NOTES: CH 7 part 2 - Transport Across the Cell Membrane (7.3-7.5)
The Permeability of the Lipid Bilayer
● Hydrophobic (nonpolar) molecules, such as hydrocarbons, can and pass through the membrane rapidly
● Polar molecules, such as sugars,
Transport proteins:
● membrane proteins that transport across biological membranes:
-may provide
-may bind to a substance and physically move it across the membrane
-
Movement across the cell membrane can be:
1) PASSIVE
●
2) ACTIVE
● energy-requiring process during which a transport protein pumps a molecule across a membrane, against its
conc. gradient; is energetically “uphill”
7.3 - PASSIVE TRANSPORT (types):
DIFFUSION: net movement of a substance down a concentration gradient
-results from
-results from random molecular movement
-continues until (molecules continue to move but there is no net directional movement)
OSMOSIS:
●; water moves down its concentration gradient
-continues
-at equil. water molecules move in both directions at same rate
Effects of Osmosis on Water Balance
● The direction of osmosis is determined only by a difference in total solute concentration
● Water diffuses across a membrane from the region of to the region of
Water Balance of Cells Without Walls
●Isotonic solution: as that inside the cell; no net water movement across the plasma membrane
●Hypertonic solution: solute concentration is greater than that inside the cell;
●Hypotonic solution: solute concentration is less than that inside the cell;
WATER MOVES FROM HYPO TO HYPERTONIC!!!
● Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment
● To maintain their internal environment, such organisms must have adaptations for , the control of water balance
● The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
Water Balance of Cells with Walls
● Cell walls help maintain water balance
● A plant cell in a hypotonic solution swells until the wall opposes uptake; (firm)
● If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
● In a hypertonic environment, ; eventually, the membrane , a usually lethal effect called plasmolysis
RECAP:In cells with cell walls:
● in a HYPERTONIC environment, occurs; cells shrivel and usually die
● in a HYPOTONIC environment, , causing it to swell; cell becomes more TURGID.
FACILITATED DIFFUSION:
● diffusion of solutes across a membrane, ;
● Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
● Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
7.4 - Active transport uses energy to move solutes against their gradients
● Facilitated diffusion is still passive because the solute moves down its concentration gradient
● Some transport proteins, however, can move solutes against their concentration gradients
The Need for Energy in Active Transport
● Active transport moves substances
● Active transport requires energy, usually in the
● Active transport is performed by specific proteins embedded in the membranes
Examples of Active Transport protein “pumps”:
1) Sodium-Potassium Pump:
-actively pumps Na+ ions out / K+ ions in
-in every pump cycle,
-Na+ and K+ are moved against their gradients (both concentration and electric potential!)
Maintenance of Membrane Potential by Ion Pumps
● Membrane potential is the voltage difference across a membrane
● Two combined forces, collectively called the , drive the diffusion of ions across a membrane:
-A (the ion’s concentration gradient)
-An (the effect of the membrane potential on the ion’s movement)
●Membrane Potential: voltage across membrane; in most cells the interior is negatively charged w/respect to outside
-favors diffusion of cations into cell and anions out of cell
●Electrochemical Gradient: diffusion gradient resulting from the
**The Na+-K+ pump maintains the membrane potential…HOW?** (Think about it!)
ELECTROGENIC PUMPS:
● An electrogenic pump is a transport protein that
● The main electrogenic pump of plants, fungi, and bacteria is a .
2) Proton Pump: pumps protons (H+ ions) out of the cell, creating a proton gradient (protons are more concentrated outside the membrane than inside)…this is an energetically “uphill” process!
-protons then diffuse back into cell
-the force of the proton pushing back through the membrane is used to power the production of ATP
3) Cotransport / Coupled Channels: process where a single ATP-powered pump actively transports one solute and indirectly drives the transport of other solutes against their conc. gradients.
-Example:
7.5 - Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
● Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
● Large molecules, such as polysaccharides and proteins,
BULK TRANSPORT also includes….EXOCYTOSIS & ENDOCYTOSIS:
● transport of large molecules (e.g. proteins and polysaccharides) across cell membrane
Exocytosis / Endocytosis* macromolecules by fusion of vesicles cromolecules by w/the plasma membrane
* vesicle buds from ER or Golgi and migrates to plasma membrane
* used by secretory cells to export products (e.g. . ) / * macromolecules by forming vesicles derived from plasma membrane
* vesicle forms in localized region of plasma membrane
* used by cells to incorporate extracellular substances (e.g. macrophage )
EXOCYTOSIS
● In exocytosis, transport vesicles migrate to the membrane,
fuse with it, and release their contents
ENDOCYTOSIS
● In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
● Endocytosis is a reversal of exocytosis, involving different proteins
Three types of Endocytosis:
1) Phagocytosis: part of the cell membrane engulfs large particles or even entire cells ()
2) Pinocytosis: part of the cell membrane engulfs small dissolved substances or fluid droplets in vesicles
()
3) Receptor-Mediated Endocytosis: importing of specific macromolecules by receptor proteins bind to a specific substance which triggers the inward budding of vesicles formed from (how mammalian cells take up cholesterol)