Rate of Photosynthesis

Think of photosynthesis like cooking - you need the right ingredients and conditions to make it work properly. Three main factors control how fast plants can photosynthesise: light intensity, CO₂ concentration, and temperature.

Light intensity provides the energy plants need to make food. As you increase light levels, the rate of photosynthesis speeds up steadily - but only to a point. Beyond that sweet spot, adding more light won't help because something else becomes the limiting factor.

You can test this in the lab by moving a lamp closer to or further from a plant. The graph shows the rate increasing with light intensity until it levels off completely.

Chlorophyll can also limit photosynthesis rates. Diseases like Tobacco Mosaic Virus or nutrient deficiencies can damage chloroplasts or reduce chlorophyll production, meaning less sunlight gets absorbed.

Key Tip: When one factor levels off on a graph, it means something else is now limiting the reaction - look for the next factor that needs increasing!

CO₂ concentration works similarly to light - more CO₂ increases the photosynthesis rate until the graph flattens out again. Once there's plenty of CO₂ and light available, temperature usually becomes the limiting factor.

Temperature affects the enzymes that control photosynthesis. Too cold and they work slowly; too hot (around 45°C) and they get destroyed completely. This creates an optimal temperature range where photosynthesis works most efficiently.