Topic 3.8 Photosynthesis

Question 1

3.8.1 Photosynthesis involves the conversion of ______ energy into ______ energy.

Answer 1

Photosynthesis involves the conversion of light energy into chemical energy.

Question 2

3.8.2 Light from the Sun is composed of a range of __________ (colours).

Answer 2

Light from the Sun is composed of a range of wavelengths (colours).

Question 3

3.8.3 ________ is the main photosynthetic pigment.

Answer 3

Chlorophyll is the main photosynthetic pigment.

Question 4

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

Answer 4

Red end of spectrum: absorbed by chlorophyll and used for photosynthesis

Green middle of spectrum: reflected by chlorophyll, colour is visible

Blue end of spectrum: absorbed by chlorophyll and used for photosynthesis

Chlorophyll = green pigment → reflects green light and absorbs other wavelengths of visible light spectrum.

Question 5

3.8.5 State that light energy is used to produce ___, and to split ______ ________ (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

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

Answer 6

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

Question 7

3.8.7 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

Measure by O₂ production: Aquatic plants (e.g. Myriophyllum) release O₂ bubbles during photosynthesis. Collect bubbles to measure volume.

Measure by CO₂ uptake: Leaves take in CO₂ from the air or water around them; difficult to measure directly. If CO₂ is absorbed from water, the pH of the water rises. Monitor with pH indicators or with pH meters.

Measure by biomass: batches of plants are harvested at series of times; biomass of batches measured. Rate of increase in biomass = indirect measure of photosynthetic rate.

Note

Day: photosynthetic rate is high

Night: photosynthetic rate may drop to 0

A plant has fairly consistent rate of cell respiration (day & night) at a relatively low level; therefore, must make correction for cell respiration when measuring photosynthetic rate

Question 8

3.8.8 Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis

Answer 8

Light

Low-medium intensities: photosynthetic rate rises, proportional to light intensity

High intensity: rate reaches plateau (all enzymes in use)

CO₂

Very low concentration: no photosynthesis

Low to fairly high: photosynthesis positively correlates with concentration

Very high: rate plateaus

Temperature

Increasing: as temp increases, rate increases more and more steeply (curved)

Optimum temperature: top of curve, best conditions

Above optimum: rate falls steeply (denaturation of enzymes)