1 The nature and variety of living organisms
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(a) Characteristics of living organisms
1.1
Understand how living organisms share the following characteristics: • they require nutrition • they respire • they excrete their waste • they respond to their surroundings • they move • they control their internal conditions • they reproduce • they grow and develop.
(b) Variety of living organisms
1.2Eukaryotic organism features
Describe the common features of eukaryotic organisms: plants, animals, fungi and protoctists. Plants are multicellular; their cells contain chloroplasts, carry out photosynthesis and have cellulose cell walls; they store carbohydrate as starch or sucrose. Examples include cereals such as maize and herbaceous legumes such as peas or beans. Animals are multicellular; their cells lack chloroplasts and cell walls and cannot photosynthesise; they usually have nervous co-ordination, can move from place to place and often store carbohydrate as glycogen. Examples include mammals such as humans and insects such as houseflies and mosquitoes. Fungi cannot photosynthesise; their bodies are usually mycelia of thread-like, multinucleate hyphae, although some are single-celled; their cell walls contain chitin; they use saprotrophic nutrition by secreting digestive enzymes and absorbing the products, and may store glycogen. Examples include Mucor and yeast. Protoctists are microscopic single-celled organisms. Amoeba has animal-cell-like features, Chlorella has chloroplasts and plant-like features, and the pathogen Plasmodium causes malaria.
1.3Prokaryotic organism features
Describe the common features of prokaryotic organisms such as bacteria. Bacteria are microscopic and single-celled. They have a cell wall, cell membrane, cytoplasm and plasmids, but no nucleus; their DNA forms a circular chromosome. Some photosynthesise, but most feed on living or dead organisms. Examples include rod-shaped Lactobacillus bulgaricus, used to make yoghurt, and spherical Pneumococcus, which causes pneumonia.
1.4Pathogens
Understand that a pathogen causes disease and may be a fungus, bacterium, protoctist or virus. Viruses are non-living particles smaller than bacteria. They are parasites that reproduce only inside living cells and can infect every type of organism. They have no cellular structure, but have a protein coat and contain either DNA or RNA. Examples include tobacco mosaic virus, which prevents chloroplast formation and discolours tobacco leaves; influenza virus; and HIV, which causes AIDS.