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

Photosynthesis in higher plants is a chapter that explains how green plants convert light energy into chemical energy through a series of well-organised processes. It covers important concepts such as the structure of the chloroplast, the role of photosynthetic pigments like chlorophyll and the steps involved in light and dark reactions. This concept is important to understanding how plants produce food and oxygen. It also explains the C3, C4, and CAM pathways of carbon fixation, factors affecting photosynthesis, and the role of ATP and NADPH. This is an important chapter of Biology in Class 11 CBSE.

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This Story also Contains

1. Photosynthesis in Higher Plants
2. The Process of Photosynthesis in Higher Plants
3. Photorespiration
4. Crassulacean Acid Metabolism
5. Factors Affecting Photosynthesis in Higher Plants
6. Recommended Video on Photosynthesis in Higher Plants

Photosynthesis in Higher Plants

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.

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.

Also Read:

• Difference Between Photosynthesis and Cellular Respiration

• Chlorophyll Structure

• Absorption spectrum, action spectrum, and differentiate between the absorption spectrum and the action spectrum

The Process of Photosynthesis in Higher Plants

Photosynthesis in higher plants involves the following processes:

• Light Reaction

• Dark Reaction

Light Reaction

• This 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 molecules.

• 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 indicates the main difference between Non-cyclic Photophosphorylation and 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 this 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.

• 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 primarily taken up by phosphoenolpyruvate, which is 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 the C4 pathway is given below-

Photorespiration

• 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.

Crassulacean Acid Metabolism

• 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.

• Both C4 and CAM pathways include processes such as initial carbon fixation into a 4-carbon compound, but differ in timing: C4 separates processes by organellar space, and CAM separates the CO₂ fixation by time.

• During the day, CAM plants close their stomata and use the stored CO₂ for the Calvin cycle, allowing photosynthesis to continue without water loss.

• Examples of CAM plants include succulents like cacti, pineapple, and Agave, which thrive in dry environments.

• The difference between C3, C4, and CAM pathways lies mainly in how and when they fix CO₂, helping plants adapt to different environmental conditions.

Factors Affecting Photosynthesis in Higher Plants

The following list the factors affecting photosynthesis in higher plants:

• Light intensity- Higher light intensity increases the rate of photosynthesis up to a certain point, as it provides the energy needed for the reaction.

• Carbon dioxide concentration- Carbon dioxide is a raw material for photosynthesis, increasing its concentration usually hastens the process until other factors become limiting.

• Water availability- Water is essential for the photolysis step in the light reactions. Lack of water can slow down or even stop photosynthesis and lead to stomatal closure.

• Temperature- Photosynthesis is enzyme-driven, so the rate increases up to an optimum point and then declines.

• Chlorophyll content- The presence of chlorophyll is important as it captures light energy. Less chlorophyll means reduced photosynthesis.

Also Read:

Recommended Video on Photosynthesis in Higher Plants