Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport

Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport 1 Unit 3 Study Guide Excluded: Pages 26 Pathways for production of ATP – up to page 32 Plasma Membrane Phospholipid bi layer All cell membranes have the same lipid bilayer structure. The bilayer consists of two sheets of phospholipid molecules oriented in opposite directions. The heads (spherical) of the phospholipids face outward. They are attracted to the watery environment inside and outside the cell. The hydrophobic phospholipid tails are sandwiched between the heads. This minimizes their interactions with water. Membrane proteins can be classified into two broad categories— - Integral (intrinsic) - Peripheral (extrinsic) Integral membrane proteins, also called intrinsic proteins, have one or more segments that are embedded in the phospholipid bilayer. Most integral proteins contain residues with hydrophobic side chains that interact with fatty acyl groups of the membrane phospholipids, thus anchoring the protein to the membrane. Most integral proteins span the entire phospholipid bilayer. Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport 2 Peripheral membrane proteins, or extrinsic proteins, do not interact with the hydrophobic core of the phospholipid bilayer. Instead they are usually bound to the membrane indirectly by interactions with integral membrane proteins or directly by interactions with lipid polar head groups. Membrane proteins: • Form water-filled channels that serve as: – – – – – Carrier molecules aka carrier protein Docking-marker acceptors Membrane-bound enzymes Receptor sites Cell adhesion molecules Types of vital proteins:  Adhesion Protein Adhesion proteins help one cell adhere to another cell or to a protein component of an extracellular matrix. 3 main types: – Tight junctions (Tight junctions also hold cells together, but they form a nearly leakproof intercellular seal by fusion of adjacent cell membranes.) – Desmosomes (Adhering junctions hold cells together mechanically and are associated with intracellular fibres of the cytoskeleton.) – Gap junctions (Which allow small molecules to pass from one cell to the next through a channel.) Communication Protein Communication proteins in the plasma membrane of one cell match up with identical proteins in the plasma membrane of an adjoining cell. Together the proteins form a channel that connects the cytoplasm of the two cells and enables transmission of signals between them. An example is a gap junction.  Receptor Protein Receptor proteins embedded in a membrane are docks for diverse hormones and other signals.  Recognition Protein Recognition proteins are identity tags by which cells recognize nonself and self. They are glycoproteins with sugar side chains projecting above the membrane. Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport 3  Passive Transport Protein Passive transport proteins are channels that passively enable one or more substances to cross a membrane. Some are always open, others have molecular gates that close and open in controlled ways.  Active Transport Proteins Active transport proteins are also called ATPase pumps. Energy provided by the ATP makes them actively pump solutes across the membrane. Types of Cell Transport There are six basic types of cell transport: 1. Diffusion, which is the movement of molecules down their concentration gradient without the use of energy. It is a passive process during which molecules move from a region of higher concentration to a region of lower concentration. The rate of diffusion of substances varies from membrane to membrane because of different selective permeabilities. 2. Osmosis, which is the passive diffusion of water down its concentration gradient across selectively permeable membranes. Water moves from a region of high water concentration to a region of low water concentration. Thinking about osmosis another way, water will flow from a region with a lower solute concentration (hypotonic) to a region with a higher solute concentration (hypertonic). This process does not require the input of energy. For example, visualize two regions—one with 10 particles of sodium per liter of water; the other with 15. Osmosis would drive water from the region with 10 particles of sodium toward the region with 15 particles of sodium. 3. Facilitated diffusion, that is, the diffusion of particles across a selectively permeable membrane with the assistance of the membrane's transport proteins. These proteins will not bring any old molecule looking for a free pass into the cell; they are specific in what they will carry and have a binding site designed for molecules of interest. Like diffusion and osmosis, this process does not require the input of energy. 4. Active transport, which is the movement of a particle across a selectively permeable membrane against its concentration gradient (from low concentration to high). This movement requires the input of energy, which is why it is termed "active" transport. As is often the case in cells, adenosine triphosphate (ATP) is called on to provide the energy for this reactive process. These active-transport systems are vital to the ability of cells to maintain particular concentrations of substances despite environmental concentrations. For example, cells have a very high concentration of potassium and a very low concentration of sodium. Diffusion would like to move sodium in and potassium out to equalize the concentrations. The all-important sodium-potassium pump actively moves Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport 4 potassium into the cell and sodium out of the cell against their respective concentration gradients to maintain appropriate levels inside the cell. This is the major pump in animal cells. 5. Endocytosis, a process in which substances are brought into cells by the enclosure of the substance into a membrane-created vesicle that surrounds the substance and escorts it into the cell (Figure 6.2). This process is used by immune cells called phagocytes to engulf and eliminate foreign invaders. 6. Exocytosis, a process in which substances are exported out of the cell (the reverse of endocytosis). A vesicle again escorts the substance to the plasma membrane, causes it to fuse with the membrane, and ejects the contents of the substance outside the cell (Figure 6.2). In exocytosis, the vesicle functions like the trash chute of the cell. Tonicity vs Osmosis Osmotic pressure is the pressure of a solution against a semipermeable membrane to prevent water from flowing inward across the membrane. Tonicity is the measure of this pressure. If the concentration of solutes on both sides of the membrane is equal, then there is no tendency for water to move across the membrane and no osmotic pressure. The solutions are isotonic with respect to each other. Usually there is a higher concentration of solutes on one side of the membrane than the other. Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport 5 Diffusion Versus Osmosis Diffusion is the movement of particles from a region of higher concentration to one of lower concentration. For example, if you add sugar to water, the sugar will diffuse throughout the water until the concentration of sugar in the water is constant throughout the solution. During osmosis, as with diffusion, there is a tendency of particles to seek the same concentration throughout the solution. However, the particles may be too large to cross a semipermeable membrane separating regions of solution, so water moves across the membrane. If you have a sugar solution on one side of a semipermeable membrane and pure water on the other side of the membrane, there will always be pressure on the water side of the membrane to try to dilute the sugar solution. Does this mean all of the water will flow into the sugar solution? Probably not, because the fluid may be exerting pressure on the membrane, equalizing the pressure. As an example, if you put a cell in fresh water, the water will flow into the cell, causing it to swell. Will all of the water flow into the cell? No. Either the cell will rupture or else it will swell to a point where the pressure exerted on the membrane exceeds the pressure of the water trying to enter the cell. Of course, small ions and molecules may be able to cross a semipermeable membrane, so solutes such as small ions (Na+, Cl-) behave much like they would if simple diffusion were occurring. The tonicity of solutions with respect to each other may be expressed as hypertonic, isotonic or hypotonic. The effect of different external solute concentrations on red blood cells serves as a good example for hypertonic, isotonic and hypotonic solution.  Hypertonic Solution or Hypertonicicty When the osmotic pressure of the solution outside the blood cells in higher than the osmotic pressure inside the red blood cells, the solution is hypertonic. The water inside the blood cells exits the cells in an attempt to equalize the osmotic pressure, causing the cells to shrink or crenate.  Isotonic Solution or Isotonicity When the osmotic pressure outside the red blood cells is the same as the pressure inside the cells, the solution is isotonic with respect to the cytoplasm. This is the usual condition of red blood cells in plasma. Lesson 2 Chapter 2 – The Plasma Membrane and Membrane Transport 6  Hypotonic Solution or Hypotonicity When the solution outside of the red blood cells has a lower osmotic pressure than the cytoplasm of the red blood cells, the solution is hypotonic with respect to the cells. The cells take in water in an attempt to equalize the osmotic pressure, causing them to swell and potentially burst. Clinical Application – Most common disease due to a defect in plasma membrane transport is “CF” (Cystic Fibrosis) – A fatal disease due to defect in membrane protein – The plasma membrane is impermeable to Cl- and Na+