Membrane Composition and Structure

1. Membrane Composition and Structure Cell membranes are bilayered, dynamic structures that: Perform vital physiological roles Form boundaries between cells and their environments Regulate movement of molecules into and out of cells Lipids, proteins, and carbohydrates in various combinations make these tasks possible.

2. Membrane Composition and Structure The lipid portion of a cellular membrane provides a barrier for water-soluble molecules. Lipids are like the water of a lake in which proteins �float.� This general design is called the fluid mosaic model. Membrane proteins are embedded in the lipid bilayer. Carbohydrates attach to lipid or protein molecules on the membrane, generally on the outer surface.

4. Membrane Composition and Structure Most of the lipid molecules found in biological membranes are phospholipids. Each has a hydrophilic region, where the phosphate groups are located, and a hydrophobic region, the fatty acid �tails.� The phospholipids organize themselves into a bilayer. The interior of the membrane is fluid, which allows some molecules to move laterally in the membrane. http://www.johnkyrk.com/cellmembrane.html

6. Membrane Composition and Structure . Cholesterol may decrease fluidity. For any given membrane, fluidity also decreases with declining temperature. Fluidity and permeability decreases with increased amounts of phospholipids with saturated fatty acid tails.

7. Membrane Composition and Structure All biological membranes contain proteins. The ratio of protein to phospholipid molecules varies depending on membrane function. Many membrane proteins have hydrophilic and hydrophobic regions.

8. Membrane Composition and Structure Integral membrane proteins have hydrophobic regions of amino acids that penetrate or entirely cross the phospholipid bilayer. Transmembrane proteins have a specific orientation, showing different �faces� on the two sides of the membrane. Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer.

10. Membrane Composition and Structure Markers Plasma membrane glycoproteins, glycolipids, lipoprotiens, enable cells to be recognized by other cells and proteins.

11. Cell Recognition and Adhesion The plasma membrane also allows for cell adhesion and recognition. In cell recognition, one cell specifically binds to another cell of a certain type. In cell adhesion, the relationship between the two cells is �cemented�.

12. Passive Processes of Membrane Transport Diffusion The movement of substances from an area of high concentration to an area of low concentration. .

13. Passive Processes of Membrane Transport A biological membrane may be permeable to some molecules and impermeable to others. Unsaturated phospholipids = higher permeability. Saturated phospholipids = low permeability.

14. Passive Processes of Membrane Transport Small molecules can move across the lipid bilayer by simple diffusion. (i.e. H2O, O2, CO2) The more lipid-soluble the molecule, the more rapidly it diffuses. An exception to this is water. Polar and charged molecules such as amino acids, sugars, and ions do not pass readily across the lipid bilayer.

15. Passive Processes of Membrane Transport Osmosis is the diffusion of water from an area of high water concentration to an area of low water concentration. Osmosis is a completely passive process and requires no metabolic energy.

16. Passive Processes of Membrane Transport Isotonic solutions have equal solute concentrations. A hypertonic solution has a greater total solute concentration than the solution to which it is being compared. A hypotonic solution has a lower total solute concentration than the solution to which it is compared.

18. Passive Processes of Membrane Transport . One way for these important raw materials to enter cells is through the process of facilitated diffusion. Facilitated diffusion depends on two type of membrane proteins: channel proteins and carrier proteins.

19. Passive Processes of Membrane Transport Channel proteins are integral membrane proteins that form channels lined with polar amino acids. Nonpolar (hydrophobic) amino acids face the outside of the channel, toward the fatty acid tails of the lipid molecules. http://www.stolaf.edu/people/giannini/flashanimat/transport/channel.swf

22. Passive Processes of Membrane Transport Facilitated diffusion is diffusion of substances with the assistance of a carrier or channel protein. Carrier proteins allow diffusion in both directions.

24. Active Transport In contrast to diffusion, active transport requires the expenditure of energy. Ions or molecules are moved across the membrane against the concentration gradient. ATP is the energy currency used either directly or indirectly to achieve active transport.

25. Active Transport Three different protein-driven systems are involved in active transport: Uniport transporters move a single type of solute, such as calcium ions, in one direction. http://www.stolaf.edu/people/giannini/flashanimat/transport/caryprot.swf Symport transporters move two solutes in the same direction. http://www.stolaf.edu/people/giannini/flashanimat/transport/symport2.swf Antiport transporters move two solutes in opposite directions, one into the cell, and the other out of the cell. http://www.stolaf.edu/people/giannini/flashanimat/transport/antiport.swf

27. Active Transport If ATP is used directly for the pumping system, as in the sodium�potassium pump, the system is a primary active transport system. Only cations, such as sodium, potassium, and calcium, are transported directly by pumps that use a primary active transport system. http://www.stolaf.edu/people/giannini/flashanimat/transport/atpase.swf

29. Active Transport Secondary active transport systems use established gradients to move substances. This form of transport uses ATP indirectly. The ATP molecules are consumed to establish the ion gradient. The gradient is then used to move a substance, as described for the symport and antiport systems. http://www.stolaf.edu/people/giannini/flashanimat/transport/secondary%20active%20transport.swf

31. Endocytosis and Exocytosis The group of processes called endocytosis brings macromolecules, large particles, small molecules, and even other cells into the eukaryotic cell. There are three types of endocytosis: phagocytosis, pinocytosis, and receptor-mediated endocytosis. http://www.cellsalive.com/qtmovs/mac_mov.htm

33. Endocytosis and Exocytosis During phagocytosis is the engulfing of solid particles. Pinocytosis is cellular drinking. The engulfing of liquid droplets.

34. Endocytosis and Exocytosis Receptor-mediated endocytosis is similar to pinocytosis, but it is highly specific. Receptor proteins are exposed on the outside of the cell in regions called coated pits. Clathrin molecules form the �coat� of the pits. Coated vesicles form with the macromolecules trapped inside.

37. Endocytosis and Exocytosis Exocytosis is the process by which materials packaged in vesicles are secreted from the cell.

38. Membranes Are Not Simply Barriers Membranes have many functions, including: Information processing Energy transformation The inner mitochondrial membrane helps convert the energy of fuel molecules to the energy in ATP. The thylakoid membranes of chloroplasts are involved in the conversion of light energy in photosynthesis. Membranes are involved in organizing chemical reactions, allowing them to proceed rapidly and efficiently.