The basic structural unit of all living organisms. The human body consists of billions of cells, structurally and functionally integrated to perform the complex tasks necessary for life. In spite of variation in size and function, most human cells have a similar basic structure. Each cell is an invisibly small bag containing liquid cytoplasm, surrounded by a cell membrane that regulates the passage of useful substances (such as oxygen and nutrients) into the cell; and waste materials (such as carbon dioxide) and manufactured substances (such as hormones) out of the cell. Some cells, such as those lining the small intestine, have microvilli, projections that increase the cells’ surface area to facilitate absorption. All cells, except red blood cells, have a nucleus, a control centre that governs all major cell activities by regulating the amount and types of proteins made in the cell. Inside the nucleus are the chromosomes, which are made of the nucleic acid DNA. This contains the instructions for protein synthesis, which are carried into the cytoplasm by a type of RNA, another nucleic acid, and are decoded in particles called ribosomes. The nucleus also contains a spherical structure called the nucleolus, which plays a role in the production of ribosomes. The cell also contains various organelles, each with a specific role. Energy is generated from the breakdown of sugars and fatty acids by mitochondria. Substances that would damage the cell if they came into contact with the cytoplasm are contained in particles called lysosomes and peroxisomes. A system of membranes in the cytoplasm called the endoplasmic reticulum transports materials through the cell. Flattened sacs called the Golgi complex receive and process proteins dispatched by the endoplasmic reticulum. Products for export, such as enzymes and hormones, are secreted by vesicles at the cell surface. Other materials, water, and waste products are transported and stored in the cytoplasm by vacuoles. The cytoplasm itself has a network of fine tubes (microtubules) and filaments (microfilaments) known as the cytoskeleton, which gives the cell a definite shape.


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