The equation: exactly what to write
Examiners want the full word equation with the conditions above the arrow: carbon dioxide + water → glucose + oxygen, in the presence of light energy and chlorophyll. The two conditions (light and chlorophyll) are written by the arrow, not as reactants, and dropping them is a common one-mark loss.
(Extended only) Learn the balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Define photosynthesis as the process by which plants make carbohydrates (glucose) from raw materials using energy from light. Note the wording: light energy is transferred, and chlorophyll absorbs light energy and transfers it to chemical energy in glucose. The glucose is then used in respiration, converted to starch for storage, or to cellulose, proteins and other molecules.
Limiting factors: the graph skill that separates grades
A limiting factor is the factor in shortest supply that limits the rate of photosynthesis. The three you must know are light intensity, carbon dioxide concentration and temperature. On a rate-against-light-intensity graph:
- The rising part: light intensity is the limiting factor. More light, faster rate.
- The plateau: light is no longer limiting; another factor (CO₂ or temperature) has become limiting.
The high-value mark is naming which factor limits the plateau, not just saying 'it levels off'. (Extended only) Temperature affects photosynthesis because the reactions are enzyme-controlled. Too high and the enzymes denature, so the rate falls. Greenhouse questions test application: growers add CO₂, provide lighting and control temperature to maximise the rate. When two graph lines differ (e.g. higher CO₂), explain that the limiting factor has been removed, allowing a higher plateau.
Leaf structure and adaptations
Leaf questions are pure structure-to-function. Bank these adaptations:
| Feature | Adaptation for photosynthesis |
|---|---|
| Large, flat lamina | Large surface area to absorb light |
| Thin leaf | Short diffusion distance for gases |
| Palisade layer near top | Packed with chloroplasts to absorb maximum light |
| Stomata (lower surface) | Allow CO₂ in and O₂/water vapour out |
| Air spaces in spongy mesophyll | Allow gas diffusion within the leaf |
| Network of veins (xylem & phloem) | Transport water in and glucose/sucrose out |
Examiners reward the feature paired with its reason. Chloroplasts and chlorophyll link back to cell structure. Plants also need mineral ions: nitrate ions for amino acids/proteins and magnesium ions for chlorophyll: these two are the most-asked, with deficiency symptoms (poor growth; yellow leaves/chlorosis respectively).
Testing a leaf for starch (the method in order)
This practical sequence must be in the correct order, and each step has a reason examiners ask for:
- Boil the leaf in water: to kill the cells / stop reactions / break down the cell membranes.
- Boil in ethanol (with the Bunsen turned off, using a water bath, because ethanol is flammable). To remove the chlorophyll/decolourise the leaf so the colour change is visible.
- Dip in hot water: to soften the leaf.
- Add iodine solution: starch turns it blue-black; orange-brown means no starch.
The safety reason for turning off the flame before using ethanol is a standard mark. This test underpins experiments showing light, chlorophyll and CO₂ are needed: variegated leaves (only green parts have starch) prove chlorophyll is needed; destarching the plant first (keeping it in the dark) is the essential control. To practise these stepwise answers, book a free trial.
Showing the necessity of light, CO₂ and chlorophyll
These investigation questions follow a pattern: destarch the plant first (leave in darkness 24–48 hours so existing starch is used up), then change one variable.
- Light needed: cover part of a leaf with foil; only the exposed part tests positive for starch.
- Chlorophyll needed: use a variegated leaf; only the green (chlorophyll-containing) parts make starch.
- CO₂ needed: enclose one leaf with soda lime (absorbs CO₂) and a control with sodium hydrogencarbonate (supplies CO₂); only the leaf with CO₂ makes starch.
The exam skill is identifying the control and explaining that destarching is needed so any starch found must have been made during the experiment. Forgetting to mention destarching is a frequent loss. Always state what each result proves, not just what colour appears.