Taylor Chapter 6

CHAPTER 6:

• Lipid – Carbon containing compound: Insolubule in water. o Most prominent for role in cell membranes

• Hydrocarbons – nonpolar because electrons are shared equally in C-H bonds

• Fatty acid – lipid consisting of a hydrocarbon chain bonded to a polar carboxyl functional

  group.
  o Nonpolar tails

• 3 types of lipids:
  o Steroids – role as hormones in cell signaling
  o Fats – energy storage
  o Phospholipids – crucial component to plasma membrane

  ▪ Hydrophilic head

  ▪ Hydrophobic tail

• Amphipathic – containing both hydrophobic and hydrophilic regions

  o Ex. Phospholipids, Cholesterol
  o When placed in water there are two outcomes:

  ▪ Micelles: Hydrophilic heads interact with water, hydrophobic tails interact with one another. -- CIRCLE

  ▪ Lipid bilayers: hydrophilic heads interact with water while hydrophobic tails interact with one another. -- LAYER

• Foundation of cellular membranes.

• Vesicles – small bubble-like structures consisting of lipid bilayers, tails are sheilded from

  water and heads remain in contact with water.

• Liposomes – artificially generated membrane-bound vesicles, provide a 3-D model that

  mimics a membrane-bound cell.

• Permeability – tendency to allow a given substance to pass through it.

• Selective permeability – some substances cross the membrane more easily than other

  substances do.

o Charged solutes (ions) can’t effectively pass through bilayers without assistance

from membrane proteins.
o Lower permeability = easier to get through o Higher permeability = harder to get through.

• Adding cholesterol molecules to artificial membranes drastically reduces their permeability.

  o Lower temperature = lower permeability/may even solidify

• Diffusion – spontaneous movement of molecules and ions.

  o Moves substances in either direction across a membrane to make the cell interior and exterior environments more similar.

• Concentration gradient – difference in solute concentrations

o Net movement - A greater number of solutes move away from regions of high concentration than solutes moving in the opposite direction.

o Diffusion down a concentration gradient, away from high concentration, is a spontaneous process because it results in an increase in entropy.

• Passive transport – when substances diffuse across a membrane in the absence of an outside energy source.

• At equilibrium, movement doesn’t stop but is equally moving in any direction.

• Osmosis – the diffusion of water across a selectively permeable membrane.

o when solutions of different solute concentrations are separated by a membrane that permits water to cross.

o Dilutes the higher concentration of solute as water diffuses across the membrane. o Entropy will increase as the difference in solute concentrations decreases.

• The First lipid bilayers were important as they were a container for replicating the firs “living” molecule.

• Protocells – simple vesicle-like structures that harbor nucleic acids. o Had simple, permeable membranes

• The hydrophobic region of an amphathic protein can be anchored into a lipid bilayer.

• Integral membrane proteins – any membrane protein that spans the entire lipid bilayer

  (transmembrane protein).

• Peripheral membrane proteins – proteins that bind to membrane lipids or integral

  membrane proteins without passing through it.

• Ion channels – allow ions to routinely cross membrane by way of specialized

  transmembrane proteins.

o Form openings in a membrane
o Ions diffuse through the openings from regions of high concentration to regions of

low concentration & from areas of like charge to areas of unlike charge. • Electrochemical gradient – combined concentration and electrical gradient

• Channel proteins – some are for ions and others for small polar molecules
o Each channel protein has a structure that permits only a particular type of ion or

small molecule to pass through.
o Allow movement through a selective pore

• Aquaporin – allows water to cross the plasma membrane but excludes other molecules and most ions.

  o Key side chains in the interior of the pore function as a filter.

• Gated channels – open or close in response to a signal

• Facilitated diffusion – when transmembrane proteins assist the passive transport of

  substances that otherwise would not cross a membrane readily.

• Carrier proteins - facilitate diffusion

  o Selectively pick up a solute on one side of the membrane then drop it off on the other side.

• Active transport – transport of molecules against a gradient.

• Secondary active transport (cotransport) - ATP pump provides the energy in the form of a

  gradient that is used to power the movement of a different solute in a directed manner, often against its gradient.