Photosynthesis at Higher Level: The same ideas as SL, with ocean phytoplankton, Brazil's sugar cane and the Atacama Desert. At HL this is the base for primary productivity: how fast producers turn light into biomass.
Practise this as you read
- Follow the energy from light to glucose to biomass.
- Link every climate factor to photosynthesis.
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How energy gets into living things: Photosynthesis is how the Sun's energy gets into an ecosystem. Only autotrophs can do it.
The points to remember
- Almost all the energy in ecosystems comes from the Sun.
- Photosynthesis converts light energy into chemical energy in glucose.
- It happens in chloroplasts, where the green pigment chlorophyll absorbs light.
- It uses carbon dioxide and water, and gives out oxygen (the word equation is on 2.2.3).
- Only autotrophs (plants, algae, some bacteria) can do it: it is the main way energy enters ecosystems.
- The chemical energy is then passed along food chains when the producer is eaten.
Remember it as: Light goes in, sugar comes out.
Real example: tiny ocean phytoplankton carry out about half of all the photosynthesis on Earth, feeding almost every food chain in the sea.
What to learn, and what not to: The green chloroplasts hold chlorophyll. You do not need the chemical steps inside them: learn what goes in, what comes out, and the energy change.
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A plant does not keep all its glucose as glucose. Some is used straight away; the rest is built into its body, its biomass.
From glucose to biomass
- Some glucose is respired at once, to give the plant energy (2.2.6): it is not stored.
- The rest is converted into other carbon compounds.
- Starch stores energy, as in potatoes and rice grains; cellulose builds cell walls and wood.
- With nitrogen from the soil, glucose becomes proteins; plants also make fats and oils, often in seeds.
- These compounds make up the plant's biomass: stored chemical energy that consumers can eat.
Real example: sugar cane stores its glucose as sucrose in its stems. Brazil, the world's largest grower, turns much of it into fuel for cars.
Not all of it is stored: Say 'some of the glucose is stored as biomass'. The plant respires the rest to stay alive, so it never becomes biomass.
How fast producers make new biomass is their primary productivity. Climate limits it by limiting photosynthesis. Insolation is one of the factors.
What limits photosynthesis
- Photosynthesis needs light, water, carbon dioxide and warmth.
- Low rainfall or drought: too little water for photosynthesis.
- Low temperature: photosynthesis slows, and frozen water cannot be taken up by roots.
- Low insolation: too little light energy for photosynthesis.
- Very high temperature: the rate of photosynthesis falls, and more water evaporates.
Name the factor AND link it: 'Low rainfall' alone scores nothing. Write 'low rainfall means too little water for photosynthesis'.
Real example: parts of the Atacama Desert in Chile receive less than 1 mm of rain a year. With almost no water for photosynthesis, large areas have no plants at all.
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Tropical rainforests have the highest primary productivity of any land biome. The reason is the climate: it suits photosynthesis all year.
Why rainforests are so productive
- Primary productivity mainly comes from photosynthesis: link every point to it.
- High temperatures speed up the rate of photosynthesis.
- High rainfall gives the water photosynthesis needs.
- High insolation and stable temperatures all year: photosynthesis and growth go on continuously.
Use only what the figure shows: If the figure shows climate, answer with climate. A point about how many species live in a rainforest does not count.
Real example: Borneo's rainforest lies on the equator, so it has about 12 hours of daylight every day of the year and no winter: photosynthesis never stops.
How this comes up: Paper 2, Section B (a): outline how light energy becomes the biomass of a producer [4].
Oil palms grown in Malaysia store large amounts of oil in their fruit.
Outline how light energy is converted into the biomass of a producer such as the oil palm.
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