The Endomembrane system Organelle recap? Ribosomes Ribosomes: a complex of ribosomal RNA (rRNA, synthesised in the nucleolus) and proteins Ribosomes carry out protein synthesis in: The cystol/plasm (free ribosomes) On the outside of the rough endoplasmic reticulum Free ribosomes make proteins used within the cell e.g. enzymes Bound ribosomes make proteins which are often secreted e.g insulin made in pancreatic cells Endoplasmic reticulum Form Accounts for more than half of the total membrane in many eukaryotic cells The ER membrane is continuous with the nuclear envelope There are two distinct regions of ER: Smooth ER: lacks ribosomes Rough ER: ribosomes are bound to the outer membrane Functions: Smooth endoplasmic reticulum