You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Microorganisms are organisms that are too small to be seen with the naked eye. This lesson provides a comprehensive overview of the four major groups of microorganisms — bacteria, viruses, fungi and protoctista — as required by the Edexcel A-Level Biology (9BI0) specification. Understanding the structural differences between these groups is essential for explaining how they cause disease and how they can be controlled.
Microorganisms are diverse and do not form a single taxonomic group. The key groups relevant to A-Level Biology are:
| Group | Cell type | Key features |
|---|---|---|
| Bacteria | Prokaryotic | No membrane-bound nucleus; cell wall of peptidoglycan (murein) |
| Viruses | Acellular (not cells) | No cellular structure; contain DNA or RNA within a protein coat |
| Fungi | Eukaryotic | Cell wall of chitin; heterotrophic; may be unicellular (yeast) or multicellular |
| Protoctista | Eukaryotic | Diverse group; some photosynthetic (algae), some parasitic (Plasmodium) |
Bacteria are prokaryotic organisms — they lack a membrane-bound nucleus and membrane-bound organelles.
| Structure | Description | Function |
|---|---|---|
| Cell wall | Made of peptidoglycan (murein) — a polymer of sugars cross-linked by short peptide chains | Maintains cell shape; prevents lysis due to osmotic pressure |
| Cell surface membrane | Phospholipid bilayer | Controls entry and exit of substances; site of some metabolic reactions |
| Capsule (slime layer) | Polysaccharide layer outside the cell wall (not always present) | Protection from phagocytosis; aids adhesion to surfaces |
| Cytoplasm | Gel-like matrix | Site of metabolic reactions |
| Ribosomes (70S) | Smaller than eukaryotic ribosomes (80S) | Protein synthesis |
| Nucleoid | Region containing a single, circular chromosome (DNA) — not enclosed by a membrane | Carries the main genetic information |
| Plasmids | Small, circular, extra-chromosomal DNA molecules | Carry genes for antibiotic resistance, toxin production; can be transferred between bacteria |
| Flagellum (plural: flagella) | Protein filament | Locomotion |
| Pili | Short, hair-like protein projections | Adhesion to surfaces; conjugation (transfer of plasmids between cells) |
| Mesosome | Infolding of the cell membrane (now debated as an artefact) | Historically associated with respiration and cell division |
Bacteria are classified into two major groups based on their cell wall structure, revealed by Gram staining:
| Feature | Gram-positive | Gram-negative |
|---|---|---|
| Peptidoglycan layer | Thick | Thin |
| Outer membrane | Absent | Present (contains lipopolysaccharides/endotoxin) |
| Gram stain colour | Purple/violet | Pink/red |
| Antibiotic susceptibility | Generally more susceptible to penicillin | Often more resistant (outer membrane acts as barrier) |
Exam Tip: Gram-negative bacteria are often harder to treat with antibiotics because their outer membrane provides an additional barrier. This is clinically important and may appear in exam questions about antibiotic resistance.
| Shape | Name | Example |
|---|---|---|
| Spherical | Cocci | Staphylococcus aureus |
| Rod-shaped | Bacilli | Escherichia coli, Mycobacterium tuberculosis |
| Spiral | Spirilla | Treponema pallidum |
| Comma-shaped | Vibrios | Vibrio cholerae |
Viruses are acellular — they are not cells and are not classified as living organisms by most biologists. They are obligate intracellular parasites, meaning they can only replicate inside a host cell.
| Structure | Description |
|---|---|
| Nucleic acid core | Either DNA or RNA (never both); may be single-stranded or double-stranded |
| Capsid | Protein coat surrounding the nucleic acid; composed of capsomeres |
| Envelope (some viruses) | Lipid bilayer derived from the host cell membrane; contains glycoprotein spikes |
| Glycoprotein spikes | Surface proteins used for attachment to host cell receptors |
| Virus | Nucleic acid | Target cells | Disease |
|---|---|---|---|
| HIV | RNA (retrovirus) | T helper cells (CD4⁺) | AIDS |
| Influenza | RNA | Respiratory epithelial cells | Flu |
| SARS-CoV-2 | RNA | Respiratory epithelial cells (ACE2 receptor) | COVID-19 |
| TMV (Tobacco mosaic virus) | RNA | Plant mesophyll cells | Mosaic disease in tobacco |
| Bacteriophage (phage) | DNA | Bacteria | Lysis of bacterial cells |
Exam Tip: Viruses are not killed by antibiotics. Antibiotics target bacterial structures (e.g. cell walls, ribosomes) that viruses do not possess. Antiviral drugs work by inhibiting specific stages of the viral replication cycle.
Fungi are eukaryotic organisms with cell walls made of chitin. They are heterotrophic, obtaining nutrients by saprotrophic (decomposer) or parasitic nutrition.
| Feature | Detail |
|---|---|
| Cell wall | Made of chitin (a polysaccharide of N-acetylglucosamine) |
| Cell membrane | Contains ergosterol (not cholesterol, as in animal cells) |
| Nucleus | Membrane-bound; eukaryotic |
| Organelles | Mitochondria, ER, Golgi, ribosomes (80S) |
| Body form | Unicellular (yeast) or multicellular (moulds and mushrooms) |
| Hyphae | Thread-like filaments forming the body of multicellular fungi; may be septate (divided) or coenocytic (undivided, multinucleate) |
| Mycelium | Network of hyphae |
Saprotrophic fungi secrete extracellular enzymes (exoenzymes) onto dead organic material, digesting it externally and absorbing the soluble products. This makes them crucial decomposers in ecosystems.
Protoctista (also called protists) are a diverse group of eukaryotic organisms that do not fit into the plant, animal, or fungi kingdoms. They include both autotrophic and heterotrophic forms.
| Type | Characteristics | Examples |
|---|---|---|
| Algae (plant-like) | Photosynthetic; contain chloroplasts | Chlorella, Spirogyra, diatoms |
| Protozoa (animal-like) | Heterotrophic; motile | Plasmodium (malaria), Amoeba, Trypanosoma |
Plasmodium is a protoctistan parasite that causes malaria. It has a complex life cycle involving two hosts:
| Stage | Host | Detail |
|---|---|---|
| Sexual reproduction | Female Anopheles mosquito | Gametes form in the mosquito gut; sporozoites migrate to salivary glands |
| Asexual reproduction | Human | Sporozoites infect liver cells, then merozoites infect red blood cells, causing their lysis |
The destruction of red blood cells causes the characteristic symptoms of malaria: fever, chills, anaemia, and fatigue.
| Feature | Bacteria | Viruses | Fungi | Protoctista |
|---|---|---|---|---|
| Cell type | Prokaryotic | Acellular | Eukaryotic | Eukaryotic |
| Nucleus | No (nucleoid) | No | Yes | Yes |
| Cell wall | Peptidoglycan | No (capsid) | Chitin | Varies (cellulose in algae; absent in protozoa) |
| Ribosomes | 70S | None | 80S | 80S |
| Reproduction | Binary fission | Host-dependent replication | Spores / budding | Binary fission / sexual |
| Size range | 0.2–10 µm | 20–300 nm | 2 µm – metres | 1 µm – cm |
| Metabolism | Own metabolism | No own metabolism | Own metabolism | Own metabolism |
| Term | Definition |
|---|---|
| Prokaryote | An organism whose cells lack a membrane-bound nucleus and membrane-bound organelles |
| Obligate intracellular parasite | An organism (e.g. a virus) that can only reproduce inside a living host cell |
| Peptidoglycan | The polymer that forms the bacterial cell wall; also called murein |
| Capsid | The protein coat of a virus |
| Chitin | The polysaccharide that forms the cell wall of fungi |
| Saprotrophic nutrition | Feeding by secreting enzymes onto dead organic matter and absorbing the soluble products |