C1.3 Photosynthesis

Photosynthesis converts light energy into chemical energy by using pigments, thylakoid reactions and Calvin-cycle carbon fixation to build organic compounds from carbon dioxide.

Learning objectives

C1.3.1Light energy → chemical energy• Photosynthesis converts light energy into chemical energy in carbon compounds• Photoautotrophs use chlorophyll in chloroplasts or cyanobacterial membranes C1.3.2CO₂ → glucose• Carbon dioxide is reduced to carbohydrate using hydrogen from water• Glucose represents the main stored product, though many compounds are synthesized C1.3.3Oxygen as by-product• Oxygen is released from photolysis of water, not directly from CO₂• Oxygenic photosynthesis occurs in plants, algae, and cyanobacteria C1.3.4Photosynthetic pigment separation• Paper chromatography separates chlorophylls and accessory pigments• Rf values compare pigment movement relative to solvent front C1.3.5Absorption of specific wavelengths• Chlorophyll and accessory pigments absorb specific wavelengths• Absorbed light excites electrons for light-dependent reactions C1.3.6Absorption vs. action spectra• Absorption spectra show wavelengths absorbed by pigments• Action spectra show wavelengths most effective for photosynthesis rate C1.3.7Limiting factors investigation• Light intensity, CO₂ concentration, and temperature can limit photosynthesis• Investigations change one variable and estimate rate from O₂ production or CO₂ uptake C1.3.8CO₂ enrichment experiments• CO₂ enrichment tests whether increased CO₂ raises photosynthesis or growth• Greenhouse and FACE experiments compare realistic crop and ecosystem responses C1.3.9(HL)—Photosystems• Photosystems are pigment-protein arrays in thylakoid membranes• Antenna pigments pass energy to reaction-centre chlorophyll C1.3.10(HL)—Advantages of pigment arrays• Pigment arrays broaden wavelength absorption and funnel energy efficiently• A single chlorophyll molecule cannot sustain the full light reaction system C1.3.11(HL)—Photolysis of water• Photosystem II uses light energy to split water• Photolysis supplies replacement electrons, protons, and oxygen C1.3.12(HL)—ATP production in thylakoids• Electron transport pumps protons into the thylakoid space• Proton flow through ATP synthase produces ATP by photophosphorylation C1.3.13(HL)—NADP reduction• Photosystem I re-excites electrons for NADP reduction• NADP accepts electrons and H⁺ to form reduced NADP/NADPH C1.3.14(HL)—Thylakoids as light-dependent systems• Thylakoids organize photosystems, electron carriers, and ATP synthase• Key outputs on the stromal side are ATP and reduced NADP C1.3.15(HL)—Carbon fixation by Rubisco• Rubisco fixes CO₂ to RuBP, forming glycerate 3-phosphate• Rubisco is abundant but slow and limited by low CO₂ C1.3.16(HL)—Triose phosphate synthesis• Glycerate 3-phosphate is reduced to triose phosphate• ATP supplies energy and reduced NADP supplies hydrogen C1.3.17(HL)—RuBP regeneration• Most triose phosphate regenerates RuBP so the Calvin cycle continues• ATP is required for regeneration of the CO₂ acceptor C1.3.18(HL)—Synthesis of other compounds• Triose phosphate is converted into sugars, starch, lipids, and organic acids• Mineral nutrients allow synthesis of amino acids and other compounds C1.3.19(HL)—Interdependence of light and light-independent reactions• Light-dependent reactions supply ATP and reduced NADP for the Calvin cycle• The Calvin cycle returns ADP and NADP and depends on CO₂/hydrogen carbonate availability

Photosynthesis converts light energy into chemical energy stored in organic carbon compounds. It supplies the carbon compounds and most of the chemical energy that enter ecosystems.

Photoautotroph	Where chlorophyll captures light
plant or alga	thylakoid membranes inside chloroplasts
cyanobacterium	internal photosynthetic membranes; no chloroplast

Chlorophyll-containing thylakoids in a plant chloroplast and photosynthetic membranes in a cyanobacterium absorb light.

light absorbed by pigments → excited electrons → ATP and reduced NADP → CO₂ reduced into carbon compounds

Overall matter equation

6CO₂ + 6H₂O + light energy →
C₆H₁₂O₆ + 6O₂

Light-dependent reactions use water and release oxygen, while ATP and reduced NADP support carbon dioxide fixation in the stroma.

Atom or product	Source and fate
carbon	CO₂ is fixed and reduced into carbohydrate
hydrogen	water supplies hydrogen carried by reduced NADP
released O₂	produced by photolysis of water, not removed from CO₂

Plants, algae and cyanobacteria carry out oxygenic photosynthesis. Glucose is a useful summary product, but fixed carbon also feeds the synthesis of many other compounds.

Pigments separate because each partitions differently between a moving solvent and the stationary phase. A pigment that is more soluble in the solvent and less strongly attracted to the stationary phase moves farther.

A pigment chromatogram labels the origin, pigment distance and solvent-front distance used to calculate Rf.

$R_f = \frac{d_{pigment}}{d_{solvent\ front}}$

Extract pigments in an organic solvent and place a concentrated spot on the origin.
Keep the origin above the solvent; let the solvent rise.
Mark the solvent front immediately, then measure from the origin to each spot's centre.
Compare colour and Rf only with standards run under the same solvent, temperature and stationary phase.

Absorption spectra show carotenoids, chlorophyll b and chlorophyll a absorbing different parts of the visible spectrum, especially blue and red wavelengths.

A photon is absorbed only when its energy matches an allowed change in a pigment's electrons. Absorption raises an electron to an excited state, beginning the energy transfers of the light-dependent reactions.

Chlorophylls absorb strongly in blue and red regions but weakly in much of the green region, so green light is reflected or transmitted. Accessory pigments absorb additional wavelengths and transfer excitation energy toward chlorophyll a.

Spectrum	What is varied	What the y-axis measures	What it tests
absorption	wavelength	light absorbed by a pigment or extract	which wavelengths pigments capture
action	wavelength	rate of photosynthesis, such as O₂ production or CO₂ uptake	which wavelengths drive the whole process

Both normally show strong effects in blue and red regions and weaker effects in much of the green region. The curves are related, not identical: an action spectrum integrates all pigments, energy transfer and every later rate-limiting step.

To compare wavelengths fairly, keep incident photon supply, temperature, CO₂ availability and plant material constant; a coloured filter that also changes intensity confounds the conclusion.

A limiting factor is the condition in shortest effective supply relative to the requirements of photosynthesis. Increasing it raises the rate only until another factor becomes limiting.

Photosynthesis rate rises with light intensity and then plateaus; high carbon dioxide permits a higher plateau than low carbon dioxide.

Region of curve	Interpretation
steep rise as light increases	light is limiting
plateau at low CO₂	added light cannot overcome carbon limitation
higher plateau at high CO₂	relieving CO₂ limitation reveals a higher maximum under those conditions

Estimate photosynthesis from oxygen volume produced per unit time or carbon dioxide removed per unit time. Bubble number is less valid because bubble size varies.

An illuminated Elodea shoot supplies oxygen to a capillary, where gas-bubble displacement is measured with a microburette and stopwatch.

Experimental role	Example
independent variable	light intensity, CO₂ concentration or temperature
dependent variable	initial O₂-production or CO₂-uptake rate
controls	plant species and amount, measurement time, wavelength, the other limiting factors
reliability	repeat each treatment and compare means with variation

Allow the apparatus to equilibrate, change one independent variable across a suitable range, measure an initial rate, reset the gas bubble or sensor, and repeat. Use a heat shield or water bath when changing lamp distance so temperature does not change with light.

Photosynthesis rate rises with temperature to an optimum and then falls steeply above the optimum.

Temperature region	Why rate changes
below optimum	greater kinetic energy increases successful enzyme–substrate collisions
optimum	combined photosynthetic processes reach their highest rate under these conditions
above optimum	carbon-fixation performance falls and other heat stresses increase; do not assume immediate wholesale enzyme denaturation

Light and CO₂ usually give rising curves that level off as limitation changes. Temperature affects enzyme-controlled reactions, so its curve normally rises to an optimum and then declines.

Extra CO₂ can raise photosynthesis or growth only while light, temperature, water, mineral nutrients and biochemical capacity remain sufficient. An enrichment result therefore supports a conditional prediction, not unlimited future growth.

Towers around a forest plot release carbon dioxide into open air for a FACE enrichment experiment.

Design	Strength	Main limitation
greenhouse or chamber	close control of CO₂ and other conditions	enclosure changes light, temperature, airflow and scale
FACE field plot	exposes intact crops or ecosystems under realistic open-air conditions	rainfall, light and other variables cannot be controlled and must be monitored

Compare enriched and ambient-CO₂ replicates, measure photosynthesis and biomass over time, and report variation. Differences among species, years or water treatments show interaction with other limiting factors.

light energy → excited pigment electrons → ATP and reduced NADP → CO₂ reduced using hydrogen from water → organic carbon compounds; photolysis of water releases O₂

A chromatogram separates pigments; Rf compares their movement under matched conditions.
An absorption spectrum measures captured wavelengths; an action spectrum measures whole-process rate.
A limiting-factor experiment changes one condition, controls the others and measures O₂ production or CO₂ uptake per unit time.

When a rate curve plateaus, identify which factor was relieved and which may now be limiting. When evaluating CO₂ enrichment, separate strong control from ecological realism and do not extrapolate beyond the tested conditions.

A photosystem is a pigment–protein array in a thylakoid membrane. Antenna chlorophylls and accessory pigments transfer excitation energy to a special reaction-centre chlorophyll; the excited reaction centre transfers an electron to a primary acceptor.

Many antenna pigments absorb light and funnel excitation energy to a reaction-centre chlorophyll that emits an excited electron.

Different pigments broaden the range of wavelengths captured.
Many antenna molecules increase the chance that light reaches one reaction centre.
Ordered energy transfer concentrates excitation where charge separation can begin.
Proteins hold pigments and acceptors in positions that make transfer efficient.

Energy moves among antenna pigments; the electron that leaves comes from the reaction-centre chlorophyll. A single isolated chlorophyll lacks the organized donors, acceptors and carriers required for sustained photosynthesis.

$2H_2O \rightarrow 4H^+ + 4e^- + O_2$

Product of photolysis	Immediate role
electrons	replace electrons emitted by PSII; later pass through carriers to PSI
H⁺	contributes to the high proton concentration inside the thylakoid space
O₂	leaves as a by-product or is used in aerobic respiration

Light excites PSII reaction-centre electrons → a primary acceptor removes them → the water-splitting complex supplies replacement electrons → electron flow can continue.

Photosystem II acts before photosystem I in linear electron flow; its number reflects discovery order, not pathway order. Photolysis supplies electrons and protons but does not make glucose directly.

water → PSII → electron carriers → PSI → NADP

Light excites electrons at both photosystems. Energy released between PSII and PSI drives proton translocation; PSI re-excites the electrons so NADP can accept them with stromal H⁺ to form reduced NADP.

A thylakoid membrane contains PSII, electron carriers, PSI, NADP reductase and ATP synthase, with protons accumulated in the thylakoid space.

Stromal-side product	How it is formed	Next use
ATP	H⁺ returns through ATP synthase	energy for Calvin-cycle reactions
reduced NADP	NADP accepts electrons from PSI and H⁺	reducing power for triose-phosphate synthesis

Process	Effect on thylakoid-space H⁺
water is split beside PSII	releases H⁺ into the space
electron transport	uses released energy to move H⁺ from stroma into the space
NADP reduction on stromal side	removes H⁺ from stroma, strengthening the difference

Because the thylakoid membrane restricts free proton diffusion, the space becomes more acidic and positive relative to the stroma. This electrochemical gradient stores potential energy.

$ADP + P_i \xrightarrow[H^+\ flow]{ATP\ synthase} ATP$

H⁺ can return mainly through ATP synthase. Proton flow drives enzyme rotation and conformational change, coupling dissipation of the gradient to photophosphorylation. ATP is released on the stromal side, beside the Calvin-cycle enzymes that use it.

Feature	Non-cyclic flow	Cyclic flow
photosystems	PSII and PSI	PSI only
electron path	water → NADP	returns to PSI through carriers
photolysis and O₂	required; O₂ released	not required; no O₂ from the cycle
products	ATP and reduced NADP	ATP only

If the Calvin cycle uses proportionally more ATP than reduced NADP, or NADP is temporarily unavailable, cyclic flow can reinforce proton pumping and ATP formation without adding more reduced NADP.

Carbon fixation occurs in the chloroplast stroma. Rubisco catalyses addition of CO₂ to ribulose bisphosphate (RuBP), a five-carbon CO₂ acceptor.

$5C + 1C \rightarrow 6C \rightarrow 2 \times 3C$

Fixation moves inorganic carbon into organic molecules. Glycerate 3-phosphate is the first stable product; Rubisco does not make glucose in one reaction.

Rubisco is relatively slow and performs poorly when CO₂ is scarce, so photosynthetic tissues contain it in very high concentration. This connects carbon supply to the limiting-factor patterns established earlier.

Calvin-cycle phase	Carbon change	Input from light-dependent reactions
reduction	glycerate 3-phosphate → triose phosphate	ATP supplies energy; reduced NADP supplies electrons and hydrogen
regeneration	most triose phosphate → RuBP	ATP
product exit	a minority of triose phosphate leaves the cycle	carbon skeleton available for synthesis

Glycerate 3-phosphate is reduced to triose phosphate using ATP and reduced NADP; ATP also supports regeneration of five-carbon RuBP while some triose phosphate exits to products.

After three CO₂ are fixed, six triose phosphates are formed: five regenerate three RuBP, while one is the net three-carbon gain. Two net triose phosphates can contribute carbon for one hexose phosphate.

The cycle must regenerate its acceptor. Removing every triose phosphate as product would stop further CO₂ fixation even if light, ATP and reduced NADP remained available.

Triose phosphate is the immediate exportable carbon product of the Calvin cycle. Its carbon skeletons enter other metabolic pathways; photosynthesis does not simply accumulate free glucose.

Product family	Additional input or transformation	Example fate
carbohydrates	rearrangement and polymerization	sucrose for transport; starch for storage; cellulose for walls
lipids	conversion through glycerol and fatty-acid pathways	membranes and energy stores
amino acids	nitrogen from nitrate/ammonium plus carbon skeletons	proteins and enzymes
organic acids	entry into respiratory and biosynthetic pathways	metabolic intermediates

All carbon in these products can be traced back to CO₂ fixed in the Calvin cycle, but mineral nutrients supply atoms such as nitrogen and phosphorus. Carbon fixation alone is therefore insufficient for complete plant growth.

Light-dependent reactions: thylakoid membrane	Direction	Calvin cycle: stroma
ATP	→	supplies energy for reduction and RuBP regeneration
reduced NADP	→	supplies electrons and hydrogen for reduction
ADP + Pᵢ	←	returned after ATP use
NADP	←	returned after reduced NADP is oxidized

Missing input	First system affected	Consequence for the partner
light	electron excitation stops	ATP and reduced NADP run out, so carbon reduction stops
CO₂	carbon fixation slows or stops	NADP and ADP regeneration decline; linear electron flow cannot continue normally
intact thylakoid gradient	ATP production falls	reduction and RuBP regeneration become ATP-limited

“Light-independent” means that Calvin-cycle reactions do not absorb photons directly. It does not mean they normally continue without the products of light-dependent reactions.

Electrons: H₂O → PSII → carriers → PSI → reduced NADP → glycerate 3-phosphate reduction. Oxygen is the remaining product of water oxidation.

Protons: water splitting and electron transport raise [H⁺] in the thylakoid space → H⁺ returns through ATP synthase → ATP forms on the stromal side.

Carbon: CO₂ + RuBP → 2 glycerate 3-phosphate → triose phosphate → most carbon regenerates RuBP; net triose phosphate feeds carbohydrates, lipids, amino acids and organic acids.

If this fails…	Trace the immediate consequence
PSII photolysis	replacement electrons, lumen H⁺ contribution and O₂ production fall
electron transport	proton pumping and ATP production fall
PSI/NADP reduction	Calvin-cycle reducing power falls
RuBP regeneration	CO₂ acceptor is depleted and fixation stops
exchange of NADP/ADP	both reaction sets slow despite occupying different chloroplast compartments

8 marks

Explain the processes by which light energy is converted into chemical energy.

4 marks

Outline how photosynthesis produces glucose.

1 mark

Which group(s) produce(s) oxygen as a by-product of photosynthesis?

I. Algae
II. Cyanobacteria
III. Fungi

3 marks

Describe the process used to obtain this chromatogram.

4 marks

Outline how plants make use of the different wavelengths of light.

3 marks

Draw a fully labelled graph of the action spectrum for photosynthesis.

9 marks

Explain methods by which the rate of photosynthesis can be measured, including conditions that affect the rate.

2 marks

Using the data in the graph, discuss whether rising carbon dioxide levels in the atmosphere will increase growth rates in maize.

3 marks

Outline the relationship between structure and function in photosystems.

8 marks

Some of the water carried to the leaves of a plant is used in photosynthesis. Explain the role of water in the light-dependent reactions of photosynthesis.

8 marks

Explain chemiosmosis as it occurs in photophosphorylation.

1 mark

What occurs during photosynthesis?

1 mark

State two products that pass from the light-dependent to the light-independent stages of photosynthesis.
1.
2.

8 marks

With reference to Calvin's experiment, explain the fixation of carbon dioxide in photosynthesis.

1 mark

What occurs in the light-independent reactions of photosynthesis?

1 mark

The Calvin cycle continues throughout the life of a plant. In this cycle, triose phosphate is produced, which is used to form glucose. In total, how many RuBP molecules are regenerated in the process of creating one glucose molecule?

7 marks

Explain how the light-independent reactions of photosynthesis rely on the light-dependent reactions.

Next Topic

—C2.1 Chemical signalling [HL only]Read

C1.3 Photosynthesis

Learning objectives

Photosynthesis Stores Light Energy in Carbon Compounds

Carbon Comes from CO₂; Released Oxygen Comes from Water

Chromatography Separates a Leaf's Pigment Mixture

Pigments Absorb Selected Wavelengths, Not Every Colour

Absorption and Action Spectra Answer Different Questions

A Plateau Means the Limiting Factor Has Changed

Measure Photosynthesis as a Rate, Not a Bubble Count Alone

Temperature Produces an Optimum, Not a Plateau

CO₂ Enrichment Tests a Conditional Prediction

SL Checkpoint: Read Photosynthesis as Matter, Energy and Evidence

A Photosystem Funnels Excitation to One Reaction Centre

Photolysis Replaces the Electrons Lost by Photosystem II

Thylakoids Convert Electron Flow into ATP and Reduced NADP

The Thylakoid Membrane Stores a Proton-Motive Difference

Cyclic Electron Flow Adjusts ATP Supply Without Making Reduced NADP

Rubisco Fixes CO₂ by Attaching It to RuBP

ATP and Reduced NADP Convert Fixed Carbon into Triose Phosphate

Triose Phosphate Feeds a Network of Plant Biosynthesis

The Two Reaction Sets Exchange Energy Carriers in a Closed Partnership

HL Summary: Follow Electrons, Protons, Carbon and Carriers

Next Topic

Light energy → chemical energy

CO₂ → glucose

Oxygen as by-product

Photosynthetic pigment separation

Absorption of specific wavelengths

Absorption vs. action spectra

Limiting factors investigation

CO₂ enrichment experiments

Photosystems exam focus

Photolysis of water

ATP production in thylakoids

NADP reduction

Thylakoids as light-dependent systems

Carbon fixation by Rubisco

Triose phosphate synthesis

RuBP regeneration

Interdependence of light and light-independent reactions