Membrane Transport Flashcards
Does homeostasis = equilibrium?
No
- ICF and ECF are in osmotic equilibrium
- chemical and electrical are in disequilibrium
Osmosis:
simple diffusion of H2O through selectively permeable membrane
Which direction does osmosis flow?
from high [H2O] to low [H2O]
Normal osmolarity for ECF and ICF
300 mOsm
Osmolarity equilibrium:
- osmolarity for ECF and ICF have to be equal
- if different, then H2O moves to fix it
Osmotic pressure:
- amount of pressure needed to stop osmosis from happening
- measured in mmHg
- 1 mOsm/L = 19.3 mmHg
What monitors osmotic pressure in the body?
osmoreceptors in hypothalamus
Oncotic pressure:
osmotic pressure of plasma proteins
Isoosmotic can be
isotonic / hypotonic
Hyperosmotic can be
isotonic / hypotonic / hypertonic
Hypoosmotic can be
hypotonic
Tonicity:
- describes volume change of a cell
- no unit for it
- affects steady state volume of cell
How is tonicity determined?
by # of nonpermeable ECF solutes
How does isotonic solution affect cell?
no osmosis, so no change in cell
How does hypotonic solution affect cell?
H2O moves into cell and causes swelling / lysis
How does hypertonic solution affect cell?
H2O moves out of cell and causes shriveling / crenulating
3 types of transport processes for molecules:
- vesicular: bulk
- passive: doesn’t need E
- active: needs E
Vesicular transport:
- bulk
- moves substance across membrane w/ vesicle
- membrane will alter to form vesicle
- uses E by breaking down ATP/GTP
- needs increase in intracellular [Ca2+]
- ex: endocytosis, exocytosis, transcytosis
Endocytosis:
- ex of vesicular transport
- receptors regulate endocytosis
- infolding of membrane to allow large molecule to enter the cell
- phagocytosis: brings solids into cell (cell eating)
- pinocytosis: brings liquid into cell (cell drinking)
Exocytosis:
- ex of vesicular transport
- fusion of vesicle w/ membrane to eject molecule from cell
- needs Ca2+ and ATP
- constitutive: product made and released immediately
- regulated: product made and stored until signaled for release
Transcytosis:
- ex of vesicular transport
- moves substance across cell
Passive transport:
- doesn’t need E
- uses diffusion and osmosis
Diffusion:
- movement of solutes from high [ ] to low [ ]
- driven by gradients until equilibrium is reached
Where does E for diffusion come from?
brownian motion: random thermal motion of atoms/molecule
Chemical gradient is…
difference in [ ] that causes net movement from higher [ ] to lower [ ]
Electrical gradient is…
- difference in charge causes net movement
- like charges repel and unlike charges attract
What is the electrochemical gradient?
a combination of electrical and chemical gradient
Simple diffusion
- carrier independent
- substance moves through membrane / channel
- lipid soluble: moves between membrane
- non-lipid soluble: moves through membrane via channels and pores
T/F: simple diffusion uses active transport
F, uses passive transport
Channel proteins:
- transmembrane protein w/ central H2O passage / pore
- aquaporins: H2O channels used for osmosis
- gated ion channels: allow channel to open (permeable) /close (nonpermeable)
Gated ion channels are classified by:
- selectivity to one or more ions
- conductance
- rectification
Selectivity of ions are based on:
- diameter of ion
- shape of channel
- electrical charge of channel
What does conductance measure?
measures how readily an ion moves through channel
Rectification is…
- the directional ease of movement through channel
- inward rectifier: movement of ions into cell is easier than out
- outward rectifier: movement of ions out of cell is easier than in
Types of gated channels:
- leak
- voltage gated
- ligand
- mechanical
- intracellular messenger
Leak gated channels:
- spontaneously open and close
- mostly for H2O
- high probability for opening
Voltage gated channels:
- responds to alterations in membrane potential
- change in charge polarity will cause it to open
Ligand gated channels:
- open by binding to signaling molecule
- ex: neurotransmitters, hormones, drugs
Mechanically gated channels:
- responds to physical conformation
- ex: stretch or pressure
Intracellular messenger gated:
- responds to change in intracellular signal
- ex: increase in intracellular Ca2+/cAMP
Facilitated diffusion:
channel/carrier dependent (for molecules too big/polar for pores)
Carrier proteins:
- transmembrane protein w/ binding sites for molecule to be moved
- undergoes conformational change
Ability of carrier proteins to move molecules is affected by…
- competitive inhibition
- non-competitive inhibitions
- saturation: not as many carriers as molecules
- specificity: carriers are specific to molecules
Competitive inhibtion:
something else attaches to binding site instead of substrate
Non-competitive inhibition:
something else attaches to different area on carrier and changes shape so substrate can’t bind
Flick’s law determines…
- net movement when there’s no electrical or pressure difference
- J=PA[C1-C2] or J= - DA[change in C/change in X]
Net flux is inversely proportionate to…
- molecular weight
- higher the molecular weight = lower speed
Net flux is directly proportionate to…
- temp: higher temp = higher brownian movement
- lipid solubility: higher solubility = higher diffusion rate b/c higher area for exchange
What is the impact of electrical charge:
- affect ions that diffuse through ion channels according to electrical/electrochemical differences
- not accounted for by Flick’s Law
- rate affected by # of protein channels present in membrane
Nernst equation is used to determine…
- electrochemical equilibrium potential of any ion
- also the electrical difference across the membrane where ion will reach its dynamic equilibrium
Nernst equation:
- assumes free permeability of ion
- Eion=[-61.5/z]log([ioninside]/[ionoutside])
Diffusion trapping:
- alteration of solute after it’s moved across membrane, which preserves gradient
- allows for increased diffusion and increased rate of diffusion
Active transport:
- movement of substance across membrane against electrochemical gradient
- needs a pump type of carrier protein
Primary active transport:
- all transported molecule are moving against the gradient
- needs pump powered by ATP
T/F: primary active transport get E indirectly from hydrolysis of ATP
F, gets E directly from hydrolysis of ATP
Secondary active transport:
- uses carrier that has binding sites for two molecules
- one molecule is moved with the gradient and drives the transport (Na+)
- the other is linked and moves against its gradient
Secondary active transport gets E ______ from hydrolysis of ATP
indirectly
- Na+ gets pumped back out of cell w/ Na+/K+ ATPase
Types of pumps and exchangers:
- uniporters
- symporters
- antiporter/exchanger
Uniporters:
- is a primary active transporter
- bind and transport only one substance against gradient
- ex: Ca2+ ATPase and SERCA
Symporters:
- binds 2+ different substances on same side of membrane
- both move in same direction
- secondary active transporter: one will move with its gradient and one will move against its gradient
Symporters are also called…
cotransporters
Antiporter/exchanger:
- binds 2 substances from different sides of membrane
- can use both primary and secondary active transport
- primary: both against gradient
- secondary: one w/ gradient and one against
Antiporter/exchanger is also called…
countertransporters
Na+/K+ ATPase
- most common antiporter/exchanger
- establishes RMP
- primary active transport
- 3 Na+ moves out and 2 K+ moves in
- accounts for 1/3 of body E supply
- decrease in activity = more positive membrane potential
Resting membrane potential (RMP):
- cellular proteins that are stuck inside cell usually have net negative charge
- inhibits movement of cations (K+) out of cell
- favors movement of anions (Cl-) out of cell
Excitable cells:
- nerve and muscle
- have lower RMP than non-excitable cells
Examples of tissues that are excitable:
- skeletal muscle
- spinal nerves
- cardiac ventricular myocytes
- neurons of CNS
- smooth muscle
Factors that contribute to RMP:
- difference in permeability of membrane to ions b/c of large conductance of K+ via leak channels
- proteins trapped in cell and act as anions, which makes inside more negative
- electrogenic pump (Na+/K+ ATPase)
- equilibrium potential of all permeant ions (increase in permeant ions = increase in ability to force membrane)
Membrane potential is weighted average of _____
equilibrium of potential of all permeant ions
- Na+, K+, and Cl- are important
Weighting factor:
accounts for relative permeability of ion
