Extract and interpret graphs of photosynthesis rate involving one limiting…

The idea

Photosynthesis rate depends on light intensity, carbon dioxide concentration and temperature. When one factor is limiting, increasing it raises the rate until another factor becomes limiting, so the graph plateaus. To extract information, read the axis labels and units, then read a value from the curve at a stated condition. To interpret, describe the trend: as the variable on the x-axis increases, rate rises steeply at first, then levels off. The plateau shows the named factor is no longer limiting; another factor, such as temperature or carbon dioxide concentration, has become limiting. For example, on a rate against light intensity graph at 20 °C, the curve rises then flattens; adding more light cannot increase rate because carbon dioxide concentration or temperature now limits it. Compare curves at different conditions to identify which factor limits.

What earns the marks

• State the independent variable on the x-axis and the dependent variable on the y-axis, including units, before reading any value.
• Describe the initial trend as the limiting factor increases: rate increases because the factor is in short supply and is limiting.
• Identify the plateau: rate stops increasing because a different factor has become limiting.
• Name the factor that becomes limiting at the plateau, using the context given in the graph or question.
• When comparing two curves, link a higher curve to a condition that allows a faster rate, such as a higher temperature or carbon dioxide concentration.
• Read values accurately from the curve, interpolating between grid lines where needed, and quote the unit.

Common mistakes and how to put them right

• Saying the rate 'stops' at the plateau instead of saying it levels off or remains constant.

  Put it right by stating the rate is constant because another factor is now limiting.

• Naming the x-axis factor as limiting at the plateau.

  Put it right by naming a different factor, such as carbon dioxide concentration or temperature.

• Ignoring units when reading values.

  Put it right by copying the unit from the y-axis, for example arbitrary units or mm³ of oxygen per minute.

Exam tips

• Annotate the graph: mark the steep section, the plateau and the point where the curve flattens.
• Use the phrase 'becomes limiting' when explaining a plateau, and name the factor explicitly.
• If asked to compare curves, quote a value from each curve at the same x-value to support your comparison.