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Cycles and decomposition: 2. The Carbon Cycle

Doc Brown's GCSE level Biology exam study revision notes

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INDEX of notes on natural cycles, their importance, decay and decomposers


(1) The Carbon Cycle and its important role in the Earth's biosphere

  • Know and understand that the constant cycling of carbon is called the carbon cycle.

    • Carbon is an important element in many of the compounds of living organisms and because there is only a fixed amount (ignoring fossil fuel burning) in the 'biosphere' it must be constantly recycled.

    • Know and understand that in the carbon cycle:

    • The natural ways in which carbon dioxide is captured from the atmosphere

      • (a) Most carbon dioxide is removed from the environment by photosynthesis in green plants on land and algae and phytoplankton in water.

        • Most carbon is 'captured' by photosynthesis in the leaves/stems of plants on land on land.

        • The carbon dioxide dissolves in water so photosynthesising green algae and other forms of plankton in lakes, seas and oceans make a major contribution to capturing and recycling carbon too.

        • Note that photosynthesis is relative rapid way in which carbon is absorbed from the atmosphere.

        • The carbon from the carbon dioxide is used to make carbohydrates, fats and proteins, which make up the body of plants and algae and ultimately the food of animals.

          • Its all about creating biomass for most food chains.

        • Photosynthesis uses sunlight energy to convert water and carbon dioxide into sugars like glucose, the 'waste product' being oxygen - though plants need oxygen for their respiration at night!

        • The simplest equation to illustrate photosynthesis is

        • water + carbon dioxide (+ sunlight) == chlorophyll ==> glucose + oxygen

        • When green plants and algae are eaten by animals and these animals are eaten by other animals, some of the carbon becomes part of the fats and proteins that make up their bodies, so carbon based materials and energy are moved up the food chain as biomass transfers.

        • The growth of photosynthesising organism in the seas and oceans is big contributor to the carbon sink (carbon stores) of the planet - all are removing carbon dioxide from the atmosphere..

          • Kelp, a large brown seaweed, grows profusely in shallow nutrient rich seawater.

          • Per unit area kelp stores more carbon than an equivalent growth of plants on land.

          • Trees are another good land-based carbon store.

        • For more see Photosynthesis - importance, rate factors explained  gcse biology revision notes.

        • The carbon store of the oceans is not only increased via photosynthesising organisms, carbon dioxide is slightly soluble in water adding to the carbon sink.

          • Unfortunately, this makes the water slightly more acidic (slightly lower pH) and, combined with warming oceans, this is having a disastrous effect on corals and their rich ecosystems.

        • The formation of limestone and fossil fuels also contribute to the 'carbon sink' and are described next.

      • (b) Some of the carbon dioxide dissolved in the sea becomes calcium carbonate and other calcium minerals and ends up as a sedimentary rock like limestone.

        • All the shells of marine organisms contain carbonates (mainly calcium carbonate CaCO3) e.g. corals and microscopic algae are covered with calcium carbonate. Over millions of years these shells fall to the ocean floor and the debris is compressed to form layers of the sedimentary rock limestone.

        • This mineralisation does store carbon, but this is a very slow geological process over thousands-millions of years.

        • However, carbon dioxide in rain forms 'carbonic acid' that weathers the limestone away releasing some of the carbon dioxide back into the atmosphere - polluted acid rain from burning fossil fuels, due to sulfur dioxide, accelerates the weathering of limestone - just look at medieval statues in ex highly industrial towns!

        (c) The remains of dead animals and plants can also store lots of carbon e.g. peat forms over thousands of years in bogs, and over millions of years some of these remains end up as coal and oil and gas hydrocarbons.

        • Not all the remains of plants and animals get broken down and the carbon oxidised to carbon dioxide.

        • BUT, although this process stores vast amounts of carbon, it is a very slow chemical-geological process over thousands-millions of years and incredibly slow compared to the rate we are burning fossil fuels.

        • In terms of the Earth's ecosystems, the decomposition of these carbon based organic materials from dead plants and animals ensures that habitats can be maintained to support the organisms that live in that particular ecosystem.

          • If this decomposition did not take place via microorganisms, all the waste would stack up and the nutrients needed to sustain new life would not be recycled - so we don't want all the carbon stored.

        • The case of peat bogs is of increasing concern to environmentalists.

          • In marshy areas, it takes thousands of years to create peat bogs form and they store large amounts of carbon (another part of our planet's carbon sink).

          • The low oxygen level and acidic marshy conditions prevent decomposer microorganisms from completely breaking down the plant material.

          • Peat is harvested as a fuel and a cheap compost for gardeners - it does improve soil quality making it more organic and increasing food production (at least on a small scale).

          • The draining of bog lands and the removal of peat is decreasing the carbon sink.

          • Another worrying trend is the warming of arctic tundra lands and the melting of permafrost.

          • Permafrost a thick subsurface layer of soil that remains below the freezing point of water (<0oC) throughout the year, occurring chiefly in polar regions of the northern hemisphere.

          • This is releasing carbon into the atmosphere from oxidation of organic material AND methane gas (CH4) a powerful greenhouse gas.

          • This leads onto the section ....

    • The many ways in which carbon dioxide is returned to the atmosphere

      • (a) When green plants and algae respire, some of this carbon from the glucose from photosynthesis becomes carbon dioxide and is released back into the atmosphere.

        • the overall simplest equation for respiration is the opposite of photosynthesis

        • glucose + oxygen ==> water + carbon dioxide (+ energy)

      • (b) When animals respire some of their biomass carbon becomes carbon dioxide and is released into the atmosphere - summary equation for aerobic respiration as above.

      • (c) When plants, algae and animals die, microorganisms feed on their bodies and carbon dioxide is released into the atmosphere as carbon dioxide when these microorganisms respire - aerobic decomposition.

        • By the time the microorganisms and detritus feeders have broken down the waste products and dead bodies of organisms in ecosystems and recycled the materials as plant nutrients, all the energy originally absorbed by green plants and algae has been transferred, and much of the carbon returned to the atmosphere as carbon dioxide.

        • Animals produce waste (e.g. droppings) that is also broken down by the same microorganisms and detritus feeders.

        • Many carbon compounds are recycled in waste materials from plants and animals to maintain healthy ecosystems.  The decay processes ensure the constant recycling of nutrients for new growth in plants and ultimately providing for animals - the respiration of plants, animals and microorganisms returns carbon dioxide to the atmosphere.

      • (a) to (c) are natural processes that work in harmony, mainly with photosynthesising organisms.

        • What we see is a massive complex system of the continuous recycling of carbon and energy through soil, water and air - transferred by the many complex food chains (of food webs) of plants, algae, animals and the microorganisms and detritus feeders.

      • (d) to (f) are all due to human activity AND not in balance with photosynthesising organisms!

      • (d) Combustion of wood and fossil fuels releases carbon dioxide into the atmosphere.

        • It takes fossil fuels millions of years to form, but using biofuels is MUCH faster!

        • Ignoring fossil fuels, through the carbon cycle, carbon is recycled over and over again, through the atmosphere and food chains over a relatively short period of time.

        • The balance has been shifted in favour of atmospheric carbon dioxide (slowly rising) because of the rapidity with which we are burning fossil fuels which take so long to form.

        • Although described as renewable, even burning biofuels, still releases carbon dioxide back into the atmosphere.

      • (e) Large quantities of carbon dioxide are released in the manufacture of lime and cement - mainly from the latter due the enormous amount of building going on around the world.

        • In both production processes, calcium carbonate (limestone) decomposes giving of carbon dioxide.

        • CaCO3(s)  ==> CaO(s)  +  CO2(g)

      • (f) There are significant changes going on in the way that we manage land.

        • Deforestation of many of the world's greatest forests in South America and Asia is happening now and at an alarming rate.

        • Large areas of forest are being destroyed in favour of removing trees to clear the land to raise cattle and grow crops.

        • This is removing a major source of carbon storage via photosynthesis.

        • The trees are often burned or left to rot AND the soil erodes away exposing it to the weather and oxygen rich atmosphere, so even more organic material in the soil gets oxidised  to carbon dioxide - the microorganisms are decaying residual plant material like leaf litter and they will also release CO2 from aerobic respiration.

        • These processes considerably decrease biodiversity.

See also

FOSSIL FUELS, coal, oil & natural gas and the Carbon Cycle  (gcse chemistry revision)

and Limestone and lime - their chemistry and uses  (gcse chemistry revision notes)


Summary of learning objectives and key words or phrases

Be able to construct a simple diagram of the carbon cycle and be able to interpret and explain a more complex diagram of the carbon cycle described above.

  • Be able to show an understanding of how carbon is recycled (CARBON CYCLE diagram above):

  • a) during photosynthesis plants remove carbon dioxide from the atmosphere

    • carbon dioxide + water == light energy/chlorophyll  ==> glucose + oxygen

    • This is the process by which plants make food, for themselves, and for most animal life, including us too!

    • Note that the only way carbon dioxide is removed from the air is photosynthesis in green land based plants or marine organisms like phytoplankton (this point ignores long term formation of carbonate rocks like limestone).

  • b) carbon compounds pass along a food chain

    • All food chains involve the passing of carbon compounds e.g. sugars, carbohydrates, fats and proteins up to the next trophic level i.e. the consecutive eating along a food chain (and waste produced on the way).

      • e.g. grass ==> cow ==> human

  • c) during plant or animal aerobic respiration organisms release carbon dioxide into the atmosphere

    • sugars e.g. glucose + oxygen ==> carbon dioxide + water (+ energy)

    • this is the main aerobic energy releasing process in most living organisms.

  • d) decomposers release carbon dioxide into the atmosphere - slow aerobic respiration

    • Microorganisms like bacteria and fungi in the soil feed off decaying plant material and animal droppings or remains.

    • Most dead plant matter consists of cellulose which most animals can't digest, but bacteria and fungi, do have the enzymes to break it down and without their help there would be no carbon cycle.

    • Most of these bacteria and fungi respire aerobically so they need a good supply of oxygen to produce the carbon dioxide essential to keeping the carbon cycle going.

  • e) combustion of fossil fuels releases carbon dioxide into the atmosphere

    • Coal, formed millions of years from the remains of tropical plant material, mainly consists of carbon,  Burning coal produces a lot of pollution as the greenhouse gas carbon dioxide.

      • The main reaction on burning is ...

      • carbon + oxygen ==> carbon dioxide

      • C(s) + O2(g) ==> CO2(g)

    • Natural gas (mainly methane) and petrol molecules like octane (and lots of other molecules) from oil and gas reserves.

    • methane + oxygen ===> water + carbon dioxide

    • octane + oxygen ===> water + carbon dioxide


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