Photosynthesis Rate Factors

Introduction

The rate of photosynthesis dictates how quickly plants produce glucose and oxygen. Several environmental factors influence this rate, and when one of these factors is in short supply, it becomes a limiting factor. The key factors are:

• Light availability
• Water availability
• Temperature
• Carbon dioxide concentration

KEY TAKEAWAY: Photosynthesis rate is affected by multiple factors, and the one in shortest supply becomes the limiting factor, dictating the overall process rate.

Light Availability

• Light is essential for the light-dependent reactions of photosynthesis.
• Increasing light intensity generally increases the rate of photosynthesis, up to a point.
• Light saturation point: The point at which increasing light intensity no longer increases the rate of photosynthesis because other factors become limiting.
• Different wavelengths of light are absorbed by different photosynthetic pigments (e.g., chlorophyll).

Graph

• A simple graph shows the rate of photosynthesis on the y-axis and light intensity/illuminance on the x-axis.
• The graph typically shows a positive correlation initially, plateauing at the light saturation point.
• (See Figure 4.27 in textbook segment 1)

VCAA FOCUS: Understand how to interpret graphs showing the relationship between light intensity and photosynthesis rate.

Water Availability

• Water is a reactant in the light-dependent reactions.
• Water availability directly impacts the rate of photosynthesis.
• Water stress causes plants to close their stomata to conserve water.
• Closed stomata limit CO2 intake, indirectly reducing the rate of photosynthesis.
• Excess water can also harm plants, decreasing the photosynthesis rate.

COMMON MISTAKE: Students often forget that water stress leads to stomata closure, which then limits CO2 intake.

Temperature

• Photosynthesis involves enzymes, which are sensitive to temperature.
• There is an optimal temperature range for photosynthesis.
• Low temperatures slow down enzyme activity.
• High temperatures can cause enzymes to denature, reducing or stopping photosynthesis.

Graph

• A graph shows the rate of photosynthesis on the y-axis and temperature on the x-axis.
• The graph typically shows a bell-shaped curve, peaking at the optimal temperature.
• (See Figure 4.35b in textbook segment 4)

EXAM TIP: When answering questions about temperature, always mention the role of enzymes and the concept of denaturation.

Carbon Dioxide Concentration

• Carbon dioxide is a reactant in the light-independent reactions (Calvin cycle).
• Increasing CO2 concentration generally increases the rate of photosynthesis, up to a point.
• C3 plants are particularly susceptible to changes in CO2 concentration.
• C4 and CAM plants have adaptations that make them less sensitive to CO2 limitations.

Graph

• A graph shows the rate of photosynthesis on the y-axis and CO2 concentration on the x-axis.
• The graph typically shows a positive correlation initially, plateauing as other factors become limiting.
• (See Figure 4.35a in textbook segment 4)

C3, C4, and CAM Plants

STUDY HINT: Use a table like the one above to compare the adaptations of C3, C4, and CAM plants.

Limiting Factors

• A limiting factor is an environmental condition that restricts the rate of photosynthesis when it is in short supply.
• The limiting factor can change depending on the circumstances.
• For example, on a sunny day, CO2 concentration or temperature might be the limiting factor, while on a cloudy day, light availability might be the limiting factor.

REMEMBER: If a factor is not limiting, increasing it will not increase the rate of photosynthesis.

Sample Problem Analysis

• Problem a: The amount of oxygen produced is highest in a plant at 35 °C. However, at both 25 °C and 45 °C, the rate of oxygen production is reduced.
  • Explanation: Photosynthesis is enzyme-driven. 35°C is the optimal temperature. 25°C is below optimal, slowing enzyme activity. 45°C is above optimal, leading to enzyme denaturation.
• Problem b: Increasing the input of carbon dioxide increases the rate of photosynthesis until this rate suddenly levels off.
  • Explanation: CO2 is a reactant in the Calvin cycle. Initially, increasing CO2 increases the reaction rate. The rate levels off when another factor (e.g., light, temperature) becomes limiting.
• Problem c: Plants that have excess water availability do not photosynthesise as well as a plant with slightly less water.
  • Explanation: Excess water can lead to waterlogging, reducing oxygen availability to roots, impairing their function and thus reducing plant health and photosynthetic rate. Also, too much water can damage the photosynthetic machinery itself.

APPLICATION: Understanding these factors is crucial in agriculture for optimizing plant growth and crop yields.