Photosynthesis — Notes

Photosynthesis is the process by which green plants make glucose from carbon dioxide and water, using light energy absorbed by chlorophyll, and release oxygen as a by-product.

Glucose is converted to starch for storage. Light energy is converted into chemical energy stored in food.

Significance: the source of food for almost all living things; keeps up the oxygen of the air; removes carbon dioxide; traps solar energy; the energy in fossil fuels came from photosynthesis long ago.

Chloroplast: an oval organelle bounded by a double membrane (envelope). Inside are grana (stacks of disc-shaped thylakoids whose membranes contain chlorophyll), joined by stroma lamellae, lying in the colourless stroma, which contains enzymes (and starch grains).

• Light phase → grana. Dark phase → stroma.

Stomata: in light, potassium ions enter the guard cells, water follows by osmosis, the guard cells become turgid and the stomata open, letting carbon dioxide in and oxygen out.

Takes place in the grana and needs light.

1. Activation of chlorophyll: chlorophyll absorbs light energy and becomes activated.
2. Photolysis of water: the energy splits water into hydrogen (ions and electrons) and oxygen.
3. Release of oxygen: the oxygen escapes from the leaf. The oxygen of photosynthesis therefore comes from water.
4. Formation of ATP and NADPH: light energy is stored in ATP; hydrogen is carried by NADPH.

Products: ATP, NADPH (used in the dark phase) and oxygen (released).

Takes place in the stroma and does not need light directly, but uses ATP and NADPH from the light phase (so it stops soon after the light phase stops).

• Carbon dioxide combines with hydrogen from NADPH, using energy from ATP, to form glucose; glucose is stored as starch. (Detailed equations of this phase are not in the syllabus.)

• Broad, flat lamina: large surface to absorb light.
• Thin: short distance for carbon dioxide and light to reach all cells.
• Palisade cells packed with chloroplasts, just below the upper surface where light is strongest.
• Spongy tissue with large air spaces for diffusion of gases.
• Many stomata for exchange of carbon dioxide and oxygen.
• A network of veins: xylem brings water and minerals; phloem carries away food.
• Leaves arranged so that they do not shade one another.

Destarching: keep the plant in darkness for one to two days so that its leaves have no starch.

Starch test: (1) boil the leaf in water (kills the cells, makes them permeable); (2) boil in alcohol in a water bath (removes chlorophyll; alcohol is flammable); (3) dip in warm water (softens the brittle leaf); (4) add iodine solution: blue-black = starch present; brown = no starch.

Light is necessary: black paper strip on both sides of a leaf of a destarched plant; after sunlight, the covered part has no starch.

Carbon dioxide is necessary: Moll's half-leaf experiment: half the leaf inside a bottle containing potassium hydroxide (absorbs carbon dioxide), cork sealed with Vaseline; that half has no starch. (Or two destarched plants under bell jars, one with potassium hydroxide and one, the control, with water.)

Chlorophyll is necessary: variegated leaf (for example croton): only the green parts turn blue-black.

Oxygen is released: hydrilla under a funnel in water in sunlight; bubbles collect in a test tube; the gas rekindles a glowing splinter. The rate rises with more light (for example 40 bubbles per minute with a lamp at 10 cm, 8 at 40 cm) and with sodium bicarbonate, which supplies carbon dioxide.

Carbon circulates between the air (as carbon dioxide), living things and fossil fuels.

• Photosynthesis by green plants removes carbon dioxide from the air.
• Feeding passes carbon from plants to animals.
• Respiration by plants, animals and decomposers returns carbon dioxide to the air.
• Decomposition of dead bodies and wastes by bacteria and fungi returns carbon dioxide.
• Some dead matter, buried for millions of years, forms fossil fuels; their combustion returns carbon dioxide.

Cutting forests and burning more fuel add carbon dioxide faster than photosynthesis removes it.