Cells as the building blocks of every living thing, the parts every cell shares, how cells specialise, and how tissues build organs. Concept tutorials with clickable cell and tissue diagrams.
All living things are made of cells, and cells share a nucleus, membrane and cytoplasm (AC9S8U01).
Most cells are about a hundredth to a tenth of a millimetre across, too small to see without a microscope: your body has about 37 trillion of them. Cells were only discovered once microscopes could magnify enough.
Cell theory: all living things, including plants, fungi and bacteria, are made of cells. Some organisms are a single cell; others are made of many.
Plant and animal cells share three parts: the nucleus (the control centre, holding DNA), the cell membrane (controls what gets in and out) and the cytoplasm (where reactions happen).
Plant cells have three parts animal cells don't: a cell wall for support, chloroplasts for photosynthesis and a large vacuole that keeps the cell firm. Both kinds have mitochondria (for energy) and ribosomes (for making proteins), and how many of each a cell has depends on its job.
Specialised cells form tissues, and tissues build organs (AC9S8U02).
Almost all your cells have the same DNA, but they specialise: each develops a shape and contents that suit its job, like a red blood cell packed with haemoglobin or a nerve cell with a long axon.
A tissue is a group of similar specialised cells working together to do one job. Muscle tissue contracts, epithelial tissue lines and protects, xylem carries water, phloem carries sugar. A tissue can do what a single cell can't do on its own.
An organ is made of several different tissues working together. A leaf has epidermis, palisade and spongy mesophyll and vascular tissue; the heart has cardiac muscle, a lining, connective tissue and nervous tissue.
An organ system is a group of organs that work together to do a job the whole organism needs. In an animal, the circulatory system (heart, blood vessels, blood) delivers oxygen and nutrients; in a plant, the vascular system (xylem and phloem in roots, stems and leaves) carries water and sugars. If one organ fails, the system fails.
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