5.4.1

Nitrogen Cycle

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Saprobionts

Saprobionts are microorganisms (e.g. bacteria and fungi) that feed on dead or decaying organic matter. Saprobionts play an important role in decomposition and in recycling nutrients.

Importance of nutrient recycling

Importance of nutrient recycling

  • Nutrient recycling is an important process in ecosystems because it allows the nutrients to be continuously reused.
  • Nutrients are recycled in the nitrogen cycle by saprobionts.
Saprobiont nutrition

Saprobiont nutrition

  • Saprobionts secrete enzymes onto the dead organic matter. This is called extracellular digestion.
    • The enzymes break down large, insoluble molecules into smaller, soluble molecules.
    • These smaller molecules can then be absorbed by the saprobiont.
    • The saprobiont will then either store the molecules or use them in respiration.
Role in nitrogen cycle

Role in nitrogen cycle

  • Saprobionts are important in the ammonification stage of the nitrogen cycle.
    • Through extracellular digestion, saprobionts break down organic matter that contains nitrogen (e.g. urea, proteins, nucleic acids).
    • Ammonium ions are released as a result of extracellular digestion.
  • These ions then move onto the next stage of the nitrogen cycle: nitrification.

Nitrogen Cycle

The nitrogen cycle shows how nitrogen is transferred between organisms and their ecosystem. There are four steps in the nitrogen cycle, each of which involves bacteria:

1) Nitrogen fixation

1) Nitrogen fixation

  • Atmospheric nitrogen (N2) is converted to ammonia (NH3) by nitrogen-fixing bacteria in the soil.
  • Bacteria (e.g. Rhizobia) form a mutualistic relationship with leguminous plants by inhabiting their root nodules.
    • Leguminous plants (e.g. alfalfa, peas, beans) are a family of plants.
  • The bacteria provide a source of ammonium ions for the plants and the plant provides sugars.
2) Ammonification

2) Ammonification

  • When an organism dies or produces waste (e.g. faeces), it is decomposed by saprobionts.
  • This releases the nutrients contained inside.
  • Saprobionts decompose the organisms' biomass by extracellular digestion and inorganic ammonium ions are released into the soil.
  • Ammonification is a by-product of saprobiont nutrition.
3) Nitrification

3) Nitrification

  • Ammonium ions that are released from ammonification are converted into nitrates by bacteria in the soil.
    • Nitrifying bacteria (e.g. Nitrosomonas) firstly convert NH4+ ions to nitrites.
    • Another bacteria (e.g. Nitrobacter) then convert nitrites to nitrates.
4) Denitrification

4) Denitrification

  • Denitrifying bacteria convert nitrates in the soil back into atmospheric nitrogen (N2).
  • This takes place in anaerobic conditions (e.g waterlogged soils).
Jump to other topics
1

Biological Molecules

1.1

Monomers & Polymers

1.1.1Monomers & Polymers1.1.2Condensation & Hydrolysis Reactions
1.2

Carbohydrates

1.2.1Structure of Carbohydrates1.2.2Types of Polysaccharides1.2.3End of Topic Test - Monomers, Polymers and Carbs1.2.4Exam-Style Question - Carbohydrates1.2.5A-A* (AO3/4) - Carbohydrates
1.3

Lipids

1.3.1Triglycerides & Phospholipids1.3.2Types of Fatty Acids1.3.3Testing for Lipids1.3.4Exam-Style Question - Fats1.3.5A-A* (AO3/4) - Lipids
1.4

Proteins

1.4.1The Peptide Chain1.4.2Investigating Proteins1.4.3Primary & Secondary Protein Structure1.4.4Tertiary & Quaternary Protein Structure1.4.5Enzymes1.4.6Factors Affecting Enzyme Activity1.4.7Enzyme-Controlled Reactions1.4.8End of Topic Test - Lipids & Proteins1.4.9A-A* (AO3/4) - Enzymes1.4.10A-A* (AO3/4) - Proteins1.4.11Diagnostic Misconceptions - Enzyme Inhibitors
1.5

Nucleic Acids

1.5.1DNA & RNA1.5.2Nucleotides1.5.3Polynucleotides1.5.4DNA Replication1.5.5Exam-Style Question - Nucleic Acids1.5.6A-A* (AO3/4) - Nucleic Acids
1.6

ATP

1.6.1Structure of ATP1.6.2Hydrolysis of ATP1.6.3Resynthesis of ATP1.6.4End of Topic Test - Nucleic Acids & ATP
1.7

Water

1.7.1Importance of Water1.7.2Structure of Water1.7.3Properties of Water1.7.4A-A* (AO3/4) - Water
1.8

Inorganic Ions

1.8.1Inorganic Ions1.8.2End of Topic Test - Water & Inorganic Ions
2

Cells

2.1

Cell Structure

2.1.1Introduction to Cells2.1.2Eukaryotic Cells & Organelles2.1.3Eukaryotic Cells & Organelles 22.1.4Prokaryotes2.1.5A-A* (AO3/4) - Organelles2.1.6Methods of Studying Cells2.1.7Microscopes2.1.8End of Topic Test - Cell Structure2.1.9Exam-Style Question - Cells2.1.10A-A* (AO3/4) - Cells
2.2

Mitosis & Cancer

2.2.1Mitosis2.2.2Stages of Mitosis2.2.3Investigating Mitosis2.2.4Cancer2.2.5A-A* (AO3/4) - The Cell Cycle
2.3

Transport Across Cell Membrane

2.3.1Cell Membrane Structure2.3.2A-A* (AO3/4) - Membrane Structure2.3.3Diffusion2.3.4Osmosis2.3.5Active Transport2.3.6End of Topic Test - Mitosis, Cancer & Transport2.3.7Exam-Style Question - Membranes2.3.8A-A* (AO3/4) - Membranes & Transport2.3.9A-A*- Mitosis, Cancer & Transport
2.4

Cell Recognition & the Immune System

2.4.1Immune System2.4.2Phagocytosis2.4.3T Lymphocytes2.4.4B Lymphocytes2.4.5Antibodies2.4.6Primary & Secondary Response2.4.7Vaccines2.4.8HIV2.4.9Ethical Issues2.4.10End of Topic Test - Immune System2.4.11Exam-Style Question - Immune System2.4.12A-A* (AO3/4) - Immune System2.4.13Diagnostic Misconceptions - Humoral vs Cellular
3

Substance Exchange

3.1

Surface Area to Volume Ratio

3.1.1Size & Surface Area3.1.2A-A* (AO3/4) - Cell Size
3.2

Gas Exchange

3.2.1Single-Celled Organisms3.2.2Multicellular Organisms3.2.3Control of Water Loss3.2.4Human Gas Exchange3.2.5Ventilation3.2.6Dissection3.2.7Measuring Gas Exchange3.2.8Lung Disease3.2.9Lung Disease Data3.2.10End of Topic Test - Gas Exchange3.2.11A-A* (AO3/4) - Gas Exchange
3.3

Digestion & Absorption

3.3.1Overview of Digestion3.3.2Digestion in Mammals3.3.3Absorption3.3.4End of Topic Test - Substance Exchange & Digestion3.3.5A-A* (AO3/4) - Substance Ex & Digestion
3.4

Mass Transport

3.4.1Haemoglobin3.4.2Oxygen Transport3.4.3The Circulatory System3.4.4The Heart3.4.5Blood Vessels3.4.6Cardiovascular Disease3.4.7Heart Dissection3.4.8Xylem3.4.9Phloem3.4.10Investigating Plant Transport3.4.11End of Topic Test - Mass Transport3.4.12A-A* (AO3/4) - Mass Transport3.4.13Diagnostic Misconceptions - Concentration Gradient3.4.14Diagnostic Misconceptions - Cardiac Cycle3.4.15Diagnostic Misconceptions - Carrying Capacity3.4.16Diagnostic Misconceptions - Translocation
4

Genetic Information & Variation

4.1

DNA, Genes & Chromosomes

4.1.1DNA4.1.2Genes4.1.3Non-Coding Genes4.1.4The Genetic Code4.1.5A-A* (AO3/4) - DNA
4.2

DNA & Protein Synthesis

4.2.1Protein Synthesis4.2.2Transcription & Translation4.2.3End of Topic Test - DNA, Genes & Protein Synthesis4.2.4Exam-Style Question - Protein Synthesis4.2.5A-A* (AO3/4) - Coronavirus Translation4.2.6A-A* (AO3/4) - Transcription4.2.7A-A* (AO3/4) - Translation
4.3

Mutations & Meiosis

4.3.1Mutations4.3.2Meiosis4.3.3A-A* (AO3/4) - Meiosis4.3.4Meiosis vs Mitosis4.3.5End of Topic Test - Mutations, Meiosis4.3.6A-A* (AO3/4) - DNA,Genes, CellDiv & ProtSynth4.3.7Diagnostic Misconceptions - Meiosis vs Mitosis
4.4

Genetic Diversity & Adaptation

4.4.1Genetic Diversity4.4.2Natural Selection4.4.3A-A* (AO3/4) - Natural Selection4.4.4Adaptations4.4.5Investigating Natural Selection4.4.6End of Topic Test - Genetic Diversity & Adaptation4.4.7A-A* (AO3/4) - Genetic Diversity & Adaptation
4.5

Species & Taxonomy

4.5.1Courtship Behaviour4.5.2Phylogeny4.5.3Classification4.5.4DNA Technology4.5.5A-A* (AO3/4) - Species & Taxonomy
4.6

Biodiversity Within a Community

4.6.1Biodiversity4.6.2Index of diversity4.6.3Agriculture4.6.4End of Topic Test - Species,Taxonomy& Biodiversity4.6.5A-A* (AO3/4) - Species,Taxon&Biodiversity
4.7

Investigating Diversity

4.7.1Genetic Diversity4.7.2Quantitative Investigation
5

Energy Transfers (A2 only)

5.1

Photosynthesis

5.1.1Overview of Photosynthesis5.1.2Photoionisation of Chlorophyll5.1.3Production of ATP & Reduced NADP5.1.4Cyclic Photophosphorylation5.1.5Light-Independent Reaction5.1.6A-A* (AO3/4) - Photosynthesis Reactions5.1.7Limiting Factors5.1.8Photosynthesis Experiments5.1.9End of Topic Test - Photosynthesis5.1.10A-A* (AO3/4) - Photosynthesis
5.2

Respiration

5.2.1Overview of Respiration5.2.2Anaerobic Respiration5.2.3A-A* (AO3/4) - Anaerobic Respiration5.2.4The Link Reaction5.2.5The Krebs Cycle5.2.6Oxidative Phosphorylation5.2.7Respiration Experiments5.2.8End of Topic Test - Respiration5.2.9A-A* (AO3/4) - Respiration5.2.10Diagnostic Misconceptions - Aerobic vs Anaerobic
5.3

Energy & Ecosystems

5.3.1Biomass5.3.2Production & Productivity5.3.3Agricultural Practices
5.4

Nutrient Cycles

5.4.1Nitrogen Cycle5.4.2Phosphorous Cycle5.4.3Fertilisers & Eutrophication5.4.4End of Topic Test - Nutrient Cycles5.4.5A-A* (AO3/4) - Energy,Ecosystems&NutrientCycles
6

Responding to Change (A2 only)

6.1

Nervous Communication

6.1.1Survival6.1.2Plant Responses6.1.3Animal Responses6.1.4Reflexes6.1.5End of Topic Test - Reflexes, Responses & Survival6.1.6Receptors6.1.7The Human Retina6.1.8Control of Heart Rate6.1.9End of Topic Test - Receptors, Retina & Heart Rate
6.2

Nervous Coordination

6.2.1Neurones6.2.2Action Potentials6.2.3Speed of Transmission6.2.4End of Topic Test - Neurones & Action Potentials6.2.5Synapses6.2.6Types of Synapse6.2.7Medical Application6.2.8End of Topic Test - Synapses6.2.9A-A* (AO3/4) - Nervous Comm&Coord
6.3

Muscle Contraction

6.3.1Skeletal Muscle6.3.2Sliding Filament Theory6.3.3Contraction6.3.4Slow & Fast Twitch Fibres6.3.5End of Topic Test - Muscles6.3.6A-A* (AO3/4) - Muscle Contraction
6.4

Homeostasis

6.4.1Overview of Homeostasis6.4.2Blood Glucose Concentration6.4.3Controlling Blood Glucose Concentration6.4.4End of Topic Test - Blood Glucose6.4.5Primary & Secondary Messengers6.4.6Diabetes Mellitus6.4.7Measuring Glucose Concentration6.4.8Osmoregulation6.4.9Controlling Blood Water Potential6.4.10ADH6.4.11End of Topic Test - Diabetes & Osmoregulation6.4.12A-A* (AO3/4) - Homeostasis6.4.13Diagnostic Misconceptions - Effect of ADH
7

Genetics & Ecosystems (A2 only)

7.1

Genetics

7.1.1Key Terms in Genetics7.1.2Inheritance7.1.3Linkage7.1.4Multiple Alleles & Epistasis7.1.5Chi-Squared Test7.1.6End of Topic Test - Genetics7.1.7A-A* (AO3/4) - Genetics7.1.8Diagnostic Misconceptions - Dihybrid Inheritance7.1.9Diagnostic Misconceptions - Sex Linkage
7.2

Populations

7.2.1Populations7.2.2Hardy-Weinberg Principle
7.3

Evolution

7.3.1Variation7.3.2Natural Selection & Evolution7.3.3End of Topic Test - Populations & Evolution7.3.4Types of Selection7.3.5Types of Selection Summary7.3.6Overview of Speciation7.3.7Causes of Speciation7.3.8Diversity7.3.9End of Topic Test - Selection & Speciation7.3.10A-A* (AO3/4) - Populations & Evolution7.3.11Diagnostic Misconceptions - Types of Speciation
7.4

Populations in Ecosystems

7.4.1Overview of Ecosystems7.4.2Niche7.4.3Population Size7.4.4Investigating Population Size7.4.5End of Topic Test - Ecosystems & Population Size7.4.6Succession7.4.7Conservation7.4.8End of Topic Test - Succession & Conservation7.4.9A-A* (AO3/4) - Ecosystems
8

The Control of Gene Expression (A2 only)

8.1

Mutation

8.1.1Mutations8.1.2Effects of Mutations8.1.3Causes of Mutations8.1.4Diagnostic Misconceptions - Gene Expression
8.2

Gene Expression

8.2.1Stem Cells8.2.2Stem Cells in Disease8.2.3End of Topic Test - Mutation & Gene Epression8.2.4A-A* (AO3/4) - Mutation & Stem Cells8.2.5Regulating Transcription8.2.6Epigenetics8.2.7Epigenetics & Disease8.2.8Regulating Translation8.2.9Experimental Data8.2.10End of Topic Test - Transcription & Translation8.2.11Tumours8.2.12Correlations & Causes8.2.13Prevention & Treatment8.2.14End of Topic Test - Cancer8.2.15A-A* (AO3/4) - Gene Expression & Cancer
8.3

Genome Projects

8.3.1Using Genome Projects
8.4

Gene Technology

8.4.1Recombinant DNA8.4.2Producing Fragments8.4.3Amplification8.4.4End of Topic Test - Genome Project & Amplification8.4.5Using Recombinant DNA8.4.6Medical Diagnosis8.4.7Genetic Fingerprinting8.4.8End of Topic Test - Gene Technologies8.4.9A-A* (AO3/4) - Gene Technology
9

Mathematical Skills

9.1

Mathematical Skills

9.1.1Diagnostic Misconceptions - Converting Units9.1.2Diagnostic Misconceptions - Correct Hypothesis9.1.3Diagnostic Misconceptions - Interpret Hypothesis9.1.4Diagnostic Misconceptions - Calculate Uncertainty

Practice questions on Nitrogen Cycle

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Which of these are NOT activities of saprobionts?Multiple choice
  2. 2
    Which of the following statements about saprobionts are true?True / false
  3. 3
    Stages of the Nitrogen CyclePut in order
  4. 4
    Which of the following is true of the nitrogen cycle?True / false
  5. 5
    Which stage in the nitrogen cycle converts atmospheric nitrogen into ammonia?Multiple choice
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Agricultural Practices

Phosphorous Cycle