Unit 6 Autotrophic nutrition
Key unit competence: Evaluate the process of photosynthesis and the environmental factors that influence its rate.
Key ideas
- Autotrophs make organic compounds from inorganic ones: photoautotrophs (green plants, algae, cyanobacteria) use light energy (photosynthesis); chemoautotrophs (e.g. nitrifying bacteria Nitrosomonas, Nitrobacter; sulphur bacteria) use energy from oxidising inorganic chemicals.
- Chloroplast structure: double envelope; stroma (fluid with enzymes such as RuBisCO, circular DNA, 70S ribosomes, starch grains, lipid droplets); thylakoids stacked into grana joined by intergranal lamellae; thylakoid membranes contain photosystems (chlorophyll a and b, carotenoids), electron carriers and ATP synthase.
- Photosynthetic pigments: chlorophyll a (primary pigment at reaction centres), chlorophyll b, carotenoids (carotene, xanthophylls; accessory pigments that widen the absorbed wavelengths and protect from excess light). An absorption spectrum shows how much light of each wavelength a pigment absorbs (peaks in blue and red); an action spectrum shows the rate of photosynthesis at each wavelength; their similarity shows these pigments drive photosynthesis. Green light is mostly reflected.
- Light-dependent reactions (thylakoid membranes): light excites electrons in photosystem II (P680); photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂) replaces them and releases oxygen; electrons pass along an electron transport chain, pumping H⁺ into the thylakoid space; H⁺ flow back through ATP synthase makes ATP (photophosphorylation, chemiosmosis); photosystem I (P700) re-excites electrons, which reduce NADP to reduced NADP (NADPH). Non-cyclic photophosphorylation makes ATP, NADPH and O₂; cyclic (PSI only) makes ATP only.
- Light-independent reactions (Calvin cycle) in the stroma: CO₂ fixation: CO₂ + ribulose bisphosphate (RuBP, 5C) → two molecules of glycerate-3-phosphate (GP, 3C), catalysed by RuBisCO; reduction: GP → triose phosphate (TP) using ATP and reduced NADP; regeneration: most TP regenerates RuBP using ATP; some TP is used to make glucose, sucrose, starch, cellulose, amino acids and lipids.
- C4 plants (maize, sugar cane, sorghum): CO₂ is first fixed in mesophyll cells by PEP carboxylase into a 4C compound (oxaloacetate/malate), which is carried to bundle sheath cells (Kranz anatomy) and releases CO₂ around RuBisCO; this prevents photorespiration and is efficient in hot, bright conditions. CAM plants (cacti, pineapple, Aloe) open stomata at night, store CO₂ as malic acid, and close stomata in the day to save water.
- Limiting factors: light intensity, CO₂ concentration and temperature; the rate is limited by the factor in shortest supply (Blackman's law). Light wavelength and water also affect the rate. Glasshouses raise yields by extra light, CO₂ enrichment (paraffin burners), heating, irrigation, pest control.
- Tests: starch test on leaves (destarch, expose, boil in water, decolourise in hot ethanol, iodine → blue-black); oxygen from aquatic plants (Elodea): count bubbles or collect gas in a capillary/photosynthometer; it relights a glowing splint. Chromatography (paper or thin-layer) separates chloroplast pigments: each has an Rf value = distance moved by pigment ÷ distance moved by solvent front.
- Importance: photosynthesis makes food for almost all food chains, releases oxygen, removes CO₂ (reduces global warming), formed fossil fuels, and provides timber, fibres, medicines and biofuels.
Common misconceptions
- "The oxygen released in photosynthesis comes from carbon dioxide." No: it comes from the photolysis of water.
- "The light-independent reactions happen at night." No: they need ATP and reduced NADP from the light reactions, so they occur mainly in the light; they just do not use light directly.
- "Plants absorb green light best." No: chlorophylls absorb blue and red light; green is mostly reflected.
- "C4 plants do not use the Calvin cycle." No: they still use the Calvin cycle in bundle sheath cells; the C4 pathway concentrates CO₂ for it.
Worked examples
Example 1. In a chromatogram the solvent front moved 8.0 cm and a yellow pigment moved 7.6 cm. Calculate its Rf value and suggest the pigment.
Solution
Rf = distance moved by pigment ÷ distance moved by solvent front = 7.6 ÷ 8.0 = 0.95. A yellow-orange pigment with a high Rf (near the front) is carotene.
Example 2. Explain what happens to the concentrations of GP and RuBP when the light is switched off.
Solution
Without light, ATP and reduced NADP are no longer produced, so GP cannot be reduced to TP: GP accumulates. RuBP is still used to fix CO₂ but cannot be regenerated from TP, so RuBP falls.
Practice questions
Describe the structure of a chloroplast and relate it to its function.
Show answer
Double envelope; thylakoid membranes stacked into grana with pigments, electron carriers and ATP synthase (light reactions; large surface area); stroma with RuBisCO and Calvin cycle enzymes; starch grains; DNA and ribosomes to make some proteins.
Common mistake: Saying the Calvin cycle occurs in the thylakoids.
Distinguish between absorption and action spectra.
Show answer
Absorption spectrum: amount of light absorbed by a pigment at each wavelength. Action spectrum: rate of photosynthesis at each wavelength. Their close match shows the pigments absorb the light used for photosynthesis.
Common mistake: Saying they are identical graphs.
Describe the light-dependent reactions.
Show answer
Light excites electrons in PSII; photolysis replaces them, releasing O₂ and H⁺; electrons pass along carriers, pumping H⁺ into thylakoids; ATP synthase makes ATP as H⁺ flows out; PSI re-excites electrons, which reduce NADP to NADPH.
Common mistake: Saying oxygen comes from CO₂.
Describe the Calvin cycle.
Show answer
CO₂ is fixed to RuBP by RuBisCO forming two GP; GP is reduced to triose phosphate using ATP and reduced NADP; most TP regenerates RuBP using ATP; some TP makes glucose and other organic molecules.
Common mistake: Saying glucose is the first product.
Compare C3, C4 and CAM plants.
Show answer
C3: CO₂ fixed directly by RuBisCO (GP first product), photorespiration in heat. C4: CO₂ fixed by PEP carboxylase in mesophyll, then released in bundle sheath; efficient in heat and light (maize). CAM: CO₂ fixed at night into malic acid, stomata closed in day (cactus, pineapple).
Common mistake: Saying C4 plants open stomata at night.
Explain the effect of increasing light intensity on the rate of photosynthesis.
Show answer
At low light, rate rises with intensity (light limiting); at high light the rate levels off because CO₂, temperature or chlorophyll becomes limiting; very high light may damage pigments.
Common mistake: Saying the rate keeps rising indefinitely.
Describe how to separate chloroplast pigments by paper chromatography.
Show answer
Grind leaves with propanone/sand, filter; spot the extract repeatedly on a pencil line on chromatography paper; place in solvent below the line in a closed container; remove when solvent nearly reaches top; mark solvent front; calculate Rf values.
Common mistake: Drawing the origin line in ink.
State four ways in which glasshouse growers increase crop yields.
Show answer
Extra lighting; CO₂ enrichment; heating to optimum temperature; irrigation and fertiliser; pest control.
Common mistake: Increasing temperature without limit.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Splitting of water by light energy is: a. photolysis 2. GP is reduced to triose phosphate using: b. at night 3. The factor in shortest supply that limits the rate is the: c. limiting factor 4. CAM plants open their stomata: d. ATP and reduced NADP Show answer
1 → a (photolysis) · 2 → d (ATP and reduced NADP) · 3 → c (limiting factor) · 4 → b (at night)
- It releases oxygen.
- This is the reduction stage.
- Blackman's law.
- To save water in dry habitats.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
glycerate-3-phosphate (GP) · glowing splint · lighted splint with a pop · stroma- The fluid part of the chloroplast where the Calvin cycle occurs is the .
- Oxygen produced by pondweed relights a .
- CO₂ combines with RuBP to form two molecules of .
Show answer
1: stroma · 2: glowing splint · 3: glycerate-3-phosphate (GP)
- It contains RuBisCO.
- It is a test for oxygen.
- GP is a 3-carbon compound.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Stacks of thylakoids are called: a. blue and red light 2. Cyclic photophosphorylation produces: b. ATP only 3. Chlorophyll absorbs mainly: c. grana 4. Maize and sugar cane are examples of: d. C4 plants Show answer
1 → c (grana) · 2 → b (ATP only) · 3 → a (blue and red light) · 4 → d (C4 plants)
- Light-dependent reactions occur here.
- Only photosystem I is involved.
- Green is reflected.
- They have Kranz anatomy.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
ATP and reduced NADP · RuBisCO · chemoautotrophs · heterotrophs- The products of the light-dependent reactions used in the Calvin cycle are .
- Organisms that use energy from oxidising inorganic chemicals to make food are .
- The enzyme that fixes CO₂ to RuBP is .
Show answer
1: ATP and reduced NADP · 2: chemoautotrophs · 3: RuBisCO
- Oxygen is a by-product.
- Example: nitrifying bacteria.
- The most abundant protein on Earth.