Energy Flow and Nutrient Cycles, Grade 10: Food Chains, Pyramids and Cycles

Two pots on a school lab bench in sunlight: a bean plant with a clear plastic bag tied over its leaves, misted with water droplets, and a pot with a bare stick under a dry plastic bag

This page covers the third part of the Grade 10 topic Biosphere to ecosystems: how energy flows through an ecosystem, food chains, food webs and ecological pyramids, and the water, oxygen, carbon and nitrogen cycles. It ends with two short classroom demonstrations.

Subject Life Sciences
Grade and term Grade 10, Term 3
Topic Biosphere to ecosystems, part 3 of 3: energy flow and nutrient cycles
Practicals Classroom demonstrations: transpiration in a plastic bag, and root nodules on a legume. The fieldwork for this topic is on the study an ecosystem page
Time Transpiration: 10 minutes to set up, results after 1 to 2 hours in the sun. Root nodules: about 20 minutes

Trophic levels

A trophic level is a feeding level: the position an organism takes in a food chain, counted from the producers.

Trophic level Feeds on Examples
Producers, first level Make their own food by photosynthesis Grass, trees, algae
Primary consumers, second level Producers. These are herbivores Grasshopper, zebra, impala
Secondary consumers, third level Herbivores. These are carnivores Lizard, jackal, lion
Tertiary consumers, fourth level Other carnivores A hawk that eats lizards

Omnivores, such as baboons and people, eat both plants and animals, so they feed at more than one level. Decomposers, mainly bacteria and fungi, get their energy by breaking down dead organisms and waste from every level.

Food chains and food webs

A food chain shows who eats whom, for example:

grass → zebra → lion

The arrows show the direction in which energy flows, from the organism that is eaten to the organism that eats it. Most animals eat more than one kind of food and are eaten by more than one predator, so in a real ecosystem the chains link up into a food web. A food web gives a truer picture of an ecosystem than a single chain does.

Food web diagram linking grass, grasshoppers, impala, zebra, mice, lizards, jackals, lions and a hawk, with arrows showing the direction energy flows

Energy flow

Almost all the energy in an ecosystem comes from the Sun. Producers capture it by photosynthesis and store it in food. When one organism eats another, only about 10% of the energy at one trophic level is passed on to the next. The rest is lost:

  • Most is used in respiration, for moving and staying alive, and is lost as heat.
  • Some parts are not eaten, such as roots, bark, bones and fur.
  • Some food is not digested and leaves the body in the faeces.
  • Organisms that die without being eaten pass their energy to the decomposers.

Pyramid of energy with four levels: producers 10 000 kJ, primary consumers 1 000 kJ, secondary consumers 100 kJ and tertiary consumers 10 kJ

Trophic level Energy, using the 10% rule
Producers 10 000 kJ
Primary consumers 1 000 kJ
Secondary consumers 100 kJ
Tertiary consumers 10 kJ

This is why food chains seldom have more than four or five links: there is too little energy left at the top to support another level. It is also why a field of maize feeds more people when they eat the maize than when it is fed to cattle and they eat the beef.

Class activity: a field of grass captures 100 000 kJ of energy from the Sun. Ask learners to work out how much reaches a grasshopper, a lizard that eats grasshoppers, and a hawk that eats lizards. The answers are 10 000 kJ, 1 000 kJ and 100 kJ.

Ecological pyramids

An ecological pyramid shows the trophic levels as bars stacked on top of each other, with the producers at the bottom. There are three kinds, each drawn from a different measurement.

Pyramid What each bar shows
Numbers The number of organisms at each level
Biomass The total mass of the organisms at each level
Energy The energy passing through each level in a given time

A pyramid of biomass is not always pyramid-shaped. In parts of the open sea, the animals and bacteria in the plankton can outweigh the tiny algae they depend on, so the pyramid is upside down. A pyramid of energy is never upside down, because energy is lost at every level, so each bar is smaller than the one below it.

Nutrient cycles

Energy flows through an ecosystem in one direction and is lost as heat. Matter is different: the Earth gets almost no new supply of it, so the same water, carbon, oxygen and nitrogen are used over and over. They move between living things and the environment in cycles.

The water cycle

Diagram of the water cycle showing evaporation from the sea, transpiration from plants, condensation into clouds, precipitation as rain, run-off into rivers and infiltration into groundwater

Process What happens
Evaporation The Sun heats water in the sea, rivers, dams and soil, and it becomes water vapour in the air
Transpiration Plants take up water through their roots and lose water vapour from their leaves
Condensation Water vapour cools and forms tiny droplets, which make clouds
Precipitation Water falls back to the ground as rain, hail or snow
Run-off and infiltration Water flows over the ground into rivers and the sea, or soaks into the soil and becomes groundwater

Animals also return water to the environment when they breathe out, sweat and pass urine.

The oxygen cycle

Diagram of the oxygen cycle: producers release oxygen into the air in photosynthesis, and plants, animals and decomposers take it in for respiration

Producers release oxygen into the air during photosynthesis. Plants, animals and decomposers take oxygen in for respiration, and burning fuels also uses oxygen up. Photosynthesis replaces the oxygen that respiration and burning remove.

The carbon cycle

Flow chart of the carbon cycle: carbon dioxide in the air is taken in by plants in photosynthesis, passed to animals by feeding, and returned to the air by respiration, decomposition and the burning of fossil fuels

Carbon leaves the air by Carbon returns to the air by
Photosynthesis: producers take in carbon dioxide and make it into food Respiration by plants, animals and decomposers
Decomposition of dead organisms and waste
Burning of fossil fuels such as coal, oil and gas, and of wood

Carbon passes along food chains as animals eat plants and each other. Over millions of years, buried remains of organisms can become coal, oil and gas, which store carbon until they are burnt.

The nitrogen cycle

Air is about 78% nitrogen, but plants and animals cannot use nitrogen gas directly. They need nitrogen to make proteins, and plants take it up from the soil as nitrates. Bacteria do most of the work of the cycle.

Flow chart of the nitrogen cycle: nitrogen gas is fixed by bacteria and lightning, plants absorb nitrates, animals eat plants, decomposers make ammonium compounds, nitrifying bacteria make nitrates, and denitrifying bacteria return nitrogen gas to the air

Step What happens
Nitrogen fixation Nitrogen-fixing bacteria turn nitrogen gas into compounds plants can use. Some live free in the soil, and some live in root nodules on legumes such as beans, peas and clover. Lightning also fixes a small amount
Absorption Plants absorb nitrates through their roots and use them to make proteins. Animals get their nitrogen by eating plants or other animals
Decomposition Decomposers break down dead organisms, faeces and urine, and release ammonium compounds into the soil
Nitrification Nitrifying bacteria turn ammonium compounds into nitrites and then into nitrates
Denitrification Denitrifying bacteria, mainly in waterlogged soil, turn nitrates back into nitrogen gas, which returns to the air

Classroom demonstration 1: transpiration in a plastic bag

Aim

To show that plants give off water vapour from their leaves, which is how plants take part in the water cycle.

What you need

  • A leafy potted plant, such as a bean plant
  • A second pot of the same soil with no plant, and a bare stick about the height of the plant pushed into it
  • 2 clear plastic bags, about 20 × 30 cm
  • 2 elastic bands or pieces of string
  • Water and a sunny windowsill or bench

Method

  1. Water both pots the evening before.
  2. Slip a plastic bag over the leaves of the plant and tie it gently around the stem just above the soil, so the soil is outside the bag and cannot add water to it.
  3. Slip the second bag over the stick and tie it around the stick just above the soil in the same way. This is the control.
  4. Stand both pots side by side in the sun.
  5. Look at both bags after 1 hour and again after 2 hours.

What you should see

Bag After 1 to 2 hours
Over the plant Misty inside, with water droplets on the plastic
Over the stick Dry

Why it happens

The plant takes up water through its roots and carries it up the stem to the leaves. Water evaporates from the leaf cells and the vapour leaves through tiny pores called stomata. Inside the bag the vapour cannot escape, so it condenses on the plastic. The dry control bag shows that the water came from the plant, not from the soil, the air or the bag. Out in the open the vapour joins the air, so plants return a large amount of water to the atmosphere.

Safety

  • Keep plastic bags away from small children.
  • Wash your hands after handling soil.

Classroom demonstration 2: root nodules on a legume

Aim

To look at the root nodules where nitrogen-fixing bacteria live.

What you need

  • A bean or pea plant at least 6 weeks old grown in garden soil, or a clover plant from the school lawn
  • A garden trowel or spade
  • A bucket of water
  • A white tile and a sharp knife, used by the teacher
  • A hand lens, if you have one

Method

  1. Dig up the plant with a wide ball of soil around the roots, so the fine roots are not torn off.
  2. Soak the roots and soil in the bucket of water for 10 minutes.
  3. Swirl the roots gently until the soil falls away.
  4. Look along the roots for small round or finger-shaped lumps.
  5. The teacher cuts one lump in half on the tile.
  6. Look at the colour inside.

What you should see

What you look at What you see
The roots Small round or finger-shaped lumps a few millimetres long. These are the root nodules
Inside a nodule Pink or red if the bacteria are fixing nitrogen. Young nodules are white or grey inside, and old ones that have stopped fixing nitrogen turn green

Why it happens

Nitrogen-fixing bacteria called Rhizobium enter the roots of legumes, and the plant grows nodules around them. The bacteria turn nitrogen gas from the air in the soil into compounds the plant uses to make proteins, and the plant gives the bacteria sugars in return. The pink colour comes from a pigment that controls how much oxygen reaches the bacteria. Because legumes add nitrogen compounds to the soil, farmers often grow them in rotation with crops such as maize.

Safety

  • Only the teacher uses the knife, cutting on the tile and away from the hand.
  • Wash your hands after handling soil.

Free worksheet and marking memo

Related practicals

References