4.2 Energy Flow

Question 1

What is the main source of energy in most ecosystems?

Answer 1

Sunlight

Question 2

What happens to energy in photosynthesis?

Answer 2

Light energy is absorbed by photoautotrophs and is converted into chemical energy in photosynthesis by using the light energy to make organic compounds (e.g. sugars) from inorganic sources (e.g. CO₂)

• Heterotrophs ingest these organic compounds in order to obtain their chemical energy
• When organic compounds are broken down via cell respiration, ATP is produced to fuel metabolic processes

Question 3

How does chemical energy flow

Answer 3

Chemical energy in carbon compounds flows through food chains by means of feeding.

Question 4

Trophic levels

Definition + explanation of levels

Answer 4

The position an organism occupies within a feeding sequence (food chain)
- 1st trophic level: producers
- 2nd trophic level: primary consumers
- Subsequent trophic levels: further consumers (secondary, tertiaty, etc.)

Question 5

Food chains

Definition

Answer 5

A linear feeding relationship between species in a community
- Arrows represent the transfer of energy and matter as one organism is eaten by another (arrows point in the direction of energy flow)
- The first organism in a food chain is always a producer, followed by consumers (primary, secondary, tertiary, etc.)

Question 6

Example of a food chain

Answer 6

Blue grass → Rabbit → Fox
Producer/1st trophic level → Primary consumer/2nd trophic level → Secondary consumer/3rd trophic level

• Blue grass can be described as an autotroph while both the rabbit and the fox are heterotrophs.
• Longer food chains are possible, although there is a limit as energy is lost as it passes up the chain.

Food chains are often completed by detritivores or saprotrophs

Question 7

Food webs

Definition

Answer 7

A network of food chains which are interconnected at various trophic levels, so as to form a number of feeding connections amongst different organisms of a biotic community.
- They allow an organism to obtain its food from more than one type or organism of the lower trophic level.

Question 8

Energy released from carbon compounds (e.g. sugars and lipids) by respiration is used in living organisms and converted to heat

Explain

Answer 8

• Energy stored in organic molecules can be released by cell respiration to produce ATP
• This ATP is then used to fuel metabolic reactions required for growth and homeostasis
• A by-product of these chemical reactions is heat (thermal energy), which is released from the organism

Question 9

The main reasons that energy is lost in food chains

Answer 9

• Excretion: part of the energy stored is excreted as part of the organism’s faeces and hence is lost
• Undigested food: some parts of animals are undigestible (bones, fur, shells)
• Movement
• Heat released during respiration

Energy loss is why most terrestrial ecosystems have no more than five trophic levels, and marine ecoystems generally have no more than seven.
Higher trophic levels receive less energy/biomass and so need to eat larger quantities. Because they need to eat more, they expend more energy/biomass hunting for food. If the energy required to hunt exceedes the energy available from the food eaten, the trophic level becomes unviable.

Question 10

Explain:
- Living organisms cannot convert heat to other forms of energy
- Heat is lost from ecosystems

Answer 10

The chemical energy produced by an organism can be converted into a number of forms, including kinetic energy, electrical energy, and light energy
- All of these reactions are exothermic, and release thermal energy (heat) as a by-product
- Living organisms cannot turn this heat into other forms of usable energy
- This heat energy is released from the organism and is lost from the ecosystem

Therefore, ecosystems require a continuous influx of energy from an external source

Question 11

Energy loss and biomass

Answer 11

As energy is lost between trophic levels, higher trophic levels store less energy as carbon compounds and so have less biomass.
- Biomass: the total mass of a group of organisms - consisting of the carbon compounds contained in the cells and tissues
- Biomass diminishes along food chains with the loss of carbon dioxide, water and waste products (e.g. urea) to the environment

Question 12

Pyramids of energy

Answer 12

A graphical representation of the amount of energy at each trophic level of a food chain.
Pyramids of energy will never appear inverted as some of the energy stored in one source is always lost upon transfer.
The bottom level will always represent the producers, with subsequent levels representing consumers (primary, secondary, etc.)