Models Of Membrane Structure

• In electron micrographs, all cell membranes have similar structure: an extremely thin sheet (~5-7 nm) with "railroad track" appearance    (protected).
• All models of membrane structure built around notion of lipid bilayer.
  View schematic diagram of lipid bilayer
• Phospholipids spontaneously form bilayers. Similar to soap bubbles: thin, flexible, fluid, not very durable or strong.
• Animal membranes strengthened by cholesterol.
• Biological membranes include proteins; provide structural integrity, variety of functions.
• Fluid Mosaic model: proteins "float" in a 2-dimensional sheet of lipids.
• View animation of fluid membrane structure (protected)
• Composition of typical membrane:
  1. ~50% lipid (largely phospholipid; in animal cells, 1/3 cholesterol)
  2. ~50% protein
• Proteins function in variety of ways: (see Fig. 8.8 in text)
  Some are integral; span entire membrane. Include transport proteins (permeases).
• Some are peripheral; include receptor proteins for hormones, matrix of structural proteins that attach to membrane and provide shape, etc.

Diffusion And Size Limits

• Diffusion is the driving force for substances to move around in cells.
• Diffusion results from random motion of molecules. If some regions more concentrated than others, diffusion will tend to cause equilibrium (see text Fig. 8.10). View animation of diffusion across a membrane (Campbell website activity)
• Einstein's equation for diffusion in 2 dimensions: d² = 6Dt
  (d = distance traveled; t = time; D = diffusion coefficient characteristic for each type of substance)
• Sample calculations: for sucrose, D = 2.38 x 10^-6 cm²/sec. How long will it take for sucrose to diffuse length of 3 cells: 1 µm (typical bacterium); 10 µm (small animal cell, e.g. lymphocyte); 1 meter (length of spinal neuron)?
  1. Rearrange equation to solve for t: t = d²/6D
  2. Convert distance to cm so units are compatible: 1 µm = 10^-4 cm; 10 µm = 10^-3 cm; 1 m = 10² cm
  3. Plug in values and solve:
     For 1 µm cell: t = 0.7 x 10^-3 sec, or 0.7 millisecond
     For 10 µm cell: t = 0.07 sec, or 70 millisecond
     For 1 m cell: t = 7 x 10⁹ sec, or 2.2 years
• Conclusions:
  1. diffusion rapidly moves molecules around in small cells, but rate increases as the square of distance, so expanding cell diameter by 10 slows down diffusion through cell by factor of 100.
  2. Large cells cannot rely on diffusion to transport material throughout cell; nerve cells must have some other mechanism (motor molecules such as kinesin that transport materials attached to microtubules) to move materials throughout cell.

Osmosis And Water Balance

• water flows smoothly across cell membranes without needing any carrier = osmosis. Other polar or charged molecules (unless lipid soluble).
• View osmosis movie
• Three situations can result from water movement.
  1. Isotonic environment. Water concentration outside = water conc. inside cell.
     • Since most cells contain about 0.9% dissolved salts + solutes, isotonic environments must contain 0.9% salt. In this situation, water flow out = water flow in. For human cells, this is desirable state.
     • Laboratory and clinical workers often use Ringer's solution to bathe exposed tissues, provide isotonic environment.
  2. Hypotonic environment. Water concentration outside cell is higher (e.g. pure water) than inside cell. Or, solute concentration outside cell is lower than inside cell.
     • Result: water moves in at greater rate that moves out.
     • 2 possible results:
       1. if cell lacks a wall, will swell up. Can cause lysis (swelling leading to breakage) if no way to remove excess water (e.g. in blood cells). Freshwater protists (e.g. paramecium) have contractile vacuoles to pump water back out, prevent lysis.
       2. if cell has a wall, water pressure will push membrane tightly against wall, lead to turgor. This is desirable state for walled cells.
     • View animation of hypotonic effects on animal and plant cells (Campbell website activity)
  3. Hypertonic environment. Water concentration outside cell is lower (e.g. brine, syrup) than inside cell. Or, solute concentration outside cell is higher than inside cell.
     • Result: water moves out at greater rate that moves in.
     • 2 possible results:
       1. if cell lacks a wall, will shrivel up like a raisin. Causes crenation in blood cells. Cells stop metabolizing, but not immediately killed. Can restore activity by placing back in isotonic environment.
       2. if cell has a wall, membrane will shrink away from wall as water leaves cell, rigid wall remains where it is, leads to plasmolysis. This is undesirable state for walled cells, cells stop metabolizing.
       3. View animation of hypertonic effects on animal and plant cells (Campbell website activity)
     • View interactive animation of hypertonic and hypotonic effects on a cell (Campbell website activity)
     • View red blood cell crenation movie

Membranes separate compartments of different concentration

• Many substances occur at very different concentrations across cell membranes.
• Examples: ion gradients across a human cell

• These gradients are maintained by membrane transport of each ion. How?

Movement Of Small Molecules Across Membranes can involve simple diffusion or protein-mediated transport

1. Passive Diffusion. Lipophilic solutes cross the membrane freely by dissolving in the lipid bilayer. This is passive diffusion. Examples: ethanol (alcohol, contains both polar and non-polar regions); also fatty acids, glycerol, steroids, etc. Also nonpolar gases like O=O (O₂)
   View animation of lipophilic molecules crossing membrane (protected)
2. Selective transport by protein carriers = "permeases". Polar or ionic small solutes may be transported across membranes if specific protein carriers are in the membrane.Examples: sugars, amino acids, ions.
   View animation of transport of polar and charged Molecules (protected)
3. Many substances cannot cross the membrane. Examples: large molecules such as proteins, nucleic acids. Also small polar molecules or ions for which there is no protein carrier.
   View animation of membrane barrier to large molecules (protected)

Some protein transporters require energy; others do not

1. facilitated diffusion. Membrane has specific protein carrier, will bind to molecule and bring it across cell membrane. No energy required. No preferential direction. If molecule is more concentrated outside than inside cell, net movement will be out of cell.
   View animation of facilitated diffusion (Campbell website activity)
2. active transport. Membrane has specific protein carrier, also a requirement for energy (ATP or other form of energy). Will move solute against a concentration gradient, so can concentrate material even if diffusion would favor opposite direction of flow.
   View animation of active transport (Campbell website activity)
   • Example: Na⁺, K⁺ ATPase in nerve cells. Pumps Na⁺ to outside, K⁺ in, maintains electrical potential against diffusion. When nerve cell "fires", momentary gates open to let diffusion occur. Then pumps are turned back on to restore potential.

Active transport can involve ATP pumps, symport, or antiport

1. ATP pumps.
   • ATP-powered pumps (ATPases) couple the splitting, or hydrolysis, of ATP with the movement of ions across a membrane against a concentration gradient.
   • ATP is hydrolyzed directly to ADP and inorganic phosphate, and the energy released is used to move one or more ions across the cell membrane.
   • As much as 25% of a cell's ATP reserves may be spent in such ion transport.
   • Examples include:
     1. The Na⁺-K⁺ ATPase pumps Na⁺ out of the cell while it pumps K⁺ in. Because the pump moves three Na⁺ to the outside for every two K⁺ that are moved to the inside, it creates an overall charge separation known as polarization. This electrical potential is required for nervous system activity, and supplies energy needed for other types of transport such as symport and antiport.
        View animation of ATP pump   (protected)
     2. Ca⁺⁺ ATPases are responsible for keeping intracellular Ca⁺⁺ at low levels, a necessary precondition for muscle contraction.
2. Symport.
   • To transport some substances against a concentration gradient, cells use energy already stored in ion gradients, such as proton (H⁺) or sodium (Na⁺) gradients, to power membrane proteins called transporters.
   • When the transported molecule and the co-transported ion move in the same direction, the process is known as symport.
   • Example: transport of amino acids across the intestinal lining in the human gut.
   • View animation of symport   (protected)
3. Antiport
   Cell uses movement of an ion across a membrane and down its concentration gradient to power the transport of a second substance "uphill" against its gradient.
   • In this process, the two substances move across the membrane in opposite directions.
   • Example: transport of Ca²⁺ ions out of cardiac muscle cells. Muscle cells are triggered to contract by a rise in intracellular Ca²⁺ concentration, so it is imperative that Ca²⁺ be removed from the cytoplasm so that the muscle can relax before contracting again. This antiport system is so effective that it can maintain the cellular concentration of Ca²⁺ at levels 10,000 times lower than the external concentration.
   • View animation of antiport   (protected)

Movement Of Large Molecules: Endocytosis, Exocytosis, Phagocytosis, Carrier-Mediated Endocytosis

• Large molecules (proteins, nucleic acids, polypeptides larger than a few amino acids, polysaccharides larger than a few sugars) are not carried by transport proteins.
• There are mechanisms for moving larger molecules, but they don't enter into cytoplasm.
  1. Exocytosis: membrane vesicle fuses with cell membrane, releases enclosed material to extracellular space. Ex: release of digestive enzymes from pancreatic cells; mucus, milk, hormones, etc.
     View animation of exocytosis (Campbell website activity)
  2. Endocytosis: cell membrane invaginates, pinches in, creates vesicle enclosing contents. Three common situations:
     1. Phagocytosis: Typically works on debris, bacteria, other particulate matter. Contents of the "phagosome" are usually fused with lysosome to create "phagolysosome", where material is broken down. Especially common in white blood cells such as macrophages and other leukocytes.
        View animation of phagocytosis (Campbell website activity)
     2. Pinocytosis: similar to phagocytosis, but ingests fluid rather than particulate matter. "Cell drinking". Ex. cells lining blood capillaries take fluid from blood (but not red cells), move fluid across their cytoplasm, release into extracellular space surrounding cells outside the capillary.
        View animation of pinocytosis (Campbell website activity)
     3. Carrier-mediated endocytosis (CME), aka receptor-mediated endocytosis: very specialized system. Certain important molecules or ions are not brought into cell by transport processes, but by CME.
        View animation of receptor-mediated endocytosis (Campbell website activity)
        • Ex. iron is carried through blood tightly bound to transferrin protein carrier. To get iron into cells, cell membrane contains special receptor proteins that bind transferrin, move towards special regions of membrane under which lie clathrin proteins. Endocytosis occurs inside clathrin "cage", moves inside cell. Cage eventually recycles back to cell surface, returning transferrin proteins to cell exterior. However, iron is released inside cell, exits from vesicles, becomes bound to ferritin.
        • View TEM of clathrin cage assembly
        • View diagram showing clathrin assembly and intracellular movement
        • View movie showing assembly of clathrin molecules into clathrin "cage" structure" (Mov file, 443 Kbs)