Photosynthesis and limiting factors

In a nutshell

Photosynthesis is an important process that happens in green plants and algae, it produces glucose. The glucose is used to make larger molecules that make up the organism's biomass (the mass of living material). There are many factors, including carbon dioxide, light intensity and temperature that can impact the rate that photosynthesis occurs.

Photosynthesis

Definition

Photosynthesis is the process of making glucose and oxygen using light energy from the sun and chlorophyll. This process is carried out by green plants and algae. 

This mass of living material contains energy which will be passed down the food chain to the organisms that eat the plants and then to the organisms who eat those organisms. 

This means that photosynthetic organisms are the main producers of food for nearly all life on earth.

Photosynthesis happens inside chloroplasts which contain chlorophyll. The chlorophyll will absorb the sunlight. Photosynthesis is an example of an endothermic reaction as the energy is taken in during the reaction.

Limiting factors of photosynthesis

The rate of photosynthesis can be impacted by multiple factors such as light intensity, concentration of carbon dioxide and temperature.

All of these factors that affect the rate of photosynthesis can become the limiting factor of the process. The limiting factor is anything that stops it going any faster.

Light

Photosynthesis requires energy which it gets from light. If the light level is raised, the rate of photosynthesis will increase as there will be more energy for photosynthesis to use (1.). The rate is said to be directly proportional to light intensity. However, this is only true to a certain point as there will be another factor, like carbon dioxide or temperature (2.), that is impacting the rate of photosynthesis. This causes a plateau in the rate of photosynthesis (3.).

The impact of light intensity on the rate of photosynthesis can be investigated in the lab using a lamp and changing the distance between the lamp and the plant. 

Inverse square law

Note: The inverse square law is for higher tier students only. 

The distance of the plant from the lamp is inversely proportional to the light intensity. This means that as the distance of the plant from the lamp increases, the light intensity decreases. 

However, light intensity decreases in proportion to the square of the distance between the plant and the lamp. This is called the inverse square law.

​​$$light\ intensity\propto \frac{1}{{distance (d)}^2}$$​​

So, if you halve the distance between the plant and the lamp, the light intensity will be four times greater.

Example

The distance between the lamp and the plant is $$10\ cm$$​. The distance is then halved to $$5\ cm$$​. What is the change in light intensity?

​$$\begin{aligned}\text{light intensity}&\propto \frac{1}{\text{5}^2}\\\\&= \underline{0.04}\end{aligned}$$

$$\begin{aligned}\text{light intensity}&\propto \frac{1}{\text{10}^2}\\\\ &= \underline{0.01}\\\end{aligned}$$​​​

​$$\underline{Therefore,\ the\ light\ intensity\ is\ four\ times\ greater\ when\ the\ distance\ is\ halved.}$$​​

Carbon dioxide

Carbon dioxide is one of the reactants of photosynthesis. Increasing the concentration of carbon dioxide will also increase the rate of photosynthesis (1.) as there is more of the reactant to react. However, as with the light, at a certain point increasing the concentration of carbon dioxide won't have a further impact as it will be the light or temperature acting as the limiting factor and impacting the rate of photosynthesis (2.).

Temperature

Temperature is a limiting factor of photosynthesis as the temperature impacts the activity of enzymes needed for the process. The enzymes will work more slowly at lower temperatures but they will denature if the temperature is too high (1.). Denaturation will usually happen at $$45\ \degree C$$​.