3.8: Photosynthesis

Question 1

State that Photosynthesis involves the conversion of light energy into chemical energy.

Answer 1

• Photosynthesis involves the conversion of light into chemical energy.
• Photosynthesis is the process by which plants synthesise organic compounds (e.g. glucose) from inorganic compounds (CO2 and H2O) in the presence of sunlight

Photosynthesis is a two step process:
1. The light dependent reactions convert the light energy into chemical energy (ATP)
2. The light independent reactions use the chemical energy to synthesise organic compounds (e.g. glucose)
+ The organic molecules produced in photosynthesis can be used in cellular respiration to provide the energy needed by the organism

Question 2

State that light from the Sun is composed of a range of wavelengths (colours).

Answer 2

Light from the sun is composed of a range of wavelengths.

Question 3

State that chlorophyll is the main photosynthetic pigment.

Answer 3

Chlorophyll is the main photosynthetic pigment.

Question 4

Outline the differences in absorption of red, blue and green light by chlorophyll.

Answer 4

• The main colours of light absorbed by chlorophyll are red and blue light.
• The main colour of light not absorbed (it is reflected) by chlorophyll is green light-
• This explains why leaves are green - excepting when the presence of other pigmented substances (e.g. anthocyanins) produces a different colour
• Deciduous trees stop producing high amounts of chlorophyll in the winter (due to insufficient sunlight), allowing other photosynthetic pigments (e.g. xanthophylls, carotenoids) to come to the fore, which changes the colour of the leaf.

Question 5

State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

Answer 5

Light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen.

Question 6

State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

Answer 6

ATP and hydrogen are used to fix carbon dioxide to make organic molecules.

Question 7

Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass.

Answer 7

Measuring CO2 Uptake:

• CO2 uptake can be measured by placing a plant in an enclosed space with water
• Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which increases the acidity of the resulting solution
• The change in pH can therefore provide a measure of CO2 uptake by a plant (increased CO2 uptake = more alkaline pH)

Measuring O2 Production:

• O2 production can be measured by submerging a plant in an enclosed space with water attached to a sealed gas syringe
• Any oxygen gas produced will bubble out of solution and can be measured by a change in water level (via the position of the meniscus)

Measuring Biomass (Indirect):

• Glucose production can be indirectly measured by a change in a plant’s biomass (weight)
• This requires the plant to be completely dehydrated prior to weighing to ensure the change in biomass reflects a change in organic matter and not water content
• An alternative method for measuring glucose production is to determine the change in starch levels in a plant (glucose is stored as starch)
• Starch can be identified via iodine staining (resulting solution turns purple) and quantitated using a colorimeter

Question 8

Outline the effect of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis:

Answer 8

As temperature increases, the rate of photosynthesis increases more and more steeply until the optimum temperature is reached. If temperature keeps increasing above the optimum temperature then photosynthesis starts to decrease very rapidly.

As light intensity increases so does photosynthesis until a certain point. At a high light intensities photosynthesis reaches a plateau and so does not increase any more. At low and medium light intensity the rate of photosynthesis is directly proportional to the light intensity.

As the carbon dioxide concentration increases so does the rate of photosynthesis. There is no photosynthesis at very low levels of carbon dioxide and at high levels the rate reaches a plateau.