The endomembrane system

Several membrane-bound organelles work together as a team, passing materials along: this is the endomembrane system. It includes the ER, the Golgi complex, lysosomes and vacuoles. Mitochondria, chloroplasts and peroxisomes work on their own and are not part of it.

Endoplasmic reticulum (ER): a network of tiny tubes and sacs spread through the cytoplasm, dividing it into the space inside the ER (luminal) and outside it (extra-luminal). Rough ER is studded with ribosomes: it makes and sends out proteins, and joins the outer nuclear membrane. Smooth ER has no ribosomes: it makes lipids, and in animals steroid hormones.

Golgi apparatus (seen by Camillo Golgi, 1898): a stack of flat, disc-shaped sacs (cisternae), 0.5 to 1.0 µm across. Each stack has a curved cis (forming) face, which receives material from the ER, and an opposite trans (maturing) face, which sends it out. The Golgi is the cell's packing and dispatch centre: it modifies and packages materials, and forms glycoproteins and glycolipids.

Lysosomes: small sacs that bud off from the Golgi, full of digesting enzymes (hydrolases: lipases, proteases, carbohydrases) that work best in acidic conditions. They can digest carbohydrates, proteins, lipids and nucleic acids, breaking down worn-out parts and food particles.

Vacuoles: spaces bound by a single membrane, the tonoplast, holding water, sap, waste and other materials. In plant cells the vacuole can fill up to 90% of the cell. In Amoeba, a contractile vacuole pumps out extra water (osmoregulation) and waste; many protists form food vacuoles around the food they swallow.