Biology · Lesson 4

Survey of organisms: viruses, monera, protista, fungi, plants

JAMBWAECNECO

What the syllabus asks

JAMB: Variety of organisms: viruses, monera, protista, fungi, plants

WAEC: Organisation of life and classification: viruses; Kingdom Monera (bacteria, blue-green algae); Kingdom Protista (Amoeba, Paramecium, Euglena, Plasmodium, Chlamydomonas, Spirogyra); Kingdom Fungi (Rhizopus, yeast, mushroom); Kingdom Plantae (bryophytes, pteridophytes, gymnosperms, angiosperms: monocots and dicots); increasing complexity

NECO: Organisation of life and classification: viruses; Kingdom Monera (bacteria, blue-green algae); Kingdom Protista (Amoeba, Paramecium, Euglena, Plasmodium, Chlamydomonas, Spirogyra); Kingdom Fungi (Rhizopus, yeast, mushroom); Kingdom Plantae (bryophytes, pteridophytes, gymnosperms, angiosperms: monocots and dicots); increasing complexity

What you will learn

State the features of viruses and why they are not placed in any kingdom.
Describe bacteria and blue-green algae as prokaryotes, with examples and importance.
Describe the structure and life processes of named protists.
Describe the features of fungi and the life cycle of Rhizopus.
Compare bryophytes, pteridophytes, gymnosperms and angiosperms.
Distinguish monocots from dicots and describe trends towards complexity.

Key points

Viruses: non-cellular, DNA or RNA in a protein coat, reproduce only in host cells, can be crystallised; not affected by antibiotics.
Monera: prokaryotes (no nuclear membrane); bacteria (cocci, bacilli, vibrios, spirilla) and cyanobacteria (Nostoc, Anabaena).
Amoeba: pseudopodia, contractile vacuole; Paramecium: cilia, macro- and micronucleus.
Euglena: flagellum, eyespot, chloroplasts, no cellulose wall; Plasmodium: parasite without locomotory organelles.
Fungi: hyphae, chitin walls, glycogen, saprophytic or parasitic; Rhizopus (sporangia, zygospores), yeast (budding), mushroom (gills).
Bryophytes: rhizoids, no vascular tissue, gametophyte dominant, need water.
Ferns: vascular tissue, true roots/stems/fronds, spores in sori, sporophyte dominant.
Gymnosperms: naked seeds on cones; angiosperms: flowers and fruits; monocot vs dicot by cotyledons, veins, roots, bundles.

Start here: Viruses: on the border of life

Viruses cause diseases from the common cold to HIV/AIDS, measles, polio, yellow fever, Lassa fever and COVID-19. Yet biologists argue about whether they are alive at all.

Features of viruses
• Extremely small (about 20–300 nm), visible only with an electron microscope
• Not cells: no cytoplasm, no cell membrane, no ribosomes
• Consist of nucleic acid (either DNA or RNA, never both) inside a protein coat (capsid)
• Reproduce only inside living host cells (obligate parasites)
• Can be crystallised and stored like chemicals

Viruses show one key living property, reproduction, but only by taking over a host cell’s machinery. Outside a cell they are inert. For this reason they are placed outside the five kingdoms.

Virus	Disease / host
HIV	AIDS (humans)
Poliovirus	Poliomyelitis
Measles virus	Measles
Lassa virus	Lassa fever (spread by rat urine/droppings)
Tobacco mosaic virus (TMV)	Mosaic disease of tobacco
Cassava mosaic virus	Cassava mosaic disease
Bacteriophage	Infects bacteria

Antibiotics do not kill viruses, because viruses lack the cell walls and ribosomes that antibiotics attack. Viral diseases are prevented with vaccines (polio, measles, yellow fever) and some are controlled with antiviral drugs (e.g. antiretrovirals for HIV).

Worked example 4.1

Give two reasons why viruses are regarded as non-living outside a host.

They show no metabolism (no feeding, respiration or excretion).
They can be crystallised like a chemical substance.

No metabolism; can be crystallised

⚠️ Common mistake: saying "viruses are the smallest bacteria". Viruses are not bacteria and are not cells.

Try it yourself. Why are antibiotics useless against measles?

Show solution

Measles is caused by a virus; antibiotics act on bacterial structures (cell walls, ribosomes) that viruses do not have.

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Practice questions

1. Which organism causes malaria?

Plasmodium
Trypanosoma
Amoeba proteus
Vibrio cholerae

Show answer

A: Plasmodium
A protozoan carried by female Anopheles mosquitoes.

2. Viruses can reproduce only

in soil
in water
by binary fission
inside living host cells

Show answer

D: inside living host cells
They are obligate parasites.

3. Bacteria reproduce mainly by

spore formation only
sexual reproduction
binary fission
budding

Show answer

C: binary fission
The cell divides into two.

4. A useful activity of bacteria is

causing cholera
nitrogen fixation
causing tuberculosis
spoiling food

Show answer

B: nitrogen fixation
Rhizobium in legume nodules.

5. Fungi obtain food

saprophytically or parasitically
by photosynthesis
by chemosynthesis only
by engulfing prey

Show answer

A: saprophytically or parasitically
They secrete enzymes and absorb food.

Key words

Virus: A non-cellular infectious particle of nucleic acid in a protein coat.
Capsid: The protein coat of a virus.
Prokaryote: An organism whose cells lack a true nucleus.
Eukaryote: An organism whose cells have a nucleus bounded by a membrane.
Binary fission: Asexual division of one cell into two.
Pseudopodium: A temporary projection of cytoplasm used for movement and feeding.
Cilia: Short hair-like structures that beat for movement.
Flagellum: A long whip-like structure for movement.
Hypha: A thread-like filament of a fungus.
Mycelium: A mass of hyphae.
Saprophyte: An organism that feeds on dead organic matter.
Rhizoid: A root-like hair that anchors mosses and fungi.

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