A2 Energy Transfer and Nutrient Cycles

Question 1

Define ecosystem

Answer 1

All organisms living in a particular area and all non-living conditions

Question 2

Define biomass

Answer 2

Mass of living material - chemical energy stored in plant

Question 3

Define producers

Answer 3

Organisms that make their own food (plants and algae)

Question 4

Define consumers

Answer 4

Organisms that feed on producers or other consumers on the food chain

Question 5

Describe the Process in Which Biomass can be Measured as Dry Mass (Calorimeter)

Answer 5

Measured in terms of mass of carbon an organism contains or dry mass of tissue per unit area

• Dry mass = mass - water
• Sample of organism dried in oven at low temp. Sample weighed regularly. Once mass is constant, water has been removed
• Sample can be scaled up to give biomass of whole of total population. Typical units - kg/m squared
• Mass of carbon generally 50% of dry mass

Question 6

Define Gross Primary Production

Answer 6

Total amount of chemical energy converted from light energy by plants, in a given area

Question 7

Define Respiratory Loss

Answer 7

Energy lost to the environment as heat through respiration

Question 8

Define Net Primary Production

Answer 8

• NPP = GPP - R
• Energy available for growth and reproduction - stored in plants biomass
• Also energy available to organisms at next stage in food chain

Question 9

Define Net Production for Consumers

Answer 9

• N = I - (F + R)
• Store chemical energy in their biomass
• Approx. 90% of energy available lost, not transferred to next trophic level
• Not all food ingested
• Of food ingested: some is indigestible, egested as faeces, some energy lost to environment through respiration or excretion of urine
• Energy left stored in biomass and available to next trophic level - net production

Question 10

Describe the Role of Fungi and Bacteria in Nutrient Recycling (Saprobionts)

Answer 10

• Many are saprobionts (decomposers), feed on remains of dead plants, animals and waste products, breaking them down
• Saprobionts secrete enzymes and digest food externally, then absorb the nutrients needed - extracellular digestion. Organic molecules broken down to inorganic ions

Question 11

Explain what is Meant by Symbiotic Relationships Between Fungi and Roots of Plants (Mycorrhizae)

Answer 11

• Fungi made up of hyphae (long, thin strands), connect to plant’s roots
• Hyphae increase SA - plant absorbs more ions from soil and uptake of water increases
• In return, fungi obtain organic compounds, such as glucose, from plant

Question 12

Describe the First Stage of the Nitrogen Cycle (Nitrogen Fixation)

Answer 12

• Nitrogen gas in atmosphere converted to nitrogen-containing compounds
• Carried out by bacteria (Rhizobium)
• Turn nitrogen into ammonia, then forms ammonium ions, that can be used by plants
• Rhizobium found in root nodules of leguminous plants
• Form mutualistic relationship with plants - provide nitrogen compounds to plants in exchange for carbohydrates

Question 13

Describe the Second Stage of the Nitrogen Cycle (Ammonification)

Answer 13

• Nitrogen compounds from dead organisms converted into ammonia by saprobionts, goes on to form ammonium ions
• Same process occurs for animal waste products

Question 14

Describe the Third Stage of the Nitrogen Cycle (Nitrification)

Answer 14

• Ammonium ions in soil converted to nitrogen compounds to be used by plants
• First nitrifying bacteria (Nitrosomonas) change ammonium ions into nitrites
• Second nitrifying bacteria (Nitobacter) changes nitrites into nitrates

Question 15

Describe the Fourth Stage of the Nitrogen Cycle (Denitrification)

Answer 15

• Nitrates in soil converted to nitrogen gas by denitrifying bacteria - use nitrates in soil to carry out respiration and produce nitrogen gas
• Happens under anaerobic conditions (e.g. waterlogged soils)

Question 16

Describe the Process of the Phosphorus Cycle

Answer 16

1. Phosphate ions in rocks released into soil by weathering
2. Ions taken into plants through roots. Mycorrhizae increase rate
3. Ions transferred through food chain
4. Ions lost from animals through dead organisms and waste products
5. Saprobionts break down organic compounds, releasing ions into soil for assimilation by plants. Microorganisms also release ions from waste
6. Weathering of rocks releases ions into waters - taken up by aquatic producers (algae), passed along food chain to birds
7. Sea birds waste known as guano, contains lots of phosphate ions

Question 17

Define Artificial and Natural Fertilisers

Answer 17

• Artificial: inorganic, contain pure chemicals as powders or pellets
• Natural: organic, include manure, composted vegetables, crop residues and sewage sludge

Question 18

Explain how Nutrients are Lost when Crops are Harvested

Answer 18

• Crops take in minerals from the soil
• Crops harvested, removed from field rather than dying and decomposing
• Mineral ions (phosphates and nitrates) not returned to soil by decomposers
• Mineral ions also lost when animals or animal products are removed from land
• Animals eat grass, taking in nutrients
• When they are removed, nutrients aren’t replaced through remains or waste products

Question 19

What are Some Environmental Issues with the Use of Fertilisers (Name 3)

Answer 19

1. Leaching could occur if more fertilisers is used than necessary
2. Leaching is when water-soluble compounds in soil are washed away (rain) into nearby waters
3. This can lead to eutrophication
4. Leaching more likely if fertiliser is applied just before heavy rainfall
5. Artificial fertilisers more likely to leach as inorganic ions in them are relatively soluble, meaning excess minerals not used immediately can leach easier
6. Natural fertilisers have nitrogen and phosphorus that need to be decomposed by microorganisms before absorption - release into soil for plants more controlled, leaching is less likely
7. Leaching of phosphates less likely than nitrates - phosphates are less soluble in water
8. Fertilisers change the balance of nutrients in the soil - too much of a nutrient can kill crops and other plants

Question 20

Describe the Process of Eutrophication

Answer 20

1. Mineral ions leached from fertilised fields - causes rapid growth of algae in waters
2. Algae block sunlight from reaching plants below
3. Plants die - unable to photosynthesise enough
4. Bacteria feed on dead plants - increased bacteria decrease oxygen conc. through aerobic respiration
5. Fish and other aquatic organisms die - isn’t enough dissolved oxygen