Photosynthesis

Photosynthesis is the process by which plants make their food by converting light energy into chemical energy. The raw materials to make food are carbon dioxide and water.

Chlorophyll is the chemical that absorbs light energy making it available to do the work of food manufacture.

The food produced is glucose, a carbohydrate. Oxygen is also made, some of it is used by the plant for its aerobic respiration and the excess is released into the air.

` chlorophyll

Equation for Photosynthesis Carbon Dioxide + Water → Glucose + Oxygen

light

The carbon dioxide for photosynthesis is obtained by the plant from the air.

The water is obtained from the soil. Light is supplied by the sun.

The plant makes chlorophyll from the food it has made by photosynthesis and magnesium it has absorbed from the soil. The glucose can be used in respiration, converted to fats, proteins and other useful substances or the excess can be stored as starch.

Photosynthesis removes carbon dioxide from the air. Therefore photosynthesis has a major influence on the greenhouse effect. Any reduction in photosynthesis will result in an increase in the level of carbon dioxide in the air and an enhancement in the greenhouse effect leading to global warming. Therefore deforestation is seen as a major threat to our climate.

MandPractivity 7 Show that Starch is produced by a Photosynthesising Plant

1. Place a potted plant in darkness, so that it cannot photosynthesise, for 48 hours at room temperature.

2. Remove a leaf and test it for starch using iodine.

a) Place the leaf in boiling water – this kills and softens the leaves.

b) Place the leaf in boiling alcohol – this removes the green chlorophyll turning the leaf white and so the colour of the iodine stain can be seen.

c) Place the leaf in boiling water – this removes the alcohol, softening the leaf and allows easy staining.

d) Using a dropper, add yellow-brown iodine stain.

3. The leaf stains a yellow-brown colour indicating that starch is not present.

No photosynthesis – no starch production. The leaves of the plant do not have starch.

4. Now place the potted plant in good light, at room temperature for six hours.

5. Remove a leaf and test it for starch, as before, using iodine.

6. The leaf stains blue-black indicating that starch is present.

This shows that the plant made starch during its six hours of photosynthesis when it was exposured to light - remember that starch was not present in the leaves when the plant was taken out of darkness and placed in light.

Results: a non-photosynthesising plant does not make starch, a photosynthesising plant does make starch.

Conclusion: photosynthesis produces starch.

Leaves - ‘green solar panels for photosynthesis’ Leaves are the major organs of photosynthesis.

Leaves have a large surface area for absorbing sunlight.

They have tiny pores (stomata), through which carbon dioxide rapidly enters the leaf for photosynthesis and the excess oxygen is allowed to escape from the leaf.

The photosynthetic cells contain chloroplasts, the specialised chlorophyll-rich green photosynthetic machines. Leaves are rich in chlorophyll which is the green that absorbs the light for photosynthesis.

A tropism is a growth response of a plant to an external stimulus. The tropism is described as positive it the growth towards the stimulus and negative if away from the stimulus. Phototropism is the growth response of a plant to light.

Geotropism is the growth response of a plant to gravity. Stems are positively phototropic and negatively geotropic. Therefore stems grow towards the light and upwards away from gravity. This is an advantage to the plant because this response places the leaves into better light resulting in great food production by photosynthesis.

Roots are negatively phototropic and positively geotropic. Therefore exposed roots grow away from light and so more likely to find soil. Roots grow downward in the soil in the direction of gravity more deeply colonising the soil improving anchorage and increasing the absorption of water and minerals. Tropisms are specialised plant responses to particular changes in the external environment and so improve a plant’s chances of survival and reproduction.

Investigate the Growth Response of Plants to Gravity (Geotropism)

1. Soak pea seeds in water overnight at room temperature.

2. Place the soaked seeds at different angles against the inside wall of a beaker with the seeds held gently in position by wet cotton wool.

3. Place the beaker in a dark box at room temperature for a few days.

Results: Stems grow up, roots grow down. Investigate the Growth Response of Plants to Light (Phototropism)

1. Set up four dishes with moist cotton wool.

2. Sprinkle 10 cress seeds onto each – you now have four ‘mini-gardens’.

3. (a) One dish is not covered and is exposed to light equally in all directions.

(b) A second dish is covered by a light proof cardboard box.

(c) A third is cover by a similar box but a wide hole in one side of the box.

(d) A fourth is again covered but the box has a wide hole in the top.

4. Leave the mini-gardens in a bright room at room temperature for a week and check each day plus make sure the cotton wool is kept moist.

Results:

(a) Seedling stems grow straight with many green leaves.

(b) Seedling stems grow straight, very tall, spindly, few pale small leaves.

(c) Seedling stems grow towards the hole with many green leaves.

(d) Seedling stems grow up tall towards the hole.