(i) Intracellular membranes

• Eukaryotic cells have a system of internal membranes, which increases the total area of membrane.

  • Because of their size, eukaryotes have a relatively small surface area to volume ratio. 

  • The plasma membrane of eukaryotic cells is therefore too small an area to carry out all the vital functions carried out by membranes.

• The endoplasmic reticulum (ER) forms a network of membrane tubules continuous with the nuclear membrane

• The Golgi apparatus is a series of flattened membrane discs

• Lysosomes are membrane-bound organelles containing a variety of hydrolases that digest proteins, lipids, nucleic acids and carbohydrates

• Vesicles transport materials between membrane compartments

(ii) Synthesis of membrane components

• Lipids and proteins are synthesised in the ER

  • Rough ER (RER) has ribosomes on its cytosolic face while smooth ER (SER) lacks ribosomes.

• Lipids are synthesised in the smooth endoplasmic reticulum (SER) and inserted into its membrane

• The synthesis of all proteins begins in cytosolic ribosomes

• The synthesis of cytosolic proteins is completed there, and these proteins remain in the cytosol

• Transmembrane proteins carry a signal sequence, which halts translation and directs the ribosome synthesising the protein to dock with the ER, forming RER

  • A signal sequence is a short stretch of amino acids at one end of the polypeptide that determines the eventual location of a protein in a cell.

• Translation continues after docking, and the protein is inserted into the membrane of the ER

(iii) Movement of proteins between membranes

• Once the proteins are in the ER, they are transported by vesicles that bud off from the ER and fuse with the Golgi apparatus

• As proteins move through the Golgi apparatus they undergo post-translational modification

  • Molecules move through the Golgi discs in vesicles that bud off from one disc and fuse to the next one in the stack. 

  • Enzymes catalyse the addition of various sugars in multiple steps to form the carbohydrates.

• The addition of carbohydrate groups is the major modification

• Vesicles that leave the Golgi apparatus take proteins to the plasma membrane and lysosomes

• Vesicles move along microtubules to other membranes and fuse with them within the cell

(iv) The secretory pathway

• Secreted proteins are translated in ribosomes on the RER and enter its lumen

  • Peptide hormones and digestive enzymes are examples of secreted proteins.

• The proteins move through the Golgi apparatus and are then packaged into secretory vesicles

• These vesicles move to and fuse with the plasma membrane, releasing the proteins out of the cell

• Many secreted proteins are synthesised as inactive precursors and require proteolytic cleavage to produce active proteins

  • Proteolytic cleavage is another type of post-translational modification. 

  • Digestive enzymes are one example of secreted proteins that require proteolytic cleavage to become active.

  • Specific names of digestive enzymes are not required.