5.3 energy & ecosystems

Question 1

describe the three groups that organisms can be divided into according to how they obtain their energy and nutrients.

Answer 1

• producers - photosynthetic organisms that manufacture organic substances using light energy, water, carbon dioxide, and mineral ions.
• consumers - organisms that obtain their energy by consuming other organisms.
• saprobionts - decomposers which break down complex materials in dead organisms.

Question 2

describe the differences between a food chain and a food web.

Answer 2

• a food chain describes a feeding relationship in which the producers are eaten by primary consumers.
• these in turn are eaten by secondary consumers, who are then eaten by tertiary consumers.
• a food web describes the linking of organisms within a habitat based on the linking of food chains within an ecosystem.

Question 3

define the term ‘biomass’.

Answer 3

biomass - the total mass of living material in a specific area at a given time.

Question 4

how is biomass measured?

Answer 4

biomass is measured using dry mass per given area, in a given time.

Question 5

give the process used to estimate the chemical energy stored in a sample of dry mass.

Answer 5

calorimetry.

Question 6

photosynthesising plants usually convert between 1% and 3% of the sun’s energy into organic matter. give two reasons why most of the sun’s energy is not converted into organic matter via photosynthesis.

Answer 6

• only some wavelengths of light on the visible light spectrum can be absorbed by plants and used for photosynthesis.
• factors, such as low carbon dioxide levels, may limit the rate at which photosynthesis occurs.

Question 7

define the following terms:

• gross primary production (GPP)
• net primary productivity (NPP)

Answer 7

gross primary production - the total quantity of the chemical energy store in plant biomass in a given area or volume, in a given time.

net primary productivity - the chemical energy store which is left over once energy losses from respiration are taken into account.

Question 8

give the equation used to calculate net primary productivity.

Answer 8

net primary productivity (NPP) = gross primary production (GPP) - respiratory losses (R)

Question 9

define the term ‘trophic level’, and describe how the trophic level of an organism is determined.

Answer 9

• the trophic level of an organism refers to the position the organism occupies in a food chain.
• an organism’s trophic level is determined by its function in the food chain and its nutritional relationship to the producer.

Question 10

give three reasons why the percentage of energy transferred at each trophic level of a food chain is relatively low.

Answer 10

• some parts of an organism cannot be digested by consumers, and are therefore lost in faeces.
• some energy is lost in excretory materials, such as urine.
• some energy losses occur through heat loss to the environment, a direct result of respiration.

Question 11

give the equation used to calculate the net production of consumers.

Answer 11

net production = chemical energy store of ingested food - (energy lost from excretion + energy lost through respiration)

Question 12

give the equation used to calculate the percentage efficiency of the energy transfer between each trophic level of a food chain.

Answer 12

% efficiency = energy available after the transfer / energy available before the transfer (x 100)

Question 13

give three reasons why factory farming, the practice of rearing livestock in small enclosures, increases the efficiency of the energy-conversion rate between trophic levels in a food chain.

Answer 13

• rearing livestock in small enclosures means that less energy is used in muscle contraction, as movement is restricted.
• the environment can be kept warm in order to reduce heat loss from the body.
• feeding can be controlled, so that the animals receive the optimum amount of food for the maximum gain of body mass.

Question 14

give another farming practice, other than factory farming, which can be used to increase the efficiency of energy transfer between trophic levels.

Answer 14

simplifying food webs, by eliminating organisms which compete with the plant or animal that is being farmed.

Question 15

describe the role of saprobiontic organisms.

Answer 15

• when producers and consumers die, their complex molecules are broken down by saprobiontic microorganisms.
• these microorganisms release the nutrient in its original simple form, so that it can be taken up and used again by plants.

Question 16

why do living organisms require a source of nitrogen?

Answer 16

living organisms require a source of nitrogen in order to manufacture proteins, nucleic acids and other nitrogen-containing compounds.

Question 17

very few organisms are able to directly use nitrogen gas to manufacture nitrogen-containing compounds. give the organism, and describe the process through which nitrogen enters the living component of an ecosystem.

Answer 17

• nitrogen enters the living component of an ecosystem through plants, which take up nitrogen in the form of nitrate ions found in the soil.
• these ions are absorbed, using active transport, by the roots of the plant.
• nitrogen is passed onto the next level of the ecosystem through consumers, which obtain nitrogen-containing compounds by eating and digesting plants.

Question 18

nitrate ions are very soluble and easily leach through soil. describe the differences in how the concentration of nitrate ions is restored in natural as opposed to agricultural ecosystems.

Answer 18

• in natural ecosystems, the concentration of nitrate ions in the soil are largely restored through the recycling of nitrogen-containing compounds through decomposition of organic material.
• in agricultural ecosystems, the concentration of nitrate ions can be further increased by the addition of nitrogen-containing fertilisers to the soil.

Question 19

give the four main stages of the nitrogen cycle.

Answer 19

• ammonification.
• nitrification.
• nitrogen fixation.
• denitrification.

Question 20

define the term ‘ammonification’.

Answer 20

ammonification refers to the production of ammonia from organic nitrogen-containing compounds.

Question 21

describe the process by which nitrogen returns to the non-living component of an ecosystem.

Answer 21

• saprobiontic organisms, mainly fungi and bacteria, feed on faeces and dead organic material, releasing ammonia, which then forms ammonium ions in the soil.
• this is how nitrogen returns to the non-living component of an ecosystem.

Question 22

give the type of reaction which converts ammonium ions into nitrate ions, including the organism which carries out this conversion.

Answer 22

• the conversion of ammonium ions to nitrate ions occurs via an oxidation reaction.
• this conversion is carried out by free-living soil microorganisms, known as nitrifying bacteria.

Question 23

give the two stages which occur in the conversion of ammonium ions to nitrate ions.

Answer 23

• oxidation of ammonium ions to nitrite (NO₂⁻) ions.
• oxidation of nitrite ions to nitrate (NO₃⁻) ions.

Question 24

describe the two main types of microorganism which carry out nitrogen-fixation.

Answer 24

free-living nitrogen-fixing bacteria:

• these bacteria reduce gaseous nitrogen to ammonia, which they then use to manufacture amino acids.
• nitrogen-rich compounds are released from these bacteria when they die and decay.

mutualistic nitrogen-fixing bacteria:

• these bacteria are found on the nodules of the roots of plants such as peas and beans, forming a symbiotic relationship with the plant.
• these bacteria obtain carbohydrates from the plant, and the plant acquires amino acids from the bacteria.

Question 25

explain why, in order for farming land to be productive, the soil on which crops are grown must be kept well aerated.

Answer 25

• in order for farming land to be productive, the soil on which crops are grown must be kept well aerated to prevent a buildup of denitrifying bacteria.
• denitrifying bacteria convert soil nitrates into gaseous nitrogen, which reduces the availability of nitrogen-containing compounds for plants.

Question 26

explain why phosphorus is an important biological element.

Answer 26

phosphorous is an important biological element because it is a component of range of biological molecules, including ATP, phospholipids and nucleic acids.

Question 27

phosphorous exists mostly in the form of phosphate (PO₄³⁻) ions. describe the process through which animals gain phosphorous, in order to manufacture phosphorous-containing biological molecules.

Answer 27

• phosphorous exists mainly in the form of phosphate ions in the form of sedimentary rock deposits.
• the weathering and erosion of these rocks dissolves the phosphate ions, which allow them to become available for absorption by plants, which incorporate them into their biomass.
• the phosphate ions are then passed onto the animals which feed on these plants.

Question 28

excess phosphate ions are excreted by animals. give an example of a waste material that these phosphate ions may accumulate in.

Answer 28

guano, which is formed from the excretory products of some sea birds.

Question 29

describe how excreted phosphate ions are reformed as sedimentary rocks, in order for the phosphorous cycle to continue.

Answer 29

• phosphate ions excreted by animals are released via decomposition, and dissolved out of rocks.
• these ions are then transported by streams and rivers into lakes and oceans, where they reform as sedimentary rocks, allowing the cycle to continue.

Question 30

explain how mycorrhizae play a part in nutrient cycles.

Answer 30

• mycorrhizae are associations between certain types of fungi and the majority of the roots of plants.
• mycorrhizae act as extensions of the plant’s root system, by increasing the total surface area for the absorption of water and mineral ions.
• they play a part in nutrient cycles by increasing the plant’s uptake of ions, such as phosphate ions.

Question 31

describe how the relationship between plants and mycorrhizal fungi is a mutualistic one.

Answer 31

the relationship between plants and mycorrhizal fungi is a mutualistic one because the plant benefits from improved water and organic ion uptake, whilst the fungus receives organic compounds, such as sugar and amino acids, from the plant.

Question 32

describe the two types of fertiliser used in agricultural ecosystems.

Answer 32

• natural (organic) fertilisers - consist of the dead and decaying remains of plants and animals, as well as animal wastes, such as manure.
• artificial (inorganic) fertilisers - mined from rocks and deposits, before being converted into different forms and blended together to give a balance of minerals suitable for a particular crop.
• nitrogen, potassium and phosphorous are almost always present in artificial fertilisers.

Question 33

give three detrimental effects caused by an increased use of nitrogen-containing fertilisers in agricultural ecosystems.

Answer 33

• reduced species diversity - nitrogen-rich soils favour the growth of grasses, nettles and other rapidly growing species; these out-compete many other species, which then die as a result.
• leaching - the loss of water-soluble ions from soil, which may lead to the pollution of watercourses, such as rivers or streams.
• eutrophication - an excessive richness of nutrients in a body of water, caused by the leaching of fertilisers into watercourses.