Key Idea: Photosynthesis is how plants and algae make their own food. The one idea the whole topic hangs on is that it is an energy conversion: it turns light energy into chemical energy stored in glucose. From that single idea everything else follows — which colours of light are captured, the gases that go in and out, what slows the rate down, what the fixed carbon is built into, and why a leaf is shaped the way it is. Topic 3.3 (C1.3) is a Paper 1 data favourite (read a spectrum or a rate graph) and a regular Paper 2 topic (outline the word equation, or explain how a leaf is adapted for photosynthesis).
⚡ Photosynthesis as an energy conversion
Photosynthesis does not create energy — it converts it. Light energy from the Sun is absorbed by chlorophyll and locked away as chemical energy in the bonds of glucose. The raw materials are carbon dioxide (from the air) and water (from the roots); the products are glucose and oxygen.
Photosynthesis converts light energy into chemical energy: carbon dioxide and water (raw materials) become glucose (a chemical-energy store) and oxygen (released as waste).
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Key Idea: carbon dioxide + water →(light, chlorophyll)→ glucose + oxygen The arrow's label reminds you that light (absorbed by chlorophyll) drives the reaction — it is the energy input, not a raw material that ends up inside the glucose.
Light energy in → chemical energy out. Light is absorbed and converted, but — unlike CO₂ and water — it is never built into the glucose. The oxygen released comes from the water that is split.
🌈 Light absorption & pigments
A leaf does not use every colour of light. Chlorophyll absorbs blue and red light strongly and reflects green — and the reflected green is what reaches your eye, which is why leaves look green. The colour you see from any pigment is the light it does NOT absorb.
Chlorophyll's absorption spectrum: tall peaks in the blue (~450 nm) and red (~660 nm) are the wavelengths it absorbs and uses; the green dip (~550 nm) is the light it reflects, so leaves look green.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Feature | Absorption spectrum | Action spectrum |
|---|---|---|
| What the y-axis shows | Amount of light absorbed by a pigment | Rate of photosynthesis |
| Plotted against | Wavelength (colour) of light | Wavelength (colour) of light |
| Typical shape | High in blue and red, low in green | High in blue and red, low in green |
| Why they match | Only absorbed light supplies energy for photosynthesis — so the wavelengths absorbed are the wavelengths that drive it | Photosynthesis is fastest at exactly the wavelengths the pigments absorb |
Accessory pigments such as carotenoids (orange/yellow) absorb some of the blue-green light that chlorophyll misses and pass the energy on. The result: the plant captures a wider range of wavelengths than chlorophyll could alone, so less light energy is wasted.
Absorbtion spectrum = what a pigment absorbs. Action spectrum = the action (rate) of photosynthesis. They line up because no absorption, no action.
🫧 Products & measuring the rate
Photosynthesis releases oxygen (a waste product) and absorbs carbon dioxide (a raw material) — the opposite of respiration. Because of these two gas changes, you can measure the rate by tracking either gas, or the pH of the water.
| What you measure | How | A faster rate shows as |
|---|---|---|
| Oxygen produced | Count gas bubbles from an aquatic plant, or collect and measure the gas volume | More bubbles per minute / a greater volume of gas |
| Carbon dioxide taken up | A CO₂ (hydrogencarbonate) indicator changes colour with the CO₂ level | A bigger colour change as more CO₂ is removed |
| pH of the water | Measure with a probe or indicator | A rising pH — removing CO₂ makes the water less acidic |
Dissolved CO₂ is slightly acidic (it forms carbonic acid). When a plant photosynthesises and removes CO₂, there is less acid, so the pH rises. The bubbles streaming off an illuminated pondweed are oxygen — a classic sign that photosynthesis is happening, and counting them measures the rate.
📈 Limiting factors & the rate curve
Photosynthesis needs light, carbon dioxide and a suitable temperature. At any moment the rate is set by whichever of these is in shortest supply — the limiting factor. Only raising the limiting factor speeds the rate up.
Rate of photosynthesis vs light intensity: it rises while light is the limiting factor, then plateaus once light is plentiful and another factor (CO₂ concentration or temperature) limits the rate instead.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Region of the curve | What the rate does | Limiting factor |
|---|---|---|
| Steep rising part (low light) | Increases as light intensity increases | Light intensity |
| Flat plateau (high light) | Stays the same even as light keeps increasing | CO₂ concentration (or temperature) |
| Temperature past the optimum | Falls — can drop to zero | Temperature too high (enzymes denature) |
On a rate-vs-light graph: while it climbs, light is limiting; once it goes flat, light is plentiful and CO₂ or temperature limits the rate. A higher-CO₂ curve plateaus higher up. Temperature is special: warming towards the optimum speeds the rate, but too high denatures the enzymes, so the rate falls (it does not hold a plateau).
🌻 From CO₂ to biological molecules
Every carbon atom in a plant began as CO₂ in the air. Building that carbon into an organic molecule is carbon fixation, and it happens during photosynthesis. The first molecule made is glucose — the hub from which the plant builds everything else.
| The glucose is used to make… | Molecule | Why the plant makes it |
|---|---|---|
| Energy (broken down now) | (respiration) | Releases energy to power the plant's cells |
| A storage carbohydrate | Starch | Compact, insoluble energy store |
| A structural carbohydrate | Cellulose | Builds strong cell walls |
| A protein (needs nitrogen too) | Amino acids → proteins | Growth and enzymes; needs N from the soil |
| An energy-dense store | Lipids (oils) | Most energy per gram — packed into seeds |
Key Idea: To make an oil: glucose → glycerol + fatty acids → join (condensation) → lipid (oil), stored often in seeds. Lipids store the most energy per gram of any food molecule — which is why seeds pack their energy as oil, not starch. Naming the intermediates (glucose, then glycerol + fatty acids) is what scores the middle marks in the classic 4-mark 'Outline' question.
Carbon enters as CO₂, becomes glucose first, and glucose is the hub for starch, cellulose, protein and oils. Making protein also needs nitrogen, not just carbon.
🍃 Leaf structure for photosynthesis
A leaf is broad, flat and thin — broad and flat for a large light-catching area, thin so light and CO₂ reach the chloroplasts over a short distance. Inside, the layers are arranged so light reaches the chloroplasts with as little wasted as possible.
A leaf in cross-section: transparent cuticle and upper epidermis let light through; the palisade mesophyll (tall, chloroplast-packed cells near the top) does most photosynthesis; the spongy mesophyll's air spaces supply CO₂ through the stomata.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Leaf layer (top to bottom) | What it is | How it helps photosynthesis |
|---|---|---|
| Waxy cuticle + upper epidermis | Clear, chloroplast-free coating and cell layer | Transparent → light passes through to the cells below |
| Palisade mesophyll | Tall cells packed with chloroplasts, just under the top | Brightest light + most chloroplasts → does most photosynthesis |
| Spongy mesophyll | Loose, rounded cells with large air spaces | Air spaces let CO₂ diffuse to every cell |
| Veins (xylem + phloem) | A vascular bundle through the mesophyll | Xylem brings water; phloem carries away the sugars |
| Lower epidermis + stomata | Surface layer with pores and guard cells | Stomata let CO₂ in; guard cells balance CO₂ supply vs water loss |
Palisade = packed and high (chloroplasts at the top, catching the light). Spongy = spaces and low (air gaps lower down, supplying the CO₂). To identify the palisade mesophyll on a cross-section, look for tall cells packed with chloroplasts, just below the upper surface.
✍️ Worked examples
IB-style question — photosynthesis as an energy conversion
Outline photosynthesis as a conversion of energy, naming the raw materials and the products. [4]
How to score all four marks:
Photosynthesis converts light energy into chemical energy.
The chemical energy is stored in glucose (in its bonds).
The raw materials used are carbon dioxide and water.
The products made are glucose and oxygen. (Award 1 mark per distinct point, max 4.)
Photosynthesis converts light energy into chemical energy stored in glucose; it uses carbon dioxide and water as raw materials and makes glucose and oxygen.
IB-style question — read the absorption spectrum
An absorption spectrum of chlorophyll is high at ~450 nm (blue) and ~660 nm (red) but very low at ~550 nm (green). Using the data, explain why a leaf containing this chlorophyll looks green. [3]
Model answer:
At about 550 nm (green) the absorption is very low, so chlorophyll absorbs little green light.
Light that is not absorbed is reflected, so the green light is reflected off the leaf.
The colour we see is the reflected light, so the reflected green reaches our eyes and the leaf looks green. (Mark 1: low absorption of green. Mark 2: green is reflected. Mark 3: reflected green is what we see.)
Chlorophyll absorbs very little green light (the ~550 nm dip), so green is reflected rather than absorbed; the reflected green light is what reaches our eyes, so the leaf looks green.
IB-style question — explain the limiting-factor curve
A graph of photosynthesis rate against light intensity rises steeply, then levels off to a plateau. Explain the shape of this curve. [3]
Model answer:
On the rising part, light intensity is the limiting factor, so increasing light increases the rate.
At the plateau the rate stops climbing because light is no longer limiting — adding more has no effect.
On the plateau another factor — the carbon dioxide concentration (or temperature) — is now limiting the rate. (Award 1 mark per linked point, max 3.)
On the rising part, light is limiting, so more light gives a faster rate. At the plateau, light is no longer limiting; CO₂ concentration or temperature now limits the rate, so adding more light does nothing.
IB-style question — build an oil from atmospheric carbon
Outline how a plant uses carbon taken from the atmosphere to build oils. [4]
How to score all four marks:
Carbon dioxide is absorbed from the air and fixed during photosynthesis.
The fixed carbon is first used to make glucose.
Glucose is converted into glycerol and fatty acids (the building blocks of a lipid).
Glycerol and fatty acids join (by condensation) to form a lipid (oil), stored often in seeds. (Award 1 mark per distinct step, max 4.)
CO₂ from the air is fixed in photosynthesis; the fixed carbon makes glucose; glucose is converted to glycerol and fatty acids; these join to form a lipid (oil), stored in seeds.
✅ Quick self-check
Tap each card to check yourself across all six micros.
In terms of energy, what does photosynthesis do? It converts light energy into chemical energy, stored in the bonds of glucose. Raw materials: CO₂ and water; products: glucose and oxygen.
Why do leaves look green? Chlorophyll absorbs blue and red light but reflects green. The reflected green light is what reaches our eyes, so the leaf looks green.
Why do the absorption and action spectra match? Only absorbed light can power photosynthesis, so the wavelengths a pigment absorbs (blue and red) are the wavelengths at which photosynthesis is fastest.
How can you measure the rate of photosynthesis? Oxygen produced (count bubbles / collect gas), CO₂ taken up (a CO₂ indicator changes colour), or pH (rising, because removing CO₂ makes the water less acidic).
On a rate-vs-light graph, what limits the rate where? On the rising part, light intensity is limiting. On the flat plateau, light is no longer limiting — CO₂ concentration or temperature limits the rate instead.
How does a plant build an oil from atmospheric carbon? CO₂ is fixed in photosynthesis → glucose is made → glucose is converted to glycerol and fatty acids → these join to form a lipid (oil), stored in seeds.
Which leaf layer does most photosynthesis, and how do you spot it? The palisade mesophyll — tall cells packed with chloroplasts just below the upper surface, in the brightest light.
One picture for the whole topic: light + CO₂ + water in; glucose (the chemical-energy store and the carbon 'hub' molecule) + oxygen out.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Exam Tips
- Light is the ENERGY INPUT, not a raw material — it is converted to chemical energy, never built into the glucose.
- Leaves look green because chlorophyll REFLECTS green; never say a green leaf 'absorbs green'.
- Absorption spectrum = light absorbed by a pigment; action spectrum = rate of photosynthesis. Both peak in blue and red.
- Bubbles from an illuminated aquatic plant are OXYGEN — always name the gas; more bubbles per minute = a faster rate.
- Removing CO₂ makes water LESS acidic, so the pH RISES — the most common direction error in this topic.
- Identify the limiting factor by WHERE the point sits: rising part = light; flat plateau = CO₂ or temperature.
- Temperature past the optimum makes the rate FALL (enzymes denature) — it does not hold a plateau like light does.
- For 'Outline how plants build oils from atmospheric carbon [4]', name EVERY intermediate: CO₂ → glucose → glycerol + fatty acids → lipid.
- Spot the palisade mesophyll as TALL, chloroplast-PACKED cells JUST BELOW the upper surface — not the loose, air-spaced layer below.
Key Idea: Photosynthesis converts light energy into chemical energy in glucose (CO₂ + water →(light, chlorophyll)→ glucose + oxygen). Chlorophyll absorbs blue and red light and reflects green; accessory pigments widen the catch. The rate — measured by O₂ released, CO₂ taken up, or pH — is set by the limiting factor (light, CO₂ or temperature), giving the rise-then-plateau curve. The fixed carbon flows through glucose (the hub) into starch, cellulose, protein and oils, and the whole reaction is housed in a leaf built to capture light: transparent top layers, a chloroplast-packed palisade mesophyll, and air-spaced spongy mesophyll that supplies CO₂.