Cells and organisation, Year 10: photosynthesis and limiting factors

C₆H₁₂O₆

HintThree elements: six atoms of the first, twelve of the second, six of the third.

The whyGlucose is the sugar made in photosynthesis and used in respiration. Its molecule contains carbon, hydrogen and oxygen atoms. AQA expects you to recognise this formula alongside CO₂, H₂O and O₂.

endothermic

HintThe first part of the word means 'inside' or 'into'.

The whyIn photosynthesis, energy is transferred from the environment to the chloroplasts by light. Respiration is the opposite kind of reaction: it transfers energy out to the surroundings.

By light

HintThink about what a plant kept in a dark cupboard is missing.

The whyChlorophyll in the chloroplasts absorbs the energy, which is then used to make glucose from carbon dioxide and water. Without this supply of energy, the reaction cannot happen.

CO₂; O₂

HintOne is the gas a plant takes in through its stomata; the other is the gas it gives out.

The whyThis is the word equation written with chemical symbols: carbon dioxide + water → glucose + oxygen. AQA asks you to recognise the four symbols, not to balance the equation.

Light supplies the energy; it is not a substance that is used up

HintReactants are made of atoms. Ask whether this one is.

The whyThe left-hand side of an equation lists the substances whose atoms are rearranged into the products. Light contributes no atoms. It is the energy input that drives the reaction, so it is shown as a condition.

A limiting factor

HintThe first word of the term says what it does to the rate.

The whyFor photosynthesis the factors are temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll. Whichever is in shortest supply sets the rate; increasing any other one makes no difference.

Light is no longer the limiting factor

HintOn the flat part, giving the plant more of what is on the x-axis changes nothing.

The whyWhile the line is rising, more light gives more photosynthesis, so light is what is holding the rate back. Once the line is flat, something else, such as carbon dioxide concentration or temperature, is in short supply.

The enzymes that control photosynthesis are denatured

HintThe process is a series of reactions, each needing a protein catalyst with a precise shape.

The whyAt low temperatures the particles move slowly, so the reactions are slow. Warming speeds them up, but beyond the optimum the enzymes change shape and stop working, so the rate drops sharply.

They contain less chlorophyll, so they absorb less light

HintAsk what gives a leaf its green colour, and what that substance is for.

The whyChlorophyll is the pigment that absorbs the light energy for photosynthesis. The amount of it is one of the four factors that affect the rate. Diseases such as tobacco mosaic virus, and a shortage of minerals, can reduce it.

Temperature also affects the rate, so two variables would be changing at once

HintA fair test changes one thing only.

The whyThe independent variable is light intensity. Temperature and carbon dioxide concentration are control variables and must be kept the same, for example by using a lamp that gives out little heat, or by standing the tube in a large beaker of water.

12

HintA rate is an amount divided by the time it took.

The whyRate = number of bubbles ÷ time = 36 ÷ 3. Counting bubbles is quick but rough, because bubbles differ in size; collecting the gas and measuring its volume gives a more accurate rate.

Carbon dioxide concentration: giving more of it raises the rate, while giving more light does not

HintCompare the two flat lines and ask what is different between them.

The whyA flat line means more light makes no difference, so light is not limiting there. The other curve shows that with ten times the carbon dioxide the rate goes higher, so on the lower curve it is the gas that is in short supply.

Light intensity

HintIn this region, ten times as much of the gas makes no difference at all to the rate.

The whyWhere the two curves coincide, the rate depends only on how bright it is, and it rises as the brightness rises. Extra carbon dioxide cannot help because there is not enough energy arriving to use it.

square

HintAs a relationship it is written: intensity ∝ 1/d².

The whyThis is the inverse square law. Light spreads out in all directions, so at twice the distance the same light covers four times the area. It explains why moving a lamp a little further from a plant cuts the light it receives so sharply.

4

HintWork out how many times further away the lamp now is, then apply the law's power to that.

The whyThe distance has doubled. Light intensity ∝ 1/d², so the intensity is divided by 2² = 4: the pondweed now receives a quarter of the light it did.

No: the profit falls by £30 a week

HintCompare what is spent with what is gained.

The whyThe gain of £90 is less than the cost of £120, so the profit changes by 90 − 120 = −£30 a week. Growers raise light, temperature or carbon dioxide only while the extra yield is worth more than it costs. If another factor becomes limiting, a further increase adds little or no yield and the money is wasted.