Cambridge IGCSE0610

Energy flow

Biology 0610 Chapter Notes

What this chapter covers

Energy flowFood chains and food websNutrient cyclesPopulations

ShareWhatsApp Post

Energy flow notes

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Energy flow notes as text: skim, search, and jump between subtopics.

~29 min read

1. The Sun as the energy source

The Sun is the principal source of energy input to biological systems.

Energy enters a food web as sunlight. Producers are not the source of energy for a food web.

Photosynthesis is the process that uses this energy. It is not the source of the energy.

Examiner insight

“Some candidates would benefit from realising that producers are not the ultimate source of energy in a food web.” “Other common incorrect responses were ‘photosynthesis’ and ‘fossil fuels’.”

Worked example 11 mark

A food web from a desert ecosystem contains needle grasses, saxaul trees, gazelles and golden eagles. State the source of energy for this food web.

1
sunlight / Sun [1]

2. Energy flow through organisms

In photosynthesis, light energy from the Sun is transferred to chemical energy in the producer.

When a consumer eats, chemical energy is transferred from one organism to the next. The arrows in a food chain show this flow of energy.

Energy flow is linear: it starts with sunlight and does not go round in a circle.

Sun xrightarrowlight energy producer xrightarrowchemical energy consumer

organisms xrightarrowheat energy environment

Examiner insight

“Fewer candidates specified that light energy is transferred to chemical energy.” “While most candidates correctly identified energy flow as being linear, starting with sunlight, some believed it to be circular.”

Worked example 11 mark

A food chain is given.

grass → wildebeest → lion

What do the arrows represent?

A digestion

B energy flow

C heat loss

D respiration

1
B: energy flow. [1]

3. Transfer of energy to the environment

Organisms give off heat energy to the environment. This is the eventual transfer to the environment of the energy that came in as light.

So energy is not passed on in full: energy is lost at each trophic level.

A crop plant is a producer, so a person eating it is a primary consumer. The food chain is shorter, so less energy is lost.

Examiner insight

“Some candidates stated that crop plants contained more energy than rainbow trout without explaining in terms of energy losses along the food chain.” “Candidates often gave examples of the causes of inefficient energy flow although sometimes these included growth, which is an example of energy that can be transferred to the next trophic level.”

Worked example 13 marks

Explain why it is more energy efficient for humans to eat crop plants than it is for humans to eat rainbow trout.

1
crop plants are producers / at a lower trophic level than rainbow trout [1]
2
shorter food chain / humans become a primary consumer instead of a secondary consumer [1]
3
energy is lost at each trophic level / energy transfer is inefficient [1]

4. Energy at each trophic level

A pyramid of energy shows the energy in each trophic level. It does not show how energy is lost.

Less energy is available at each level up the pyramid, because energy is lost at each trophic level.

Examiner insight

“Most candidates selected incorrect statements about a pyramid of energy. Many thought that they show how energy is lost.”

Worked example 11 mark

What is a correct statement about a pyramid of energy for an ecosystem?

A It shows the amount of energy entering the first trophic level.

B It shows the energy in each trophic level.

C The energy available at each trophic level increases further up a pyramid of energy.

D The height of the bars show how much energy is available at each trophic level.

1
B: it shows the energy in each trophic level. [1]

Recap

The Sun is the principal source of energy input to biological systems.
Photosynthesis transfers light energy to chemical energy in producers.
Chemical energy is transferred from one organism to the next along a food chain.
The arrows in a food chain show the flow of energy.
Energy flow is linear: it starts with sunlight and ends in the environment.
Heat energy is given off to the environment, and energy is lost at each trophic level.

5. Food chains and producers

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. The arrows show the energy transfer, so each arrow points to the organism that feeds.

A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis.

The Sun is the source of energy for a food web. The producer is not the source.

grass → rabbit → fox → flea

Examiner insight

“Some listed the organisms in the wrong order, and some gave the arrows in the wrong direction.” “Inaccurate responses such as photosynthesis were not credited.”

Worked example 11 mark

A food web from a desert ecosystem begins with needle grasses and saxaul trees.

State the source of energy for this food web.

1
sunlight / the Sun, not the producers [1]

6. Food webs

A food web is a network of interconnected food chains. One organism can appear in more than one chain.

To interpret a food web, follow the arrows. The producer is the only organism that makes its own nutrients. Every other organism is a consumer.

An organism that is in several chains can feed at more than one trophic level.

Examiner insight

“Candidates should be reminded to look at the arrows showing energy transfer rather than the position of organism name on the page.” “Consumer means all organisms in the food web, except the producer.”

Worked example 11 mark

In a food web, rabbits and grasshoppers eat grass. Stoats eat rabbits. Foxes eat rabbits, grasshoppers and stoats. Eagles eat rabbits and stoats.

Which statement about this food web is correct?

A It has five carnivores. · B It has five consumers.

C It has one herbivore. · D It has two producers.

1
grass is the only producer, so D is wrong
2
2 herbivores (rabbits, grasshoppers) and 3 carnivores (stoats, foxes, eagles), so C and A are wrong
3
all 2 + 3 = 5 are consumers, so the answer is B [1]

7. Consumers and trophic levels

A consumer is an organism that gets its energy by feeding on other organisms. Consumers are classed as primary, secondary, tertiary and quaternary according to their position in a food chain.

A trophic level is the position of an organism in a food chain, food web or ecological pyramid. The producer is the first trophic level. A primary consumer is the second, and so on.

undersetproducergrass → undersetprimary consumerrabbit → undersetsecondary consumerfox → undersettertiary consumerflea

Examiner insight

“Possibly, these candidates did not know that the producers also represent a trophic level.” “Apex consumer or predator were seen quite often but these general terms could not be credited as they did not describe the trophic level.”

Worked example 11 mark

Frogs are amphibians and secondary consumers. Herons are birds that eat frogs.

What describes the heron?

A quaternary consumer and carnivore · B quaternary consumer and herbivore

C tertiary consumer and carnivore · D tertiary consumer and herbivore

1
frogs are secondary consumers, so a heron that eats frogs is a tertiary consumer
2
herons eat other animals, so they are carnivores
3
the answer is C [1]

8. Herbivores, carnivores and decomposers

A herbivore is an animal that gets its energy by eating plants. A carnivore is an animal that gets its energy by eating other animals. Both are consumers.

A decomposer is an organism that gets its energy from dead or waste organic material. Decomposers release carbon dioxide.

Examiner insight

“Although most candidates knew that decomposers feed on dead organic material, only very few added that they receive their energy from this material.” “Many candidates gave examples of the organisms that get energy from dead or waste organic material rather than naming the term decomposer.”

Worked example 12 marks

The food web does not include any decomposers.

Describe what is meant by a decomposer.

1
an organism that gets its energy [1]
2
from dead or waste organic matter (organisms, animals or plants) [1]

9. Humans, overharvesting and foreign species

Overharvesting is taking too many of a food species. Its numbers decrease, so the organisms that eat it have fewer to eat and decrease too. The organism it eats is eaten less, so its numbers may increase.

A foreign species introduced to a habitat joins the food web. Trace its effect in the same way. If it eats the same food as a species already there, that population is likely to decline due to a lack of food. If it is a new predator, the species it eats decrease.

Follow the effect one link at a time and name the organisms.

Examiner insight

“One misconception was that harvesting the rainbow trout increased their population, so bald eagles would have more to eat and their population would increase.” “A minority of candidates did not understand the impact of overharvesting salmon and described the reverse impact.”

Worked example 12 marks

A food chain is phytoplankton → krill → herring → Atlantic salmon → great cormorant.

Some humans also eat herring. Suggest two effects on the food chain if humans harvest too many herring.

1
salmon decrease: fewer herring to eat [1]
2
cormorants decrease [1]
3
also credited: krill increase, phytoplankton decrease. Any two effects score.

10. Pyramids of numbers and biomass

A pyramid of numbers shows the number of organisms at each trophic level. A pyramid of biomass shows the biomass at each trophic level.

Draw each as horizontal bars, one per trophic level, with the producer at the bottom. Label each bar with the organisms it represents. In a pyramid of biomass the bars get shorter from the bottom to the top. The top bar is a consumer, never a decomposer.

A pyramid of numbers is not always a pyramid shape. In the woodland food chain below, a few oak trees feed many caterpillars, so the oak tree bar is narrow. The pyramid of biomass for the same food chain has its widest bar at the bottom.

Examiner insight

“Some candidates drew the pyramid with the widest box at the top and others drew a correctly shaped pyramid but labelled it with the producer at the top.” “Few candidates understood that the top of the pyramid represents carnivores.”

Worked example 13 marks

The table gives the biomass of the organisms in a food chain in a meadow.

Describe how to draw a pyramid of biomass for this food chain. Use a scale of 1 cm to 100 kg.

organism	biomass / kg
grass	800
grasshoppers	200
frogs	50

1
draw horizontal bars, one for each organism, each labelled [1]
2
grass at the bottom, then grasshoppers, then frogs, with bars getting shorter upwards [1]
3
bar widths to scale: 800 ÷ 100 = 8 cm, 200 ÷ 100 = 2 cm, 50 ÷ 100 = 0.5 cm [1]

11. Pyramids of energy

A pyramid of energy shows the energy at each trophic level. Draw it as horizontal bars, centred, in order, with the producer at the bottom. Label each trophic level and plot the widths to scale.

It is better than a pyramid of numbers or biomass because not all biomass has the same energy content. It also takes time into account, so it is more reliable and compares species more accurately.

Examiner insight

“The best responses appreciated that time is taken into account when using a pyramid of energy, thus making them more reliable than pyramids of biomass.” “Few candidates suggested that pyramids of energy take into account the rate of reproduction, life spans or time.”

Worked example 14 marks

A food chain is phytoplankton → zooplankton → cuttlefish. The table gives the energy in each organism.

(i) Use the information to construct a pyramid of energy on the grid. Label each trophic level.

(ii) State one reason for using a pyramid of energy rather than a pyramid of biomass.

organism	energy / arbitrary units
phytoplankton	960
zooplankton	120
cuttlefish	10

1
(i) three horizontal bars, centred, in order: phytoplankton at the bottom, then zooplankton, then cuttlefish [1]
2
each bar labelled with its trophic level: producer, primary consumer, secondary consumer [1]
3
widths to scale: with 1 small square for 20 units, 960 ÷ 20 = 48, 120 ÷ 20 = 6 and 10 ÷ 20 = 0.5 small squares [1]
4
(ii) any one from: not all species have the same energy content / shows the energy lost or transferred at each trophic level / more accurate for comparing species [1]

12. Energy loss and eating crops

Energy is lost at each transfer from one trophic level to the next, so the transfer is not efficient. Energy is released by respiration. Some is lost by movement and excretion, and some parts of an organism are not eaten. Growth is not a loss: energy in growth can be transferred to the next trophic level.

Very little energy reaches the top of a food chain. There is not enough for another trophic level, so food chains usually have fewer than five trophic levels.

People who eat crop plants are primary consumers. People who eat livestock fed on crops are secondary consumers. The chain is shorter, so less energy is lost.

percentage transferred = (energy in the higher level)/(energy in the lower level) × 100

Examiner insight

“Candidates often gave examples of the causes of inefficient energy flow although sometimes these included growth, which is an example of energy that can be transferred to the next trophic level.” “A common misconception was to state that energy is used in respiration rather than released by respiration.”

Worked example 13 marks

Explain why it is more energy efficient for humans to eat crop plants than it is for humans to eat rainbow trout.

1
crop plants are producers, in the first trophic level, lower than rainbow trout [1]
2
shorter food chain: humans become a primary consumer instead of a secondary consumer [1]
3
energy is lost at each trophic level: energy transfer is inefficient [1]
4
or a named example of energy loss, such as heat from respiration. Any three of the four points score.

Recap

A food chain shows the transfer of energy from one organism to the next, beginning with a producer. Each arrow points to the organism that feeds.
A producer makes its own organic nutrients, usually using energy from sunlight, through photosynthesis. The Sun is the source of energy.
A food web is a network of interconnected food chains. Every organism except the producer is a consumer.
Consumers are primary, secondary, tertiary and quaternary. The producer is the first trophic level.
Herbivores eat plants. Carnivores eat other animals. Decomposers get energy from dead or waste organic material.
Overharvesting lowers the harvested species. Its predators decrease and its prey may increase. Trace a foreign species through the food web in the same way.
Pyramids use horizontal bars with the producer at the bottom. Biomass bars get shorter upwards. A pyramid of numbers is not always a pyramid shape.
Extended: a pyramid of energy takes time into account and compares species more reliably.
Extended: energy is lost at each transfer, so chains have fewer than five levels and crops are more energy efficient than livestock.

13. Carbon dioxide into plants and back

Producers take carbon dioxide from the atmosphere by photosynthesis. The carbon becomes carbon compounds in green plants.

Plants and animals both return carbon dioxide to the atmosphere by respiration.

Photosynthesis removes carbon dioxide. Respiration adds it.

Examiner insight

“The majority of candidates appreciated that producers remove carbon dioxide from the atmosphere, some incorrectly thought that decomposers remove carbon dioxide from the atmosphere.”

Worked example 11 mark

In the carbon cycle, what is the simple substance absorbed by producers?

A carbohydrate · B carbon dioxide · C nitrate · D protein

1
B: carbon dioxide is absorbed by producers. [1]

14. Feeding and decomposition

Feeding passes carbon compounds from green plants to animals, and from animals to other animals.

Carbon leaves consumers through egestion, excretion and decomposition. Decomposition of dead organic material releases carbon dioxide into the air.

Decomposition is not a way of removing carbon dioxide.

Examiner insight

“This proved to be a challenging question with many candidates not appreciating that carbon atoms leave the consumers through egestion, excretion and decomposition.” “Few candidates appreciated that decomposition releases carbon dioxide into the atmosphere.”

Worked example 12 marks

Name the process that passes carbon compounds from green plants to animals. Name the process that releases carbon dioxide from dead organic material.

1
feeding [1]
2
decomposition [1]

15. Fossil fuels and combustion

Formation of fossil fuels is gradual. Carbon compounds in plants and animals become coal, oil and gas.

Combustion of fossil fuels releases carbon dioxide into the atmosphere. Factories and cars burn fossil fuels.

Write combustion, not burning. Combustion is not respiration or decomposition.

Examiner insight

“Although many candidates could correctly link this description of combustion of fossil fuels to the diagram a large number thought that this could be plant respiration or decomposition.” “Some confused the carbon cycle with the water cycle.”

Worked example 11 mark

Which process produces the carbon dioxide from the factory and the car?

A combustion · B condensation · C decomposition · D precipitation

1
A: combustion of fossil fuels. [1]

16. Nitrogen fixation

Nitrogen fixation changes nitrogen in the air into compounds in the soil. Two things do it: lightning and bacteria.

Lightning changes nitrogen in the atmosphere to nitrates in the soil. Nitrogen fixing bacteria change atmospheric nitrogen to ammonium ions.

Fixation starts with nitrogen gas. It does not produce nitrogen gas.

Examiner insight

“Many candidates appreciated that lightning brings about the conversion of nitrogen in the atmosphere to nitrates in the soil. A minority of candidates incorrectly believed this to be the role of nitrifying bacteria.” “Some candidates were unsure about the nitrogen cycle with some incorrectly believing that nitrogen fixation produces nitrogen gas from nitrate ions.”

Worked example 12 marks

State the two ways in which nitrogen fixation happens.

1
lightning [1]
2
bacteria (root nodules alone does not earn the mark) [1]

17. Nitrification and uptake by plants

Nitrifying bacteria carry out nitrification. They convert ammonia (ammonium ions) to nitrate ions.

Plants absorb nitrate ions from the soil. They use the nitrogen to make amino acids and then proteins.

Nitrification is not nitrogen fixation.

Examiner insight

“Nitrifying bacteria convert ammonia to nitrates.” “Most candidates correctly identified nitrification. Some candidates incorrectly selected option A which is nitrogen fixation.”

Worked example 11 mark

In the nitrogen cycle, bacteria convert ammonia to nitrate ions. What is the name of this process?

A deamination · B denitrification · C nitrification · D nitrogen fixation

1
C: nitrification. [1]

18. Feeding, deamination and decomposition

Animals get nitrogen by feeding on plants. They digest proteins and use the amino acids to make their own proteins.

Excess amino acids are broken down by deamination.

When plants and animals die, decomposers break down the protein. This is decomposition. It forms ammonium ions.

Examiner insight

“Some candidates omitted the necessary details, such as the conversion of proteins to ammonium ions and deamination.” “Others missed that it is excess amino acids that are deaminated.”

Worked example 13 marks

Describe how the protein in a dead animal is changed into nitrate ions in the soil.

1
decomposers break down the protein (decomposition) [1]
2
to form ammonium ions [1]
3
nitrifying bacteria convert the ammonium ions to nitrate ions (nitrification) [1]

19. Denitrification and the roles of bacteria

Denitrification changes nitrate ions in the soil into nitrogen gas in the air. It is done by denitrifying bacteria.

Four roles of microorganisms: decomposition (protein to ammonium ions), nitrification (ammonium ions to nitrate ions), nitrogen fixation (nitrogen gas to compounds in the soil) and denitrification (nitrate ions to nitrogen gas).

Name the ion or the gas. Do not write that nitrogen is denitrified.

Examiner insight

“Many candidates incorrectly referred to nitrogen throughout their answer, for example nitrogen is denitrified to nitrate.” “Many candidates correctly selected option A, but some confused the processes of denitrification and nitrogen fixation.”

Worked example 12 marks

Name the process that changes nitrate ions in the soil into nitrogen gas. Name the type of organism that carries it out.

1
denitrification [1]
2
bacteria (denitrifying bacteria) [1]

Recap

Photosynthesis removes carbon dioxide from the atmosphere. Respiration adds it.
Feeding passes carbon compounds along. Decomposition releases carbon dioxide.
Formation of fossil fuels is gradual. Combustion of fossil fuels releases carbon dioxide.
Extended: nitrogen fixation by lightning and bacteria makes compounds from nitrogen in the air.
Extended: nitrification converts ammonia to nitrate ions. Plants absorb nitrate ions to make amino acids and proteins.
Extended: animals feed and digest proteins. Excess amino acids are deaminated. Decomposition of protein forms ammonium ions.
Extended: denitrification changes nitrate ions to nitrogen gas. Bacteria carry out decomposition, nitrification, fixation and denitrification.

20. Population

A population is a group of organisms of one species, living in the same area, at the same time.

All the three-spined stickleback fish living in a lake are one population. All the fish in the lake, or all the organisms in the lake, are not one population.

Examiner insight

“Some candidates understood that a population would be all the three-spined stickleback fish living in a lake but many candidates incorrectly believed that a population is all the organisms living in a lake or all the fish living in a lake.” “However, some responses did not specify that a population refers to a single species and very few mentioned that it is the group of organisms found at the same time.”

Worked example 11 mark

Which statement describes the term population?

A a group of organisms of one species, living in different areas at the same time · B a group of organisms of one species, living in the same area at the same time · C the number of different species that live in the same area at the same time · D the number of individuals of all the species that live in the same area at the same time

1
B: one species, the same area, the same time. [1]

21. Community and ecosystem

A community is all of the populations of different species in an ecosystem.

An ecosystem is a unit containing the community of organisms and their environment, interacting together.

Examiner insight

“Common errors were to refer to a single species rather than many species or all species and to omit reference to an ecosystem or place.”

Worked example 12 marks

In the wild, Arabian oryx all live in similar ecosystems. Describe what is meant by an ecosystem.

1
a unit containing the community of organisms and their environment [1]
2
interacting together [1]

22. Factors affecting population growth

Four factors affect the rate of population growth: food supply, competition, predation and disease.

Predation, disease or a lack of food act on the stationary and death phases of a growth curve. An increase in predators could result in the death phase.

Examiner insight

“Most candidates could name factors that could decrease population size. Occasionally, the terms predator and prey were confused.”

Worked example 11 mark

Which factor could result in the death phase of a sigmoid population growth curve?

A absence of limiting factors · B abundance of food · C decrease in competition for space · D increase in predators

1
D: an increase in predators. [1]

23. The sigmoid growth curve

A population growing in an environment with limited resources follows a sigmoid curve. Its four phases, in order, are lag (the slow start), exponential (log) (the steep rise), stationary (the flat top) and death (the fall).

Examiner insight

“Some confused the lag and log stages and some gave simple descriptions of the stages, for example: increasing stage and number stays the same.”

Worked example 11 mark

A yeast cell was introduced into a flask containing nutrients. The flask was kept at optimum conditions. The graph shows the yeast population size over a period of time. Which phase of the growth curve is missing from the graph?

A death phase · B exponential (log) phase · C lag phase · D stationary phase

1
C: the curve is already rising steeply when it begins, so the slow lag phase is missing. [1]

24. Reading population graphs

Read a population graph by its shape. A rising line means the birth rate is greater than the death rate. A flat line means the birth rate equals the death rate. A falling line means the death rate is greater than the birth rate.

The y-axis is the number of organisms in the population, not the growth of one organism.

Examiner insight

“Some candidates incorrectly believed that the reproduction rate was greater than the death rate during the stationary phase.” “A significant number of candidates misread the graph and decided that the giant panda population in 1977 was either 2500 or 2400.”

Worked example 11 mark

The graph shows the changes in a population of brine shrimp over a period of time. Over which section or sections is the birth rate greater than the death rate?

A 1, 2 and 3 · B 1 and 2 only · C 1 only · D 2 only

1
C: the number of brine shrimp rises only in section 1. It is constant in section 2 and falls in section 3. [1]

25. Explaining the phases

Lag: the population takes time to reproduce or acclimatise.

Exponential (log): there are no limiting factors and plenty of nutrients, so reproduction is rapid. This phase is limited only by the rate of reproduction.

Stationary: the birth rate is equal to death rate. A limiting factor, such as nutrients, stops any further increase.

Death: the death rate is greater than the birth rate. Nutrients are no longer available and the concentration of toxic waste products increases.

Examiner insight

“Common misconceptions included stating that there was no further reproduction at the point the stationary phase was reached, or to describe the y-axis as if it referred to the growth of a single organism, rather than the number of organisms.” “The term resources is too general and was not accepted as an alternative to named resources, such as nutrients and oxygen.”

Worked example 15 marks

A sample of phytoplankton was grown in a container with conditions similar to those found in the wild. The phytoplankton were provided with constant light. The population of phytoplankton was measured as the number of organisms per cm³. Figure 5.3 shows the results of the investigation.

Identify and explain the phases of growth shown by the phytoplankton in Figure 5.3.

1
phases identified as lag, exponential (log), stationary and death [1]
2
lag: takes time for the population to reproduce or acclimatise [1]
3
exponential (log): rapid reproduction, as there are no limiting factors [1]
4
stationary: birth rate is equal to death rate [1]
5
death: death rate is greater than the birth rate, as nutrients or carbon dioxide are no longer available [1]

Recap

Population: one species, the same area, the same time.
Community: all the populations of different species in an ecosystem.
Ecosystem: a unit containing the community and its environment, interacting together.
Growth is affected by food supply, competition, predation and disease.
Sigmoid curve, in order: lag, exponential (log), stationary, death.
Rising line: birth rate greater than death rate. Flat: equal. Falling: death rate greater.
Extended: the exponential phase has no limiting factors. A limiting factor holds the stationary phase.
Extended: in the death phase the death rate is greater than the birth rate.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

State the principal source of energy input to biological systems.1 mark
In a food chain, state what each arrow represents.1 mark
A leaf carries out photosynthesis. Name the form of energy the leaf takes in and the form of energy stored in the leaf.2 marks
A rabbit eats grass and a fox eats the rabbit. Name the form of energy transferred from the rabbit to the fox.1 mark
A student says that energy goes round and round a food chain in a circle. Correct the student.2 marks
Name the form of energy that organisms give off to the environment.1 mark
A pyramid of energy for a woodland has a wide bar for the trees and a narrow bar for the owls. Explain why the owls’ bar is narrower.3 marks
A farmer can feed people with wheat, or with cattle that have eaten wheat. Explain why more people can be fed from the wheat.3 marks
A locust eats leaves that contain 500 kJ of chemical energy. The table gives what happens to this energy. (a) Calculate the energy stored in new body tissue. (b) A lizard eats the locust. State the greatest amount of this energy that can be transferred to the lizard, and name the form of the energy. (c) State what eventually happens to all 500 kJ. | what happens to the energy | energy / kJ | | --- | --- | | given off to the environment as heat | 165 | | left in the locust’s waste | 300 | | stored in new body tissue (growth) | ? |4 marks
A cave is completely dark. Bats sleep in the cave by day. At night they fly out and eat beetles that have fed on leaves. Bat droppings fall to the cave floor. Fungi grow on the droppings, and cave crickets eat the fungi. A student says: ‘The droppings are the source of energy for the crickets, so this food web does not need the Sun.’ Describe the flow of energy from its source to the crickets, and then to the environment, to explain why the student is wrong.5 marks
Define the term producer.1 mark
A heron eats frogs. Frogs eat grasshoppers. Grasshoppers eat grass. Construct the food chain.2 marks
A food chain is wheat → mouse → snake → eagle. (a) Name the herbivore. (b) Name the tertiary consumer.2 marks
In a pond, snails and tadpoles eat pondweed. Beetles eat snails. Fish eat tadpoles. Herons eat beetles and fish. (a) State the number of consumers in this food web. (b) Name the secondary consumers. (c) State the trophic level of the herons.3 marks
(a) Name the term for an organism that gets its energy from dead or waste organic material. (b) A student writes: ‘A decomposer feeds on dead or waste organic material.’ State what the decomposer gets from this material.2 marks
In a lake, algae are eaten by water fleas. Water fleas are eaten by small fish. Small fish are eaten by perch. (a) People catch too many perch. Predict the effect on the numbers of small fish, water fleas and algae. Give a reason for each. (b) A second lake has the same food chain and no perch are caught. A foreign species of fish that eats only water fleas is introduced. Predict the effect on the numbers of small fish. Give a reason.4 marks
The table gives the biomass of the organisms in a food chain on a farm. (a) Name the pyramid that these data are used to draw. (b) State which organism is the top bar. (c) Compare the length of the bar for owls with the bar for wheat. | organism | biomass / kg | | --- | --- | | wheat | 600 | | mice | 60 | | owls | 6 |3 marks
In a pond, the producers contain 7600 kJ of energy. The primary consumers contain 540 kJ. Calculate the percentage of energy transferred from the producers to the primary consumers. Give your answer to one significant figure.2 marks
The table gives the energy in each trophic level of a food chain in a seagrass bed. (a) Calculate the percentage of energy transferred from the sea snails to the crabs. (b) A pyramid of energy is drawn for this food chain. The bar for seagrass is 10 cm wide. Calculate the width of the bar for the crabs. (c) An animal that eats only octopuses would receive 10% of their energy. Calculate the energy that would reach this fifth trophic level. Use your answer to explain why the food chain has only four trophic levels. | trophic level | organism | energy / kJ | | --- | --- | --- | | producer | seagrass | 15 000 | | primary consumer | sea snails | 1800 | | secondary consumer | crabs | 270 | | tertiary consumer | octopuses | 27 |7 marks
A field of soya plants contains 90 000 MJ of energy. People can eat the soya, or feed it to chickens and eat the chickens. Assume that 10% of the energy in one trophic level is transferred to the next. (a) Calculate the energy transferred to people when they eat the soya, and when they eat the chickens. (b) A student writes: ‘People who eat the soya get all 90 000 MJ. Chickens lose energy through respiration, movement and growth.’ Identify the two mistakes. (c) Explain why it is more energy efficient for people to eat the soya. Use your answers to (a).7 marks
Name the process by which green plants take carbon dioxide from the air.1 mark
State two processes in the carbon cycle that add carbon dioxide to the atmosphere.2 marks
A student says that decomposers remove carbon dioxide from the atmosphere. Explain why the student is wrong.2 marks
Coal and oil contain carbon compounds. Name the process that turns them into carbon dioxide in the atmosphere. Name the process that makes coal and oil from carbon compounds in animals.2 marks
A rabbit eats grass. Describe how carbon in the grass can return to the atmosphere as carbon dioxide.3 marks
Bacteria in the soil convert ammonium ions to nitrate ions. Name the process and name the type of bacteria.2 marks
Describe how nitrogen in the protein of a plant is passed to a herbivore and used by it.3 marks
Bean plants have bacteria in their root nodules. Name the process these bacteria carry out and state the change they make to nitrogen gas. State one other way this process happens.3 marks
A farmer adds nitrate ions to a field of bare, wet soil. No plants grow there and no water drains away. After four weeks the soil has fewer nitrate ions, and bubbles of gas have left the soil. (a) Name the gas made from the nitrate ions, the process and the type of microorganism responsible. [3] (b) The farmer then grows bean plants, which have bacteria in their root nodules. The plants use nitrogen from the nodules to make protein, then die and are left on the field. Later the soil gains nitrate ions. Describe how nitrogen from the air became nitrate ions in the soil. [3]6 marks
A fish tank has no plants. Protein-rich food is added daily and some is left uneaten. Water plants are added on day 21. The table gives the ion concentrations in the water. (a) Explain why ammonium ions form in the water. [2] (b) Explain the changes in both ions between day 7 and day 21. [2] (c) A student writes: 'Protein turns straight into nitrate ions.' Use day 7 to explain why this is wrong. [1] (d) Another student says the fall in nitrate ions after day 21 must be denitrification. Give a different explanation and state what happens to the nitrogen. [2] | day | ammonium ions / mg per dm³ | nitrate ions / mg per dm³ | | --- | --- | --- | | 7 | 3.0 | 0.5 | | 14 | 1.2 | 6.0 | | 21 | 0.2 | 12.0 | | 35 | 0.2 | 4.0 |7 marks
State what is meant by a population.1 mark
A pond holds one species each of frog, water snail, pondweed and dragonfly. (a) State how many populations the pond holds. (b) State the term for all of them together.2 marks
A student writes: ‘An ecosystem is all the organisms living in a forest.’ State two things the student has left out.2 marks
Foxes are introduced to an island where rabbits live. State the factor that foxes add to the rabbit population and state how it affects the rate of population growth.2 marks
Name, in order, the four phases of a sigmoid population growth curve.4 marks
The graph shows the number of bacteria in a flask over 9 hours. (a) Identify the phase between 2 hours and 4 hours. (b) State how the birth rate compares with the death rate between 8 hours and 9 hours.2 marks
The graph shows the number of deer in a forest over 10 years. (a) State the largest number of deer. (b) Describe how the number of deer changed between year 6 and year 10. Use figures from the graph.3 marks
Bacteria are added to a flask of nutrient broth. Explain why the population grows exponentially at first.2 marks
Bacteria were grown in a sealed flask of glucose solution. The table gives the number of live bacteria and the glucose concentration over 20 hours. (a) Identify the phase between 0 and 4 hours. Explain why the number rises so little although the glucose concentration is high. (b) A student writes: ‘Between 12 and 16 hours the bacteria have stopped reproducing.’ Explain why the student is wrong. (c) Use the table to explain the change in the number of live bacteria between 16 and 20 hours. | time / hours | live bacteria / millions per cm³ | glucose / g per dm³ | | --- | --- | --- | | 0 | 2 | 40 | | 4 | 3 | 39 | | 8 | 48 | 30 | | 12 | 90 | 12 | | 16 | 90 | 3 | | 20 | 35 | 0 |6 marks
The graph shows the number of yeast cells in two flasks over 24 hours. Both flasks start with the same number of cells in the same volume of sugar solution. The sugar concentration in flask B is twice that in flask A. (a) Between 4 and 12 hours the two populations rise at the same rate. Explain why the extra sugar in flask B makes no difference. (b) Explain why the population in flask B reaches a higher maximum than the population in flask A. (c) Calculate the percentage decrease in the population in flask A between 20 and 24 hours. (d) State how the birth rate compares with the death rate in flask A between 20 and 24 hours, and give one reason for this.8 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Characteristics of living organisms Cell structure Diffusion Biological molecules Enzymes Photosynthesis

Read the notes

1. The Sun as the energy source#

2. Energy flow through organisms#

3. Transfer of energy to the environment#

4. Energy at each trophic level#

5. Food chains and producers#

6. Food webs#

7. Consumers and trophic levels#

8. Herbivores, carnivores and decomposers#

9. Humans, overharvesting and foreign species#

10. Pyramids of numbers and biomass#

11. Pyramids of energy#

12. Energy loss and eating crops#

13. Carbon dioxide into plants and back#

14. Feeding and decomposition#

15. Fossil fuels and combustion#

16. Nitrogen fixation#

17. Nitrification and uptake by plants#

18. Feeding, deamination and decomposition#

19. Denitrification and the roles of bacteria#

20. Population#

21. Community and ecosystem#

22. Factors affecting population growth#

23. The sigmoid growth curve#

24. Reading population graphs#

25. Explaining the phases#

End-of-chapter exercise

Go deeper

Workbook

Chapter at a Glance

Free notes are just the start.

Related chapters

1. The Sun as the energy source

2. Energy flow through organisms

3. Transfer of energy to the environment

4. Energy at each trophic level

5. Food chains and producers

6. Food webs

7. Consumers and trophic levels

8. Herbivores, carnivores and decomposers

9. Humans, overharvesting and foreign species

10. Pyramids of numbers and biomass

11. Pyramids of energy

12. Energy loss and eating crops

13. Carbon dioxide into plants and back

14. Feeding and decomposition

15. Fossil fuels and combustion

16. Nitrogen fixation

17. Nitrification and uptake by plants

18. Feeding, deamination and decomposition

19. Denitrification and the roles of bacteria

20. Population

21. Community and ecosystem

22. Factors affecting population growth

23. The sigmoid growth curve

24. Reading population graphs

25. Explaining the phases