ch 6 - interactions btwn cells and extracellular environment

diffusion

passive movement of a solute down its concentration gradient (high → low concentration). (bc it wants equal concentration)

• simple

• facilitated

total body water

• 67% is intracellular fluid (ICF) within cells

• 33% is extracellular fluid (ECF) outside of cells

  • 20% of ECF is blood plasma

  • 80% of ECF is interstitial fluid (ISF) around cells

  • many substances are dissolved in ICF and ECF (gases, nutrients, ions, proteins, waste, hormones)

interstitial fluid (ISF)

this fluid is derived from fluid in the bloodstream, and makes up 80% of ECF

blood plasma

makes up 20% of ECF, it contains fluid and plasma and the many substances in fluid. it comes from the blood vessel

movement of substances

• substances constantly move btwn the intracellular environment and the extracellular environment, crossing the plasma membrane

• membrane is selectively permeable

• movement via these processes:

  • diffusion: simple and facilitated

  • osmosis

  • active transport: primary and secondary

  • epithelial transport

  • exocytosis and endocytosis

simple diffusion

due to random thermal motion. higher concentration of solute molecules move toward lower concentration; over time solute molecules will evenly distribute themselves and there will be no net diffusion

facilitated diffusion

requires carrier molecule or transporter, like GLUT for glucose. “helper molecule”

rate of diffusion

___ is a function of:

• concentration difference btwn the 2 regions

• permeability of membrane to the substance

• temp (higher = more movement = faster flux)

• surface area btwn the 2 regions (larger = faster)

• medium through which solute must pass (air = faster than water)

• distance (longer = slower)

diffusion through the membranes

• diffusion thru phospholipid bilayer

• diffusion thru ion channels

diffusion through the phospholipid bilayer

• nonpolar substances like O2 and steroid hormones are much mroe soluble (like dissolved like)

• small, polar molecules also diffuse easily

diffusion through ion channels

• ions diffuse through ion channels (integral proteins)

• channel selectivity (e.g., selective to Na+)

• membrane potential also influences the movement of ions (charge of the cell, usually negative in the cell)

  • difference in electrical charge across the membrane (mV)

  • its an electrical force acting on the ions

gas exchange by diffusion

ex: more o2 outside cell, less inside. so o2 diffuses into the cell. more CO2 is inside the cell than outside so it diffuses OUT of the cell

ion channels

_ ___ are integral proteins that regulate the passage of ions through the plasma membrane.

• can be leak or gated

• most channels are selective

facilitated diffusion

• solute needs a helper, usually the solute is too big or there is no channel

• solute binds to carrier protein, carrier protein changes shape and allows the solute molecule to get inside the cell

• concentrations of solute outside the cell do change, like after a meal there is an influx of glucose. glucose gets in → glycolysis → cycle repeats

carrier protein

___ ___ in the membrane allow passive transport (no ATP required) of glucose into the cell. glucose binds to the carrier and then diffuses down (along, with) its concentration gradient

osmosis

diffusion of WATER down its concentration gradient

• membrane must be selectively permeable to water, and relatively impermeable to the solute

• higher water concentration → lower water concentration

  • = moving from lower solute concentration to a region of higher solute concentration (wants equilibrium)

osmolarity

total solute concentration of a solution (all solutes) = sum of all molarities of all solutes, per Liter of solution

• osmol/L = osm

• 1 osmole (osmol) = 1 mole of solute particles

• 1 mole = amount of compound equal to a substance’s molecular weight

• osmolarity determines water movement

• equal osmolarity in 2 solutions also means equal water concentration in 2 solutions.

  • total number of solute particles per unit volume is equal

osmolarity and cell volume

• osmolarities of ICF and ECF are about 300 mOsm

• can change for many reasons

  • tonicity: refers to relative concentrations of two solutions, which creates osmotic pressure and affects the volume of a cell

tonicity

refers to relative concentrations of 2 solutions, which creates osmotic pressure and affects the volume of a cell

• isotonic: equal osmolarities for internal and external solutions

• hypertonic: higher osmolarity in the external solution

• hypotonic: lower osmolarity in its external solution

isotonic

equal osmolarities (concentration of solute particles in a solvent) for internal and external solutions

hypertonic

higher osmolarity (concentration of solute particles in solvent) in the external solution

hypotonic

lower osmolarity (concentration of solute particles in solvent) in the external solution

primary active transport

transport of a solute AGAINST its concentration gradient, requiring ATP

• carrier: integral protein serving as a pump (e.g., Ca2+, H+, Na+/K+ pumps)

• ATP powers pump

• pumps are also ATPase enzymes that break down into ATP energy

• there is also secondary active transport involving second solute

primary

what kind of active transport pump transports Ca2+ AGAINST its concentration gradient. ATP is broken down to power the pump, and Ca2+ is released into the ECF

sodium potassium pump

the Na+/K+ pump exchanges 3 intracellular Na+ for 2 K+ to create a charge difference, or potential, across the membrane. (3Na+ leaves, and 2K+ enters thru primary active transport)

epithelial transport

transport of substances across epithelial cells

• epithelial cells line hollow organs and tubes

• regulate absorption or secretion of substances

• substances must cross this epithelial membrane via epithelial transport to move btwn blood and organs

  • ex: movement of nutrients across epithelium of small intestine into bloodstream

bulk transport

• endocytosis - brings fluid into cell

• exocytosis - fluid released outside the cell = secretion

endocytosis

portions of cell membrane fold into cell, forming small pockets that pinch off to form vesicles that enclose a small amount of ECF (containing cholesterol, for example), brings that fluid into the cell

exocytosis

membrane bound vesicles (containing hormones, neurotransmitters, etc.) fuse with membrane and release their contents outside of the cell. fluid released outside the cell = secretion

membrane potential

difference in charge across the cell membrane

• inside of cell is negatively charged compared to outside of cell

• concentrations of ions and permeability of the membrane affects the membrane potential

• results from unequal concentrations of Na+, K+ and other ions across the membrane:

  • Na+/K+ pump moves 3Na+ out for every 2K+ moved in → inside more negative

  • pump contains a constant membrane potential

  • there are fixed anions (negative) in the cell. they attract K+ because the membrane is highly permeable to it

    • K+ builds up in cell then some diffuses out thru ion channels

resting membrane potential

-65 to -85 mV

cell signaling

chemical communication btwn cells

• chemical signals are molecules released by cells to travel to a target to cause a response

• target cell has receptors for the signal molecules (receptors are proteins in plasma membrane or in cytoplasm)

• signal binds to receptor, cell detects signal, and transduces it into a physiological response

chemical signals

molecules released by cells to travel to a target to cause a response

target cell

has receptors for the signal molecules (receptors are proteins in plasma membrane or in cytoplasm)

transducer

converts one form of energy into another

• (chemical) signal from cell → physiological response