The eukaryotic cell, membrane and main organelles

A eukaryotic cell has three parts: the plasma membrane, the cytoplasm and the nucleus. Cells vary in shape and size: most are 10–100 µm, but very large cells such as a bird’s egg also exist. The plasma membrane is built of a double layer of phospholipids with proteins embedded in and on it (the fluid-mosaic model); it separates the cell from its surroundings, lets substances through selectively (selective permeability) and receives signals. Substances cross by diffusion and osmosis on their own, or by active transport using ATP energy: for example, K⁺ ions are more concentrated inside the cell and Na⁺ ions outside, a difference kept up by the sodium–potassium pump. Large particles enter by phagocytosis (solid particles, e.g. amoeba feeding or white blood cells engulfing a bacterium) and pinocytosis (droplets of liquid); in exocytosis substances are expelled. A plant cell has a cellulose wall outside the membrane, a fungal cell a chitin-like wall and a bacterial cell a murein wall.