Photosynthesis — the endothermic reaction
By the end of this topic you'll write both equations, explain why the reaction is endothermic, and link the leaf's structure to its job.
Part 1The reaction that feeds (almost) everything
Photosynthesis is the reaction that green plants and algae use to make their own food. It happens in the chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs light, and that light energy is transferred to the chemical store of the glucose that is made.
Because energy is taken in from the surroundings (from light), photosynthesis is an endothermic reaction. The raw materials are carbon dioxide (taken from the air) and water (taken up by the roots). The products are glucose and oxygen.
The equations to know
- carbon dioxide + water → glucose + oxygen recall
- The word equation. Light (absorbed by chlorophyll) drives the reaction — it sits above the arrow, not in the equation itself.
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ given
- The symbol equation. It is balanced: six carbons, twelve hydrogens and eighteen oxygens on each side.
⚠ Watch out — light is not a reactant
Don't write "light" inside the equation as if it were a chemical. Light is the energy source, and it's transferred from the surroundings, which is exactly why the reaction is endothermic. A common mix-up: chlorophyll is the green pigment; the chloroplast is the structure that holds it.
Why is photosynthesis described as an endothermic reaction?
- AIt releases energy to the surroundings as heat
- BIt takes in energy from the surroundings (light is absorbed)
- CIt produces oxygen, which is flammable
- DIt happens inside warm-blooded organisms
Show answer
Part 2The leaf — built for the job
A leaf is shaped to photosynthesise efficiently. It is broad and flat to catch as much light as possible and to give a large surface area for gases to diffuse in and out. Tiny holes called stomata on the underside let carbon dioxide diffuse in and oxygen diffuse out; their size is controlled by guard cells. The cells near the top, the palisade cells, are packed with chloroplasts because that's where most light reaches.
⚠ Watch out — name the gas the right way round
In photosynthesis, carbon dioxide goes in and oxygen comes out — the opposite of respiration. In bright light, a plant photosynthesises faster than it respires, so the net movement is CO₂ in, O₂ out.
Which feature of a leaf increases the surface area for carbon dioxide to diffuse in?
- AThe waxy upper surface
- BThe air spaces in the spongy layer and the stomata
- CThe green colour of the chlorophyll
- DThe thickness of the leaf
Show answer
Write the word equation for photosynthesis.
carbon dioxide + water → glucose + oxygen (light absorbed by chlorophyll drives it).Photosynthesis is endothermic. What does that mean here?
Energy is taken in from the surroundings — light energy is absorbed by chlorophyll and transferred to the chemical store of glucose.Where in a plant cell does photosynthesis happen, and what pigment absorbs the light?
In the chloroplasts; the green pigment chlorophyll absorbs the light.Give the balanced symbol equation for photosynthesis.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Rate of photosynthesis & limiting factors
Why a rate stops rising even when you give a plant more light — and the practical that proves it.
Part 1The three limiting factors
The rate of photosynthesis is how fast a plant makes glucose (we often measure it by how fast oxygen is given off). Three things can slow it down. Whichever is in shortest supply at that moment is the limiting factor — the one holding the rate back.
The three limiting factors are light intensity, carbon dioxide concentration and temperature. Increase the limiting one and the rate goes up — until something else becomes limiting and the rate levels off (it reaches a plateau).
Temperature works differently at the top end: it's also an enzyme-controlled process, so if the temperature gets too high (above about 45 °C) the enzymes denature and the rate falls sharply rather than levelling off.
⚠ Watch out — the plateau still has a cause
When the graph levels off, light is no longer the limiting factor — carbon dioxide or temperature now is. Don't just say "the rate stops increasing"; say which factor has become limiting. And remember a graph can only tell you a factor might be limiting if changing it changes the rate.
Higher tier — light intensity and the inverse square law
Light intensity falls off quickly as you move a lamp away from a plant. It follows the inverse square law: the intensity is proportional to 1 ÷ distance².
light intensity ∝ 1 / distance²
So if you double the distance, the light intensity drops to a quarter (½² = ¼). If you triple it, intensity falls to a ninth. This is why moving a lamp twice as far away in the pondweed practical cuts the bubble rate far more than you might expect.
Part 2Measuring the effect of light
Effect of light intensity on the rate of photosynthesis
Aim: investigate how light intensity affects the rate of photosynthesis using pondweed (or algal balls).
- Place a piece of pondweed (e.g. Elodea) in a beaker of water with a measured amount of sodium hydrogencarbonate, which supplies a steady level of carbon dioxide.
- Put a lamp a set distance from the pondweed and leave it a few minutes to settle.
- Count the oxygen bubbles given off per minute (or measure the volume of gas collected) — this is your measure of the rate.
- Move the lamp to a new distance, recording the distance each time, and repeat the bubble count.
- Plot a graph of rate against light intensity (or against 1/distance²).
Control / improve: keep the temperature constant by putting a clear glass tank of water (a heat shield) between the lamp and the beaker, since lamps warm the water. Keeping CO₂ and temperature fixed makes light intensity the only variable you change.
In the pondweed experiment, the bubble rate stops rising when the lamp is brought closer. Which is the most likely reason?
- AThe pondweed has run out of water
- BCarbon dioxide or temperature is now the limiting factor
- CLight is being destroyed by the chlorophyll
- DOxygen has stopped being made completely
Show answer
Name the three factors that can limit the rate of photosynthesis.
Light intensity, carbon dioxide concentration and temperature.What does "limiting factor" mean?
The factor in shortest supply that is holding back the rate — increasing it would increase the rate.Why does the rate of photosynthesis fall if the temperature gets too high?
The enzymes denature (their shape is destroyed), so they can no longer catalyse the reaction.In the required practical, how do you keep the temperature roughly constant?
Put a tank of water (a heat shield) between the lamp and the pondweed to absorb the lamp's heat.[HT] The lamp is moved from 10 cm to 20 cm from the pondweed. By roughly what factor does the light intensity change?
Intensity ∝ 1/distance², so doubling the distance gives one quarter the intensity.
What a plant does with glucose
Five fates for the sugar a plant makes — and the one that needs nitrate from the soil.
Part 1Five uses of glucose
The glucose made in photosynthesis is the plant's raw material. It's put to five uses, and you need all of them.
Respiration. Some glucose is used straight away in respiration to release energy for the plant's life processes. Making cellulose. Glucose is converted into cellulose to strengthen the cell walls, especially important in fast-growing plants.
Making amino acids (and then proteins). Glucose is combined with nitrate ions taken from the soil to make amino acids, which are then joined into proteins. Stored as starch. Glucose is turned into starch for storage in roots, stems and leaves — starch is insoluble, so it doesn't affect the cell's water balance. Making fats and oils. Glucose is used to produce lipids (fats and oils), often stored in seeds as an energy reserve.
⚠ Watch out — proteins need nitrate
Glucose alone can become cellulose, starch and fats, but to make proteins the plant also needs nitrate ions absorbed from the soil. A plant short of nitrate grows poorly because it can't make enough protein. Also: starch is for storage; cellulose is for structure — don't swap them over.
A plant stores glucose as starch rather than leaving it as glucose. Why is that an advantage?
- AStarch is sweeter and tastes better
- BStarch is insoluble, so it doesn't draw water into the cell by osmosis
- CStarch can be used directly in respiration without change
- DStarch contains nitrate
Show answer
List the five uses of glucose in a plant.
Respiration, making cellulose, making amino acids/proteins (with nitrate), storage as starch, and making fats and oils.What does a plant need, in addition to glucose, to make proteins?
Nitrate ions absorbed from the soil.What is glucose converted into for storage, and where might it be stored?
Into starch, stored in roots, stems and leaves.Name the substance glucose is turned into to strengthen plant cell walls.
Cellulose.
Aerobic & anaerobic respiration
The exothermic reaction in every living cell — with oxygen and without — plus fermentation in yeast and muscle.
Part 1Aerobic respiration
Respiration happens in every cell, all the time, transferring energy from glucose so the organism can use it — for movement, keeping warm, and building larger molecules. It is an exothermic reaction: energy is released to the surroundings.
Aerobic respiration uses oxygen and releases the most energy. Most of it takes place in the mitochondria.
The equations to know
- glucose + oxygen → carbon dioxide + water recall
- Aerobic respiration, word equation. The reaction releases energy (exothermic).
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O given
- Aerobic respiration, symbol equation — it's photosynthesis run backwards.
- glucose → lactic acid recall
- Anaerobic respiration in muscle cells (no oxygen). Releases much less energy.
⚠ Watch out — respiration is not breathing
Respiration is a chemical reaction inside cells; breathing (ventilation) is just moving air in and out of the lungs. They're different things — don't use the words as if they mean the same. And respiration is exothermic (releases energy), the opposite of endothermic photosynthesis.
Part 2Anaerobic respiration & fermentation
When oxygen runs short — for example during hard exercise — cells respire anaerobically (without oxygen). This is the incomplete breakdown of glucose, so it releases much less energy than aerobic respiration.
In muscle cells, anaerobic respiration produces lactic acid. In yeast and plant cells, it produces ethanol and carbon dioxide — this is called fermentation, and it's used by humans to make bread (the CO₂ makes dough rise) and alcoholic drinks (the ethanol).
Fermentation in yeast
- glucose → ethanol + carbon dioxide recall
- Anaerobic respiration in yeast cells (fermentation). Releases little energy, but the products are economically useful.
⚠ Watch out — different products in different cells
Anaerobic respiration in your muscles makes lactic acid (no CO₂). Anaerobic respiration in yeast makes ethanol + carbon dioxide. Both release less energy than aerobic respiration because the glucose is only partly broken down.
Why does anaerobic respiration release less energy than aerobic respiration?
- AIt uses more oxygen
- BThe glucose is only partly (incompletely) broken down
- CIt happens outside the mitochondria, which destroys energy
- DLactic acid contains no energy at all
Show answer
Write the word equation for aerobic respiration.
glucose + oxygen → carbon dioxide + water (energy released — exothermic).Is respiration exothermic or endothermic, and what does that mean?
Exothermic — energy is released to the surroundings.What is produced by anaerobic respiration in muscle cells?
Lactic acid (glucose → lactic acid).Write the equation for fermentation in yeast and give one human use.
glucose → ethanol + carbon dioxide. Used to make bread (CO₂ rises the dough) or alcoholic drinks (ethanol).In which part of the cell does most aerobic respiration take place?
In the mitochondria.
The body's response to exercise
Why your heart pounds and you keep panting after you stop — heart rate, breathing, lactic acid and oxygen debt.
Part 1Getting more oxygen and glucose to the muscles
During exercise the muscles respire faster, so they need more oxygen and glucose delivered and more carbon dioxide removed. The body responds in three ways: heart rate increases, breathing rate and depth (breath volume) increase, and the muscles' stored glycogen is converted back to glucose for respiration.
A faster, deeper breathing rate gets more oxygen into the blood; a faster heart rate pumps that oxygenated blood to the muscles more quickly. Together they keep aerobic respiration going as long as possible.
⚠ Watch out — say "breathing rate AND depth"
During exercise you breathe both faster and more deeply (bigger breaths), not just faster. For full marks, mention both. And it's glycogen → glucose in muscles — glycogen is the muscle's stored carbohydrate.
Part 2Lactic acid and oxygen debt
If exercise is hard enough that oxygen can't be delivered fast enough, muscles start to respire anaerobically, building up lactic acid. Lactic acid causes muscle fatigue (and that achy feeling). Because it's only partial breakdown, it also leaves the body with an oxygen debt.
The oxygen debt is the extra oxygen your body needs after exercise to deal with the lactic acid that built up. That's why you keep breathing hard for a while after you stop — you're "repaying" the debt.
Higher tier — how the lactic acid is removed
After exercise, blood flowing through the muscles carries the lactic acid to the liver. In the liver, the lactic acid is converted back into glucose.
The "extra" oxygen you take in (the oxygen debt, sometimes called excess post-exercise oxygen consumption) is used to react with the accumulated lactic acid and remove it, which is why heart rate and breathing stay high until it's cleared.
Worked example — explaining a recovery graph
After a sprint, an athlete's breathing rate stays high for several minutes before returning to normal. Explain why.
Why does a runner keep breathing heavily for a few minutes after a hard race finishes?
- ATo cool the body down by losing heat through the breath
- BTo take in extra oxygen to remove the lactic acid (repay the oxygen debt)
- CTo breathe out the extra glucose made during exercise
- DBecause the heart has stopped working temporarily
Show answer
State three ways the body responds to exercise.
Heart rate increases, breathing rate and depth increase, and stored glycogen is converted to glucose.What substance builds up in muscles during long, hard exercise, and what does it cause?
Lactic acid, which causes muscle fatigue and an oxygen debt.Define oxygen debt.
The extra oxygen the body needs after exercise to react with and remove the lactic acid that built up.[HT] Where is lactic acid taken to be dealt with, and what happens to it there?
It's carried in the blood to the liver, where it is converted back into glucose.
Metabolism — all the reactions at once
The big picture: how cells build up and break down molecules, powered by the energy from respiration.
Part 1What metabolism means
Metabolism is the sum of all the chemical reactions happening in a cell or in the body. These reactions are controlled by enzymes, and the energy that drives them is transferred by respiration.
Metabolic reactions fall into two types. Synthesis (building up) reactions join small molecules into larger ones — these need an energy input. Breakdown reactions split large molecules into smaller ones — these often release energy. Respiration sits at the centre, supplying the energy for the building-up reactions.
Metabolism in plants & animals — the key conversions
- Glucose → starch, glycogen and cellulose
- Small sugars joined into larger storage and structural carbohydrates.
- Glucose + nitrate → amino acids → proteins
- In plants; the amino acids are then built into proteins.
- Lipids from one glycerol + three fatty acids
- How fats and oils are built up.
- Breaking down excess protein
- In animals, surplus protein is broken down to form urea, which is excreted.
⚠ Watch out — metabolism is the whole set
Metabolism means all the reactions added together, not a single reaction. It includes both building-up and breaking-down. And remember the energy that powers metabolism is transferred by respiration — so respiration underpins the whole topic.
Joining many glucose molecules together to make starch is an example of which kind of metabolic reaction?
- AA breakdown reaction that releases energy
- BA synthesis (building-up) reaction that needs energy
- CAnaerobic respiration
- DPhotosynthesis
Show answer
Define metabolism.
The sum of all the chemical reactions in a cell or the body, controlled by enzymes.Which process supplies the energy needed to drive metabolic reactions?
Respiration transfers the energy used in building-up reactions.Give one example of a building-up (synthesis) reaction in metabolism.
Any of: glucose → starch / glycogen / cellulose; glucose + nitrate → amino acids → proteins; making lipids from glycerol and fatty acids.What happens to excess protein in the body, and what is formed?
It is broken down to form urea, which is then excreted.