The nature and variety of living organisms

• Living organisms share the following characteristic: Require nutrition, respire, excrete their waste, respond to their surroundings, move, control their internal conditions, reproduce, grow and develop.
• A eukaryotic organism is an organism that contains a nucleus and other membrane-bound organelles.
• Plants are multicellular eukaryotes. They have organelles called chloroplasts and cell walls made of cellulose. Plants have specialised reproductive organs. Plants produce food through photosynthesis.
• Animals are multicellular eukaryotes that lack cell walls. All animals are heterotrophs. Animals have sensory organs, the ability to move and internal digestion. Animals have sexual reproduction.
• Heterotroph: an organism that obtains energy and nutrients from other organisms.
• Autotroph: An organism that makes its own food using energy from the sun.
• Fungi are eukaryotic, non-vascular, non-motile and heterotrophic organisms. They are either unicellular or filamentous. Fungi reproduce by means of spores. Fungi exhibit the phenomenon of alternation of generation. Fungi lack chlorophyll and cannot perform photosynthesis.
• Non-vascular: lacking blood vessels or a vascular system.
• Non-motile: not able to move by itself.
• Filamentous: An elongated, thin series of cells attached one to another or a very long thin cylindrical single cell.
• Protoctists are eukaryotic (have a nucleus). Most have mitochondria. They can be parasites and they all prefer aquatic or moist environments.
• Plants: these are multicellular organisms; their cells contain chloroplasts and are able to carry out photosynthesis; their cells have cellulose cell walls; they store carbohydrates as starch or sucrose. Examples include flowering plants, such as cereal (eg: maize) and herbaceous legume (eg: peas or beans).
• Animals: these are multicellular organisms; their cells do not contain chloroplasts and are not able to carry out photosynthesis; they have no cell walls; they usually have nervous co-ordination and are able to move from one place to another; they often store carbohydrate as glycogen. Examples include mammals (eg: humans) and insects (eg: houseflies and mosquitoes)
• Fungi: these are organisms that are not able to carry out photosynthesis; their body is usually organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei; some examples are single-celled; their cells have walls made of chitin; they feed by extracellular secretion of digestive enzymes onto food material and absorption of the organic product; this is known as saprotrophic nutrition; they may store carbohydrate as glycogen. (Eg: Mucor - which has the typical fungal hyphal structure, Yeast - which is single-celled )
• Protoctists: these are microscopic single-celled organisms. Some, like Amoeba, that live in pond water, have features like an animal cell, while others, like Chlorella, have chloroplasts and are more like plants. A pathogenic example is Plasmodium, responsible for causing malaria.
• Pathogens include: fungi, bacteria, protoctists, viruses
• Bacteria: these are microscopic single-celled organisms; they have a cell wall, cell membrane, cytoplasm and plasmids; they lack a nucleus but contain a circular chromosome of DNA; some bacteria can carry out photosynthesis but most feed off other living or dead organisms. (Eg: Lactobacillus bulgaricus - a rod-shaped bacterium used in the production of yoghurt from milk. Pneumococcus - a spherical bacterium that acts as the pathogen causing pneumonia.
• Viruses: these are not living organisms. They are small particles, smaller than bacteria; they are parasitic and can reproduce only inside living cells; they infect every type of living organism. They have a wide variety of shapes and sizes; they have no cellular structure but have a protein coat and contain one type of nucleic acid, either DNA or RNA. (Eg: tobacco mosaic virus- causes discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts. Influenza virus - causes ‘flu’. HIV virus - causes AIDS.