Summary
Highlights
This revision lesson covers Unit 1 of Edexcel IGCSE Biology, focusing on the characteristics and variety of living organisms. The eight characteristics of life are summarized by the acronym Mr. Nerg, which includes movement, respiration, sensitivity, homeostasis, reproduction, excretion, nutrition, and growth.
Movement is defined as changing position, not moving from place to place. Respiration is the release of energy as ATP from food, typically glucose, through metabolic reactions in the mitochondria. It's crucial not to say 'making energy' as energy is transformed, not created.
Sensitivity involves responding to changes in surroundings or a stimulus. Homeostasis is maintaining a constant internal environment, such as body temperature or blood glucose levels. Reproduction is the production of new organisms or offspring. Excretion is the removal of metabolic waste from the body, like carbon dioxide or urine.
Nutrition is providing food for the organism. Heterotrophs absorb food (e.g., humans), while autotrophs make their own (e.g., plants via photosynthesis and mineral absorption). Growth is a permanent increase in the number or size of cells. All living organisms also contain DNA and can die.
Living organisms are classified into five kingdoms. Eukaryotes have cells with a distinct nucleus and membrane, including plants, animals, fungi, and protists. Prokaryotes, like bacteria, do not have a true nucleus.
Animals are multicellular eukaryotes that lack chloroplasts, cannot photosynthesize, and have no cell walls. They exhibit nervous coordination, can move, and store carbohydrates as glycogen. Examples include mammals and insects.
Plants are multicellular eukaryotes with chloroplasts for photosynthesis using chlorophyll. Their cells have cellulose cell walls for support, and they store carbohydrates as starch and sucrose. Examples include flowering plants and cereals.
Fungi are saprophytic organisms that feed by excreting digestive enzymes and absorbing the products through hyphae. They lack chloroplasts, have chitin cell walls, and store carbohydrates as glycogen. Examples include mucor and yeast.
Protists are usually aquatic, microscopic single-celled organisms, often called the 'dustbin group' as they don't fit neatly into other kingdoms. They can have animal-like characteristics (e.g., amoeba) or plant-like characteristics with chloroplasts (e.g., chlorella). Plasmodium, which causes malaria, is a pathogenic example.
Bacteria are unicellular prokaryotes with cell walls made of peptidoglycan, a cell membrane, cytoplasm, and plasmids. They may have a protective slime capsule and flagella for movement. Instead of a nucleus, they have a circular bacterial chromosome. Some photosynthesize, but most are saprophytes. Examples include Lactobacillus bulgaricus and pneumococcus.
Viruses are small, non-cellular particles, not considered living as they cannot carry out the eight characteristics of life. They are parasitic, reproduce inside host cells, and infect all living organisms. They have a protein coat (capsid) and contain either DNA or RNA. Examples include tobacco mosaic virus, influenza, HIV, and SARS virus. Viruses have an envelope for host cell entry, a capsid for protection and structure, and nucleic acid (DNA or RNA) for genetic coding.
A pathogen is an organism that causes disease. This includes fungi (e.g., athlete's foot, ringworm), bacteria (e.g., salmonella), protists (e.g., amoebic dysentery, malaria-causing plasmodium), and viruses (e.g., HIV, SARS virus). Understanding these buzzwords is crucial for exams.