What Photosynthesis Is and Where It Happens: Quick Revision

A brief note on What Photosynthesis Is and Where It Happens. Read it before a test. Try each question at the end before you open its answer.

Early experiments

• Priestley (1770): a mint plant restores the air that a candle or an animal makes unfit.
• Ingenhousz (1779): sunlight is needed, and only the green parts release oxygen.
• Sachs (1854): glucose is made in the green parts and stored as starch.
• Engelmann: bacteria gather in the blue and red parts of the spectrum. The first action spectrum.
• Van Niel: bacteria with hydrogen sulphide give out sulphur. In green plants the oxygen comes from water.

The reaction

• Overall: $6\,\text{CO}_2 + 12\,\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{H}_2\text{O} + 6\,\text{O}_2$.
• Water is oxidised. Carbon dioxide is reduced.

The chloroplast

• Two membranes, the stroma, thylakoids stacked as grana, joined by stroma lamellae. Lumen inside each thylakoid.
• Thylakoid membranes: chlorophyll, light reactions, ATP and NADPH.
• Stroma: the enzymes that build sugar. Chloroplasts are mainly in the mesophyll.

Pigments

• Chlorophyll a (bright or blue green), chlorophyll b (yellow green), xanthophylls (yellow), carotenoids (yellow to yellow-orange).
• Rf = pigment distance $\div$ solvent distance. Always below $1$.
• Absorption spectrum: light absorbed. Action spectrum: rate of photosynthesis. Chlorophyll a peaks in blue and red.
• Accessory pigments widen the range of light used and protect chlorophyll a from photo-oxidation.

A worked example

A leaf fixes $60$ millimoles of carbon dioxide. How much glucose is made, in millimoles and in milligrams?

1. Moles. Six carbon dioxide make one glucose, so $60 \div 6 = 10$ mmol.
2. Mass. $10\ \text{mmol} \times 180\ \text{mg/mmol} = 1800\ \text{mg}$.
3. Check. The oxygen released is $60 \times 32 = 1920\ \text{mg}$, a little more than the glucose.

Practice with new numbers

A program to try

The program works out the glucose and the oxygen for the carbon dioxide that is fixed.

Mistakes to avoid

• The oxygen comes from water, not from carbon dioxide.
• Six carbon dioxide make one glucose. Do not equate one with one.
• Chlorophyll is in the thylakoid membranes, the sugar enzymes are in the stroma.
• Rf is below 1. The pigment distance goes on top.

Check yourself

1. What did Ingenhousz add to Priestley's work?

2. Where does the oxygen released in photosynthesis come from?

3. In which part of the chloroplast is sugar made?

4. Which pigment travels the farthest on a chromatogram?

5. Why does the action spectrum rise above the absorption curve of chlorophyll a in the blue-green region?