Section 5: Energy and Ecosystems + Nutrient Cycles

Question 1

How do different types of autotrophs (producers) to synthesise their own food

Answer 1

• Photoautotrophs use light energy and chemoautotrophs use inorganic molecules

Question 2

How can biomass be measured?

Answer 2

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

Question 3

How is dry mass of tissue obtained?

Answer 3

• Sample of organism dried in an oven set to a low temperature
• Sample reweighed at regular intervals
• All water is removed when mass remains constant
• Mass of carbon is 50% of dry mass

Question 4

How is dry mass more representative

Answer 4

• Because water content of sample varies

Question 5

How do you measure the chemical energy stored in dry biomass using calorimetry

Answer 5

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

Question 6

What is gross primary production?

Answer 6

• Chemical energy store in plant biomass in a given area/ volume in a given time

Question 7

What is net primary production?

Answer 7

• Chemical energy store in plant biomass after respiratory losses to the environment
• NPP - GPP - R
• NPP is available for plant growth and reproduction
• The NPP is also available to other trophic levels in the ecosystem

Question 8

How do you work out the net production of consumers?

Answer 8

• N = I - (F+R)
• I = The chemical energy store in food
• F = The chemical energy lost to the environment as faeces and urine
• R = Respiratory losses

Question 9

What are the units of rate productivity

Answer 9

KJ ha^-1year^-1

• KJ = unit of energy
• Per unit area (ha) = different environments vary in size
• Per year = more representative as it takes into account the effect of seasonal variation on biomass so environments can be compared

Question 10

Why is the energy transfer between the sun and producer inefficient

Answer 10

• Wrong wavelength of light
• Light does not hit chlorophyll
• Light is reflected back into space
• Lost as heat

Question 11

Why is the energy transfer between producer - primary consumer - secondary consumer inefficient

Answer 11

• Respiratory loss - for metabolism
• Lost as heat
• Not all plant/ animal eaten e.g bones
• Some food not digested - faeces

Question 12

What are some farming practices that increase energy efficiency in crops?

Answer 12

• Simplifying food webs to reduce energy/ biomass losses to non-human food chains
• Herbicides - kill weeds - less competition - more energy to create biomass
• Fungicides
• Pesticides
• Fertilisers

Question 13

What are some farming practices that increase energy efficiency in livestock?

Answer 13

• Restrict movement
• Keep warm
• Slaughter when young
• Selective breeding
• Treated with antibiotics to prevent loss of energy due to pathogens

Question 14

What is the role of saprobionts in recycling chemical elements?

Answer 14

• Feed on remains of dead plants/ animals and their waste products and break down organic molecules
• By secreting enzymes for extracellular digestion - absorb soluble nutrients

Question 15

What is the role of mycorrhizae in recyling chemical elements

Answer 15

• Symbiotic relationship between fungi and roots of plants = mycorrhizae
• Fungi acts as an extension of the plant roots - increase surface area - absorption of water
• Mutualistic relationship - plant provides fungi with carbs

Question 16

What are the main stages of the nitrogen cycle?

Answer 16

• Nitrogen Fixation
• Ammonification
• Nitrification
• Denitrification

Question 17

What is Nitrogen Fixation?

Answer 17

• Nitrogen gas converted to nitrogen containing compounds e.g ammonia
• By nitrogen fixing bacteria
  
  • Can be free living in the soil
  • Or mutualistic - nodules on roots of legumes. Plant gets amino acids and bacteria gets carbs

Question 18

What is ammonification?

Answer 18

• Nitrogen containing compounds e.g proteins from dead organisms is broken down
• Converted to ammonia which forms ammonium ions
• By saprobionts

Question 19

What is Nitrification?

Answer 19

• Ammonium ions in the soil turn into nitrites then nitrates by nitrifying bacteria
• Bacteria need oxygen
• Nitrates absorbed by plant root hair cells by active transport
• Adding more O2 - nitrifying bacteria increases and denitrifying to decrease

Question 20

What is denitrification?

Answer 20

• Nitrates in the soil - nitrogen gas by denitifying bacteria - anaerobically respire

Question 21

What is the importance of the nitrogen cycle?

Answer 21

• Nitrogen gas is unreactive and not easily converted into other compounds
• Nitrates can be used to make proteins and nucleic acids for growth

Question 22

What are the stages of the Phosphorous cycle?

Answer 22

• Phosphate ions in the rocks released by erosion/weathering
• Phosphate ions taken up by plants into their biomass
• Phosphate ions transferred through food chain
• Some phosphate ions lost in waste products - saprobionts decompose
• Weathering of rocks can release phosphate into seas, lakes and rivers

Question 23

What is net primary production?

Answer 23

• Chemical energy store in plant biomass after respiratory losses to the environment
• NPP - GPP - R
• NPP is available for plant growth and reproduction
• The NPP is also available to other trophic levels in the ecosystem