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Photosynthesis
In photosynthesis, plants use light energy, absorbed by chlorophyll in chloroplasts, to make glucose from carbon dioxide and water, releasing oxygen: . Glucose is used for respiration or converted to starch (storage), cellulose (cell walls), sucrose (transport) and, with nitrate ions, amino acids. The rate is controlled by limiting factors: light intensity, carbon dioxide concentration and temperature; whichever is in shortest supply limits the rate.
Cell Structure
All living things are made of cells. Animal cells have a cell membrane (controls what enters and leaves), cytoplasm (where reactions happen), a nucleus (contains DNA, controls the cell), mitochondria (aerobic respiration) and ribosomes (protein synthesis). Plant cells also have a cellulose cell wall (support), chloroplasts (photosynthesis) and a large permanent vacuole (cell sap, keeps cells firm). Bacterial cells have no nucleus: their DNA is a single loop, often with plasmids.
Leaf Structure and Mineral Ions
Leaves are adapted for photosynthesis: broad and thin (large surface area, short diffusion distance), with a waxy cuticle, a transparent upper epidermis, a palisade mesophyll layer packed with chloroplasts, a spongy mesophyll with air spaces for gas exchange, stomata (opened and closed by guard cells) and vascular bundles (xylem brings water, phloem removes sugars). Plants also need nitrate ions to make amino acids (deficiency: stunted growth, yellow older leaves) and magnesium ions to make chlorophyll (deficiency: yellowing between veins).
Exothermic and Endothermic Reactions
Exothermic reactions release heat to the surroundings (temperature rises; negative), e.g. combustion, neutralisation. Endothermic reactions take in heat (temperature falls; positive), e.g. thermal decomposition, photosynthesis. Breaking bonds takes in energy; making bonds releases it. . Energy released into water: .
Food Chains and Energy Flow
Energy enters ecosystems as sunlight captured by producers (plants) in photosynthesis. It passes along food chains to primary consumers (herbivores), secondary consumers and tertiary consumers (carnivores). Each feeding level is a trophic level. Many chains link into a food web. At each transfer only about 10% of the energy passes on; the rest is lost in respiration, movement, heat, and uneaten or undigested parts. So food chains are short, and pyramids of energy and biomass narrow upwards. Decomposers release energy from dead material.
Nutrient Cycles
Elements are recycled in ecosystems. Carbon cycle: CO₂ is removed from air by photosynthesis; returned by respiration (plants, animals, decomposers) and combustion of fuels; passed along food chains by feeding; trapped in fossil fuels if decomposition doesn't happen. Nitrogen cycle: nitrogen-fixing bacteria (in soil and root nodules of legumes) turn N₂ into nitrogen compounds; decomposers turn proteins and urea into ammonium; nitrifying bacteria convert ammonium → nitrite → nitrate, absorbed by plants; denitrifying bacteria return nitrate to N₂ in waterlogged soil. Water cycle: evaporation, transpiration, condensation, precipitation.
Pollution and Conservation
Humans destroy habitats for farming, building, mining and logging. Deforestation reduces biodiversity, increases CO₂ (less photosynthesis, burning), causes soil erosion and flooding. Eutrophication: fertilisers or sewage add nutrients → algal bloom → algae block light, plants die → decomposers respire and use up oxygen → fish die. Greenhouse gases (CO₂, methane) cause climate change. Plastics don't biodegrade and harm wildlife. Conservation protects species and habitats with reserves, laws, seed banks, captive breeding and education; sustainable use includes fishing quotas, net mesh sizes and replanting forests.
Syllabus topics
| Topic | Board | Paper | Lesson |
|---|---|---|---|
| Photosynthesis | Cambridge 5090 | Multiple Choice | Available |
| Photosynthesis | Cambridge 5090 | Theory | Available |
| Photosynthesis | Cambridge 0610 | Extended | Available |
| Photosynthesis | Cambridge 0610 | Core | Available |
| Photosynthesis | Edexcel 4BI1 | Paper 1 (core content) | Available |
| Photosynthesis | Edexcel 4BI1 | Paper 2 (all content) | Available |
| Photosynthesis | Cambridge 9700 | A Level Structured Questions | Planned |
Questions
| ID | Question | Topic | Marks | Difficulty |
|---|---|---|---|---|
| SA-OB-PH-002 | Light is absorbed by… | Biology: photosynthesis and limiting factors Word and balanced equations for photosynthesis | 1 | L1 Foundation |
| SA-OB-LE-001 | Most photosynthesis takes place in the… | Biology: leaf structure and mineral requirements Leaf tissues and their adaptations | 1 | L1 Foundation |
| SA-OB-PH-001 | The gas released by photosynthesis is… | Biology: photosynthesis and limiting factors Word and balanced equations for photosynthesis | 1 | L1 Foundation |
| SA-OB-CE-002 | Photosynthesis takes place in the… | Biology: cell structure Structures in animal, plant and bacterial cells and their functions | 1 | L1 Foundation |
| SA-OB-PH-012 | Plants are "destarched" before experiments by… | Biology: photosynthesis and limiting factors Investigating photosynthesis (starch test, pondweed) | 1 | L1 Foundation |
| SA-OB-PH-006 | On a graph of rate against light intensity, the curve levels off. Light is now… | Biology: photosynthesis and limiting factors Limiting factors: light, carbon dioxide, temperature | 1 | L2 Standard |
| SA-OB-PH-007 | Glucose is stored in plants as… | Biology: photosynthesis and limiting factors Word and balanced equations for photosynthesis | 1 | L2 Standard |
| SA-OB-PH-008 | Plants need nitrate ions to make glucose into… | Biology: photosynthesis and limiting factors Word and balanced equations for photosynthesis | 1 | L2 Standard |
| SA-OB-PH-009 | A lamp is moved from 30 cm to 15 cm from a plant. By what factor does the light intensity increase?… | Biology: photosynthesis and limiting factors Investigating photosynthesis (starch test, pondweed) | 2 | L2 Standard |
| SA-OB-PH-010 | Pondweed releases 45 bubbles in 3 minutes. Calculate the rate in bubbles per minute.… | Biology: photosynthesis and limiting factors Investigating photosynthesis (starch test, pondweed) | 2 | L2 Standard |
| SA-AB-PR-012 | Petri dishes of bacteria are incubated at 25 °C in schools to… | A Level Biology: practical techniques and data handling Core practicals: osmosis, enzymes, respiration, photosynthesis | 1 | L2 Standard |
| SA-OB-SC-005 | Palisade mesophyll cells are adapted for photosynthesis because they… | Biology: specialised cells, organisation and magnification Adaptations of specialised cells | 1 | L2 Standard |
| SA-OB-PH-003 | In 6CO_2+6H_2O C_6H_12O_6+\_\,O_2, what number goes in front of O_2?… | Biology: photosynthesis and limiting factors Word and balanced equations for photosynthesis | 1 | L2 Standard |
| SA-OB-PH-004 | In the starch test on a leaf, the leaf is warmed in ethanol to… | Biology: photosynthesis and limiting factors Investigating photosynthesis (starch test, pondweed) | 1 | L2 Standard |
| SA-OB-PH-005 | A variegated leaf is tested with iodine. Only the green parts turn blue-black. This shows… | Biology: photosynthesis and limiting factors Investigating photosynthesis (starch test, pondweed) | 1 | L2 Standard |