Photosynthesis is an energy transfer process: light energy ends up as chemical energy in organic molecules. It all happens inside chloroplasts, whose membranes, pigments and fluid are each arranged for one part of the job.
Photosynthesis transfers light energy into chemical energy. In the light-dependent stage, on the thylakoid membranes (stacked into grana), light energy is used to make ATP and reduced NADP. In the light-independent stage (the Calvin cycle) in the stroma, that ATP and reduced NADP are used to turn carbon dioxide into complex organic molecules. Light is absorbed by chloroplast pigments in the thylakoids: chlorophyll a (the primary pigment) and the accessory pigments chlorophyll b, carotene and xanthophyll, which widen the range of wavelengths used. An absorption spectrum shows how much light a pigment absorbs at each wavelength; an action spectrum shows the rate of photosynthesis at each wavelength. The pigments can be separated by chromatography and identified by their Rf values.
Rotate the chloroplast and click each structure: the envelope, the grana, the lamellae, the stroma, the starch grains, the lipid droplets, the ribosomes and the loop of DNA. Read what each one does in the panel.
| Structure | Feature | How it suits its function |
|---|---|---|
| envelope | two membranes | controls which substances enter and leave the chloroplast |
| thylakoids and grana | many flattened membrane sacs, stacked into grana and linked by lamellae | a huge membrane surface area holding photosystems, electron carriers and ATP synthase: the site of the light-dependent stage |
| thylakoid space | a small enclosed space inside each thylakoid | protons pumped into it build up a steep concentration gradient quickly, used to make ATP |
| stroma | fluid matrix around the grana | contains rubisco and the other enzymes of the Calvin cycle: the site of the light-independent stage, close to the ATP and reduced NADP made in the thylakoids |
| circular DNA and 70S ribosomes | in the stroma | code for and make some chloroplast proteins |
| starch grains and lipid droplets | in the stroma | store the products of photosynthesis |
Learn the sites exactly: the thylakoids (membranes and spaces, in grana) are the site of the light-dependent stage; the stroma is the site of the light-independent stage. The light-independent stage is not "the dark reaction": it needs a constant supply of products from the light-dependent stage, so it stops soon after the light goes off.
Drag the wavelength line from 400 nm to 700 nm. The pigment balls in the antenna complex glow when they absorb that wavelength. Compare the four absorption spectra with the white action spectrum, and find the green gap near 550 nm.
| Pigment | Colour | Absorbs mainly | Role |
|---|---|---|---|
| chlorophyll a | blue-green | blue-violet (about 430 nm) and red (about 660 nm) | primary pigment; at the reaction centre of each photosystem |
| chlorophyll b | yellow-green | blue (about 450 nm) and red-orange (about 640 nm) | accessory pigment |
| carotene | orange | blue and blue-green (about 450-490 nm) | accessory pigment |
| xanthophyll | yellow | blue and blue-green (about 440-480 nm) | accessory pigment |
Press Run the solvent and watch it rise up the paper, carrying the pigments different distances. When it stops, click each spot to read its Rf value.
An Rf value only identifies a pigment if it is compared with known values for the same solvent and the same paper or plate. Change the solvent and every Rf changes. Measure both distances from the origin line, not from the bottom of the paper.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the chloroplast simulator, which structure is the site of the light-dependent stage?
Drag the wavelength line to about 550 nm. Why do the pigments barely glow?
What does the white action spectrum in the simulator show?
On the chromatogram, carotene has Rf 0.95 and chlorophyll b 0.45. What does this tell you?
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
A pigment spot moves 3.6 cm from the origin while the solvent front moves 8.0 cm. What is its Rf value?
Which statement correctly links a feature of the chloroplast to its function?
Why does the action spectrum for photosynthesis show some activity at wavelengths where chlorophyll a absorbs very little?
Why is the origin line on a chromatogram drawn in pencil?
This skill is now recorded on your profile. Keep the chain going.