Life Processes in Animals and Plants / Nutrition
Nutrition is the process by which organisms acquire and use the materials and energy needed to grow, reproduce and maintain themselves. The PMDC MDCAT 2026 syllabus expects you to differentiate autotrophic and heterotrophic nutrition, master photosynthesis — including the light reactions on the thylakoid membrane and the Calvin (dark) cycle in the stroma — and recognise factors that limit photosynthetic rate. This is high-yield: 3-5 MCQs in a typical paper.
Autotrophic Nutrition
An autotroph ("self-feeder") synthesises its own organic food from simple inorganic raw materials. Autotrophs are the producers in every ecosystem.
- Photoautotrophs
- Use light energy — green plants, algae, cyanobacteria. Capture CO2 + H2O and produce glucose + O2.
- Chemoautotrophs
- Use chemical energy from oxidising inorganic substances (NH3, H2S, Fe2+). Examples: nitrifying bacteria Nitrosomonas and Nitrobacter.
- Heterotrophs
- Cannot synthesise food; depend on organic matter made by autotrophs. Animals, fungi and most bacteria.
Carbon dioxide (CO2) is absorbed through stomata. Water (H2O) is taken up by roots and transported via xylem. Light energy — chiefly red and blue wavelengths — is absorbed by chlorophyll. Mineral ions (Mg2+ for chlorophyll, N for proteins) are obtained from soil.
Photosynthesis
Photosynthesis is the conversion of light energy into chemical energy stored in glucose. Overall equation:
6 CO2 + 6 H2O ⟶light, chlorophyll C6H12O6 + 6 O2
A double-membrane organelle. Inside is the stroma (a fluid matrix) and a system of flattened sacs called thylakoids stacked into grana. Light reactions occur on thylakoid membranes; the Calvin cycle (dark reactions) occurs in the stroma. Chloroplasts also contain their own circular DNA and 70S ribosomes.
Chlorophyll a (P680, P700) is the primary pigment that drives the light reactions. Chlorophyll b, carotenoids and xanthophylls are accessory pigments that broaden the absorption spectrum and protect chlorophyll from photo-oxidation. The action spectrum (rate of photosynthesis vs wavelength) closely matches the chlorophyll absorption spectrum — peaks in the blue (~430 nm) and red (~660 nm) regions.
Light and Dark Reactions
Photosynthesis runs in two coupled stages: the light-dependent reactions (which need light directly) and the light-independent (Calvin) cycle (which uses the products of the light reactions but does not itself need light).
Light reactions
Site: thylakoid membrane. Inputs: light, H2O, NADP+, ADP + Pi. Outputs: O2, NADPH, ATP.
Two protein-pigment complexes embedded in thylakoid membranes:
- PSII (P680): absorbs light at 680 nm. Uses energy to split water (photolysis): 2 H2O → 4 H+ + 4 e− + O2. Releases the O2 we breathe.
- PSI (P700): absorbs at 700 nm. Re-energises electrons that have travelled down the chain, allowing them to reduce NADP+ to NADPH.
Excited electrons from PSII pass through plastoquinone, the cytochrome b6f complex, and plastocyanin to PSI — pumping H+ into the thylakoid lumen. The proton gradient drives ATP synthase on the thylakoid membrane to make ATP — this is non-cyclic photophosphorylation. PSI also operates a cyclic path that produces ATP only (no NADPH, no O2) when extra ATP is needed.
Dark (Calvin) cycle
Site: stroma. Inputs: CO2, ATP, NADPH. Outputs: glucose precursor (G3P), ADP, NADP+. Three phases:
- Carbon fixation: CO2 joins the 5C sugar RuBP, catalysed by the most abundant enzyme on Earth — RuBisCO. The unstable 6C intermediate immediately splits into two 3C molecules of 3-PGA.
- Reduction: Each 3-PGA is phosphorylated by ATP and reduced by NADPH to G3P (glyceraldehyde-3-phosphate). For every 6 CO2 fixed, the cycle uses 18 ATP and 12 NADPH; 2 G3P leave the cycle to make glucose.
- Regeneration of RuBP: The remaining G3P molecules are rearranged using more ATP back into RuBP so the cycle can continue.
| Property | Light reactions | Dark cycle (Calvin) |
|---|---|---|
| Site | Thylakoid membrane (grana) | Stroma |
| Need light directly? | Yes | No (uses products of light reactions) |
| Inputs | Light, H2O, NADP+, ADP + Pi | CO2, ATP, NADPH |
| Outputs | O2, NADPH, ATP | G3P (→ glucose), ADP, NADP+ |
| Key event | Photolysis of water; electron transport; photophosphorylation | Carbon fixation; reduction; RuBP regeneration |
| Key enzyme / complex | Photosystems II & I, ATP synthase | RuBisCO |
| Pigments involved | Chlorophyll a (P680, P700), accessory pigments | None directly |
| ATP per CO2 fixed | Produces ~3 ATP | Consumes 3 ATP |
C3 vs C4 plants
| Property | C3 plants | C4 plants |
|---|---|---|
| First CO2 acceptor | RuBP (5C) | PEP (3C, in mesophyll) |
| First stable product | 3-PGA (3C) | Oxaloacetate (4C) |
| CO2-fixing enzyme | RuBisCO (low CO2 affinity) | PEP carboxylase (high affinity, no O2 competition) |
| Anatomy | Single mesophyll layer | Kranz anatomy — mesophyll + bundle-sheath cells |
| Photorespiration | High (especially in heat) | Very low |
| Optimum conditions | Cool, moist, moderate light | Hot, dry, high light |
| Water-use efficiency | Lower | Higher |
| Examples | Rice, wheat, soybean, most temperate plants | Maize, sugarcane, sorghum, millet |
Factors affecting the rate of photosynthesis
According to Blackman’s law of limiting factors, the slowest factor sets the overall rate.
- Light intensity — rate rises until light saturation.
- CO2 concentration — usually the limiting factor in well-lit fields (atmospheric CO2 ~0.04%).
- Temperature — optimum around 25-35°C; enzymes (especially RuBisCO) denature beyond ~40°C.
- Water availability — drought closes stomata, restricting CO2 entry.
- Chlorophyll content and mineral ions (Mg2+, N).
Worked MCQs
Five MCQs covering the high-yield testing patterns for nutrition and photosynthesis.
Q1. The light reactions of photosynthesis occur in which part of the chloroplast?
Photosystems, electron carriers and ATP synthase are all embedded in the thylakoid membrane. Light energy is absorbed there, water is split, and ATP plus NADPH are produced. The Calvin cycle, in contrast, runs in the stroma.
Q2. Oxygen released during photosynthesis comes from which molecule?
Photolysis of water at PSII releases molecular oxygen: 2 H2O → 4 H+ + 4 e− + O2. Isotope-labelling experiments (using 18O) confirmed that O2 originates from water, not from CO2.
Q3. The enzyme responsible for fixing CO2 in the Calvin cycle is:
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) attaches CO2 onto RuBP. PEP carboxylase fixes CO2 in C4 plants but only as a pre-concentration step. RuBisCO is widely regarded as the most abundant protein on Earth.
Q4. In non-cyclic photophosphorylation, the final electron acceptor is:
Electrons from PSII flow through the electron transport chain to PSI and finally reduce NADP+ to NADPH. NADPH then supplies the reducing power needed in the Calvin cycle. Oxygen is the final acceptor in respiration, not photosynthesis.
Q5. Which factor is most often the limiting factor for photosynthesis in a well-lit, well-watered crop field on a warm day?
When light, water and temperature are abundant, the atmospheric CO2 level (~0.04%) becomes the bottleneck. This is why greenhouse growers enrich the air with CO2 to boost yields.
Quick Recap
- Autotrophs make their own food: photoautotrophs (light) and chemoautotrophs (chemical energy).
- Overall photosynthesis: 6 CO2 + 6 H2O → C6H12O6 + 6 O2.
- Light reactions on thylakoid membrane → ATP, NADPH, O2.
- PSII splits water (photolysis); PSI re-energises electrons to reduce NADP+.
- Calvin cycle in stroma: fix (RuBisCO), reduce (ATP + NADPH), regenerate (RuBP).
- C4 plants pre-fix CO2 via PEP carboxylase — less photorespiration.
- Limiting factors: light, CO2, temperature, water.