Transport through cell membranes Flashcards
Cell membranes are:
-composed of phospholipids and proteins
-selectively permeable
Two ways of moving across membrane:
- passive transport
-down a concentration gradient ([high]->[low])
-does not require cellular energy
-may or may not use membrane proteins - active transport
-up a concentration gradient ([low]->[high])
-requires cellular energy (ATP)
-uses membrane proteins
Diffusion
the passive movement of molecules
-molecules move from areas of high concentration to areas of low concentration
Simple vs facilitated diffusion
-Simple:
–movement through intermolecular spaces or membrane openings
–no interaction with carrier proteins
-Facilitated:
–interaction with carrier proteins
Simple diffusion (intramolecular spaces):
-movement through spaces between phospholipids
-molecules must be lipid soluble
-ex: diffusion of O2, nitrogen, CO2, and alcohols
Simple diffusion (protein channels):
-movement through tubular proteins that span entire membrane
-selective: size and electrical charges
-may be gated
-ex: aquaporins
allow passage of water through cell membranes
Gated channels:
Voltage-gated
and
Chemical (ligand) gated
Voltage-gated:
-open/close in response to changes in electrical potential across cell membrane
ex: Na+ and K+ channels
Chemical (ligand) gated:
-open/close in response to binding of chemical
ex: acetylcholine channel
Facilitated diffusion
-carrier mediated diffusion
-molecules infuse using carrier protein
-carrier protein changes shape during transport
-specific
-ex: transport of glucose and amino acids
Rate of facilitated diffusion:
the speed at which the carrier protein can move the substance
Diffusion rate effectors:
-temperature
-molecule size
-concentration gradient
-membrane electrical potential
-pressure differential
Membrane electrical potential example:
-concentration of negative ions is the same on both sides of the membrane
-positive charge applied to right side, negative charge to left
—creates electrical gradient
-negatively charged ions will be attracted towards positive charge on right
–net diffusion from left to right
-this creates concentration gradient, moving ions from right to left
–the two forces: electrical and concentration gradients will eventually balance each other
Osmosis
-diffusion of water through a semipermeable membrane
-uses aquaporins (channels)
-travels down concentration gradient
-movement in and out of cell is normally balanced so no net movement occurs
-net movement of water caused by a concentration difference of water
Solvent vs solute
solvent: a fluid substances dissolve in
ex: water
solute: a substance dissolved in a solvent
ex: salt or sugar
Osmosis across a ——— membrane
semi-permeable
water can pass through membrane, but other solutes cannot
Osmosis with solute concentrations:
osmosis will move towards higher solute concentration
from [high] water to [low] water
Osmotic pressure
-due to nondiffusbale solutes (can’t cross boundary)
-OP pulls fluid across boundary
pressure that develops due to osmosis, the more solutes inside a cell, the higher its osmotic pressure
Hydrostatic pressure
-due to fluid pressing against boundary
-HP pushes fluid across the boundary
-pressure exerted by water against plasma membrane
Filtration:
the movement (down a pressure gradient) of fluid across filtration membranes cause by hydrostatic pressure
-ex: capillaries
Isotonic
-solution and cell have same solute concentration
-no net movement of water
hypotonic
-solution has lower solute concentration than inside the cell
-water moves into the cell by osmosis
-leads to swelling and bursting (lysis)
hypertonic
-solution has greater solute concentration than inside the cell
-water moves out of cell by osmosis
-cell shrivels and becomes crenate
How much NaCl and glucose in isotonic, hypotonic, hypertonic
isotonic: 0.9% NaCl and 5% glucose
hypotonic: <0.9% and 5%
hypertonic: >0.9% and 5%
Solution most commonly given when blood volume needs to be increased quickly
isotonic
solution given to patients with edema (swelling ) to pull water back into blood, but can potentially over increase vascular volume
hypertonic
solutions given with caution because they can result in dangerous lysing of red and white blood cells
hypotonic
Diffusion continued until molecules reach ——
equilibrium
active transport
-requires cellular energy
-moves against concentration gradient [low] to [high]
-uses carrier proteins
-two types: primary and secondary
Primary active transport
-carrier protein uses ATP directly to move molecules against their concentration gradient
-ex: Na-K+ pump
Na+-K+ Pump
-located in plasma membrane
-1 ATP powers pump to transport:
—-3 Na+ ions out of cell
—-2 K+ ions into cell
—-both against concentration gradient
-establishes negative electrical voltage inside cell
-regulates volume of cell: activated by increase in cell volume
Na+-K+ steps:
- carrier in membrane binds intracellular sodium
- ATP phosphorylates protein with bound sodium
- Phosphorylation causes conformational change in protein reducing its affinity for Na+. The Na+ then diffuses out
- the conformational has higher affinity for K+. extracellular potassium binds to exposed sites
- binding of potassium causes dephosphorylation of protein
- dephosphorylation of protein triggers change back to original conformation, with low affinity for K+. K+ diffuses into the cell, and the cycle repeats
Secondary active transport
-uses the energy from one molecule/driver moving down its concentration gradient to power movement of another molecule up its concentration gradient
-ATP used indirectly to create concentration gradient of driver
-Sympoter: transport substance in same direction as driver
-antiporter: transport substance in opposite direction of driver
-ex: sodium-glucose co-transport
Vesicular transport
-one more way to move across membrane
endocytosis:
phagocytosis
pinocytosis
phagocytosis: “cell-eating”
-pseudopods form and flow around SOLID particles. forms a vesicle (phagosome) which is pulled into cell
pinocytosis: “cell drinking”
-fluid phase endocytosis
-brings extracellular fluid and dissolved solutes; fuses with endoscope
uses:
-sample environment
-main way in which nutrient absorption occurs in the small intestine
Exocytosis:
-process where material is ejected from cell
-substance being ejected is enclosed in secretory vesicle
-commonly exocytosed substances: hormones, neurotransmitters, mucus, cellular wastes
