All Molecules Move Continuously by Simple Diffusion

• Heat energy causes molecules to move randomly (sometimes called Brownian motion)
• If the concentration of molecules is different in 2 regions (this produces a concentration gradient), diffusion will cause molecules to move from a region of high concentration to one of low concentration
• The higher the concentration gradient the more rapid the net diffusion
• Diffusion evens out the concentrations so they are equal everywhere (maximum entropy)
• If 2 different substances have the same concentration gradient, usually one will move faster than the other
  • Differences in speed between molecules with the same concentration gradient are given in terms of diffusion constants
  • A substance with a high diffusion constant moves faster than one with a low diffusion constant
  • In general, large molecules move slower than small molecules
  • Diffusion across a membrane is called permeability, and membrane diffusion constants are called permeability constants

Membrane Fluxes are Driven by Forces Existing Across the Cell Membrane

• A flow or flux of a material is proportional to the gradient of force acting on the molecule. The forces arise because conditions are different on the 2 sides of the membrane:
  • Concentration differences
  • Voltage differences
  • Pressure differences
  • Osmotic pressure differences
• Typical forces and fluxes are (the forces are the differences in concentration or other conditions, divided by the membrane thickness).
  
  

  Type of Flux	Conditions Different	Force
  Diffusional Flux	Concentrations, C	Concentration gradient
  Electrical Current	Voltage, E	Voltage gradient
  Bulk (Volume) Flow	Pressure, P	Pressure gradient
  Osmosis (Volume) Flow	Osmotic Pressure, OP	Osmotic pressure gradient

  
  
• More than one force may act upon a molecule. For example, an ion will be driven by both voltage and concentration gradients. Water flow typically is affected by both pressure and osmotic pressure gradients. Since very large amounts of water flow in osmosis there will be significant changes in the volume of the cell.

Simple Diffusion Across Membranes is Called Permeability

• The flow or flux of materials across membranes by simple diffusion is called the permeability
• Simple diffusion spontaneous; it does not require energy from ATP
• In simple diffusion the flux is proportional to the concentration gradient; no saturation is seen
  • The permeability coefficient is the diffusion constant in the membrane divided by the membrane thickness; substances penetrating the membrane rapidly have high permeability constant
  • Flux = (Permeability Constant) x (Concentration Difference)
    
• The flux is always downhill, from a high concentration to a lower one

Hydrophobic Substances Have a High Permeability Through Bilayer Membranes

• Hydrophobic chemicals cross membranes faster than chemicals that like water
• First stage of flow across a membrane is partition from the water into the membrane; hydrophobic molecules partition better into the lipid membrane
• Hydrophobic substances have high permeability coefficients
  
  
  
• Many biological chemicals are deliberately made hydrophobic to increase their rate of penetration into cells.
  • Examples: many drugs, pesticides such as DDT
• Hydrophobicity measured by oil/water partition

Osmosis Moves Water Across Biological Membranes and Causes Volume Changes

• Osmosis = movement of water from low osmotic pressure (dilute solution) to high osmotic pressure (concentrated solution)

• It is useful to think of a dilute solution as having a high water concentration and a concentrated solution as having a lower water concentration. Then the water flow goes from high water to low water concentration. A more accurate picture is that solutes lower the free energy of water.
  • In a mixture the substance present in the greatest amount is called the solvent; in biology the solvent is almost always water
  • Solutes are the dissolved substances in the solvent.
  • Osmotic pressure is computed by adding up the total concentration of solutes (ions are counted separately)
    • Solution with low solute concentration has low osmotic pressure; the water has high free energy
    • Solution with high solute concentration has high osmotic pressure; the water has low free energy
• Osmosis is a type of bulk flow; it is not the same as simple diffusion
• Osmosis is passive: doesn't require ATP energy

Cells Swell in Hypotonic Solutions and Shrink in Hypertonic Ones

• All cells have osmotic problems, especially those which live in dilute solutions (such as freshwater)
• There is so much water that osmosis often causes significant volume changes, causing swelling or shrinking
• Biologists sometimes talk of osmosis in terms of tonicity:
  • If the external solution balances the osmotic pressure of the cytoplasm it is said to be isotonic.
  • If the external solution is more dilute than the cytoplasm it is hypotonic
  • If the external solution is more concentrated it is hypertonic.
  • Cell B is in an isotonic solution. What kind of solutions are cells A and C in?

Cells Have Developed Different Ways of Combating Osmosis

• Osmotic swelling dilutes the cytosol and can eventually cause the cell to burst
• Cells have different ways of preventing excessive swelling:
  • Cell walls of plant, fungal and bacterial cells are rigid and prevent swelling- the walls are strong enough to allow a fairly high pressure gradient
  • Some protozoa have contractile vacuoles which store excess water and then squirt it out (see picture of Paramecium, text p. 150)
  • Most cells pump ions out of the cell, which reduces the internal osmotic pressure; the most important pump is the Na/K pump (see below)

In Facilitated Diffusion Special Proteins Help Move Substances Across Membranes

• Protein transport molecules are used to carry many substances across membranes
• Very specific: allows cell to select substances taken up
• Sensitive to inhibitors that react with protein side chains
• ATP energy not required
• Transport rate reaches a maximum when all of the protein transporters are being used
• Cannot transport molecules against a concentration gradient (no "uphill" transport)
• Example: glucose transporters
  • Glucose is hydrophilic, has very low permeability across lipid bilayer
  • 7 proteins transporting glucose across cell membranes are known
  • Each has 12 hydrophobic sections imbedded in the membrane
  • One of the glucose transporters (GLUT 4) is controlled by insulin in muscle and fat tissue; insulin causes more glucose transporters to be inserted into the cell membrane- this helps to lower blood glucose
    
    
• Some transport proteins form channels with "gates"; gates normally closed but open in response to electrical or chemical stimulus
  • Na and K channels of nerves- responsible for nerve action potentials
• Some transporters carry more than 1 type of molecule (coupled transport)
  • Example: some sugar and amino acid transporters carry Na ion in addition to the organic molecule; both must be present

Endocytosis Can Bring Macromolecules Into the Cell

• In endocytosis the cell membrane bends inward (invaginates), forming a vesicle (endosome) containing extracellular fluid and other substances
• Can bring in large molecules such as proteins; such molecules would normally not diffuse across cell membranes
• Some endocytic vesicles are coated with receptor molecules that selectively bind molecules
  • Example: LDL receptors (LDL = low density lipoprotein)
• The macromolecules are usually digested by lysosomes

Active Transport Uses Energy to Pump Molecules Against a Concentration Gradient

• Membrane pumps are proteins that use ATP energy to move substances across the cell membrane
• Like facilitated diffusion the transport rate has a maximum which occurs when all of the transport molecules are being used
• Also sensitive to inhibition by protein reagents
• Can pump substances from a low concentration to a high concentration ("uphill" transport): called active transport
• Example: the Na/K pump (see figure on p. 152 of text):
  • Found in all cells
  • In humans may account for as much as 30% of basal metabolism
  • Pump is an ATPase because it splits ATP in its operation
  • Pumps 3 Na ions out of cell and 2 K ions in with each pump cycle:
    • 3 Nas bind to sites exposed on inside of cell
    • ATP binds and is hydrolyzed to ADP, leaving a phosphate bound to the pump
    • Pump changes shape, exposing sites to outside of cell
    • 3 Nas leave pump and 2 Ks are bound (to different sites)
    • Phosphate is split from pump
    • Shape changes again, exposing sites to inside
    • 2Ks leave pump to inside
  • Inhibited by drugs such as digitalis, ouabain
  • Purposes:
    • Helps keep osmotic pressure of cell low (protects against swelling)
    • Charges the membrane electrically
    • Sodium gradient used for secondary active transport of amino acids and sugars

Active Transport Produces Concentration Gradients Across Membranes

• Because active transport can pump uphill using ATP for an energy source it produces concentration gradients
• Example 1: Na/K pump:
  • Reduces Na inside cell and raises K
  • Typically K inside the cell will be ~140 mM and outside it will be only ~5 mM (humans)
  • Na will be ~150 mM outside and ~10 mM inside the cell
• Example 2: Ca pump:
  • Ca outside the cell is usually around 1 to 2 millimoles/liter
  • Inside the cell Ca is kept more than 1000 times lower by a Ca pump
  • Important because Ca within cell is very toxic if it gets too high
• Concentration gradients represent stored energy- can be tapped for doing work

Concentration Gradients of Ions Will Electrically Charge Membranes

• Voltage differences are caused by separation of charges
• Ion pumps can charge cell membranes in 2 ways:
  • They may pump more ions in one direction than the other
  • They produce ion gradients- as ions diffuse back through the membrane they carry charge. One side will end up negative and the other positive
  • Example: the Na/K pump sets up a gradient of K ions with a high concentration inside the cell
    • Some K will diffuse back out of the cell, carrying + charge
    • The outside of the cell will become positive and the inside negative
    • Most cells are charged with the inside negative due to the K gradient
    • Diffusion of Na would tend to make the inside positive, but Na is less important because it has a much lower permeability
• Ion gradients are stored electrical energy, like batteries

Many Molecules Enter Cells by Secondary Active Transport

• The Na concentration gradient is used to produce secondary active transport of sugars and amino acids
• Some sugar and amino acid transporters must bind Na as well as the sugar or amino acid (coupled transport)
• Both Na and the organic molecule must be present at the same time and on the same side of the membrane
• Since there is more Na outside the cell, sugars and amino acids get transported mainly from the outside to the inside
• The sugar and amino acid transporters do not use ATP directly, but ATP is required to set up the Na gradient

Knowledge of Transport Mechanisms is Important in Medicine

• Many drugs are made to be hydrophobic so that they cross membranes more easily
  • Example: general anesthetics
• Except for blood flow almost all water movement in body is osmosis
  • Kidney is an osmotic machine: adjusts body water volume by osmosis
  • Medical problems involving osmosis: pulmonary edema, childhood diarrhea, cholera, inflammation of tissues
• Partial inhibition of the Na/K pump with cardiac glycosides strengthens the heartbeat
• Mechanism:
  • inhibition of pump -> more Na inside cell -> exchange with Ca -> more Ca inside heart cell -> stronger contraction
• Cholera treatment by oral rehydration therapy (ORT)
  • Cholera patients loose enormous amounts of water by osmotic diarrhea
    • They die because of dehydration
  • Cholera toxin causes secretion of Cl into intestine -> osmosis & fluid loss
  • To treat this you should give fluids containing both sugar and NaCl
  • This causes transport of both sugar and NaCl into the blood (coupled transport)
  • Blood osmotic pressure rises causing osmotic flow of fluid back into the blood, reversing the loss
  • ORT can save as many as 95% of cholera victims

Comparison of Simple Diffusion, Facilitated Transport & Active Transport

Property	Simple Diffusion	Facilitated Transport	Active Transport
Requires special membrane proteins	No	Yes	Yes
Highly selective	No	Yes	Yes
Transport saturates	No	Yes	Yes
Can be inhibited	No	Yes	Yes
Uphill transport	No	No	Yes
Requires ATP energy	No	No	Yes

• Note that most of the special properties of facilitated and active transport (those checked "yes") are due to the protein nature of the transport molecules.