Topic 1 - Detailed Notes - Nutrition in Plants.docx

Edexcel IGCSE Biology

Topic 1: Nutrition in Plants

Notes

This work by PMT Education is licensed under CC BY-NC-ND 4.

Nutrition

Photosynthesis in plants (2.18-2.21)

Photosynthesis is the process of making glucose from sunlight in the leaves of the plant. It is an endothermic reaction in which light energy is converted into chemical energy within the chloroplasts.

The equation for photosynthesis is:

light

carbon dioxide + water → glucose + oxygen

Carbon dioxide = CO2, Water = H2O, Oxygen = O2 Glucose = C6H12O6

Factors affecting photosynthesis:

Temperature:

• With an increase in temperature, the rate of photosynthesis increases.

• However, as the reaction is controlled by enzymes, this trend only continues up to a certain temperature until the enzymes begin to denature and the rate of reaction decreases.

Light intensity:

• For most plants, the higher the light intensity, the rate of photosynthesis increases.

• As the distance between the light source and the plant increases, the light intensity decreases,

i.e. It is inversely proportional to the square of the distance: light intensity ∝ 1/distance²

• This means that if a lamp is 2 metres away from a plant, then light intensity of the lamp is a ¼ of its original value → 1/2² = ¼

Carbon dioxide concentration:

• Carbon dioxide is also needed to make glucose (see equation).

• As the concentration of carbon dioxide increases, the rate of reaction increases.

Practicals investigating photosynthesis:

• Use water plants, such as Elodea which release bubbles of oxygen when photosynthesising

• A lamp with an LED bulb is set up beside the beaker of water containing the water plant. An LED is best as it will not raise the temperature of the water.

• Sodium hydrogen carbonate (NaHCO₃) is added to the water to supply carbon dioxide

• This can set up can be used to investigate the evolution of oxygen from a water plant.

Investigating the effect of light intensity:

1. Place pondweed in water and set up a desk lamp next to alongside a ruler so that you can measure the distance between the light and the beaker

2. Move the lamp away by 10cm

3. Leave for 5 minutes to allow for the pondweed to adapt

4. Count the number of bubbles given off in 1 minute and record

5. Repeat steps 2-4

Investigating the effect of carbon dioxide:

Complete the experiment above but instead of testing the variable of light intensity by using a lamp, use different concentrations of sodium hydrogen carbonate solution, whilst keeping the other variables the same.

Investigating starch production:

1. Cover half of a small leaf with foil

2. Place the plant on a windowsill for 48 hours so that light can reach it

3. Put the leaf into boiling water to kill and preserve it.

4. Put the leaf in a boiling tube containing hot ethanol for 10 minutes (this removes the chlorophyll pigment).

5. Dip the leaf in boiling water to soften it.

6. Put leaf in a Petri dish and cover with iodine solution.

7. The covered half of the leaf will remain orange-brown, whereas the exposed half will change to blue-black (as iodine solution changes colour in the presence of starch, as photosynthesis turned the glucose into starch for storage)

Experiment provide photosynthesis requires chlorophyll

Repeat the experiment above but with a variegated leaf. Variegated plants are white and green and only contain chlorophyll in the green parts. Therefore only the green areas of the plant will test positive for starch (i.e. turn blue-black) as a result of photosynthesis occuring. The white areas that do not contain chlorophyll remain an orange-brown colour.

Leaf structures

Mineral ions

Magnesium:

• Required for chlorophyll production

• Deficiency: causes leaves to turn yellow

Nitrate:

• Required to produce amino acids

• Deficiency: causes stunted growth and turns leaves yellow