Photosynthesis in Higher Plants - Complete NEET Guide with Diagrams & Practice Questions

Introduction

Welcome, future doctors! The chapter 'Photosynthesis in Higher Plants' is not just a fundamental topic in biology but a high-yield area for the NEET exam. Accounting for an average of 2-3 questions each year, a strong grasp of this chapter can significantly boost your score. Photosynthesis is the engine of life on Earth, the process by which green plants convert light energy into chemical energy, providing the food we eat and the oxygen we breathe. This guide will walk you through every critical concept, from the foundational experiments that unveiled its mysteries to the intricate molecular details of the C3 and C4 pathways. We'll break down complex mechanisms like the Z-scheme and chemiosmosis with clear diagrams and provide memory aids to help you retain key facts, ensuring you are fully prepared to tackle any question NEET throws your way.

Key Concepts

1. What is Photosynthesis?

Photosynthesis is a physico-chemical process used by green plants, algae, and some bacteria to convert light energy into chemical energy. This energy is stored in the form of sugar (carbohydrates), which is synthesized from carbon dioxide and water.

Site of Photosynthesis: The primary site is the chloroplast, located mainly in the mesophyll cells of leaves.
Overall Equation: The process is more than just a single step. The universally accepted equation highlights the key reactants and products:

6CO₂ + 12H₂O → (in the presence of Light & Chlorophyll) → C₆H₁₂O₆ + 6H₂O + 6O₂

NEET Pointer: Note the use of 12 water molecules as a reactant. This is crucial because experiments using radioisotopes (¹⁸O) proved that all the oxygen (O₂) evolved during photosynthesis comes from the splitting of water (H₂O), not carbon dioxide (CO₂).

2. Early Experiments - Milestones in Photosynthesis Research

Our understanding of photosynthesis was built upon the work of several pioneering scientists.

Joseph Priestley (1770): His bell jar experiments with a candle, a mouse, and a mint plant demonstrated that plants restore to the air whatever breathing animals and burning candles remove. He essentially discovered the role of air in the process.
Jan Ingenhousz (1779): He expanded on Priestley's work, showing that sunlight is essential for this process and that only the green parts of the plant could release oxygen.
Julius von Sachs (1854): He provided evidence that glucose is produced in the green parts of the plant (chloroplasts) and is usually stored as starch.
T.W. Engelmann (1883): He performed a brilliant experiment using a prism to split light into its spectral components and illuminated a green alga, Cladophora. He used aerobic bacteria to detect the sites of O₂ evolution and found that bacteria accumulated mainly in the blue and red light regions, thus giving the first action spectrum of photosynthesis.
Cornelius van Niel (1897-1985): Based on his studies of purple and green sulfur bacteria, he proposed that O₂ comes from H₂O. These bacteria used H₂S instead of H₂O as the hydrogen donor, producing sulfur, not oxygen. This led to the general equation: 2H₂A + CO₂ → 2A + CH₂O + H₂O.

3. Photosynthetic Pigments & Their Role

Pigments are substances that absorb specific wavelengths of light. The green colour of leaves is not due to a single pigment.

Chlorophyll a: (Bright or blue-green) The main or chief pigment. It acts as the reaction center and is directly involved in converting light energy to chemical energy.
Accessory Pigments:
- Chlorophyll b: (Yellow-green)
- Xanthophylls: (Yellow)
- Carotenoids: (Yellow to yellow-orange)
Function of Accessory Pigments:
1. They absorb light at different wavelengths and transfer the energy to chlorophyll a, broadening the spectrum of light used for photosynthesis.
2. They protect chlorophyll a from photo-oxidation (damage by excess light).

Absorption Spectrum vs. Action Spectrum

Absorption Spectrum: A graph showing the amount of light absorbed by a pigment at different wavelengths. Chlorophyll a and b show maximum absorption in the blue and red regions.
Action Spectrum: A graph showing the rate of photosynthesis at different wavelengths of light. This spectrum also shows peaks in the blue and red regions, closely matching the absorption spectrum of chlorophylls, proving their crucial role.

4. The Light-Dependent Reactions (Photochemical Phase)

This phase occurs in the grana/thylakoid membranes of the chloroplasts and involves two main events: light absorption and energy conversion.

Products: ATP (energy currency), NADPH (reducing power), and O₂ (by-product).
Photosystems: Pigments are organized into two discrete Light Harvesting Complexes (LHC) known as Photosystem I (PS I) and Photosystem II (PS II).
- Each photosystem has an antenna (all pigments except one molecule of chlorophyll a) and a reaction center (a single special chlorophyll a molecule).
- PS I: The reaction center is P700 (absorbs light at 700 nm).
- PS II: The reaction center is P680 (absorbs light at 680 nm).

The Z-Scheme of Electron Transport (Non-Cyclic Photophosphorylation)

This is the primary pathway for the light reaction, involving both PS II and PS I. The name 'Z-scheme' comes from the shape formed when the components are plotted on a redox potential scale.

PS II Activation: P680 absorbs light, and its electrons get excited.
Electron Acceptor: The excited electrons are captured by a primary electron acceptor.
Photolysis of Water: To replace the electrons lost by P680, water is split on the inner side of the thylakoid membrane: 2H₂O → 4H⁺ + O₂ + 4e⁻. This is the source of O₂.
Electron Transport System (ETS): Electrons travel from the primary acceptor of PS II down an ETS (containing cytochromes) to PS I. This "downhill" journey releases energy, which is used to pump protons into the thylakoid lumen, creating a proton gradient.
PS I Activation: P700 in PS I absorbs light and its electrons are excited. These electrons are replaced by the ones coming from the ETS.
NADPH Formation: The excited electrons from PS I are passed to another acceptor and finally reduce NADP⁺ to NADPH via the enzyme NADP⁺ reductase.

Cyclic Photophosphorylation

When: Occurs when only PS I is functional, for example, in stroma lamellae (which lack PS II and NADP⁺ reductase) or when light of wavelength beyond 680 nm is available.
Process: The electron from PS I is cycled back to the PS I complex through the ETS.
Products: Only ATP is synthesized. No NADPH or O₂ is produced.

Feature	Non-Cyclic Photophosphorylation	Cyclic Photophosphorylation
Photosystems Involved	PS I and PS II	Only PS I
Electron Source	H₂O	P700 of PS I
Oxygen Evolved	Yes	No
Final Electron Acceptor	NADP⁺	P700 of PS I
Products	ATP, NADPH, O₂	Only ATP
Location	Grana Thylakoids	Stroma Lamellae & Grana

5. Chemiosmotic Hypothesis - How ATP is Made

This hypothesis explains how ATP is synthesized in chloroplasts. It relies on the creation of a proton gradient across the thylakoid membrane.

Protons (H⁺) accumulate inside the thylakoid lumen due to:

Splitting of water releases H⁺ into the lumen.
Electron transport pumps H⁺ from the stroma into the lumen.
Consumption of H⁺ in the stroma during the formation of NADPH.

This creates a high concentration of H⁺ in the lumen (low pH) and a low concentration in the stroma (high pH). This gradient holds potential energy.

ATP Synthase (CF₀ - CF₁ particle): This enzyme complex allows protons to diffuse back into the stroma.

CF₀: An embedded transmembrane channel that allows facilitated diffusion of H⁺.
CF₁: A protruding headpiece on the stroma side. The energy released by the movement of protons through CF₀ causes a conformational change in CF₁, which drives the synthesis of ATP from ADP and inorganic phosphate (Pi).

6. The Light-Independent Reactions (Biosynthetic Phase)

This phase, also known as the Calvin Cycle or C3 Pathway, occurs in the stroma of the chloroplast. It uses the ATP and NADPH from the light reaction to "fix" atmospheric CO₂ into carbohydrates.

The Calvin Cycle (C3 Pathway)

Primary CO₂ Acceptor: A 5-carbon sugar, Ribulose-1,5-bisphosphate (RuBP).
First Stable Product: A 3-carbon compound, 3-Phosphoglyceric acid (3-PGA). This is why it's called the C3 pathway.
Key Enzyme: RuBisCO (Ribulose bisphosphate Carboxylase-Oxygenase), the most abundant enzyme on Earth.

Three Stages of the Calvin Cycle:

Carboxylation: RuBisCO catalyzes the addition of CO₂ to RuBP, forming an unstable 6C intermediate that immediately splits into two molecules of 3-PGA.
Reduction: This is a two-step process where 3-PGA is converted into triose phosphates (the sugar). This step utilizes 2 ATP and 2 NADPH for every CO₂ molecule fixed.
Regeneration: The CO₂ acceptor, RuBP, is regenerated from the triose phosphates. This step requires 1 ATP for every CO₂ molecule fixed.

NEET Stoichiometry: To make one molecule of glucose (C₆H₁₂O₆), the cycle must turn 6 times.

Input: 6 CO₂ + 18 ATP + 12 NADPH

Output: 1 Glucose + 18 ADP + 12 NADP⁺

7. The C4 Pathway (Hatch and Slack Pathway)

This is an adaptation in plants living in dry, tropical regions (e.g., maize, sugarcane, sorghum) to minimize water loss and avoid photorespiration.

Special Feature: Kranz Anatomy C4 plants have a unique leaf structure. The vascular bundles are surrounded by large bundle sheath cells, which are themselves surrounded by mesophyll cells.

Bundle Sheath Cells: Have thick walls impervious to gas exchange, no intercellular spaces, and a large number of chloroplasts.

Steps of the C4 Pathway:

In Mesophyll Cells: The primary CO₂ acceptor is a 3-carbon molecule, Phosphoenolpyruvate (PEP). The enzyme is PEP carboxylase (PEPcase). (Note: Mesophyll cells in C4 plants lack RuBisCO).
PEP and CO₂ form Oxaloacetic acid (OAA), a 4-carbon acid (hence the C4 pathway).
OAA is converted to other 4C acids like malic acid or aspartic acid.
In Bundle Sheath Cells: The 4C acid is transported here and is decarboxylated (CO₂ is removed). This release of CO₂ raises the CO₂ concentration inside the bundle sheath cells.
This concentrated CO₂ now enters the Calvin cycle (C3 pathway), which is carried out by RuBisCO in the bundle sheath cells.
The remaining 3C molecule is transported back to the mesophyll to regenerate PEP, a process that requires ATP.

Significance of C4 Pathway: It is a CO₂-concentrating mechanism. It ensures that RuBisCO in the bundle sheath cells is always exposed to a high concentration of CO₂, preventing the wasteful process of photorespiration.

8. Photorespiration

Photorespiration is a wasteful process that occurs in C3 plants when the O₂ concentration is high and the CO₂ concentration is low.

RuBisCO's Dual Nature: The active site of RuBisCO can bind to both CO₂ and O₂. Its binding is competitive.
Process: When RuBisCO binds to O₂, it initiates a pathway where RuBP is converted into one molecule of PGA (3C) and one molecule of phosphoglycolate (2C).
Outcome: This pathway does not produce any sugar, ATP, or NADPH. Instead, it consumes ATP and releases CO₂. It is therefore considered a wasteful process.
Why C4 Plants Avoid It: The C4 pathway's CO₂ pump ensures that the CO₂ concentration around RuBisCO is always high, promoting its carboxylase activity and minimizing its oxygenase activity.

9. Factors Affecting Photosynthesis

Blackman's Law of Limiting Factors: "If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value: it is the factor which directly affects the process if its quantity is changed."

Light: The rate increases with light intensity until it reaches a saturation point (~10% of full sunlight), after which other factors (like CO₂) become limiting. Very high light can cause photo-oxidation.
Carbon Dioxide: This is the major limiting factor in nature.
- C3 plants show saturation only beyond 450 µlL⁻¹ and benefit from CO₂ enrichment.
- C4 plants show saturation at around 360 µlL⁻¹, indicating they are more efficient at lower CO₂ levels.
Temperature: The dark reactions are enzymatic and thus temperature-sensitive. C4 plants have a higher temperature optimum (30-40°C) than C3 plants (20-25°C).
Water: Water stress causes stomata to close, which reduces CO₂ availability. This is usually an indirect limiting factor.

Memory Techniques

Calvin Cycle Stages: C-R-R → Carboxylation, Reduction, Regeneration.
C4 Pathway Products: "Remember OAA and Malic Acid are MACHO 4-carbon acids." (M = Malic, A = Aspartic, C = Carbon, H=Hatch, O=OAA).
Z-Scheme Flow: Visualize a tilted 'Z'. PS II → ETS → PS I → NADPH. It starts low (PSII), goes up, comes down (ETS), goes up again (PSI), and finally comes down to make NADPH.
C3 vs C4 Acceptors:
- C3: The acceptor RuBP is "Ready and waiting" for CO₂ in a 3-step cycle.
- C4: The acceptor PEP "Peps up" the CO₂ in the mesophyll for the main event in the bundle sheath.

Previous Year Questions (NEET)

Q1. (NEET 2021) Which of the following statements is incorrect? (a) The stroma lamellae have PS I only and lack NADP reductase. (b) Grana lamellae have both PS I and PS II. (c) Cyclic photophosphorylation involves both PS I and PS II. (d) Both ATP and NADPH + H+ are synthesized during non-cyclic photophosphorylation.

Answer: (c) Cyclic photophosphorylation involves both PS I and PS II. Explanation: Cyclic photophosphorylation involves only Photosystem I (PS I). The electron excited from P700 is cycled back to the same photosystem. PS II is not involved. Statements (a), (b), and (d) are correct descriptions of the light reactions.

Q2. (NEET 2020) In a chloroplast, the highest number of protons are found in: (a) Stroma (b) Lumen of thylakoids (c) Intermembrane space (d) Antennae complex

Answer: (b) Lumen of thylakoids Explanation: During the light-dependent reactions, protons are actively pumped from the stroma into the thylakoid lumen, and the photolysis of water also releases protons into the lumen. This creates the highest concentration of protons inside the lumen, which drives ATP synthesis.

Q3. (NEET 2019) Which of the following is not a product of the light reaction of photosynthesis? (a) ATP (b) NADH (c) NADPH (d) Oxygen

Answer: (b) NADH Explanation: The light reaction of photosynthesis produces ATP and NADPH as the energy-rich compounds and Oxygen as a by-product. NADH is a coenzyme primarily associated with cellular respiration, not photosynthesis. Be careful to distinguish between NADPH (photosynthesis) and NADH (respiration).

Key Takeaways

Photosynthesis converts light energy into chemical energy, occurring in chloroplasts.
The oxygen evolved during photosynthesis comes from the splitting of water (H₂O).
Light reactions (in thylakoids) produce ATP, NADPH, and O₂.
Dark reactions (Calvin Cycle, in stroma) use ATP and NADPH to fix CO₂ into sugar.
C3 Plants: Use RuBisCO to fix CO₂ to RuBP, forming 3-PGA. They undergo photorespiration.
C4 Plants: Use PEPcase to fix CO₂ to PEP, forming OAA. They have Kranz anatomy and avoid photorespiration by concentrating CO₂ in bundle sheath cells.
Photorespiration is a wasteful process where RuBisCO binds O₂ instead of CO₂, reducing photosynthetic output in C3 plants.
CO₂ is the most significant limiting factor for photosynthesis in nature.

Photosynthesis In Higher Plants