Transport Across Membranes Flashcards
2 main categories of transport across membranes:
1) Passive Transport
2) Active Transport
Passive Transport
The diffusion of a substance across a membrane with NO energy investment
Types of passive transport
1) Diffusion
2) Osmosis
3) Facilitated diffusion
Diffusion
Transport of a solute down a concentration gradient
–> Solute goes from areas of high conc. to low conc.
In diffusion, all molecules move___________ but as a population move ___________
RANDOMLY, but as a population diffusion becomes DIRECTIONAL
Each substances diffuses down…
ITS OWN gradient
–> Unaffected by the gradient of others
Equilibrium
Molecules are still moving but concentrations remain constant
Osmosis
The diffusion of free water across a selectively permeable membrane
Osmosis is helpful when…
solute cannot diffuse through a membrane
–> H2O will then move through the membrane to attempt to dilute the more concentrated side
Tonicity
The ability of a surrounding solution to cause a cell to gain or lose water
–> Depends largely on concentration of solutes that cannot cross the membrane
Isotonic
Concentrations are ~ equal on either side of membrane
= NO NET MOVEMENT of water across membrane
Hypertonic
A solution that has > conc. of solute than other solution across a membrane
= Water moves INTO hypertonic solution
Hypotonic
A solution that has < conc. of solute than another solution across a membrane
= Water moves OUT OF hypotonic solution
Most animals have their cells in a ______________ extracellular fluid
ISOTONIC: to maintain a stable cell volume
Osmotic Pressure
The tendency for a solution to take up water when separated from pure water by a selectively permeable membrane
Animal Cell in Hypotonic Solution
= LYSED
–> Too much water entering cell causes it to swell and then burst
Animal Cell in Isotonic Solution
= NORMAL
–> Maintains proper cell volume
Animal Cell in Hypertonic Solution
= SHRIVELED
–> Too much water leaves cell causing it to shrink in on itself
Plant Cell in Hypotonic Solution
= TURGID (ideal condition) – Very firm
–> More water entering cell creates turgor pressure that pushes up against cell wall
–> Keeps plants “perky”
Plant Cell in Isotonic Solution
= FLACCID (limp)
–> Not enough water to produce the turgor pressure needed
Plant Cell in Hypertonic Solution
= PLASMOLYZED
–> Cell inside shrinks but cell wall maintains shape (too much water leaving cell)
Facilitated Diffusion
Diffusion aided by transport proteins (“to make faster”)
Facilitated diffusion can allow for…
polar/charged molecules to diffuse through a membrane using the transport proteins as a “door”
2 types of transport proteins:
1) Channel Proteins
2) Shuttle/Carrier Proteins
Channel Proteins
Provide hydrophilic pathways to allow for very fast diffusion
Carrier Proteins
AKA Shuttle Proteins
–> Undergo a subtle change in shape that translocates molecules across a membrane
Facilitated diffusion still…
MOVES DOWN CONC GRADIENT (cannot go against the gradient)
Active Transport
“pumping” a solute/molecule across a membrane going AGAINST the conc. gradient
–>*Requires energy input to do so
Active transport enables a cell to…
maintain internal concentrations of small solutes that differ from concentrations in its environment
Sodium-Potassium Pump
Import K+ and Exports Na+
–> Cells want high conc. of K+ and low conc. of Na+
Sodium-Potassium Pump is also known as…
Sodium-Potassium ATPase
(Due to hydrolysis of ATP)
Sodium-Potassium Pump Process (4 Steps)
1) 3 Na+ bind to pump (INSIDE cell)
2) ATP phosphorylation of pump causes conformation change = Na+ released to environ.
3) 2 K+ bind to pump with new conform. = triggers dephosphorylation = return to old conform.
4) K+ released into cell
REPEAT
ATP is not the only driver of active transport:
Membrane potential/electrochemical gradient can be coupled to transport molecules
Membrane Potential
The voltage across a membrane
Membrane potential is due to…
Charge differences between cytoplasmic and extracellular membrane sides
–> as a result of unequal cation/anion distribution
If inside of cell is negatively charged (which it is), membrane potential favors…
the passive transport of CATIONS INTO cell
and ANIONS OUT of cell
2 forces driving diffusion of ions:
1) Conc. gradient
2) Membrane potential
Electrochemical Gradient
The combination of the forces of conc. gradient and membrane potential acting on an ion
–> *Ions diffuse down their electrochemical gradient
If conc. gradient and electrical forces oppose…
Active transport may be needed
If conc. gradient and electrical forces align…
Ions diffuse down electrochemical gradient
Electrogenic Pump
Transport protein that generates voltage across a membrane through the active transport of ions
Electrogenic pumps help to store…
ENERGY –> Through generation of the voltage
–> This energy can be tapped for cellular work
Co-transport
Couples favorable movement of one molecule down its conc. gradient and unfavorable movement of another molecule down its conc. gradient
Co-Transport Example with Sucrose
1) H+ it pumped across membrane creating positive charge and high conc. on one side
2) H+ diffuses back through a co-transporter in which sucrose follows along
3) Sucrose is now across membrane
Co-Transporter
Transport protein that couples the transport of molecules
2 main types of active transport:
1) Co-transport
2) Bulk transport
Bulk Transport
A category of transport of large quantities of materials/large molecules/particles across a membrane
Types of Bulk Transport
1) Exocytosis
2) Endocytosis
a. Pinocytosis
b. Phagocytosis
c. Receptor-mediated endocytosis
Exocytosis
Process of cells secreting (excreting) certain molecules by fusion of vesicles with the plasma membrane
(which causes materials inside to release out of cell)
Endocytosis
The taking in of molecules and particulate matter by forming new vesicles from the plasma membrane
Phagocytosis
The engulfing of a particle (usually food) by extending membrane around it and packaging it within a membranous sac called a food vacuole
(NON SPECIFIC)
Pseudopodium
The membrane extension in phagocytosis
Pinocytosis
The engulfing of liquid droplets by infolding part of the membrane to form “pits” that pinch into the cell as a vesicle
(NON SPECIFIC)
Clathrin
AKA Coat Protein
–> Coats the membrane (on cytoplasmic side) that then form coated vesicles when pinched into cell
Receptor-Mediated Endocytosis
SPECIFIC engulfing of substances due to receptors embedded in plasma membrane
(vesicle formation process is similar to pinocytosis)
Receptor-mediated endocytosis is the…
main type of transport of macromolecules across a membrane
Pinocytosis and phagocytosis are…
NON SPECIFIC
