The photosynthesis equation and limiting factors

• Carbon dioxide diffuses into the leaf through the stomata.
• Water is absorbed by root hair cells and travels up the xylem.
• Light is absorbed by chlorophyll in the chloroplasts, mostly in the palisade cells near the top of the leaf.
• Oxygen is a by-product and diffuses out through the stomata.

• Respiration, to release energy.
• Starch for storage (insoluble, so it doesn't affect water movement).
• Cellulose to strengthen cell walls.
• Amino acids for proteins (combined with nitrate ions from the soil).
• Fats and oils for storage, especially in seeds.

The rate of photosynthesis is limited by whichever factor is in shortest supply:

• Light intensity: more light, faster photosynthesis, until another factor limits it.
• Carbon dioxide concentration: the same pattern.
• Temperature: the rate rises up to an optimum, then falls as enzymes denature.

On a graph, the rate rises and then levels off; where it levels off, something else has become limiting. Higher-tier students also use the inverse square law: halving the distance from a lamp increases light intensity four times.

• Testing a leaf for starch: boil it in water, then in ethanol to remove chlorophyll, rinse, and add iodine solution. Blue-black shows starch was made.
• Measuring the rate: count oxygen bubbles from pondweed at different distances from a lamp, or collect the gas in a measuring cylinder. Keep the temperature and CO₂ constant.

The photosynthesis equation is the reverse of aerobic respiration. Plants respire all the time, day and night; they photosynthesise only in the light. See aerobic vs anaerobic respiration.

• Saying plants get their food from the soil (they make glucose; minerals come from the soil).
• Saying plants don't respire.
• Forgetting that light is needed but isn't a reactant in the symbol equation.

• Broad and thin: a large surface area to absorb light, and a short distance for gases to diffuse.
• Palisade cells near the top, packed with chloroplasts.
• Air spaces in the spongy layer let carbon dioxide reach the cells.
• Stomata, opened and closed by guard cells, let gases in and out.
• Veins (xylem and phloem) bring water and carry sugars away.

1. Name the two products of photosynthesis.
2. A plant in bright light and warm conditions stops increasing its rate of photosynthesis when the light gets brighter. Suggest the limiting factor.
3. What colour does iodine solution turn if starch is present?

Answers: glucose and oxygen; probably carbon dioxide concentration (or temperature); blue-black.