Biology · Chapter 23
Study notes aligned to the official NEB syllabus.
Photosynthesis is the anabolic process by which green plants, algae and some bacteria trap light energy and convert it into chemical energy stored in carbohydrates. It is the primary route by which solar energy enters the biosphere.
Overall summary equation:
$$6CO_2 + 12H_2O \xrightarrow{\text{light, chlorophyll}} C_6H_{12}O_6 + 6O_2 + 6H_2O$$
Photosynthesis occurs in the chloroplast and has two stages: the light reaction (photochemical phase, in the thylakoid membrane) and the dark reaction (biosynthetic/Calvin-Benson phase, in the stroma).
Pigments absorb light of specific wavelengths and are held in the thylakoid membrane as part of two photosystems.
| Pigment | Nature | Distribution |
|---|---|---|
| Chlorophyll a | Blue green, primary pigment | All oxygenic photosynthetic organisms |
| Chlorophyll b | Yellow green, accessory | Higher plants and green algae |
| Chlorophyll c | Accessory | Brown algae, diatoms |
| Carotenes | Orange yellow carotenoid | All chloroplasts, precursor of vitamin A |
| Xanthophylls | Yellow carotenoid, oxygenated | All chloroplasts, gives autumn leaf colour |
| Phycobilins (phycocyanin, phycoerythrin) | Water soluble accessory pigments | Cyanobacteria, red algae |
Chlorophyll a is the only pigment that can directly take part in the light reaction; the other, accessory pigments absorb light of wavelengths chlorophyll a cannot use well and pass the energy on to it, while also protecting it from photo-oxidation.
Two photosystems sit in the thylakoid membrane, each built from a light harvesting antenna complex plus a reaction centre:
This is the main pathway and uses both photosystems in series.
$$2H_2O \rightarrow 4H^+ + 4e^- + O_2$$
$$NADP^+ + 2e^- + H^+ \rightarrow NADPH$$
Net products of non cyclic photophosphorylation are ATP, NADPH and O2 (the O2 comes from water splitting, not from CO2).
Only PS I takes part. The electron ejected from excited P700 passes through ferredoxin, then back through the cytochrome b6f complex and plastocyanin, and returns to P700 instead of moving on to NADP+. Because the electron cycles back to its own photosystem:
This pathway supplements ATP supply whenever the chloroplast needs more ATP relative to NADPH than the Calvin cycle alone would use.
Photosynthesis is the anabolic process by which green plants, algae and some bacteria trap light energy and convert it into chemical energy stored in carbohydrates. It is the primary route by which solar energy enters the biosphere.
Overall summary equation:
Photosynthesis occurs in the chloroplast and has two stages: the light reaction (photochemical phase, in the thylakoid membrane) and the dark reaction (biosynthetic/Calvin-Benson phase, in the stroma).
Pigments absorb light of specific wavelengths and are held in the thylakoid membrane as part of two photosystems.
| Pigment | Nature | Distribution |
|---|---|---|
| Chlorophyll a | Blue green, primary pigment | All oxygenic photosynthetic organisms |
| Chlorophyll b | Yellow green, accessory | Higher plants and green algae |
| Chlorophyll c | Accessory | Brown algae, diatoms |
| Carotenes | Orange yellow carotenoid | All chloroplasts, precursor of vitamin A |
| Xanthophylls | Yellow carotenoid, oxygenated | All chloroplasts, gives autumn leaf colour |
| Phycobilins (phycocyanin, phycoerythrin) | Water soluble accessory pigments | Cyanobacteria, red algae |
Chlorophyll a is the only pigment that can directly take part in the light reaction; the other, accessory pigments absorb light of wavelengths chlorophyll a cannot use well and pass the energy on to it, while also protecting it from photo-oxidation.
Two photosystems sit in the thylakoid membrane, each built from a light harvesting antenna complex plus a reaction centre:
This is the main pathway and uses both photosystems in series.
Net products of non cyclic photophosphorylation are ATP, NADPH and O2 (the O2 comes from water splitting, not from CO2).
Only PS I takes part. The electron ejected from excited P700 passes through ferredoxin, then back through the cytochrome b6f complex and plastocyanin, and returns to P700 instead of moving on to NADP+. Because the electron cycles back to its own photosystem:
This pathway supplements ATP supply whenever the chloroplast needs more ATP relative to NADPH than the Calvin cycle alone would use.