How can we increase food supply sustainably, and how does photosynthesis make food possible?
Food supply, sustainable food production and the demands of a growing human population, the dependence of food production on photosynthesis, the capture and conversion of light energy in the light reactions and carbon fixation, and the factors limiting productivity in crops and livestock.
An SQA Higher Biology answer on food supply and photosynthesis, covering sustainable food production for a growing population, the dependence of food production on photosynthesis, the capture and conversion of light energy and carbon fixation, and the factors that limit productivity.
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What this key area is asking
The SQA wants you to explain why food production must be sustainable for a growing population, explain how food production depends on photosynthesis, describe the capture and conversion of light energy and carbon fixation, and describe the factors that limit productivity in crops and livestock.
Sustainable food supply
As the human population grows, food production must increase, but it must do so sustainably, meaning without exhausting soils, water or other resources that future production depends on. Producing plant crops is generally more efficient than rearing livestock, because energy is lost between trophic levels (as heat in respiration, in movement and in undigested waste). Feeding people directly on plants therefore feeds far more people from the same area than feeding them on animals that ate those plants.
Photosynthesis: the light reactions
The light reactions take place on the membranes inside the chloroplast, where the pigments are held. The ATP and the hydrogen on NADP that they produce are the two products passed on to the next stage; without them, carbon fixation cannot run, which is why photosynthesis needs light.
Photosynthesis: carbon fixation
In the cycle:
- The enzyme RuBisCO joins carbon dioxide to the acceptor molecule RuBP.
- The ATP and hydrogen (from NADPH) are used to convert the product into G3P, a sugar.
- Some G3P is used to make glucose and other products (which the plant uses for energy, growth and storage), and some is used to regenerate RuBP so the cycle can continue.
Because carbon fixation depends on the ATP and hydrogen from the light reactions but does not itself need light directly, the two stages are tightly linked: the light reactions trap the energy, and carbon fixation uses it to build sugar.
Factors limiting productivity
The rate of photosynthesis, and therefore the productivity of a crop, can be limited by:
- Light intensity, which provides the energy for the light reactions.
- Carbon dioxide concentration, which supplies the carbon for carbon fixation.
- Temperature, which affects the enzymes such as RuBisCO.
At any time, the factor in shortest supply is the limiting factor, and increasing it raises the rate until another factor becomes limiting. Growers exploit this in greenhouses by adding carbon dioxide, extra light and warmth to push up yield.
Examples in context
Example 1. Greenhouse tomato production. Commercial tomato growers raise yield by controlling the limiting factors of photosynthesis. They add extra carbon dioxide to the air, provide artificial lighting on dull days, and keep the greenhouse warm. By making sure none of light, carbon dioxide or temperature is in short supply, they push the rate of photosynthesis higher and produce far more fruit per plant than would be possible outdoors. This is a direct application of the limiting factor idea.
Example 2. Feeding a growing population with cereals. Cereal crops such as wheat and rice are the staple foods for most of the world because growing them is an efficient use of land. Eating cereals directly places people just one trophic level above the producers, so little energy is lost. If instead the cereal were fed to cattle and people ate the beef, most of the energy would be lost between trophic levels, and the same land would feed far fewer people. This is why sustainable food planning emphasises plant crops.
Try this
Q1. Name the two products of the light reactions that are used in carbon fixation. [2 marks]
- Cue. ATP and hydrogen (carried by NADP).
Q2. Explain why producing crops can feed more people than producing livestock from the same area. [2 marks]
- Cue. Energy is lost between trophic levels, so feeding people directly on plants wastes less energy than feeding them on animals.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20194 marksDescribe the events of the light reactions of photosynthesis and explain how their products are used in carbon fixation.Show worked answer →
A 4-mark answer needs the light reaction events plus the link to carbon fixation.
In the light reactions, chlorophyll absorbs light energy and uses it to excite electrons. This energy is used to generate ATP and to split water (photolysis), which releases oxygen as a by-product and provides hydrogen that is picked up by the carrier NADP, forming NADPH.
The ATP and the hydrogen carried by NADP are then passed to carbon fixation, where they are used to convert the product of carbon dioxide fixation into sugar (G3P).
Markers reward chlorophyll absorbing light and exciting electrons, ATP generated, water split to give oxygen and hydrogen on NADP, and the use of ATP and hydrogen in carbon fixation.
SQA Higher 20214 marksA crop converts 1.8 percent of the light energy reaching a field into plant biomass. The field receives 4000 megajoules of light energy. Calculate the energy stored as biomass, and explain why growing crops can feed more people than rearing livestock on the same land.Show worked answer →
This is a percentage and trophic-level question.
Step 1. Find the energy stored: 1.8 percent of 4000 megajoules is megajoules.
Step 2. Growing crops can feed more people because energy is lost between trophic levels (as heat in respiration, in movement and in undigested material). When people eat plants directly, they occupy one trophic level above the producer; when people eat animals that ate the plants, an extra trophic level is added and most of the energy is lost on the way.
Markers reward the calculation giving 72 megajoules and a clear explanation of energy loss between trophic levels.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)