5 Energy transfer: 13 Energy and Ecosystems

Question 1

What do plants do in an ecosystem?

Answer 1

Synthesise organic compounds from atmospheric, or aquatic, carbon dioxide.

Question 2

What are the sugars synthesised by plants used for?

Answer 2

Most are used by the plant as respiratory substrates.
The rest are used to make other groups of biological molecules which form the biomass of the plants.

Question 3

What can biomass be measured in terms of?

Answer 3

Mass of carbon
or
dry mass of tissue per given area.

Question 4

How can the chemical energy store in dry biomass be estimated?

Answer 4

Using calorimetry.

Question 5

What is gross primary production (GPP)?

Answer 5

The chemical energy store in plant biomass, in a given area or volume.

Question 6

What is net primary production (NPP)?

Answer 6

The chemical energy store in plant biomass after respiratory losses to the environment have been taken into account.

Question 7

What is the equation for NPP?

Answer 7

NPP = GPP - R

R - respiratory losses

Question 8

What is NPP available for?

Answer 8

Plant growth and reproduction,
and other trophic levels in the ecosystem (e.g. herbivores and decomposers).

Question 9

What is the equation for the net production of consumers?

Answer 9

N = I - (F + R)

N - net production of consumers
I - chemical energy store in ingested food
F - chemical energy lost to the environment in faeces and urine
R - respiratory losses

Question 10

What is primary and secondary productivity and what is it measured as?

Answer 10

The rate of primary / secondary production.
Measured as biomass in a given area in a given time. (e.g. kJ / ha / year)

Question 11

How do farming practices increase the efficiency of energy transfer by simplifying food webs to reduce energy losses?

Answer 11

1. Using chemical pesticides reduces pest numbers so less biomass is lost from crops so NPP is greater.
2. Using herbicides reduces competition from weeds.
3. Biological agents are also used to reduce the number of pests and weeds.

Question 12

How do farming practices increase the efficiency of energy transfer by reducing respiratory losses?

Answer 12

1. Animals are kept in pens to restrict movement so their rate of respiration doesn’t increase.
2. Pens are kept indoors so less energy is wasted generating body heat.

Question 13

What happens to nutrients?

Answer 13

They are recycled within natural ecosystems.

Question 14

What is the process of the nitrogen cycle?

Answer 14

1. Nitrogen fixation
   - atmospheric nitrogen gas is turned into ammonia by nitrogen-fixing bacteria.
2. Ammonification
   - nitrogen compounds from dead organisms and animal waste are turned into ammonia by saprobionts, which form ammonium ions.
3. Nitrification
   - nitrifying bacteria change ammonium ions into nitrites then into nitrates.
4. Denitrification
   - denitrifying bacteria convert nitrates in the soil into nitrogen gas, under anaerobic conditions.

Question 15

What is the process of the phosphorus cycle?

Answer 15

1. Phosphate ions in rocks are released into the soil by weathering.
2. Plants take up phosphate ions via the roots. (Mycorrhizae increase the rate of uptake of phosphate ions).
3. Phosphate ions are transferred through the food chain.
4. Phosphate ions are lost in waste products.
5. Saprobionts break down organic compounds in dead organisms and animal waste, releasing phosphate ions into the soil again for plants.
6. Weathering also releases phosphate ions into water, which is taken up by aquatic producers and passed along the food chain to birds.
7. Waste produced by sea birds is ‘guano’ and contains a high concentration of phosphate ions.

Question 16

What is the role of saprobionts in decomposition?

Answer 16

They break down dead organisms and animal waste, allowing the chemical elements to be released and recycled.

Question 17

What is the role of mycorrhizae in plants?

Answer 17

It facilitates the uptake of water and inorganic ions.
It’s a fungi which connects to the plant’s roots, increasing the surface area.
It’s a mutualistic relationship as the fungi obtain organic compounds from the plants.

Question 18

What is saprobiontic nutrition?

Answer 18

Obtaining nutrients from dead organic matter using extracellular digestion.

Extracellular digestion is when saprobionts excrete enzymes and digest their food externally, then absorb the nutrients they need.

Question 19

What are fertilisers used for?

Answer 19

When crops are harvested, their ions can’t be returned to the soil so fertilisers replace the nitrates and phosphates lost.

Question 20

What are the environmental issues arising from the use of fertilisers?

Answer 20

Leaching:
- water soluble compounds are washed away into nearby pond and rivers
- could lead to eutrophication

Eutrophication:
1. mineral ions leached from fields stimulate the rapid growth of algae in ponds and rivers.
2. the algae block light from reaching the plants below.
3. the plants die because they can’t photosynthesise.
4. bacteria feed on the dead organisms, reducing oxygen concentration through aerobic respiration.
5. other aquatic organisms die due to lack of oxygen.