recommended Video on Photosynthesis in Higher Plants