Question

 1. What are the components which make up a membrane?

 2. How are proteins "associated" with a membrane?

 3. What role(s) do membranes play in a cell?

 4. How do molecules cross the membrane? Do all need a channel or transporter? Why?

 5. What is aquaporin, what role do aquaporin play and what structure and class does it belong to?

 6. How are channels different from transporters?

 7. What do transporters transport? Is transport active or passive?

 8. What are Na/K ATPases and how do they function in the cell?

 9. What role does ATP play with transporters? Why is ATP used/needed in the transport process?

 10. What structure do the channels/transporters form to span the membrane?

 11. Do all cells express the same channels/ transporters? Why is this important?

Answer 1

1. Chemically, plasmamembrane and other membranes of the organelles are made up of proteins, lipids and carbohydrates. Lipids: Four major type of lipids are commonly present in the plasmamembrane. These are phospholipids, sphingolipids, glycolipids and sterols. The relative proportions of these lipids vary between different membranes. Proteins: According to their position in the plasma membrane, proteins are divided into two categories: Integral proteins and peripheral proteins. Integral proteins are generally transmembrane proteins and hard to detach from the membrane while peripheral proteins are loosely associated with the membrane and easily detached from the membrane. On the basis of their functions, membrane proteins are classified into three main types: structural proteins, enzymes and transporters. Carbohydrates: Carbohydrates in the plasmamembrane are present as short, branched or unbranched chains of sugars attached to exterior ectoproteins (forming glycoproteins) or to the polar ends of phospholipids (forming glycolipids). No carbohydrate is located at the cytoplasmic or inner surface of the plasmamembrane.

2. Proteins are associated to the membrane are classified into two types: Integral or intrinsic proteins and Peripheral or extrinsic proteins. Again both of these types are divided into two categories: Ectoproteins (lying to the external cytoplasmic surface of the plasmamembrane) and Endoproteins (lying to internal cytoplasmic surface of the plasmamembrane). The intrinsic proteins tend to attache firmly with the plasma membrane, while the extrinsic proteins have a weaker association and are bound to lipids of membrane by electrostatic interaction.

3. Roles that membrane plays: (A) Compartment to the cell: The plasmamembrane encloses entire content of the cell. (B) Schafold for biochemical activities: Because of their construction, membranes provide the cell with an extensive framework or scaffolding within which components can be ordered for effective interaction. (C) Provide selectively permeable barrier: Membranes prevent the unrestricted exchange of molecules from one side to the other. (D) Transport solutes: The plasma membrane contains the machinery for physically transporting substances from one side of the membrane to another, often from a region where the solute is present at low concentration into a region where that solute is present at much higher concentration. (E) Intracellular interaction: The plasma membrane allows cells to recognize and signal one another, to adhere when appropriate, and to exchange materials and information.

4. Several different processes are known by which substances move across membranes: simple diffusion through the
lipid bilayer;

simple diffusion through an aqueous, proteinlined channel;

diffusion that is facilitated by a protein transporter; and

active transport, which requires an energy-driven protein “pump” capable of moving substances against a concentration gradient

No, all the molecules do not require channels or transporters because there are several molecules (O2, CO2 H2O) can freely diffuse through the lipid bilayer without use of protein channels.

5. Aqua porins are integral membrane proteins that serves as the channels for the transport of water molecules. These are the channel proteins made up of six transmembrane alpha- helices.

6. Ion channels are pore forming transmembrane channels which allows transport of ions from higher concentration to the lower; while transporters are transmembrane proteins allowing transport of molecules through membrane against the concentration gradient. Channels do not utilize energy for transport, while transporters require energy in terms of ATP.

7. Transporters are associated with the transport of ions, peptides, small molecules, lipids and macromolecules across the membrane. Transport through the transporters is an active process, because it requires ATP for the transport of molecules against the concentration gradient.

8. Na/K ATPases are the active transporters of sodium and potassium. They utilize energy in terms of ATP to transport 2 molecules of sodium to the outside of the cell and 2 molecules of potassium into the cell. This is how they maintain low sodium ion concentration and high potassium ion concentration inside the cell.

9. Unlike facilitated diffusion, however, movement of a solute against a gradient requires the coupled input of energy. Consequently, the endergonic movement of ions or other solutes across the membrane against a concentration gradient is coupled to an exergonic process, such as the hydrolysis of ATP, the absorbance of light, the transport of electrons, or the flow of other substances down their gradients. ATP is used to phosphorylate the transporter proteins. Upon phosphorylation, the pump protein changes its conformation, which becomes the basis for opening and closing of the pump.

10. The membrane channels and transporters form gate like structures into the plasma membrane. These gates are opened upon exposure the the specific stimulus or signals. Ion channels just simply open up when the concentration of ion goes high at one side compare to the other of the plasmamembrane. while, the transport proteins requires ATP for transport of the molecules.

11. No, not all the cells in the body express same channels/transporters. Because, the functionality and structures of the cell differs largely into the body. The cells of intestinal lumen and the cells of nervous systems express totally different channels and transporter proteins because these cells are not similar, neither functionally nor structurally. Same way RBCs and muscle cells also do not express the same transmembrane proteins.