Nutrition in Plants — Photosynthesis

What is photosynthesis?

Photosynthesis is the process by which green plants (and algae) make their own food. It uses light energy, absorbed by the green pigment chlorophyll, to convert simple inorganic molecules into glucose. Because plants make their own food, they are called producers, and they form the start of almost every food chain.

The word equation is:

$$\text{carbon dioxide} + \text{water} \xrightarrow[\text{chlorophyll}]{\text{light}} \text{glucose} + \text{oxygen}$$

The balanced symbol equation is:

$$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$

Light is not a reactant, so it is written above the arrow, along with chlorophyll. Light energy is transferred to chemical energy stored in the glucose. Photosynthesis is essentially the reverse of aerobic respiration.

Key terms

Photosynthesis — making glucose from carbon dioxide and water, using light energy trapped by chlorophyll.

Chlorophyll — the green pigment in chloroplasts that absorbs light energy.

Producer — an organism that makes its own food by photosynthesis.

The role of chlorophyll and light

Chlorophyll is found inside chloroplasts, mostly in the cells of the leaf. It absorbs mainly red and blue light (and reflects green, which is why leaves look green). The absorbed light energy is used to split water and to drive the reactions that build glucose. Without chlorophyll, or without light, no photosynthesis can take place — this is the basis of several classic experiments later in this chapter.

What the plant does with the glucose

The glucose made in photosynthesis is used in several ways:

1. Respiration — glucose is broken down to release energy for the cell's activities.
2. Stored as starch — excess glucose is converted to insoluble starch. Starch is ideal for storage because it is compact, insoluble, and does not affect the water balance (osmosis) of the cell.
3. Made into cellulose — used to build strong cell walls.
4. Made into sucrose — the soluble sugar transported around the plant in the phloem.
5. Used to make proteins — glucose provides the carbon skeleton, but the plant must also absorb nitrate ions from the soil to supply nitrogen for amino acids and proteins.
6. Made into lipids (fats and oils) — for storage, especially in seeds.

Exam tip

A common question asks why glucose is stored as starch rather than as glucose. The mark is for: starch is insoluble, so it does not dissolve and does not draw water into the cell by osmosis.

Limiting factors

The rate of photosynthesis depends on three main factors. A limiting factor is the factor in shortest supply — the one holding back the rate. At any moment, only one factor is usually limiting.

For light and carbon dioxide, the graph rises steeply, then levels off (plateaus) when something else becomes limiting. For temperature, the rate rises to an optimum (around 25–35 °C in many plants) and then drops as enzymes are denatured.

Watch out

When a graph "levels off", light is no longer the limiting factor. The rate is now limited by something else, usually carbon dioxide concentration or temperature. To raise the plateau, you must increase that other factor.

Real world

Commercial growers use these ideas in greenhouses: extra lighting, paraffin heaters that release carbon dioxide, and controlled warmth all push up the rate of photosynthesis so crops grow faster and yields increase.

Leaf structure and adaptations

Leaves are the main organ of photosynthesis and are beautifully adapted for the job.

Mineral requirements

Plants make carbohydrates, but they cannot make proteins or chlorophyll without mineral ions absorbed from the soil by the roots.

Exam tip

Link the symptom to the job: no magnesium means no chlorophyll, so leaves go yellow; no nitrate means no protein, so the plant cannot grow properly. Stating the ion and the use earns full marks.

Practical: testing a leaf for starch

A plant making glucose stores it as starch, so testing for starch shows where photosynthesis has happened. The starch test uses iodine solution, which turns from orange-brown to blue-black if starch is present.

Method:

1. Drop the leaf in boiling water to kill it and stop reactions (and break down the cell walls).
2. Heat the leaf in ethanol in a water bath (no Bunsen flame near it — ethanol is flammable) to remove the green chlorophyll, so colour changes are easy to see.
3. Dip the leaf in hot water again to soften it.
4. Spread it on a white tile and add iodine solution.

A blue-black colour means starch is present; orange-brown means it is absent.

Watch out

Ethanol is highly flammable, so it is heated using a water bath, never directly over a flame.

Practical: investigating what photosynthesis needs

Before each experiment the plant is destarched by leaving it in the dark for 24–48 hours, so any starch found afterwards must have been made during the test.

Light is needed — cover part of a leaf with foil. After light exposure, only the uncovered part turns blue-black with iodine.
Chlorophyll is needed — use a variegated leaf (green and white patches). Only the green parts (with chlorophyll) test positive for starch.
Carbon dioxide is needed — enclose one destarched plant with soda lime (absorbs $CO_2$) and another with sodium hydrogencarbonate (releases $CO_2$). Only the leaf with $CO_2$ available makes starch.

Worked example

A variegated leaf is exposed to light, then tested with iodine. The green areas turn blue-black; the white areas stay orange-brown.

Explain why. The white areas have no chlorophyll, so they cannot absorb light energy and cannot photosynthesise. With no glucose made, no starch is stored, so iodine stays orange-brown. The green areas have chlorophyll, photosynthesise, store starch, and turn blue-black. This shows chlorophyll is essential for photosynthesis.