Q.Figure 11.10 shows the effect of light on the rate of photosynthesis. Based on the graph, answer the following questions:

(a) Light is a limiting factor at point A — the initial, rising part of the curve where the rate of photosynthesis increases as light intensity increases. (b) In region A, light is the limiting factor: the rate is directly proportional to light intensity here, so light (present at sub-optimal levels) controls the rate. (c) C represents the light-saturation point / plateau — the maximum rate, beyond which increasing light does not raise photosynthesis (other factors like CO2 or temperature now limit); D represents the light intensity on the x-axis at which this saturation (maximu …

Figure 11.10 plots the rate of photosynthesis (y-axis) against light intensity (x-axis). The curve rises steeply from the origin, then bends (around B) and flattens into a plateau (C).

1. Where is light a limiting factor? Light is a limiting factor at point A, which lies on the initial, ascending part of the curve. Here every increase in light intensity produces a proportional increase in the rate of photosynthesis — a sign that light, being at sub-optimal levels, is the factor limiting the rate. At B and C the curve has levelled off, so light is no longer limiting there.
2. Limiting factor in region A. In region A the rate rises in direct (linear) proportion to light intensity, showing that the process is being held back by the amount of light. Therefore the limiting factor in region A is light itself. Other factors (CO2 concentration, temperature) are adequate here and are not limiting.
3. What do C and D represent?

• C lies on the flat plateau of the curve. It represents the light-saturation point — the stage at which the rate of photosynthesis has reached its maximum and does not increase further with additional light. Beyond this point, factors other than light (such as CO2 concentration or temperature) become limiting. (Recall that light saturation occurs at only about 10 per cent of full sunlight.) …

Method 1: Step-by-step

1. Read the axes of Fig 11.10: y-axis = rate of photosynthesis, x-axis = light intensity.
2. Trace the curve's shape: a steep, near-linear rise (region A), a bend, then a flat plateau (points B to C).
3. (a) Where the curve is still rising (A) the rate tracks light intensity directly, so light is limiting there; on the plateau (B, C) light is no longer limiting.
4. (b) In region A the rate rises in direct proportion to light intensity, so light itself (present at sub-optimal levels) is the limiting factor there. …