Revision:Membranes 3

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Explain the dynamic relationship between nucleus, rough endoplasmic Reticulum, golgi apparatus and cell surface membrane

Membranes regulate molecular traffic at that boundary.

Proteins move from the cis to the trans face of the Golgi in transport vesicles that bud and fuse with other membranes much in the same manner as the transport from the ER to Golgi. Often, this trip across the Golgi acts to concentrate the secretory product. At the trans face, the product may move into one specific area and be enclosed in a vacuole. The vacuole may then become a secretory vesicle that moves to the plasma membrane, where the contents can be discharged from the cell. This is a closely regulated process; discharge takes place only at the appropriate membrane to avoid the dangerous or wasteful discharge of products. For example, pancreatic enzymes are secreted from the acinar cells into ductules, which carry the enzymes not into the blood but into the intestine.The stimulus for discharge is most often a nervous or hormonal signal. Discharge itself is generally triggered by a rise in the concentration of calcium ions in the cell. The action is one of exocytosis: the vesicle and the plasma membrane fuse, forming a channel by which the proteins and glycoproteins in the vesicle can be released.As secretory vesicles fuse with the plasma membrane, the area of the plasma membrane increases. Normal size is regained by the recycling of membrane components through endocytosis. Regions bud from the plasma membrane and then fuse with the membrane of the Golgi or lysosomes.

Describe the ways in which Vesicles are used to transport materials within a cell and to the cell surface

• Vesicles are involved in cell-mediated transport (see endocytosis and exocytosis).
• Vesicles move from the golgi complexes to the surfaces of the cell.
• When a vesicle reaches the cell surface, its membrane fuses with the membrane of the cell, thus expelling its contents to the outside.
  • This is exocytosis.
  • In endocytosis, material to be taken into the cell induces the membrane to bulge inward, producing a vesicle enclosing the substance.

Describe the membrane proteins and their positions with membranes

• Allow different types of molecules to enter and leave the cell.
• Membranes are continuous.
• Materials flow through channels by vesicles.
• Pores exist.
• Some intrinsic proteins are anchored.

Outline the functions of membrane proteins

Membrane proteins act as:

• Hormone binding sites
• Antibody recognition sites
• Catalysts for biochemical reactions
• Electron carrier sites

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