Photosynthesis overview and pigmentsAQA A-Level Biology: Revision notes
Section 1
Photoautotrophs and evolution
A photoautotroph makes organic molecules from inorganic ones using light energy. Photosynthesis is common to all photoautotrophs, including cyanobacteria, algae and plants. They all use chlorophyll, and a very similar process, which suggests it was inherited from a common ancestor. This is indirect evidence for evolution, as it is inferred from similarities between living organisms.
Section 2
Light absorption by chlorophyll
Light is absorbed by chlorophyll, and the energy absorbed is linked to the production of ATP. Photosynthetic pigments absorb different wavelengths of light. Chlorophyll a and b absorb mainly red and blue light and reflect green. Accessory pigments such as carotene widen the range of wavelengths absorbed.
Section 3
How ATP is produced
ATP is made when protons diffuse down an electrochemical gradient through ATP synthase, an enzyme embedded in the thylakoid membrane of chloroplasts. ATP synthase catalyses the formation of ATP from ADP and inorganic phosphate.
The same mechanism makes ATP in the membranes of other organelles, such as the mitochondria.
Protons move down their gradient through ATP synthase. Do not write that they are pumped through ATP synthase.
Section 4
Efficiency of photosynthesis
Photosynthesis is not 100% efficient. Light is lost because it is reflected, transmitted through the leaf, absorbed by structures that do not photosynthesise, or not of a wavelength chlorophyll absorbs. Some absorbed energy is lost as heat, and limiting factors such as carbon dioxide concentration or temperature stop all the light being used. A typical crop stores only a few per cent of the light that falls on it.
Section 5
Required practical 7: chromatography of pigments
Pigments are extracted from leaves with a solvent and spotted on a chromatography plate. The solvent moves up the plate and the pigments move different distances, because they differ in solubility.
Worked example. A pigment moves 4.5 cm and the solvent front 9.0 cm: .
Comparing leaves from shade-tolerant and sun plants, or of different colours, shows different pigments. Shade-tolerant plants often have more chlorophyll b, to absorb more of the dim light.
Measure the pigment to the centre of the spot and the solvent front from the same origin line. Rf has no unit and is never greater than 1.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Photosynthesis overview and pigments
- Cyanobacteria, algae, mosses and flowering plants all carry out photosynthesis. In every group, light is absorbed by chlorophyll, and the energy is used to make ATP by a very similar set of reactions involving membrane-bound proteins.Explain how the similarities in photosynthesis between these groups suggest that photoautotrophs share a common ancestor.2 marks
- Isolated chloroplasts were kept in the dark in a buffer at pH 4 until the space inside their thylakoids also reached pH 4. They were then transferred to a buffer at pH 8 containing ADP and inorganic phosphate, still in the dark. ATP was made for a short time.Explain why ATP was made in this experiment even though there was no light.2 marks
- A student separated the photosynthetic pigments from the leaves of two plants by thin-layer chromatography. The solvent front travelled 9.0 cm from the origin. The extract from a plant grown in full sun gave spots at 8.1 cm (carotene), 6.3 cm (chlorophyll a) and 5.4 cm (chlorophyll b) from the origin. The extract from a shade-tolerant plant gave the same three spots, but its chlorophyll b spot was much larger and darker.Calculate the Rf value of chlorophyll a in the extract from the plant grown in full sun. Show your working.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).