Chapter 4: Movement of molecules Flashcards
Purposes of membranes
- membranes serve as boundaries between the cell interior and extracellular fluid (plasma membrane) and between organelle interiors and the cytosol
- -> regulate what substances enter and leave cells/organelles, how much and how fast
Passive mechanisms of membrane transport
- don’t require energy
- diffusion, osmosis, facilitated diffusion
active mechanisms of membrane transport
- require energy
- active transport, exocytosis, endocytosis
- exo and endocytosis are both bulk transport
Active transport and facilitated diffusion
- both require carrier molecules and are considered transport mechanisms
Diffusion
- movement of molecules from one location to another as a result of random thermal motion; movement of molecules from higher to lower concentration (decrease the concentration gradient)
- used to move substances in and out of capillaries and blood and cells
- most common process most likely
- may or may not occur across a semi-permeable membrane
- uniform concentration is eventually reached
Flux
- amount of material crossing a surface in a unit of time
- magnitude determined by concentration gradient
net flux
- the difference between two one-way fluxes
- the net amount of material transferred from one location to another
- net flux always occurs from regions of higher concentration to regions of lower concentration
- concentration difference determines the magnitude of net flux
- at a given concentration gradient, also influenced by temperature, mass, surface area, and medium
diffusion rate
affected by distance
diffusion time
square of the distance over which molecules diffuse
diffusion through lipid bilayer
- plasma membrane is semipermeable/differentially permeable
- some, not all, molecules can diffuse across the plasma membrane
- small, uncharged, lipid-soluble molecules diffuse easily across lipid bilayer (nonpolar)
- charged (polar) molecules have more difficulty diffusing across lipid bilayer…slowly or not at all
- macromolecules can’t freely cross membrane
Polar molecules and membrane
- lower permeability and harder to cross membrane
Protein Channels
- used by ions because harder for them to pass because charge
- made of integral membrane proteins, either a single one or usually a protein aggregate
- small diameters
- ion selectivity
Electrochemical gradient
- intracellular fluid is usually negatively charged
- extracellular fluid usually positively charged
- influences ion movement
- direction and magnitude of ion fluxes across membranes depend on both the concentration difference and membrane potential (electrical difference)
membrane potential
- separation of electrical charge across the plasma membrane
regulation of diffusion through ion channels
channel gating
- ligand-gated channels
- voltage-gated channels
- mechanically-gated channels
ligand-gated channels
- open and close in response to a certain binding chemical ligate to protein of channel
voltage-gated channels
- open and close in response to membrane potential
mechanically-gated channels
- open and close due to the stretching of the membrane
Mediated transport mechanisms
- responsible for the transport of some ions, and for polar molecules such as amino acids and glucose that are too large to move through channels
- use transporters/carrier molecules (integral mem proteins, undergo conformational change, can move molecules in either direction across mem, chemical specificity)
how mediated transport mechanisms work
- the solute binds to a specific site on the transporter protein
- the protein changes shape, and the solute is released on the other side of the membrane
magnitude of solute flux determined by
- extent of saturation of transporter binding sites (solute concentration and affinity of transporters for the solute)
- number of transporters in membrane
- rate at which conformational change in the transport protein occurs
- finite number of tranporters in any mem for a given solute –> maximal flux (saturation)
mediated transport vs simple diffusion
- in mediated transport, the # of available transporters places an upper limit on the flux magnitude
- in simple diffusion, flux magnitude is limited only by the concentration gradient
2 types of mediated transport
- facilitated diffusion
- active transport (primary and secondary)
Facilitated diffusion
- uses carrier molecules/transporters to transport substances from areas of higher to lower concentration
- doesn’t require ATP
active transport
- uses carrier molecules to transport substances from areas of low to high concentration (against gradient!)
- i.e. sodium/potassium pump
- primary active transport requires ATP
- secondary active transport uses an electrochemical gradient across a membrane to drive the process
NA/K pump
- maintain proper concentrations
- usually high sodium out and potassium in
- concentration moves sodium in and potassium out but need to work against gradient
- uses ATP
cotransport
- form of secondary active transport
- sodium and second solute cross membrane in same direction (symport)
countertransport
- form of secondary active transport
- sodium and second solute move in opposite directions across membrane (antiport)
Osmosis
- the diffusion of water through a semipermeable membrane (movement of water from high to low concentration)
- responsible for much of the movement of fluids
- as solute increases, the water decreases. Water concentration determines the direction of osmosis
aqua porins
- membrane proteins that create channels for water
osmolarity
- the total solute concentration of a solution
- 1 osmo = 1 mol of solute particles
- 1 M solution NaCl contains 2 osmol of solute per liter; osmolarity=2OSM
Osmotic pressure
- the pressure that must be applied to the solution to prevent the net flow of water into it
- attractive force to water
- the higher osmolarity, the higher the osmotic pressure, the more water will move into solution via osmosis
- a tendency of a solution to attract water to move into it via osmosis
extracellular osmolarity and cell volume
- non-penetrating solutes can’t cross plasma membrane (Na+, Cl-, K+ behave as nonpenetrating solutes
- penetrating solutes cross plasma membrane
- at equilibrium, the intracellular and extracellular fluids have an osmolarity of ~300 mOsm
tonicity
- the concentration of nonpenetrating solutes in a solution relative to the concentration of those solutes within the cell
isotonic solution
- has same nonpenetrating solute concentration as the cell
- perfect concentration, stays the same
hypotonic solution
- has a lower nonpenetrating solute concentration than the cell
- cell swells and bursts
hypertonic solution
- has a higher nonpenetrating solute concentration than the cell
- cell gets skinny and shriveled and shrinks
isoosmotic, hypoosmotic, hyperosmotic
- osmolarity of a solution relative to that of the cell in regard to all solutes
Exocytosis
- moves larger molecules or groups of molecules OUT of the cell (bulk transport)
- uses vesicles which fuse with plasma mem
- requires energy ATP
- secretions ~ cells release subs that they generate into extracellular fluid
endocytosis
- moves larger molecules, whole cells, or groups of molecules INTO the cell
- requires ATP
3 mechanisms endocytosis may occus
- pinocytosis
- receptor-mediated
- phagocytosis
pinocytosis
- cell drinking
- invagination of the plasma mem forms a vesicle that encases a portion of the extracellular fluid and substances suspended in it
- nonselective
receptor-mediated endocytosis
- requires specific receptors
- specific and selective
phagocytosis
- cell eating
- cells engulf whole cells, pathogens, parts of cells, or large organic molecules
transcytosis
- endocytosis and exocytosis move a molecule across a cell
- capillary exchange of fatty acids, albumin, and hormones such as insulin
luminal/apical/mucosal membrane
lines the hollow organs
basolateral/serosal membranes
opposite the luminal mem, usually adjacent to blood
lateral membrances
face adjacent epithelial cells
epithelial transport pathways
- paracellular and transcellular
paracellular pathway
- diffusion between adjacent epithelial cells
transcellular pathway
- movement through an epithelial cell (involves movement of the substance across the luminal and basolateral mem)
epithelial cells
- form lining of hollow organs and tubes and therefore epithelial cells regulate absorption/secretion across those surfaces
