The hydrophobic core impedes the transport of ions and polar molecules which are hydrophilic.  Hydrophilic molecules pass easily because they can dissolve in the membrane.  Examples are O and hydrocarbons.  Very small polar but uncharged molecules (CO₂ and water) are small enough to pass between lipids in the bilayer.  The bilayer is quite impermeable to large polar but uncharged molecules (glucose and other sugars).

Solutions and Osmosis

hyper tonic = more solute concentration
hypo tonic = less solute concentration
is tonic = same solute concentration

This can also refer to the Concentration of unbound water wgeb referring to water concentration as a result of solute concentration. Osmosis is diffusion of water where the direction of movement is determined by total difference in solute concentration. Diffusion is the movement of something down its concentration gradient (higher concentration to lower concentration).

Where the solution is hypertonic there is less free water.  Where the solution is hypotonic there more free water than needed to have equal free water.  Thus where the solutions are isotonic to each other, each has the same amount of free water.

Water Balance of Cells Without Walls

cell	environment	explanation
isotonic	isotonic	In an isotonic solution, the volume of the animal cell is stable.
hypotonic	hypertonic	The cell loses HOH, shrivels, and dies.  This is one reason why salinity can kill animals.
hypertonic	hypotonic	HOH flows faster into the cell than it leaves and the cell swells and lyses (bursts) like a water balloon.

Seawater is isotonic to many invertebrates.
Terrestrial cells bathe in an isotonic extracellular fluid.

Osmoregulation,  an example of it:  Paramecium uses a contractile vacuole to pump out excess HOH because its membrane is more permeable.

Water Balance of Cells With Walls

environment	explanation
hypertonic	plasmolysis (plasma membrane pulls from the cell wall
isotonic	flaccidity (limpness; no net tendency for HOH to enter the cell; wilting)
hypotonic	turgidity (firm; the wall functions in water balance; healthy state for most plant cells; non-woody plants depend on turgidity for mechanical support--plant cells must be hypertonic to the solution)

Passive Transport

No E (energy) needed for this type of transport; it is the net diffusion across a membrane, occurring because the thermal motion of the molecules is random but a solute will go from a more concentrated area to one with less concentration (diffusing down its concentration gradient).

Transport Proteins

The protein on the right binds to the molecule and physically transports it across the membrane.  It is powered by the hydrolysis of a Phosphate group from ATP to form ADP.

Blue = hydrophilic regions

Specific Proteins that Facilitate Passive Transport

This is called facilitated diffusion.  The proteins involved are similar to enzymes: they are specialized, have a specific binding site, are susceptible to competitive inhibition; however, it is Catalysis of a Physical rxn v. Catalysis of a Chemical rxn.  The protein probably undergoes subtle changes in shape that translocates the binding site.  This could be triggered by binding and releasing the transportee.

Selective corridors (see diagram of Transport Proteins)
Gated Channels = electrical or chemical stimulus causes them to open

Diseases related to the absence of a transport protein
Cystinuria = the cystine (amino acid transporter) in kidneys
Cells usually reabsorb the amines from urine and return them to the blood but amino acids crystallize in the kidneys forming stones.

Active Transport consists of proteins that can move solutes against their concentration gradient. This "uphill" movement is exergonic via direct phosphorylation (Phosphate + Protein)

1. Binding of cytoplasmic Na stimulates phosphorylation.

2. Phosphorylation changes the conformation of the protein translocating the binding site.
   

3. Na expelled and K binds to phosphorylized protein.
4. K binding triggers removal of Phosphate group.
5. Loss of phosphate group returns the protein to its original conformation.
6. K is released inside the cell.

Some ion pumps generate voltage

Voltage is electrical potential E requiring the separation of opposite charges.  Cytoplasm is negative compared to the extracellular fluid because of an uneven distribution of anions and cations.  Membrane potential is the voltage difference across the cell membrane (-50 to -200 millivolts).  Membrane potential acts like a battery, affecting the traffic of uncharged particles and favors cations entering and anions exiting.

Two forces drive Diffusion creating the ElectroChemical Gradient

1. concentration gradient of ions (chemical)
2. effect of membrane potential on the ions movement (electrical)
3. Ions actually diffuse down their electrochemical gradient and not down their concentration gradient.
4. some membrane proteins contribute to membrane potential (pumps)
5. Example:  Na-K pump (is the major pump in animals)
   1. for each 6 steps there is one positive charge out of the cell
   2. this process stores E in the form of voltage
   3. electrogenic pump = protein generating voltage by concentrating substances on one side of a membrane
   4. is used for Cotransport
   5. proton pump = major electrogenic pump of plants, bacteria, and fungi
      it actually transport H ions out of the cell generating a net transfer of one charge and is used to power ATP synthesis reversely (that will be discussed later).

Cotransport couples "downhill" diffusion with "uphill" transport of a substance.  One ATP-powered pump can indirectly drive active transport of several solutes

Example (above) is in plant cells:
          The proton pump indirectly permits sucrose to enter the plant cell.  The proton "has the tickets to get through the front door so he takes a friend with him." (what a nice ion!)

Transportation of Large Molecules: Two methods.

1. exocytosis : when the cell secretes macromolecules (eg insulin, neuron, cell wall creation in plants) by the fusion of vesicles with the plasma membrane
2. endocytosis : when the cell ingests macromolecules by forming vesicles from the plasma membrane

The following are 3 types of endocytosis

pinocytosis (cellular drinking), above

• the cell "gulps" droplets of EC (extracellular) fluid into tiny vesicles
• any and all solutes dissolved in the solvent contained in the droplet are taken into the cell
• this is unspecific in the substances transported

phagocytosis (cellular eating), above

receptor-mediated endocytosis, above

• very specific
• substances that bind to the receptors are called ligands
• the fuzzy layer of coat protein helps deepen the pit on the cell membrane reminiscent of pinocytosis
• enables the cell to acquire molecules in bulk even if they are not concentrated in the environment
• example :  cells take in cholestoral to synthesize membranes and other steroids
  cholestoral travels in the blook via LDL (low-density lipoproteins) through the process of endocytosis
  hypercholesterolemia = when LDLs can't enter the cells, causing a massive buildup of cholestoral in the blood stream leading ot atherosclerosis