Photosynthesis in Higher Plants

Photosynthesis in Higher PlantsNEET Botany · Class 11 · NCERT Chapter 13

Overview Notes NEET Questions Interactive Learning

8 interactive concept widgets for Photosynthesis in Higher Plants. Drag any slider, change any number, and watch the formula and the answer update live. Built so you understand how each NEET problem actually works, not just the final number.

On this page

Pigment absorption spectrum: Chl a, Chl b, carotenoids

Z-scheme: PS II to PS I electron flow, cyclic vs non-cyclic

Calvin cycle step builder with ATP/NADPH tracker

C3 vs C4 vs CAM: full comparison and quiz

Blackman's law: interactive limiting factor simulator

Photorespiration flow: chloroplast, peroxisome, mitochondria

Photosynthesis energy budget: ATP, NADPH, glucose

Photosynthesis in Higher Plants — 12-question NEET quiz

Photosynthetic pigments and absorption spectrum

Explore the 4 photosynthetic pigments (Chlorophyll a, Chlorophyll b, Carotenes, Xanthophylls), their absorption peaks, and Engelmann's experiment that proved which wavelengths drive photosynthesis.

Photosynthesis

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

Primary

Chlorophyll b

Accessory

Carotenes

Accessory

Xanthophylls

Accessory

Select all pigments

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 trap: Green light is least effective for photosynthesis because chlorophylls reflect it.

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.)

Z-scheme: cyclic and non-cyclic photophosphorylation

Step through the Z-scheme electron transport chain from water to NADPH. Toggle between non-cyclic (PS II + PS I, ATP + NADPH + O2) and cyclic (PS I only, ATP only) photophosphorylation.

Photosynthesis

Z-scheme and photophosphorylation explorer

Click any component to learn what it does. Switch between non-cyclic and cyclic photophosphorylation.

Non-cyclic (ATP + NADPH)

Cyclic (ATP only)

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)

O2 released

ATP produced

NADPH produced

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.

Calvin cycle: CO2 fixation, reduction, RuBP regeneration

Interactive Calvin cycle builder. Slide through 1 to 12 CO2 molecules and see cumulative ATP, NADPH, and G3P accounting. Click each of the 3 stages for equations, enzymes, and NEET focus points.

Photosynthesis

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

(3 per CO2)

NADPH consumed

(2 per CO2)

G3P produced (gross)

(1 per CO2)

G3P net output

(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)

CO2 (1C)

RuBP (5C)

2 x 3-PGA (6C total)

then:

1 G3P (3C) exits

5C regenerated back to RuBP

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.)

C3 vs C4 vs CAM comparison lab

Compare the three carbon fixation strategies: C3 (wheat, rice), C4 (maize, sugarcane), and CAM (cactus, agave). Explore primary CO2 acceptor, first stable product, Kranz anatomy, photorespiration, and water use efficiency. Includes a quick quiz.

Photosynthesis

C3 vs C4 vs CAM comparison lab

Explore the three photosynthetic pathways, compare them side by side, and test yourself with a quick quiz.

Plant type detail

Side-by-side compare

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.

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.

Blackman's limiting factor graph engine

Adjust light intensity, CO2 concentration, and temperature with sliders. The graph shows live photosynthesis rate and highlights the current limiting factor, demonstrating Blackman's Law of 1905.

Limiting factors

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

Limiting factor

75%

Temperature

100%

Overall photosynthesis rate

75%

Current limiting factor:

CO2 concentration

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.

Photorespiration: RuBisCO carboxylase vs oxygenase

Toggle between RuBisCO's carboxylase mode (Calvin cycle) and oxygenase mode (photorespiration). See the 3-organelle C2 pathway and why C4 plants suppress this wasteful process.

Photorespiration

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)

High CO2 / Low O2 / C3 plants

Reaction flow

RuBP (5C)

CO2 acceptor

Chloroplast stroma

↓

+ 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

CO2 is first fixed in mesophyll cells by PEP carboxylase (no photorespiration there)

OAA (4C) is transported to bundle sheath cells

CO2 is released from OAA inside bundle sheath cells, concentrating CO2 ~10x vs mesophyll

RuBisCO in bundle sheath cells is exposed only to high CO2

Oxygenase activity of RuBisCO is suppressed at high CO2 concentration

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.

ATP/NADPH energy budget: light reactions and Calvin cycle

Calculate ATP and NADPH required for any number of CO2 molecules fixed. See how non-cyclic photophosphorylation supplies the Calvin cycle. Compare cyclic vs non-cyclic output.

Energy budget

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

NADPH needed

ATP per CO2

NADPH per CO2

Net G3P produced

Glucose equivalents (per 6 CO2)

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

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.

Photosynthesis in Higher Plants NEET quiz

12-question scored quiz covering pigments, light reactions, Calvin cycle, C4 and CAM plants, photorespiration, Engelmann's experiment, and Blackman's limiting factors.