| 1.2 Variety of living organisms |
|---|
The living world is classified into major groups based on cell structure and how organisms obtain their food.
Plants
- Are multicellular
- Photosynthesise using chloroplasts
- Have cellulose cell walls
- Store carbohydrates as starch or sucrose
Animals
- Are multicellular
- Lack chloroplasts and cell walls, so cannot photosynthesise
- Have nervous co-ordination and can move from place to place
- Store carbohydrate as glycogen
Fungi
- Cannot photosynthesise
- Have cell walls made of chitin
- Feed by saprotrophic nutrition (secreting digestive enzymes onto food material and absorbing the products)
- May be single-celled (e.g. yeast) or multicellular with a body (mycelium) made of thread-like hyphae
- May store carbohydrate as glycogen
Protoctists
- Are microscopic single-celled eukaryotes
- May resemble plant cells (e.g. Chlorella, which has chloroplasts) or animal cells (e.g. Amoeba)
- A few are pathogenic, for example Plasmodium, which causes malaria
Bacteria
- Are microscopic single-celled prokaryotes — they lack a true nucleus
- Have a circular chromosome of DNA, plasmids, and a cell wall
- Some can photosynthesise, but most feed on other living or dead organisms
Viruses
- Are not living organisms
- Have no cellular structure
- Possess only a protein coat surrounding one type of nucleic acid (DNA or RNA)
- Can reproduce only inside living host cells (they are parasitic)
Key Definition A pathogen is any organism or agent that causes disease, and pathogens can be found among fungi, bacteria, protoctists, and viruses.
At a Glance
| Group | Cell Type | Cell Wall | Nutrition | Storage / Key Feature |
|---|---|---|---|---|
| Plants | Multicellular | Cellulose cell walls | Photosynthesise using chloroplasts | Store carbohydrates as starch or sucrose |
| Animals | Multicellular | No cell walls | Cannot photosynthesise (no chloroplasts); feed on other organisms | Store glycogen; have nervous co-ordination |
| Fungi | Single-celled (e.g. yeast) or multicellular with a hyphal mycelium | Chitin cell walls | Feed by saprotrophic nutrition; cannot photosynthesise | May store carbohydrate as glycogen |
| Protoctists | Single-celled eukaryotes | Variable — present in plant-like (e.g. Chlorella), absent in animal-like (e.g. Amoeba) | May resemble plant cells or animal cells | Plasmodium is a pathogenic example, causing malaria |
| Bacteria | Single-celled prokaryotes — lack a true nucleus | Have a cell wall | Some photosynthesise but most feed on other organisms | Have a circular chromosome and plasmids |
| Viruses | No cellular structure | Absent (no cellular structure) | None — non-living and acellular | Possess only a protein coat and one type of nucleic acid (DNA or RNA); can reproduce only inside living cells |
Understanding the features that define each group is essential for classification questions and for linking organisms to the diseases they cause.