NEET Biology: Photosynthesis in Higher Plants – Complete Guide, Notes & Solved PYQs

Introduction to Photosynthesis in Higher Plants for NEET UG

Photosynthesis is arguably the most vital anabolic bio-process on Earth, serving as the primary source of food for all living organisms and releasing oxygen into the atmosphere. For medical aspirants preparing for the National Eligibility cum Entrance Test (NEET UG), Photosynthesis in Higher Plants is a fundamental chapter under the Plant Physiology module in the Class 11 NCERT Biology syllabus.

Photosynthesis is defined as a physicochemical process by which green plants, algae, and photosynthetic bacteria convert radiant solar energy into stable chemical energy stored in organic molecules like glucose. The overall simplified equation for oxygenic photosynthesis is:

$$6 \text{CO}_2 + 12 \text{H}_2 \text{O} xrightarrow[ \text{Chlorophyll}]{ \text{Light}} \text{C}_6 \text{H}_{12} \text{O}_6 + 6 \text{H}_2 \text{O} + 6 \text{O}_2uparrow$$

Understanding the physiological, biochemical, and structural adaptations involved in light absorption, electron transport, photophosphorylation, and carbon fixation pathways ($ \text{C}_3$, $ \text{C}_4$, and CAM) is critical for answering complex assertion-reasoning, diagrammatic, and conceptual MCQs in the NEET examination.

NEET Weightage & Expected Questions

In the NEET UG question paper, Plant Physiology carries a weightage of roughly 12% to 15% of total Biology marks. Specifically, Photosynthesis in Higher Plants routinely yields 2 to 4 direct or application-based questions (translating to 8 to 16 marks).

• Expected Question Types: Match the following (e.g., pigments vs. maximum absorption wavelength), sequence identification (Z-scheme electron transport chain), tabular comparison ($ \text{C}_3$ vs. $ \text{C}_4$ pathways), diagram identification (chloroplast structure, Hatch-Slack pathway, Kranz anatomy), and direct enzyme characteristics ($ \text{RuBisCO}$ vs. $ \text{PEPcase}$).
• Rank Boosting Potential: Mastering this chapter helps students score high accuracy because all questions strictly align with NCERT textbook line items. Distinguishing subtle details—such as ATP/NADPH consumption ratios or primary carbon dioxide acceptors—can give you a crucial edge over thousands of candidates.

Core Concepts & Key Mechanisms Explained

1. Chloroplast Structure & Photosynthetic Pigments

Photosynthesis occurs within specialized organelles called chloroplasts present predominantly in the mesophyll cells of leaves. A chloroplast is enclosed by a double membrane system (outer and inner) and contains two distinct structural sub-regions:

• Membranous System (Grana & Stroma Lamellae): Responsible for trapping light energy and synthesizing ATP and NADPH during the photochemical phase (light reaction).
• Fluid Matrix (Stroma): Contains enzymatic machinery required for reducing carbon dioxide into carbohydrates through enzymatic steps (dark reaction).

Paper chromatography reveals that leaf pigments are not uniform; rather, four distinct pigments drive photosynthesis:

• Chlorophyll a (Bright or blue-green): The primary photosynthetic pigment forming the essential reaction center.
• Chlorophyll b (Yellow-green): An accessory pigment that absorbs light and transfers energy to Chlorophyll a.
• Xanthophylls (Yellow): Accessory pigments that protect chlorophyll from photo-oxidation.
• Carotenoids (Yellow to yellow-orange): Accessory pigments that act as shield pigments preventing photo-destruction of chlorophyll a molecules.

Light Harvesting Complexes (LHC): Hundreds of accessory pigment molecules bound to proteins form the LHC within Photosystem I (PS I) and Photosystem II (PS II). The single Chlorophyll a molecule forms the reaction center. PS I has an absorption peak at $700 \text{ nm}$ ($ \text{P}_{700}$), while PS II has an absorption peak at $680 \text{ nm}$ ($ \text{P}_{680}$).

2. Light Reaction (Photochemical Phase)

The light reaction includes light absorption, water splitting, oxygen release, and the synthesis of high-energy chemical intermediates ($ \text{ATP}$ and $ \text{NADPH}$).

The Z-Scheme (Non-Cyclic Photophosphorylation)

1. Light Absorption: $ \text{PS II}$ ($ \text{P}_{680}$) absorbs red light ($680 \text{ nm}$), causing its electrons to become excited and jump to an orbit farther from the atomic nucleus.
2. Electron Transport Chain (ETC): These excited electrons are accepted by a primary electron acceptor ($ \text{Pheophytin}$) and transferred down an electron transport system consisting of cytochromes ($ \text{Plastoquinone} ightarrow \text{Cytochrome } b_6f ightarrow \text{Plastocyanin}$).
3. Transfer to PS I: The electrons are passed down to $ \text{PS I}$ ($ \text{P}_{700}$), which has simultaneously absorbed light of wavelength $700 \text{ nm}$.
4. NADP+ Reduction: Electrons excited from $ \text{PS I}$ pass to another acceptor ($ \text{Ferredoxin}$) and move downhill to reduce $ \text{NADP}^+$ into $ \text{NADPH} + \text{H}^+$ catalyzed by the enzyme $ \text{NADP}^+$ reductase (located on the stroma side of the thylakoid membrane).

Splitting of Water (Photolysis): To supply electrons continuously to $ \text{PS II}$, water is split on the inner side of the thylakoid membrane:

$$2 \text{H}_2 \text{O} longrightarrow 4 \text{H}^+ + \text{O}_2 + 4e^-$$

Cyclic Photophosphorylation

When only $ \text{PS I}$ is functional (occurring mainly in stroma lamellae membranes which lack $ \text{PS II}$ and $ \text{NADP}^+$ reductase enzyme), the excited electron does not pass to $ \text{NADP}^+$. Instead, it is cycled back to the $ \text{PS I}$ complex through the electron transport chain. Consequently, only ATP is synthesized, while no NADPH or $ \text{O}_2$ is produced. This cyclic flow also occurs when light wavelength beyond $680 \text{ nm}$ is provided.

3. Chemiosmotic Hypothesis (ATP Synthesis)

Proposed by Peter Mitchell, this mechanism explains ATP synthesis across the thylakoid membrane driven by a proton gradient:

• Proton Accumulation in Thylakoid Lumen: Protons ($ \text{H}^+$) accumulate inside the lumen due to three simultaneous events:
  • Water splitting occurs on the lumen side of the membrane.
  • As electrons move through cytochromes, protons are transported across the membrane from the stroma into the lumen by the hydrogen carrier ($ \text{Plastoquinone}$).
  • $ \text{NADP}^+$ reductase enzyme removes protons from the stroma to form $ \text{NADPH}$.
• Proton Motive Force: Creates a significant drop in $ \text{pH}$ inside the lumen relative to the stroma.
• ATP Generation: Breakdown of this proton gradient occurs via proton movement through the transmembrane channel of the $ \text{F}_0$ portion of $ \text{ATP Synthase}$. The conformational change in the $ \text{F}_1$ headpiece drives the enzymatic synthesis of $ \text{ATP}$ from $ \text{ADP}$ and inorganic phosphate ($ \text{P}_i$).

4. Dark Reaction (Biosynthetic Phase)

The dark reaction does not directly require light but depends strictly on the products of the light reaction ($ \text{ATP}$ and $ \text{NADPH}$) to fix $ \text{CO}_2$ into carbohydrates.

Calvin Cycle ($ \text{C}_3$ Pathway)

Occurs in all photosynthetic plants within the chloroplast stroma. It comprises three main stages:

1. Carboxylation: Fixation of $ \text{CO}_2$ into a stable $3$-carbon intermediate ($3$-phosphoglyceric acid or $ \text{PGA}$). The primary $ \text{CO}_2$ acceptor is a $5$-carbon ketose sugar, Ribulose-1,5-bisphosphate ($ \text{RuBP}$), catalyzed by $ \text{RuBP}$ carboxylase-oxygenase ($ \text{RuBisCO}$).$$ \text{RuBP (5C)} + \text{CO}_2 + \text{H}_2 \text{O} xrightarrow{ \text{RuBisCO}} 2 \times \text{3-PGA (3C)}$$
2. Reduction: A series of reactions leading to glucose formation. For fixing each $ \text{CO}_2$ molecule, $2 \text{ ATP}$ (for phosphorylation) and $2 \text{ NADPH}$ (for reduction) are consumed.
3. Regeneration: Regeneration of the $ \text{CO}_2$ acceptor molecule $ \text{RuBP}$ requires $1 \text{ ATP}$ per cycle.

Net energy requirement for synthesizing $1$ molecule of Glucose ($6 \text{ CO}_2$ fixed):

• $ \text{CO}_2$ inputs: $6$
• $ \text{ATP}$ consumed: $6 \times 3 = 18 \text{ ATP}$
• $ \text{NADPH}$ consumed: $6 \times 2 = 12 \text{ NADPH}$

Hatch-Slack Pathway ($ \text{C}_4$ Pathway)

Plants adapted to dry tropical regions (e.g., Maize, Sugarcane, Sorghum) utilize the $ \text{C}_4$ pathway to maximize $ \text{CO}_2$ fixation and prevent water loss.

• Kranz Anatomy: Leaves feature large bundle sheath cells arranged in a wreath around vascular bundles. These cells have thick walls impervious to gaseous exchange, no intercellular spaces, and high densities of agranal chloroplasts. Mesophyll cells contain granal chloroplasts.
• Primary Acceptance: In mesophyll cells, the primary $3$-carbon $ \text{CO}_2$ acceptor is Phosphoenolpyruvate ($ \text{PEP}$), catalyzed by PEP carboxylase ($ \text{PEPcase}$). Note: Mesophyll cells lack $ \text{RuBisCO}$.$$ \text{PEP (3C)} + \text{HCO}_3^- xrightarrow{ \text{PEPcase}} \text{Oxaloacetic Acid (OAA - 4C)}$$
• Transport & Decarboxylation: $ \text{OAA}$ is converted to malic acid or aspartic acid, transported into bundle sheath cells, and decarboxylated to release $ \text{CO}_2$ and pyruvate ($3 \text{C}$).
• Calvin Cycle Execution: Released $ \text{CO}_2$ enters the standard Calvin cycle in bundle sheath cells, where $ \text{RuBisCO}$ is present. Pyruvate is transported back to mesophyll cells and converted back to $ \text{PEP}$ consuming $2 \text{ ATP}$ equivalents per $ \text{CO}_2$.

Net energy requirement for synthesizing $1$ molecule of Glucose in $ \text{C}_4$ plants:

• $ \text{ATP}$ consumed: $30 \text{ ATP}$ ($18 \text{ ATP}$ for Calvin cycle + $12 \text{ ATP}$ for PEP regeneration)
• $ \text{NADPH}$ consumed: $12 \text{ NADPH}$

5. Photorespiration ($ \text{C}_2$ Cycle)

Photorespiration is a light-dependent, non-photosynthetic wasteful pathway active in $ \text{C}_3$ plants under high oxygen concentration, high temperature, and low $ \text{CO}_2$ levels.

• $ \text{RuBisCO}$ has active binding sites for both $ \text{CO}_2$ and $ \text{O}_2$. When relative $ \text{O}_2$ concentration increases, $ \text{RuBisCO}$ acts as an oxygenase.
• $ \text{RuBP}$ binds with $ \text{O}_2$ to yield $1$ molecule of Phosphoglyceric acid ($3 \text{C}$) and $1$ molecule of Phosphoglycolate ($2 \text{C}$).
• This process involves three organelles in strict sequential order: Chloroplast $ ightarrow$ Peroxisome $ ightarrow$ Mitochondria.
• Result: No ATP or NADPH is generated; instead, $ \text{CO}_2$ is released with consumption of ATP. Thus, photorespiration reduces photosynthetic efficiency. $ \text{C}_4$ plants avoid photorespiration by maintaining high $ \text{CO}_2$ concentration at the $ \text{RuBisCO}$ active site in bundle sheath cells.

Important Formulas & Key Terms Table

Solved Step-by-Step Previous Years Questions (PYQs)

Question 1 (NEET 2021)

Which of the following statements is incorrect regarding the light reactions of photosynthesis?

• (A) $ \text{PS II}$ absorbs light at $680 \text{ nm}$.
• (B) Photolysis of water occurs on the inner side of the thylakoid membrane.
• (C) $ \text{NADP}^+$ reductase enzyme is located on the stroma side of the thylakoid membrane.
• (D) Cyclic photophosphorylation involves both $ \text{PS I}$ and $ \text{PS II}$.

Correct Answer: (D)

Step-by-Step Solution:

1. Evaluate Cyclic Photophosphorylation: Cyclic photophosphorylation occurs predominantly in the stroma lamellae.
2. Stroma lamellae membranes lack both $ \text{PS II}$ and the enzyme $ \text{NADP}^+$ reductase.
3. Hence, cyclic photophosphorylation involves only $ \text{PS I}$, recirculating electrons back through the cytochrome complex to generate ATP without reducing $ \text{NADP}^+$ or splitting water.
4. Statements A, B, and C are completely accurate descriptions per NCERT text. Thus, statement D is incorrect and the correct option.

Question 2 (NEET 2020)

How many ATP and NADPH molecules are required for the synthesis of one molecule of Glucose through the Calvin cycle?

• (A) $18 \text{ ATP}$ and $12 \text{ NADPH}$
• (B) $12 \text{ ATP}$ and $18 \text{ NADPH}$
• (C) $30 \text{ ATP}$ and $12 \text{ NADPH}$
• (D) $6 \text{ ATP}$ and $6 \text{ NADPH}$

Correct Answer: (A)

Step-by-Step Solution:

1. To fix one single molecule of $ \text{CO}_2$, the Calvin cycle requires:
   • $2 \text{ ATP}$ for the reduction phase ($2 \times 3$-PGA to $2 \times$ Triose phosphate).
   • $2 \text{ NADPH}$ for the reduction phase.
   • $1 \text{ ATP}$ for the regeneration of $ \text{RuBP}$.
2. Total required per $ \text{CO}_2$ molecule $= 3 \text{ ATP} + 2 \text{ NADPH}$.
3. Synthesis of 1 Glucose ($ \text{C}_6 \text{H}_{12} \text{O}_6$) requires fixing $6$ molecules of $ \text{CO}_2$:$$ \text{Total ATP} = 6 \times 3 = 18 \text{ ATP}$$$$ \text{Total NADPH} = 6 \times 2 = 12 \text{ NADPH}$$

Question 3 (NEET 2019)

In $ \text{C}_4$ plants, carbon dioxide fixation takes place in:

• (A) Stroma of mesophyll chloroplasts
• (B) Bundle sheath cells only
• (C) Epidermal cells
• (D) Guard cells

Correct Answer: (A)

Step-by-Step Solution:

1. Initial primary $ \text{CO}_2$ fixation in $ \text{C}_4$ plants takes place in the mesophyll cytoplasm/chloroplasts where bicarbonate ($ \text{HCO}_3^-$) reacts with Phosphoenolpyruvate ($ \text{PEP}$) catalyzed by $ \text{PEPcase}$.
2. Secondary fixation via the standard Calvin cycle occurs later inside the bundle sheath cells where $ \text{RuBisCO}$ resides.
3. Therefore, the primary fixation site is the mesophyll cell. Option (A) correctly answers initial fixation.

Common NEET Traps & Mistakes to Avoid

• Trap 1: Confusing Initial $ \text{CO}_2$ Acceptor in $ \text{C}_3$ vs. $ \text{C}_4$ Pathways. Remember: $ \text{C}_3$ uses $ \text{RuBP}$ ($5 \text{C}$ sugar) while $ \text{C}_4$ uses $ \text{PEP}$ ($3 \text{C}$ compound). Do not confuse acceptor carbon numbers with the first stable product carbon numbers ($ \text{3-PGA}$ vs $ \text{OAA}$).
• Trap 2: Organelle Order in Photorespiration. Questions often randomize the structural order of photorespiration. Memorize the strict order using the trick CPM: Chloroplast $ ightarrow$ Peroxisome $ ightarrow$ Mitochondria.
• Trap 3: Products of Cyclic Photophosphorylation. Cyclic flow generates ONLY ATP. It does NOT yield NADPH, nor does it split water to evolve $ \text{O}_2$.
• Trap 4: Location of RuBisCO in $ \text{C}_4$ Plants. $ \text{RuBisCO}$ is completely absent in the mesophyll cells of $ \text{C}_4$ plants; it is exclusively localized within bundle sheath cells. Conversely, $ \text{PEPcase}$ is present in mesophyll cells.
• Trap 5: ATP Count Variations for $ \text{C}_4$ Glucose Synthesis. Standard $ \text{C}_3$ cycle takes $18 \text{ ATP}$ for $1$ glucose. $ \text{C}_4$ requires an \textra $2 \text{ ATP}$ per $ \text{CO}_2$ ($12 \text{ ATP}$ \textra total) to regenerate PEP from pyruvate. Total $ \text{ATP}$ for $ \text{C}_4$ glucose $= 18 + 12 = 30 \text{ ATP}$.

High-Yield NEET Practice MCQs with Answer Keys

Q1. Oxygenic photophosphorylation and water splitting are directly associated with:

(A) $ \text{PS I}$ located on stroma lamellae
(B) $ \text{PS II}$ located on the inner side of thylakoid membrane
(C) $ \text{RuBisCO}$ located in stroma
(D) Ferredoxin on outer thylakoid surface

Answer: (B)
Explanation: Water-splitting complex (oxygen-evolving complex) is physically associated with $ \text{PS II}$ and is located on the inner side (lumen side) of the thylakoid membrane.

Q2. Phosphoenolpyruvate carboxylase ($ \text{PEPcase}$) enzyme differs from $ \text{RuBisCO}$ because:

(A) $ \text{PEPcase}$ can bind to both $ \text{CO}_2$ and $ \text{O}_2$
(B) $ \text{PEPcase}$ operates only in high $ \text{O}_2$ conditions
(C) $ \text{PEPcase}$ lacks oxygenase activity and shows high affinity for $ \text{CO}_2$
(D) $ \text{PEPcase}$ is localized inside mitochondrial matrix

Answer: (C)
Explanation: $ \text{PEPcase}$ has no oxygenase activity, meaning it does not bind $ \text{O}_2$. This prevents wasteful photorespiration in $ \text{C}_4$ plants even under high oxygen/low carbon dioxide ratios.

Q3. During non-cyclic photophosphorylation, the primary electron acceptor from excited $ \text{P}_{680}$ is:

(A) Plastocyanin
(B) Plastoquinone
(C) Pheophytin
(D) Ferredoxin

Answer: (C)
Explanation: When $ \text{PS II}$ ($ \text{P}_{680}$) absorbs light, its excited electron is initially accepted by the primary acceptor Pheophytin, which subsequently passes it to Plastoquinone ($ \text{PQ}$).

Q4. According to Blackman's Law of Limiting Factors, if a chemical process is affected by more than one factor, its rate is determined by the factor which is:

(A) Present in maximum quantity
(B) Nearest to its minimal value
(C) Independent of ambient temperature
(D) Present in optimal concentration

Answer: (B)
Explanation: Blackman (1905) stated that if a biological process is conditioned by multiple factors, its rate is limited by the pace of the slowest factor—i.e., the factor present at sub-optimal or minimal level.

Q5. Which pigment appears blue-green in a chromatogram and acts as the chief reaction center pigment?

(A) Chlorophyll b
(B) Xanthophyll
(C) Carotenoid
(D) Chlorophyll a

Answer: (D)
Explanation: Chlorophyll a displays a distinct blue-green color on paper chromatography and serves as the primary reaction center pigment ($ \text{P}_{680}$ / $ \text{P}_{700}$).

Summary & Final NEET Revision Tips

• Light Reaction Summary: Takes place in thylakoid membranes (grana and stroma lamellae). Inputs: Light, $ \text{H}_2 \text{O}$, $ \text{ADP}$, $ \text{NADP}^+$. Outputs: $ \text{O}_2$, $ \text{ATP}$, $ \text{NADPH}$.
• $ \text{C}_3$ Cycle Quick Math: $1 \text{ CO}_2 ightarrow 3 \text{ ATP} + 2 \text{ NADPH}$. For $1$ Glucose ($6 \text{ CO}_2$) $ ightarrow 18 \text{ ATP} + 12 \text{ NADPH}$.
• $ \text{C}_4$ Cycle Quick Math: $1 \text{ CO}_2 ightarrow 5 \text{ ATP} + 2 \text{ NADPH}$. For $1$ Glucose ($6 \text{ CO}_2$) $ ightarrow 30 \text{ ATP} + 12 \text{ NADPH}$.
• Photorespiration Sequence Trick: Chloroplast $ ightarrow$ Peroxisome $ ightarrow$ Mitochondria (CPM). Consumes $ \text{ATP}$ and $ \text{O}_2$, releases $ \text{CO}_2$, produces zero sugar or $ \text{NADPH}$.
• Key NCERT Lines to Remember: Light saturation occurs at 10% of full sunlight. Carbon dioxide is the major limiting factor in nature for photosynthesis, as atmospheric levels ($0.03%$ to $0.04%$) are below saturation levels for $ \text{C}_3$ plants.