How do bacteria cycle nitrogen?

Bacteria fix nitrogen into ammonia, change it to nitrate that plants absorb, and return it to the air as nitrogen gas. Nitrogen fixation turns N2 into ammonia. Nitrification turns ammonia into nitrite then nitrate in aerobic soil. Plants take in nitrate to make protein (assimilation).

Energy transferred in ecosystems Notes

Oxford AQA IGCSE Biology: Revision notes

Key facts

Energy enters ecosystems by photosynthesis and flows along food chains and food webs.
Only about 10% of energy passes to the next trophic level; the rest is lost.
Efficiency $= \dfrac{\text{energy in higher level}}{\text{energy in lower level}} \times 100$ .
The carbon cycle: photosynthesis removes carbon dioxide; respiration, decomposition and combustion return it.
The nitrogen cycle needs bacteria; the water cycle is driven by the Sun.

Food chains and webs

Producers capture light energy by photosynthesis, and it passes along food chains. Only about 10% reaches each next level.

A food chain is a linear sequence: producer, primary consumer, secondary consumer. A food web links several chains and is more stable, since there are alternative feeding routes.

Energy flow is one-directional and cannot be recycled. Numbers and biomass fall at each level.

Energy at each level if 10% is transferred (example).

1
Grass
producer
2
Rabbit
primary consumer
3
Fox
secondary consumer

A food chain

Food chain

Simple and linear
One feeding route

Food web

Interconnected chains
More realistic and stable

In the chain grass, rabbit, fox, the rabbit is the

Energy losses

Energy is lost through respiration, egestion, excretion and parts not eaten, so only about 10% is passed on. That limits chains to three to five levels.

Respiration releases energy as heat for movement and keeping warm. Egestion is undigested food passing out as faeces. Excretion removes nitrogen-containing waste such as urea. Some parts are not eaten at all.

About 90% is lost at each step, so there is too little energy to support more than three to five trophic levels, and top predators are the fewest.

1
Energy taken in
from the food eaten
2
Lost in respiration
released as heat
3
Lost in egestion and excretion
faeces and urea
4
Parts not eaten
bones, roots, shells
5
Passed on
only about 10% reaches the next level

Where the energy goes at each trophic level

Respiration

Energy released as heat
Movement and warmth

Egestion

Undigested food lost in faeces

Excretion

Urea and other waste

Not eaten

Bones, roots, shells

Worked example

1000 kJ reaches the primary consumers. How much reaches the tertiary consumers if 10% passes on at each step?

1
Primary to secondary: $10\% \text{ of } 1000 = 100$ kJ.
2
Secondary to tertiary: $10\% \text{ of } 100 = 10$ kJ.

Why are food chains rarely longer than five levels?

Efficiency of transfer

Efficiency is the energy in the higher level divided by the energy in the lower level, times 100.

Typical efficiency is about 10%. Most absorbed energy is used in respiration, and only a small share goes into new biomass.

Show the formula, the substitution and the multiplication by 100.

Energy at each trophic level for 10% efficiency (illustrative values)

Efficiency (%) $\dfrac{\text{energy in higher level}}{\text{energy in lower level}} \times 100$

Worked example

Grass contains 10 000 kJ and rabbits contain 1000 kJ. Calculate the efficiency of the transfer.

1
Efficiency $= \dfrac{1000}{10\,000} \times 100$ .

Worked example

A food chain has 50 000 kJ at the producer level and 2500 kJ at the primary consumer level. Calculate the efficiency.

1
$2500 \div 50\,000 = 0.05$ .
2
$0.05 \times 100$ .

1000 kJ is passed from a level with 5000 kJ. What is the efficiency?

The carbon cycle

Photosynthesis removes carbon dioxide from the air; respiration, decomposition and combustion return it; fossilisation stores it.

Photosynthesis fixes carbon in glucose. Respiration by all organisms releases carbon dioxide. Decomposers (bacteria and fungi) release it as they break down dead matter. Combustion of fossil fuels and wood releases it fast. Fossilisation locks carbon away over millions of years.

Link them as a cycle, not as separate facts.

1
Atmosphere
carbon dioxide
2
Photosynthesis
carbon fixed in plants and algae
3
Feeding
carbon passes to animals
4
Respiration and decomposition
carbon dioxide returned to the air
5
Fossilisation and combustion
long-term storage, then release by burning

Then back to step 1

The carbon cycle

Which process removes carbon dioxide from the air?

The nitrogen cycle

Bacteria fix nitrogen into ammonia, change it to nitrate that plants absorb, and return it to the air as nitrogen gas.

Nitrogen fixation turns $\ce{N2}$ into ammonia. Nitrification turns ammonia into nitrite then nitrate in aerobic soil. Plants take in nitrate to make protein (assimilation).

Decomposers return nitrogen as ammonia. Denitrification turns nitrate into $\ce{N2}$ in oxygen-poor soil. Legumes in crop rotation improve soil fertility.

1
Atmospheric nitrogen
N₂ gas, 78% of the air
2
Nitrogen fixation
bacteria make ammonia
3
Nitrification
ammonia, then nitrite, then nitrate
4
Assimilation
plants build nitrate into protein; animals eat plants
5
Decomposition
ammonia released from dead matter and waste
6
Denitrification
nitrate converted to nitrogen gas

Then back to step 1

The nitrogen cycle

Nitrogen fixation

Rhizobium, Azotobacter
N₂ to NH₃

Nitrification

Nitrosomonas, Nitrobacter
NH₃ to nitrite to nitrate
Aerobic

Decomposition

Decomposers
Protein to NH₃

Denitrification

Denitrifying bacteria
Nitrate to N₂
Anaerobic

Which process turns nitrate back into nitrogen gas?

The water cycle

The Sun's energy evaporates water, vapour condenses into clouds, and precipitation returns it to the land and sea.

Evaporation from oceans, lakes and rivers, and transpiration from plants, put water vapour into the air. It cools and condenses to form clouds, then falls as precipitation and is collected in oceans, rivers and groundwater.

Water changes state but not chemical composition. Deforestation and damming can disrupt the cycle.

1
Evaporation and transpiration
the Sun's energy turns water into vapour
2
Condensation
vapour cools and forms clouds
3
Precipitation
rain or snow falls
4
Collection
water stores in oceans, lakes, rivers and groundwater

Then back to step 1

The water cycle

What drives the water cycle?

Try an exam question

Explain why food chains rarely have more than five trophic levels.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.