Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 578–601 of the course textbook. Image credits are on slide 25.
Energy entering and passing along
C4.2.1
Ecosystems are open systems
p. 579
Energy and matter can both enter and leave an ecosystem. In a closed system, only energy can do so.
C4.2.2
Sunlight sustains most ecosystems
pp. 579–580
Exceptions include caves, and ocean depths below the reach of light. There, organic matter brought in, or chemical energy used by bacteria, supplies the energy.
C4.2.3
Chemical energy flows through food chains
pp. 580–581
Chemical energy passes to a consumer as it feeds on the previous organism in the chain.
C4.2.4
Food chains and food webs
pp. 581–582
Chains and webs show feeding relationships. Arrows show the direction of transfer of energy and biomass.
C4.2.5
Energy for decomposers
pp. 582–583
Decomposers receive energy as carbon compounds in faeces, dead parts of organisms and dead whole organisms.
How organisms obtain energy and carbon
C4.2.6
Autotrophs
p. 584
Autotrophs use an external energy source to make carbon compounds from simple inorganic molecules. Energy is needed for carbon fixation, and for reactions that build macromolecules.
C4.2.7
Photoautotrophs and chemoautotrophs
p. 585
Photoautotrophs use light. Chemoautotrophs use oxidation reactions, which release energy. Iron-oxidizing bacteria are an example.
C4.2.8
Heterotrophs
p. 585
Heterotrophs digest carbon compounds from other organisms, internally or externally, and assimilate the products to build the compounds they need.
C4.2.9
Energy release in cell respiration
p. 585
Autotrophs and heterotrophs both release energy by oxidizing carbon compounds in cell respiration.
C4.2.10
Trophic levels
p. 586
Levels are producer, primary consumer, secondary consumer and tertiary consumer. Many organisms have a varied diet, so they can feed at more than one level.
Energy lost at each stage
C4.2.11
Energy pyramids
pp. 586–588
Pyramids are stepped and show energy per unit area per unit time, such as kJ m−2 yr−1. Producers are at the bottom.
C4.2.12
Why energy falls along a chain
pp. 589–590
Some organisms or parts are not eaten, some food is not digested, and carbon compounds are oxidized in respiration. Decomposers and detritus feeders are not usually placed in food chains, though they do transform energy.
C4.2.13
Heat loss
pp. 590–591
Energy transfers are not 100% efficient. Heat is produced when ATP is made in respiration, and when ATP is used in cells. This heat passes to the environment, and organisms cannot reuse it.
C4.2.14
Limits on trophic levels
p. 591
At each stage there are fewer or smaller organisms, so there is less biomass. Energy per unit mass is not reduced. Too little total energy remains to support many levels.
C4.2.15
Primary production
pp. 592–593
This is the build-up of carbon compounds in autotrophs, measured in g m−2 yr−1. Biomes vary in how much they accumulate.
C4.2.16
Secondary production
pp. 593–594
This is the build-up of carbon compounds in heterotrophs. It is lower than primary production, because biomass is lost as carbon dioxide and water in respiration.
The carbon cycle
C4.2.17
Carbon cycle diagrams
pp. 594–595
Show how carbon is recycled by photosynthesis, feeding and respiration.
C4.2.18
Sinks and sources
pp. 595–596
If photosynthesis exceeds respiration, an ecosystem has a net uptake and acts as a sink. If respiration exceeds photosynthesis, it has a net release and acts as a source.
C4.2.19
Combustion
pp. 597–598
Burning biomass, peat, coal, oil and natural gas releases carbon dioxide. These stores formed at different times. Lightning can start fires, but human activity greatly raises the amount of combustion.
C4.2.20
The Keeling curve
pp. 599–600
Carbon dioxide at Mauna Loa has been measured since 1958. The long-term rise reflects combustion. The yearly rhythm reflects photosynthesis, which is greater in the northern summer, and respiration, which exceeds it in winter.
C4.2.21
Photosynthesis and respiration depend on each other
p. 601
Aerobic respiration uses oxygen from photosynthesis. Photosynthesis uses carbon dioxide from respiration. The amounts each year are huge.
C4.2.22
All elements are recycled
p. 601
All elements that organisms use are recycled, not just carbon, and decomposers play a key role.
Videos
Select a video to play it here. Select it again to close it.
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
A printable worksheet covering all twenty-two statements, from quick recall to data questions and a longer answer. It has 52 marks and takes about 55 minutes, or you can do one section at a time.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 8 marks in total. Your teacher has the mark scheme.
How fast are nutrients recycled?
8 marks
Image: Bernard Spragg. NZ from Christchurch, New Zealand, Public domain, Wikimedia Commons.
The mean residence time (MRT) is the average time a nutrient stays in dead organic matter, such as leaf litter, before decomposers release it back to the soil for plants. The table shows MRTs in four biomes.
MRT / years
Boreal forest
Temperate forest
Chaparral (scrub)
Tropical rainforest
Nitrogen
230
5.5
3.8
0.4
Phosphorus
320
5.8
3.6
0.6
Potassium
94
1.3
1.4
0.2
Identify the biome in which nutrients are recycled fastest. [1]
Compare the MRTs in the temperate forest and the chaparral. [2]
Suggest two reasons for the very long MRTs in boreal forest. [2]
Radioactive caesium-137 from weapons tests settled on Arctic land in the early 1960s. It was measured in lichen, in caribou (which eat lichen) and in people who eat caribou.
Deduce the trophic level of the lichen and of the people. [2]
Suggest why caesium-137 in people peaked a year after it peaked in lichen. [1]
Practical
Analysing the Keeling curve
Data35 minC4.2.20
You will need: Monthly atmospheric carbon dioxide data from NOAA, spreadsheet
Plot one recent year, then a span of decades
Describe the seasonal cycle and the long-term rise
Calculate the mean yearly increase
Explain each pattern using photosynthesis, respiration and combustion
Safety: No practical hazards.
Carry this out with your teacher's supervision and follow your school's lab rules.
Nature of science and TOK
Nature of science
Measurements
Charles Keeling began measuring carbon dioxide at Mauna Loa in 1958. His careful, continuous record became a key piece of climate evidence.
Theory of knowledge
Why does long-term data have special value?
Nobody could know in advance how much the record would matter. Consider how knowledge depends on funding work with no immediate payoff.
Syllabus statements
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
C4.2.1Ecosystems as open systems that energy and matter can enter and leavep. 579
C4.2.2Sunlight as the main energy source for most ecosystemsp. 579
C4.2.3How chemical energy flows along food chainsp. 580
C4.2.4Drawing food chains and food webs to show who feeds on whatp. 581
C4.2.5How decomposers get energy from the carbon compounds in dead organismsp. 582
C4.2.6Autotrophs: organisms that use an outside energy source to build carbon compounds from simple inorganic moleculesp. 584
C4.2.7Photoautotrophs use light, while chemoautotrophs use oxidation reactions, as their energy sourcep. 585
C4.2.8Heterotrophs: organisms that build what they need from carbon compounds taken from other organismsp. 585
C4.2.9How autotrophs and heterotrophs both release energy by oxidizing carbon compounds in respirationp. 585
C4.2.10Sorting organisms into trophic levelsp. 586
C4.2.11Drawing energy pyramidsp. 586
C4.2.12Why less energy is available at each step along a food chainp. 589
C4.2.13How respiration in autotrophs and heterotrophs turns chemical energy into heat that is lostp. 585
C4.2.14Why energy losses limit how many trophic levels an ecosystem can havep. 591
C4.2.15Primary production: carbon compounds building up in the biomass of autotrophsp. 592
C4.2.16Secondary production: carbon compounds building up in the biomass of heterotrophsp. 593
C4.2.17Drawing diagrams of the carbon cyclep. 594
C4.2.18How ecosystems can act as carbon sinks or carbon sourcesp. 595
C4.2.19How burning biomass, peat, coal, oil and natural gas adds carbon dioxide to the airp. 597
C4.2.20Explaining the Keeling curve through photosynthesis, respiration and combustionp. 599
C4.2.21How aerobic respiration relies on oxygen from photosynthesis, and photosynthesis on carbon dioxide from respirationp. 601
C4.2.22How ecosystems recycle every element that organisms needp. 601
HL extension
C4.2 has no HL-only content. SL and HL students learn the same material here.
