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Short Notes Photosynthesis in Higher Plants - Short Notes for NEET
Definition and Overview
- Photosynthesis: Process by which autotrophs convert light energy into chemical energy (glucose)
- Equation: 6CO2 + 12H2O + Light energy → C6H12O6 + 6O2 + 6H2O
- Anabolic process: Building complex molecules from simpler ones
- Endergonic process: Requires energy input
Site of Photosynthesis
| Level | Location | Details |
|---|---|---|
| Organ | Leaves (mainly) | Also occurs in green stems and sepals |
| Tissue | Mesophyll cells | Palisade and spongy parenchyma |
| Organelle | Chloroplast | Double membrane-bound; contains chlorophyll |
| Sub-organelle | Grana (thylakoids) | Light reactions occur here |
| Sub-organelle | Stroma | Dark reactions (Calvin cycle) occur here |
Chloroplast Structure
- Outer membrane: Permeable
- Inner membrane: Selectively permeable
- Stroma: Fluid-filled matrix; contains enzymes, DNA, ribosomes
- Thylakoids: Flattened membranous sacs; contain photosynthetic pigments
- Grana: Stacks of thylakoids (singular: granum)
- Stroma lamellae: Thylakoids connecting different grana
Photosynthetic Pigments
| Pigment Type | Examples | Color | Light Absorbed |
|---|---|---|---|
| Chlorophyll a | Primary pigment | Blue-green | Blue (430 nm) and Red (662 nm) |
| Chlorophyll b | Accessory pigment | Yellow-green | Blue (453 nm) and Red (642 nm) |
| Carotenoids | Xanthophylls (yellow) Carotenes (orange) | Yellow-orange | Blue-green light (450-570 nm) |
- Chlorophyll a: Reaction center pigment; directly participates in light reactions
- Accessory pigments: Chlorophyll b and carotenoids; absorb light and transfer energy to chlorophyll a; broaden absorption spectrum
- Absorption spectrum: Graph showing wavelengths absorbed by pigments
- Action spectrum: Graph showing effectiveness of different wavelengths in driving photosynthesis
Overview of Photosynthesis Phases
| Phase | Also Called | Location | Light Required? | Products |
|---|---|---|---|---|
| Light Reaction | Photochemical phase; Hill reaction | Grana (thylakoids) | Yes | ATP, NADPH, O2 |
| Dark Reaction | Biosynthetic phase; Calvin cycle; C3 cycle | Stroma | No (but occurs in light too) | Glucose (carbohydrates) |
Light Reaction (Photochemical Phase)
Key Events in Light Reaction
- Photolysis of water: 2H2O → 4H+ + 4e- + O2
- Oxygen evolution: O2 released as by-product
- Photophosphorylation: Formation of ATP using light energy
- Reduction of NADP+: NADP+ + 2e- + H+ → NADPH + H+
Photosystems
| Feature | Photosystem I (PS I) | Photosystem II (PS II) |
|---|---|---|
| Reaction center | P700 (absorbs 700 nm) | P680 (absorbs 680 nm) |
| Location | Unstacked thylakoid regions | Stacked thylakoid regions (grana) |
| Function | Reduces NADP+ to NADPH | Photolysis of water; O2 evolution |
| First to evolve | Yes (evolutionarily older) | No (evolved later) |
| First to function | No (in Z-scheme) | Yes (in Z-scheme) |
Non-Cyclic Photophosphorylation (Z-Scheme)
- Most common pathway
- Both PS I and PS II involved
- Electron flow: Unidirectional (non-cyclic)
- Products: ATP + NADPH + O2
Steps of Non-Cyclic Photophosphorylation:
- Light excites PS II (P680); electrons ejected to high energy level
- Photolysis of water replaces lost electrons: H2O → 2H+ + ½O2 + 2e-
- Electrons pass through electron transport chain: Pheophytin → Plastoquinone (PQ) → Cytochrome b6f complex → Plastocyanin (PC)
- Energy released during electron transport pumps H+ into thylakoid lumen
- ATP synthesis via chemiosmosis (ATP synthase)
- Electrons reach PS I (P700)
- Light excites PS I; electrons ejected again
- Electrons pass through: Ferredoxin (Fd) → Ferredoxin-NADP reductase (FNR)
- NADP+ reduced to NADPH
Cyclic Photophosphorylation
- Only PS I involved
- Electron flow: Cyclic (returns to PS I)
- Products: Only ATP (no NADPH, no O2)
- Occurs when: NADP+ is not available; extra ATP needed
Steps of Cyclic Photophosphorylation:
- Light excites PS I (P700)
- Electrons ejected to ferredoxin
- Instead of going to NADP+, electrons return to PS I via: Cytochrome b6f complex → Plastocyanin
- ATP generated during electron transport
- No photolysis, no NADPH, no O2
Comparison: Cyclic vs Non-Cyclic Photophosphorylation
| Feature | Non-Cyclic | Cyclic |
|---|---|---|
| Photosystems | Both PS I and PS II | Only PS I |
| Electron flow | Unidirectional (H2O → NADP+) | Cyclic (returns to PS I) |
| Photolysis | Occurs | Does not occur |
| O2 evolution | Yes | No |
| ATP formation | Yes | Yes |
| NADPH formation | Yes | No |
Chemiosmotic Hypothesis (Peter Mitchell)
- Explains ATP synthesis during photophosphorylation
- Mechanism:
- Electron transport creates proton gradient across thylakoid membrane
- H+ accumulates in thylakoid lumen (low pH, high H+)
- Stroma has low H+ concentration (high pH)
- Proton motive force created
- H+ flows down gradient through ATP synthase (CF0-CF1 complex)
- Energy released used to phosphorylate ADP to ATP
- ATP synthase: CF0 (membrane channel) + CF1 (catalytic head)
Dark Reaction (Biosynthetic Phase) - Calvin Cycle
Overview
- Discovered by Melvin Calvin, Benson, and Bassham
- Also called C3 cycle (first stable product is 3-carbon compound)
- Occurs in stroma of chloroplast
- Light-independent but requires ATP and NADPH from light reaction
- Uses CO2 to synthesize glucose
Three Stages of Calvin Cycle
| Stage | Key Events | Enzyme |
|---|---|---|
| 1. Carboxylation (CO2 fixation) | • CO2 acceptor: RuBP (Ribulose 1,5-bisphosphate) - 5C • CO2 + RuBP → 2 molecules of 3-PGA (3-phosphoglyceric acid) - 3C • First stable product: 3-PGA • Rate-limiting step | RuBisCO (Ribulose bisphosphate carboxylase-oxygenase) Most abundant enzyme on Earth |
| 2. Reduction | • 3-PGA reduced to G3P (Glyceraldehyde 3-phosphate) - 3C • Uses ATP and NADPH from light reaction • 3-PGA + ATP → 1,3-bisphosphoglycerate • 1,3-bisphosphoglycerate + NADPH → G3P • Some G3P used to form glucose; rest regenerates RuBP | Phosphoglycerate kinase G3P dehydrogenase |
| 3. Regeneration | • G3P molecules rearranged to regenerate RuBP • Requires ATP • Cycle continues | Multiple enzymes including RuBP kinase |
Calvin Cycle - Quantitative Details
- For 1 glucose molecule (6C):
- 6 turns of Calvin cycle needed
- 6 CO2 fixed
- 18 ATP consumed
- 12 NADPH consumed
- For 1 turn of cycle:
- 3 ATP and 2 NADPH used
- 1 CO2 fixed
C4 Pathway (Hatch-Slack Pathway)
Overview
- Discovered by Hatch and Slack
- Found in tropical plants adapted to high temperature, low CO2, and high light intensity
- First stable product: OAA (Oxaloacetic acid) - 4-carbon compound
- Spatial separation: CO2 fixation in mesophyll cells; Calvin cycle in bundle sheath cells
C4 Plant Anatomy
- Kranz anatomy: Special leaf structure
- Bundle sheath cells surround vascular bundles
- Mesophyll cells surround bundle sheath cells
- Large bundle sheath cells with many chloroplasts
- Chloroplasts in bundle sheath lack grana (or reduced grana)
Steps of C4 Pathway
| Step | Location | Process | Enzyme |
|---|---|---|---|
| 1. CO2 fixation | Mesophyll cells | CO2 + PEP (3C) → OAA (4C) First stable product | PEP carboxylase (High affinity for CO2; no oxygenase activity) |
| 2. OAA reduction | Mesophyll cells | OAA → Malate (4C) or Aspartate (4C) | Malate dehydrogenase |
| 3. Transport | Between cells | Malate/Aspartate transported to bundle sheath cells | - |
| 4. Decarboxylation | Bundle sheath cells | Malate → Pyruvate (3C) + CO2 CO2 released | Malic enzyme |
| 5. Calvin cycle | Bundle sheath cells | CO2 enters Calvin cycle; fixed by RuBisCO | RuBisCO |
| 6. Pyruvate return | Back to mesophyll | Pyruvate → PEP (regenerated) Uses ATP | Pyruvate phosphate dikinase |
Advantages of C4 Pathway
- More efficient CO2 fixation in high temperature and light
- No photorespiration (PEP carboxylase has no oxygenase activity)
- High CO2 concentration in bundle sheath cells favors carboxylase activity of RuBisCO
- Better water use efficiency (stomata can remain partially closed)
- Higher productivity in tropical conditions
Examples of C4 Plants
- Maize (corn), Sugarcane, Sorghum, Amaranthus
Comparison: C3 vs C4 Plants
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First stable product | 3-PGA (3C) | OAA (4C) |
| CO2 fixation enzyme | RuBisCO | PEP carboxylase (then RuBisCO) |
| Cell types involved | Mesophyll only | Mesophyll + Bundle sheath |
| Kranz anatomy | Absent | Present |
| Photorespiration | Occurs (high rate) | Negligible/absent |
| CO2 compensation point | High (25-100 ppm) | Low (0-10 ppm) |
| Optimum temperature | 20-25°C | 30-40°C |
| Water use efficiency | Lower | Higher |
| Examples | Wheat, Rice, Soybean, most trees | Maize, Sugarcane, Sorghum |
Photorespiration
- Definition: Light-dependent uptake of O2 and release of CO2 by photosynthetic tissues
- Occurs in: C3 plants (minimal in C4 plants)
- Location: Chloroplasts, peroxisomes, and mitochondria
- Cause: RuBisCO acts as oxygenase when O2/CO2ratio is high
- RuBP + O2 → 3-PGA (3C) + Phosphoglycolate (2C)
- Consequences:
- Wasteful process - no ATP or NADPH produced
- Loss of fixed carbon
- Reduces photosynthetic efficiency by 25-50%
- Conditions favoring photorespiration:
- High temperature
- High light intensity
- Low CO2 concentration
- High O2 concentration
Factors Affecting Photosynthesis
Law of Limiting Factors (Blackman's Law)
- When a process depends on multiple factors, rate is limited by the factor present in minimum quantity
- At any given time, only one factor is limiting
| Factor | Effect on Photosynthesis |
|---|---|
| Light Intensity | • Increases rate up to saturation point (light saturation) • Beyond saturation, no effect (other factors become limiting) • Very high intensity can damage chlorophyll • C4 plants have higher saturation point than C3 |
| CO2 Concentration | • Normal atmospheric CO2: 0.03-0.04% (300-400 ppm) • Increases rate up to 0.5% (saturation) • Beyond 1%, inhibitory • Major limiting factor in C3 plants |
| Temperature | • Optimum: 25-35°C for C3; 30-45°C for C4 • Below optimum: rate decreases (enzyme activity low) • Above optimum: enzymes denature; rate decreases • Affects dark reaction more (enzymatic) |
| Water | • Essential as raw material (photolysis) • Maintains turgidity; stomatal opening • Water stress closes stomata; reduces CO2 uptake |
| Chlorophyll | • More chlorophyll → higher photosynthesis • Deficiency reduces light absorption |
| O2 | • High O2 inhibits photosynthesis (competitive inhibition) • Promotes photorespiration in C3 plants |
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FAQs on Short Notes Photosynthesis in Higher Plants - Short Notes for NEET
| 1. What is the overall equation for photosynthesis in higher plants? |  |
Ans. The overall equation for photosynthesis in higher plants can be expressed as 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. This indicates that carbon dioxide and water, in the presence of light energy, are converted into glucose and oxygen.
| 2. What are the stages of photosynthesis? | |
Ans. Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). The light-dependent reactions take place in the thylakoid membranes and convert light energy into chemical energy in the form of ATP and NADPH. The light-independent reactions occur in the stroma, where ATP and NADPH are used to convert carbon dioxide into glucose.
| 3. How does chlorophyll function in photosynthesis? | |
Ans. Chlorophyll is the green pigment found in chloroplasts that plays a crucial role in photosynthesis by absorbing light, primarily in the blue and red wavelengths. It facilitates the conversion of light energy into chemical energy by exciting electrons when they absorb photons, which then participate in the electron transport chain during light-dependent reactions.
| 4. What role does water play in photosynthesis? | |
Ans. Water serves as a raw material for photosynthesis. It is split during the light-dependent reactions to provide electrons and protons, releasing oxygen as a by-product. The equation for this process is 2H₂O → 4H⁺ + 4e⁻ + O₂. Water also helps maintain turgor pressure in plant cells, which is essential for structural support.
| 5. Why is photosynthesis significant for life on Earth? | |
Ans. Photosynthesis is significant for life on Earth as it is the primary source of organic matter for almost all organisms. It produces glucose, which serves as food for plants and, indirectly, for animals. Additionally, photosynthesis generates oxygen as a by-product, which is essential for the respiration of most living organisms, thus supporting life and maintaining the balance of oxygen and carbon dioxide in the atmosphere.
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