4.1 Photosynthesis

Question 1

Photosynthesis

Answer 1

Plants are autotrophs – this means that they can make their own food using light, water and carbon dioxide.
This is why they are called producers in food chains.
Photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light.
The leaves of the plant are where most photosynthesis takes place, in specialised mesophyll cells which are packed with chloroplasts containing chlorophyll to absorb as much light energy as possible.
The sugars produced by photosynthesis are used to make all the substances a plant needs, as well as being used in respiration to release energy.

Carbon dioxide + water —— oxygen + glucose
6CO2 + 6H2O —— 6O2 + C6H12O6

Question 2

Factors affecting the rate of photosynthesis (temperature)

Answer 2

The temperature of the environment affects how much kinetic energy all particles have – so temperature affects the speed at which carbon dioxide and water move through a plant.
The lower the temperature, the less kinetic energy particles have, resulting in fewer successful collisions occurring over a period of time.
Increasing temperature increases the kinetic energy of particles, increasing the likelihood of collisions between reactants and enzymes which results in the formation of products.
At higher temperatures, however, enzymes that control the processes of photosynthesis can be denatured (where the active site changes shape and is no longer complementary to its substrate) – this reduces the overall rate.

Question 3

Factors affecting rates of photosynthesis (light intensity)

Answer 3

The intensity of the light available to the plant will depend on the amount of energy that it has to carry out photosynthesis.
The more light a plant receives, the faster the rate of photosynthesis.
This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply.

Question 4

Factors affecting rates of photosynthesis (CO2)

Answer 4

Carbon dioxide is one of the raw materials required for photosynthesis.
This means the more carbon dioxide that is present, the faster the reaction can occur.
This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is now in short supply.

Question 5

Factors affecting rates of photosynthesis (chlorophyll)

Answer 5

The number of chloroplasts (as they contain the pigment chlorophyll which absorbs light energy for photosynthesis) will affect the rate of photosynthesis.
The more chloroplasts a plant has, the faster the rate of photosynthesis.
The amount of chlorophyll can be affected by:
diseases (such as tobacco mosaic virus)
lack of nutrients (such as magnesium)
loss of leaves (fewer leaves means fewer chloroplasts)

Question 6

Greenhouses

Answer 6

Commercial horticulturists will grow their plants in a greenhouse.
This means that they are able to control as many of the limiting factors of photosynthesis as possible.
Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit.

Question 7

Properties of greenhouses

Answer 7

shades removed from ceiling to allow maximum light.
artificial light for winter/darker hours.
ventilation helps prevent plants from getting to hot.
water system to ensure plants are watered.
it traps the suns heat so that the temperature is not a limiting factor.
protects the plants from pests and diseases.

Question 8

uses of glucose in plants

Answer 8

Used for respiration (both aerobic and anaerobic).
Converted into insoluble starch for storage in the stems, leaves and roots.
Used to produce fat or oil for storage (especially in seeds).
Used to produce cellulose, which strengthens the cell wall.
Combined with nitrate ions absorbed from the soil to produce amino acids for protein synthesis.

Question 9

Practical 6 (rate of photosynthesis)

Answer 9

Aim: Investigate the effect of light intensity on the rate of photosynthesis using an aquatic organism such as pondweed.
Procedure - Place a piece of pondweed into a beaker of water.
Use a light a set distance from the plant.
Record the number of bubbles observed in three minutes.
Repeat steps for different distances.

Improvements - Use a gas syringe to collect the volume of gas produced
Repeat the experiment at least twice for each distance and calculate the mean number of bubbles.
Use of a glass tank between lamp and plant to prevent heating of the plant, or using an LED bulb that releases very little heat energy.