PHOTOSYNTHESIS IN HIGHER PLANTS · Comparison Tables

1. Light Reactions vs Carbon Reactions

Feature	Light reactions	Carbon reactions / Calvin cycle
Site	Thylakoid/granal membrane system	Chloroplast stroma
Direct energy source	Light	ATP and NADPH supplied by light reactions
Main events	Light absorption, water splitting, O₂ release, electron transport, ATP and NADPH formation	CO₂ fixation, reduction to carbohydrate and RuBP regeneration
Products	ATP, NADPH and O₂	Triose phosphate/carbohydrate; ADP, Pi and NADP⁺ returned
Misleading name	Also called photochemical reactions	Conventionally called dark reactions, but do not occur only in darkness

2. Chlorophyll a vs Accessory Pigments

Feature	Chlorophyll a	Chlorophyll b, xanthophylls and carotenoids
Status	Chief photosynthetic pigment	Accessory pigments
Colour in chromatogram	Bright/blue green	Chlorophyll b: yellow green; xanthophylls: yellow; carotenoids: yellow to yellow-orange
Role	Reaction-centre pigment that initiates photochemical events	Absorb additional wavelengths and transfer energy to chlorophyll a
Protection	Receives transferred energy	Protect chlorophyll a from photo-oxidation

Spectrum clue The action spectrum shows highest photosynthesis mainly in blue and red light, demonstrating the contribution of all pigments.

3. Photosystem II vs Photosystem I

Feature	Photosystem II	Photosystem I
Reaction centre	P680 chlorophyll a	P700 chlorophyll a
Peak red-light absorption	680 nm	700 nm
Electron source/destination	Receives replacement electrons from photolysis of water	Passes re-excited electrons to reduce NADP⁺ in non-cyclic flow
Water splitting and O₂ evolution	Associated with PS II	Not associated with PS I
Role in flow	Starts non-cyclic electron transport	Participates in non-cyclic flow and alone participates in cyclic flow

4. Non-cyclic vs Cyclic Photophosphorylation

Feature	Non-cyclic	Cyclic
Photosystems involved	PS II and PS I	PS I only
Electron path	Electrons move from water through PS II and PS I to NADP⁺	Excited electrons return through carriers to PS I
ATP formation	Yes	Yes
NADPH formation	Yes	No
O₂ evolution	Yes, due to water splitting	No
Significance	Provides both ATP and NADPH for carbon fixation	Provides extra ATP to meet greater ATP requirement of Calvin cycle

5. Three Stages of Calvin Cycle

Stage	What happens	Key molecule / energy use
Carboxylation	CO₂ combines with RuBP to form stable organic intermediate and two molecules of 3-PGA	RuBisCO; RuBP is 5-carbon acceptor
Reduction	Intermediates convert to carbohydrate/triose phosphate	2 ATP and 2 NADPH used per CO₂ fixed
Regeneration	CO₂ acceptor RuBP is reformed	1 ATP used per CO₂ fixed
Overall cost	Per CO₂: 3 ATP + 2 NADPH. Per glucose from 6 CO₂: 18 ATP + 12 NADPH.

6. C3 Plants vs C4 Plants

Feature	C3 plants	C4 plants
First stable fixation product	3-PGA (3-carbon acid)	OAA (4-carbon acid)
Primary CO₂ acceptor	RuBP, 5-carbon	PEP, 3-carbon
Initial carboxylation enzyme and site	RuBisCO in mesophyll cells	PEP carboxylase in mesophyll cells
Calvin cycle site	Mesophyll cells	Bundle sheath cells
Leaf anatomy	No Kranz anatomy	Kranz anatomy; thick-walled, chloroplast-rich bundle sheath cells without intercellular spaces
Photorespiration	Present	Absent/negligible
Temperature and productivity	Lower temperature optimum; lower high-light productivity	Tolerates high temperature, high light and shows greater biomass productivity

7. Carboxylase vs Oxygenase Activity of RuBisCO

Feature	Carboxylase activity	Oxygenase activity / photorespiration
Competing gas bound to RuBisCO	CO₂	O₂
RuBP product	Two molecules of 3-PGA	One phosphoglycerate and one 2-carbon phosphoglycolate
Energy and carbon consequence	Supports sugar formation in Calvin cycle	Uses ATP and releases CO₂; synthesises neither sugar nor ATP/NADPH
In C3 plants	Competes with oxygenase activity	Occurs because O₂ can bind RuBisCO
In C4 plants	Favoured by high CO₂ concentration in bundle sheath cells	Minimised because C4-acid decarboxylation concentrates CO₂ around RuBisCO

8. Factors Affecting Photosynthesis

Factor	NCERT effect on photosynthesis
Light	Rate rises linearly at low intensity, then plateaus when another factor limits. Light saturation is about 10% of full sunlight; excess can break down chlorophyll.
CO₂	Major limiting factor. Atmospheric level is 0.03-0.04%; increase to 0.05% raises fixation but prolonged higher levels may harm. C4 saturates near 360 microlitres L^-1; C3 beyond 450 microlitres L^-1.
Temperature	Carbon reactions are strongly temperature-controlled. C4 plants have a higher optimum than C3 plants; habitat adaptation affects optimum.
Water	Water stress closes stomata, lowering CO₂ availability; wilting reduces leaf area and metabolic activity.
Blackman's law	When several factors act together, rate is determined by the factor nearest its minimum/sub-optimal level.

Remember Internal factors include leaf and chloroplast characteristics, internal CO₂ and chlorophyll quantity; external factors include light, temperature, CO₂ and water.