NCERT Class 11 Biology Chapter 13 Notes Photosynthesis In Higher Plants - Download PDF Notes

Download NCERT Notes for Class 11 Biology Chapter 11 PDF

This chapter goes into how plants release energy by breaking down the food they produce. It includes detailed processes like glycolysis, fermentation, and the Krebs cycle. Students can download the NCERT notes for class 11 Biology chapter 11 PDF from the link given below. With everything explained step by step, it becomes easier to revise when they have the notes in one file. That’s why it’s helpful to have the NCERT Class 11 Biology Notes during exams.

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Photosynthesis in Higher Plants Class 11 Notes

This chapter talks about how plants make their food using sunlight, water, and carbon dioxide. Photosynthesis in Higher Plants explains steps like light reactions and the Calvin cycle in an easy way. Students will see how chlorophyll and other pigments help capture light energy. Photosynthesis in Higher Plants Class 11 Notes also include clear diagrams and important terms for better understanding.

Photosynthesis

Photosynthesis is a biochemical process by which green plants, algae, and some bacteria transform light energy into chemical energy in the form of glucose. Photosynthesis also produces oxygen as a byproduct, which is vital for the existence of all aerobic organisms.

Photosynthesis in Higher Plants: Early Experiments

• Joseph Priestley (1770) showed plants restore air damaged by burning candles or animals; he used a mint plant in a bell jar to keep a mouse alive, proving plants purify air.

• Jan Ingenhousz (1779) proved that sunlight is essential for this air-purifying effect and showed that only the green parts of plants release oxygen in sunlight.

• Julius von Sachs (1854) confirmed that green parts of plants (chlorophyll in chloroplasts) make glucose, stored as starch.

• T.W. Engelmann (1880s) used a prism and green algae with bacteria to show that oxygen production is highest in red and blue light — the first action spectrum of photosynthesis.

• By the mid-1800s, scientists knew plants make carbohydrates from CO₂ and water using light energy, summarised by the equation:

$6{\mathrm{CO}}_2+12{\mathrm{H}}_{2}O\stackrel{\text{Light}}{\to }{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{H}}_{2}O+6{\mathrm{O}}_2$

• Cornelius van Niel (1930s) explained that hydrogen from water reduces CO₂, and O₂ comes from water, not CO₂. Radioisotope experiments later confirmed this.

• The above equation uses 12 H₂O to show that 6 become part of glucose formation while 6 are released as water, reflecting the entire photosynthesis process.

Importance of Photosynthesis

• Photosynthesis is the beginning of all food chains since plants and algae form organic matter that feeds herbivores and, consequently, carnivores.

• Photosynthesis maintains the oxygen content in the atmosphere, making aerobic respiration possible.

• It maintains the equilibrium of CO₂ content in the atmosphere, minimising the greenhouse effect.

• Fossil fuels such as coal and petroleum have been formed through the contribution of ancient plants and algae, by photosynthesis.

• Crop productivity is dependent on increased photosynthetic efficiency, which provides improved crop yield, essential to ensure food security globally.

Where does Photosynthesis take place?

Photosynthesis takes place in the chloroplasts, which are specialised organelles found in plant cells. Leaf mesophyll cells have the maximum number of chloroplasts and are therefore the main site for photosynthesis.

Structure of Chloroplast

• The outer membrane is permeable to small molecules.
• The inner Membrane is selectively permeable, regulating material exchange.
• Thylakoids are flattened sacs that house chlorophyll, the location of the light reactions.
• Grana are the thylakoid stacks where photophosphorylation takes place.
• Stroma is a fluid-filled area in which the Calvin cycle (dark reaction) occurs.

Photosynthetic Pigments

Plants employ various photosynthetic pigments to trap light energy:

1. Chlorophyll a: The primary pigment responsible for trapping light.
2. Chlorophyll b: Assists in transferring energy to chlorophyll a.
3. Carotenoids (Xanthophylls & Carotenes): Accessory pigments that capture light of varying wavelengths and guard against damage to chlorophyll.

Phases of Photosynthesis

Photosynthesis occurs in two main stages:

1. Light Reactions (Photochemical Phase)

• It takes place in the thylakoid membranes of the chloroplast.

• Light is trapped, and its energy is utilised for generating ATP (energy currency) and NADPH (reducing power).

• Water molecules are cleaved (Photolysis), with oxygen gas as a by-product.

2. Dark Reactions (Biosynthetic Phase or Calvin Cycle)

• It takes place in the stroma of the chloroplast.

• ATP and NADPH from light reactions are utilised to fix CO₂ into glucose through the Calvin cycle.

• Does not need light directly, but relies on byproducts of the light reaction.

Light Reaction (Hill Reaction)

The light-dependent reactions involve two photosystems:

Photosystem I (PSI)

• Absorbs 700 nm wavelength light.

• Synthesises NADPH for the dark reaction.

Photosystem II (PSII)

• Absorbs 680 nm wavelength light.

• Photolysis of water molecules, oxygen release.

The Electron Transport and Splitting of Water

• In Photosystem II (PS II), chlorophyll a absorbs red light at 680 nm, exciting its electrons to higher energy levels.

• These excited electrons are captured by a primary acceptor and then passed down an electron transport chain with cytochromes, moving “downhill” in redox potential.

• Electrons reach Photosystem I (PS I), whose chlorophyll a absorbs red light at 700 nm and gets excited again; these electrons go to another acceptor and finally reduce NADP⁺ to NADPH ⁺ H⁺.

• This movement of electrons from PS II to PS I and then to NADP+ is called the Z-scheme, due to its zig-zag flow of electrons.

• To keep supplying electrons, PS II uses the splitting of water (photolysis), providing electrons to PS II, protons for the proton gradient, and oxygen as a photosynthesis product.

2H₂O→4H⁺The water-splitting complex is linked with PS II on the inner side of the thylakoid membrane; the released protons collect in the lumen, while oxygen diffuses out.

Types of Photophosphorylation

• In cyclic photophosphorylation, PSI is used alone, and only ATP is synthesised.

• In Non-Cyclic Photophosphorylation, both PSI and PSII are used, and ATP, NADPH, and O₂ are synthesised.

Chemiosmotic Hypothesis

• The chemiosmotic hypothesis explains how ATP is made in chloroplasts, similar to respiration, but on thylakoid membranes.

• Here, protons (H⁺) build up inside the lumen of thylakoids, unlike mitochondria, where they build up in the intermembrane space.
  Proton gradient formation steps:

1. Water splitting (photolysis) inside the thylakoid lumen releases protons there.

2. As electrons move through the transport chain, proton carriers move protons from the stroma to the lumen.

3. NADP⁺ reduction on the stroma side also uses protons from the stroma, reducing their concentration there.

4. These steps create a high proton concentration (low pH) in the lumen and a low proton concentration in the stroma.

5. Protons flow back to the stroma through the CF₀ channel of ATP synthase, releasing energy that triggers CF₁ to produce ATP.

• ATP synthase has two parts

  1. CF₀ (membrane-embedded, acts as a channel)

  2. CF₁ (protrudes into the stroma, catalyses ATP formation)

• Chemiosmosis thus needs:

  1. a membrane (thylakoid)

  2. a proton pump (ETS)

  3. a proton gradient

  4. ATP synthase

• This mechanism links the light reaction (producing ATP and NADPH) with the dark reaction (using ATP and NADPH to fix CO₂ and make sugars).

${\mathrm{NADP}}^{+}+2e^{-}+2H^{+}\to \mathrm{NADPH}+H^{+}$

Dark Reaction (Calvin Cycle or C₃ Cycle)

The Calvin cycle takes place in the stroma and involves three steps:

• Carbon Fixation: CO₂ is fixed by RuBP (Ribulose-1,5-bisphosphate) and is reduced to 3-PGA.

• Reduction: ATP and NADPH are utilised to produce G3P (glyceraldehyde-3-phosphate), a glucose precursor.

• Regeneration: RuBP is regenerated to allow the cycle to repeat.

C₄ Pathway (Hatch and Slack Pathway)

• The C₄ Pathway is present in tropical crops (e.g., sugarcane, maize).

• Substitutes PEP carboxylase for RuBisCO in the first CO₂ fixation, minimising photorespiration.

• Easier under hot and dry conditions.

CAM Pathway (Crassulacean Acid Metabolism)

• The CAM Plants are the desert-adapted plants (e.g., pineapple, cactus).

• Stomata are opened during the night for CO₂ fixation without transpiration.

• CO₂ is saved in the form of organic acids and utilised when in sunlight.

Photorespiration

• Occurs when RuBisCO binds with O₂ instead of CO₂.

• Leads to the loss of fixed carbon and energy.

• More common in C₃ plants under high temperatures and low CO₂ conditions.

Factors Affecting Photosynthesis

• Light Intensity: Increases the rate up to a saturation point.

• Carbon Dioxide Concentration: Higher CO₂ enhances photosynthesis.

• Temperature: Optimal range depends on the plant type.

• Water Availability: Essential for photolysis and stomatal function.

Differences Between C₃, C₄, and CAM Plants

There are some important differences between C₃, C₄, and CAM Plants, which are given in the table below for better understanding:

Factors Affecting Photosynthesis

• Internal factors: leaf number, size, age, orientation, mesophyll cells, chloroplasts, internal CO₂ level, and chlorophyll content (linked to the plant’s genetics).

• External factors: sunlight, temperature, CO₂ concentration, and water availability.

• According to Blackman’s Law of Limiting Factors (1905), when multiple factors affect a process, the one closest to its minimal level limits the rate.

• Light: quality, intensity, and duration matter; usually not limiting except in dense shade; very high light can damage chlorophyll.

• CO₂ major limiting factor; higher CO₂ boosts photosynthesis, especially in C₃ plants; greenhouse crops benefit from CO₂ enrichment.

• Temperature: affects enzyme-controlled dark reactions; C₄ plants tolerate higher temperatures; tropical plants have higher optima than temperate plants.

• Water: indirectly limits photosynthesis by closing stomata (less CO₂), causing wilting and reduced leaf area.

Also Read

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Chapter 11: Previous Year Questions and Answers

Some of the questions that have come in past years from the chapter are given below. By referring to the NCERT Class 11 Biology Chapter 11 Notes Photosynthesis in Higher Plants, students can understand the concepts clearly.

Question 1. Which pigment acts directly to convert light energy to chemical energy?

Option 1. Carotenoid

Option 2. Chlorophyll B

Option 3. Chlorophyll A

Option 4. Xanthophyll

Answer :

Chlorophyll is a pigment that is essential to photosynthesis and is responsible for the highest amount of light absorption. The blue (about 430 nm) and red (approximately 662 nm) portions of the spectrum are where this pigment absorbs light the best. Chlorophyll a quickly transforms light energy into chemical energy because it can directly participate in the photochemical events of photosynthesis.

Hence, the correct answer is option 3. Chlorophyll A

Question 2. The reaction that is responsible for the primary fixation of CO₂ is catalysed by

Option 1. PGA synthase

Option 2. RuBP carboxylase and PEP carboxylase

Option 3. RuBP carboxylase

Option 4. PEP carboxylase

Answer :

PEP carboxylase in C₄ plants is highly efficient in fixing CO₂, even at low concentrations, by forming oxaloacetate in mesophyll cells. In contrast, RuBP carboxylase (Rubisco) in C₃ plants fixes CO₂ directly during the Calvin cycle but is less efficient due to its oxygenase activity, leading to photorespiration. This differentiation allows C₄ plants to thrive in hot, dry environments by minimising photorespiration.

Hence, the correct answer is option 2, RuBP carboxylase and PEP carboxylase

Question 3. The correct sequence of flow of electron in the light reaction is

Option 1. PSI, ferredoxin, PSII

Option 2. PSI, plastoquinone, cytochromes, PS II, ferredoxin

Option 3. PS II, plastoquinone, cytochromes, PS I, ferredoxin

Option 4. PS I, plastoquinone, cytochromes, PS I, ferredoxin

Answer :

In the light reaction, the proper order of electron flow is Ferredoxin, PSI, PSII, Plastoquinone, and Cytochromes.
Thylakoids are subject to light-dependent light responses. They include:
Water photolysis: Water splitting into H₂ and O₂
Assimilation power production: ATP and NADPH
The P⁶⁸⁰ photo center of photosystem II absorbs the electron generated during the photolysis of water.

Hence, the correct answer is option (3) PS II, plastoquinone, cytochromes, PS I, ferredoxin

Also Read:

• NCERT Solution
• NCERT Solutions for Class 11
• NCERT Solutions for Class 11 Biology

Importance of Class 11 Biology Chapter 11 Notes

These notes make the concepts easy to follow and simple to understand during preparation. The important points are given below for quick review.

• Class 11 Biology Chapter 11 Notes break down topics into simple explanations, making it easier to understand the concepts.

• They help in building a strong foundation for understanding the subject.

• The Class 11 Biology Chapter 11 Notes are arranged in a logical sequence, making it easier for students to systematically.

• All the important points are highlighted to make learning effective.

Chapter-Wise NCERT Class 11 Notes Biology

Given below are the chapter-wise NCERT Class 11 Biology notes for quick and easy revision.