C4.2 Transfers of energy and matter

Question 1

C4.2.1—Ecosystems as open systems in which both energy and matter can enter and exit

Answer 1

Students should know that in closed systems only energy is able to pass in and out.

Question 2

C4.2.2—Sunlight as the principal source of energy that sustains most ecosystems

Answer 2

Include exceptions such as ecosystems in caves and below the levels of light penetration in oceans.
NOS: Laws in science are generalized principles, or rules of thumb, formulated to describe patterns
observed in nature. Unlike theories, they do not offer explanations, but describe phenomena. Like
theories, they can be used to make predictions. Students should be able to outline the features of useful
generalizations.

Question 3

C4.2.3—Flow of chemical energy through food chains

Answer 3

Students should appreciate that chemical energy passes to a consumer as it feeds on an organism that is
the previous stage in a food chain

Question 4

C4.2.4—Construction of food chains and food webs to represent feeding relationships in a community

Answer 4

Represent relationships in a local community if possible. Arrows indicate the direction of transfer of energy
and biomass.

Question 5

C4.2.5—Supply of energy to decomposers as carbon compounds in organic matter coming from dead
organisms

Answer 5

Include faeces, dead parts of organisms and dead whole organisms.

Question 6

C4.2.6—Autotrophs as organisms that use external energy sources to synthesize carbon compounds from
simple inorganic substances

Answer 6

Students should understand that energy is required for carbon fixation and for the anabolic reactions that
build macromolecules.

Question 7

C4.2.7—Use of light as the external energy source in photoautotrophs and oxidation reactions as the
energy source in chemoautotrophs

Answer 7

Students should understand that oxidation reactions release energy, so they are useful in living organisms.
Include iron-oxidizing bacteria as an example of a chemoautotroph.

Question 8

C4.2.8—Heterotrophs as organisms that use carbon compounds obtained from other organisms to
synthesize the carbon compounds that they require

Answer 8

Students should appreciate that complex carbon compounds such as proteins and nucleic acids are
digested either externally or internally and are then assimilated by constructing the carbon compounds
that are required.

Question 9

C4.2.9—Release of energy in both autotrophs and heterotrophs by oxidation of carbon compounds in cell
respiration

Answer 9

Students are not required to be familiar with photoheterotrophs.

Question 10

C4.2.10—Classification of organisms into trophic levels

Answer 10

Use the terms “producer”, “primary consumer”, “secondary consumer” and “tertiary consumer”. Students
should appreciate that many organisms have a varied diet and occupy different trophic levels in different
food chains.

Question 11

C4.2.11—Construction of energy pyramids

Answer 11

Application of skills: Students should use research data from specific ecosystems to represent energy
transfer and energy losses between trophic levels in food chains.

Question 12

C4.2.12—Reductions in energy availability at each successive stage in food chains due to large energy
losses between trophic levels

Answer 12

Decomposers and detritus feeders are not usually considered to be part of food chains. However, students
should understand the role of these organisms in energy transformations in food chains. Consider the
causes of energy loss.

Question 13

C4.2.13—Heat loss to the environment in both autotrophs and heterotrophs due to conversion of
chemical energy to heat in cell respiration

Answer 13

Include the idea that energy transfers are not 100% efficient so heat is produced both when ATP is
produced in cell respiration and when it is used in cells.

Question 14

C4.2.14—Restrictions on the number of trophic levels in ecosystems due to energy losses

Answer 14

At each successive stage in food chains there are fewer organisms or smaller organisms. There is therefore
less biomass, but the energy content per unit mass is not reduced.

Question 15

C4.2.15—Primary production as accumulation of carbon compounds in biomass by autotrophs

Answer 15

The units should be mass (of carbon) per unit area per unit time and are usually g m−2 yr−1. Students should
understand that biomes vary in their capacity to accumulate biomass. Biomass accumulates when
autotrophs and heterotrophs grow or reproduce.

Question 16

C4.2.16—Secondary production as accumulation of carbon compounds in biomass by heterotrophs

Answer 16

Students should understand that, due to loss of biomass when carbon compounds are converted to
carbon dioxide and water in cell respiration, secondary production is lower than primary production in an
ecosystem.

Question 17

C4.2.17—Constructing carbon cycle diagrams

Answer 17

Students should illustrate with a diagram how carbon is recycled in ecosystems by photosynthesis,
feeding and respiration.

Question 18

C4.2.18—Ecosystems as carbon sinks and carbon sources

Answer 18

If photosynthesis exceeds respiration there is a net uptake of carbon dioxide and if respiration exceeds
photosynthesis there is a net release of carbon dioxide.

Question 19

C4.2.19—Release of carbon dioxide into the atmosphere during combustion of biomass, peat, coal, oil and
natural gas

Answer 19

Students should appreciate that these carbon sinks vary in date of formation and that combustion
following lightning strikes sometimes happens naturally but that human activities have greatly increased
combustion rates.

Question 20

C4.2.20—Analysis of the Keeling Curve in terms of photosynthesis, respiration and combustion

Answer 20

Include analysis of both the annual fluctuations and the long-term trend.

Question 21

C4.2.21—Dependence of aerobic respiration on atmospheric oxygen produced by photosynthesis, and of
photosynthesis on atmospheric carbon dioxide produced by respiration

Answer 21

The fluxes involved per year are huge, so this is a major interaction between autotrophs and heterotrophs.

Question 22

C4.2.22—Recycling of all chemical elements required by living organisms in ecosystems

Answer 22

Students should appreciate that all elements used by living organisms, not just carbon, are recycled and
that decomposers play a key role. Students are not required to know details of the nitrogen cycle and
other nutrient cycles.