4.3 Carbon cycling
Essential idea: Continued availability of carbon in ecosystems depends on carbon cycling.
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4.3.U1 Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds
4.3.U1 Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds
Be able to:
State the role of photosynthesis in the carbon cycle.
Autotrophs absorb carbon dioxide from the atmosphere and convert it into carbohydrates, lipids, and all the other carbon compounds that they require. This has the effect of reducing the carbon dioxide concentration of the atmosphere.
4.3.U2 In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogencarbonate ions.
4.3.U2 In aquatic ecosystems carbon is present as dissolved carbon dioxide and hydrogencarbonate ions.
Be able to:
Outline the process that converts CO2 to hydrogen carbonate ion in water, leading to a reduction of the pH in the water.
Carbon dioxide dissolves in water and some of it will remain as a dissolved gas, however the remainder will combine with water to form carbonic acid
(CO2 + H2O ⇄ H2CO3)
Carbon dioxide is soluble in water – can remain in water as dissolved gas or can combine with water to form carbonic acid (H2CO3)
Carbonic acid can dissociate to form hydrogen and hydrogen carbonate ions (H+ and HCO3-) -> explains how CO2 can reduce the pH of water
Dissolved CO2 and hydrogen carbonate ions are absorbed by aquatic plants / other autotrophs that live in water
Use them to make carbohydrates and other carbon compound
4.3.U3 Carbon dioxide diffuses from the atmosphere or water into autotrophs.
4.3.U3 Carbon dioxide diffuses from the atmosphere or water into autotrophs.
Be able to:
State that in diffusion, molecules move from an area of higher concentration to an area of lower concentration.
Autotrophs, such as all plants and algae, convert inorganic carbon dioxide into organic compounds via photosynthesis. These organic compounds include the carbohydrates, lipids and proteins required by the organism for survival.
Autotrophs use CO2 in the production of carbon compounds by photosynthesis or other processes. – Reduces the concentration of CO2 inside autotrophs and sets up a concentration gradient between cells in autotrophs and the air/water around
CO2 diffuses from the atmosphere or water into autotrophs
In land plants – occurs in the stomata in the underside of leaves
Aquatic plants – entire surface of leaves / stems usually permeable to CO2 (diffusion can be through any part of the plants.
4.3.U4 Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere
4.3.U4 Carbon dioxide is produced by respiration and diffuses out of organisms into water or the atmosphere
Be able to:
State that carbon dioxide is a waste product of aerobic cellular respiration.
State that carbon dioxide diffuses out of cells into the atmosphere or water.
All organisms produce the chemical energy (ATP) required to power metabolic processes via the process of cell respiration.
CO2 is a waste product of aerobic cell respiration – produced in all cells that carry out aerobic cellular respiration.
Non-photosynthetic cells in producers (eg root cells)
Animal cells
Saprotrophs (eg fungi that decompose dead organic matter)
CO2 produced by cell respiration diffuses out of cells and passes into the atmosphere or water that surrounds the organism.
4.3.U5 Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.
4.3.U5 Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere or accumulates in the ground.
Be able to:
Outline the role of methanogenic archaea in the transformation of organic material into methane
Methane is produced widely in anaerobic environments . It is a waste product of a type of anaerobic respiration
Three groups of anaerobic prokaryotes involved:
Bacteria that convert organic matter into mixture of organic acids, alcohol, hydrogen, and carbon dioxide
Bacteria that use organic acids and alcohol to produce acetate, carbon dioxide, and hydrogen
Archaeans that produce methane from carbon dioxide, hydrogen and acetate. They do this using one of the chemical reactions below:
CO2 + 4H2 -> CH4 + 2H20
CH3COOH -> CH4 + CO2
Archaeans in this group are methanogenic – they carry out methanogenesis in many anaerobic environments:
Mud along shores / in lake beds
Swamps, mires, mangrove forests and other wetlands where soil and/or peat deposits are waterlogged
In the guts of termites and of ruminant mammals like cattle / sheep
Landfill sites where organic matter is in wastes that have been buried
Some methane produced by archaeans is released into the atmosphere
Methane produced from organic waste in anaerobic digesters is not allowed to escape is instead burned as a fuel
4.3.U6 Methane is oxidized to carbon dioxide and water in the atmosphere.
4.3.U6 Methane is oxidized to carbon dioxide and water in the atmosphere.
Be able to:
State that methane is oxidized to carbon dioxide in the atmosphere.
When methane is released into the atmosphere as a result of anaerobic reactions, it only persists for ~12 years. Methane will be naturally oxidised to form carbon dioxide and water (CH4 + 2 O2 → CO2 + 2 H2O)
Monatomic oxygen and highly reactive hydroxyl radicals are involved in methane oxidation
4.3.U7 Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils.
4.3.U7 Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils.
Be able to:
Define peat.
Outline formation of peat.
In many soils, saprotrophic bacteria and fungi will decompose dead organisms and return nutrients to the soil for cycling. This decomposition process requires oxygen (cell respiration is required to fuel digestive reactions)
Peat forms when organic matter is not fully decomposed because of anaerobic conditions in waterlogged soils
large quantities of organic matter accumulate and become compressed to form a dark brown acidic material called peat
Dead leaves from plants is eventually digested by saprotrophic bacteria and fungi
Saprotrophs obtain oxygen for respiration from airspaces in soil
In some environments, water cannot drain out of soils – become waterlogged and anaerobic
Saprotrophs can’t thrive – dead organic matter isn’t fully decomposed
Acidic conditions tend to develop – further inhibiting saprotrophs and methanogens that might break down the organic matter
Large quantities of partially decomposed organic matter have accumulated in some ecosystems and become compressed to form a dark brown acidic material called peat
Conditions go from being aerobic to being anaerobic.
Becomes more and more acidic -> methanogens cannot work any longer. (The methanogens are releasing carbon dioxide which is what causes the water to become more acidic)
4.3.U8 Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.
4.3.U8 Partially decomposed organic matter from past geological eras was converted either into coal or into oil and gas that accumulate in porous rocks.
Be able to:
Outline formation of coal.
Outline formation of oil and natural gas.
Oil (i.e. petroleum) and natural gas form as the result of the decay of marine organisms on the ocean floor.
Carbon / some carbon compounds are chemically stable and can remain unchanged in rocks for hundreds of millions of years
Large deposits of carbon from past geological eras – a result of incomplete decomposition of organic matter. Burial in sediments became rock
Coal formed when deposits of peat are buried under other sediments. Peat is compressed/heated and can eventually turn into coal.
Oil and natural gas are formed in the mud at bottom of seas and lakes. Conditions are usually anaerobic so decomposition is often incomplete. More mud/other sediments are deposited, partially decomposed matter is compressed/heated. Chemical changes occur and produce complex mixtures of liquid carbon compounds or gases.
-> crude oil and natural gas. *Methane forms the largest part of natural gas.
4.3.U9 Carbon dioxide is produced by the combustion of biomass and fossilized organic matter.
4.3.U9 Carbon dioxide is produced by the combustion of biomass and fossilized organic matter.
Be able to:
Define combustion.
State the products of a combustion reaction.
State sources of fuel for a combustion reaction.
When organic compounds rich in hydrocarbons are heated in the presence of oxygen, they undergo a combustion reaction. This reaction is exergonic (produces energy) and releases carbon dioxide and water as by-products
The carbon dioxide is typically released into the atmosphere, increasing the concentration of the gas in the air
If organic matter is heated to ignition temperature in the presence of oxygen – it will burn. -> Oxidation reactions are called combustion
Products of combustion are CO2 and H2O
Biomass: total dry mass of a group of organisms
Units of grams/meter squared
land and terrestrial unit (g/m^2) = g m^-2
Aquatic unit g m^3
This is one way that carbon enters the atmosphere
4.3.U10 Animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate and can become fossilized in limestone.
4.3.U10 Animals such as reef-building corals and mollusca have hard parts that are composed of calcium carbonate and can become fossilized in limestone.
Be able to:
State that hard shells, such as in mollusk and coral, are made of calcium carbonate.
Some animals like Molusca have hard body parts composed of calcium carbonate (CaCO3).
Hard corals that build reefs (exoskeletons made by secreting calcium carbonate)
When these animals die, soft parts are decomposed quickly.
In acidic conditions, calcium carbonate dissolves
In neutral/alkaline conditions, calcium carbonate is stable and deposits of it from hard animal parts settle on the sea bed. In shallow tropical seas calcium carbonate is deposited by precipitation in the water and results in limestone rock.
4.3.S1 Estimation of carbon fluxes due to processes in the carbon cycle. (Carbon fluxes should be measured in gigatonnes.)
4.3.S1 Estimation of carbon fluxes due to processes in the carbon cycle. (Carbon fluxes should be measured in gigatonnes.)
Be able to:
List seven flux processes in the carbon cycle.
State the unit of measure for carbon flux values.
Carbon fluxes describe the rate of exchange of carbon between the various carbon sinks / reservoirs. There are four main carbon sinks – lithosphere (earth's 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 compounds 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
4.3.S2 Analysis of data from air monitoring stations to explain annual fluctuations
4.3.S2 Analysis of data from air monitoring stations to explain annual fluctuations
Be able to:
Sketch a graph of the annual fluctuation in atmospheric carbon dioxide concentration.
Explain the annual fluctuation in atmospheric carbon dioxide concentration in the northern hemisphere.
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- Can you match the graph with the correct description?
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
Data is now being regularly collected at a variety of field stations globally, using standardised measurement techniques
All stations show a clear upward trend in atmospheric CO2 concentrations year on year, with annual fluctuations
Different monitoring stations may have slightly different trends due to seasonal variations and the distribution of local vegetation
Analysing Carbon Data
Carbon data can be plotted and analysed using the online database at CDIAC (Carbon Dioxide Information Analysis Centre)
This website stores data on atmospheric CO2 levels, which can be imported into an Excel spreadsheet in order to graph
How to use the CDIAC database:
Access the CDIAC website
Click on data tab and then ‘Atmospheric Trace Gases and Aerosols’
Select ‘Carbon dioxide’ from the list of greenhouse gases
Choose Scripps Institution of Oceanography Network
Open graphice data from a particular site (e.g. Christmas Island)
4.3.S1 Construct a diagram of the carbon cycle.
4.3.S1 Construct a diagram of the carbon cycle.
Be able to:
Draw a diagram of the terrestrial carbon cycle.
Draw a diagram of the aquatic carbon cycle.
Define pool and flux.
The carbon cycle is a biogeochemical cycle whereby carbon is exchanged between the different spheres of the Earth. The four spheres are the atmosphere (air), lithosphere (ground), hydrosphere (water / oceans) and biosphere (living things). 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
Details include:
interaction of living organisms and the biosphere through:
photosynthesis - atmospheric carbon dioxide --> organisms
cell respiration - organisms --> atmospheric carbon dioxide
fossilization - carbon-containing molecules --> fossil fuels
combustion - carbon-containing organisms & fossil fuels --> atmospheric carbon dioxide
sedimentation - carbon-containing molecules --> mineral deposits
volcanoes - carbon-containing mineral deposits --> atmospheric carbon dioxide
*memorizing quantitative data is not required
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