Energy Transfer and Nutrient Cycles

Question 1

What is an ecosystem?

Answer 1

An ecosystem includes all the organisms living in a particular area and all the abiotic factors.

Question 2

What are producers?

Answer 2

Organisms which make their own food using energy from the sun and carbon dioxide from the air or dissolved in water. They are autotrophs.

Question 3

What is biomass?

Answer 3

Biomass is the mass of carbon/mass of dry tissue in a given area at a given time. It is essentially the mass of ‘living material’ in an organism.

Question 4

What are the units for biomass?

Answer 4

g m-2/kg m-2

Question 5

Why is dry mass used to measure biomass rather than wet mass?

Answer 5

Water content varies massively between individuals and between the same individual at different times. Dry mass is therefore a much more consistent (and therefore better), measure of biomass.

Question 6

How could you measure the biomass of a tissue sample?

Answer 6

Heat to 100 degrees C to evaporate the water, then weigh and heat to constant mass (at this point you know all the water has been evaporated. You then divide this mass by the area of the tissue sample.

Question 7

What are photoautotrophs?

Answer 7

Organisms which can make their own organic compounds using light energy and simple inorganic molecules. In any ecosystem, plants synthesise organic compounds from atmospheric or aquatic carbon dioxide.

Question 8

What are heterotrophs?

Answer 8

Organisms which eat other organisms as their source of food. Both primary (herbivores) and secondary (carnivores and omnivores) consumers are heterotrophs.

Question 9

What are saprobionts?

Answer 9

Saprobionts include some bacteria and some fungi. They are decomposers which break down dead organic material and wastes from organisms at various trophies levels. They help release nutrients from material which can be recycled. They excrete enzymes, which digest the material externally. They then absorb the products of digestion.

Question 10

What is a tropic level?

Answer 10

The stage in a food chain where an organism is found

Question 11

Roughly how much energy is passed on from one trophic level to the next?

Answer 11

Only around 10% of energy can be passed from one level to the next.

Question 12

Why is so little energy passed from one trophic level to the next?

Answer 12

Movement, heat loss, respiration, metabolism, not all material is digested, energy lost in faeces, not all of the organism is eaten (e.g. bones), some energy excreted in urine, some organisms die/are decomposed

Question 13

Why do food chains usually only have a maximum of 5 trophic levels?

Answer 13

At the top of the food chain, there is not enough biomass/energy left to support a new level

Question 14

How can calorimetry be used to calculate the chemical energy stored in dry biomass?

Answer 14

Take a fresh sample of tissue at heat to 100 degrees C atoms evaporate the water. Weigh and heat until no further change in mass. Put a known mass of the dry sample into a calorimeter and burn in pure oxygen to achieve complete combustion.
Heat energy transferred = mass of water being heated x specific heat capacity of water x temperature change
To work out energy released per gram of tissue, divide by the mass of the tissue

Question 15

How are bomb calorimeters set up to ensure a valid measurement of heat energy released?

Answer 15

A stirrer is used to distribute heat evenly in the water, insulation/air layer is used to prevent loss or gain of heat by conduction or convection, water is used as the substance being heated as it has a high specific heat capacity, so buffers changes in temperature. This means that water will give a valid measurement for a wide range of energy released values.

Question 16

What is gross primary production (GPP)?

Answer 16

The total energy in a given area in a given time, fixed by plants and converted to biomass. This is about 40% of energy available at the start.

Question 17

What is net primary production (NPP)?

Answer 17

Total energy (after respiratory losses) available for plant growth and reproduction. <10% of this passes to primary consumers.

Question 18

What is the equation for NPP?

Answer 18

NPP = GPP -R
Net primary production = gross primary production - respiratory losses

Question 19

What is secondary production?

Answer 19

The chemical energy stored in consumer biomass is a given area.

Question 20

What is the equation for the net secondary production of consumers (N)?

Answer 20

N = I - (F + R)
Net secondary production of consumers = chemical energy stored in ingested food - chemical energy lost to the environment in faeces and urine - respiratory losses to the environment (as heat)

Question 21

What is productivity?

Answer 21

Productivity is the chemical energy stored in biomass that has been produced in a given area at a given time.

Question 22

What are the units for productivity?

Answer 22

KJ ha-1 year-1
Kilojoules per hectare per year

Question 23

Why does so little light energy reaching a plant end up as chemical energy stored in a plant?

Answer 23

Sunlight can be reflected rather than absorbed, not all wavelengths of light can be absorbed by chlorophyll. Light may miss the chlorophyll molecules. Low CO2 concentration or temperature may limit the rate of photosynthesis

Question 24

How can respiratory losses be reduced in farming?

Answer 24

Restrict animal movement so less respiration is needed. Keep animals warm indoors so less energy is needed to be released by respiration to help animals maintain body temperature.

Question 25

How can you calculate the efficiency of energy transfer between trophic levels?

Answer 25

% efficiency = (net production of trophic level/net production of previous trophic level) x100

Question 26

Why does energy efficiency between trophic levels generally increase as you go up a food chain?

Answer 26

Cellulose in plants is difficult to digest, so more energy is lost by egestion (in faeces). Meat is more protein-rich, and protein is easier to digest.

Question 27

What are food chains and food webs?

Answer 27

Food chains and food webs show energy transfer between trophic levels. Food chains show simple lines of energy transfer, while food webs show lots of food chains in an ecosystem and how they overlap.

Question 28

How can energy efficiency be increased by reducing energy lost to other organisms?

Answer 28

Food webs can be simplified by eradicating pests. Using pesticides or herbicides can remove that part of the food chain, increasing yields and the amounts of energy available for that trophic level.

Question 29

What are mycorrhizae?

Answer 29

Fungi which form a mutualistic relationship with a plant. They from long, thin strands called hyphae, which connect to the plant’s roots. This greatly increases the plant’s root surface area which helps the plants absorb more water and mineral ions from the soil. The fungi gain organic compounds, such as glucose, from the plant.

Question 30

Why do organisms need nitrogen?

Answer 30

Nitrogen is need to make amino acids and therefore proteins. Nitrogen is also need to make the bases of DNA. However, organisms can’t use atmospheric nitrogen (it is very unreactive). They take up nitrogen in the form of ammonium and nitrate ions.

Question 31

How can nitrogen in the atmosphere be fixed into the soil?

Answer 31

N2 can be fixed by lightning into nitrates (NO3-) in the soil. Nitrogen is also fixed by nitrogen-fixing bacteria found in root nodules of leguminous plants or in the soil. These bacteria fix nitrogen gas from the air into ammonia, which dissolves in water and readily forms ammonium ions, which can be oxidised to nitrate ions.

Question 32

How do ammonium ions get converted into nitrate ions?

Answer 32

Ammonium ions in the soil are nitrified by nitrifying bacteria into nitrites (NO2-), then nitrates (NO3-), which can be taken up by and used by plants.

Question 33

How can nitrogen in the soil move throughout food webs?

Answer 33

Nitrogen in the soil is in the form of nitrate ions, which are taken up by plants by active transport. This nitrogen is then used in biological processes to form plant biomass. This nitrogen is transferred to animals, then to other animals through feeding.

Question 34

How can the nitrogen in plant/animal biomass be put back into the soil?

Answer 34

Nitrogen is lost from animals through excretion and egestion. When the animals and plants die, saprobionts decompose the protein/DNA in dead remains and in waste products, releasing ammonium ions, which can be nitrified (ammonification).

Question 35

How does nitrogen in the soil get back into the atmosphere?

Answer 35

Denitrifying bacteria convert nitrates in the soil back into nitrogen gas, which is released into the atmosphere (denitrification).

Question 36

How do nitrogen-fixing bacteria and plants have a mutualistic relationship?

Answer 36

Nitrogen-fixing bacteria fix atmospheric nitrogen into ammonium ions, which can be nitrified and taken up by the plant. The bacteria get a constant source of organic compounds like glucose for respiration.

Question 37

What conditions do denitrifying bacteria thrive in?

Answer 37

Denitrifying bacteria thrive in anaerobic conditions. This often occurs in waterlogged soil. Digging drainage ditches and ploughing to add more oxygen to the soil are methods used by farmers to reduce denitrification, ensuring their crops get enough nitrogen.

Question 38

Ammonium ions can be taken up by plants and used to make amino acids, but nitrate is easier to absorb - why?

Answer 38

Nitrate ions are more water soluble

Question 39

Why are phosphates needed by organisms?

Answer 39

ATP, ADP, DNA, RNA, Phospholipids, muscle contraction, nerve signalling, metabolic pathways, controls enzyme reactions, calcium phosphate in bones and teeth

Question 40

How do phosphates in rocks get into water systems?

Answer 40

Weathering, erosion and use of fertilisers moves phosphates from rocks into water systems. Phosphates dissolved in water can also deposit back onto rocks (sedimentation)

Question 41

How can dissolved phosphate ions in water get into food webs?

Answer 41

The dissolved phosphate ions get into soil and are then absorbed into plants by active transport. The phosphorus is stored in the plant’s biomass and is transferred to other organisms by feeding.

Question 42

How can phosphate ions in animals/plants get back into soil?

Answer 42

Excretion and decomposition by saprobionts takes phosphate ions in dead organic material/waste and puts it back into the soil, where it dissolves in water.

Question 43

How can phosphate ions in the soil get back onto rocks?

Answer 43

Phosphate ions in soil dissolve in water and deposit on rocks by deposition and sedimentation. Guano (sea bird waste) has a high proportion of phosphate ions and returns phosphate ions to soils/water systems.

Question 44

How can phosphate ions in the soil get back onto rocks?

Answer 44

Phosphate ions in soil dissolve in water and deposit on rocks by deposition and sedimentation. Guano (sea bird waste) has a high proportion of phosphate ions and returns phosphate ions to soils/water systems.

Question 45

What are the differences between the nitrogen cycle and the phosphorus cycle?

Answer 45

• PC has no atmospheric involvement, NC uses nitrogen gas in the atmosphere
• Phosphate has low solubility in water, nitrate is highly soluble in water
• PC is slower, NC is faster
• phosphorus is very reactive and never exists naturally in its elemental form, nitrogen gas is very unreactive and exists naturally in its elements,a form

Question 46

Why are fertilisers important to maintaining nutrient cycles?

Answer 46

Farming removes crops from the soil before they die, so they take nutrients from the soil as they grow, but here are not returned. This leads to the soil being nutrient-deficient, which leads to poor plant growth. Fertilisers put nutrients back into the soil.

Question 47

What causes eutrophication?

Answer 47

Leaching of fertilisers into waterways leads to excess mineral ions/ nutrients in lakes and rivers. This causes the rapid growth of algae (algal bloom). The algae cover the surface if the lake/river, blocking sunlight from reaching plants below the surface of the water. These plants die because they cannot photosynthesise enough. Bacteria decompose the dead plant matter. These bacteria increase in number and aerobically respire, reducing the oxygen concentration in the water. Fish and other aquatic organisms die because there is not enough oxygen, so they cannot respire enough to survive.