1. 4.3 – Carbon Cycle
Essential Idea: continued availability of carbon in ecosystems depends on carbon cycling
Nature of Science: Making accurate, quantitative measurements—it is important to obtain reliable data on the concentration of carbon dioxide and methane in the atmosphere. (3.1)

2. Carbon Carbon: Atomic #6 – ability to form up to 4 covalent bonds
Carbon is the backbone of all of the organic compounds and many biological mocelcules;
Examples of common organic molecules
 amino acids, carbohydrates, lipids,

3. Where do we find Carbon
Carbon can be found throughout the different environments on earth
hydrosphere: water
biosphere: all the places where life is found
lithosphere: all the places where rocks are found
atmosphere: air
Do Workbook Activity 127 AND Note that this connects to Topic 2

4. U4.3.1 Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds.
What do we mean when we say that carbon cycles?
the cycle of carbon in the earth's ecosystems in which carbon dioxide is fixed by photosynthetic organisms to form organic nutrients and is ultimately restored to the inorganic state (as by respiration, protoplasmic decay, or combustion
Name some examples of autotrophs
Plants, some bacteria and protists including algae
What role do autotrophs play in the carbon cycle? Photosynthesis is the main means autotrophs produce organic compounds and oxygen from carbon dioxide and water
Autotrophsan organism that is able to form nutritional organic substances from simple inorganic substances such as carbon dioxide.

5. U4.3.2 In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogen carbonate ions.
What happens when carbon dioxide from the air diffuses (dissolves) into the water
 react with a water molecule to form H2CO3 which, in solution, will dissociate like an acid (carbonic acid).
Can you think of a consequence of rising atmospheric carbon dioxide levels?
Increase ocean acidity
FYI HCO3- is hydrogen carbonate ion

6. U4.3.3 Carbon dioxide diffuses from the atmosphere or water into autotrophs.
Why do producers need carbon dioxide?
Heterotrophs cannot synthesize their own organic molecules and instead obtain carbon compounds via feeding
Since autotrophs use carbon dioxide for photosynthesis, the levels of carbon dioxide within the organism should always be low
In other words, carbon dioxide should always be at a higher concentration in the atmosphere (or water)
This concentration gradient ensures that carbon dioxide will passively diffuse into the autotrophic organism as required

7. S4.3.1 Skill: Construct a diagram of the carbon cycle.
Carbon is exchanged between a variety of forms, including:
Atmospheric gases – mainly carbon dioxide (CO2), but also methane (CH4)
Oceanic carbonates – including bicarbonates dissolved in the water and calcium carbonate in corals and shells
As organic materials – including the carbohydrates, lipids and proteins found in all living things
As non-living remains – such as detritus and fossil fuels
Different processes facilitate the cycling of carbon between the different forms (e.g. feeding, combustion, etc.)

8. S4.3.1 Skill: Construct a diagram of the carbon cycle.

9. S4.3.1 Skill: Construct a diagram of the carbon cycle.

10. U4.3.4 Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere.
What process absorbs carbon dioxide from the atmosphere?
Photosynthesis
What process releases carbon into the atmosphere?
Cellular Respiration

11. U4.3.4 –Extension Details
All organisms may produce the chemical energy (ATP) required to power metabolic processes via the process of cell respiration
Cell respiration involves the breakdown of organic molecules (e.g. sugars) and produces carbon dioxide as a by-product
The build up of CO2 in respiring tissues creates a concentration gradient, allowing it to be removed by passive diffusion
In autotrophs, the uptake of CO2 by photosynthesis may at times be balanced by the production of CO2 by respiration
This is known as the compensation point, at which the net carbon dioxide assimilation is zero (intake = output)
Similarly, the amount of carbon dioxide in the environment will be determined by the level of these two processes:
If there is more net photosynthesis than cell respiration occuring in the biosphere, atmospheric carbon dioxide levels should drop
If there is more net respiration than overall photosynthesis occuring, atmospheric carbon dioxide levels should increase

12. U4.3.5 Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.
Methane is produces by Archaen Microorganisms that produce methane (CH4) as a metabolic by-product in anaerobic conditions
Anaerobic conditions where methanogens may be found include:
Wetlands (e.g. swamps and marshes)
Marine sediments (e.g. in the mud of lake beds)
Digestive tract of ruminant animals (e.g. cows, sheep, goats)

13. U4.3.5 Extension Info
Methanogens produce methane from the by-products of anaerobic digestion, principally acetic acid and carbon dioxide:
Acetic acid → Methane and Carbon Dioxide  (CH3COO– + H+  →  CH4 + CO2)
Carbon Dioxide and Hydrogen → Methane and Water  (CO2 + 4 H2  →  CH4 + 2 H2O)
Methane may either accumulate under the ground or diffuse into the atmosphere
When organic matter is buried in anoxic conditions (e.g. sea beds), deposits of methane (natural gas) may form underground
Rising global numbers of domesticated cattle may be increasing the levels of methane being released into the atmosphere

15. U4.3.6 Methane is oxidized to carbon dioxide and water in the atmosphere
only persists for ~12 years
naturally oxidized to form carbon dioxide and water  
(CH4 + 2 O2  →  CO2 + 2 H2O)
This is why methane levels in the atmosphere are not very large, even though significant quantities are being produced

16. U4.3.7 Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils.
Water is unable to drain out of soils  anaerobic
Saprotrophs die  Organic matter is not fully decomposed
Conditions become more acidic  more saprotrophs and methanogens die
Accumulation of partially decomposed matter & pressure  formation of peat

17. Peat as a Fossil Fuel

18. U4.3.8 Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.
Since the organic matter is not fully decomposed in waterlogged soils, carbon- rich molecules remain in the soil and form peat
Coal and Oil Formation
When deposits of peat are compressed under sediments, the heat and pressure force out impurities and remove moisture
The remaining material has a high carbon concentration and undergoes a chemical transformation to produce coal
Oil, coal, natural gas and other fossil fiels are:  a nonrenewable energy source since they take millions of years to form.

20. U4.3.8 Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.
Mining for them – destroys ecosystems, takes energy, releases chemicals and polution Burning them – release carbon dioxide and other green-house gasses into the atmosphere

21. U4.3.9 Carbon dioxide is produced by the combustion of biomass and fossilized organic matter.
When Biomass (organic matter used as a fuel) is burned, emissions significantly influence the Earth's atmosphere and climate.
Note:
As burning occurs, it can release hundreds of years worth of stored carbon dioxide into the atmosphere in a matter of hours.

22. Combustion Sources 1. Fossil Fuels
Organic compounds can become rich in hydrocarbons when compacted underground for millions of years
The heat and pressure over time triggers a chemical transformation that results in the compaction of the organic matter
The resulting products of this process are fossil fuels (coal, oil and natural gas)
Because this geological process takes millions of years to occur, fossil fuels are a non-renewable energy source
2.  Biomass
An alternative to relying on fuels produced by geological processes is to manufacture fuels from biological processes
Living organisms produce hydrocarbons as part of their total biomass (either for use or as a waste product)
These hydrocarbons can be extracted and purified to produce an alternative fuel source (e.g. bioethanol and biodiesel)
Provided new raw materials are provided and waste products are removed, this source of energy is renewable

24. In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogen carbonate ions
Carbon dioxide dissolves in water and will combine with water to form carbonic acid  (CO2 + H2O  ⇄  H2CO3)
Carbonic acid will then dissociate to form hydrogen carbonate ions  (H2CO3  ⇄  HCO3– + H+)
This conversion also releases hydrogen ions (H+), which is why pH changes when CO2 is dissolved in water (> acidic)
Autotrophs absorb both dissolved carbon dioxide and hydrogen carbonate ions and use them to produce organic compounds 

25. U Animals such as reef-building Corals and Mollusca have hard parts that are composed of calcium carbonate and can become fossilized in limestone.
When the hydrogen carbonate ions come into contact with the rocks and sediments on the ocean floor, they acquire metal ions  calcium carbonate  development of limestone
Living animals may also combine the hydrogen carbonate ions with calcium to form calcium carbonate hardened exoskeleton of coral, and mollusca shells
When the organism dies and settles to the sea floor, these hard components may become fossilized in the limestone

27. 4.3 A1-  Estimation of carbon fluxes due to processes in the carbon cycle
Carbon fluxes describe the rate of exchange of carbon between the various carbon sinks / reservoirs
There are four main carbon sinks – lithosphere (earth crust), hydrosphere (oceans), atmosphere (air), biosphere (organisms)
The rate at which carbon is exchanged between these reservoirs depends on the conversion processes involved:
Photosynthesis – removes carbon dioxide from the atmosphere and fixes it in producers as organic compounds 
Respiration – releases carbon dioxide into the atmosphere when organic compouns are digested in living organisms
Decomposition – releases carbon products into the air or sediment when organic matter is recycled after death of an organism
Gaseous dissolution – the exchange of carbon gases between the ocean and atmosphere
Lithification – the compaction of carbon-containing sediments into fossils and rocks within the Earth’s crust (e.g. limestone)
Combustion – releases carbon gases when organic hydrocarbons (coal, oil and gas) are burned as a fuel source

28. It is not possible to directly measure the size of the carbon sinks or the fluxes between them – instead estimates are made
Because carbon fluxes are large and based on measurements from many different sources, estimates have large uncertainties 

29. Estimating carbon fluxes requires an understanding of the factors that can affect the exchange of carbon between different sinks. Some of the main causes for flux change include climate conditions, natural events and human activity 
Climate Conditions
Rates of photosynthesis will likely by higher in summer seasons, as there is more direct sunlight and longer days
Oceanic temperatures also determine how much carbon is stored as dissolved CO2 or as hydrogen bicarbonate ions
Climate events like El Nino and La Nina will change the rate of carbon flux between ocean and atmosphere
Melting of polar ice caps will result in the decomposition of frozen detritus
Natural Events
Forest fires can release high levels of carbon dioxide when plants burn (loss of trees also reduces photosynthetic carbon uptake)
Volcanic eruptions can release carbon compounds from the Earth’s crust into the atmosphere
Human Activity
Clearing of trees for agricultural purposes (deforestation) will reduce the removal of atmospheric CO2 via photosynthesis
Increased numbers of ruminant livestock (e.g. cows) will produce higher levels of methane
The burning of fossil fuels will release carbon dioxide into the atmosphere

30. 4.3 A2 - Analysis of data from air monitoring stations to explain annual fluctuations
Atmospheric CO2 concentrations have been measured at the Mauna Loa Observatory (in Hawaii) since 1958 by Charles Keeling
From these continuous and regular measurements a clear pattern of carbon flux can be seen:
CO2 levels fluctuate annually (lower in the summer months when long days and more light increase photosynthetic rates)
Global CO2 trends will conform to northern hemisphere patterns as it contains more of the planet’s land mass (i.e. more trees)
CO2 levels are steadily increasing year on year since the industrial revolution (due to increased burning of fossil fuels)
Atmospheric CO2 levels are currently at the highest levels recorded since measurements began

32. Analysing Carbon Data
Carbon data can be plotted and analysed using the
online database at CDIAC 
This website stores data on atmospheric CO2 
levels, which can be imported into an Excel
spreadsheet in order to graph like the one here
How to use the CDIAC database:
Access the CDIAC website (click on the link to redirect)
Click on ‘Atmospheric Trace Gases and Aerosols’ (under ‘Data' tab at top of page)
Select ‘Carbon dioxide’ from the list of greenhouse gases
Choose a monitoring station / network (e.g. Scripps Institution of Oceanography Network)
Download data from a particular site (e.g. South Pole, Antarctica)
Paste data of interest into an Excel spreadsheet to produce a graphical display (e.g. Jan 2000 – Dec 2007)