4.2 energy flow

State how energy in carbon compounds enters most biological communities.

State how energy in carbon compounds enters most biological communities.

In most ecosystems, chemical potential energy stored in carbon compounds enters biological communities through photosynthesis in producers.

Define “food chain”.

A food chain is a sequence of organisms through which nutrients and energy pass as one organism eats another. Food chains show which organisms eats which organism. Food chains always begin with a producer and follow the flow of energy through trophic levels.

Define “food web”.

A food web represents the interconnected feeding relationships within an ecological community.

Outline the flow of energy through a food chain.

1. Light energy is converted by an autotrophs to chemical energy stored in carbon compounds (such as glucose) through photosynthesis.

2. Energy is transferred to other organisms through feeding; producers are eaten by primary consumers, these by secondary consumers, these by tertiary consumers...

3. Cellular respiration releases energy from the carbon compounds to produce ATP for use by organisms for metabolism, growth, repair, and/or movement

4. Approximately 90% of the energy at each trophic level is lost as heat, biomass not consumed (i.e. bones/hair) or biomass lost as waste (i.e. in faeces/urine)

List three reasons why living organisms need energy for cell activities.

Maintaining a living system requires an input of energy for cell activities such as synthesising large molecules (such as DNA and proteins), active transport of molecules and ions across the cell membrane, movement of structures within the cell (such as vesicles and chromosomes) and contraction of proteins (such as during cytokinesis).

Outline how light energy is converted to other energy.​

Living organisms can perform various energy conversions:

Light energy -> chemical energy (photosynthesis)

Chemical energy -> kinetic energy (muscle contraction)

Chemical energy -> electrical energy (neuron action potential)

Chemical energy -> heat energy (cell respiration)

State the function of ATP.

Adenosine triphosphate (ATP) is a nucleotide that functions as the main source of energy for most metabolic processes. ATPs are consumed by energy-requiring (endergonic) processes and produced by energy-releasing (exergonic) processes in the cell.

Outline how ATP is formed, referencing exothermic and endothermic reactions.

Formation of ATP:
ADP + Pi —> ATP + H2O
(this reaction is endothermic, with the energy input coming from the oxidation of carbon compounds such as carbohydrates and lipids)

Hydrolysis of ATP
ATP + H2O —> ADP + Pi
(this reaction is exothermic)

Outline the reason why respiration releases heat.

Heat energy is released as glucose burns. This is combustion. During cell respiration glucose is broken down gradually by a series of reactions, each catalysed by a different enzyme. This releases energy in small amounts so that it can be used by cells.

However, metabolism is far from efficient at capturing the energy in glucose in the form of ATP. A large fraction of the energy is lost as heat in the process of making ATP.

State the reason why heat created by living organisms is eventually lost from the ecosystem.

All energy released by metabolism for use in cell activities will ultimately be lost from an ecosystem because living organisms cannot turn this heat into other forms of usable energy.

Define biomass.

Biomass is the mass of living biological organisms in a given area or ecosystem at a given time.

Outline three reasons why the amount of energy decreases at higher trophic levels.

Energy decreases as it moves up trophic levels because energy is lost as metabolic heat at each trophic level.

Additionally, some chemical energy is excreted in faeces at each trophic level.

Lastly, not all of the biomass at each level is consumed during feeding, the unconsumed biomass is a source of loss of chemical energy from the trophic level.

Explain why there is a limited number of organisms in a food chain.​

Because only a small proportion (10 %) of energy can pass from one trophic level to the next, the consumers at the top of an energy pyramid have much less energy available to support them than those closer to the bottom.

The amount of useful energy left can’t support another level. There is not enough energy for a 4th/5th/later stages of a food chain.

Most of the energy that enters a community is ultimately lost to the living world as heat
released by cell respiration, biomass not consumed (i.e. bones/hair) or biomass lost as waste (i.e. in feces/urine).