4.1 Species, communities and ecosystems


Autotrophs and heterotrophs

Refer to Unit 4.2 - Carbon Fixation 

Consumers, detritivores and saprotrophs


They eat other living organisms or recently living organisms to get the carbon compounds they need for energy


Detritivores are heterotrophs that obtain their organic nutrients from detritus by internal digestion however unlike saprotrophs, they do not secrete enzymes into the environment to break down food, but rather digest the food inside themselves)


Saprotrophs are heterotrophs that obtain their organic nutrients from dead organisms by external digestion. Also called decomposers as they feed on dead organic matter. Fungi and bacteria are examples of saprotrophs.

4.2 Energy flow

Energy flow basics:

Ecosystems require a continuous supply of energy to fuel life processes and to replace energy lost as heat

Sunlight: is the original source of energy for almost all communities

How does enegy flow through a community? 

  1. Light energy is captured by the producers that can turn sunlight into food.
  2. Next, consumers gain their energy from the Sun via eating the producers that have this energy in them
  3. All organisms lose heat in respiration or when they die and therefore this process gets repeated

Food chains and loss of energy between trophic levels

Energy is lost as heat from respiration, incomplete digestion and egestion of waste products along the food chain. This means that, as we move up the pyramid, each successive trophic level only has around 10% of the energy of the level before it. 

How are trophic levels related to the flow of energy through an ...

4.3 Carbon Cycling

Carbon Cycle

The Carbon Cycle • A* Biology

Pool: is a reserve of the element.

Flux: is the transfer of the element from one pool to another. An example of carbon flux is the absorption of carbon dioxide from the atmosphere and its conversion by photosynthesis to plant biomass.

Reservoir:  a very large pool or store of an element. In this case, it can be in an inorganic form, such as carbon dioxide, or it can be organic, such as the biomass of the autotrophs. There are four major reservoirs, of carbon on Earth. These are: 1. In rocks (this includes fossil fuels) 2. Dissolved in ocean water 3. As plants, sticks, animals, and soil (which can be lumped together and called the land biosphere) 4. As a green house gas in the atmosphere.

Carbon fixation

Autotrophs: An autotroph or producer, is an organism that produces complex organic compounds from simple substances present in its surroundings.  Most autotrophs use a process called photosynthesis to make their food, e.g plants.

Heterotrophs: A heterotroph is an organism that cannot produce its own food, relying instead on the intake of nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are secondary and tertiary consumers.

Process: Autotrophs convert carbon dioxide into carbohydrates and other carbon compounds. This reduces the carbon concentration of the atmosphere. However, autotrophs also respire and produce carbon dioxide

Release of carbon dioxide from cell respiration

Carbon dioxide diffuses from the atmosphere or water into autotrophs. In land plants that have leaves this diffusion usually happens through stomata in the underside of the leaves. However in aquatic plants the whole surface of the leaves and stems is usually permeable (allows liquids or gases to pass through it) to carbon dioxide.

Cellular respiration: This takes place in all living plant cells to produce carbon dioxide, just like in all fungi/animal cells. Plant cells contain chloroplasts and mitochondria. When plant cells respire they absorb oxygen which the mitochondria use to produce carbon dioxide. During the day, carbon dioxide can be passed on directly to chloroplasts for photosynthesis. In which case, carbon dioxide is not released from the leaves (as photosynthesis mainly occurs here). When it is not possible to photosynthesize carbon dioxide produced during respiration is a waste product of metabolism and diffuses out of the plant into the atmosphere or water.


Methane | BioNinja

Methane (gas): CH4

Anaerobic conditions: Without the presence of oxygen.

Methanogenic archaeans: All of the methanogens are lithotrophs microorganisms that produce methane as a metabolic byproduct in anoxic conditions to make their own energy.

Methane is produced from organic matter in anaerobic conditions by methanogenic archaeans and some diffuses into the atmosphere.

3 different groups of anaerobic prokaryotes are involved in the process:

  1. Organic matter is first converted into a mixture of organic acids, alcohol, hydrogen and CO2 by a group of bacteria.
  2. Other bacteria then convert these organic acids and alcohol into acetate, carbon dioxide and hydrogen.
  3. Finally, methanogenic archaea can produce methane either through the reaction of carbon dioxide and hydrogen {1} or through the breakdown of acetate {2}.

{1} - CO2 + 4 H2 → CH4 + 2 H2O

{2} - CH3COOH → CH4 + CO2

Common places where methanogenic archaeans carry out methanogenesis in anaerobic environments:

Peat formation

Saprotrophs: organisms that feeds on or derives nourishment from decaying organic matter.

Peat is formed when partially decomposed organic matter is compressed in anaerobic waterlogged soils to form a brown soil like carbon rich matter.

In aerobic conditions organic matter is eventually digested by saprotrophic bacteria and fungi. Saprotrophs take out the oxygen they require for respiration from air spaces in the soil, but in some environments water is unable to drain out of the soil and it become waterlogged and anaerobic. Acidic conditions tend to develop, further inhibiting saprotrophs or methanogens being able to break down the organic matter.  New layers of leaf litter and other organic debris fall on top of this older layer of material, further compressing it forming the peat. Peat exists across wide areas of our planet (3% of the earth's land surface) and can reach depths of 10 m.


When biomass and fossil fuels are ignited. Carbon dioxide is produced by the combustion of biomass and fossilized matter. 

Fossil fuel + Oxygen + heat → Carbon dioxide + Water

Example with methane: 

CH+ 2 O2 + heat → CO+ 2 H2O

Limestone formation: 

Some andimals have hard body parts composed of calcium carbonate (CaCO3) such as:

When these animals die their soft parts decompose. In acidic conditions the calcium carbonate will disolve away but in neutral or alkaline conditions it is stable and you will find deposits of the compound form on the sea bed and becomes part of the sedimentary rock. Sometimes in shallow tropical seas, calcium carbonate can be deposited by precipitation in the water. Calcium carbonate depositing in non acidic conditions results in the formation of limestone rock. 

Chemical process of Limestone formation: 

CO2+H2OH2CO Carbonic acidH2CO3 H+ (lowers PH) + HCO3- (Bicarbonate)Bicarbonate can be broken down to form Carbonate and hydrogenH++ CO32- HCO3-Ca2+(aq) (found natuarlly in water) + CO32-CaCO3 (calcium carbonate)

After many years with high pressure and high heat limestone rock is formed. Figure 3. shows the white limestone cliffs of Dover, United Kingdom that has been formed over millions of years of this process. 


England's Iconic White Cliffs of Dover Eroding at Alarming Rate ...







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