C4.2 - energy flow in ecosystems (4f)

Question 1

are ecosystems open or closed systems?

Answer 1

ecosystems are open systems in which both energy and matter can enter and exit
- in closed systems, only energy is able to pass in and out

Question 2

what is the the principal source of energy that sustains most ecosystems?
what are 2 exceptions?

Answer 2

sunlight is the principal source of energy that sustains most ecosystems
- exceptions include ecosystems in caves and below the levels of light penetration in oceans

Question 3

what are laws?

Answer 3

laws in science are generalised principles, or rules of thumb, formulated to describe patterns observed in nature

Question 4

how do laws differ from theories?

Answer 4

unlike theories, laws do not offer explanations, but describe phenomena
- like theories, laws can be used to make predictions

Question 5

what is a heteretroph?

Answer 5

organisms that use carbon compounds obtained from other organisms to synthesize the carbon compounds they need - they are also termed as consumers

Question 6

how does the concept of a heterotroph link to energy flow in an ecosystem?

Answer 6

chemical energy from ingested nutrients are used for digesting complex carbon compounds, either internally or externally, and the assimilated to construct the carbon compounds required

Question 7

what are autotrophs?

Answer 7

autotrophs are organisms that use external energy sources to synthesise carbon compounds from simple inorganic substances

Question 8

why does an autotroph need energy?

Answer 8

energy is required for carbon fixation and for the anabolic reactions that build macromolecules

Question 9

what are the 2 modes of autotrophs?

Answer 9

chemoautotrophs - use oxidation reactions as the energy source, in chemosynthesis

photoautotrophs - use light energy as the external energy source, in photosynthesis

Question 10

how do autotrophs and heterotrophs release energy from carbon compounds?

Answer 10

both autotrophs and heterotrophs release energy by oxidation of carbon compounds in cell respiration

Question 11

how does energy flow through ecosystems?

Answer 11

1. energy from the sun is captured by autotrophs
2. light energy is converted to chemical energy using photosynthesis
3. chemical energy passes to a consumer as it feeds on an organism that is in the previous stage of a food chain
4. this energy transfer via feeding is repeated multiple times along a food chain
5. respiration releases chemical energy as ATP and heat is lost from the food chain. ATP is used to carry out life functions

Question 12

why is energy described as flowing in an ecosystem?

Answer 12

energy cannot be recycled - it enters a food chain and is lost from that food chain

Question 13

what do food chains and webs represent?

Answer 13

food chains and webs show feeding relationships in a community

Question 14

what do the arrows of food chains and webs indicate?

Answer 14

the direction of arrow shows the direction of transfer of energy and biomass

Question 15

what are advantages and disadvantages of food webs and chains?

Answer 15

advantages:
- realistic representation
- food webs summarize all possible food chains in the community

disadvantages:
- some communities are too complex to represent
- only shows qualitative information, not quantitative data
- the impact of saprotrophs or abiotic factors are not taken into account

Question 16

why is the number of trophic levels restricted?

Answer 16

due to large energy losses between trophic levels (90%), meaning there is a reduction in energy availability at each successive stage of a food chain

Question 17

why are there fewer or smaller organisms at each successive stage of a food chain?

Answer 17

production of carbon dioxide by respiration results in the loss of chemical energy contained within the molecules, reducing biomass in a food chain as less energy is available for building biological molecules
- however, the energy content per unit mass is not reduced

Question 18

why doesn’t all the energy flow along a food chain?

Answer 18

both autotrophs and heterotrophs release energy by oxidation of carbon compounds in cell respiration, and as energy transfers are not 100% efficient, heat is produced both when ATP is produced in cell respiration and when it is used in cells
- decomposers gain energy using carbon compounds from detritus (organic matter) of dead organisms, which are exctracted during sapotrophic nutrition
- decomposers and detrivores and not considered to be a part of the food chain

Question 19

what is a trophic level?

Answer 19

a trophic level is the position an organism occupies in the food chain or web, and to find the trophic level of an organism you must know:

Question 20

what do you need to know to find the trophic level of an organism?

Answer 20

• what it feeds on
• the trophic level of what it eats
• many organisms have a varied diet and occupy different trophic levels in different food chains

Question 21

what are energy pyramids?

Answer 21

energy pyramids represent energy transfer and energy losses between trophic levels in food chains
- they show the amount of energy at each trophic level, measured over a period of time, using the width of the bars
- they are always classic pyramid shape, with a larger bar at the bottom representing the producers, and successively smaller bars representing each consumer trophic level

Question 22

what information is needed to construct an energy pyramid?

Answer 22

to construct one, you need to know:
- the food chain
- the energy content in the biomass of all organisms at each trophic level
- the size of the study area
- the period of time that the sample was taken

• thus the units recorded are: KJ m⁻² yr⁻¹.

Question 23

what are the advantages and disadvantages of the use of pyramids of energy as models of energy flow in an ecosystem?

Answer 23

advantages:
- can show the energy loss / transfer between trophic levels
- allows quantitative comparison between ecosystems
- shows change over time

disadvantages:
- difficult to produce accurately
- requires destructive methods to obtain data
- cannot represent organisms that feed at different trophic levels
- do not show all the feeding relationships within the community

Question 24

what are the causes of energy losses between trophic levels?

Answer 24

reductions in energy availability at each successive stage in food chains is due to large energy losses between trophic levels, as only 10% is available to the next trophic level

causes of energy loss include:
- heat lost by respiration
- some energy is made available to decomposers, via excretion, faeces, undigested food or uneaten parts of an organism

Question 26

what are the roles of decomposers and detritivores?

Answer 26

the role of these organisms in energy transformations in food chains is to transfer energy from waste, and dead bodies
- decomposers and detritus feeders are not usually considered to be part of food chains