Biology IB HL - 4.2 Energy Flow

All green plants, and some bacteria, are photoautotrophic – they use sunlight as a source of energy

This makes light the initial source of energy for almost all communities

In a few ecosystems the producers are chemoautotrophic bacteria, which use energy derived from chemical processes

Light energy is absorbed by photoautotrophs and is converted into chemical energy via photosynthesis

This light energy is used to make organic compounds (e.g. sugars) from inorganic sources (e.g. CO2)

Heterotrophs ingest these organic compounds in order to derive their chemical energy (ATP)

When organic compounds are broken down via cell respiration, ATP is produced to fuel metabolic processes

Energy enters most ecosystems as sunlight, where it is converted into chemical energy by producers (via photosynthesis)

This chemical energy is stored in carbon compounds (organic molecules) and is transferred to heterotrophs via feeding

The position an organism occupies within a feeding sequence is known as a trophic level

Producers always occupy the first trophic level in a feeding sequence

Primary consumers feed on producers and hence occupy the second trophic level

Further consumers (e.g. secondary, tertiary, etc.) may occupy subsequent trophic levels

A food chain shows the linear feeding relationships between species in a community

Arrows represent the transfer of energy and matter as one organism is eaten by another (arrows point in direction of energy flow)

The first organism in a food chain is always a producer, followed by consumers (primary, secondary, tertiary, etc.)

Energy stored in organic molecules (e.g. sugars and lipids) can be released by cell respiration to produce ATP

This ATP is then used to fuel metabolic reactions required for growth and homeostasis

A by-product of these chemical reactions is heat (thermal energy), which is released from the organism

Not all energy stored in organic molecules is transferred via heterotrophic feeding – some of the chemical energy is lost by:

Being excreted as part of the organism’s faeces

Remaining unconsumed as the uneaten portions of the food

kinetic, electrical and light energy

during muscular contractions

during the transmission of nerve impulses

producing bioluminescence

Living organisms cannot turn this heat into other forms of usable energy

This heat energy is released from the organism and is lost from the ecosystem (unlike nutrients, which are recycled)

Hence ecosystems require a continuous influx of energy from an external source (such as the sun)

Typically energy transformations are ~10% efficient, with about 90% of available energy lost between trophic levels

The amount of energy transferred depends on how efficiently organisms can capture and use energy (usually between 5 – 20%)

As energy is lost between trophic levels, higher trophic levels store less energy as carbon compounds and so have less biomass

Biomass is the total mass of a group of organisms – consisting of the carbon compounds contained in the cells and tissues

Because carbon compounds store energy, scientists can measure the amount of energy added to organisms as biomass

Biomass diminishes along food chains with the loss of carbon dioxide, water and waste products (e.g. urea) to the environment

Because energy and biomass is lost between each level of a food chain, the number of potential trophic levels are limited

Higher trophic levels receive less energy / biomass from feeding and so need to eat larger quantities to obtain sufficient amounts

Because higher trophic levels need to eat more, they expend more energy (and biomass) hunting for food

If the energy required to hunt food exceeds the energy available from the food eaten, the trophic level becomes unviable

A pyramid of energy is a graphical representation of the amount of energy at each trophic level of a food chain

They are expressed in units of energy per area per time (e.g. kJ m–2 year–1)

YES

| Pyramids of energy will never appear inverted as some of the energy stored in one source is always lost upon transfer

Each level should be roughly one tenth of the size of the preceding level (as energy transformations are ~10% efficient)

The bottom level will always represent the producers, with subsequent levels representing consumers (primary, secondary, etc.)