5.3 Energy and ecosystems

Question 1

Describe how biomass is formed in plants

Answer 1

● During photosynthesis, plants make organic (carbon) compounds from atmospheric or aquatic CO2 ● Most sugars synthesised are used by the plant as respiratory substrates
● Rest used to make other groups of biological molecules (eg. carbs, lipids & proteins) → form biomass

Question 2

How can biomass be measured?

Answer 2

Mass of carbon or dry mass of tissue per given area

Question 3

Describe how dry mass of tissue can be measured

Answer 3

1. Sample dried in an oven eg. at 100oC (avoid combustion)
2. Sample weighed and reheated at regular intervals
3. Until mass remains constant (all water evaporated)

Question 4

Explain why dry mass is more representative than fresh (wet) mass

Answer 4

Water volume in wet samples will vary but will not affect dry mass

Question 5

Describe how the chemical
energy stored in dry biomass can be estimated

Answer 5

Using calorimetry:
1. Known mass of dry biomass is fully combusted (burnt)
2. Heat energy released heats a known volume of water
3. Increase in temperature of
water is used to calculate
chemical energy of biomass

Question 6

Explain how features of a calorimeter enable
valid measurement of heat energy released

Answer 6

● Stirrer → evenly distributes heat energy (in water)
● Air / insulation → reduces heat loss & gain to & from surroundings
● Water → has a high specific heat capacity

Question 6

What is gross primary production (GPP)?

Answer 6

● Chemical energy store in plant biomass, in a given area or volume, in a given time ○ Total energy transferred into chemical energy from light energy during photosynthesis

Question 7

What is net primary production (NPP)?

Answer 7

Chemical energy store in plant biomass after respiratory losses to environment taken into account

Question 8

State the formula for NPP

Answer 8

NPP = GPP – R R = respiratory losses to the environment

Question 9

Explain the importance of NPP in ecosystems

Answer 9

● NPP is available for plant growth and reproduction
● NPP is also available to other trophic levels in the ecosystem, such as herbivores and decomposers

Question 10

What is primary or secondary productivity?

Answer 10

The rate of primary or secondary production, respectively

Question 11

State the units used for primary or secondary productivity

Answer 11

kJ ha-1 year-1(unit for energy, per unit area, per year)

Question 12

Explain why these units for primary or secondary productivity are used

Answer 12

● Per unit area → takes into account that different environments vary in size
○ Standardising results to enable comparison between environments
● Per year → takes into account effect of seasonal variation (temperature etc.) on biomass ○ More representative and enables comparison between environments

Question 13

Explain why most light falling on producers is not used in photosynthesis

Answer 13

● Light is reflected or wrong wavelength
● Light misses chlorophyll / chloroplasts / photosynthetic tissue
● CO2 concentration or temperature is a limiting factor

Question 14

State the formula for net production of consumers (N)

Answer 14

N = I – (F + R)I = the chemical energy store in ingested food
F = the chemical energy lost to the environment in faeces and urine

Question 15

State the formula for efficiency of energy transfer

Answer 15

Energy or biomass available after transfer / energy or biomass available before transfer
x 100 if a %

Question 16

Explain why energy transfer between trophic levels is inefficient

Answer 16

● Heat energy is lost via respiration
● Energy lost via parts of organism that aren’t eaten (eg. bones)
● Energy lost via food not digested → lost as faeces
● Energy lost via excretion eg. urea in urine

Question 17

Explain how crop farming practices increase energy transfer efficiency

Answer 17

● Simplifying food webs to reduce energy / biomass losses to non-human food chains eg. ○ Herbicides kill weeds → less competition (eg. for light) so more energy to create biomass ○ Pesticides kill insects (pests) → reduce loss of biomass from crops
○ Fungicides reduce fungal infections → more energy to create biomass
● Fertilisers e.g. nitrates to prevent poor growth due to lack of nutrients

Question 18

Explain how livestock farming practices increase energy transfer efficiency

Answer 18

● Reducing respiratory losses within a human food chain (so more energy to create biomass): ○ Restrict movement and keep warm → less energy lost as heat from respiration
○ Slaughter animal while still growing / young, when most of their energy is used for growth ○ Treated with antibiotics → prevent loss of energy due to pathogens
○ Selective breeding to produce breeds with higher growth rates