Photosynthesis: rates, controls and pigment evidence

What you’ll learn

• Calculate a photosynthetic rate from gas-volume data.
• Distinguish net measurements from gross photosynthesis.
• Evaluate light, carbon dioxide, temperature and wavelength comparisons.

Understand the idea

Core practical 10 investigates photosynthetic rate in an aquatic plant while varying light intensity, wavelength, temperature or carbon dioxide availability. Oxygen collected per unit time can indicate the rate, but some oxygen may dissolve and respiration consumes some of what is produced. Bubble number is a weaker volume measure when bubble sizes vary. Compare independent samples on a stated basis, such as equal leaf area or a rate per unit mass, and allow conditions to stabilise before interpreting a steady rate.

Change one intended factor while controlling the others. Moving a lamp can change both light and temperature; coloured filters can change light intensity as well as wavelength. A fair wavelength comparison therefore needs a suitable measure of the light reaching the plant, rather than assuming all filters transmit equally. A rate plateau at high light can indicate that another factor is limiting, but it does not identify that factor without further evidence.

The observed oxygen balance in light is a net rate: photosynthetic production minus respiratory consumption. Adding a separately estimated respiratory rate gives an estimate of gross production only if the assumptions about respiration and other losses hold. Pigment chromatography provides complementary evidence about absorbed wavelengths. An Rf value uses distances from the same origin and supports identification only when compared under matching chromatographic conditions.

Key formulae

$$r_{\mathrm{net}}=\frac{\Delta V_{\mathrm{O_2}}}{\Delta t}$$

$$r_{\mathrm{gross}}=r_{\mathrm{net}}+r_{\mathrm{respiration}}$$

$$R_f=\frac{\mathrm{distance\ moved\ by\ pigment}}{\mathrm{distance\ moved\ by\ solvent\ front}}$$

Worked example

An aquatic-plant record shows a net oxygen gain of $2.40\ \mathrm{cm^3}$ in $6.00\ \mathrm{min}$. Its separately measured respiratory oxygen consumption is $0.080\ \mathrm{cm^3\,min^{-1}}$. Assume respiration is the same in light and dark, and other oxygen losses are negligible. Estimate gross oxygen production.

1. The net rate is $2.40/6.00=0.400\ \mathrm{cm^3\,min^{-1}}$.
2. The plant also uses $0.080\ \mathrm{cm^3\,min^{-1}}$ in respiration under the stated assumption.
3. Gross production is $0.400+0.080=0.480\ \mathrm{cm^3\,min^{-1}}$.

Answer: Estimated gross rate $0.480\ \mathrm{cm^3\,min^{-1}}$.