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Ordinary Level Revision Notes
Leaving Cert Ordinary Level Biology
Chapter 12: Diversity of Organisms
Microorganisms & Fungi
Subtopics Covered
- Learning Outcomes
- Big Picture: Microorganism Groups
- Classification Overview
- Monera: Bacteria Structure
- Endospore Formation
- Bacterial Growth Curve
What This Pack Includes
- Structured chapter notes formatted for ExamsLogic website reading
- Exam-focused diagrams, definitions, and worked examples
- Interactive practice sections carried over from the source notes
- Independent study guidance based on the official curriculum
- Print-friendly layout for future PDF export when needed
This publication is an independent educational resource developed by ExamsLogic and compiled by experienced educators. It is based on publicly available official curricula, including Cambridge, Pearson Edexcel, IB, and the Irish Leaving Certificate. This product is not endorsed by, affiliated with, or sponsored by any examination board or governing authority. All registered trademarks remain the property of their respective owners.
1. Learning Outcomes
- Describe bacteria as members of Monera and identify their main structures.
- Explain bacterial features such as flagella, plasmid DNA and endospores.
- Describe the four stages of a bacterial growth curve: lag, log, stationary and decline.
- Compare batch and continuous food processing.
- Describe fungi using Rhizopus and yeast as examples.
- Distinguish saprophytic and parasitic nutrition.
- Compare asexual and sexual reproduction in fungi at Ordinary Level standard.
- Describe amoeba structure, pseudopodia movement and contractile vacuole osmoregulation.
- Describe viruses as non-cellular particles made of a protein coat and nucleic acid.
- Outline viral replication and explain medical and economic importance of microorganisms.
OL Focus: This chapter is about recognising structures, comparing organism groups, and explaining life processes in simple exam language.
2. Big Picture: Microorganism Groups
Bacteria
Single-celled organisms without a true nucleus. Some are useful, while others cause disease.
Fungi
Organisms such as moulds and yeast. They feed by absorbing digested food.
Protista + Viruses
Amoeba is a single-celled protist. Viruses are non-cellular particles that reproduce only inside host cells.
Biology link: Microorganisms are small, but their effects are massive: food production, decomposition, disease, biotechnology and ecosystems all depend on them.
3. Classification Overview
| Group | Example | Main features |
|---|---|---|
| Monera | Bacteria | Single-celled, no true nucleus, DNA not enclosed in a nucleus. |
| Fungi | Rhizopus, yeast | No chlorophyll, absorb food, cell walls present. |
| Protista | Amoeba | Single-celled eukaryote with nucleus and organelles. |
| Viruses | Influenza virus, bacteriophage | Non-cellular; protein coat and nucleic acid; reproduce inside host cells. |
Common Mistake: Calling viruses cells. Viruses are not cells. They are non-cellular particles and need a host cell to reproduce.
4. Monera: Bacteria Structure
Bacteria are very small single-celled organisms. They do not have a true nucleus. Their DNA is found free in the cytoplasm, often with extra small DNA loops called plasmids.
| Structure | Function |
|---|---|
| Cell wall | Gives shape and protection. |
| Cell membrane | Controls movement of substances in and out. |
| Cytoplasm | Site of many chemical reactions. |
| Chromosomal DNA | Main genetic material. |
| Plasmid DNA | Small circular DNA that may carry useful genes, such as antibiotic resistance genes. |
| Flagellum | Used for movement in some bacteria. |
| Capsule/slime layer | Extra protection in some bacteria. |
Exam Tip: When drawing bacteria, include cell wall, cell membrane, cytoplasm, DNA, plasmids and possibly flagellum.
5. Endospore Formation
Some bacteria can form endospores when conditions become unfavourable. An endospore is a resistant survival structure.
| Condition | Why endospores help |
|---|---|
| Lack of food | Allows bacteria to survive until food returns. |
| Dry conditions | Protects genetic material from damage. |
| High temperature or harsh chemicals | Endospores are more resistant than normal bacterial cells. |
Unfavourable conditions→
Endospore forms→
Bacterium survives→
Conditions improve→
Growth resumes
Examiner Secret: Endospores are for survival, not normal reproduction.
6. Bacterial Growth Curve
Bacteria reproduce rapidly by binary fission when conditions are suitable. Their population growth often follows four phases.
| Phase | What happens? | Reason |
|---|---|---|
| Lag phase | Little increase in number. | Bacteria adjust to conditions and make enzymes. |
| Log phase | Rapid exponential growth. | Plenty of nutrients and favourable conditions. |
| Stationary phase | Population size levels off. | Food becomes limited and waste builds up. |
| Decline phase | Number of living bacteria decreases. | Nutrients run out and toxic waste increases. |
Examiner Trap: The log phase is the fastest growth phase, not the lag phase.
7. Batch vs Continuous Food Processing
Microorganisms are used in food and industrial processing. Two common methods are batch processing and continuous processing.
| Feature | Batch processing | Continuous processing |
|---|---|---|
| Method | All ingredients added at the start; product removed at the end. | Fresh nutrients are added continuously and product is removed continuously. |
| Control | Easier to clean and control between batches. | Needs careful constant monitoring. |
| Use | Useful for smaller or specific batches. | Useful for large-scale constant production. |
| Risk | If contaminated, one batch may be lost. | Contamination can affect a large amount of product. |
Exam Tip: Batch = start and finish as one batch. Continuous = nutrients in and product out all the time.
8. Fungi: Main Features
Fungi are organisms that do not have chlorophyll. They cannot photosynthesise, so they feed by absorbing nutrients from other materials.
| Feature | Fungi |
|---|---|
| Cell type | Eukaryotic cells with nuclei. |
| Cell wall | Present. |
| Chlorophyll | Absent. |
| Nutrition | Absorptive nutrition after external digestion. |
| Examples | Rhizopus mould, yeast, mushrooms. |
Biology link: Fungi are important decomposers because they break down dead organic material and recycle nutrients.
9. Rhizopus Structure
Rhizopus is a mould that grows as thread-like structures called hyphae. A mass of hyphae is called mycelium.
| Structure | Function |
|---|---|
| Hyphae | Thread-like filaments that grow through food. |
| Mycelium | Network of hyphae. |
| Sporangium | Produces spores for reproduction. |
| Spores | Dispersal and reproduction structures. |
| Rhizoids | Anchor the fungus and absorb digested food. |
Common Mistake: Saying Rhizopus has roots. Rhizoids are not true roots; they anchor and absorb digested food.
10. Yeast Structure and Importance
Yeast is a single-celled fungus. It is important in baking and brewing because it can ferment sugars.
| Yeast structure | Function |
|---|---|
| Cell wall | Protection and shape. |
| Cell membrane | Controls entry and exit of substances. |
| Cytoplasm | Site of cell reactions. |
| Nucleus | Contains genetic material. |
| Vacuole | Storage and cell support. |
| Bud | New yeast cell forming during asexual reproduction. |
11. Fungal Nutrition: Saprophytic vs Parasitic
| Type of nutrition | Meaning | Example |
|---|---|---|
| Saprophytic | Feeds on dead organic material. | Rhizopus growing on bread. |
| Parasitic | Feeds on a living host and may harm it. | Fungal infections in plants or animals. |
Fungus releases enzymes→
Food digested outside body→
Soluble nutrients absorbed
Exam Tip: Fungi digest food outside their bodies using enzymes, then absorb the soluble products.
12. Fungal Reproduction
| Reproduction type | How it happens | Example |
|---|---|---|
| Asexual reproduction | One parent produces offspring without gamete fusion. | Yeast budding; Rhizopus spore production. |
| Sexual reproduction | Genetic material from two compatible individuals combines. | Some fungi produce sexual spores. |
Examiner Secret: Yeast budding is a very common example of asexual reproduction in fungi.
13. Protista: Amoeba Anatomy
Amoeba is a single-celled protist. It has no fixed shape and moves using pseudopodia.
| Structure | Function |
|---|---|
| Cell membrane | Controls movement of substances in and out. |
| Cytoplasm | Contains cell contents and reactions. |
| Nucleus | Controls cell activities and contains genetic material. |
| Pseudopodia | Temporary projections used for movement and feeding. |
| Food vacuole | Contains food being digested. |
| Contractile vacuole | Removes excess water for osmoregulation. |
14. Amoeba Movement, Feeding and Osmoregulation
| Process | Explanation |
|---|---|
| Pseudopodia movement | Amoeba extends part of its cytoplasm to form a temporary projection, then flows into it. |
| Feeding | Pseudopodia surround food, forming a food vacuole. |
| Osmoregulation | The contractile vacuole collects excess water and expels it from the cell. |
Pseudopodia extend→
Food surrounded→
Food vacuole forms→
Food digested
Examiner Trap: The contractile vacuole is not mainly for food digestion. It removes excess water.
15. Viruses: Non-Cellular Particles
Viruses are not classified as cells. They are non-cellular particles made of nucleic acid surrounded by a protein coat. They can reproduce only inside living host cells.
| Virus component | Function |
|---|---|
| Protein coat / capsid | Protects the genetic material. |
| Nucleic acid | Genetic material, either DNA or RNA depending on the virus. |
| Attachment proteins | Help virus attach to a host cell. |
Examiner Secret: Viruses are often described as non-living outside host cells because they cannot carry out normal life processes independently.
16. Viral Replication Cycle
Viruses reproduce by infecting host cells and using the host machinery to make new virus particles.
Attachment→
Entry/injection→
Host makes viral parts→
Assembly→
Release
| Stage | What happens? |
|---|---|
| Attachment | Virus attaches to a specific host cell. |
| Entry / injection | Viral genetic material enters the host cell. |
| Replication | The host cell makes viral nucleic acid and proteins. |
| Assembly | New virus particles are put together. |
| Release | New viruses leave and may infect other cells. |
Exam Tip: A virus must enter or use a host cell to reproduce. That is the key difference from bacteria and fungi.
17. Medical and Economic Importance
| Organism group | Useful importance | Harmful importance |
|---|---|---|
| Bacteria | Yoghurt, cheese, decomposition, biotechnology. | Food poisoning, disease, spoilage. |
| Fungi | Bread, brewing, decomposition, antibiotics. | Food spoilage, crop disease, infections. |
| Protista | Food chains and research examples. | Some cause disease. |
| Viruses | Research, vaccines, gene technology tools. | Human, animal and plant diseases. |
Exam Tip: Always give a named example if asked for importance, such as yeast in bread making or bacteria in yoghurt production.
Examiner Trap: Antibiotics affect bacteria, not viruses. Viral diseases usually require prevention, vaccines or antiviral treatments.
18. Interactive Simulator: Organism Group Detective
Identify the Group
Clue: Non-cellular particle with protein coat and nucleic acid.
Feedback
Ready
Use cell type, structures and reproduction method to identify the group.
19. Interactive Simulator: Growth Curve Phase Checker
Choose the Phase
Description: Bacteria divide rapidly because nutrients are plentiful.
Feedback
Growth Curve
Remember: Lag adjusts, Log grows fast, Stationary levels off, Decline decreases.
20. High-Value Exam Guidance
Exam Tip 1: Bacteria have DNA but no true nucleus.
Exam Tip 2: Plasmids are small circular pieces of DNA in bacteria.
Exam Tip 3: Label bacterial growth curve phases in order: Lag, Log, Stationary, Decline.
Exam Tip 4: Yeast is a single-celled fungus and reproduces by budding.
Examiner Trap 1: Viruses are not bacteria. Antibiotics do not treat viral infections.
Examiner Trap 2: Amoeba pseudopodia are for movement and feeding, not photosynthesis.
Examiner Trap 3: Endospores are survival structures, not ordinary spores for fungal reproduction.
Common Mistake 1: Saying fungi photosynthesise. Fungi lack chlorophyll.
Common Mistake 2: Confusing log and stationary phases in bacterial growth.
Examiner Secret 1: Saprophytic nutrition is linked to decomposition and recycling nutrients.
Examiner Secret 2: For virus questions, say “protein coat and nucleic acid” early.
Examiner Secret 3: Contractile vacuole = osmoregulation. Food vacuole = digestion.
21. MCQs with Instant Answers
1. Bacteria belong to the group:
Answer: A. Bacteria are classified in Monera.
2. Plasmids are:
Answer: A. Plasmids are small circular DNA molecules found in bacteria.
3. The fastest bacterial population increase occurs during:
Answer: B. The log phase has rapid exponential growth.
4. Yeast reproduces asexually by:
Answer: A. Yeast commonly reproduces by budding.
5. Saprophytic nutrition means feeding on:
Answer: A. Saprophytes feed on dead organic matter.
6. Amoeba moves using:
Answer: B. Amoeba moves using pseudopodia.
7. The contractile vacuole in Amoeba is mainly used for:
Answer: A. The contractile vacuole removes excess water for osmoregulation.
8. Viruses are made mainly of:
Answer: A. A virus has nucleic acid enclosed in a protein coat.
9. Continuous processing means:
Answer: A. In continuous processing, nutrients are added and products removed continuously.
10. Rhizopus produces spores in:
Answer: A. Rhizopus produces spores in sporangia.
22. Structured Exam Questions
- Draw and label a bacterial cell. Include at least five labels. [6]
- Explain the functions of plasmid DNA and flagella in bacteria. [4]
- Describe endospore formation and explain its importance. [4]
- Describe the four phases of a bacterial growth curve. [8]
- Compare batch and continuous food processing. [6]
- Describe the structure of Rhizopus. [6]
- Compare saprophytic and parasitic nutrition in fungi. [4]
- Describe asexual reproduction in yeast. [4]
- Describe Amoeba movement and feeding using pseudopodia. [5]
- Explain the role of the contractile vacuole in Amoeba. [3]
- Describe the basic structure of a virus. [4]
- Outline the viral replication cycle. [5]
- Give two useful and two harmful effects of microorganisms. [8]
23. Detailed Mark Scheme
Q1. Correct bacterial shape [1]. Labels: cell wall [1], cell membrane [1], cytoplasm [1], DNA [1], plasmid [1], flagellum/capsule [1]. Max 6.
Q2. Plasmid is small circular DNA [1] that may carry useful genes such as antibiotic resistance [1]. Flagellum helps movement [2].
Q3. Endospore is a resistant survival structure [1]. Formed in unfavourable conditions [1]. Protects DNA/cell contents [1]. Allows bacterium to survive until conditions improve [1].
Q4. Lag: little growth/adjusting [2]. Log: rapid exponential growth [2]. Stationary: birth rate equals death rate/nutrients limited/waste builds [2]. Decline: death rate exceeds reproduction/nutrients exhausted/toxic waste [2].
Q5. Batch: ingredients added at start and product removed at end [2]. Continuous: nutrients added and products removed continuously [2]. One valid comparison advantage/disadvantage [2].
Q6. Rhizopus has hyphae [1], mycelium [1], sporangia [1], spores [1], rhizoids [1], absorbs digested food/grows on organic material [1].
Q7. Saprophytic feeds on dead organic matter [2]. Parasitic feeds on living host and may harm it [2].
Q8. Yeast reproduces by budding [1]. A small bud grows from parent cell [1]. Nucleus divides/genetic material copied [1]. Bud separates to form new cell [1].
Q9. Amoeba extends pseudopodia [1], cytoplasm flows into them for movement [1]. Pseudopodia surround food [1]. Food vacuole forms [1]. Food is digested inside vacuole [1].
Q10. Contractile vacuole collects excess water [1], expels it from the cell [1], maintaining water balance/osmoregulation [1].
Q11. Virus is non-cellular [1]. Has protein coat/capsid [1]. Contains nucleic acid/DNA or RNA [1]. May have attachment proteins [1].
Q12. Attachment to host [1], entry/injection of genetic material [1], viral parts copied by host [1], assembly of new viruses [1], release from host cell [1].
Q13. Useful: food production/decomposition/antibiotics/biotechnology [1 each, max 4]. Harmful: disease/food spoilage/crop disease/food poisoning [1 each, max 4].
Q2. Plasmid is small circular DNA [1] that may carry useful genes such as antibiotic resistance [1]. Flagellum helps movement [2].
Q3. Endospore is a resistant survival structure [1]. Formed in unfavourable conditions [1]. Protects DNA/cell contents [1]. Allows bacterium to survive until conditions improve [1].
Q4. Lag: little growth/adjusting [2]. Log: rapid exponential growth [2]. Stationary: birth rate equals death rate/nutrients limited/waste builds [2]. Decline: death rate exceeds reproduction/nutrients exhausted/toxic waste [2].
Q5. Batch: ingredients added at start and product removed at end [2]. Continuous: nutrients added and products removed continuously [2]. One valid comparison advantage/disadvantage [2].
Q6. Rhizopus has hyphae [1], mycelium [1], sporangia [1], spores [1], rhizoids [1], absorbs digested food/grows on organic material [1].
Q7. Saprophytic feeds on dead organic matter [2]. Parasitic feeds on living host and may harm it [2].
Q8. Yeast reproduces by budding [1]. A small bud grows from parent cell [1]. Nucleus divides/genetic material copied [1]. Bud separates to form new cell [1].
Q9. Amoeba extends pseudopodia [1], cytoplasm flows into them for movement [1]. Pseudopodia surround food [1]. Food vacuole forms [1]. Food is digested inside vacuole [1].
Q10. Contractile vacuole collects excess water [1], expels it from the cell [1], maintaining water balance/osmoregulation [1].
Q11. Virus is non-cellular [1]. Has protein coat/capsid [1]. Contains nucleic acid/DNA or RNA [1]. May have attachment proteins [1].
Q12. Attachment to host [1], entry/injection of genetic material [1], viral parts copied by host [1], assembly of new viruses [1], release from host cell [1].
Q13. Useful: food production/decomposition/antibiotics/biotechnology [1 each, max 4]. Harmful: disease/food spoilage/crop disease/food poisoning [1 each, max 4].
24. Mastery Checklist
- I can describe and label a bacterial cell.
- I can explain the role of plasmids and flagella.
- I can explain endospore formation.
- I can describe the four phases of bacterial growth.
- I can compare batch and continuous processing.
- I can describe Rhizopus and yeast structure.
- I can distinguish saprophytic and parasitic nutrition.
- I can explain yeast budding.
- I can label Amoeba and explain pseudopodia movement.
- I can explain osmoregulation using the contractile vacuole.
- I can describe virus structure.
- I can outline viral replication.
- I can give useful and harmful examples of microorganisms.