Hi Tara! Here is the shape of this topic in one paragraph, so you always know where you are. A plant cannot eat. It has to build its food, and it does that in the chloroplasts of its leaf cells using carbon dioxide from the air, water from the soil and energy transferred from light by the green pigment chlorophyll. The immediate product is glucose — not “food”, not “sugar”, glucose — and the plant then converts it into starch to store, cellulose to build with, sucrose to transport and nectar to bribe insects, or breaks it down again in respiration. Two mineral ions matter: nitrate to make amino acids and magnesium to make chlorophyll. The rate of the whole process is set by whichever of light intensity, carbon dioxide concentration or temperature is in shortest supply at that moment — the limiting factor. And the leaf itself is a beautifully engineered gas-exchange and light-capture organ: wide and thin, packed with chloroplasts at the top, riddled with air spaces below, and plumbed with xylem and phloem. Let’s build all of it properly.
The Big Idea: Building Matter Out of Air
An oak tree can weigh forty tonnes. Dig up the soil it grew in and weigh that too, and you will find the soil has barely lost anything. So where did forty tonnes of oak come from?
Almost all of it came out of the air. Carbon dioxide is a gas, so it feels weightless, but every molecule carries a carbon atom, and a leaf spends its life catching those atoms and bolting them together into sugar. The water arrives from the soil through the roots. Neither of those raw materials contains any usable energy, which is the whole problem — and the solution is light.
“The plant makes food” is the single most common wasted sentence in this topic. Food is not a chemical. The product of photosynthesis is glucose, a carbohydrate, and the mark scheme prints that word. “Sugar” is usually refused too, because sucrose and starch are also sugars-in-the-loose-sense and they are made later, from the glucose. Say glucose for the product and carbohydrate for the class of molecule, and you will never lose this mark again.
Its twin: the raw materials are carbon dioxide and water. Light is not a raw material — it is not a substance and nothing is built from it. Light supplies the energy. A question asking for the raw materials wants two things, and writing “carbon dioxide, water and sunlight” can cost you the mark.
Chlorophyll and the Chloroplast
Chlorophyll is a green pigment found in chloroplasts. Its job is to transfer energy from light into energy in chemicals — that is the exact syllabus wording, and it is worth copying, because the sloppy version (“chlorophyll absorbs sunlight and makes food”) skips the energy transfer that the mark is for.
Why is chlorophyll green? Because it absorbs red and blue light strongly and absorbs green much less: more of the green is reflected or passes through the leaf, and that is the colour that reaches your eye. So a leaf given only green light photosynthesises more slowly than one given red or blue light of the same brightness, because less of the light is absorbed. It does not stop completely: the green light that IS absorbed is still used. Write “absorbs less green light”, never “cannot use green light”. That comparison is a favourite challenge-paper setup.
Two words, one letter apart, and examiners swap them deliberately to see whether you know the difference. The chloroplast is the organelle — the green disc you can see under a microscope. The chlorophyll is the pigment molecule inside it. A chloroplast contains chlorophyll; chlorophyll never contains a chloroplast. “Pigment” is a molecule, “organelle” is a structure.
What Happens to the Glucose
The glucose does not sit around. Within minutes it is converted into something more useful, and the syllabus lists exactly five fates. Learn them as a list of five, because “state three uses of the carbohydrate made in photosynthesis” is a routine three-mark question.
| Converted to… | Why that molecule | Where it happens / goes |
|---|---|---|
| Starch | an energy store — insoluble, so it has no effect on water potential and cannot leak out of the cell | stored in leaves during the day, and in seeds, tubers and storage roots |
| Cellulose | to build cell walls — long straight chains of glucose that give the wall its strength | every new cell the plant makes |
| Used in respiration | glucose is broken down to release energy for the plant’s own metabolism | mitochondria, in every living plant cell, day and night |
| Sucrose | the transport sugar — soluble, so it can be moved, but not immediately used up | carried in the phloem to roots, fruits, growing points |
| Nectar | a sugary bribe, made to attract insects for pollination | nectaries in the flowers |
This is a two-mark classic. Starch is insoluble, so (1) it cannot diffuse out of the cell and is not carried away in the phloem, and (2) it does not change the water potential of the cell, so it does not cause water to move into the cell by osmosis. Glucose stored at the same concentration would do both. The mirror question — “why transport sucrose rather than starch?” — has the mirror answer: sucrose is soluble, so it can actually move, and it is not immediately used in respiration the way glucose is.
Two Mineral Ions You Must Know By Name
Glucose contains only carbon, hydrogen and oxygen. A plant also needs proteins, and proteins contain nitrogen. There is no nitrogen in carbon dioxide and none in water, so it has to come from somewhere else — from ions absorbed from the soil by the roots (by active transport, as you learned in Topic 3).
| Deficiency | What you see | Why |
|---|---|---|
| Nitrate deficiency | stunted growth, small plant, often with older leaves yellowing | no nitrogen → few amino acids → few proteins → little new cytoplasm, so growth stops |
| Magnesium deficiency | yellow leaves (chlorosis), especially between the veins | no magnesium → no chlorophyll → the green colour is missing and photosynthesis slows |
Nitrate → Nitrogen → amiNo acids → proteiNs → growth. Magnesium → chlorophyll → the green. Without nitrate the main sign is poor growth: a small, weak plant, often with its older leaves turning yellow. Without magnesium the main sign is yellow leaves, especially between the veins, because chlorophyll cannot be made, while the plant may still be a normal size at first. Look at growth first: badly stunted points to nitrate. That one line answers most deficiency questions.
Testing a Leaf for Starch — and Why Destarching Comes First
Almost every photosynthesis experiment ends the same way: test a leaf for starch. Starch is the marker, because it is only made if glucose was made, and glucose is only made if photosynthesis happened. The test itself has four steps and each one has a reason. Examiners ask for the reasons.
Before any of these experiments the plant is kept in complete darkness for 24–48 hours. This is destarching. In the dark the plant cannot photosynthesise, but it goes on respiring, so it uses up the starch already stored in its leaves. Only then can you be sure that any starch you find at the end was made during the experiment.
Skip this step and a positive iodine test proves nothing at all — the starch might have been there since last Tuesday. Cambridge awards a whole mark for it and most candidates never write it down. Learn the sentence: “the plant is destarched by leaving it in the dark for 48 hours so that any starch found at the end must have been made during the investigation.”
Controls: The Second Mark Nobody Remembers
A control is not “a second experiment to compare with”. A control is a set-up that is identical in every way except for the one factor you are testing, so that if the results differ, the factor must be the cause. Every one of the three investigations has an obvious control built into it, and it is worth naming explicitly.
| Testing the need for… | How the factor is removed | The control | Predicted result |
|---|---|---|---|
| Light | a strip of aluminium foil or black card fixed over part of one leaf | the uncovered part of the same leaf — same plant, same leaf, same everything except light | covered part stays orange-brown; uncovered part goes blue-black |
| Chlorophyll | use a variegated leaf, whose white regions have no chlorophyll | the green regions of the same leaf | white region stays orange-brown; green region goes blue-black |
| Carbon dioxide | seal the leaf in a bag or flask containing soda lime, which absorbs carbon dioxide | an identical leaf sealed in an identical bag containing sodium hydrogencarbonate (or nothing), so CO₂ is available | soda-lime leaf stays orange-brown; control leaf goes blue-black |
In both cases the control is part of the same leaf. That automatically keeps the age, the water supply, the temperature, the light source and the individual plant identical — every variable except the one being tested is controlled without any effort. When a challenge question asks you to evaluate a design, this is the point to make: a control on the same leaf is stronger than a control on a different plant, because a different plant differs in a hundred unmeasured ways.
The carbon dioxide experiment cannot do this, which is exactly why it needs the most careful control. Sealing one leaf in a bag changes the humidity and the temperature inside as well as the CO₂, so the control leaf must be sealed in an identical bag — otherwise you have tested “being in a bag” rather than “having carbon dioxide”.
The Rate of Photosynthesis — Three Factors
How fast a plant photosynthesises is not fixed. Three environmental factors control it, and the syllabus names all three: light intensity, carbon dioxide concentration and temperature. The standard way to measure the rate in the laboratory is to count bubbles of oxygen released per minute by a piece of pondweed such as Elodea, or better, to collect the gas in a syringe and measure its volume per minute.
| Factor | Shape of the graph | Explanation the mark scheme wants |
|---|---|---|
| Light intensity | rises steeply, then levels off (plateau) | more light → more energy transferred by chlorophyll → faster rate; at the plateau light is no longer limiting because another factor is now in shortest supply |
| CO₂ concentration | rises, then levels off — same shape | carbon dioxide is a raw material; more of it means more can be converted per second, until light or temperature becomes limiting |
| Temperature | rises to an optimum then falls sharply | photosynthesis is controlled by enzymes; warmth increases kinetic energy and collision frequency, but above the optimum the enzymes are denatured — the active site changes shape and the substrate no longer fits |
Investigating the Rate of Photosynthesis with Pondweed
Set-up. Put a 5 cm piece of pondweed (for example Elodea) upside down, cut end up, in a boiling tube of sodium hydrogencarbonate solution, which supplies carbon dioxide. Collect the gas that comes off in an inverted measuring cylinder, a capillary tube or a gas syringe. The gas is mainly oxygen.
- Light intensity. Place a lamp at 10, 20, 30, 40 and 50 cm from the tube (better still, measure the light intensity at the tube with a light meter). Stand a beaker of water between the lamp and the tube to absorb the lamp’s heat, and check with a thermometer that the temperature stays the same.
- Carbon dioxide concentration. Keep the lamp at one distance. Use sodium hydrogencarbonate solutions of different concentrations, for example 0, 0.2, 0.4, 0.6 and 0.8 %.
- Temperature. Keep the lamp and the sodium hydrogencarbonate the same. Stand the tube in a water bath at 10, 20, 30, 40 and 50 °C.
Each time: leave the pondweed for 5 minutes at the new setting so that it adjusts. Then measure the volume of gas collected in a fixed time (for example 5 minutes). Repeat each setting three times and calculate the mean. Rate = volume ÷ time.
Variables, for the light experiment. Independent: light intensity (distance of the lamp). Dependent: volume of oxygen collected per minute. Controlled: temperature, carbon dioxide concentration (same concentration and volume of sodium hydrogencarbonate solution), the same piece of pondweed, the same length of time at each setting.
Counting bubbles is quicker but less accurate, because bubbles are not all the same size.
Expected results. Light intensity and carbon dioxide: the rate rises, then levels off. Temperature: the rate rises to an optimum, then falls as the enzymes are denatured.
Worked example. A student collected 6.0 cm³ of gas in 5 minutes with the lamp at 20 cm. Rate = 6.0 ÷ 5 = 1.2 cm³ per minute.
Light and carbon dioxide graphs plateau. The temperature graph peaks and then crashes. That difference is worth a mark on its own, and the reason is that temperature is the only one of the three that acts on the enzymes rather than on the supply of a raw material or of energy. Never write “the enzymes are killed” — enzymes are not alive. They are denatured.
Limiting Factors (Supplement)
This is the classic lost mark. A limiting factor does not have to run out, be used up or reach zero — in ordinary air carbon dioxide is only 0.04 %, and on a bright, warm day, once light is plentiful, it is usually the limiting factor, even though it never runs out. The idea is relative shortage: of everything the reaction needs, one thing is in shorter supply than the rest, and that one thing sets the pace. Think of a production line — the slowest worker sets the output, even though nobody has stopped working.
The second half of the trap is the phrase at that moment. The limiting factor changes. Just after dawn, light is limiting. By noon on a bright day, light is plentiful and carbon dioxide has become limiting. In an unheated greenhouse in winter, temperature is limiting. A question that says “identify the limiting factor at point X” is asking you to read that moment off the graph, not to name your favourite factor.
The trap: answering “buy more lamps” or “the lamps must be broken”. The data already tell you light is not the constraint — trust the data over your instinct.
The Hydrogencarbonate Indicator Experiment
Hydrogencarbonate indicator solution is a pH indicator that is exquisitely sensitive to dissolved carbon dioxide. It does not detect oxygen, it does not detect starch, and it does not detect photosynthesis directly — it detects the CO₂ concentration of the water around it, and you infer the rest.
| Colour | Carbon dioxide level | What is happening |
|---|---|---|
| Purple / magenta | low CO₂ | photosynthesis is faster than respiration — CO₂ is being taken from the water faster than it is added |
| Red / orange-red | atmospheric CO₂ | no net change — either nothing living is present, or photosynthesis exactly balances respiration (the compensation point) |
| Yellow | high CO₂ | respiration is faster than photosynthesis, or there is no photosynthesis at all — CO₂ is being added to the water |
Four sealed test tubes, each containing the same volume of red hydrogencarbonate indicator and a bung:
Tube 1 — pondweed, in bright light → goes purple. Photosynthesis exceeds respiration, so CO₂ falls.
Tube 2 — pondweed, wrapped in foil (dark) → goes yellow. Only respiration occurs, so CO₂ rises. This tube proves that plants respire all the time, which is a favourite follow-up question.
Tube 3 — pondweed in dim light or in green light → stays red. Photosynthesis and respiration are balanced — the compensation point.
Tube 4 — no pondweed, in bright light → stays red. This is the control: it shows the colour change in the other tubes was caused by the plant and not by the light, the warmth of the lamp or the passage of time.
Every tube must be sealed with a bung, or carbon dioxide from the room diffuses in and swamps the effect. And all four should sit in a water bath or beaker of water between the lamp and the tubes, so that heat from the lamp does not raise the temperature and change the rate independently.
A Leaf Is a Solar Panel That Also Has to Breathe
A leaf has to do four jobs at once, and they pull against each other. It must catch as much light as possible. It must let carbon dioxide in to every photosynthesising cell. It must get water to those cells and carry the sugar away. And it must do all of that without drying out and dying. Every structure you are about to learn is a solution to one of those four problems, and if you can say which problem it solves, you can answer any leaf question.
The Cross-Section, Layer by Layer
Reading a photomicrograph of a leaf section
Paper 4 often shows a photograph instead of a drawing. Look from the top down:
- a thin, clear line with no cells in it is the cuticle;
- one row of small, clear cells with no green dots is the upper epidermis;
- one or two rows of tall, closely packed cells full of green dots (chloroplasts) is the palisade mesophyll;
- rounded cells with fewer green dots and large gaps between them are the spongy mesophyll and its air spaces;
- a ring of cells in the middle is the vein (vascular bundle): the cells with thick walls and wide, empty-looking openings on the upper side are xylem, and the smaller cells below them are phloem;
- a gap in the bottom row, between two curved cells, is a stoma with its guard cells.
| Structure | What it is | Adaptation for photosynthesis — the explanation mark |
|---|---|---|
| Waxy cuticle | a transparent waterproof layer secreted over the epidermis, thickest on the upper surface | waterproof, so it reduces water loss by evaporation; transparent, so it does not block light reaching the mesophyll |
| Upper epidermis | a single layer of flat cells with no chloroplasts | having no chloroplasts makes it transparent, so light passes straight through to the palisade layer; it also protects the leaf and stops pathogens entering |
| Palisade mesophyll | tall, column-shaped cells packed tightly just under the upper epidermis, containing the most chloroplasts | positioned nearest the light and packed with chloroplasts, so it absorbs the most light; the tall shape means light passes through many chloroplasts in one cell; this is the main site of photosynthesis |
| Chloroplasts | organelles containing chlorophyll | contain the chlorophyll that transfers energy from light into chemicals — they can move within the cell towards the light |
| Spongy mesophyll | irregular, loosely packed cells with fewer chloroplasts | the loose packing creates the air spaces, and the cells still photosynthesise using the light that gets past the palisade layer |
| Air spaces | the interconnected gaps between the spongy mesophyll cells | allow rapid diffusion of carbon dioxide from the stomata to every mesophyll cell, and give a large moist surface area for gases to dissolve into and cross the cell surface membranes |
| Stomata (one stoma) | pores in the epidermis, mostly on the lower surface | allow carbon dioxide to diffuse in and oxygen to diffuse out; being on the shaded lower surface reduces water loss |
| Guard cells | the pair of sausage-shaped cells around each stoma — the only epidermal cells with chloroplasts | they change shape to open and close the stoma, controlling gas exchange and water loss; they become turgid and curve apart in the light and flaccid and close in the dark or when water is short |
| Lower epidermis | single layer of cells on the underside, containing the stomata, with a thin cuticle | protects the leaf while allowing gas exchange through the stomata it contains |
| Vascular bundle | the vein — xylem and phloem together, surrounded by supporting tissue | supplies the leaf and supports it, holding the wide flat blade out in the light |
| Xylem | dead, hollow, lignified tubes; sits on the upper side of the bundle | carries water and mineral ions (including nitrate and magnesium) up from the roots to the mesophyll cells |
| Phloem | living tubes with sieve plates; sits on the lower side of the bundle | carries sucrose and amino acids away from the leaf to the rest of the plant (translocation) |
In a leaf vein the xylem is always the upper half and the phloem the lower half. That orientation is examined in diagram-labelling questions and you can work it out from the stem: xylem is on the inside of a stem bundle and the leaf is an outgrowth of the stem, so the inside becomes the top. For the contents: Xylem carries water up from the roots and it is dead; Phloem carries the products (sucrose, amino acids) in either direction and it is alive.
Guard Cells and the Stoma
The stoma is the leaf’s dilemma made visible. Open it and carbon dioxide floods in, but water vapour floods out. Close it and you save the water but starve the chloroplasts. The guard cells are the valve that manages that trade-off, and they do it with osmosis — which is why this is such a good synoptic link back to Topic 3.
Why a turgid guard cell curves away from the pore
A guard cell has an unevenly thickened wall: the inner wall, facing the pore, is thick and inflexible; the outer wall is thin and stretchy. When the cell takes in water by osmosis and becomes turgid, only the thin outer wall can stretch. So the cell bulges outwards and, because the two ends stay joined, it bends into a banana shape — and the pore between the pair opens.
Lose that water and turgor pressure falls, the cells go flaccid, the bowing straightens out and the two cells fall back against each other. The pore closes. That is the entire mechanism, and it is pure Topic 3 osmosis wearing a Topic 6 costume.
Guard cells are also the only cells in the epidermis with chloroplasts, which is a favourite one-mark question. It is a neat piece of design: the chloroplasts let the guard cell respond to light directly, opening the pore when photosynthesis is about to need carbon dioxide.
Almost every leaf-structure question is really two questions stacked: identify the structure and explain how it adapts the leaf for photosynthesis. If a question is worth two marks and you have written one noun, you have half an answer. The fix is a sentence pattern you can apply to any structure:
“[Structure] is [description], which means [property], so [consequence for photosynthesis].”
For example: “The upper epidermis is a single layer of cells with no chloroplasts, which makes it transparent, so light passes straight through to the palisade cells below.” That sentence would score both marks in any year.
The Six Sentences That Earn Most of the Marks
Topic 6 is unusually formulaic. A surprisingly large share of the available marks come from six sentences, and if you can produce them under pressure the rest is application. Write these out until they are automatic.
| # | Say this | Not this |
|---|---|---|
| 1 | “carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll” | “CO₂ + water → food + oxygen” |
| 2 | “chlorophyll transfers energy from light into energy in chemicals” | “chlorophyll absorbs sunlight and makes food” |
| 3 | “the plant is destarched by 48 hours in the dark, so any starch found must have been made during the investigation” | “the plant was left in a dark cupboard” |
| 4 | “a limiting factor is the factor in shortest supply at that moment, which therefore limits the rate” | “the factor that runs out” |
| 5 | “above the optimum the enzymes are denatured — the active site changes shape so the substrate no longer fits” | “the enzymes are killed” |
| 6 | “the control is identical except for the one factor being tested, so any difference must be caused by that factor” | “the control is there to compare with” |
State — one short fact, no reason needed. Do not waste time explaining.
Describe — say what happens, including the shape of any data. On a graph question this means quoting figures: “the rate rises steeply from 0 to 20 bubbles per minute between 0 and 4 arbitrary units, then levels off at 20”.
Explain — say why. Every “explain” answer needs a because, even if you have to write the word.
Suggest — you are being asked to apply what you know to something not in the syllabus. There is usually more than one acceptable answer. Do not panic and leave it blank; a sensible biological reason will score.
Compare — both halves in the same sentence, with a comparative word. “The palisade layer has more chloroplasts than the spongy layer” scores; two separate sentences often do not.
Predict — say what would happen and use the trend in the data to justify it.
How to Attack a Rate-of-Photosynthesis Graph
These questions look different every year and are always the same underneath. Work through them in this fixed order and you will not go wrong.
| Step | What to do | What it earns |
|---|---|---|
| 1 | Read both axes, including the units. Is the x-axis light intensity, CO₂ or temperature? | stops you writing the answer to a different question |
| 2 | Identify the shape: rises-then-plateaus, or rises-then-falls? | plateau → light or CO₂; peak then fall → temperature, and the fall means denaturing |
| 3 | Describe with figures quoted from the graph, including the value where the shape changes | the description marks, which are free but only if you quote numbers |
| 4 | On the rising part, name the factor on the x-axis as limiting. On the flat part, say it is no longer limiting and name what is | the explanation marks |
| 5 | If there is more than one curve, ask which change moved the line — that change is the new limiting factor | the application marks, which is where challenge papers live |
Counting bubbles of gas from pondweed is quick but crude: bubbles vary in size, so twice as many bubbles is not necessarily twice as much oxygen. If a question asks you to improve the method, the top answer is to collect the gas and measure its volume with a syringe or capillary tube per unit time. That single improvement is worth a mark on almost every experimental-design question in this topic.
Two more improvements that are always creditworthy: use a water bath or a beaker of water between lamp and plant so that the lamp’s heat does not change the temperature as you change the light intensity, and allow the plant time to equilibrate at each new setting before you start counting.
Vocabulary Cambridge Will Not Let You Blur
| Pair | The difference that carries the mark |
|---|---|
| chloroplast / chlorophyll | organelle / the green pigment molecule inside it |
| glucose / starch | the immediate soluble product / the insoluble storage form made from it |
| glucose / sucrose | made in photosynthesis and used in respiration / made from glucose for transport in phloem |
| stoma / stomata | singular / plural. “A stomata” is a grammatical error examiners notice |
| guard cell / stoma | the cell / the pore between the pair of cells. They are not the same thing |
| palisade / spongy mesophyll | tall, tightly packed, most chloroplasts, near the light / irregular, loosely packed, fewer chloroplasts, air spaces |
| xylem / phloem | dead, water and mineral ions, upwards, upper side of bundle / living, sucrose and amino acids, either direction, lower side |
| nitrate / magnesium ions | amino acids and proteins, so growth / chlorophyll, so green colour |
| soda lime / sodium hydrogencarbonate | removes CO₂ / supplies CO₂. Opposites, and one letter of carelessness apart |
| denatured / killed | enzymes are molecules, not organisms. Only “denatured” scores |
| limiting / used up | in shortest supply relative to the others / reduced to zero. Only the first is the definition |
| respiration / photosynthesis | happens in all living cells all the time / happens in chloroplasts in the light only. A plant does both at once in daylight |
This is the single biggest conceptual mistake in Topic 6, and it hides behind almost every gas-exchange question. Plants do not photosynthesise by day and respire by night. They respire continuously, and in daylight they photosynthesise as well. What changes is the balance:
• Bright light: photosynthesis > respiration → net uptake of CO₂, net release of O₂.
• Dim light (compensation point): photosynthesis = respiration → no net gas exchange.
• Dark: respiration only → net release of CO₂, net uptake of O₂.
The word that saves you every time is net. “The plant gives out oxygen in the light” is imprecise; “there is a net release of oxygen because photosynthesis exceeds respiration” is the answer.