IB Biology · Theme C: Interaction and interdependence · SL and HL
C1.3 Photosynthesis
A one-page summary of C1.3 Photosynthesis, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
How is energy from sunlight absorbed and used in photosynthesis?
How do abiotic factors interact with photosynthesis?
What C1.3 covers
Photosynthesis as an energy transformation
- C1.3.1Photosynthesis is an energy transformation
- C1.3.2Carbon dioxide becomes glucose using hydrogen from water
- C1.3.3Oxygen is a by-product, not the point
Pigments, absorption and action spectra
- C1.3.4 · Application of skillsChromatography separates the pigments a leaf contains
- C1.3.5Chlorophyll absorbs blue and red, and reflects green
- C1.3.5An absorption spectrum graphs what a pigment absorbs
- C1.3.5 · Data bookletThe visible spectrum is in the data booklet
- C1.3.6 · Application of skillsAbsorption and action spectra measure different things
Limiting factors and investigating photosynthesis
- C1.3.7 · NOSGood experimental design changes one factor at a time
- C1.3.7Reading limiting-factor curves
- C1.3.8Carbon dioxide enrichment experiments predict tomorrow's plants
Light-dependent reactions: photosystems, photolysis and chemiosmosis
- C1.3.9–10 · HLA photosystem is an array, not a single molecule
- C1.3.11 · HLSplitting water at photosystem II makes oxygen a waste product
- C1.3.12 · HLA proton gradient across the thylakoid membrane makes ATP
- C1.3.12 · HLElectrons can take a cyclic or non-cyclic route
- C1.3.13 · HLPhotosystem I reduces NADP
- C1.3.14 · HLOne membrane runs the whole light-dependent system
The Calvin cycle and interdependence
- C1.3.15 · HLRubisco fixes carbon dioxide onto a five-carbon sugar
- C1.3.16 · HLReduction converts GP into triose phosphate
- C1.3.17 · HLMost of the triose phosphate goes back to regenerating RuBP
- C1.3.18 · HLTriose phosphate builds more than just glucose
- C1.3.19 · HLLight-independent doesn't mean independent of light
C1.3 Photosynthesis: summary
The equation & energy transformation
- CO₂ + H₂O + light → glucose + O₂; H comes from water, O₂ is a by-product.
- Photosynthesis is an energy transformation: light energy → chemical energy in carbon compounds.
Pigments & spectra
- Chromatography: Rf = pigment distance ÷ solvent distance.
- Absorption spectrum (extracted pigment) vs action spectrum (living rate) — similar shape, different method.
Limiting factors
- Light and CO₂ limit the rate until another factor takes over (plateau); temperature has an optimum, then rate falls.
- FACE experiments test CO₂ enrichment under real field conditions.
HL · Light-dependent reactions
- Photosystem II: photolysis of water → O₂, H⁺, e⁻ (replace lost electrons).
- Chemiosmosis: H⁺ gradient drives ATP synthase; photosystem I reduces NADP⁺ to NADPH.
HL · Calvin cycle
- Rubisco fixes CO₂ onto RuBP → GP → triose phosphate (TP), using ATP + NADPH.
- 5 TP → 3 RuBP regenerates the cycle; TP also builds amino acids (+ mineral N), lipids.
HL · Interdependence
- No light → no ATP/NADPH → Calvin cycle stops.
- No CO₂ → photosystem II backs up and stops functioning too.
Key terms
- Pigment
- A coloured molecule, such as chlorophyll or a carotenoid, that absorbs specific wavelengths of light.
- Rf value
- Distance travelled by a pigment ÷ distance travelled by the solvent front in chromatography.
- Absorption spectrum
- A graph of how strongly an extracted pigment absorbs light across the visible spectrum.
- Action spectrum
- A graph of a living organism's rate of photosynthesis at each wavelength of light.
- Limiting factor
- Whichever variable — light, CO₂ or temperature — is currently holding back the rate of photosynthesis.
- FACE
- Free-air carbon dioxide enrichment: raising CO₂ around crops in open, unenclosed field plots.
- Photolysis
- The light-driven splitting of water at photosystem II into oxygen, H⁺ and electrons.
- Chemiosmosis
- ATP production driven by H⁺ flowing down its concentration gradient through ATP synthase.
- Photophosphorylation
- ATP synthesis driven by light — cyclic (photosystem I only) or non-cyclic (both photosystems).
- Rubisco
- The carboxylase/oxygenase enzyme that fixes CO₂ onto RuBP to start the Calvin cycle.
- RuBP
- Ribulose bisphosphate: the five-carbon sugar CO₂ is added onto during carbon fixation.
- Triose phosphate (TP)
- The three-carbon sugar that is the Calvin cycle's actual direct product, not glucose.
Sample exam questions
Three of the 56 multiple-choice questions for C1.3. Try each one before opening the answer.
Question 1. What is the role of reduced NADP made in the light-dependent reactions?
- A reducing agent in the Calvin cycle
- A structural part of the thylakoid
- An oxidising agent that removes electrons
- The final electron acceptor of PSII
Show the answer
Answer: A. It supplies hydrogen and electrons to reduce GP to triose phosphate.
Question 2. In the light-dependent reactions, how is ATP made?
- Direct conversion of light into ATP
- Oxidation of glucose in the stroma
- Chemiosmosis, driven by a proton gradient
- Substrate-level phosphorylation in the stroma
Show the answer
Answer: C. Protons diffuse from the lumen through ATP synthase, which phosphorylates ADP.
Question 3. Of every six triose phosphate molecules made in the Calvin cycle, how many are used to regenerate RuBP?
- All six regenerate RuBP.
- One regenerates RuBP; five leave.
- Five regenerate RuBP; one leaves.
- Three regenerate RuBP; three leave.
Show the answer
Answer: C. Five TP are converted to three RuBP using ATP; one TP leaves the cycle.
Linking questions
Questions that connect C1.3 to other parts of the course, the kind that come up in Paper 2.
- Photosynthesis produces glucose, which is used in cell respiration (C1.2). Compare the ATP yield from the complete oxidation of one glucose molecule with the ATP cost of synthesising one glucose molecule via photosynthesis. (see C1.2)
- The thylakoid membrane contains phospholipids and embedded proteins similar to the cell membrane (B2.1). Explain why the thylakoid membrane must be impermeable to H⁺, and how this relates to chemiosmosis. (see B2.1)
- Stomata (B3.1) allow CO₂ entry for photosynthesis but also allow water loss by transpiration. Discuss this trade-off, and how the plant's stomata respond when water is scarce. (see B3.1)
- Photosynthesis removes CO₂ from the atmosphere while respiration returns it (C4.2). Explain how these two processes interact in the global carbon cycle and how human activities have altered this balance. (see C4.2)
Practise C1.3
56 quiz questions6 data questions5 exam questionsmarkschemes included
Study notes, every question and full markschemes for C1.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.