The chloroplast and pigments
Photosynthesis and the chloroplast
- Photosynthesis happens inside the chloroplast.
- Its structure has two parts for the two stages of the reaction.
- Coloured pigments trap the light that powers it all.
Inside a chloroplast
- Flat sacs called thylakoids stack into a granum (plural grana) — the thylakoid membranes hold the pigments and run the first stage.
- The fluid around them is the stroma — where the second stage happens.
Practice
In a chloroplast, the stacks of thylakoids (grana) are where:
The thylakoid membranes hold the pigments and carry out the light-dependent stage; the stroma runs the Calvin cycle.
The two stages
- light-dependent stage (in the thylakoids): uses light energy to make ATP and reduced NADP.
- light-independent stage / Calvin cycle (in the stroma): uses that ATP and reduced NADP to build organic molecules from carbon dioxide.
Practice
The light-dependent stage produces:
It uses light to make ATP and reduced NADP, which power the Calvin cycle.
Practice
The Calvin cycle (light-independent stage) takes place in the:
The Calvin cycle runs in the stroma, fixing CO₂ using the ATP and reduced NADP made earlier.
Pigments and their spectra
- Pigments: chlorophyll a and b (absorb red and blue), plus carotene and xanthophyll (absorb other colours).
- An absorption spectrum shows how much light each pigment absorbs; an action spectrum shows how fast photosynthesis goes. They match — proof the pigments drive it.
- Chromatography separates pigments; each has an Rf value = distance pigment moved ÷ distance solvent moved.
Practice
The absorption spectrum and action spectrum match closely. This shows that:
Where pigments absorb most (blue/red), photosynthesis is fastest — so those pigments drive the reaction.
Practice
An Rf value in chromatography is:
Rf = distance the pigment moved ÷ distance the solvent moved; it identifies each pigment.
You've got it
Key idea
- thylakoids stack into grana (light-dependent stage); the stroma runs the Calvin cycle
- light-dependent → ATP + reduced NADP; light-independent → builds molecules from CO₂
- pigments: chlorophyll a/b + carotene/xanthophyll; absorption & action spectra match
- chromatography + Rf value (pigment distance ÷ solvent distance) identify pigments