5.3 Energy and ecosystems

Question 1

What are the different types of producers (autotrophs)?

Answer 1

Photoautotrophs use light energy to synthesise their own food, while chemoautotrophs use inorganic molecules

Question 2

What do plants synthesise through photosynthesis?

Answer 2

Plants synthesise organic compounds from carbon dioxide

Question 3

What are most sugars synthesised by?

Answer 3

Using respiratory substrates

Question 4

What are heterotrophs?

Answer 4

They cannot synthesise their own energy, but must obtain it from autotrophs or other heterotrophs; they act as consumers in food webs

Question 5

What can biomass be measured in terms of?

Answer 5

• Mass of carbon
• Dry mass of tissue per given area

Question 6

How do you measure biomass?

Answer 6

1. Sample of organism dried in an oven set to a low temperature (to avoid combustion)
2. Sample reweighed at regular intervals eg/ everyday
3. All water removed when mass remains constant
4. Mass of carbon taken to be 50% of dry mass

Question 7

What is the chemical energy stored in dry biomass can be estimated by?

Answer 7

Using calorimetry

• Sample of dry biomass burnt
• Energy released is used to heat a known volume of water
• Change in temperature of water used to calculate the chemical energy

Question 8

What is GPP?

Answer 8

Gross primary production

Chemical energy store in plant biomass, in a given area / volume, in a given time
i.e. the total energy resulting from photosynthesis

Question 9

What is NPP?

Answer 9

Net primary production

Chemical energy store in plant biomass after respiratory losses (R) to the environment have been taken into account

Question 10

What is the equation to find NPP?

Answer 10

NPP = GPP - R

Question 11

What is the NPP available for?

Answer 11

The NPP is available for plant growth and reproduction

The NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers

Question 12

How do you work out the net production of consumers?

Answer 12

N=I–(F+R)

I = the chemical energy store in ingested food
F = the chemical energy lost to the environment in faeces and urine
R = respiratory losses to the environment

Question 13

What are rates of productivity recorded by?

Answer 13

kJ ha-1 year-1

kJ: a unit for energy
Per unit area (e.g. ha): different environments vary in size; standardizes results so environments can be compared
Per year: more representative as takes into account the effect of seasonal variation (temperature etc.) on biomass so environments can be compared

Question 14

Why is energy transfer between trophic levels is inefficient?

Answer 14

Sun -> producer:
Wrong wavelength of light
Light strikes non-photosynthetic
region e.g. bark
Light reflected
Lost as heat

Producer -> primary consumer -> secondary consumer:
Respiratory loss - energy used for metabolism
Lost as heat
Not all plant/animal eaten eg/bones
Some food not digested (faeces)

Question 15

Why is energy transfer between trophic levels is inefficient?

Answer 15

Sun -> producer:
Wrong wavelength of light
Light strikes non-photosynthetic
region e.g. bark
Light reflected
Lost as heat

Producer -> primary consumer -> secondary consumer:
Respiratory loss - energy used for metabolism
Lost as heat
Not all plant/animal eaten eg/bones
Some food not digested (faeces)

Question 16

How could you use crops to increase energy transfer efficiency?

Answer 16

Simplifying food webs to reduce energy/biomass losses to non-human food chains:
- Herbicides kill weeds less competition more energy to create biomass
- Fungicides reduce fungal infections more energy to create biomass
- Pesticides reduce loss of biomass from crops

Fertilisers e.g. nitrates to prevent poor growth due to lack of nutrients