Photosynthesis and Limiting Factors | Light Intensity | Carbon dioxide Concentration | Temperature

Define photosynthesis

The root words for "photosynthesis" is made up of two words "photo" and "synthesis”. The word "photo" is derived from the Greek word "phōs," meaning "light” and "synthesis" comes from the Greek word "synthesis", which means "putting together" or "combination." So, "photosynthesis" literally means "putting together with light," referring to the process by which plants synthesize organic molecules using light energy.

Photosynthesis is the process in which green plants, algae, and some bacteria use carbon dioxide from the atmosphere, water from the soil, and energy from sunlight to produce glucose and oxygen as byproducts. The chemical equation for photosynthesis is:

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

This process takes place in chloroplasts, the organelles found in plant cells, particularly in the leaves. This process is crucial for life on Earth as it provides oxygen for respiration and serves as the foundation of the food chain, as plants are primary producers, providing energy and nutrients for other organisms.

What does limiting factor mean?

A limiting factor refers to any essential component or condition that is in short supply relative to the demand placed on it. In the context of photosynthesis, a limiting factor is a factor that constrains the rate of photosynthesis when it is in short supply, even if other factors are optimal.


Photosynthesis and limiting factors

The rate of photosynthesis is influenced by several limiting factors, including light intensity, carbon dioxide concentration, and temperature. Let’s discuss them one by one.

1. Light intensity as a limiting factor for photosynthesis

Light intensity directly affects photosynthesis by providing the energy needed for chlorophyll molecules to initiate the conversion of carbon dioxide and water into glucose and oxygen. Higher light intensity leads to increased rates of the light-dependent reactions, which produce energy-rich molecules essential for carbon fixation in the Calvin cycle. 

However, there's a saturation point where further increases in light intensity don't enhance photosynthesis, as other factors like carbon dioxide availability become limiting.

Extremely high light intensity can also cause photo-damage. Therefore, while light intensity is crucial for photosynthesis, its impact is subject to the availability of other necessary resources and the plant's capacity to manage excessive light energy.

2. Carbon dioxide as a limiting factor for photosynthesis

Carbon dioxide concentration directly influences the rate of photosynthesis because it is one of the key reactants in the process. When carbon dioxide levels are high, plants can more readily uptake and utilize it in the Calvin cycle, the light-independent reactions of photosynthesis.

At higher concentrations, the enzyme RuBisCO, which catalyzes the fixation of carbon dioxide, operates more efficiently, leading to increased rates of carbon fixation and ultimately higher rates of glucose production.

However, as carbon dioxide concentrations decrease, the carbon dioxide become the limiting factor and slows down the rate of photosynthesis. This relationship highlights the importance of maintaining adequate carbon dioxide levels for optimal plant growth and productivity.

3. Temperature as a limiting factor for photosynthesis

Temperature significantly affects the rate of photosynthesis due to its impact on enzyme activity and metabolic processes within plant cells. As temperature increases within a certain range, the rate of photosynthesis generally increases because the enzymes involved in the process become more active, facilitating faster chemical reactions. This temperature dependence is particularly evident in the light-independent reactions of photosynthesis, such as the Calvin cycle, where enzymes like RuBisCO function optimally within specific temperature ranges.

However, beyond an optimal temperature range, the rate of photosynthesis begins to decline. This decline is primarily due to the denaturation of enzymes involved in photosynthesis. High temperatures can cause the three-dimensional structure of enzymes to unravel, rendering them non-functional. Additionally, extreme heat can lead to the disruption of cell membranes and the loss of water through transpiration, further compromising photosynthetic efficiency.

Conversely, at low temperatures, enzyme activity decreases, slowing down the rate of photosynthesis. Cold temperatures can also affect membrane fluidity and decrease the solubility of gases, including carbon dioxide, in water, which can limit the availability of substrates for photosynthesis.

In summary, while moderate temperatures enhance the rate of photosynthesis by promoting enzyme activity and metabolic reactions, both excessively high and low temperatures can inhibit the process. This temperature sensitivity underscores the importance of maintaining suitable environmental conditions for optimal plant growth and productivity.

Some questions and answers

1. What are the 3 limiting factors of photosynthesis?

A. Light intensity, carbon dioxide concentration, and temperature are three limiting factors of photosynthesis.

2. How does light intensity affect the rate of photosynthesis?

A. Light intensity directly influences the rate of photosynthesis because it provides the energy needed for the light-dependent reactions to occur. An increase in light intensity generally leads to a higher rate of photosynthesis until a saturation point is reached.

3. Explain the relationship between carbon dioxide concentration and the rate of photosynthesis.

A. Carbon dioxide concentration affects the rate of photosynthesis because it is one of the reactants used in the Calvin cycle. Lower carbon dioxide concentrations decrease the rates of photosynthesis as it becomes the limiting factor.

4. What is the effect of temperature on photosynthesis?

Answer: Temperature impacts the rate of photosynthesis by affecting enzyme activity. Within an optimal temperature range, higher temperatures increase the rate of photosynthesis, while lower temperatures slow it down. Extreme temperatures can denature enzymes, leading to a decrease in photosynthetic activity.

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