Vesicular transport

• Protect the amino acids from water
• Allows large proteins to easily pass through the plasma membrane (they are too large to diffuse through easily and this would be energetically costly)
• Allows specific products to be held and released all at once (secretory granules)

• Different proteins coat the outside of vesicles
• This protein coat is always lost and recycled after the vesicle has been released
• COPI - coats vesicles travelling from the Golgi to the ER
• COPII - coats vesicles travelling from the ER to the Golgi
• Clathrin - coats vesicles for almost everything else

• Clathrin is the most widely studied coat protein
• It is a complex of 6 polypeptides (3 large and 3 small), which form a triskelion shape
• Clathrin is attached to the vesicles via the protein adaptin
• The clathrin first coats the proteins for secretion and shapes them into a bud 
• As the bud continues to form the protein dynamin assembles around the neck of the bud
• Dynamin then constricts to pinch off the vesicle

• V-SNARE proteins coat vesicles
• T-SNARE proteins coat target membranes
• When V and T SNAREs meet they form a stable trans-SNARE complex
• The SNAREs then wind around each other, bringing the vesicles closer together, until water is excluded from between the plasma membranes
• Eventually the membranes are pulled so tightly that they fuse

There are several mechanisms of endocytosis:

• Pinocytosis = 'cellular drinking' the cell takes up very small molecules in tiny vesicles
• Receptor-mediated endocytosis 
• Phagocytosis = 'cellular eating' the cell is able to engulf larger molecules