Energy flow — Cambridge IGCSE Biology 0610 Revision Notes & Topical Questions

Section 1 of 17Energy flow

1Source

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

2Forms

• Forms light energy from the Sun and chemical energy in organisms.

3Direction

• Direction energy flows through organisms and is eventually transferred to the environment as heat.

4

Energy flows, while nutrients cycle.

Section 2 of 17Pyramids of numbers

Pyramid of numbers

Pyramid of numbers shows how many organisms are present at each level of a food chain.

Width of box

• Width of box the number of organisms at that level.

Shape

• Shape not always pyramid-shaped because the size of organisms matters, for example one large oak tree supports many insects.

Larger organisms

• Larger organisms generally there are fewer individuals.

Section 3 of 17Rules for drawing pyramids of numbers

Order

• Order producers at the bottom, then primary, secondary and tertiary consumers.

Labels

• Labels each bar is labelled with its trophic level.

Scale

• Scale drawn to scale if graph paper is provided.

Section 4 of 17Pyramids of biomass

Pyramid of biomass

Pyramid of biomass shows how much living matter is present at each trophic level.

Biomass

• Biomass often measured as dry mass, once water is removed.

Shape

• Shape always pyramid-shaped because mass decreases up a food chain.

Advantage over numbers

• Advantage over numbers takes account of organism size, so gives a more accurate picture of living matter.

Energy

• Energy stored in biomass, so biomass approximates the energy available at each level.

Section 5 of 17Pyramids of energy

Pyramid of energy

Pyramid of energy shows the energy stored at each trophic level.

Always pyramid-shaped

• Always pyramid-shaped it is never inverted, because energy is lost at each stage.

Advantages

• Advantages highlights energy lost at each level and allows fair comparison, since organisms store different amounts of energy per unit biomass.

Section 6 of 17Why energy transfer is inefficient

Why energy transfer is inefficient

Respiration

• Respiration energy is released for movement and heat.

Excretion

• Excretion energy is lost in metabolic wastes such as urine.

Uneaten parts

• Uneaten parts roots of plants or bones of animals never enter the consumer.

Undigested waste

• Undigested waste faeces are removed and feed decomposers.

Transfer

• Transfer only about 10% of the energy at one level is available to the next.

Section 7 of 17Consequences of energy loss

Food chains

• Food chains rarely have more than five organisms, because the final consumer would need to eat huge numbers of organisms below.

More levels

• More levels the greater the number of trophic levels, the greater the energy loss.

Humans

• Humans it is more energy efficient to eat crop plants such as wheat than cattle fed on wheat.

Exceptions

• Exceptions animals often eat plants humans cannot, like grass, or plants too widely spread, like ocean algae.

Section 8 of 17More from the specification

Combustion energy of conventional fossil fuels

• Combustion energy of conventional fossil fuels burning coal, oil,
  • and gas fossil fuels releases ancient trapped photosynthetic energy alongside carbon dioxide.

Unidirectional Energy Transfer from Sunlight

• Sunlight represents the principal initial source of biological energy;
  • photosynthetic primary producers convert radiant solar energy into chemical energy stored within organic carbohydrates.

Thermodynamic Inefficiency Across Trophic Levels

• Approximately 90% of available energy is lost between consecutive trophic levels through cellular respiration, heat dissipation, incomplete digestion,
  • and egestion, leaving only ~10% for incorporation into consumer biomass.

Limitations on Food Chain Length

• Due to persistent progressive thermodynamic energy losses, food chains rarely exceed four to five trophic levels before insufficient metabolic energy remains to sustain higher consumer populations.

Fossil fuel

• Fuel formed from long-dead organisms.

Population

• A group of organisms of one species in the same area at the same time.

Nitrogen

• Element in amino acids and nitrogenous bases.

Carbon

• Element with four valence electrons that forms the backbone of organic molecules.

Growth

• A permanent increase in size and dry mass.

Photosynthesis

• The process by which plants make carbohydrates from raw materials using energy from light.

Species

• A group of organisms that can reproduce to produce fertile offspring.

Microorganism

• A living organism so small that it can only be seen clearly with a microscope.

Bacteria

• Single-celled microorganisms, much smaller than plant or animal cells.

Section 9 of 17Source and pathway of energy

1Source

• Source almost all energy in ecosystems comes from the Sun.

2Capture

• Capture producers capture a small fraction (about 1 to 2%) of the light reaching them.

3Conversion

• Conversion photosynthesis converts light to chemical energy in organic molecules.

4Transfer

• Transfer energy passes along food chains when organisms are eaten.

5Direction

• Direction energy moves one way only, from the Sun through producers and consumers to the environment as heat.

Section 10 of 17Energy losses at each level

Respiration

• Respiration energy released for movement, growth and heat ends up as heat.

Waste

• Waste faeces and urine contain energy not absorbed.

Uneaten parts

• Uneaten parts bones, roots, shells and fur are not eaten.

Heat loss

• Heat loss particularly high in warm-blooded animals.

Transfer

• Transfer typically only about 10% of the energy at one level passes to the next.

Section 11 of 17Pyramids compared

Pyramid	Shows	Shape
Numbers	Number of organisms at each level	May be inverted, for example one tree with many insects
Biomass	Dry mass at each level	Usually narrows upward
Energy	Energy per area per year	Always narrows upward

• A pyramid of energy is always pyramid shaped because energy is lost at each stage.

Section 12 of 17Consequences of energy loss

1Top predators

• Top predators there is less energy at higher trophic levels, so there are fewer top predators.

2Eating plants

• Eating plants eating plants directly is more efficient than eating animals.

3Land use

• Land use crops feed more people per hectare than meat.

4Heat

• Heat energy lost as heat cannot be reused by other organisms, so the Sun must continually supply energy.

Section 13 of 17Efficiency

Percentage captured

• Percentage captured energy stored in new biomass divided by energy received × 100.

Transfer efficiency

• Transfer efficiency energy stored at one level divided by energy at the level below × 100.

Section 14 of 17Improving efficiency in farming

Limit movement

• Limit movement farm animals kept in confined conditions use less energy moving, so more goes into growth.

Keep warm

• Keep warm warm sheds reduce heat loss.

Direct consumption

• Direct consumption growing crops for people rather than for livestock feeds more people.

Section 15 of 17More from the specification

1

Carbon Cycle and Atmospheric Equilibrium

• Carbon dioxide is exchanged between the atmosphere and living organisms via photosynthetic fixation by producers and respiratory release by consumers and decomposers.

2

Nitrogen Cycle in Natural Ecosystems

• Nitrogen-fixing and nitrifying bacteria convert inert atmospheric nitrogen gas into soluble nitrates that living plant populations absorb to synthesize amino acids and proteins.

3

Sigmoid Population Growth Curve

• Natural biological populations exhibit a classic sigmoid growth curve consisting of lag, log, stationary equilibrium,
  • (exponential growth)
• and death phases.

4

Human Impact on Global Atmosphere

• Expanding human populations have intensified deforestation and fossil fuel combustion, driving elevated carbon dioxide levels in the global atmosphere and accelerating climate change.

5

Microorganism

• A living organism so small that it can only be seen clearly with a microscope.

6

Bacteria

• Single-celled microorganisms, much smaller than plant or animal cells.

7

Carbon cycle

• The movement of carbon between air, living things, soil and fossil fuels.

8

Fossil fuel

• Fuel formed from long-dead organisms.

9

Primary consumer

• Organism that eats producers.

10

Secondary consumer

• An organism that eats primary consumers.

11

Population

• A group of organisms of one species in the same area at the same time.

12

Nitrogen

• Element in amino acids and nitrogenous bases.

13

Carbon

• Element with four valence electrons that forms the backbone of organic molecules.

14

Species

• A group of organisms that can reproduce to produce fertile offspring.

Section 16 of 17Key terms

Term	Meaning
Producer	An organism that makes its own food, usually using sunlight.
Consumer	An organism that feeds on other organisms.
Trophic level	The feeding level of an organism in a chain.
Biomass	The mass of living matter, often dry mass.
Pyramid of energy	A diagram of energy stored at each trophic level.
Decomposer	An organism that feeds on dead or waste material.

Term	Meaning
Biomass	The mass of living material, often dry mass.
Trophic level	The feeding position in a food chain.
Pyramid of numbers	A diagram of the number of organisms at each level.
Pyramid of biomass	A diagram of dry mass at each level.
Pyramid of energy	A diagram of energy at each level.
Efficiency	The proportion of energy passed on.

Section 17 of 17Exam technique

1

Saying pyramids of energy can be inverted — they cannot.

2

Saying energy is recycled — it is lost as heat.

3

Saying plants lose energy as urine or faeces — plants do not make these.

4

Saying all energy passes to the next level — only about 10% does.

1

Saying pyramids of numbers are always pyramid shaped — they may be inverted.

2

Saying energy is lost only as heat — it is also lost in wastes and uneaten parts.

3

Saying 10% of energy is lost — about 10% is passed on and about 90% is lost.

4

Saying energy is recycled — nutrients are recycled but energy is not.