5A.2 Chloroplasts and Chlorophyll – Photosynthesis

Process used by plants to capture solar energy using chlorophyll and convert CO₂ and H₂O into organic sugars.
Word equation: ext{CO}2 + ext{H}2 ext{O}
ightarrow ext{C}6 ext{H}{12} ext{O}6 + ext{O}2
Symbol equation: 6\,\text{CO}2 + 6\,\text{H}2\text{O} \rightarrow \text{C}6\text{H}{12}\text{O}6 + 6\,\text{O}2
How light energy is used:
- Break the strong O–H bonds in water.
- Released hydrogen combines with CO₂ to form glucose.
- Oxygen is released as a waste product.

Chloroplasts: large organelles found in cells of green parts; typically 10–50 per cell; uniquely adapted for photosynthesis.
Location and role: Site of photosynthesis in plants.

Outer and inner membranes form the chloroplast envelope.
Grana: stacks of thylakoids (membrane discs) containing chlorophyll.
Lamellae: extensions of thylakoid membranes that join grana; act as a skeleton and help optimize light capture.
Stroma: fluid matrix surrounding grana; contains enzymes needed for photosynthesis.

Chlorophyll is not a single molecule; it is a mixture of pigments:
1. Chlorophyll a (blue-green)
2. Chlorophyll b (yellow-green)
3. Carotenoids (orange carotene, yellow xanthophyll)
4. Phaeophytin (grey; breakdown product)
Each pigment absorbs light at different wavelengths; together increase total light capture.

Chlorophyll a is found in all photosynthesising plants and in the largest amount.
Other pigments vary among plants, explaining leaf colour variation; e.g., aquatic plants may appear red or brown to adapt to habitat.

Absorption spectrum: amount of light of different wavelengths absorbed by a pigment.
Different pigments have different absorption spectra, enabling broader light utilisation.

Visible light is part of the electromagnetic spectrum.
Pigments absorb light at certain wavelengths; the visible range is approximately 400\ \text{nm} to 700\ \text{nm}.

Action spectrum: plant’s rate of photosynthesis as a function of light wavelength.
Action spectra closely correlate with the absorption spectra of pigments.
Having multiple pigments extends the action spectrum and maximises photosynthesis across more wavelengths.

T. W. Engelmann compared photosynthesis rates under different wavelengths using algae.
Used bacteria that move toward oxygen to indicate sites of high photosynthetic activity.
Result: demonstrated an action spectrum linking wavelength to photosynthetic rate.

Chromatography separates mixtures based on size/charge; used to separate chloroplast pigments from leaves.
Pigments travel up a solid medium at different speeds in a suitable solvent.

After chromatography, determine pigment Rf values and compare to known pigments in the same solvent.
Definition: Rf = \frac{d{pigment}}{d{solvent}}
Range: 0 < Rf < 1

Two chlorophyll-containing complexes: Photosystem I (PSI) and Photosystem II (PSII).
Each has a distinct pigment composition and absorbs light at different wavelengths to maximise light capture.
Peak absorption:
- PSI: 700\ \text{nm}
- PSII: 680\ \text{nm}

Seaweeds contain chloroplast pigments similar to land plants and some different pigments.
Possible question topics:
- Which method to separate chloroplast pigments by solubility? (Chromatography)
- Interpreting chromatogram with pigments such as beta-carotene, chlorophyll a, chlorophyll b, xanthophylls among seaweeds and spinach.
- Compare/contrast pigment types in seaweeds versus spinach, using diagram evidence.

Chloroplast structure supports efficient light capture and pigment separation.
Pigments broaden light absorption via absorption spectra;
multiple pigments enable wider action spectra and higher photosynthesis rates.
Chromatography and Rf values identify individual pigments.
Photosystems I and II optimize absorption at 700 nm and 680 nm respectively.