Edexcel IGCSE Biology

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Edexcel IGCSE Biology

Ø  Variety of living organisms:

·      Plants are multicellular organisms. They contain chloroplasts and are able to carry out photosynthesis. They have cellulose cell walls and store carbohydrates as starch or sucrose.

·      Animals are also multicellular organisms, but do not contain chloroplasts or have cell walls. They usually have nervous coordination and are able to move from one place to another, and often store carbohydrates as glycogen.

·      Fungi 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 fungi are unicellular. They have cell walls made of chitin, and feed by extracellular secretion of digestive enzymes onto food material and absorption of organic products (saprotrophic nutrition). They may store carbohydrate as glycogen. Examples include Mucor (hyphal structure) and yeast (unicellular).

·      Bacteria are microscopic single-celled organisms, containing a cell wall, a cell membrane, a cytoplasm, and plasmids. They lack a nucleus but have a circular chromosome of DNA. Some can photosynthesise but most feed off other living or dead organisms. Examples include Lactobacillus bulgaricus, a rod-shaped bacterium used in the production of yoghurt from milk, and Pneumococcus, a spherical bacterium that acts as the pathogen causing pneumonia.  

·       Protoctists are microscopic single. 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.

·      Viruses are small particles, smaller than bacteria. They are parasitic and can only reproduce inside living cells. They infect every type of living organism, and 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. Examples include the tobacco mosaic virus that causes the discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts; the influenza virus that causes ‘flu’ and the HIV virus that causes AIDS.

Ø  Levels of organisation:

·      In multicellular organisms, the cells become specialised to carry out a particular function.

·      These group together to form a tissue, which performs a particular job (e.g. muscle tissue contracts).

·      Different tissues work together to form an organ. Organs carry out functions in the body. An organ system involves several different organs working together to carry out a particular function.

Ø  Biological molecules:

·      Carbohydrates are made up of atoms of carbon, hydrogen, and oxygen. Simple carbohydrates like glucose and sucrose are sugars. Complex carbohydrates like starch, glycogen, and cellulose are long chains of sugars such as glucose and fructose joined together.

·      Proteins are long chains of amino acids, and contain carbon, hydrogen, oxygen, nitrogen, and sulfur.

·      Lipids are made up of fatty acids and glycerol, and contain carbon, hydrogen, and oxygen.

·      Reddish-brown iodine


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