energy and ecosystems

Question 1

1. What is biomass?

Answer 1

The total mass of living material in a specific area at a given time (g m-2year-1)

Question 2

2. How do you determine the dry mass of a sample of plant material?

Answer 2

Heat the sample to 100oC, weigh at intervals until there is no further change in mass

Question 3

3. What is the advantage of using the dry mass?

Answer 3

For comparison, as the amount of water will vary

Question 4

4. How can the chemical energy stored in dry mass be determined?

Answer 4

• Use a bomb calorimeter
• A sample of dried plant or animal tissue is burnt inside the bomb in pure oxygen (to ensure it fully combusts).
• The heat released is transferred to water in the insulated bucket.
• A stirrer distributes the heated water evenly
• The temperature change of the water is recorded and used to calculate the energy transferred to the water.
• Measured in KJ Kg-1

Question 5

5. What do producers do?

Answer 5

They manufacture organic substances using light energy, water, carbon dioxide and mineral ions

Question 6

6. How is net primary production calculated?

Answer 6

NPP = GPP – R
(net primary production = gross primary production - respiratory losses)

Question 7

7. What are consumers?

Answer 7

Organisms that obtain their energy by feeding on other organisms.

Question 8

8. How is net production in consumers calculated?

Answer 8

N= I – (F+R)

Question 9

9. What is a trophic level?

Answer 9

A stage in a food chain

Question 10

10. How can efficiency be improved?

Answer 10

Restrict movement, keep animals warm, control feeding, exclude predators, reduce the number of trophic levels, simplify food webs by killing pests that are also eating crops.

Question 11

11. What is primary and secondary productivity

Answer 11

The rate of primary or secondary production. The units are KJ Kg-1 year-1

Question 12

12. How is the efficiency of an energy transfer in a food chain calculated?

Answer 12

Efficiency = energy available before the transfer x100
Energy available after the transfer

Question 13

13. State 3 reasons why only a small amount of the light energy falling on a leaf is converted to chemical potential energy in biomass of the plant.

Answer 13

some is reflected, some is the wrong wavelength for the pigments present, some is transmitted through the leaf, light may not strike chlorophyll, another factor may be limiting.

Question 14

14. State the reasons why only around 10% of the energy in a lettuce is transferred to energy in the biomass of a rabbit.

Answer 14

not all parts of the organism are eaten (e.g bones, hair, roots) so some chemical energy is left behind in the biomass
-some parts of the organism are consumed but cannot be digested so pass out as faeces-there is chemical energy in the biomass in the faeces
-some of the energy is lost in biological molecules in excretory materials such as urine which contains urea.
-some energy losses occur due to the release of heat during respiration. This is larger in organisms with a high body temperature as heat will be continually transferred to the environment so needs to be replaced by increasing the metabolic rate.
Due to these factors, only a small amount of energy is retained in the biomass of the animal’s tissues.