Photosynthesis: Question 2

Worked solution

Part (a): Why several pigments broaden the range of usable wavelengths

Each photosynthetic pigment has a slightly different chemical structure, so each absorbs light most efficiently at different, characteristic wavelengths:

• Chlorophyll a absorbs strongly in the blue-violet and red regions of the visible spectrum.
• Chlorophyll b and the carotenoids (carotene and xanthophyll) absorb light at somewhat different wavelengths from chlorophyll a, filling in some of the wavelengths that chlorophyll a itself absorbs only weakly.

Within a photosystem, these accessory pigments (chlorophyll b and the carotenoids) are arranged close together with chlorophyll a and light-harvesting proteins. When an accessory pigment absorbs a photon of light, the energy is not wasted. It is passed on (transferred from pigment to pigment) until it reaches the chlorophyll a molecules at the reaction centre, where it excites an electron to a higher energy level. Because several pigments with different absorption peaks funnel their captured energy into the same reaction centre, a wider range of wavelengths across the visible spectrum can be captured and used to drive the light-dependent reactions than chlorophyll a could capture on its own.

Part (b): Why the action spectrum matches the absorption spectrum

The absorption spectrum shows how strongly the extracted pigments absorb light at each wavelength. The action spectrum shows how fast photosynthesis proceeds at each wavelength, for the same plant.

Photosynthesis is driven by the energy from light that is absorbed by the pigments. Light that is reflected or transmitted (not absorbed) cannot excite electrons and so cannot contribute to the light-dependent reactions. This means:

• At wavelengths where absorption is high (peaks in the absorption spectrum, e.g. blue and red light), a large amount of light energy is captured, exciting many electrons, so the rate of photosynthesis is also high (a peak in the action spectrum).
• At wavelengths where absorption is low (a trough in the absorption spectrum, e.g. green light), very little energy is captured, so the rate of photosynthesis is also low (a corresponding trough in the action spectrum).

Because the rate of the light-dependent reactions depends directly on how much light is absorbed, the two spectra rise and fall together across the visible spectrum, giving the action spectrum a shape that closely resembles the absorption spectrum.

Part (c): Why leaves appear green

Chlorophyll a and chlorophyll b absorb light strongly in the blue and red regions of the spectrum, but only weakly in the green region. The carotenoids extend the range of blue light absorbed but, like the chlorophylls, absorb very little green light either.

Since green light is only weakly absorbed by any of the leaf’s main pigments, most of it is reflected or transmitted rather than being taken up and used. It is this reflected/transmitted green light that reaches an observer’s eye, which is why leaves appear green. Even though the leaf is actively absorbing and using large amounts of blue and red light for photosynthesis.

Final answers

• (a) Different pigments absorb different wavelengths and pass their captured energy to chlorophyll a at the reaction centre, so a wider range of wavelengths can be used.
• (b) Absorbed light energy drives the rate of photosynthesis, so wavelengths absorbed strongly (peaks in the absorption spectrum) also give a high rate of photosynthesis (peaks in the action spectrum), and the two spectra match in shape.
• (c) Green light is poorly absorbed by all the main pigments, so it is mostly reflected/transmitted, making leaves appear green.