13.2. Limiting factors

Question 1

Factors affecting the rate of photosynthesis

Answer 1

1) Light intensity
2) CO2 Concentration
3) Temperature

Question 2

Light intensity on the rate of photosynthesis

Answer 2

• affects the rate of the light dependent stage, because this is driven by energy transferred in light rays

Question 3

CO2 Concentration on the rate of photosynthesis

Answer 3

• CO2 is a main reactant in photosynthesis

- Normal air only contains about 0.04% of CO2

Question 4

Temperature on the rate of photosynthesis

Answer 4

• affects the rate of the light dependent stage
• at higher temperatures, molecules have more kinetic energy so collide more often and are more likely to react when they do collide

Question 5

Limiting Factor

Answer 5

A factor that, when short in supply, limits the rate of a reaction or process

Question 6

Growing plants in protected environments

Answer 6

• An understanding of the effect of environmental factors on the rate of photosynthesis allows their management when crops are grown in protected environments, such as glasshouses.
• The aim is to increase the yield of the crop concerned
• Such glasshouse-grown crops have the added advantage that insect pests and fungal diseases are more easily controlled than is possible with field-grown crops, further improving yield.

Question 7

Growing Tomatoes in protected environments

Answer 7

• For example, many hectares of tomato plants are grown in glasshouses.
• In the most sophisticated of these, sensors monitor the light intensity, the humidity of the atmosphere and the concentration of carbon dioxide around the plants
• The plants grow hydroponically – that is, with their roots in a nutrient solution whose nutrient content can be varied at different stages of the plants’ growth
• All of these factors are managed by a computer to maximise the yield of the crop

Question 8

Investigating the rate of photosynthesis using an aquatic plant

Answer 8

Elodea, or other similar aquatic plants, can be used to investigate the effect on the rate of photosynthesis of altering the:

• light intensity – by altering the distance, d, of a small light source from the plants (light intensity is proportional to 1 d2)
• wavelength of light – by using different colour filters, making sure that they each transmit the same light intensity
• concentration of carbon dioxide – by adding different quantities of sodium hydrogencarbonate (NaHCO3) to the water surrounding the plant
• temperature of the water surrounding the plant – using a large container, such as a beaker, to help maintain the chosen temperatures

Question 9

Investigating the rate of photosynthesis using an aquatic plant precautions

Answer 9

• The aquatic plant needs to be well illuminated before use and the chosen stem needs to be cut cleanly just before putting it into a test tube.
• The bubbles given off are mostly oxygen, but contain some nitrogen.
• To prevent these gases from dissolving in the water, rather than forming bubbles, the water needs to be well aerated (by bubbling air through it) before use.

Question 10

Hill Reaction

Answer 10

• Hill showed that isolated chloroplasts had ‘reducing power’ and liberated oxygen from water in the presence of an oxidising agent.
• The ‘reducing power’ was demonstrated by using a redox agent that changed colour on reduction.
• This technique can be used to investigate the effect of light intensity or of light wavelength on the rate of photosynthesis of a suspension of chloroplast
• Hill used Fe3+ ions as his acceptor, but various redox agents, such as the blue dye DCPIP can substitute for the plant’s NADP in this system
• DCPIP becomes colourless when reduced:

Question 11

Investigating the Hill Reaction

Answer 11

• Chloroplasts can be isolated from a leafy plant, such as lettuce or spinach, by liquidising the leaves in ice-cold buffer and then filtering or centrifuging the resulting suspension to remove unwanted debris.
• Working quickly and using chilled glassware, small tubes of buffered chloroplast suspension with added DCPIP solution are placed in different light intensities or in different wavelengths of light and the blue colour assessed at intervals.
• The rate of loss of blue colour (as measured in a colorimeter or by matching the tubes against known concentrations of DCPIP solution) is a measure of the effect of the factor being investigated (light intensity or the wavelength of light) on chloroplast activity