Rate of photosynthesis

AQA·Biology (Combined Science)·4.4.1.2·Trilogy — Higher

Limiting Factors of Photosynthesis

The rate of photosynthesis depends on the conditions around a plant. A factor is limiting under the prevailing conditions if increasing it increases the rate of photosynthesis. If that factor is increased enough, the rate may stop rising because a different factor is then limiting.

For example, a plant in dim light may photosynthesise faster when given more light. Once the light is sufficiently bright, adding more may have little effect if carbon dioxide concentration or temperature limits the rate instead.

Factors that limit photosynthesis

  • Light intensity: Increasing light intensity can increase the rate because more light is available for photosynthesis. The rate eventually levels off if another factor limits it.
  • Carbon dioxide concentration: Increasing the concentration can increase the rate because more carbon dioxide is available for photosynthesis. Again, the rate may level off when another factor becomes limiting.
  • Temperature: At low temperatures, increasing temperature can increase the rate because the enzyme-controlled reactions of photosynthesis proceed faster. At excessively high temperatures, enzyme activity can decline, causing the rate of photosynthesis to fall.

Reducing limitations in agriculture

Growers can use supplementary lighting, carbon dioxide enrichment or temperature control to reduce limitations on photosynthesis. The effect of each treatment depends on the other conditions. For example, extra light may have little effect if carbon dioxide concentration remains limiting.

Data on photosynthetic rate or crop yield can help assess a treatment. A comparison between treated and untreated plants under otherwise similar conditions shows whether the treatment produced a benefit under the conditions measured. A higher photosynthetic rate or yield alone, however, does not show whether the treatment is worthwhile financially.

To judge whether the benefit justifies the cost, a grower can compare the value of any additional crop yield with the cost of the treatment. A treatment may increase yield but still cost more than the extra yield is worth. The result applies to the conditions measured: a treatment that is effective under one set of conditions may have little effect when a different factor is limiting.

Factors affecting the rate of photosynthesis

The rate of photosynthesis is how quickly a plant carries out photosynthesis. It depends on conditions including light intensity, carbon dioxide concentration and temperature. A factor that restricts the rate under the current conditions is called a limiting factor.

Light intensity and carbon dioxide concentration

Photosynthesis requires light and carbon dioxide. If light intensity is limiting, increasing it raises the rate of photosynthesis. If carbon dioxide concentration is limiting, increasing it raises the rate.

The rate does not keep rising indefinitely as either factor increases. Eventually, another factor becomes limiting, so increasing the first factor further has little or no effect. For example, increasing light intensity may initially increase the rate, but the rate may then level off because carbon dioxide is limiting. Increasing the carbon dioxide concentration could allow the rate to rise again.

Temperature and photosynthesis

Photosynthesis involves enzyme-controlled reactions. As temperature rises, these reactions generally become faster, so the rate of photosynthesis increases towards an optimum temperature.

Above the optimum, the rate falls. High temperatures can cause enzymes to lose their working shape, so they can no longer carry out their reactions effectively.

Investigating a factor affecting photosynthesis

To investigate one factor, change that factor and measure the rate in the same way at each value. Keep other relevant conditions controlled so that they do not cause the observed change.

For example, to investigate light intensity, a water plant can be exposed to different light intensities while its temperature and carbon dioxide supply are kept constant. The volume of oxygen produced in a fixed time can be used as an indicator of the photosynthetic rate. Comparing the measurements shows how the rate changes as light intensity changes.

Photosynthesis rate graphs

A photosynthesis rate graph shows how the rate changes as one factor, such as light intensity, changes. The factor being varied is plotted on the horizontal axis; the rate of photosynthesis is plotted on the vertical axis. Other factors should be kept constant so that the effect of the chosen factor can be examined.

Plotting a photosynthesis rate graph

Label each axis with the quantity and its unit. For example, the horizontal axis could show light intensity in lux, and the vertical axis could show the volume of oxygen produced in $$\mathrm{cm^3/min}$$.

Choose a consistent scale for each axis that covers all the data and uses the available space appropriately. For each pair of measurements, find the factor value on the horizontal scale and the corresponding rate on the vertical scale. Plot a point where the two positions meet.

Draw an appropriate line or curve to show the pattern. It should follow the overall trend of the points; it does not have to pass through every point if the measurements vary.

For example, suppose the data include a light intensity of $$200\ \mathrm{lux}$$ and a rate of $$4\ \mathrm{cm^3/min}$$. Plot the point at $$200$$ on the horizontal scale and $$4$$ on the vertical scale. To read that point from the graph, reverse the process: read its horizontal and vertical positions using the axis scales.

Interpreting the pattern

Read the graph from left to right to see what happens to the rate as the factor increases:

  • An upward line or curve shows that the rate rises.
  • A plateau, or level section, shows that further increases in the plotted factor do not increase the rate.
  • A downward line or curve shows that the rate falls.

A plateau may indicate that another factor is limiting photosynthesis. Increasing the factor on the horizontal axis then has no further effect because the other factor restricts the rate.

Chlorophyll and photosynthesis rate

Plants use light energy to carry out photosynthesis. Chlorophyll absorbs some of this light, making its energy available for the process.

Under otherwise comparable conditions, a smaller amount of chlorophyll means that less light can be absorbed. Less energy is then available for photosynthesis, so its rate decreases.

When more chlorophyll increases the rate

Increasing the amount of chlorophyll can increase the rate of photosynthesis if the amount of light absorbed is limiting the process. More chlorophyll can absorb more light, providing more energy for photosynthesis.

More chlorophyll does not necessarily increase the rate. If another factor is limiting photosynthesis, absorbing more light will not remove that limit, so the rate may not increase.

Measuring photosynthesis rate

Photosynthesis rate can be estimated by measuring how much a quantity changes over a known time interval. For example, photosynthesis releases oxygen and takes up carbon dioxide, so the amount of oxygen released or carbon dioxide taken up can indicate the rate.

Measuring oxygen from an aquatic plant

An aquatic plant releases oxygen as bubbles. The oxygen can be collected over a measured time and its volume recorded. Alternatively, the bubbles can be counted over the same interval. Bubble count is less precise because bubbles can differ in size: equal numbers of bubbles do not necessarily contain equal volumes of oxygen.

Calculating and comparing rates

Calculate the mean rate by dividing the measured change by the time taken:

Mean rate=measured changetime taken\text{Mean rate} = \frac{\text{measured change}}{\text{time taken}}

For example, if an aquatic plant releases $$12\ \text{cm}^3$$ of oxygen in $$4$$ minutes, its mean rate of oxygen release is:

12 cm34 min=3 cm3/min\frac{12\ \text{cm}^3}{4\ \text{min}} = 3\ \text{cm}^3/\text{min}

The unit reflects both what was measured and the time unit: here it is cubic centimetres of oxygen per minute. A bubble count would instead give a rate in bubbles per minute.

To compare rates, use a consistent measurement method and keep conditions such as temperature and light intensity controlled. If one condition is being investigated, change that condition while keeping the others the same.

Multi-factor photosynthesis rate graphs

A multi-factor graph shows how the rate of photosynthesis changes with one factor, such as light intensity, under different conditions for other factors. The horizontal axis shows the factor being changed; each curve or data series represents a stated set of conditions for the other factors.

For example, a graph might show light intensity on the horizontal axis and have separate curves for two carbon dioxide concentrations, both measured at the same temperature. To compare the effects of carbon dioxide concentration, read the rates from the two curves at the same light intensity.

Interpreting the shape of a curve

If the rate rises as the factor on the horizontal axis increases, that factor is limiting the rate over that range: increasing it allows photosynthesis to proceed faster under those conditions.

If the curve levels off, it reaches a plateau. Further increases in the horizontal-axis factor do not raise the rate under those conditions. Another factor may be restricting the rate, but the plateau alone does not identify which one.

Comparing curves

Suppose a curve at a higher carbon dioxide concentration shows a higher rate than a curve at a lower concentration. If the comparison is made at the same light intensity and temperature, the increase in rate shows that the lower carbon dioxide concentration was restricting photosynthesis at that light intensity.

If the curves show the same rate at a particular light intensity, the graph does not show an effect of changing carbon dioxide concentration at that point. The comparison may have a different result at another light intensity, so the relevant part of the graph matters.

For graphs involving three factors, check all the stated conditions before comparing curves. To judge the effect of changing one factor, keep the value on the horizontal axis and the other condition the same. A suitable comparison—not the shape of one plateau by itself—is needed to identify which changed factor was restricting the rate.

Inverse square law for light intensity

Light from a source spreads out as it travels. The farther it travels, the larger the area over which the light is spread, so the light intensity decreases.

For a source that can be approximated as a point source, intensity is inversely proportional to the square of the distance from the source:

I∝1d2I \propto \frac{1}{d^2}

Here, $$I$$ is light intensity and $$d$$ is distance from the source. If the distance doubles, the light is spread over four times the area, so the intensity falls to one quarter of its original value.

Comparing intensities at two distances

For the same light source, the relative intensities at two distances are given by:

I2I1=(d1d2)2\frac{I_2}{I_1} = \left(\frac{d_1}{d_2}\right)^2

Here, $$I_1$$ is the intensity at distance $$d_1$$, and $$I_2$$ is the intensity at distance $$d_2$$. For example, if the second distance is three times the first, $$d_1/d_2 = 1/3$$, so $$I_2/I_1 = 1/9$$. The intensity at the second distance is one ninth of the original intensity.

This relationship assumes the same source, approximated as a point source, and negligible background light.

Light intensity and photosynthesis

Moving a lamp closer to a plant increases the light intensity reaching it; moving the lamp farther away decreases it. The rate of photosynthesis does not necessarily change in the same proportion as the intensity. If another factor limits photosynthesis, increasing the light intensity may have little or no effect on the rate.