Physio: Membrane

briefly describe the cell membrane

• amphipathic phospholipid bilayer

  • interior is hydrophobic

  • exterior is polar

• contains cholesterol

  • maintains membrane fluidity and stability

describe the 2 types of proteins in plasma membranes

• integral proteins

• peripheral proteins

describe integral plasma membrane proteins

• tightly associated within the bilayer

  • inserted halfway or all the way through

describe peripheral plasma membrane proteins

• located at the membrane surface

  • loosely attached

what separates the extracellular fluid and interstititial fluid with the intracellular fluid comparments

• plasma membrane

  • responsible for the differences in the compositions

describe the four major membrane transport mechanisms

• simple diffusion

• facilitated diffusion

• primary active transport

• secondary active transport

describe simple diffusion

• movement of molecules from high concentration to low concentration

  • passive process because it is thermodynamically favorable

  • no ATP used

• will diffuse until there is NO FLUX

  • equal speed and distribution

in regards to magnitude and direction of diffusion across a cells membrane, what is the net flux directly proportional to?

• fick’s law

  • concentration difference

    • the larger the difference, the greater the net flux

  • permeability co-coefficient

    • the lipid solubility and molecule

  • area of membrane

    • greater area means more available space for diffusion

describe the types of molecules that are permeable to the phospholipid bilayer

• hydrophobic

• small

what is net flux indirectly proportional to?

• distance

  • the greater the flux, the shorter the distance

• how is this addressed?

  • most cells are close to capillary so length of time for a substance to diffuse from substance to capillary is short

describe ion channels

• the main method of facilitated simple diffusion for ions

• show selectivity for the type of ion

describe ligand gated ions

• activated in response to a chemical messenger binding to them

describe mechanically gated ion channels

• activated by the physical deformation of the surrounding plasma membrane

describe voltage gated ion channels

• activated by changes in the membrane potential

where does membrane potential come from

• membrane potentials exists across plasma membranes caused by a separation of electrical charges

• provides electrical force that influences movement of ions across the membrane

the charge of the extracellular environment is…

positive

the charge of the intracellular environment is…

negative

describe facilitated diffusion

• a type of simple diffusion that occurs down a concentration gradient

  • not coupled to ATP

• movement is bidirectional

ion channels are a type of…

facilitated diffusion

• no energy is needed

glucose transporters are a type of…

• facilitated diffusion

  • no energy needed

describe glucose facilitated diffusion

• glucose binds to a specific site on the GLUT transporter

• undergoes conformational change

• glucose is released into the cell

describe active transport

• utilizes energy to move an ion or solute against its electrochemical gradient

• make possible the large differences in the concentrations of substances of intracellular and extracellular compartments

describe primary active transport

• uphill transport across the membrane coupled to ATP hydrolysis

• example

  • NA+/K+ ATPase

    • 3 2 1 NOKIA

      • 3 Na OUT

      • 2 K IN

      • 1 ATP used

what are examples of primary active transporter pumps

• Sodium potassium pump

• Calcium pump

• Hydrogen ion pump

• Hydrogen potassium pump

what maintains the concentration gradients between the ICF and ECF

• Sodium potassium pump

  • keeps the intracellular K+ concentration high

  • keeps the extracellular Na+ concentration low

describe the primary active transport mechanism of the sodium potassium pump

• ATP binds to pump

• 3 Na+ from the inside bind to the pump

• ATP is broken down and pump is phosphorylated

• phosphorylation triggers conformational change and releases 3 Na+ from inside to outside

• 2 K+ binds to pump

• Pump is dephosphorylated

• Dephosphorylation triggers conformational change and releases 2 K+ from outside to inside

describe secondary active transport

• transport proteins utilize the passive energy gradient for one solute to facilitate the uphill transport of a second solute

• has two binding sites

what kind of energy is used in secondary active transport

• indirect

  • no ATP hydrolysis is directly used, but ATP was used to SET the gradient for the first solute

describe the step by step mechanism of the secondary active transport pump

• sodium potassium pump used ATP to set up a Na+ gradient (Na+ high concentration outside)

• Na+ and solute bind to transporter

• pump undergoes conformational change

• both Na+ and solute comes out the other side

describe how the Na+ gradient is used to power secondary active transport systems

• Na+ concentration AND voltage gradient favor Na+ entry into cells

  • Sodium potassium pump maintains a relatively low ICF Na+ concentration

  • there is a negative charge inside the cell

describe secondary symport/co-transport

• both solute and Na+ are being transported in the same direction

describe secondary anti-port

• the solute and Na+ are being transported in opposite directions

describe endocytosis

• membrane bound vesicles entering cells WITHOUT passing through the plasma membrane

describe exocytosis

• membrane bound vesicles exiting cells WITHOUT passing through the plasma membrane

Describe pinocytosis

• vesicles engulf extracellular fluid

describe phagocytosis

immune cells engulfing bacteria

describe receptor-mediated endocytosis

• specific proteins on the outer surface of plasma membrane recognize a ligand and activate membrane invagination

describe transcellular transport

• movement into a cell, through the cytosol, and exit across the opposite membrane

  • apical and basolateral membrane contain different transporters for this

describe paracellular transport

• diffusion between epithelial cell layers

  • limited by tight junctions that form a seal around the apical ends of

describe the capillary endothelium

• wall of capillary composed of a single layer of epithelial cells

  • separates the plasma from the ISF

  • has a very high permeability to solutes and water

what is the implication of the high permeability of the capillary endothelium

• differences in the composition of plasma and interstitial fluid are small

  • EXCEPT higher protein content in plasma