The cell – the basis of life
The cell is the smallest unit of life; every living organism is made of cells. Cells differ in shape according to their job: red blood cells carry oxygen, white blood cells defend the body against germs, leaf stomata cells help evaporate water and exchange gases, and the extensions of nerve cells carry nerve impulses. Organisms whose cells have no formed nucleus are prokaryotes (bacteria and cyanobacteria, once called blue-green algae); those with a formed nucleus are eukaryotes (fungi, plants, animals). A eukaryotic cell has three main parts: the cytoplasmic membrane surrounds the cell and lets needed substances through selectively; the cytoplasm is the inner fluid that holds the organelles; the nucleus stores hereditary information and directs metabolism. A plant cell also has a cellulose cell wall, plastids (green chloroplasts, coloured chromoplasts, colourless leucoplasts) and a large vacuole filled with cell sap; an animal cell has no cellulose wall or plastids and only small vacuoles. A group of cells doing the same job is a tissue, tissues form an organ, and organs form an organism.
“The cell is a city” game: in groups, match the parts of a cell to places in a city (for example, membrane – a wall with gates, nucleus – the control centre, chloroplast – a solar power station, vacuole – a water reservoir) and justify each match in one sentence. Then make a model of a plant cell from plasticine. Safety: do not put plasticine in your mouth, and wash your hands with soap afterwards.