Topic 5: Energy in Ecosystems

Question 1

What is the main source of energy for all ecosystems?

Answer 1

The Sun. Sunlight is conserved as chemical energy by plants during photosynthesis

Question 2

Define food chain

Answer 2

A feeding hierarchy with trophic levels in succession to show the flow of food energy and feeding relationships

Question 3

Define food web

Answer 3

A model showing the interconnecting food chains in an ecological community

Question 4

Define trophic level

Answer 4

Position in a food chain or ecological pyramid by a group of organisms with a similar feeding mode

Question 5

Define producer and give its other name

Answer 5

Autotroph - photosynthetic organisms that manufacture organic substances using light

Question 6

Define primary consumer, secondary consumer and tertiary consumer

Answer 6

• Primary = an organism that eats producers
• Secondary = an organism that eats primary consumers
• Tertiary = an organism that eats secondary consumers

Question 7

Define consumer

Answer 7

An organism that obtains its energy by feeding on other organisms

Question 8

Define saprobiont

Answer 8

An organism that breaks down complex material in dead organisms into simple molecules

Question 9

What will eventually happen to all energy in an ecosystem?

Answer 9

It will be lost as heat

Question 10

Define biomass

Answer 10

The total mass of living material in a specific area at a specific time

Question 11

How do we measure biomass accurately and why?

Answer 11

Dry mass - sample is dried in an oven on low heat (to prevent burning) until there is no change in mass (all water evaporated). A sample must be used as the organism is killed.

Used because fresh mass is unreliable due to varying amounts of water between organisms

Biomass is measured in dry mass per area at a given time (g/m^2)

Question 12

What is calorimetry used for and how does it work?

Answer 12

Used to estimate the chemical energy stored in dry mass.

Sample is weighed, then burnt in pure oxygen in a sealed chamber called a bomb, which is surrounded by water. The temperature change is measured.

Question 13

Give the equation used to calculate the energy content in dry mass after a calorimetry experiment

Answer 13

energy content (Jg^-1) = (4.2 x volume water x temp change) / mass of sample

Question 14

What percentage of light energy do plants convert into organic matter and why?

Answer 14

1-3% because:

• > 90% of the Sun’s energy is reflected back to space by clouds and dust or absorbed by atmosphere
• Not all wavelengths of light can be absorbed and used for photosynthesis
• Light may not fall on a chlorophyll molecule
• A limiting factor may limit the rate of photosynthesis

Question 15

What is gross primary production (GPP)?

Answer 15

The total quantity of chemical energy store in plant biomass in a given area/volume at a given time

Question 16

What is net primary production (NPP)

Answer 16

The chemical energy store left in plant biomass after respiratory losses have been taken into account

Question 17

Give the equation for net primary production

Answer 17

NPP = GPP - R

NPP = net primary production
GPP = gross primary production
R = respiratory losses

Question 18

How much energy is transferred between consumers roughly?

Answer 18

Around 10% transferred from producers to primary consumers, and up to 20% after that

Question 19

Why is the % of energy transferred between organisms in a food chain so low?

Answer 19

• Not all the organism is eaten
• Some parts are eaten but can’t be digested and are lost in faeces
• Some energy is lost in excretory materials
• Some energy losses as heat from respiration

Question 20

Why is more energy transferred between consumers than from producers to primary consumers?

Answer 20

• Carnivores use more of their food than herbivores
• Organisms with a high body temperature have a lower efficiency due to heat loss to environment

Question 21

Give the equation for net production of consumers

Answer 21

N = I - (F + R)

N = net production of consumers
I = chemical energy store of ingested food
F = energy lost in faeces/urine
R = energy lost in respiration

Question 22

What are the consequences of the inefficiency of energy transfer between trophic levels?

Answer 22

• Most food chains only have 4/5 trophic levels because there is insufficient energy to support a large enough breeding population at higher trophic levels
• Total biomass is lower at higher trophic levels
• Total amount of energy available is lower at higher trophic levels

Question 23

Give the equation for percentage efficiency

Answer 23

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

Question 24

What are some techniques used to maximise the efficiency of intensive rearing of livestock?

Answer 24

• Movement restricted (less energy used in muscle contraction)
• Kept in a warm environment (reduce heat loss from body)
• Feeding controlled (receive optimum amount and type of food for maximum growth + minimal waste)
• Predators excluded (no loss to other organisms in the food web)

Question 25

Give two examples of nutrient cycles

Answer 25

• Phosphorous cycle
• Nitrogen cycle

Question 26

What are the biological uses for phosphorous?

Answer 26

Used in:
- DNA
- RNA
- ATP
- Phospholipids

Question 27

How does phosphorous naturally exist?

Answer 27

Usually found as PO4(3-) ions in sedimentary rock deposits in the sea. Never exists as a gas.

Question 28

Describe the phosphorous cycle

Answer 28

• Phosphorous naturally exists in rock as phosphate ions
• Geological uplift and weathering deposits phosphates in soil
• These are either lost in drainage and incorporated back into sedimentary rock, or absorbed and used by plants
• Plants are eaten by animals, passing the phosphates along
• When plants and animals die / excrete waste, saprobionts return the phosphates to the soil by extracellular digestion

Question 29

What are the biological uses for nitrogen?

Answer 29

Used in:
- Proteins
- Nucleic acids (DNA, RNA)
- ATP

Question 30

Describe the steps of the nitrogen cycle

Answer 30

• Nitrogen gas in the atmosphere is absorbed by producers by nitrogen fixation
• As plants are eaten and so are their consumers, the nitrogen moves up the food chain
• When organisms die / excrete waste, saprobionts produce ammonium compounds by ammonification
• Aerobic nitrifying bacteria turn ammonium ions into nitrite ions by nitrification
• Nitrite ions are turned into nitrate ions by another type of nitrification
• Nitrate ions are either absorbed by plants or converted into nitrogen gas by denitrification

Question 31

Describe the process of nitrogen fixation

Answer 31

Nitrogen gas is turned into nitrogen-containing compounds.

Can occur by atmospheric fixation (lightning) but mostly by biological fixation:
- Aerobic nitrogen-fixing bacteria reduce atmospheric nitrogen with hydrogen and make it available to an ecosystem.
- Catalysed by nitrogenase (doesn’t function near oxygen so has adaptations to make anaerobic conditions within cell)
- Can be free living bacteria in soil or mutualistic bacteria (live on root nodules of legumes - provides amino acids + receives carbohydrates)

Question 32

Describe the process of ammonification

Answer 32

Production of ammonia from organic ammonium containing compounds (e.g urea, amino acids, nucleic acids).

• Aerobic saprobiotic organisms (eg fungi, bacteria) break down faeces/dead material into ammonia, and then ammonium ions
• This is where nitrogen enters the non-living component of the ecosystem

Question 33

Describe the process of nitrification

Answer 33

Aerobic nitrifying bacteria oxidise ammonium ions into nitrite ions (NO2(-)).

Another group of bacteria oxidise the nitrite ions into nitrate (NO3(-)) which can be absorbed by active transport into root hairs of plants

Question 34

How should soil be maintained for maximum growth of plants?

Answer 34

Ploughing aerates soil, and ensure good drainage to prevent air spaces being filled with water.

Aerated soil is needed because nitrifying bacteria are aerobic.

Nitrifying bacteria are needed to turn ammonium ions into soluble nitrate ions that can be absorbed by plants and used for growth

Question 35

Describe the process of denitrification

Answer 35

Anaerobic denitrifying bacteria use nitrates as an alternative to oxygen in the electron transport chain, ultimately reducing nitrate ions to nitrogen gas. This is release into the atmosphere.

Question 36

What happens to nitrogen in the soil when it is waterlogged?

Answer 36

When soil is waterlogged, air spaces are filled so there are fewer aerobic nitrifying and nitrogen-fixing bacteria, but more anaerobic denitrifying bacteria.

Nitrates are converted to nitrogen gas so there are fewer soluble nitrogen-containing compounds for plants

Question 37

What is mycorrhizae? What is its role in the ecosystem?

Answer 37

A type of fungi that grows around some plant roots.

They have a mutualistic relationship with the plant a both gain a nutritional advantage - are often highly specific to the plant species.

Mycorrhizae assists plant with uptake of water and inorganic ions through its hyphae - microscopic threads that grow into the root system, increasing surface area for absorption.

Saprobiotic hyphae secrete enzymes that hydrolyse biological molecules ready for absorption by the plant.

In return, mycorrhizae obtains carbohydrates from the plant.

Question 38

Why are fertilisers necessary in agriculture?

Answer 38

All plants need mineral ions, mostly nitrates.

Most farming is intensive: concentrated on a specific area of land where a large part of biomass is harvested and removed. This means the minerals aren’t replaced in the soil so crop yield falls.

Fertilisers replace these lost ions.

Question 39

Where do we get artificial fertilisers from?

Answer 39

Mined from rock deposits and enriched with nitrogen, phosphorous and potassium

Question 40

What are the benefits of artificial fertlisers?

Answer 40

• Guaranteed composition so it is easier to determine the rate of application needed, and to predict crop yield
• Concentrated sources of nutrients, so applied in smaller amounts, which lowers transport costs and damage done by heavy machinery
• Clean and convenient to handle and apply evenly

Question 41

Where do we get natural fertilisers from?

Answer 41

Made from dead and decaying remains and wastes

Question 42

Give some advantages of natural fertilisers

Answer 42

• May contain trace elements - substances important to plants in small amounts
• Adds organic matter to the crop - can improve soil structure, reducing erosion and improving water-holding properties
• Release their nutrients over a longer period of time

Question 43

Give some detrimental environmental effects of fertilisers

Answer 43

• Reduced species diversity
• Leaching
• Eutrophication

Question 44

Describe how the use of fertilisers leads to reduced species diversity

Answer 44

Nitrogen-rich soils favour the growth of grasses and nettles, which out-compete other species, which then die out.

Question 45

Describe how the use of fertilisers leads to leaching

Answer 45

• Leaching is where nutrients from fertilisers are removed from the soil.
• Ions dissolve in rainwater and are carried deep into the soil, out of reach from plant roots.
• Ions make their way to waterways and lakes, sometimes contaminating drinking water or leading to eutrophication

Question 46

Describe how the use of fertilisers leads to eutrophication

Answer 46

• Usually nitrate ions are a limiting factor for plant growth
• Nitrate concentration in water increases from leaching, so algal populations grow on the surface (algal bloom)
• Algae blocks light so photosynthetic plants die, leading to the growth of saprobiotic bacteria
• Saprobiotic bacteria use oxygen for respiration, so aerobic organisms fie
• Less competition for anaerobic organisms, who grow, decomposing more material, releasing more nitrates and toxic wastes, making the water putrid