The Calvin cycle, pigments and photosynthesis investigations

What you’ll learn

• Track carbon and energy through the Calvin cycle.
• Interpret absorption and action spectra and chromatography.
• Evaluate measurements of photosynthetic rate.

Understand the idea

Rubisco combines carbon dioxide with five-carbon RuBP. The unstable six-carbon product splits into two molecules of three-carbon GP. ATP and reduced NADP from the light reactions convert GP to GALP. Most GALP regenerates RuBP, using ATP; some contributes to sugars and to other molecules such as lipids and, with a nitrogen source, amino acids. Three fixed carbon dioxide molecules give one net three-carbon GALP while regenerating the carbon acceptor.

An absorption spectrum shows how strongly a pigment absorbs different wavelengths; an action spectrum shows photosynthetic effectiveness at different wavelengths. Accessory pigments extend the usable range, so the action spectrum reflects a pigment system. Chromatography separates pigments through differences in solubility in the mobile phase and attraction to the stationary phase. An Rf value is a ratio measured from the same origin and is compared only under matching solvent and stationary-phase conditions.

For an aquatic-plant investigation, compare oxygen production per unit time while changing one factor and controlling the others. Gas volume is more reliable than counting bubbles of unequal size. Temperature, carbon dioxide availability, wavelength and light intensity can each become limiting. Oxygen production in light measures net gas exchange because respiration continues; repeated measurements, equilibration and suitable controls improve interpretation.

Key formulae

$$\mathrm{CO_2}+\mathrm{RuBP}\xrightarrow{\mathrm{Rubisco}}2\mathrm{GP}$$

$$R_f=\frac{\text{distance travelled by pigment}}{\text{distance travelled by solvent front}}$$

$$\text{measured rate}=\frac{\text{oxygen volume}}{\text{time}}$$

Worked example

A leaf is moved from light to darkness. Predict the initial changes in GP and RuBP while carbon dioxide remains available.

1. Light-dependent production of ATP and reduced NADP stops rapidly.
2. Conversion of GP into GALP slows because ATP and reducing power are no longer replenished.
3. Remaining RuBP can still accept carbon dioxide briefly, while its regeneration slows.

Answer: Initially GP increases and RuBP decreases; these predictions concern the short period after the light is removed.