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Have you ever thought about how plants manage to make their food? The NCERT Class 11 Biology Chapter 11 Notes Photosynthesis in Higher Plants explains this clearly in a simple manner. These notes have important points, flowcharts, and diagrams so students can revise easily. The chapter covers light and dark reactions, pigments involved, and different pathways that help plants prepare glucose. The NCERT Notes match the NCERT book well, so that students can follow along without missing any important points.
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In these NCERT Class 11 Biology Notes of Chapter 11, you will see clearly explained steps of photosynthesis and examples that make things easier to remember. The notes help you organise your answers properly and include diagrams to make study time interesting. Each section is neatly highlighted and broken down so students can revise quickly before any test. With these NCERT Notes for Class 11, you can build a strong base for understanding plant physiology topics later.
Also, students can refer,
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. With everything explained step by step, it becomes easier to revise when you have the notes in one file. You don’t need to go through long chapters again, just open the file and start studying. That’s why it’s helpful to have the NCERT Class 11 Biology Chapter 12 Notes Respiration in Plants PDF.
This chapter, Photosynthesis in Higher Plants, talks about how plants make their food using sunlight, water, and carbon dioxide. It 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. The notes also include clear diagrams and important terms for better understanding. They are great for quick revision before exams and for anyone needing Class 11 Photosynthesis in Higher Plants Notes PDF.
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.
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:
6CO2+12H2O→LightC6H12O6+6H2O+6O2
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.
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.
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.
Plants employ various photosynthetic pigments to trap light energy:
Photosynthesis occurs in two main stages:
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.
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.
The light-dependent reactions involve two photosystems:
Absorbs 700 nm wavelength light.
Synthesises NADPH for the dark reaction.
Absorbs 680 nm wavelength light.
Photolysis of water molecules, oxygen release.
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.
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.
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:
Water splitting (photolysis) inside the thylakoid lumen releases protons there.
As electrons move through the transport chain, proton carriers move protons from the stroma to the lumen.
NADP+ reduction on the stroma side also uses protons from the stroma, reducing their concentration there.
These steps create a high proton concentration (low pH) in the lumen and a low proton concentration in the stroma.
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
CF0 (membrane-embedded, acts as a channel)
CF1 (protrudes into the stroma, catalyses ATP formation)
Chemiosmosis thus needs:
a membrane (thylakoid)
a proton pump (ETS)
a proton gradient
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).
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.
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.
The CAM Plants are the carnivorous plants which are adapted to deserts (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.
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.
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.
There are some important differences between C₃, C₄, and CAM Plants, which are given in the table below for better understanding:
Feature | C₃ Plants | C₄ Plants | CAM Plants |
Primary CO₂ Acceptor | RuBP | PEP | Organic acids |
First Stable Product | 3-PGA (3C) | OAA (4C) | Organic acids |
Photorespiration | High | Low | Minimal |
Water Use Efficiency | Moderate | High | Very High |
Best Environment | Cool, moist | Hot, dry | Extremely arid |
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.
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Some of the questions which have come in past years from the chapter are given below:
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:
Given below are the chapter-wise NCERT Class 11 Biology notes for quick and easy revision.
Photosynthesis is the process by which green plants convert light energy into chemical energy, producing glucose and oxygen. It sustains life by providing food, oxygen, and regulating atmospheric CO₂, playing a crucial role in the global carbon cycle.
Photosynthesis occurs in two stages: Light Reaction (Photochemical Phase) in the thylakoid membranes, producing ATP, NADPH, and O₂, and Dark Reaction (Calvin Cycle) in the stroma, where ATP and NADPH are used to fix CO₂ into glucose.
The light reaction occurs in the thylakoids, requires sunlight, and produces ATP, NADPH, and O₂. The dark reaction (Calvin cycle) occurs in the stroma, does not need light directly, and uses ATP and NADPH to synthesize glucose.
C₃ Pathway: Found in most plants; the first stable product is 3-PGA.
C₄ Pathway: Adapted for hot climates; minimizes photorespiration using PEP carboxylase.
CAM Pathway: Found in desert plants; stomata open at night to conserve water.
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