Introduction to Photosynthesis
Photosynthesis is the process by which green plants synthesize organic compounds (glucose) from inorganic raw materials (CO2 and H2O) using light energy. Oxygen is released as a byproduct. It is the most important chemical process on Earth because it is the primary source of food and oxygen for almost all living organisms.
The overall equation for photosynthesis is:
Note that 12 molecules of water are used, but 6 molecules of water are also produced. The net consumption is 6 H₂O. The 12 H₂O notation is important because it correctly accounts for the source of oxygen (water splitting).
- Primary producers: Plants are the base of almost every food chain. They convert solar energy into chemical energy stored in glucose.
- Site: Chloroplasts, found in green parts of plants, especially mesophyll cells of leaves.
- Two main stages: Light reactions (require direct light; occur in thylakoid membranes) and dark reactions / Calvin cycle (do not directly require light; occur in the stroma).
- Inputs: CO₂ (from air via stomata), H₂O (from soil via roots and xylem), and light energy (absorbed by chlorophyll).
- Outputs: Glucose (stored as starch), O₂ (released to atmosphere), and water.
Chloroplast: Structure and Function
The chloroplast is a double-membrane organelle found in the mesophyll cells of green leaves. Understanding its structure is key to understanding where each stage of photosynthesis happens.
Outer and Inner Membrane
- Outer membrane: Freely permeable to small molecules and ions. Acts as a sieve for larger molecules.
- Inner membrane: Selectively permeable. Controls what enters and leaves the organelle.
- Between the two membranes is a narrow intermembrane space.
Grana and Thylakoids
- Thylakoids: Flattened, disc-like membrane sacs stacked inside the chloroplast. Each thylakoid has a membrane and an internal space called the lumen.
- Granum (plural: grana): A stack of thylakoids. A single chloroplast may have 40 to 80 grana.
- Stroma lamellae (intergranal thylakoids): Flattened membrane tubules that connect adjacent grana.
- Thylakoid membrane is the site of all light reactions. It contains the photosystems (PS I and PS II), the electron transport chain, and ATP synthase.
- Lumen: The internal thylakoid space. H⁺ (protons) accumulate here during light reactions, creating the proton gradient that drives ATP synthesis.
Stroma
- Stroma: Fluid matrix that fills the space between the inner membrane and the thylakoids. Site of the Calvin cycle (dark reactions).
- Contains all the enzymes needed for carbon fixation, including RuBisCO (the most abundant enzyme on Earth).
- Also contains chloroplast DNA (circular, like prokaryotes), ribosomes (70S, like prokaryotes), starch granules, and lipid droplets.
Why leaves are green
Chlorophyll absorbs red light (around 660-680 nm) and blue-violet light (around 430-450 nm) most strongly. It reflects green light (around 500-560 nm) back to your eye. That reflected green is why most leaves look green.
Photosynthetic Pigments
Photosynthetic pigments are molecules that absorb light energy and transfer it to the reaction centre. There are two main categories: chlorophylls and carotenoids.
Chlorophylls
- Chlorophyll a: Blue-green in colour. Absorption peaks at 430 nm (blue-violet) and 662 nm (red). This is the PRIMARY pigment. It is the only pigment that directly participates in the light reactions by undergoing photoexcitation at the reaction centre. In reaction centres, it is called P680 (in PS II) or P700 (in PS I), where the number refers to the wavelength it absorbs.
- Chlorophyll b: Yellow-green in colour. Absorption peaks at 453 nm and 642 nm. Acts as an ACCESSORY pigment: absorbs light and passes the energy to Chl a.
Carotenoids
- Carotenes: Orange pigments. Absorb mainly blue-violet light (450-480 nm). Beta-carotene is the most common. Also serve as photoprotective agents (quench harmful triplet chlorophyll).
- Xanthophylls: Yellow pigments. Also absorb in the 450-480 nm range. Example: lutein.
- All carotenoids are accessory pigments: they transfer absorbed energy to Chl a and broaden the range of wavelengths that the plant can use for photosynthesis.
Absorption Spectrum vs Action Spectrum
- Absorption spectrum: A graph of how much light a specific pigment absorbs at each wavelength. Chlorophyll a shows peaks at 430 nm and 662 nm.
- Action spectrum: A graph of the rate of photosynthesis at each wavelength. Shows which wavelengths are most effective for driving photosynthesis.
- The action spectrum closely resembles the absorption spectrum of Chl a (not of the mixture of all pigments). This proves that Chl a is the pigment that directly drives photosynthesis.
Engelmann's Experiment (1882)
T.W. Engelmann used a prism to split white light into its spectrum and directed it onto a filament of Spirogyra algae. He added aerobic bacteria that move toward O₂. The bacteria clustered at the red (~660 nm) and blue-violet (~430 nm) ends of the spectrum.
- Bacteria clustered where O₂ was released, which is where photosynthesis was fastest.
- This proved that red and blue-violet light are most effective for photosynthesis.
- This is the first experimental demonstration of the action spectrum of photosynthesis.
NEET trap: Primary vs Accessory pigments
Chlorophyll a is the ONLY primary pigment. It is found in the reaction centres and is the only one that directly converts light energy to chemical energy. Chlorophyll b, carotenes, and xanthophylls are ALL accessory pigments. They absorb light and pass energy to Chl a. They are found in the antenna complex (light harvesting complex), not the reaction centre.
Pigment absorption spectrum explorer
Click a pigment to explore its absorption peaks, colour, and NEET facts. Use 'Select all' to overlay all spectra.
Chlorophyll a
Chlorophyll b
Carotenes
Xanthophylls
Chlorophyll a
Absorption peaks
430 nm (blue-violet) and 662 nm (red)
Apparent colour
Bright / bluish green
Location: Thylakoid membrane (reaction centre pigment)
PRIMARY pigment: directly involved in light reactions
Present in all photosynthetic organisms (including cyanobacteria)
Acts as the reaction centre in both PS I and PS II
P700 (PS I) and P680 (PS II) are both Chlorophyll a molecules
NEET trap: Chlorophyll b is an ACCESSORY pigment, not primary
Engelmann's Experiment (1883)
T.W. Engelmann used a prism to split white light into a spectrum and illuminated Spirogyra (a green alga) placed with aerobic bacteria that move toward oxygen. The bacteria clustered most densely at the red (around 660-680 nm) and blue-violet (around 430-450 nm) ends of the spectrum.
This proved that those wavelengths drive the most photosynthesis (and therefore release the most O2), matching the absorption peaks of chlorophylls. Green light (500-600 nm) produced the fewest bacteria clusters because chlorophylls reflect green light.
NEET traps to remember
!
Chlorophyll a is the ONLY primary pigment; all others (Chl b, carotenes, xanthophylls) are accessory pigments.
!
Chlorophyll a directly participates in light reactions (as P680 in PS II and P700 in PS I).
!
Accessory pigments absorb wavelengths Chl a misses and pass energy to Chl a (they do NOT directly drive the light reactions).
!
Carotenoids (carotenes + xanthophylls) also protect against photo-oxidative damage from excess light.
Try this
- Select only Chl a, then only Chl b: notice how Chl b peaks shift slightly compared to Chl a. Both absorb in the blue AND red regions.
- Select all four pigments together: the combined absorption covers almost the entire visible spectrum from 400 to 700 nm.
- Engelmann showed bacteria clustered at the red and blue ends of the spectrum. Which pigment is responsible for the red absorption peak? (Answer: Chlorophyll a at 662 nm.)
Early Experiments in Photosynthesis
Our understanding of photosynthesis was built up over centuries through careful experiments. You need to know each scientist, their experiment, and what it proved.
- van Helmont (1648): Grew a willow sapling in a weighed pot of soil. After 5 years, the plant gained ~74 kg but the soil lost only ~57 g. Concluded that the mass came mainly from water, not soil. (He did not know about CO₂ yet.)
- Joseph Priestley (1772): Placed a burning candle in a closed jar until it went out, then placed a mint plant in the same jar. After a few days, the candle could burn again and a mouse placed in the jar survived. He showed that plants "restore" air that has been damaged by burning or respiration: they release O₂.
- Jan Ingenhousz (1779): Repeated Priestley's work and showed that plants only restore air in the presence of light, not in the dark. This was the first evidence that light is required for photosynthesis.
- Julius von Sachs (1854): Demonstrated that green plants produce starch (not glucose directly) during photosynthesis. He used the iodine test on illuminated leaves to show starch accumulation. This proved that photosynthesis produces an organic product.
- Blackman (1905): Proposed the Law of Limiting Factors: when a process is controlled by more than one factor, the rate is limited by the factor present in the shortest supply. At any one moment, only one factor limits the rate.
- Ruben and Kamen (1941): Used radioactive ¹⁸O-labelled water (H₂¹⁸O). The ¹⁸O appeared in the O₂ released, not in the glucose. This proved that the O₂ released during photosynthesis comes from water (H₂O), not from CO₂.
- Robert Hill (1939), the Hill reaction: Showed that isolated chloroplasts illuminated in the presence of an artificial electron acceptor (Hill reagent, e.g. ferricyanide) could split water and release O₂, even without CO₂. This proved that O₂ evolution is a property of the chloroplast alone and does not require CO₂. It separated the light reactions from carbon fixation experimentally.
Light Reactions: Photosystems and Electron Transport Chain
Light reactions occur in the thylakoid membranes of the grana. They convert light energy into chemical energy in the form of ATP and NADPH, and release O₂ as a byproduct of water splitting.
Photosystems
A photosystem is a protein complex embedded in the thylakoid membrane. It has two parts:
- Antenna complex (light-harvesting complex): Contains hundreds of accessory pigment molecules (Chl b, carotenoids) that absorb photons and funnel the energy to the reaction centre.
- Reaction centre: Contains a special Chl a molecule. When it receives energy from the antenna complex, it ejects an electron (photoexcitation), starting the electron transport chain.
Photosystem II (PS II): P680
- Reaction centre = P680 (absorbs light of wavelength 680 nm).
- PS II is named "II" because it was discovered second, but it functions FIRST in the electron transport chain.
- When P680 absorbs light, it is excited and ejects a high-energy electron.
- The electron is replaced by splitting water (photolysis of water): 2H₂O → 4H⁺ + 4e⁻ + O₂. This is where all the O₂ you breathe comes from.
- The ejected electrons pass to plastoquinone (PQ) to enter the electron transport chain.
Electron Transport Chain (ETC)
- Plastoquinone (PQ): Mobile electron carrier in the thylakoid membrane. Carries electrons AND protons (H⁺) from the stroma side to the lumen side, contributing to the proton gradient.
- Cytochrome b6f complex: Protein complex in the thylakoid membrane. Receives electrons from PQ and passes them to plastocyanin. The energy released here drives proton pumping into the lumen.
- Plastocyanin (PC): Small, mobile copper-containing protein in the lumen. Carries electrons from Cyt b6f to PS I.
Photosystem I (PS I): P700
- Reaction centre = P700 (absorbs light of wavelength 700 nm).
- PS I receives electrons from plastocyanin and re-energizes them using absorbed light.
- The excited electrons pass to ferredoxin (Fd), then to NADP⁺ reductase.
- NADP⁺ reductase catalyzes: NADP⁺ + H⁺ + 2e⁻ → NADPH.
- NADPH is the reducing power (hydrogen carrier) used in the Calvin cycle.
The full sequence of non-cyclic electron flow is:
Z-scheme and photophosphorylation explorer
Click any component to learn what it does. Switch between non-cyclic and cyclic photophosphorylation.
PS II (P680)
Photosystem II contains the reaction centre pigment P680 (absorbs at 680 nm). When P680 absorbs a photon, it is excited and loses an electron to plastoquinone. The "hole" left is filled by electrons from water splitting.
NEET: P680 is the primary pigment of PS II. It is the strongest biological oxidising agent (needs to oxidise water). Named P680 because its reaction centre Chl a absorbs at 680 nm.
Non-cyclic products (per 2 photons absorbed)
NEET traps
!
P680 is the reaction centre of PS II (absorbs at 680 nm); P700 is the reaction centre of PS I (absorbs at 700 nm).
!
Non-cyclic photophosphorylation produces ATP, NADPH, AND O2. Cyclic produces ATP only.
!
O2 from photosynthesis comes entirely from water splitting at PS II, NOT from CO2.
!
Chemiosmosis (via ATP synthase) is how ATP is made in BOTH cyclic and non-cyclic flows.
!
The ratio of products in non-cyclic flow: for every 2 NADPH, approximately 3 ATP are made.
Try this
- Click PS II, then P680: both refer to the same reaction centre. P680 is Chlorophyll a absorbing at 680 nm.
- In the non-cyclic tab, trace the path: Water splits at PS II, electrons travel through PQ and Cyt b6f (ATP made here), then PC connects to PS I, finally Fd donates to NADP+ for NADPH.
- Switch to the cyclic tab: electrons loop from PS I through Fd back to Cyt b6f to PC and back to PS I. No NADPH is formed, only ATP.
Z-Scheme and Photophosphorylation
When the energy levels of electrons are plotted on a diagram as they flow through both photosystems and the ETC, the resulting shape looks like a "Z" laid on its side. This is why non-cyclic electron flow is called the Z-scheme.
Non-Cyclic Photophosphorylation
- Both PS II and PS I are involved.
- Electrons flow from water all the way to NADP⁺ (one-way, non-cyclic flow).
- Products: ATP + NADPH + O₂.
- Water is split (O₂ released). This replenishes PS II electrons.
- Also called the Z-scheme or non-cyclic electron transport.
- ATP is produced via chemiosmosis as electrons flow through Cyt b6f.
Cyclic Photophosphorylation
- Only PS I is involved.
- Electrons from excited P700 go: PS I → Fd → Cyt b6f → PC → back to PS I (cyclic).
- Only ATP is produced (no NADPH).
- No water splitting (no O₂ released).
- No NADPH produced.
- Occurs when the cell needs extra ATP but not NADPH (e.g. in bundle sheath cells of C4 plants).
NEET trap: Cyclic vs Non-Cyclic
Cyclic = Only PS I, only ATP, NO NADPH, NO O₂, no water splitting. Non-cyclic = Both PS I and PS II, ATP + NADPH, O₂ released, water is split. This is one of the most frequently tested distinctions in this chapter.
ATP and NADPH Energy Budget
See exactly how much ATP and NADPH the Calvin cycle needs and how the light reactions supply them.
Section 1: Calvin cycle energy requirements
CO2 molecules to fix: 6
ATP needed
18
NADPH needed
12
ATP per CO2
3
NADPH per CO2
2
Net G3P produced
4
Glucose equivalents (per 6 CO2)
1
For every 6 CO2: 18 ATP + 12 NADPH are consumed. 6 G3P are made gross, but 5 are used to regenerate RuBP. Net yield: 2 G3P, which combine to make 1 glucose.
Section 2: Light reactions supply
H2O molecules that must split to supply 12 NADPH
12
Non-cyclic photophosphorylation: per 2H2O split → 1 O2 released + 2 NADPH + approximately 3 ATP. To supply 12 NADPH, approximately 12 H2O molecules must be split, releasing 6 O2 molecules.
Section 3: Overall photosynthesis equation
6CO2 + 12H2O + light energy
↓
C6H12O6 + 6O2 + 6H2O
Light energy captured by pigments (chlorophyll a, b, carotenoids)
Light reactions: ATP + NADPH produced; water split; O2 released
Calvin cycle: ATP + NADPH drive CO2 fixation into G3P
G3P molecules combine to form glucose (sucrose, starch)
Section 4: Cyclic vs Non-cyclic photophosphorylation
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Photosystems involved | PS I + PS II | PS I only |
| Water splitting | Yes (2H2O per 2 NADPH) | No |
| O2 released | Yes | No |
| ATP produced | Yes | Yes |
| NADPH produced | Yes | No |
| When used | Normal photosynthesis | When ATP:NADPH ratio is low |
NEET tip
Cyclic photophosphorylation produces ONLY ATP. It involves only PS I (P700). No water is split and no O2 is released. It helps balance the ATP:NADPH ratio when NADPH is abundant but the Calvin cycle needs more ATP.
Chemiosmosis and ATP Synthesis
ATP synthesis in chloroplasts follows the same chemiosmotic principle as in mitochondria. The key is the buildup of a proton (H⁺) gradient across the thylakoid membrane.
How the Proton Gradient is Built
- Water splitting: 2H₂O → 4H⁺ + 4e⁻ + O₂ occurs inside the lumen. The 4H⁺ from water splitting go directly into the lumen, raising H⁺ concentration there.
- Plastoquinone (PQ): Carries electrons AND H⁺ from the stroma side to the lumen side of the thylakoid membrane. Each PQ molecule picks up 2H⁺ from the stroma and releases them into the lumen as electrons pass to Cyt b6f.
- Meanwhile, NADP⁺ reduction (using H⁺ and electrons) occurs on the stroma side, consuming H⁺ from the stroma.
- Result: High H⁺ in lumen (low pH ≈ 4), low H⁺ in stroma (higher pH ≈ 8). This is the proton-motive force.
ATP Synthase (CF₀-CF₁ Complex)
- ATP synthase spans the thylakoid membrane. The CF₀ subunit is embedded in the membrane; the CF₁ subunit projects into the stroma.
- H⁺ ions flow DOWN their concentration gradient from the lumen through the CF₀ channel into the stroma.
- This flow of protons drives the rotation of CF₁, which synthesizes ATP from ADP + Pᵢ on the STROMA side of the membrane.
- This process is called photophosphorylation (ATP synthesis driven by light-generated proton gradient).
Comparison with mitochondria
In mitochondria, the same chemiosmotic principle is called oxidative phosphorylation. H⁺ accumulates in the intermembrane space (not the matrix). In chloroplasts, H⁺ accumulates in the thylakoid lumen (not the stroma). In both cases, ATP is synthesized on the matrix/stroma side where the ATP synthase headpiece projects.
Calvin Cycle (Dark Reactions / C3 Pathway)
The Calvin cycle was discovered by Melvin Calvin, who received the Nobel Prize in 1961 for this work. It occurs in the stroma of the chloroplast and uses the ATP and NADPH produced in the light reactions to fix CO₂ into organic sugar. Since it does not directly need light to run, it is also called the dark reactions (though it does run in the light because it depends on ATP and NADPH from light reactions).
Stage 1: CO₂ Fixation (Carboxylation)
- CO₂ acceptor: RuBP (Ribulose-1,5-bisphosphate), a 5-carbon compound.
- CO₂ (1C) + RuBP (5C) → an unstable 6-carbon intermediate → 2 molecules of 3-PGA (3-phosphoglyceric acid, 3C each).
- The enzyme that catalyzes this reaction is RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase). It is the most abundant enzyme on Earth.
- 3-PGA is the first stable product of CO₂ fixation in C3 plants. Because the first product is a 3-carbon compound, these plants are called C3 plants.
Stage 2: Reduction
- 3-PGA (3C) is reduced to G3P (glyceraldehyde-3-phosphate, also 3C) using ATP and NADPH.
- For each molecule of 3-PGA reduced: 1 ATP + 1 NADPH consumed. For each CO₂ fixed: 2 molecules of 3-PGA formed, so 2 ATP + 2 NADPH are used in this step per CO₂.
- G3P is the actual sugar phosphate product. It can be used to build glucose, amino acids, fatty acids, and other organic molecules.
Stage 3: Regeneration of RuBP
- Most of the G3P molecules (10 out of every 12 G3P produced in 6 turns) are used to regenerate RuBP. This requires ATP.
- 5 G3P (3C each = 15C) → 3 RuBP (5C each = 15C), using 3 ATP.
- The remaining 2 G3P (net output) are used to make 1 molecule of glucose (6C).
Overall Balance for One Glucose
To fix 6 CO₂ and make 1 glucose (C₆H₁₂O₆):
- 6 turns of the Calvin cycle
- 18 ATP consumed (3 per CO₂: 2 in reduction + 1 in RuBP regeneration)
- 12 NADPH consumed (2 per CO₂)
- Net output: 2 G3P → combined into 1 glucose
NEET trap: C3 vs C4 first product
In C3 plants: CO₂ acceptor = RuBP (5C); first stable product = 3-PGA (3C); enzyme = RuBisCO. In C4 plants: CO₂ acceptor = PEP (3C); first stable product = OAA / oxaloacetate (4C); enzyme = PEP carboxylase. This is extremely frequently tested.
Calvin cycle (C3 pathway) step builder
Click each stage to explore what happens. Adjust the CO2 slider to see how inputs and outputs scale.
Stage 1: CO2 Fixation
CO2 from the atmosphere combines with a 5-carbon acceptor molecule RuBP (ribulose-1,5-bisphosphate). The enzyme RuBisCO catalyses this reaction. The unstable 6-carbon compound immediately splits into two molecules of 3-PGA (3-phosphoglycerate), a 3-carbon compound.
CO2 (1C) + RuBP (5C) → [6C unstable] → 2 x 3-PGA (3C each)
Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
Where: Stroma of chloroplast
NEET: 3-PGA is the FIRST STABLE product of CO2 fixation in C3 plants. RuBP is the CO2 acceptor. RuBisCO is the most abundant enzyme on Earth.
Number of CO2 molecules fixed: 6
For 6 CO2 molecules fixed:
ATP consumed
18
(3 per CO2)
NADPH consumed
12
(2 per CO2)
G3P produced (gross)
6
(1 per CO2)
G3P net output
2
(1 per 3 CO2, after RuBP regeneration)
6 CO2 fixed = 18 ATP + 12 NADPH consumed = 2 net G3P = 1 glucose equivalent (C6H12O6)
Carbon balance (per turn, 1 CO2 fixed)
+
=
then:
+
NEET key facts
!
RuBP (ribulose-1,5-bisphosphate) is the CO2 acceptor molecule in C3 plants.
!
3-PGA (3-phosphoglycerate) is the FIRST STABLE product of CO2 fixation in C3 plants (3 carbons per molecule).
!
RuBisCO is the enzyme for CO2 fixation and the MOST ABUNDANT ENZYME on Earth.
!
RuBisCO has both carboxylase activity (fixes CO2) and oxygenase activity (causes photorespiration in C3 plants).
!
Calvin cycle occurs in the STROMA of the chloroplast (dark reactions; light-independent reactions).
!
For every glucose (6C) produced: 6 CO2, 18 ATP, and 12 NADPH are consumed.
Try this
- Set CO2 to 3: you get 1 net G3P. Set it to 6: you get 2 net G3P (= 1 glucose). This is the full cycle for one glucose molecule.
- Click Stage 1 (CO2 Fixation): the equation shows CO2 + RuBP (both reactants) producing 3-PGA. Remember: RuBP has 5 carbons and CO2 has 1 carbon. What is the total carbon count on the product side?
- The Calvin cycle uses ATP and NADPH made in the light reactions. What happens to the Calvin cycle if the light reactions stop? (Clue: Stage 2 and 3 both need ATP and NADPH.)
Photorespiration: The Problem with RuBisCO
RuBisCO is a dual-function enzyme. In normal conditions, it acts as a carboxylase and fixes CO₂. But in hot, dry, bright conditions, it can also act as an oxygenase and react with O₂ instead of CO₂. This process is called photorespiration.
What Triggers Photorespiration
- On hot, bright days, plants close their stomata to prevent water loss.
- With stomata closed, CO₂ cannot enter and the CO₂ inside the leaf falls.
- Meanwhile, light reactions keep producing O₂, so O₂ inside the leaf rises.
- When the O₂/CO₂ ratio rises, RuBisCO binds O₂ more often than CO₂.
The Oxygenase Reaction
- RuBP + O₂ → 3-PGA (3C, one molecule) + phosphoglycolate (2C, one molecule). [Compare: RuBP + CO₂ → two 3-PGA molecules in carboxylation].
- Phosphoglycolate (2C) cannot enter the Calvin cycle. It is metabolized in the C2 pathway across three organelles.
Three Organelles Involved in Photorespiration
- Chloroplast: RuBisCO reacts with O₂; phosphoglycolate (2C) is formed here.
- Peroxisome: Phosphoglycolate is converted to glycolate, which is then oxidized (glycolate → glyoxylate → glycine). H₂O₂ is produced and broken down by catalase here.
- Mitochondria: Glycine is converted, releasing CO₂. This is the "wasteful" step where fixed carbon is lost.
Why Photorespiration is Wasteful
- CO₂ is released without producing ATP or NADPH or any net sugar.
- It can reduce photosynthesis efficiency by 25-50% in C3 plants under high temperature and light.
- C4 plants and CAM plants have mechanisms to avoid photorespiration by concentrating CO₂ around RuBisCO.
RuBisCO: Carboxylase vs Oxygenase
Toggle between RuBisCO's two activities to understand when photosynthesis is productive and when it wastes carbon.
Carboxylase mode (Calvin Cycle)
Oxygenase mode (Photorespiration)
Reaction flow
RuBP (5C)
CO2 acceptor
↓
+ CO2
from atmosphere
↓
RuBisCO
carboxylase activity
↓
2× 3-PGA (3C)
first stable product
↓
Calvin Cycle
reduction + regeneration
↓
G3P
→ Glucose / sucrose
Outcome
Net CO2 is fixed into organic matter. Glucose is produced. No carbon is lost. Energy is efficiently used.
Why C4 plants avoid photorespiration
1.
CO2 is first fixed in mesophyll cells by PEP carboxylase (no photorespiration there)
2.
OAA (4C) is transported to bundle sheath cells
3.
CO2 is released from OAA inside bundle sheath cells, concentrating CO2 ~10x vs mesophyll
4.
RuBisCO in bundle sheath cells is exposed only to high CO2
5.
Oxygenase activity of RuBisCO is suppressed at high CO2 concentration
6.
Result: photorespiration is effectively eliminated in C4 plants
Conditions that increase photorespiration
High temperature
Reduces CO2 solubility; increases O2 to CO2 ratio
High O2
Competes with CO2 for RuBisCO active site
Low CO2
Shifts RuBisCO toward oxygenase activity
High light intensity
Generates excess O2 from water splitting
NEET fact
Photorespiration occurs only in C3 plants. C4 plants (maize, sugarcane, sorghum) and CAM plants (Opuntia, Agave) have evolved mechanisms to concentrate CO2 around RuBisCO, suppressing the oxygenase activity entirely.
C4 Pathway (Hatch-Slack Pathway)
The C4 pathway was discovered by M.D. Hatch and C.R. Slack in Australia in 1966. It is a CO₂ concentration mechanism that allows plants to fix CO₂ efficiently even at low concentrations, preventing photorespiration.
C4 Plants and Kranz Anatomy
- Examples: Maize (Zea mays), Sugarcane (Saccharum officinarum), Sorghum, Amaranthus. These are mostly tropical and warm-climate grasses.
- Kranz anatomy: The key structural feature of C4 leaves. The vascular bundle is surrounded by a thick, compact layer of bundle sheath cells. These are surrounded in turn by mesophyll cells. The arrangement looks like a wreath (Kranz = wreath in German).
- Bundle sheath cell chloroplasts are large, thick-walled, and agranal (few or no grana). They are specialized for the Calvin cycle.
- Mesophyll cell chloroplasts are normal, with grana. They are specialized for the initial CO₂ fixation step.
Steps of the C4 Pathway
- Step 1 (Mesophyll cells): CO₂ enters the mesophyll cell. PEP (phosphoenolpyruvate, 3C) + CO₂ → [PEP carboxylase] → OAA (oxaloacetate, 4C). OAA is the FIRST STABLE PRODUCT of CO₂ fixation in C4 plants (4C, hence C4 plants). PEP carboxylase has a very high affinity for CO₂ and no oxygenase activity (cannot react with O₂).
- Step 2: OAA is converted to malate (or aspartate, depending on the species) in the mesophyll cell. Malate/aspartate is transported to bundle sheath cells through plasmodesmata (cell junctions).
- Step 3 (Bundle sheath cells): Malate is decarboxylated: malate → CO₂ + pyruvate (3C). This releases CO₂ at high concentration inside bundle sheath cells, right next to RuBisCO. RuBisCO now acts as a carboxylase (not oxygenase) because CO₂ is abundant. The Calvin cycle proceeds normally here.
- Step 4: Pyruvate (3C) is transported back to the mesophyll cell. It is converted back to PEP using ATP (specifically 2 ATP equivalent: ATP → AMP + 2Pᵢ). PEP is ready for the next round of CO₂ fixation.
Why C4 Plants Are More Efficient
- The CO₂ concentration inside bundle sheath cells is kept very high (5-10x higher than in mesophyll cells).
- This suppresses RuBisCO oxygenase activity completely. Photorespiration is essentially absent in C4 plants.
- C4 plants can photosynthesize efficiently at higher temperatures (optimum 30-45°C) and lower CO₂ concentrations than C3 plants.
- C4 plants use water more efficiently (better water use efficiency) than C3 plants.
- C4 is more energy-costly per CO₂ fixed (costs 5 ATP + 2 NADPH vs 3 ATP + 2 NADPH for C3) but the absence of photorespiration makes it a net gain under hot, bright conditions.
C3 vs C4 vs CAM comparison lab
Explore the three photosynthetic pathways, compare them side by side, and test yourself with a quick quiz.
🌾
C3 Plants
Wheat, Rice
🌽
C4 Plants
Maize, Sugarcane
🌵
CAM Plants
Cactus, Pineapple
First stable product
3-PGA (3C)
The "3" in C3 refers to 3-carbon first product: 3-phosphoglycerate
Primary CO2 acceptor
RuBP (ribulose-1,5-bisphosphate, 5C)
CO2 fixation enzyme
RuBisCO (in mesophyll cells)
Cells for fixation
Mesophyll cells only
Bundle sheath cells
Absent (no chloroplasts in bundle sheath)
Kranz anatomy
Absent
Photorespiration
High (RuBisCO also fixes O2)
Water use efficiency
Low
Temperature optimum
15-25 degrees C (cool/moderate)
Stomata timing
Open during the day
Examples
Wheat, Rice, Oat, Sunflower, Pea, Soybean, most trees
NEET traps
!
C3 first product = 3-PGA (3 carbons). C4 first product = OAA (4 carbons). These are the most-asked NEET questions on this topic.
!
PEP carboxylase (in C4 and CAM) has a much HIGHER affinity for CO2 than RuBisCO. This is why C4/CAM plants can work at low CO2 concentrations.
!
CAM stomata: open at NIGHT, closed during the day. C3 and C4 stomata: open during the day. This is a classic NEET trap.
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Kranz anatomy (bundle sheath cells with chloroplasts): C4 plants ONLY. Neither C3 nor CAM plants have Kranz anatomy.
!
Photorespiration is caused by RuBisCO fixing O2 instead of CO2. C4 and CAM plants avoid this by concentrating CO2 around RuBisCO.
Try this
- In the compare view, look at the "Stomata timing" row: C3 and C4 open during the day, CAM opens at night. This is the most common NEET trap for this chapter.
- Click C4 plants in the detail view: notice how fixation happens in TWO types of cells (mesophyll then bundle sheath). This spatial separation is the key to avoiding photorespiration.
- Take the quiz without looking at the detail view first. A score of 4+ means you know this topic well.
CAM Plants (Crassulacean Acid Metabolism)
CAM stands for Crassulacean Acid Metabolism, named after the family Crassulaceae where it was first studied. CAM plants solve the same problem as C4 plants (photorespiration and water loss) but use a different strategy: temporal separation rather than spatial separation.
CAM Plant Examples
- Cacti (Opuntia and other succulents), Agave, Pineapple (Ananas comosus), Sedum, Kalanchoe, and most other succulents.
- All are adapted to very dry (arid) environments where water conservation is critical.
Temporal Separation Strategy
- At night (stomata OPEN): Stomata open in the cool night air (reduces water loss). CO₂ enters the leaf. PEP carboxylase fixes CO₂ into OAA → malate. Malate accumulates in vacuoles as malic acid. The leaf becomes acidic by morning (the "acid" in Crassulacean Acid Metabolism).
- During the day (stomata CLOSED): Stomata close to prevent water loss during the hot day. Stored malate is released from vacuoles and decarboxylated: malate → CO₂ + pyruvate. CO₂ is released at high concentration inside the cell. Light reactions proceed (ATP and NADPH are produced). CO₂ enters the Calvin cycle via RuBisCO. Pyruvate is converted back to PEP using ATP (to repeat the cycle at night).
CAM vs C4: Key Difference
- C4: Spatial separation. Initial CO₂ fixation in MESOPHYLL cells; Calvin cycle in BUNDLE SHEATH cells. Both happen during the day. Two different cell types are used.
- CAM: Temporal separation. CO₂ fixation at NIGHT; Calvin cycle during the DAY. Same cell, same chloroplast, different times.
- Both: CO₂ is first fixed by PEP carboxylase into OAA (4C); no photorespiration; higher water-use efficiency than C3 plants.
Factors Affecting Photosynthesis (Blackman's Law)
Blackman (1905) proposed the Law of Limiting Factors: when a process is governed by several factors, the rate of the process is limited by the factor that is in shortest supply. At any given instant, only one factor is the limiting factor.
Light Intensity
- As light intensity increases from zero, the rate of photosynthesis increases linearly (light is the limiting factor).
- At a certain point (light saturation point), increasing light intensity no longer increases the rate. CO₂ concentration or temperature has now become the limiting factor.
- C4 plants have a higher light saturation point than C3 plants.
- Very high light intensities can cause photoinhibition (damage to PS II).
CO₂ Concentration
- Normal atmospheric CO₂ is about 0.04% (400 ppm). This is often a limiting factor.
- Increasing CO₂ concentration (e.g. in greenhouses, up to about 0.05%) significantly increases photosynthesis in C3 plants.
- CO₂ is the substrate for RuBisCO in the Calvin cycle; more CO₂ = faster carboxylation.
- C4 plants are less CO₂-limited at normal atmospheric concentrations because PEP carboxylase concentrates CO₂ for RuBisCO.
Temperature
- Photosynthesis involves enzyme-catalyzed reactions (especially the Calvin cycle). These are temperature-dependent.
- Optimum temperature: about 25°C for C3 plants, 30-45°C for C4 plants.
- Below the optimum: rate is slow (low enzyme activity).
- Above the optimum: enzymes denature; rate drops sharply. RuBisCO oxygenase activity also increases relative to carboxylase at high temperatures.
- Light reactions are less temperature-sensitive (photochemical steps have low temperature coefficients).
Water
- Direct role: Water is the electron donor for the light reactions (water splitting at PS II). Without water, O₂ cannot be released and electrons cannot flow.
- Indirect role: Water stress causes stomata to close (via abscisic acid). Closed stomata reduce CO₂ entry, making CO₂ the limiting factor. This is the main indirect effect of water on photosynthesis.
Mineral Nutrients
- Magnesium (Mg²⁺): Central atom of the chlorophyll porphyrin ring. Without Mg²⁺, chlorophyll cannot be synthesized.
- Iron (Fe²⁺/Fe³⁺): Required for synthesis of cytochromes (components of the ETC in thylakoids) and ferredoxin. Iron deficiency causes chlorosis.
- Nitrogen: Required for synthesis of chlorophyll, amino acids, enzymes (including RuBisCO), and ATP. Nitrogen deficiency severely limits photosynthesis.
Blackman's Limiting Factors
Adjust the three factors to see how they limit photosynthesis rate. Blackman's Law (1905): the slowest factor controls the overall rate.
Light intensity: 50%
CO2 concentration: 0.04%
Temperature: 25°C
Light
83%
CO2
75%
Temperature
100%
Overall photosynthesis rate
75%
Current limiting factor:
Score = 75%. Increasing other factors will not raise the rate further.
Reference: why CO2 is often limiting even at high light
Each curve reaches a plateau (light saturation point). Increasing light beyond that plateau has no effect when CO2 is the bottleneck.
NEET tip
The factor with the lowest score is the limiting factor. Increasing other factors above their limiting point has no effect on the overall photosynthesis rate.
Example: At full sunlight and high CO2, temperature becomes the limiting factor. This is why plants in glasshouses benefit from controlled temperature; even with abundant light and CO2, photosynthesis slows above 35°C due to enzyme denaturation.
Worked NEET Problems
NEET-style problem · Calvin Cycle
Question
Solution
NEET-style problem · C4 vs C3 Identification
Question
Solution
NEET-style problem · Z-Scheme
Question
Solution
NEET-style problem · Photorespiration
Question
Solution
NEET-style problem · Photosynthetic Pigments
Question
Solution
Cheat Sheet
Overall Equation
6CO₂ + 12H₂O + light → C₆H₁₂O₆ + 6O₂ + 6H₂O
Light Reactions Summary
PS II (P680) splits H₂O → O₂; ETC pumps H⁺; PS I (P700) → NADPH; ATP synthase → ATP
Calvin Cycle (C3)
Acceptor: RuBP (5C) | First product: 3-PGA (3C) | Enzyme: RuBisCO | 18 ATP + 12 NADPH per glucose | Site: stroma
C4 Pathway
Acceptor: PEP (3C) | First product: OAA (4C) | Enzyme: PEP carboxylase | Kranz anatomy | Examples: maize, sugarcane | No photorespiration
CAM Plants
Night: stomata open, CO₂ fixed by PEP carboxylase → malate stored | Day: stomata closed, malate → CO₂ → Calvin cycle | Examples: opuntia, agave, pineapple
Cyclic Photophosphorylation
Only PS I | Only ATP produced | No NADPH | No O₂ released | No water splitting
Non-Cyclic Photophosphorylation
Both PS I + PS II | ATP + NADPH produced | O₂ released | Water is split
Photorespiration
C3 plants only | RuBisCO + O₂ | No ATP, CO₂ released (wasteful) | 3 organelles: chloroplast + peroxisome + mitochondria
Photosynthetic Pigments
Primary: Chl a ONLY (reaction centre, P680/P700) | Accessory: Chl b, carotenes, xanthophylls (antenna complex) | All pass energy to Chl a
Blackman's Limiting Factor
Rate = limited by slowest factor (light, CO₂, or temperature) | Only ONE factor limits at any moment | CO₂ often limiting in normal air
Key Scientists
Ruben & Kamen: O₂ from H₂O (not CO₂) | Hill: O₂ from isolated chloroplasts | Blackman: limiting factors | Calvin: dark reactions (Nobel 1961) | Engelmann: action spectrum
C4 vs CAM Difference
C4: Spatial separation (mesophyll vs bundle sheath cells, same time) | CAM: Temporal separation (same cell, night vs day)
Photosynthesis NEET quiz
Question 1 of 12 · Topic: Pigments
The primary photosynthetic pigment in higher plants is:
A.
Chlorophyll b
B.
Xanthophyll
C.
Chlorophyll a
D.
Carotene
0 answered
Frequently asked questions
How often does Photosynthesis in Higher Plants appear in NEET?
Photosynthesis in Higher Plants is a High Weightage chapter with 4 to 6 questions in most NEET exams. Questions focus on photosynthetic pigments and absorption peaks, Z-scheme electron flow, Calvin cycle intermediates (RuBP, 3-PGA, G3P), the C4 Hatch-Slack pathway (PEP, OAA, Kranz anatomy), photorespiration, and Blackman's law of limiting factors. Mastering all these sub-topics is essential for a strong NEET score.
What is the difference between absorption spectrum and action spectrum?
The absorption spectrum shows which wavelengths of light a pigment absorbs. You measure this in a lab using a spectrophotometer. The action spectrum shows which wavelengths actually drive photosynthesis (measured by the rate of O2 release or CO2 uptake). The action spectrum closely matches the absorption spectrum of chlorophyll a, which confirms that chlorophyll a is the main pigment responsible for photosynthesis. Engelmann's experiment with Spirogyra and bacteria was the first to demonstrate the action spectrum.
What is the Z-scheme in photosynthesis?
The Z-scheme describes the non-cyclic flow of electrons during light reactions. It gets its name from the zigzag shape of the energy diagram. Electrons start at PS II (P680), which absorbs light and gets excited. The electrons pass through plastoquinone, the cytochrome b6-f complex, and plastocyanin to PS I (P700). PS I absorbs more light, re-energises the electrons, and passes them via ferredoxin to NADP+ reductase, which reduces NADP+ to NADPH. Water is split at PS II to replace the lost electrons, and O2 is released as a byproduct.
What are the differences between C3 and C4 plants?
In C3 plants, CO2 is fixed directly by RuBisCO in mesophyll cells to form 3-PGA (a 3-carbon compound) as the first stable product. In C4 plants, CO2 is first fixed in mesophyll cells using PEP carboxylase to form OAA (a 4-carbon compound). OAA is converted to malate, which moves to bundle sheath cells where CO2 is released and then fixed again by RuBisCO in the Calvin cycle. C4 plants (like maize and sugarcane) have Kranz anatomy (a ring of bundle sheath cells around vascular bundles), do not show photorespiration, and are more efficient in hot, dry, and high-light conditions.
What is Blackman's Law of Limiting Factors?
Blackman's Law (1905) states that when a process depends on multiple factors, its rate is limited by the factor present at the least favourable value. For photosynthesis, the main limiting factors are light intensity, CO2 concentration, and temperature. For example, even if light intensity is high, increasing CO2 will increase the photosynthesis rate until another factor (such as light or temperature) becomes limiting. This explains why rate-vs-factor graphs show a plateau where a different factor becomes the new limit.
What is photorespiration and why do C4 plants not show it?
Photorespiration occurs when RuBisCO uses O2 instead of CO2 as its substrate (oxygenase activity) in conditions of high O2 and low CO2. This produces glycolate, which is processed in peroxisomes and mitochondria, releasing CO2 and consuming ATP without producing sugar. It is a wasteful process. C4 plants avoid photorespiration because their CO2 pump (using PEP carboxylase in mesophyll cells) concentrates CO2 in bundle sheath cells, where the Calvin cycle runs. The high CO2 concentration in bundle sheath cells suppresses the oxygenase activity of RuBisCO.
What are the products of light reactions versus the Calvin cycle?
Light reactions (in thylakoid membranes) produce: ATP, NADPH, and O2. These reactions need light. The Calvin cycle (in the stroma) uses ATP and NADPH from light reactions to fix CO2 and produce G3P (glyceraldehyde-3-phosphate), which is the precursor for glucose. The Calvin cycle does not directly need light, which is why it was formerly called the "dark reaction," but this name is misleading because it runs most actively in the light when ATP and NADPH are available.
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