Transport Across the Cell Membrane 11/5/07

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1 11/5/07 "The difference between the internal and external chemical composition of a cell represents a degree of order, that can be maintained only by a barrier to free movement into and out of the cell. Notes And since cells must also assimilate matter from their external environment, to grow and reproduce, they must be able to selectively allow certain molecules and ions across this barrier, often against concentration gradients, while restricting and excluding others." A. Plasma Membrane - The plasma membrane surrounds the cell and functions as an interface between the living interior of the cell and the nonliving exterior. - All cells have one. - It regulates the movement of molecules into and out of the cell. Membrane Structure - The membranes of a cell are phospholipid bilayers that contain numerous proteins embedded within them. Some of the proteins extend all the way through the membrane; others do not. Source: Phospholipids - Most of the lipids in a membrane are phospholipids. Cell Biology 1 Transport Across the Cell Membrane

2 - Phospholipids contain glycerol, two fatty acids, and a phosphate group. The phosphate group is polar (hydrophilic), enabling it to interact with water. The fatty acid tails are nonpolar (hydrophobic) and do not interact with water. Phospholipid Bilayers - Phospholipids spontaneously form a bilayer in a watery environment. They arrange themselves so that the polar heads are oriented toward the water and the fatty acid tails are oriented toward the inside of the bilayer (see the diagram below). - In general, nonpolar molecules do not interact with polar molecules. This can be seen when oil (nonpolar) is mixed with water (polar). Polar molecules interact with other polar molecules and ions. For example table salt (ionic) dissolves in water (polar). - The bilayer arrangement shown below enables the nonpolar fatty acid tails to remain together, avoiding the water. The polar phosphate groups are oriented toward the water. Flexibility - The fatty acid tails are flexible, causing the lipid bilayer to be fluid. This makes the cells flexible. At body temperature, membranes are a liquid similar to cooking oil in consistency. Cholesterol Cell Biology 2 Transport Across the Cell Membrane

3 - In animals, cholesterol is a major membrane lipid. It may be equal in amount to phospholipids. - It is similar to phospholipids in that it one end is hydrophilic, the other end is hydrophobic. - Cholesterol makes the membrane less permeable to most biological molecules. Source: 00%20Lectures/Membranes/membrane.htm Fluid Mosaic Model - a model of the plasma membrane - based on the changing location and pattern of protein molecules in a fluid phospholipids bilayer - composed of a mosaic of lipid, protein and carbohydrate biomolecules. Parts Glycolipids (gl) - are carbohydrate-attached lipids. Their role is to provide energy and also serve as markers for cellular recognition. - also to attach cells to form tissues Glycoproteins - a group of extracellular protein- Cell Biology 3 Transport Across the Cell Membrane

4 Lipoproteins (lp) Carrier Protein (cp) Channel Protein (cp) - bind tissues - also include hormones such as, - Human Chorionic Gonadotropin - Follicle stimulating hormone - Luteinizing hormone - Thyroid stimulating hormone - a combination of fat (cholesterol) and protein that transports lipids, such as cholesterol,in the blood. - HDL (high density lipoprotein), the good cholesterols - LDL (ow density lipoprotein), the bad cholesterols - an integral protein that acts as a gateway for water, Na + and the like o e.g., aquaporins, Na + / K + pump, glucose carrier - Carrier Protein - an integral membrane proteins that bind to a "substrate" and transport it across the membrane - aka Carrier Molecule - an integral membrane protein that acts as pore B. Membrane Transport Processes 1. Diffusion - the migration of molecules or ions as a result of their own random movements, from a region of higher concentration to a region of lower concentration 2. Osmosis - is the movement of water through a semipermeable membrane (analogy a colander or sieve) - movement of water (at constant temp. and pressure) is from the solution with lower concentration of solutes to the solution of higher concentration of solutes (or from the more pure water to less pure water) The movement of water follows the concentration gradient from high concentration of water to low concentration of water Hi conc. of water water Low conc. of water Low conc. of solute water Hi conc. of solute 99% water 1% solute 90% water 10% solute Cell Biology 4 Transport Across the Cell Membrane

5 source: - the more dissolved particles in solution, the greater the tendency of water to move into it and the higher the osmotic pressure Source: NB. When doing such questions on the Gov t, check to see which solutes are actually permeable to the membrane it makes a difference 3. Tonicity - Tonicity refers to the relative concentration of solute on either side of a membrane Hypertonic Hypotonic Isotonic Cell Biology 5 Transport Across the Cell Membrane

6 Osmotic Effects i. Hypertonic Medium - such a medium is much richer in solutes than the cell (hypotonic cell) - as such osmotic flow is outward from the cell - the cell shrinks as it loses water - plasmolysis ( fluid breaks ) e.g., sea water (very salty water) ii. Hypotonic Medium - such a medium is poorer in solutes than the cell (hypertonic cell) - as such osmotic flow forces water into the cell - the cell swells as it gains water - cytolysis ( cell breaks ) e.g., fresh water [mnemonic: think hypodermic needle - injects water into] iii. Isotonic Medium - such a medium is in equilibrium with the cell - neither gaining nor losing water - the cell is iso-osmotic to the medium - has the same osmolarity as the cell - e.g., blood (salty water) Hyperosmotic or Hypertonic Extracellular Fluid ["hyper" = above, excessive] - has HIGHER osmolarity than cell; higher osmotic pressure - solute concentration outside cell > solute conc. inside cell - solution has less H2O & thus has greater tendency to GAIN it - cell loses water; shrinks or crenate; water moves into solution Hypo-osmotic or Hypotonic Extracellular Fluid ['hypo" = beneath] - has LOWER osmolarity than cell; lower osmotic pressure - solute concentration outside cell < solute conc. inside cell - solution has more water & thus has greater tendency to LOSE it - cell gains water, may cause lysis; water enters the cell Osmotic concentration = concentration of all solutes Osmotic pressure = force required to stop osmosis The above diagram shows the movement of water into or out of the cell. The example below is a red blood cell (RBC) - Solution with higher osmotic concentration is hyperosmotic - Solution with lower osmotic concentration is hypoosmotic - Solutions with equal osmotic concentrations are isosmotic - Cells shrink when hypoosmotic to environment - Cells swell when hyperosmotic to environment - Cells without walls want to be isosmotic with environment Cell Biology 6 Transport Across the Cell Membrane

7 Source: RBC in Isotonic Solution RBC in Hypertonic Solution crenation RBC in Hypotonic Solution hemolysis Some more vocab (just to confuse you some more) - A Mole of a substance may be defined as the molecular weight of that substance expressed in grams. o The molecular weight (m wt) of glucose is 180 so 180g of glucose = 1mole. The m wt of NaCl is about 58 so 58g of NaCl = 1mole. If 58g of NaCl are placed into a beaker and water added to a volume of 1l then the result will be a 1 molar solution. Which may be shown as 1mol/l or 1mol l-1 or 1M. o Moles are used as a mass unit by biologists (and Chemists) in preference to grams because a mole of any substance contains the same number of particles as a mole of any other substance. o This greatly simplifies calculations that deal with chemical reactions (including biologically relevant reactions such as buffering) and osmotic effects. o The number of particles, atoms or molecules in a mole of any substance is given by Avagadro's number which is about or 6 x Cell Biology 7 Transport Across the Cell Membrane

8 - Osmolarity is a measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane (one which allows free passage of water and completely prevents movement of solute) compared to pure water. o Osmolarity is dependent on the number of particles in solution but independent of the nature of the particles. o For example, 1 mole of glucose dissolved in 1 litre of water has an osmolarity of 1 osmole (osm) /l. If 1 mole of another sugar, such as sucrose were added to the same litre of water, the osmolarity would be 2 osm/l. o It doesn't matter that the solution contains 1 mole of glucose and 1 mole of sucrose. o If 1 mole of NaCl were dissolved in 1 litre of water it would produce a 1 mol/l NaCl solution with an osmolarity 2 osm/l because NaCl dissociates into Na+ and Cl- (two particles) in solution. This is true of all compounds that dissociate in solution. Na 2 SO 4, which dissociates into Na +, Na + and SO 2-4, to give 3 particles per molecule produces 3 osm/l for every mole dissolved in 1 litre. o o If two solutions contain the same number of particles they may be said to be iso-osmotic (isosmotic) with respect to each other. If one solution has a greater osmolarity than another solution it is hyperosmotic with respect to the weaker solution. If one solution has a lower osmolarity than another solution then it is hypo-osmotic (hyposmotic) with respect to the stronger solution. Iso, hyper and hypo osmolarity should always be stated with respect to another solution. For example, a 1 mol/l NaCl solution is hyperosmotic with respect to 1 mol/l glucose solution. - Because there are more Na+ and Clparticles relative to glucose particles - Tonicity is nearly the same as osmolarity. o For substance that cannot cross cell membranes, tonicity is practically identical to osmolarity. o Tonicity is a measure of the osmotic pressure that a substance can exert across a cell membrane, compared to blood plasma. Plasma has an osmolarity of about 0.3 osm/l, therefore a 0.15 mol/l NaCl solution may be said to be isotonic with plasma (Assuming that neither Na+ nor Cl- can cross cell membranes, which is nearly true). If a substance can cross a plasma membrane, then it cannot exert an osmotic pressure across that membrane. The solute will equilibrate across the membrane instead of forcing water to move. Urea behaves like this, so a 0.3 mol/l urea solution may be said to be iso-osmotic with plasma but it is not isotonic. Source : Pete Smith Cell Biology 8 Transport Across the Cell Membrane

9 ( mo.htm) 4. Membranes are Differentially Permeable - The plasma membrane is differentially or selectively permeable because some particles can pass through, others cannot. o It can control the extent to which certain substances pass through. - Nonpolar molecules pass through cell membranes more readily than polar molecules because the center of the lipid bilayer (the fatty acid tails) is nonpolar and does not readily interact with polar molecules. The following substances can pass through the cell membrane: - Nonpolar molecules (example: lipids) - Small polar molecules such as water o Via porins, specialized protein channels Porins are proteins which cross the cellular membrane and act as a pore through which molecules can diffuse. - Unlike other membrane transport proteins, porins are large enough to allow passive diffusion - i.e. they act as channels. - Porins are composed of beta sheets (2 proteins) these form a cylindrical tube, called a beta barrel. - The amino acid composition of the porin beta sheets is unique in that polar and nonpolar residues alternate along them. - This means the nonpolar residues face outwards so as to interact with the nonpolar lipid membrane, whilst the polar residues face inwards into the centre of the beta barrel to interact with the aqueous channel. - And water can pass through The lipid bilayer is permeable to small, uncharged (nonpolar), molecules like oxygen (O 2 ), carbon dioxide (CO 2 (these diffuse freely in and out of the cell), and ), other lipids. Lipid bilayers are not permeable to charged particles and really big particles: - Water (polar) - ions such as - K +, Na +, Ca 2+ (called cations because when subjected to an electric field they migrate toward the cathode [the negatively-charged electrode]) - Cl -, HCO 3- (called anions because they migrate toward the anode [the positively-charged electrode]) - small hydrophilic molecules like glucose - macromolecules like proteins and RNA Note though that the plasma membrane (cf fluid mosaic model) is permeable to water and other small polar molecules because of porin channels, openings in the phospholipids bilayer. The following substances cannot pass through a cell membrane without help: - Ions and charged molecules (example: salts dissolved in water) via porins/channels/carrier Cell Biology 9 Transport Across the Cell Membrane

10 proteins - Large polar molecules (example: glucose) - Macromolecules via endo/exocytosis Source: 00%20Lectures/Membranes/membrane.htm 5. Facilitated Transport/Diffusion - the transport of molecules across a cellular membrane thru specific protein channels / carrier molecule (facilitating pathway) from a region of high conc. to a region of low conc. Carrier proteins bind to a substrate to assist it thru the lipid bilayer - facilitated and active transport Channel proteins are pores through which a particle simply glides thru - facilitated diffusion - process is driven by conc. differences and does not require energy - chief difference from free diffusion is that the membrane is impermeable to the molecule except for passage thru the carrier channels, - e.g., the cell membrane, composed of the phospholipids is impermeable to the passage of water 6. Active Transport - Against the Gradient - the other half of Facilitated Transport - the pumping of molecules or ions across a cellular membrane thru a carrier protein - from a region of lower conc. to one of higher conc. - therefore against the current or concentration gradient - such a process requires energy - = Active transport Water and lipids are the two major types of solvent in the body. The lipid cell membrane separates the intracellular fluid from the extracellular fluid. Substances which are water soluble typically do not cross lipid membranes easily unless specific transport mechanisms are present. It might be expected that water would likewise not cross cell membranes easily. Indeed, in artificial lipid bilayers, water does not cross easily and this is consistent with our expectation but paradoxically, water crosses nearly all the membranes in the body with ease! How? Via aquaporins These aquaporin proteins form complexes that span the membrane and water moves through these channels passively in response to osmotic gradients. These channel proteins are present in highest concentrations in tissues where rapid transmembrane water movement is important (e.g., in renal tubules). Source: fs1.wilkes.edu/~terzaghi/bio- 226/lectures/13.html source: Cell Biology 10 Transport Across the Cell Membrane

11 C. Bulk (Vesicular) Transport - also a form of active (ATP using) transport 1. Endocytosis - material is engulfed by the plasma membrane and deposited in the cytoplasm in pockets that are pinched off as vesicles toward the interior of the cell - the vesicle is then digested by the cell - two variations on this theme: i. Pinocytosis - Greek drink cell s - cell takes in dissolved material ii. Phagocytosis - Greek eat cell - cell takes in solids source: - basically large particulate matter is tightly enclosed by the membrane bound arm of cytoplasm, and most extracellular fluid is excluded, e.g., amoebae eating a paramecium 2. Exocytosis - like endocytosis only the vesicle is not digested, but transported to and fused with, another part of the plasma membrane Cell Biology 11 Transport Across the Cell Membrane