Question

5 a well watered plant was placed in a sealed bell jar for one day. the percentage proportion of oxygen and carbon dioxide in the jar were measured at different times throughout the day. fig. 5.1 shows the results of the experiment. (a) write down the word equation for photosynthesis. 1 (b) describe and explain the change in carbon dioxide from 0600 hours to 1200 hours. 2 (c) suggest why the levels of oxygen and carbon dioxide are equal in proportion at points p and q. 2

Explanation:

Part (a)

Step1: Recall photosynthesis equation

Photosynthesis uses carbon dioxide, water, and light energy to produce glucose and oxygen. The word equation is: Carbon dioxide + Water $\xrightarrow[\text{Chlorophyll}]{\text{Light energy}}$ Glucose + Oxygen.

Step1: Describe the trend

From 0600 to 1200 hours, the percentage of carbon dioxide decreases.

Step2: Explain the trend

During this time (daylight), the plant performs photosynthesis, which consumes carbon dioxide (and produces oxygen). The rate of photosynthesis is higher than the rate of respiration, so carbon dioxide levels drop.

Step1: Analyze processes at P and Q

At points P and Q, the rates of photosynthesis and respiration are balanced.

Step2: Explain gas exchange

During respiration, the plant produces carbon dioxide and consumes oxygen; during photosynthesis, it consumes carbon dioxide and produces oxygen. When the rates of these two processes are equal, the net production/consumption of oxygen and carbon dioxide is zero, so their proportions in the jar become equal.

Answer:

Carbon dioxide + Water $\xrightarrow[\text{Chlorophyll}]{\text{Light energy}}$ Glucose + Oxygen

Part (b)