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. Chlorophyll absorbs light most strongly in which regions of the visible spectrum?
- Ultraviolet (< 400 nm) and infrared (> 700 nm) — wavelengths invisible to the human eye
- Green (~550 nm) and yellow (~580 nm) — reflecting blue and red
- Blue (~430 nm) and red (~660 nm) — reflecting green (~550 nm), which is why leaves appear green
- All wavelengths equally, making chlorophyll a universal absorber
Show the answer
Answer: C. Chlorophyll a has absorption peaks at ~430 nm (blue) and ~662 nm (red). Green light (~500-550 nm) is poorly absorbed and mostly reflected — hence leaves appear green. Accessory pigments (chlorophyll b, carotenoids) absorb wavelengths chlorophyll a misses, extending the action spectrum.
Question 2. In the light-dependent reactions, ATP is synthesised by:
- Direct conversion of light energy to ATP by chlorophyll
- Oxidation of glucose in the stroma
- Chemiosmosis — light energy drives H⁺ pumping into the thylakoid lumen; the proton gradient drives ATP synthase
- Substrate-level phosphorylation in the Calvin cycle
Show the answer
Answer: C. Light energy excites electrons in chlorophyll → electrons pass through an ETC in the thylakoid membrane → energy released pumps H⁺ from stroma into thylakoid lumen → proton gradient established → H⁺ flows back through ATP synthase → ATP produced. This is photophosphorylation, analogous to oxidative phosphorylation in mitochondria.
Question 3. If a plant is illuminated with only green light:
- Only the Calvin cycle would operate
- Carotenoids would become the primary photosynthetic pigment
- Photosynthesis would be very slow — chlorophyll reflects green light, so little energy is absorbed
- Photosynthesis would be maximised because green light has the most energy
Show the answer
Answer: C. Chlorophyll absorbs blue (~430 nm) and red (~660 nm) strongly but reflects green (~550 nm). Green light is poorly absorbed → insufficient energy for the light reactions → little ATP/NADPH produced → Calvin cycle limited.
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
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.