Skip to main content
Biology 4% exam weight

Nutrition: Photosynthesis and Chemosynthesis

Part of the NECO SSCE study roadmap. Biology topic bio-4 of Biology.

→ See in Roadmap Build 1-Month Plan
By Last updated 4% exam weight

Nutrition: Photosynthesis and Chemosynthesis

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Photosynthesis is the conversion of light energy into chemical energy by green plants, algae and some bacteria, using chlorophyll inside the chloroplast. The overall equation is:

6CO₂ + 6H₂O →(light/chlorophyll) C₆H₁₂O₆ + 6O₂

Two stages occur: the light-dependent reaction (in the thylakoid membrane — photolysis of water yields O₂, NADPH, ATP) and the light-independent reaction / Calvin cycle (in the stroma — CO₂ is fixed into glucose by the enzyme RuBisCO).

Chemosynthesis builds glucose without sunlight. Chemoautotrophic bacteria oxidise inorganic substances — Nitrosomonas oxidises NH₄⁺ to NO₂⁻; Beggiatoa oxidises H₂S to SO₄²⁻ — releasing energy that drives CO₂ fixation. It supports life in deep-sea vents and dark soils.

FeaturePhotosynthesisChemosynthesis
Energy sourceSunlightChemical oxidation
OrganismsGreen plants, algae, cyanobacteriaNitrifying, sulphur, iron bacteria
SiteChloroplastCytoplasm / cell membrane
OxygenReleased (in most cases)Not always released

High-yield pointers: know the balanced equation, the four limiting factors (light, CO₂, temperature, water), and that the compensation point is where photosynthesis rate equals respiration rate.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

The Two Stages of Photosynthesis

The light-dependent stage needs chlorophyll trapping photons. Photolysis of water splits H₂O into H⁺, electrons and O₂. Electrons flow through the electron transport chain, generating ATP and reducing NADP⁺ to NADPH. Oxygen diffuses out through the stomata, which are pores flanked by guard cells.

The light-independent stage (Calvin cycle) uses the ATP and NADPH to fix CO₂. One CO₂ molecule is attached to ribulose bisphosphate (RuBP) by RuBisCO, forming unstable 6-carbon intermediates that split into 3-phosphoglycerate (3-PGA), then glyceraldehyde-3-phosphate (G3P). Most G3P regenerates RuBP; some exits as glucose.

Limiting Factors and Their Graphs

A limiting factor is the variable in shortest supply. NECO often asks candidates to read or sketch the effect curves.

  • Light intensity: rate rises linearly until saturation, then plateaus.
  • CO₂ concentration: rate rises up to ~0.1% CO₂, then plateaus.
  • Temperature: optimum is 25–35 °C; the curve drops sharply as RuBisCO denatures above ~40 °C.
  • Water: severe water stress closes stomata, dropping CO₂ intake.

Tip: On a graph question, the flat top of any curve shows the new limiting factor — identify which input is still rising while rate is not.

Chemosynthesis in Context

Chemoautotrophs lack chlorophyll and live where sunlight is absent — hydrothermal vents, volcanic soils, sewage beds. They oxidise reduced inorganic substrates:

  • Nitrosomonas: NH₄⁺ + O₂ → NO₂⁻ + energy (≈ 274 kJ/mol).
  • Beggiatoa: H₂S + O₂ → SO₄²⁻ + energy (≈ 800 kJ/mol).

The released energy drives CO₂ fixation into organic matter, forming the base of food chains in dark ecosystems.

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Worked Numeric Example

A water plant releases 6 cm³ of O₂ per minute under bright light, but 2 cm³ per minute in dim light, while respiration consumes a constant 1 cm³ per minute.

  • Gross photosynthesis (dim) = Net + Respiration = 2 + 1 = 3 cm³/min.
  • Net photosynthesis (bright) = 6 − 1 = 5 cm³/min.
  • Compensation point is reached when net O₂ = 0, i.e. light intensity where photosynthesis = respiration = 1 cm³/min.

Edge Cases and Examiner Traps

  • C₃ vs C₄ plants: C₄ plants (maize, sugarcane) minimise photorespiration by spatial separation of initial CO₂ fixation; the NECO syllabus expects only the C₃ pathway.
  • Bacterial photosynthesis (e.g. Chromatium) is anoxygenic — it uses H₂S instead of H₂O and releases sulphur, not oxygen. Many candidates wrongly write O₂ for every photosynthesis equation.
  • Chemosynthesis ≠ photosynthesis even though both are autotrophic; confusing them costs easy marks in 2-mark comparison questions.

Common Mistakes to Avoid

  • Writing O₂ on the reactant side of the photosynthesis equation (mixing it up with respiration).
  • Claiming chemosynthesis requires sunlight — the entire point is that it does not.
  • Forgetting that the Calvin cycle occurs in the stroma, not in the thylakoid membrane.
  • Listing light intensity as the only limiting factor.

Exam Strategy for NECO SSCE Biology

This topic sits in Section B (Theory) under “Nutritional Processes” and contributes 1–3 marks per paper, usually as a structured question. Expect: (a) a balanced equation, (b) a labelled chloroplast diagram worth 3 marks, or (c) a tabular compare-and-contrast between photosynthesis and chemosynthesis. Allocate 2 minutes per mark: write the equation first, then label, then explain.

Practice Prompts

  1. State the overall equation for photosynthesis and identify the source of the oxygen released.
  2. Describe how Nitrosomonas obtains energy for food production, and explain why this matters in soil ecosystems.

Continue your study

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.

Sources & verification