Photosynthesis in Higher Plants class 11th NEET notes

What is photosynthesis in higher plants?

Photosynthesis is a vital physiological process through which green plants, algae, and some bacteria convert light energy into chemical energy in the form of glucose. This process primarily takes place in the chloroplasts of mesophyll cells in the leaves.

In higher plants, photosynthesis is more complex and efficient due to the presence of specialized structures such as stomata, vascular tissues, and well-developed leaf anatomy. These plants use chlorophyll pigments to absorb sunlight and convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆), releasing oxygen (O₂) as a by-product.

This process is essential not only for the survival of plants but also for maintaining life on Earth, as it is the primary source of organic matter and atmospheric oxygen.

Sites of Photosynthesis –

Photosynthesis in higher plants mainly occurs in the leaves, specifically within specialized cell structures called chloroplasts. These chloroplasts contain the green pigment chlorophyll, which is responsible for capturing sunlight.

The leaf is the most efficient organ for photosynthesis due to its:

Broad surface area (for maximum light absorption)
Numerous stomata (for CO₂ intake and O₂ release)
Well-organized vascular tissues (xylem and phloem for water and food transport)
Thin structure (for easy gas exchange)

Within the leaf, photosynthesis primarily occurs in:

Mesophyll cells: These cells are rich in chloroplasts.
Palisade parenchyma: Located just below the upper epidermis, contains tightly packed cells with a high number of chloroplasts.
Spongy parenchyma: Below the palisade layer, helps in gas exchange and also contains chloroplasts.

At the cellular level, the chloroplast is the actual site where photosynthesis takes place. It has two main regions:

1. Grana (stacked thylakoids): Site of the light-dependent reactions
2. Stroma (fluid-filled space): Site of the light-independent reactions (Calvin Cycle)

Photosynthetic Pigments –

Photosynthetic pigments are special molecules found in the chloroplasts of plant cells that absorb light energy and initiate the process of photosynthesis. These pigments are crucial because different pigments absorb different wavelengths of light, thereby increasing the efficiency of light absorption.

The main photosynthetic pigments in higher plants include:

Chlorophyll a – Primary pigment, essential for photosynthesis, bluish-green in color
Chlorophyll b – Accessory pigment, yellowish-green, helps in capturing additional light
Carotenoids – Accessory pigments like carotenes (orange) and xanthophylls (yellow), protect chlorophyll from damage and expand the range of light absorption
Phycobilins – Found in algae, not in higher plants

Among these, chlorophyll a is the most important pigment because it directly participates in the light reaction of photosynthesis.
These pigments are organized into light-harvesting complexes (LHCs) within the thylakoid membranes of the chloroplasts, forming the photosystems (PS I and PS II) that capture and convert light energy into chemical energy.

Light Reaction (Photochemical Phase) –

The light reaction, also known as the photochemical phase of photosynthesis, is the first stage of photosynthesis that takes place in the thylakoid membranes of the chloroplasts. This phase requires sunlight and occurs only in the presence of light.

Photosynthesis in Higher Plants class 11th NEET notes

During the light reaction, light energy is captured by photosynthetic pigments (mainly chlorophyll) and converted into chemical energy in the form of ATP and NADPH. These energy-rich molecules are then used in the dark reaction (Calvin Cycle) to synthesize glucose.

Key Events of Light Reaction:

1. Absorption of light energy by chlorophyll.
2. Excitation of electrons and transfer through the electron transport chain (ETC).
3. Photolysis of water – splitting of water molecules into H⁺, electrons, and O₂.
4. Formation of ATP by photophosphorylation.
5. Reduction of NADP⁺ to NADPH.

This process involves two photosystems:

Photosystem I (PS I)
Photosystem II (PS II)

Electron Flow:

Non-cyclic photophosphorylation – Both PS I & II, makes ATP + NADPH + O₂
Cyclic photophosphorylation – Only PS I, makes ATP only

Dark Reaction (Biosynthetic Phase) –

The dark reaction, also known as the biosynthetic phase or Calvin Cycle, is the second stage of photosynthesis. Unlike the light reaction, it does not require light directly and takes place in the stroma of the chloroplast.

In this phase, the energy-rich molecules ATP and NADPH, produced during the light reaction, are used to fix carbon dioxide (CO₂) and synthesize glucose (C₆H₁₂O₆) – the food for the plant.

Features of the Dark Reaction:

1. Occurs in the stroma of chloroplasts.
2. CO₂ is fixed into a stable organic compound.
3. ATP and NADPH are used to reduce CO₂ into glucose.
4. The main pathway is the Calvin Cycle, discovered by Melvin Calvin.

Though it’s called the "dark reaction," it can occur in the light as long as ATP and NADPH are available.

This phase is crucial as it leads to the actual formation of food in plants, making it essential for the survival of nearly all life forms on Earth.

C₄ Plants and Hatch-Slack Pathway:

C₄ plants are a special group of plants that have evolved a more efficient method of photosynthesis in hot and dry environments. These plants minimize photorespiration and increase carbon fixation efficiency by using a two-step process known as the Hatch-Slack pathway, named after its discoverers Hatch and Slack.
Unlike C₃ plants (which use only the Calvin Cycle), C₄ plants first fix CO₂ in the mesophyll cells to form a 4-carbon compound (like oxaloacetic acid). This compound is then transported to the bundle sheath cells, where CO₂ is released and enters the Calvin Cycle.

Features of C₄ Plants:

Initial CO₂ fixation occurs in the mesophyll cells.
Calvin Cycle operates in the bundle sheath cells.
Shows Kranz anatomy (distinct arrangement of mesophyll and bundle sheath).
Reduces photorespiration, making photosynthesis more efficient.

Examples of C₄ plants: Maize, Sugarcane, Sorghum, Amaranthus

This adaptation allows C₄ plants to photosynthesize efficiently under high light intensity, temperature, and limited water conditions.

Comparison between C3 and C4 plants:

Feature	C3 Plants	C4 Plants
Photosynthetic Pathway	Calvin cycle (C3 pathway)	Hatch-Slack pathway followed by Calvin cycle
First Stable Product	3-carbon compound (3-PGA)	4-carbon compound (oxaloacetic acid - OAA)
Leaf Anatomy	No Kranz anatomy	Kranz anatomy present
Cell Types Involved	Only mesophyll cells	Mesophyll and bundle sheath cells
Photorespiration	High	Very low or absent
Carbon Fixation Efficiency	Lower	Higher
Enzyme for CO₂ Fixation	RuBisCO	PEP Carboxylase (in mesophyll) & RuBisCO
Optimum Temperature	15–25°C	30–45°C
Water Use Efficiency	Low	High
Examples	Rice, wheat, soybean, barley	Maize, sugarcane, sorghum, millet

Photorespiration –

Photorespiration is a wasteful, light-dependent process in plants where the enzyme RuBisCO fixes oxygen (O₂) instead of carbon dioxide (CO₂). This leads to the formation of a 2-carbon compound (phosphoglycolate) instead of the usual 3-carbon compound (PGA), resulting in loss of energy and fixed carbon.

This process occurs mainly in C₃ plants under conditions of:

High oxygen concentration
Low carbon dioxide
High temperature and intense light

Features of Photorespiration:

Takes place in the chloroplast, peroxisome, and mitochondria.
Does not produce ATP or glucose.
Leads to a reduction in photosynthetic efficiency.

C₄ and CAM plants avoid photorespiration due to their adapted leaf anatomy and CO₂ concentrating mechanisms, which is why they are more efficient in hot and dry climates.

Factors Affecting Photosynthesis –

Photosynthesis is a complex process that is influenced by multiple internal and external factors. The rate of photosynthesis in plants depends on the availability and efficiency of essential resources such as light, carbon dioxide, water, and temperature.

These factors can be grouped into two main categories:

External (Environmental) Factors:

Light intensity: More light increases the rate up to a point.
Carbon dioxide concentration: Higher CO₂ levels generally enhance photosynthesis.
Temperature: Moderate temperatures support the enzymatic reactions involved; very high or low temperatures reduce efficiency.
Water availability: Essential for photolysis; water stress limits photosynthesis.
Oxygen concentration: High O₂ can increase photorespiration and reduce photosynthetic efficiency.