Transfer of Energy & Matter (DP IB Biology)

The arrows in a food chain represent the transfer of energy and biomass from one trophic level to the next.

Food chains show a single energy transfer pathway within a community, while food webs show how several food chains are interconnected.

Decomposers gain their energy from carbon compounds in:

• dead organisms

• dead parts of organisms

• faeces

Autotrophs are organisms that use external energy sources to produce their own carbon compounds from simple inorganic substances.

Carbon fixation is the conversion of inorganic carbon dioxide into organic carbon compounds, e.g. carbohydrates.

True.

Carbon fixation requires energy, e.g. photosynthesis uses energy from sunlight.

Anabolic reactions build large macromolecules from smaller molecules, e.g. glucose molecules being used to produce starch or cellulose.

Photoautotrophs are organisms that use light energy for the production of carbon compounds.

Chemoautotrophs gain energy from oxidation reactions.

An example chemoautotrophy is found in bacteria that produce ATP by oxidising iron.

False.

Some heterotrophs use internal digestion while others use external digestion.

Energy is released from carbon compounds in both autotrophs and heterotrophs by oxidation during the process of cellular respiration.

A trophic level is the position of an organism in a food chain, indicating the number of organisms through which energy has passed.

Producers are organisms that produce their own carbon compounds using, e.g. light energy

Primary consumers are herbivores that feed on plant tissue. The energy stored in the bodies of primary consumers can then be transferred to secondary consumers.

False.

Secondary consumers occupy the third trophic level of a food chain.

Primary consumers are found at the second trophic level.

Tertiary consumers are carnivores that feed on secondary consumers.

False.

Many organisms have a varied diet, meaning that a species may be present at more than one trophic level in different food chains within a food web.

Pyramids of energy illustrate the energy contained within the biomass of organisms in a food chain.

True.

Pyramids of energy allow us to see the energy transferred and lost at each trophic level.

As you move up a pyramid of energy to higher trophic levels, the energy available decreases.

Conventions of drawing pyramids of energy include:

• pyramids drawn to scale so that each bar is proportional in size to the energy it represents

• levels labelled: producer, primary consumer, secondary consumer etc.

• units should be energy per unit area per year, e.g. kJ m^-2 year^-1

False.

Only around 10 % of the energy stored in the tissues of an organism is available to the consumer at the next trophic level; the remaining 90 % is lost to the environment.

Only 10 % of the energy in a food organism transferred to a consumer's tissues because:

• not every part of the food organism is eaten

• consumers are not able to digest all of the food they ingest

• heat is lost by the consumer during respiration

• energy is lost by the consumer due to excretion

Energy is lost due to incomplete consumption when parts of the food organism are not eaten, such as the roots and woody parts of plants, or the bones of animals. The energy stored in the uneaten parts is lost from the food chain.

False.

Decomposers and detritus feeders are not usually considered to be part of food chains.

The excretion of waste products results in energy loss because energy is transferred to metabolic waste products, such as urea; this energy is then removed from the food organism during excretion and is not passed on.

Detritivores and saprotrophs are involved with energy loss from food chains when they:

• decompose the parts of organisms that are not eaten

• break down undigested waste material

• decompose the bodies of organisms that die and are not consumed

The energy that is transferred to decomposers during these processes is not available to the rest of the food chain.

The energy that remains in the tissues of organisms after energy losses is stored in carbon compounds, which are available to be passed on consumers at higher trophic levels.

Heat is lost to the environment during cellular respiration and when ATP is used in other cellular processes.

Food chains are limited in length because of energy losses at each trophic level. There is less energy available to consumers as you go up the food chain, meaning that it becomes too difficult for a predator to hunt enough prey to gain the energy to survive.

False.

There is less biomass at higher trophic levels, but the energy content per unit mass is not reduced.

Primary production is the accumulation of carbon compounds in the biomass of autotrophs.

The rate of primary production is expressed in units of biomass, per unit area (or volume), per unit time, e.g. g m^–2 yr^-1.

In an aquatic habitat primary production might be measured in g m^-3 yr^-1 (biomass, per cubic metre, per year).

Primary production occurs more quickly in biomes with more hours of sunlight, optimum temperatures, and higher levels of rainfall, e.g. in tropical forest biomes.

Secondary production is the accumulation of carbon compounds in the tissues of heterotrophs.

The rate of secondary production is lower that the rate of primary production due to the to loss of biomass that occurs when carbon compounds are converted to carbon dioxide and water during cell respiration.

The role of photosynthesis in the carbon cycle is the transfer of carbon from the atmosphere to the tissues of plants.

Carbon passes between living organisms in the carbon cycle by feeding.

The role of respiration in the carbon cycle is to transfer carbon from:

• the tissues of plants to the atmosphere

• the tissues of animals to the atmosphere

• the tissues of dead organisms and from waste matter to the atmosphere (when decomposers respire)

The processes are:

• A = photosynthesis

• B = respiration

• C = respiration (of decomposers)

• D = combustion

A carbon sink is a part of the carbon cycle that takes up and stores carbon.

A carbon source is a part of the carbon cycle that releases carbon.

False.

Plants function as carbon sinks only when their rate of photosynthesis is higher than their rate of respiration.

Combustion affects the carbon cycle by transferring carbon from carbon sinks to the atmosphere. Carbon sinks that can release carbon by combustion include:

• coal

• oil

• natural gas

• peat

• biomass

False.

Burning biomass is considered to have a less significant impact on atmospheric carbon dioxide than burning fossil fuels; this is because the the carbon contained in plant tissues has been removed from the atmosphere relatively recently in comparison to that stored in fossil fuels.

Human activities have increased the amount of combustion taking place, due to increased combustion of fossil fuels, and due to increased numbers of forest fires as a result of climate change. This has increased the impact of combustion on the carbon cycle.

The Keeling curve is a graph that shows the ongoing change in the concentration of carbon dioxide in Earth's atmosphere, based on continuous measurements taken at the Mauna Loa Observatory since 1958.

The seasonal fluctuations in atmospheric carbon dioxide levels shown in the Keeling curve occur due to seasonal changes in rates of photosynthesis.

The Keeling curve shows that atmospheric carbon dioxide levels decrease during the spring and summer because photosynthesis occurs at a higher rate during these seasons and removes more carbon dioxide from the atmosphere.

The overall trend in atmospheric carbon dioxide levels shown on the Keeling curve shows a steady increase in atmospheric carbon dioxide levels; this is due to human activities like the combustion of fossil fuels.

Aerobic respiration depends on photosynthesis because photosynthesis releases oxygen as a waste product.

The global interaction between autotrophs and heterotrophs, involving photosynthesis and respiration, includes:

• the combined photosynthesis of all photosynthetic organisms removes carbon dioxide from the atmosphere and releases oxygen

• the combined respiration of all aerobically respiring organisms removes oxygen and releases carbon dioxide

Carbon fluxes are the processes that transfer carbon, such as photosynthesis and respiration.

True.

All elements used by living organisms, not just carbon, are recycled in ecosystems.

During nutrient cycling decomposers break down organic molecules in dead tissues and waste, making nutrients available again to producers.