Energy transfer and Nutrient cycles

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 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 7

What is gross primary production (GPP)?

Answer 7

● 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 8

What is net primary production (NPP)?

Answer 8

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

Question 9

State the formula for NPP

Answer 9

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

Question 10

Explain the importance of NPP in ecosystems

Answer 10

● 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 11

What is primary or secondary productivity

Answer 11

The rate of primary or secondary production, respectively.

Question 12

State the units used for primary or secondary productivity

Answer 12

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

Question 13

Explain why these units for primary or secondary productivity are used

Answer 13

● 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 14

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

Answer 14

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

Question 15

State the formula for net production of consumers (N)

Answer 15

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 16

State the formula for efficiency of energy transfer

Answer 16

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

Question 17

Explain why energy transfer between trophic levels is inefficient

Answer 17

● 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 18

Explain how crop farming practices increase efficiency of energy transfer

Answer 18

● 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 19

Explain how livestock farming practices increase efficiency of energy transfer

Answer 19

● 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

Question 20

Explain the role of saprobionts in recycling chemical elements

Answer 20

● Decompose (break down) organic compounds eg. proteins / urea / DNA in dead matter / organic waste
● By secreting enzymes for extracellular digestion (saprobiotic nutrition)
● Absorb soluble needed nutrients and release minerals ions eg. phosphate ions

Question 21

Explain the role of mycorrhizae = symbiotic association between fungi and plant roots

Answer 21

● Fungi act as an extension of plant roots to increase surface area of root system
● To increase rate of uptake of water and inorganic ions
● In return, fungi receive organic compounds eg. carbohydrates

Question 22

Give examples of biological molecules that contain nitrogen

Answer 22

Amino acids / proteins or enzymes / urea / DNA or RNA / chlorophyll / ATP or ADP / NAD or NADP

Question 23

state the stages of the nutrient cycle and what happens in each

Answer 23

1. nitrogen fixation
   ● Nitrogen gas (N2) converted into ammonia (NH3), which forms ammonium ions(NH4+) in soil
   ● By nitrogen-fixing bacteria
2. Ammonification
   ● Nitrogen-containing compounds from dead organisms are broken down / decomposed
   ● Converted to ammonia, which forms ammonium ions in soil
   ● By saprobionts - secrete enzymes for extracellular digestion
3. nitrification
   ● Ammonium ions in soil converted into nitrites then nitrates, via a two-step oxidation reaction
   ○ For uptake by plant root hair cells by active transport
   ● By nitrifying bacteria in aerobic conditions (oxygen)
4. denitrification
   ● Nitrates in soil converted into nitrogen gas (reduction)
   ● By denitrifying bacteria in anaerobic conditions (no oxygen, eg. waterlogged soil)

Question 24

Describe the role of saprobionts in the nitrogen cycle.

Answer 24

1. (They use enzymes to) decompose proteins/DNA/RNA/urea;
2. Producing/releasing ammonia/ammonium

Question 25

Suggest why ploughing (aerating) soil increases its fertility

Answer 25

● More ammonium converted into nitrite and nitrate / more nitrification / more (active) nitrifying bacteria
● Less nitrate converted to nitrogen gas / less denitrification / fewer (active) nitrifying bacteria

Question 26

Describe the phosphorus cycle

Answer 26

1. Phosphate ions in rocks released (into soils / oceans) by erosion / weathering
2. Phosphate ions taken up by producers / plants / algae and incorporated into their biomass
   ○ Rate of absorption increased by mycorrhizae
3. Phosphate ions transferred through food chain eg. as herbivores eat producers
4. Some phosphate ions lost from animals in waste products (excretion)
5. Saprobionts decompose organic compounds eg. DNA in dead matter / organic waste,
   releasing phosphate ions

Question 27

Give examples of biological molecules that contain phosphorus

Answer 27

Phospholipids / DNA or RNA / ATP or ADP / NADP / TP or GP / RuBP

Question 28

Explain why fertilisers are used

Answer 28

● To replace nitrates / phosphates lost when plants are harvested and livestock are removed
○ Those removed from soil and incorporated into biomass can’t be released back into the
soil through decomposition by saprobionts
● So improve efficiency of energy transfer → increase productivity / yield

Question 29

Describe the difference between artificial and natural fertilisers

Answer 29

-natural=Contain inorganic compounds of nitrogen, phosphorus and potassium
-artificial=Organic, eg. manure, compost, sewage → ions released during decomposition by saprobionts

Question 30

Explain the key environmental issue arising from use of fertilisers

Answer 30

● Phosphates / nitrates dissolve in water, leading to leaching of nutrients into lakes / rivers / oceans
● This leads to eutrophication
1. Rapid growth of algae in pond / river (algal bloom) so light blocked
2. So submerged plants die as they cannot photosynthesise
3. So saprobionts decompose dead plant matter, using oxygen in aerobic respiration
4. So less oxygen for fish to aerobically respire, leading to their death

Question 31

Explain the key advantage of using natural fertiliser over artificial fertiliser

Answer 31

● Less water soluble so less leaching → eutrophication less likely
● Organic molecules require breaking down by saprobionts → slow release of nitrate / phosphate etc.

Question 32

Give two examples of biological molecules containing nitrogen that would
be removed when a crop is harvested.

Answer 32

DNA
ATP