Energy and ecosystems

Question 1

How do plants use the sugars from photosynthesis?

Answer 1

Primarily as respiratory substrates

To synthesise other biological molecules e.g cellulose

Question 2

What is biomass?

Answer 2

Total dry mass of tissue or mass of carbon measured over a given time in a specific area

Question 3

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

Answer 3

Using calorimetry

Energy released = specific heat capacity of water x volume of water x temperature increase of water

Question 4

Define gross primary production

Answer 4

Total chemical energy in plant biomass within a given volume or area

Question 5

Define net primary productivity

Answer 5

Total chemical energy available for plant growth, plant reproduction and energy transfer to other trophic levels after respiratory losses

Question 6

Why is most of the suns energy not converted to organic matter?

Answer 6

Most solar energy is absorbed by the atmosphere or reflected by the clouds

Photosynthetic pigments cannot absorb some wavelengths of light

Not all light falls directly on a chlorophyll molecule

Energy is lost as heat during respiration/photosynthesis

Question 7

How can the net production of consumers be calculated?

Answer 7

N = I - (F + R)

I: chemical energy from ingested food

F: energy lost as faeces and urine

R: respiratory losses

Question 8

Why does biomass decrease along a food chain?

Answer 8

Energy lost in nitrogenous waster and faeces

Some of the organism is not consumed

Energy is lost as heat

Question 9

Define primary and secondary productivity

Answer 9

Rate of primary and secondary production

Biomass in a specific area over a given time period

Question 10

Outline some common farming practices used to increase the efficiency of energy transfer

Answer 10

Exclusion of predators: no energy lost to other organisms in food web

Artificial heating: reduce energy lost to maintain constant body temperature

Restriction of movement

Feeding is controlled at the optimum

Question 11

Explain why the length of the food chain is limited

Answer 11

Energy is lost at each trophic level

So there is insufficient energy to support a higher trophic level

Question 12

Name the general stages of the phosphorous cycle

Answer 12

Weathering
Runoff
Assimilation
Decomposition
Uplift

Question 13

Why is the phosphorus cycle a slow process?

Answer 13

Phosphorus has no gas phase so there is no atmospheric cycle

Most phosphorus is stored in rocks

Question 14

What happens during weathering and runoff?

Answer 14

Phosphate compounds from sedimentary rocks leach into surface water and soil

Question 15

Explain the significance of phosphorus to living organisms

Answer 15

Plants convert inorganic phosphate into biological molecules e.g DNA, ATP

Phosphorus is passed to consumers via feeding

Question 16

What happens during uplift?

Answer 16

Sedimentary layers from oceans are brought up to land over many years

Question 17

How does mining affect the phosphorus cycle?

Answer 17

Speeds up uplift

Question 18

Name the 44 main stages in the nitrogen cycle

Answer 18

Nitrogen fixation
Ammonification
Nitrification
Dentrification

Question 19

Why can’t organisms use nitrogen directly from the atmosphere?

Answer 19

N2 is very stable due to strong covalent triple bond so can’t be broken down

Question 20

What happens during atmospheric fixation of nitrogen?

Answer 20

1. High energy of lightning breaks N2 into N
2. N reacts with oxygen to form NO2-
3. NO2- dissolves in water to form NO3-

Question 21

Outline the role of bacteria in nitrogen fixation

Answer 21

Mutualistic nitrogen-fixing bacteria in nodules of legumes and free living bacteria in soil

Use the enzyme nitrogenase to reduce gaseous nitrogen into ammonia

Question 22

How does ammonium change into nitrogen?

Answer 22

Ammonia - Nitrite - Nitrate

Question 23

Outline the role of bacteria in ammonification

Answer 23

1. Saprobionts feed on and decompose organic waste containing nitrogen
2. NH3 released
3. NH3 dissolves in water in soil to form NH4+

Question 24

Outline the role of bacteria in nitrification

Answer 24

2 step process carried out by saprobionts in aerobic conditions:

2NH4+ + 3O2 = 2NO2- + 2H2O +4H+

2NO2- + O2 = 2NO3-

Question 25

Outline the role of bacteria in dentrification

Answer 25

Anaerobic dentrifying bacteria convert soil nitrates back into gaseous nitrogen

Question 26

Explain the significance of nitrogen to living organisms

Answer 26

Plant roots uptake nitrates via active transport and use them to make biological compounds e.g amino acids, NAD/NADP and nucleic acids

Question 27

Outline the role of mycorrhizae

Answer 27

Mutualistic relationship between plant and fungus increases surface area of root system = increases uptake of water and mineral ions

Question 28

Give 3 benefits of planting a different crop on the same field each year

Answer 28

Nitrogen fixing crops e.g legumes make soil more fertile by increasing soil nitrate content

Different crops have different pathogens

Different crops use different proportions of certain ions

Question 29

Name the 2 categories of fertiliser and state the purpose of using fertiliser

Answer 29

Organic: decaying organic matter and animal waste

Inorganic: minerals from rocks, usually containing nitrogen, phosphorus, potassium

Increase gross productivity for higher yield

Question 30

At a certain point, using fertiliser no longer increases crop yield. Why?

Answer 30

A factor unrelated to the concentration of mineral ions limits the rate of photosynthesis so rate of growth cannot increase any further

Question 31

Outline 2 main environmental issues caused by the use of fertilisers

Answer 31

Leaching: nitrates dissolve in rainwater and runoff into water sources

Eutrophication: water source becomes putrid as a result of algal bloom

Question 32

What happens during eutrophication?

Answer 32

1. Aquatic plants grow exponentially since nitrate level is no longer a limiting factor
2. Algal bloom on water surface prevents light from reaching the bottom and plants die
3. Oxygen levels decrease as population of aerobic saprobionts increases to decay dead matter so fish die
4. Anaerobic organisms reproduce exponentially and produce toxic waste which makes water putrid

Question 33

How can the risk of eutrophication be reduced?

Answer 33

Sewage treatment marshes on farms

Pumping nutrient enriched sediment out of water