IB DP Biology: Photosynthesis: pigments, limiting factors, light-dependent reactions and the Calvin cycle (DP Biology) – Revision Notes

These are condensed revision notes for IB Diploma Programme Biology topic C1.3 (Photosynthesis), from the IB Biology guide, first assessment 2025. Statements C1.3.1 to C1.3.8 are SL and HL; C1.3.9 to C1.3.19 are additional higher level and are marked “HL only”. For full explanations read the study guide.

Then test yourself with the practice set. Course pages: IB DP Biology hub, printable checklist and free 10-minute diagnostics.

Key ideas at a glance

Statement	Remember
C1.3.1	Light energy becomes chemical energy in carbon compounds; this supplies most of the energy for ecosystems
C1.3.2	Carbon dioxide + water → glucose + oxygen; hydrogen from split water reduces carbon dioxide
C1.3.3	Oxygen is a by-product from water, in plants, algae and cyanobacteria
C1.3.4	Rf is pigment travel divided by solvent-front travel
C1.3.5	A pigment absorbs a wavelength only if the photon energy matches an electron transition
C1.3.6	Absorption spectrum: light absorbed. Action spectrum: rate of photosynthesis
C1.3.7	Vary one factor, control the others, measure the rate
C1.3.8	Greenhouse and FACE experiments predict responses to more carbon dioxide
C1.3.9 to C1.3.14	HL only: photosystems, photolysis, chemiosmosis, NADP reduction in the thylakoid
C1.3.15 to C1.3.19	HL only: Rubisco, triose phosphate, RuBP regeneration, other compounds, interdependence

Energy, equation and oxygen (C1.3.1 to C1.3.3)

• Word equation: carbon dioxide + water → glucose + oxygen.
• Carbon dioxide is reduced; water is split. Oxygen gas comes from water.
• Evidence: water labelled with ¹⁸O gives labelled oxygen gas.
• Oxygen is released by plants, algae and cyanobacteria.

Chromatography (C1.3.4)

Method in steps:

1. Pencil start line; small concentrated spot of extract.
2. Solvent below the line; stop near the top.
3. Mark where the solvent reached straight away.
4. Measure from the start line to each spot centre.
5. Rf is spot distance divided by front distance. It has no units and is below 1.

Order up the paper: carotene (orange-yellow, highest), xanthophyll (yellow), chlorophyll a (blue-green), chlorophyll b (yellow-green, lowest). Identify by colour and by Rf compared with values from the same solvent and paper.

Worked reminder. Front 12.0 cm, spot 9.0 cm: Rf = 9.0 ÷ 12.0 = 0.75.

Spectra (C1.3.5 and C1.3.6)

Feature	Absorption spectrum	Action spectrum
Vertical axis	Light absorbed	Rate of photosynthesis
Measured on	Extracted pigment	Whole organism or cells
Shared pattern	High in blue-violet and red	High in blue-violet and red
Green	Low absorption	Low rate, but not zero

• Only some wavelengths are absorbed because photon energy must match the energy needed to raise an electron.
• Absorbed energy excites an electron; excited electrons carry the light energy into chemical form.
• Chlorophyll reflects or transmits green light, so leaves look green.
• Axis labels: wavelength in nm, with colours marked.
• Rate from oxygen: volume ÷ time. Rate from carbon dioxide: fall in concentration ÷ time.

Worked reminder. 4.5 cm³ of oxygen in 30 minutes is 4.5 ÷ 30 = 0.15 cm³ min⁻¹ (9.0 cm³ per hour).

Limiting factors (C1.3.7)

• The limiting factor is whichever is scarcest; raising it raises the rate.
• Graph shape: rises, then plateaus when another factor limits.
• Temperature graph: rises to an optimum, then falls sharply as enzymes denature.

Factor	Technique	Common control
Light intensity	Lamp distance (intensity ∝ 1 ÷ distance²)	Heat shield, constant temperature
Carbon dioxide	Different hydrogencarbonate concentrations	Same light and temperature
Temperature	Water bath	Same light and carbon dioxide

• Measures: bubble count, gas volume, floating leaf discs, oxygen or carbon dioxide sensor.
• Independent variable: what you change. Dependent variable: what you measure. Controlled variables: kept constant.
• A hypothesis is a provisional explanation that needs repeated testing. It can come from theory or from earlier evidence.
• Doubling the distance cuts intensity to one quarter: (10 ÷ 20)² = 0.25.
• Cite figures from a graph: say where the rate rises steeply, where it levels off, and which other factor may then be limiting.
• Repeat each setting and calculate a mean so one odd result does not mislead you.

Carbon dioxide enrichment (C1.3.8)

Design	Strength	Weakness
Enclosed greenhouse	Variables easy to control	Enclosure alters wind, light, soil
FACE	Natural open-air conditions, whole ecosystems	Poorer control of variables

• Control plots receive ambient air; same species, soil and watering in all plots.
• Percentage change = (new − old) ÷ old × 100. For 8.0 to 9.2: 1.2 ÷ 8.0 = 15%.

Photosystems (C1.3.9 and C1.3.10, HL only)

• An array of chlorophyll and accessory pigments in a membrane (thylakoid membrane, or the membranes of cyanobacteria).
• A special chlorophyll is the reaction centre, where an excited electron is emitted.
• Advantage of the array: wider range of wavelengths, more photons caught, energy passed to the reaction centre. One pigment molecule alone cannot do any part of photosynthesis.

Light-dependent reactions (C1.3.11 to C1.3.14, HL only)

Process	Where	Result
Photolysis, 2H₂O → 4H⁺ + 4e⁻ + O₂	Photosystem II, thylakoid space	Electrons replace those emitted; protons in the space; oxygen waste
Chemiosmosis	Carriers and ATP synthase in thylakoid membrane	Protons pumped stroma → space; flow back through ATP synthase makes ATP
NADP reduction	Photosystem I side, stroma	NADP⁺ + 2e⁻ + H⁺ → NADPH, with H⁺ from the stroma

• Non-cyclic: electrons from photosystem II (via photolysis) end in NADPH. Products: ATP, NADPH, oxygen.
• Cyclic: electrons from photosystem I return to the chain. Product: ATP only.

Calvin cycle (C1.3.15 to C1.3.18, HL only)

Method in steps:

1. Rubisco fixes CO₂ onto RuBP (5C) to give glycerate 3-phosphate, GP (3C).
2. GP becomes triose phosphate, TP, with energy from ATP and hydrogen from reduced NADP.
3. Five TP are converted to three RuBP using ATP.
4. TP not recycled leaves the cycle to make glucose and other compounds.

• No enzyme on Earth is more abundant than Rubisco. It works slowly and is poor at low carbon dioxide, so the stroma holds a lot.
• If glucose is the product, five-sixths of all TP is converted back to RuBP.
• All carbon in the organism was fixed in the cycle. Other compounds are made by pathways from a cycle intermediate. Amino acids also need mineral nutrients.

Worked reminder. Fixing 12 CO₂ gives 24 GP, then 24 TP. 20 TP regenerate 12 RuBP (20 ÷ 24 = five-sixths). 4 TP leave, making 2 glucose. Carbon check: 12 × 5 + 12 = 72 = 24 × 3.

Interdependence (C1.3.19, HL only)

• No light: neither ATP nor reduced NADP, so GP is not converted and RuBP is not regenerated.
• No carbon dioxide: reduced NADP is not used, NADP is not regenerated, electrons have nowhere to go, and photosystem II stops.

Must-know distinctions

• Absorption spectrum versus action spectrum.
• Independent versus dependent versus controlled variable.
• Cyclic versus non-cyclic photophosphorylation.
• NADP (oxidised) versus reduced NADP.
• Photolysis (light-dependent) versus carbon fixation (Calvin cycle).

Quick self-test

1. Which molecule supplies the oxygen gas given off by a photosynthesising cell?
2. A spot moves 3.5 cm while the solvent front moves 10.0 cm. Calculate Rf.
3. Why does a leaf look green?
4. How does the intensity at 40 cm compare with that at 20 cm from a lamp?
5. Define a limiting factor.
6. Distinguish a FACE experiment from an enclosed greenhouse experiment.
7. Mean yield rises from 8.0 to 9.2 t ha⁻¹ with extra carbon dioxide. Calculate the percentage change.
8. (HL only) Where does photolysis happen and what are its three products?
9. (HL only) State the substrates of Rubisco and the product.
10. (HL only) Twelve CO₂ are fixed. How many TP are made and how many regenerate RuBP?
11. (HL only) Why does photosystem II stop when carbon dioxide is absent?

Answers

1. Water.
2. 3.5 ÷ 10.0 = 0.35.
3. Chlorophyll absorbs little green light; green is reflected or transmitted.
4. One quarter ((20 ÷ 40)² = 0.25).
5. The scarcest factor, which holds the rate below its possible value.
6. FACE releases carbon dioxide into open-air plots with natural conditions; the greenhouse is enclosed, with easier control.
7. (9.2 − 8.0) ÷ 8.0 × 100 = 15%.
8. In photosystem II, in the thylakoid space: electrons, protons and oxygen.
9. RuBP and CO₂; glycerate 3-phosphate.
10. 24 TP; 20 regenerate RuBP.
11. Reduced NADP is not used, so NADP is not regenerated; the chain fills with electrons.

Where marks are usually lost

• Writing that oxygen comes from carbon dioxide.
• Taking Rf distances from the paper edge, not the start line, or giving units.
• Describing a spectrum without quoting wavelengths.
• Confusing absorption with action spectra.
• Naming no controlled variable, or naming the dependent variable as controlled.
• Using “temperature” as an explanation without linking it to enzymes or kinetic energy.
• Dividing by the new value in a percentage change.
• Placing carbon fixation or the Calvin cycle in the thylakoid.
• Saying the Calvin cycle needs no light-dependent products.

Official syllabus

IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization. Topic C1.3 (Photosynthesis), statements C1.3.1 to C1.3.19; C1.3.9 to C1.3.19 are additional higher level.