Cycles of Nutrients

What are the two main functions of carbon?

It is a structural component of organic molecules, and it is an important element in chemical energy storage.

What reaction includes carbon?

Photosynthesis

How does carbon affect the atmosphere?

As carbon exists as carbon dioxide in the atmosphere, this gas is a greenhouse gas which traps heat on earth

What is anthropogenic activity?

Human activities that alter natural environments, especially by increasing greenhouse gases like CO₂ and CH₄, leading to changes such as global warming, pollution, and habitat loss.

How are fossil fuels made?

When dead organisms are buried in the ground over millions of years they become fossil fuels.

What reaction is responsible for returning carbon into the air?

aerobic resiration

Aerobic equation

C₆H₁₂O (aq) + O₆ (g) = 6CO₂ (g) + 6H₂O (l)

What is combustion?

a process that releases carbon dioxide into the atmosphere when fossil fuels such as coal, wood and hydrocarbons are burnt.

When does carbon fixation occur?

Carbon fixation occurs during the process of photosynthesis when plants remove carbon dioxide from the atmosphere to make carbohydrates. Sunlight provides the energy for this reaction

Photosynthesis Equation

6CO₂ (g) + 6H₂O (l) = C₆H₁₂O₆ (aq) + 6O₂ (g)

The equation when carbon dioxide is very soluble and large amounts are removed from the atmosphere when it dissolves in the oceans:

CO₂ (g) + H₂O (I) = HCO₃⁻ + H⁺ (aq)

The role of organism in the carbon cycle

Three examples

Carbon Cycle diagram

The role of nitrogen

Nitrogen is essential for living organisms because it is needed to make amino acids, peptides, polypeptides, and proteins. Green plants (producers) cannot use atmospheric nitrogen directly, so it must first be converted from dinitrogen gas (N₂) into ammonia (NH₃). Plants then use nitrogen to form proteins, which are passed to consumers through feeding.

Nitrogen exists in both inorganic forms (e.g. ammonia, nitrates) and organic forms (e.g. amino acids and nucleic acids). It is continuously recycled in ecosystems through processes such as nitrogen fixation, nitrification, ammonification, and denitrification, which are mainly carried out by microorganisms.

Nitogen Fixation

Nitrogen fixation is the process that converts atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). It is a multi-step process catalysed by the enzyme nitrogenase.

It is carried out by free-living bacteria such as Azotobacter, symbiotic bacteria such as Rhizobium in root nodules of legumes, and cyanobacteria such as Nostoc.

Lightning may also fix atmospheric nitrogen

During lightning, high-energy discharge causes nitrogen (N₂) to react with oxygen (O₂), forming nitrogen oxides. These dissolve in rainwater to produce weakly acidic solutions, adding nitrates to the soil.

Lighting equation

1. N₂ + 2O₂ → 2NO₂ + H₂O → 2HNO₃

2. N₂ + O₂ → 2NO + H₂O → 2HNO₂

Nitrification

Nitrification is a series of oxidation reactions in which ammonium compounds are converted into nitrates, releasing energy used by bacteria.

Nitrite-forming bacteria (e.g. Nitrosomonas) convert ammonia (NH₃/NH₄⁺) into nitrites (NO₂⁻). Another group of bacteria (e.g. Nitrobacter) convert nitrites into nitrates (NO₃⁻).

Nitrates are the main form of nitrogen absorbed by plants. After absorption, they are converted into ammonium and then used to form amino acids, peptides, polypeptides, and proteins.

Denitrification

Denitrification is the process where denitrifying bacteria convert nitrates (NO₃⁻) into nitrogen gas (N₂) and nitrous oxide (N₂O), releasing them back into the atmosphere.

This is carried out by bacteria such as Pseudomonas and Thiobacillus. It commonly occurs in waterlogged soils where oxygen levels are low and there is a high amount of decomposing organic matter.

Humans and the Nitrogen Cycle

Humans affect the nitrogen cycle by adding or removing nitrogen from ecosystems. The use of nitrogen-based fertilisers increases nitrogen levels in soil and contributes to increased nitrogen fixation. Cultivation of leguminous crops also raises populations of nitrogen-fixing microorganisms in the soil.

Burning fossil fuels releases nitrogen oxides (NO, NO₂), nitrous oxide (N₂O), and ammonia (NH₃) into the atmosphere. In addition, agricultural runoff increases nitrate levels in water bodies, leading to eutrophication, which reduces oxygen levels and harms aquatic organisms.

Important Role of Phosphorus

Phosphorus is an essential element found in energy-rich compounds such as ATP, which is important in cellular energy transfer. It is also a key component of nucleic acids (DNA and RNA), making it vital for genetics.

In vertebrates, about 80% of phosphorus is found in bones and teeth. Phosphorus is often a limiting nutrient in aquatic ecosystems, so its availability affects productivity.

Excess phosphorus in water promotes algal blooms. When algae die, their decomposition by bacteria reduces oxygen levels, leading to water pollution and harm to aquatic life.

Phosphorus cycle and Importance

Sedimentary cycle involving movement of phosphate between rocks, soil, water, and organisms | Required for ATP (energy transfer), DNA/RNA (genetics), and bones/teeth in vertebrates

The largest reservoir of phosphorus

Sedimentary rocks are the largest store of phosphorus

Weathering and release

Rocks release phosphate ions (PO4³⁻) into soil and water through weathering

Plant uptake

Plants absorb phosphates and incorporate them into tissues

Animal uptake

Animals obtain phosphorus through feeding on plants/other animals

Return to soil

Excretion (urine/faeces) and decomposition return phosphates to soil

Decomposition role

Fungi and bacteria break down organic matter releasing phosphate

Sedimentation

Long-term storage of phosphate in ocean sediments

Geological uplift

Sedimentary rocks are exposed on land, continuing the cycle

Human impacts on phosphorus cycle

Fertilisers, mining of phosphate rock, superphosphate production, deforestation, detergents

Agricultural runoff

Excess phosphates washed into water bodies causing eutrophication

Eutrophication

Excess nutrients (especially phosphates) cause rapid algal growth and ecosystem imbalance

Algal bloom

Rapid increase in algae due to nutrient enrichment

Decomposition of algae

Bacteria break down dead algae and consume dissolved oxygen

Oxygen depletion (anoxia)

Loss of dissolved oxygen in water leading to aquatic organism death

Fish kills

Aquatic organisms die due to lack of oxygen

Toxic gas production

Anoxic conditions produce hydrogen sulphide (H₂S), ammonia (NH₃), and thioalcohols (RSH)

Thioalcohols

Organic compounds containing sulphur instead of oxygen causing foul odours

Ammonia toxicity

Un-ionised ammonia (NH₃) is highly toxic; ionised ammonia (NH₄⁺) is less toxic

Effect of pH

High pH increases toxic NH₃; low pH increases NH₄⁺

Environmental effects of eutrophication

Reduced biodiversity, altered species composition, and ecosystem imbalance

Physical effects of eutrophication

Increased turbidity reduces light penetration affecting photosynthesis in aquatic plants

Productivity effects of eutrophication

Reduced oxygen lowers fish yield and overall ecosystem productivity

Recreational impact of eutrophication

Decline in water quality reduces recreational use of water bodies

Water (role)

Covers ~70% of Earth's surface; essential for life and must be continuously recycled and purified

Water cycle (hydrological cycle)

Continuous movement of water between atmosphere, hydrosphere, lithosphere, and biosphere in solid, liquid, and gas states

Main driving force

Solar energy (sun heats water causing evaporation)

Evaporation

Process where liquid water changes to water vapour due to heat

Evapotranspiration

Combined water loss from evaporation and plant transpiration

Transpiration

Water absorbed by plant roots and released from leaves as water vapour

Condensation

Cooling of water vapour in the atmosphere to form clouds or fog

Cloud formation

Tiny water droplets form when water vapour condenses around particles in air

Advection

Movement of water vapour/clouds through the atmosphere; redistributes water globally

Precipitation

Water returns to Earth as rain, snow, sleet, or hail when clouds are saturated

Interception

Precipitation caught by vegetation and re-evaporated before reaching ground

Runoff

Water flowing over land or through channels into rivers, lakes, and oceans

Surface runoff

Water moving over land surface into water bodies

Channel runoff

Water moving through rivers and streams

Infiltration

Process where water enters soil and moves downward

Percolation (groundwater flow)

Downward movement of water through soil and rock to form aquifers

Groundwater flow

Movement of stored water underground through aquifers back to rivers/oceans

Water storage locations

Oceans, rivers, lakes, groundwater, ice caps, atmosphere, organisms

Water cycle importance

Regulates climate, redistributes freshwater, supports ecosystems, and purifies water naturally

Natural purification

Infiltration and percolation filter water as it moves through soil and rock layers