Energy transferred in ecosystemsOxford AQA IGCSE Biology: Revision notes

Energy enters ecosystems through photosynthesis, where plants (producers) convert light energy into chemical energy stored in glucose. This energy is transferred through food chains and food webs as organisms consume one another.

Food chains show a linear sequence of energy transfer:

• Producer → Primary consumer → Secondary consumer → Tertiary consumer
• Example: Grass → Rabbit → Fox

Food webs show multiple interconnected food chains in an ecosystem, reflecting the reality that most organisms eat more than one food source. Food webs are more stable than single food chains because if one organism becomes unavailable, alternative feeding routes exist.

Key points about energy flow:

• Only about 10% of energy is transferred between trophic levels
• The remaining 90% is lost through respiration, heat production, egestion, and excretion
• Energy flow is one-directional (cannot be recycled like nutrients)
• The number of organisms and biomass decreases at each trophic level, creating an energy pyramid

Not all energy consumed by an organism is stored as new biomass. Energy is lost at each trophic level through four main routes:

1. Respiration – Organisms use energy to maintain metabolism, movement, and body temperature. This energy is released as heat and cannot be recaptured by the next trophic level.

2. Heat loss – Warm-blooded animals lose significant energy maintaining body temperature, especially in cold environments.

3. Egestion – Undigested food passes through the gut and is lost in faeces. Only absorbed nutrients are available for growth.

4. Excretion – Nitrogen-containing waste products (urea, ammonia) are excreted and lost from the organism.

Consequences for food chain length:

• Because approximately 90% of energy is lost between trophic levels, only 10% transfers to the next level
• This creates an exponential decline in available energy up the chain
• Most food chains contain only 3–5 trophic levels because energy becomes too limited to support further organisms
• Shorter food chains are more efficient and support larger populations
• Top carnivores (apex predators) have the smallest populations because they receive the least energy

Energy efficiency is the percentage of energy from one trophic level that is transferred to the next trophic level.

Formula:

Energy efficiency (%) = (Energy in trophic level / Energy in previous trophic level) × 100

Step-by-step calculation:

1. Identify the energy values for two consecutive trophic levels
2. Divide the energy of the higher trophic level by the energy of the lower trophic level
3. Multiply by 100 to convert to a percentage

Example:

• Grass (producer) contains 10,000 kJ of energy
• Rabbits (primary consumer) contain 1,000 kJ of energy
• Energy efficiency = (1,000 ÷ 10,000) × 100 = 10%

This means only 10% of the energy from grass is available to rabbits; 90% is lost through respiration, heat, egestion, and excretion.

Why efficiency is typically around 10%:

• Most organisms use 50–90% of absorbed energy for respiration
• Only 10–20% of absorbed energy is incorporated into new biomass
• The remaining absorbed energy is lost as heat and in excretion

Alternative calculation method (using biomass): If given biomass instead of energy, use the same formula with biomass values, assuming approximately 20 kJ of energy per gram of biomass in animal tissue.

The carbon cycle describes the movement of carbon between the atmosphere, living organisms, and the Earth's crust. Carbon exists mainly as carbon dioxide (CO₂) in the atmosphere and is recycled continuously through four key processes:

1. Photosynthesis

• Plants absorb CO₂ from the atmosphere through stomata
• Light energy converts CO₂ and water into glucose (organic compounds)
• Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• This removes carbon from the atmosphere and fixes it into organic matter

2. Respiration

• All living organisms (plants, animals, decomposers) break down glucose to release energy
• This oxidises organic compounds back to CO₂, which is released into the atmosphere
• Occurs in all organisms 24 hours a day
• Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

3. Decomposition

• Dead organisms and waste products are broken down by decomposers (bacteria and fungi)
• Organic carbon is oxidised back to CO₂ through respiration by decomposers
• Returns carbon to the soil and atmosphere relatively quickly (weeks to years)
• Critical for nutrient recycling in ecosystems

4. Combustion (Burning)

• Burning of fossil fuels (coal, oil, natural gas) releases carbon stored for millions of years
• Also includes burning of wood and other organic material
• Rapidly releases large quantities of CO₂ into the atmosphere
• Contributes significantly to increasing atmospheric CO₂ levels

5. Fossilisation

• Dead organisms buried under sediment over millions of years form fossil fuels
• Carbon becomes locked in rock layers (long-term storage)
• When burned, releases ancient carbon into the atmosphere

The carbon cycle summary: Atmosphere (CO₂) ↔ Living organisms (organic compounds) ↔ Decomposers and soil ↔ Fossil fuels (fossilisation)

The nitrogen cycle describes how nitrogen moves between the atmosphere, soil, and living organisms. Although nitrogen (N₂) makes up 78% of the atmosphere, most organisms cannot use it directly. Bacteria play essential roles in converting nitrogen between different chemical forms.

Key processes in the nitrogen cycle:

1. Nitrogen Fixation

• Nitrogen-fixing bacteria (free-living in soil or symbiotic in root nodules) convert atmospheric N₂ into ammonia (NH₃)
• Examples: Rhizobium (in legume root nodules), Azotobacter (free-living)
• This is the only way atmospheric nitrogen enters the biological cycle
• Energy-expensive process, requires ATP
• Essential because most organisms cannot utilise N₂ directly

2. Nitrification

• Nitrifying bacteria in soil oxidise ammonia → nitrite (NO₂⁻) → nitrate (NO₃⁻)
• Two stages:
  • Nitrosomonas converts NH₃ → NO₂⁻
  • Nitrobacter converts NO₂⁻ → NO₃⁻
• Nitrate is the form plants can absorb through roots
• Occurs in aerobic soil conditions

3. Assimilation

• Plants absorb nitrate from soil through roots
• Nitrate is reduced back to ammonia and incorporated into amino acids and proteins
• Animals obtain nitrogen by eating plants (or other animals)
• Nitrogen becomes part of proteins, DNA, and other organic compounds

4. Decomposition

• Dead organisms, faeces, and urine contain nitrogen (mainly as proteins and nucleic acids)
• Decomposer bacteria and fungi break down these materials
• Organic nitrogen is converted back to ammonia through ammonification
• Ammonia can be absorbed by plants or further oxidised by nitrifying bacteria

5. Denitrification

• Denitrifying bacteria (anaerobic bacteria in waterlogged soil) break down nitrate
• Nitrate is reduced back to nitrogen gas (N₂) and released into the atmosphere
• Occurs in oxygen-poor conditions (e.g. waterlogged soils, sediments)
• Returns nitrogen to the atmosphere, completing the cycle
• Reduces the availability of nitrogen in soil

Summary table of bacterial roles:

Why the nitrogen cycle is important:

• Nitrogen is essential for synthesising proteins and nucleic acids
• Without bacteria, plants could not access atmospheric nitrogen
• Agricultural practice (adding legumes to crop rotation) harnesses nitrogen-fixing bacteria to improve soil fertility

The water cycle (hydrological cycle) describes the continuous movement of water between the atmosphere, land, and bodies of water. Water changes between solid, liquid, and gaseous states as it moves through the cycle.

Key processes in the water cycle:

1. Evaporation

• Water from oceans, lakes, and rivers is heated by the Sun's energy
• Liquid water converts into water vapour (gaseous state)
• Rises into the atmosphere carrying thermal energy
• Increases with temperature, wind speed, and surface area
• Removes water from aquatic and terrestrial environments

2. Transpiration

• Plants absorb water through roots and transport it through xylem
• Water is released as water vapour through stomata (small pores in leaves)
• Functions: cooling the plant, transporting dissolved minerals, maintaining turgor pressure
• Increases with temperature, light intensity, wind speed, and humidity
• Together with evaporation, called evapotranspiration

3. Condensation

• Water vapour in the atmosphere cools and converts back into liquid water droplets
• Occurs when air is cooled below its dew point temperature
• Droplets form around microscopic dust particles (condensation nuclei)
• Forms clouds and mist
• Releases latent heat energy back into the atmosphere

4. Precipitation

• Water droplets in clouds combine and become heavy enough to fall
• Falls as rain (most common), snow, sleet, or hail
• Returns water to land and oceans
• Amount depends on atmospheric moisture and temperature
• On land, infiltrates soil or flows as runoff to water bodies

5. Collection

• Water collects in oceans, lakes, rivers, and groundwater stores
• Groundwater is stored in soil and rock layers (aquifers)
• Water remains in these stores until evaporation or transpiration returns it to the atmosphere
• Some water infiltrates deep into rock layers (slow movement)

The water cycle in summary:

Evaporation (land and water) + Transpiration (plants) → Condensation (atmosphere) → Precipitation (rain/snow) → Collection (oceans, lakes, soil) → Evaporation (cycle repeats)

Key points:

• The cycle is continuous and driven by the Sun's energy
• Water changes physical state but not chemical composition
• Both living organisms (transpiration) and physical processes (evaporation) are essential
• Groundwater storage is important for maintaining water availability during dry periods
• Human activities (deforestation, damming) can disrupt the natural water cycle