The Nature and Variety of Living Organisms
Characteristics and Variety of Living Organisms
Pearson Edexcel International GCSE Biology
Characteristics of living organisms
Living organisms require nutrition, respire, excrete their waste, respond to their surroundings, move, control their internal conditions, reproduce, and grow and develop.
Eukaryotic organisms
Eukaryotic cells have a nucleus, have organelles such as mitochondria and chloroplasts, and have chromosomes.
| Group | Features |
|---|---|
| Plants | multicellular; chloroplasts, so they photosynthesise; cellulose cell walls; store carbohydrate as starch or sucrose. Examples: maize, peas, beans |
| Animals | multicellular; no chloroplasts, so they cannot photosynthesise; no cell walls; nervous co-ordination; move from place to place; store carbohydrate as glycogen. Examples: humans, housefly, mosquito |
| Fungi | cannot photosynthesise; a mycelium of thread-like hyphae with many nuclei, some single-celled; cell walls of chitin; saprotrophic nutrition; store carbohydrate as glycogen. Examples: Mucor, yeast (single-celled) |
| Protoctists | microscopic single-celled; some like an animal cell (Amoeba), some with chloroplasts (Chlorella); Plasmodium |
Saprotrophic nutrition in fungi
- Hyphae grow over the food.
- They secrete enzymes onto the food: extracellular digestion.
- Absorption of the soluble products into the hyphae.
- This is saprotrophic nutrition.
Bacteria
- Microscopic single-celled and prokaryotic: no nucleus.
- Structures: cell wall, cell membrane, cytoplasm, a circular chromosome of DNA, plasmids.
- Most feed off other living or dead organisms; some photosynthesise.
- Examples: Lactobacillus bulgaricus, rod-shaped, used to make yoghurt from milk; Pneumococcus, spherical.
Viruses
- Small particles, smaller than bacteria, with no cellular structure.
- A protein coat around one type of nucleic acid: DNA or RNA.
- Not living: viruses do not respire, do not grow, do not feed, do not excrete, do not reproduce independently.
- Parasitic: they reproduce only inside living cells, and infect every type of living organism.
Pathogens
A pathogen causes disease. Pathogens include fungi, bacteria, protoctists and viruses.
| Pathogen | Group | Disease |
|---|---|---|
| Plasmodium | protoctist | malaria |
| Pneumococcus | bacterium | pneumonia |
| HIV | virus | AIDS |
| influenza virus | virus | flu |
| tobacco mosaic virus | virus | discoloured leaves of tobacco plants |
Tobacco mosaic virus prevents chloroplasts forming: less chlorophyll, so discoloured leaves. Never write only less growth for tobacco mosaic virus.
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