Chloroplasts, Photosynthetic Pigments and Action Spectra

Where exactly does photosynthesis occur?

Leaves are prominent photosynthetic organs because their mesophyll cells contain many chloroplasts positioned to receive light. Green stems and other chlorophyll-containing plant structures can also photosynthesise. At the cell scale, a chloroplast contains an outer and inner envelope, a fluid stroma and a separate internal thylakoid membrane system. Stacks of thylakoids form grana, connected by stroma lamellae. Chloroplasts may rearrange their positions under different illumination conditions, helping balance light interception and avoidance of excessive exposure.

The thylakoid membranes contain light-absorbing pigments, electron-transfer complexes and ATP synthase. Light-driven reactions produce ATP, a usable chemical-energy carrier, and NADPH, a carrier of reducing power. Water splitting also releases oxygen. The stroma contains enzymes of the Calvin cycle that use ATP, NADPH and CO₂ to produce carbohydrate intermediates. A crucial misconception is that the carbon-fixation reactions operate only at night. They do not require a photon to drive each step directly, but they depend strongly on products of light reactions and are generally active in illuminated green tissue.

Why do leaves contain several pigments?

A green leaf does not rely on one pigment. Paper chromatography can separate some of its compounds because they differ in solubility and interaction with the paper and solvent. The characteristic families introduced in the chapter are chlorophyll a, chlorophyll b, carotenes and xanthophylls. Chlorophyll a commonly appears blue-green in suitable extracts, chlorophyll b yellow-green, carotene pigments yellow-orange and xanthophylls yellow. Their exact positions on a strip depend on the solvent system and separation conditions.

A pigment absorbs particular wavelengths because photons of suitable energy can excite its electrons. Chlorophyll a absorbs especially strongly in blue and red parts of the visible spectrum while green light is less strongly absorbed and is often reflected or transmitted. Chlorophyll b and carotenoid pigments absorb overlapping but not identical ranges. The combination broadens the wavelengths that leaf light-harvesting complexes can use. These accessory pigments also participate in protection against damage from excess absorbed energy and reactive oxygen-related processes.

Absorption spectrum is not the same as action spectrum

An absorption spectrum plots how strongly a pigment absorbs different wavelengths. An action spectrum plots the effectiveness of different wavelengths in driving a biological outcome, such as rate of oxygen evolution or CO₂ uptake. Chlorophyll a has two strong visible-region absorption bands, in the blue and the red. Photosynthetic action spectra also tend to show greater effectiveness in blue and red light, as Engelmann's bacterial-accumulation experiment helped show. But the curves do not align perfectly, because the action spectrum reflects all pigments and the functioning photosynthetic apparatus, not chlorophyll a in isolation.

Engelmann illuminated a green alga with different wavelengths separated by a prism while aerobic bacteria were available to respond to oxygen release. Bacteria accumulated most where oxygen evolution was greatest, especially in blue and red spectral regions. This was an ingenious indirect readout of photosynthetic activity, not a measurement that chlorophyll molecules themselves attracted bacteria. Different algae and experimental conditions can yield somewhat different spectra because pigment composition and optical factors vary.

How light energy reaches a reaction centre

Photosynthetic pigments are organised into light-harvesting antenna complexes bound to proteins in thylakoid membranes. When an antenna pigment absorbs a suitable photon, excitation energy can pass between nearby pigments. Much of this energy is funnelled towards a specialised reaction-centre chlorophyll a pair, where a photochemical charge separation can occur. Energy transfer within an antenna is not the same as an electron physically migrating from one pigment molecule all the way through the antenna to the electron transport chain.

Most chlorophyll a molecules act as antenna pigments, but certain reaction-centre chlorophyll a molecules carry out the primary photochemical step. Photosystem II is associated with a reaction centre known as P680, while photosystem I has P700; their names refer to characteristic absorption peaks near 680 and 700 nm. The photosystems have complementary roles in the electron transport network, covered next. The distinction between antenna and reaction centre explains why many pigment types help capture light even though only specialised components initiate the key electron-transfer event.

Interpret a new set of pigment graphs

First read the axes: wavelength belongs on the horizontal axis and either absorbance or photosynthetic response on the vertical. Compare the locations of peaks without assuming equal vertical scales between different figures. If a plant has strong photosynthetic action where isolated chlorophyll a absorbs only moderately, accessory pigments may be contributing to light harvesting. Avoid claiming that green wavelengths never support photosynthesis; their effectiveness is often lower in a simple leaf-spectrum diagram but is not necessarily zero.

Quick Quiz

Model Answers

1. Thylakoid membranes absorb light and support electron flow, proton gradients and ATP/NADPH production; stroma enzymes fix CO₂ and produce carbohydrate intermediates using ATP and NADPH.

2. Absorption spectra show wavelengths absorbed by pigments; action spectra show how wavelengths drive photosynthetic activity. Chlorophyll pigments absorb strongly in blue and red, which usually produces prominent action peaks.

3. Chlorophyll b and carotenoids absorb additional photon energies and transfer excitation to reaction-centre systems; carotenoids also help dissipate potentially damaging energy.

4. Dispersed spectral light illuminated parts of an alga; aerobic bacteria collected near regions with greatest O₂ release. This associated photosynthetic oxygen evolution with blue and red wavelengths.

5. Chloroplasts still absorb some green light, and the action spectrum reflects several pigments and leaf optics, so effectiveness is typically lower but not strictly zero.