Photosynthesis in Higher Plants – Complete Notes | Biology

Autotrophic Nutrition & Types

Autotrophic vs Heterotrophic

Autotrophic — organisms that synthesize their own food from inorganic materials using an external energy source
Heterotrophic — organisms that depend on other organisms for food (cannot synthesize own food)

Type	Energy Source	Pigment	H-donor	O₂ produced?	Examples
Photoautotrophic (Oxygenic)	Sunlight	Chlorophyll (bacteriochlorophyll absent)	H₂O	Yes (O₂ byproduct)	Cyanobacteria, Algae, All plants
Photoautotrophic (Anoxygenic)	Sunlight	Bacteriochlorophyll + Carotenoids	H₂S, organic acids	No	Green sulphur bacteria, Purple sulphur bacteria, Green non-sulphur bacteria, Purple non-sulphur bacteria
Chemoautotrophic	Chemical oxidation reactions	Absent	Inorganic compounds (NH₃, H₂S, Fe²⁺)	No	Nitrifying bacteria (Nitrosomonas, Nitrobacter)

Key Points on Autotrophic Nutrition

All photoautotrophic bacteria have bacteriochlorophyll, NOT chlorophyll a
All O₂-producing organisms have chlorophyll — no exceptions
All photosynthetic higher plants have 4 types of pigments: Chl a, Chl b, Carotenoids (Carotene + Xanthophyll)
Universal pigment = Chlorophyll a (reaction centre in all photosystems)
Final equation of photosynthesis (corrected by Van Niel): 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
All O₂ released comes from water (proved by 18O isotope labelling)

Important Scientists & Experiments

Scientist	Contribution
Priestley (1770)	Mint plant can restore air quality damaged by candle/mouse — first showed plants purify air
Jan Ingenhousz (1779)	Repeated Priestley's experiment — showed only green parts of plant purify air AND only in sunlight (not in dark)
T.W. Engelmann	First action spectrum using prism, Cladophora algae + aerobic bacteria — bacteria accumulated at Blue and Red regions (most photosynthesis there)
Julius von Sachs (1854)	Glucose is formed during photosynthesis and stored as starch; chlorophyll is present in green bodies (chloroplasts)
Van Niel (1931)	Light is important for photosynthesis; source of O₂ is H₂O (not CO₂); corrected photosynthesis equation
Calvin (Melvin Calvin)	Used radioactive ¹⁴C isotope tracer technique with Chlorella algae to discover the Dark Reaction pathway (Calvin Cycle); first stable product = 3-C PGA
Hatch & Slack	Discovered C4 pathway (first stable product = 4-C OAA) in plants like sugarcane

Location of Photosynthesis & Chloroplast

Location in Plant

Photosynthesis occurs in all green parts of plant — mainly leaves
Epidermal cells do NOT have chloroplasts — except Guard cells (do have chloroplasts)
In monocot leaf — both upper and lower mesophyll cells have chloroplasts (no differentiation)
In dicot leaf — mainly mesophyll cells (palisade + spongy); palisade = more photosynthesis
Each mesophyll cell has 20–40 chloroplasts

Chloroplast Structure

Double membrane bound organelle
Outer membrane — permeable
Inner membrane — selectively permeable
Stroma — fluid-filled matrix; contains enzymes for Dark Reaction (RUBISCO etc.), DNA, ribosomes
Thylakoid membranes — system of membranes inside stroma; contain light-absorbing pigments
Grana — stacks of thylakoids (like stack of coins); singular = granum; site of Light Reaction
Stroma lamellae — unstacked membranes connecting grana; also called fret channels
Lumen — inside space of thylakoid

Photosynthetic Pigments

4 Pigments in Higher Plants

Chlorophyll a (Chl a) — C₅₅H₇₂O₅N₄Mg; blue-green colour; Universal pigment; forms Reaction Centre; absorbs blue (430 nm) and red (680–700 nm)
Chlorophyll b (Chl b) — C₅₅H₇₀O₆N₄Mg; yellow-green colour; Antenna pigment; differs from Chl a by CHO group instead of CH₃
β-Carotene (Carotenoid) — C₄₀H₅₆; orange-yellow colour; Antenna pigment; absorbs blue-violet region
Xanthophyll (Carotenoid) — C₄₀H₅₆O₂; yellow colour; Antenna pigment

Pigment Separation — Paper Chromatography

Pigments separated by paper chromatography in order of Rf value (top to bottom):
Carotene → highest Rf (most soluble in solvent, moves farthest) → orange
Xanthophyll → yellow
Chlorophyll a → blue-green
Chlorophyll b → lowest Rf (least soluble) → yellow-green

Action Spectrum vs Absorption Spectrum

Absorption spectrum — wavelengths absorbed by a pigment (plotted as absorbance vs wavelength)
Action spectrum — rate of photosynthesis at different wavelengths of light
Both Chl a and Chl b — maximum absorption in blue (430–450 nm) and red (640–680 nm)
Carotene and Xanthophyll — absorb mainly in blue-violet region
Green light is least absorbed — reflected back (that's why plants look green)
PAR = Photosynthetically Active Radiation = 400–700 nm
Action spectrum shape closely matches absorption spectrum of Chl a — confirming Chl a is the main photosynthetic pigment
Middle green region — no absorption by Chl a/b, but carotenoids transfer energy → action spectrum still shows some photosynthesis there

Photosystems

Feature	PS I (Photosystem I)	PS II (Photosystem II)
Discovery order	Discovered First	Discovered Second (but acts first in Z-scheme)
Reaction Centre	P700 (works at 700 nm)	P680 (works at 680 nm)
Location	Stroma lamellae (unstacked)	Granal thylakoid (stacked)
Antennae	Chl a, Chl b, Carotenoids	Chl a, Chl b, Carotenoids
Primary electron acceptor	FeS protein (Iron-sulphur)	Pheophytin
Role	Reduces NADP⁺ → NADPH; cyclic photophosphorylation	Photolysis of water; non-cyclic electron flow starts here

LHC — Light Harvesting Complex

LHC (Antenna complex) = hundreds of accessory pigment molecules that absorb light and funnel energy to the Reaction Centre
Reaction Centre = special Chl a molecule (P680 or P700) with primary electron acceptor + enzymes
Photosystem = LHC + Reaction Centre together
LHC is bigger than Reaction Centre
Emerson's Red Drop — below 680 nm single wavelength → quantum yield drops sharply
Emerson's Enhancement Effect — two wavelengths (680 nm + 700 nm) together → more photosynthesis than sum of each separately → proved two photosystems work together

Light Reaction

Overview

Also called Photochemical phase / Hill Reaction
Location: Thylakoid membrane (Granal thylakoid)
Products: ATP, NADPH, O₂
O₂ is a by-product (not a product used for glucose synthesis)
NADP⁺ is coenzyme = Nicotinamide Adenine Dinucleotide Phosphate
Reaction: NADP⁺ + H⁺ + 2e⁻ → NADPH

Non-Cyclic Electron Flow (Z-scheme) — Main Pathway

Steps of Non-Cyclic Flow

Light (≤680 nm) hits PS II (P680) → electrons excited → passed to primary acceptor Pheophytin
Electrons travel through ETS: Pheophytin → PQ (Plastoquinone) → Cytochrome b6f complex → PC (Plastocyanin) → PS I (P700)
Simultaneously PS II gap filled by electrons from Photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂ (occurs on lumenal side of thylakoid)
Light also hits PS I (P700) → electrons re-excited → passed to primary acceptor FeS protein
Electrons travel: FeS → Fd (Ferredoxin) → FNR (Ferredoxin NADP Reductase) → reduces NADP⁺ to NADPH
Products: ATP + NADPH + O₂ (ATP via chemiosmosis)
Called Z-scheme because when plotted on redox potential graph, path of electrons forms a Z-shape
This is Non-cyclic photophosphorylation — electrons do not return to PS II

Cyclic Electron Flow

Cyclic Flow — When light >680 nm (only PS I active)

Only PS I is activated; PS II is NOT involved
Pathway: PS I → FeS → Fd → PQ → Cytochrome b6f → PC → back to PS I
Electrons cycle back to PS I — hence "cyclic"
Products: Only ATP (NO NADPH, NO O₂)
This is Cyclic photophosphorylation
Occurs in stroma lamellae (not granal thylakoid)
Photolysis does NOT occur here
Inhibitor of PS II: DCMU (Dichloromethyl Urea) — blocks PQ; stops non-cyclic flow

Chemiosmotic Hypothesis — ATP Synthesis in Chloroplast

4 requirements for ATP synthesis: H⁺ gradient + Intact membrane + CF₀-CF₁ (ATP synthase) + Energy
3 ways proton gradient is created in chloroplast:
1. Photolysis of water in lumen → H⁺ released into lumen
2. PQ carries H⁺ from stroma to lumen as it transfers electrons
3. NADP⁺ reduction in stroma consumes H⁺ from stroma side
Result: High H⁺ in lumen, low in stroma → H⁺ flows through CF₀-CF₁ ATP synthase (thylakoid membrane) → ATP synthesis = Photophosphorylation
Location of ATP synthesis: Thylakoid membrane

Q. What is the difference between Cyclic and Non-cyclic photophosphorylation?

✅ Answer: Cyclic — only PS I, only ATP; Non-cyclic — both PS I and PS II, produces ATP + NADPH + O₂

Explanation

In non-cyclic photophosphorylation, both PS II and PS I work together. Electrons flow from water → PS II → ETS → PS I → NADPH. This one-way flow produces ATP (by chemiosmosis), NADPH (for dark reaction) and O₂ (by-product of water splitting). In cyclic photophosphorylation, only PS I is active (when light wavelength exceeds 680 nm). Electrons excited in PS I flow through a cyclic route via ferredoxin, PQ, cytochrome b6f, and plastocyanin back to PS I. This generates only ATP — no NADPH, no O₂. Cyclic flow serves mainly to produce extra ATP when the cell's ATP:NADPH ratio is low. Location of cyclic flow is stroma lamellae while non-cyclic occurs in granal thylakoid.

Dark Reaction — Calvin Cycle (C3 Cycle)

Overview

Also called Biosynthetic phase / Carbon fixation / C3 cycle
Dark Reaction is a misnomer — it does NOT directly require light but can occur in light; uses ATP and NADPH from light reaction
Location: Stroma of chloroplast
Discovered by Calvin using ¹⁴C isotope in Chlorella algae
First stable product: 3-C PGA (3-Phosphoglyceric acid) → so called C3 cycle; plants using this = C3 plants
Primary CO₂ acceptor: RuBP (Ribulose-1,5-bisphosphate) — 5C compound (ketose)
Key enzyme: RUBISCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase) — most abundant enzyme/protein on Earth

3 Steps of Calvin Cycle

Step 1 — Carboxylation

CO₂ + RuBP (5C) → 2 molecules of 3-C PGA (3-phosphoglyceric acid)
Enzyme: RUBISCO (acting as carboxylase)
This is the CO₂ fixation step

Step 2 — Reduction

3-C PGA → reduced to G3P (Glyceraldehyde-3-phosphate) / PGAL (3-phosphoglyceraldehyde)
Requires: ATP + NADPH (from light reaction)
Sugar is formed at this step

Step 3 — Regeneration of RuBP

G3P → regenerated back to RuBP (5C)
Requires: ATP
Enzyme: RUBISCO; involves phosphorylation

ATP & NADPH Budget for One Glucose (6 turns of Calvin cycle)

1 turn of Calvin cycle: 3 ATP + 2 NADPH
For 1 glucose (6 CO₂ fixed = 6 turns): 18 ATP + 12 NADPH
In C4 plants: extra 2 ATP per CO₂ fixed (for C4 cycle) = 30 ATP + 12 NADPH total for 1 glucose

Q. What is RUBISCO and why is it significant?

✅ Answer: RUBISCO = Ribulose-1,5-bisphosphate Carboxylase/Oxygenase; it is the most abundant enzyme/protein on Earth; catalyzes CO₂ fixation in Calvin cycle

Explanation

RUBISCO is the key enzyme of the Calvin cycle that catalyzes the fixation of CO₂ onto the 5-carbon RuBP molecule. It is the most abundant protein on Earth — making up about 50% of all soluble leaf protein. It can act as BOTH a carboxylase (when CO₂ concentration is high, it fixes CO₂ → PGA) and an oxygenase (when O₂ concentration is high, it uses O₂ → PGA + phosphoglycolate). The oxygenase activity leads to photorespiration — a wasteful process. C4 plants evolved a mechanism to saturate RUBISCO with CO₂ (in bundle sheath cells) so it acts primarily as a carboxylase, minimizing photorespiration. RUBISCO is present in the stroma of chloroplasts in C3 plants, but only in bundle sheath cell chloroplasts in C4 plants.

C4 Plants — Hatch & Slack Pathway

Overview of C4 Plants

Plants where first stable product of photosynthesis is a 4-carbon compound OAA (Oxaloacetic Acid)
Discovered by Hatch and Slack
Special anatomy: Kranz Anatomy — bundle sheath cells (BSC) arranged in a wreath around vascular bundle; BSC have no intercellular spaces
Two types of cells: Mesophyll cells (MC) + Bundle Sheath cells (BSC)
Examples: Sugarcane, Sorghum, Maize (not wheat, not rice)
Optimum temperature: 30–40°C

Steps of C4 Pathway

Mesophyll cells: CO₂ + PEP (3C) → OAA (4C) — enzyme: PEP Carboxylase (PEPcase) (only carboxylase, no oxygenase activity)
OAA → Malic acid (4C) — in mesophyll chloroplast
Malic acid transported from MC → BSC through plasmodesmata
Bundle Sheath cells: Malic acid → Decarboxylation → CO₂ + Pyruvic acid (3C)
CO₂ released in BSC → enters Calvin/C3 cycle (RUBISCO present only in BSC)
Pyruvic acid transported back to MC → regenerated to PEP (requires ATP)

Feature	C3 Plants	C4 Plants
First stable product	3-C PGA	4-C OAA
Primary CO₂ acceptor	RuBP (5C)	PEP (3C)
CO₂ fixation enzyme	RUBISCO (carboxylase + oxygenase)	PEPcase in MC (carboxylase only); RUBISCO in BSC
Kranz anatomy	Absent	Present
Photorespiration	Present (major loss)	Absent/negligible
Optimum temperature	25–30°C	30–40°C
Compensation point (CO₂)	25–100 ppm	0–10 ppm (very low)
ATP per glucose	18 ATP	30 ATP (extra for C4 cycle)
Chloroplasts in BSC	No BSC	Agranal (no grana; only stroma lamellae)
Examples	Wheat, Rice, Potato, Sunflower	Sugarcane, Maize, Sorghum, Amaranthus

Q. Why do C4 plants have negligible photorespiration compared to C3 plants?

✅ Answer: In C4 plants, CO₂ is concentrated in BSC cells via C4 cycle, keeping CO₂ concentration high around RUBISCO — RUBISCO then acts as carboxylase only, not oxygenase

Explanation

RUBISCO has dual activity — carboxylase (fixes CO₂) and oxygenase (uses O₂, leading to photorespiration). In C3 plants, RUBISCO is in mesophyll cells exposed to atmospheric CO₂/O₂ — when O₂ concentration is high (e.g., hot days, high light), RUBISCO acts as oxygenase → photorespiration occurs, wasting energy. In C4 plants, PEPcase in mesophyll cells fixes CO₂ into OAA/malate even at very low CO₂ concentrations. This malate is transported to BSC where it releases CO₂ → very high local CO₂ concentration around RUBISCO. At high CO₂, RUBISCO preferentially acts as carboxylase, not oxygenase → photorespiration is suppressed. This gives C4 plants a major advantage in hot, dry, high-light environments (like tropics), which is why sugarcane and maize thrive there.

CAM Plants

CAM = Crassulacean Acid Metabolism

CAM plants are Xerophytes (desert plants) — succulent plants
Examples: Bryophyllum, Cactus, Agave, Pineapple, Aloe
To avoid water loss — stomata open at NIGHT (CO₂ absorbed at night, not day)
At night: stomata open → CO₂ + PEP → OAA (malic acid) → stored in vacuole as Malic acid
At day: stomata closed → malic acid decarboxylated → CO₂ released → enters Calvin/C3 cycle in same cell
Temporal separation (time-based) vs C4 plants' spatial separation (space-based)
C3 cycle does occur in CAM plants (not at night — during day)
All CAM plants are xerophytes (halophytes = salt plants — different)

Photorespiration

Photorespiration — Wasteful Process in C3 Plants

Occurs when RUBISCO acts as oxygenase (O₂ concentration high, CO₂ concentration low)
RuBP + O₂ → 1 PGA (3C) + 1 Phosphoglycolate (2C) (by RUBISCO as oxygenase)
Phosphoglycolate → converted to glycolate → enters peroxisome → then mitochondria → CO₂ released without ATP production
Organelles involved: Chloroplast + Peroxisome + Mitochondria
Conditions causing photorespiration: High O₂, Low CO₂, High temperature, High light intensity (conditions at equator on hot afternoons)
No ATP produced, no NADPH produced — purely wasteful for plant
C4 plants — no photorespiration; C3 plants — significant photorespiration

Factors Affecting Photosynthesis

Blackman's Law of Limiting Factors

When a process is governed by a number of factors, the rate of the process is limited by the factor present in the minimum (limiting factor). Rate depends on the slowest step.

Factor	Effect on Photosynthesis	Key Details
Light Intensity	Rate ∝ light intensity (up to saturation point)	Compensation point = light intensity where photosynthesis rate = respiration rate (net CO₂ exchange = 0); Shade plants have lower compensation point; Beyond saturation point → photoinhibition (damage)
CO₂ Concentration	↑CO₂ → ↑photosynthesis up to saturation point	Atmospheric CO₂ = 0.03–0.04% (380 ppm); Compensation point (C3) = 25–100 ppm; Compensation point (C4) = 0–10 ppm; Saturation point = 1200–1500 ppm; Greenhouse gas increasing CO₂ benefits C3 plants more
Temperature	Dark reaction (enzyme-dependent) more affected by temperature than light reaction	Optimum temp for C3 = 25–30°C; for C4 = 30–40°C; Above 35°C, RUBISCO activity decreases; Light reaction enzymes not much affected (physical/photochemical process)
Water	Indirect effect — water stress → stomata close → CO₂ not entering → photosynthesis stops	Direct requirement for photolysis; afternoon stomatal closure during water stress reduces CO₂ entry

Shade Plants vs Sun Plants

Shade plants (Sciophytes): low saturation point, damaged by high light; e.g. Ferns, shade-tolerant herbs
Sun plants (Heliophytes): high saturation point, require high light; e.g. Dalbergia, Croton
Shade plant chloroplasts: more and larger; Sun plant chloroplasts: fewer

Quick Revision — Must-Know Points

All O₂ released → from Water (H₂O)

Universal pigment → Chlorophyll a

Reaction centre PS I → P700

Reaction centre PS II → P680

Primary acceptor PS II → Pheophytin

Primary acceptor PS I → FeS protein

Cyclic flow product → ATP only

Non-cyclic products → ATP + NADPH + O₂

C3 first product → 3-C PGA

C4 first product → 4-C OAA

RUBISCO = most abundant → Enzyme/Protein on Earth

CAM plants stomata open → Night

Kranz anatomy → C4 plants only

Photorespiration organelles → Chloroplast + Peroxisome + Mitochondria

Carotene Rf → Highest in chromatography

Emerson's enhancement → Two wavelengths together → more PS

Blackman's Law → Limiting factor governs rate

Photolysis of water site → Lumenal side of thylakoid