Chapter 4. Movement of Molecules Across Cell Membranes
hypotonic
This has a nonpenetrating solute concentration lower than that found in cells
surface area of the membrane
A stands for what in the Fick diffusion equation?
osmotic pressure
Application of sufficient pressure to a solution will prevent the osmotic flow of water into the solution from a compartment of pure water. What is this called?
in proportion to the square of the distance over which the molecules diffuse
Diffusion times increase in proportion to what?
It moves three Na+ ions out of the cell and two K+ ions into the cell. This results in a net transfer of positive charge to the outside of the cell; therefore, this transport process is not electrically neutral.
For each molecule of ATP hydrolyzed, how many Na+ and how many K+ does the Na+K+pump move across the cell?
by opening or closing ion channels
How can flux of ions across a membrane be altered?
by monitoring the rate at which its intracellular concentration approaches diffusion equilibrium with its concentration in the extracellular fluid.
How can the rate at which a substance diffuse across a plasma membrane be measured?
by diffusing through the lipid bilayer, and by diffusing through protein channels in the membrane
How does water cross membranes?
secondary active transport
In this transport, the downhill flow of an ion is linked to the uphill movement of a second solute either in the same direction as the ion (cotransport) or in the opposite direction of the ion (counter transport).
secondary active transport
In this type of transport, the movement of Na+ is always downhill
secondary active transport
In this type of transport, the movement of an ion down its electrochemical gradient is coupled to the transport of another molecule, such as a nutrient like glucose or an amino acid.
endocytotic vesicles
Most of these fuse with endosomes, which in turn transfer the vesicle contents to lysosomes for digestion by lysosomal enzymes.
1 mol of solute particles
One osmol is equal to what?
an increase in the volume of the compartment on the side that initially had the higher osmolarity, and a decrease in the volume on the side that initially had the lower osmolarity.
Osmosis across a membrane that is permeable to water but impermeable to solute leads to what?
membrane permeability coefficient
P stands for what in the Fick diffusion equation?
net flux of solute from a lower to a higher concentration across a membrane
Simple diffusion and facilitated diffusion don't use energy so they can't produce what?
ion channels
Some integral membrane proteins form this that allow ions to diffuse across the membrane.
ligand-gated channels
The binding of specific molecules to channel proteins may directly or indirectly produce either an allosteric or covalent change in the shape of the channel protein.
both the direction and the magnitude of the net flux
The greater the difference in concentration between any two regions, the greater the magnitude of the net flux. Therefore, the concentration difference determines what?
osmotic pressure
The greater the osmolarity of a solution, the greater its what?
the faster the net flux across the membrane for any given concentration difference and membrane surface area
The greater the permeability coefficient, the what?
difference in concentration across the membrane
The magnitude of the net flux is directly proportional to the what?
voltage-gated channels
The membrane potential can cause movement of certain charged regions on a channel protein, altering its shape
counter transport or antiport
The movement of the actively transported solute going in the opposite direction as Na+, out of the cell
cotransport or symport
The movement of the actively transported solute going in the same direction as Na+, into the cell
greater concentration to the region of less concentration
The net flux of material across the membrane is from the region of what?
the intracellular fluid has an osmolarity equal to that of the extracellular fluid
The osmolarity of the extracellular fluid is about 300 mOsm. Describe the osmolarity when water comes to diffusion equilibrium across the cell membrane.
transporters
The passage of molecules and the non diffusional movements of ions are mediated by integral membrane proteins
electrochemical gradient
The two driving forces: the concentration difference and the membrane potential are collectively known as what?
GLUTs
These are glucose transporters
Ions
These diffuse across membranes by passing through ion channels formed by integral membrane proteins.
aquaporins
These form channels through which water can diffuse.
nonpolar molecules
These have large permeability constants.
Ion channels
They can exist in an open or closed state, and changes in a membrane's permeability to ions can occur rapidly as these channels open or close.
active transport
This is a mediated transport process that moves molecules against an electrochemical gradient across a membrane by means of a transporter and an input of energy.
facilitated diffusion
This is a mediated transport process that moves molecules from higher to lower concentrations across a membrane by means of a transporter until the two concentrations become equal. Metabolic energy is not required for this process.
membrane potential
This is a separation of electrical charge that exists across plasma membranes of all cells. The magnitude is measured in millivolts.
Potocytosis
This is a special type of receptor-mediated endocytosis in which vesicles called caveolae deliver their contents directly to the cytosol.
ATPase
This is a transporter that is an enzyme that catalyzes the breakdown of ATP and in the process, phosphorylates itself.
isotonic
This is any solution that does not cause a change in cell size. They have the same concentration of non penetrating solutes as normal extracellular fluid.
diffusion equilibrium
This is reached when the concentrations of the diffusing substance in the two compartments become equal.
flux
This is the amount of material crossing a surface in a unit of time.
osmosis
This is the diffusion of water across a membrane from a region of higher water concentration to a region of lower water concentration.
primary active transport
This is the direct use of ATP.
Simple diffusion
This is the movement of molecules from one location to another by random thermal motion.
mediated transport
This is the movement of substances through a membrane by using transporters or carriers. This is dependent on the conformational changes of the transporters.
osmosis
This is the net diffusion of water across a membrane
facilitated diffusion
This is the net flux of a molecule across a membrane that always proceeds from higher to lower concentration, or "downhill" across a membrane. It uses a transporter to move solute.
channel gating
This is the process of opening and closing ion channels, like the opening and closing of a gate in a fence.
osmolarity
This is the total solute concentration in a solution. It determines the water concentration: the higher this is in a solution, the lower the water concentration.
osmolarity
This is the total solute concentration of a solution
secondary active transport
This is the use of an electrochemical gradient across a membrane.
simple diffusion
This is when movement of molecules move from one location to another due to their random thermal motion
diffusion equilibrium
This is when no further change in the solute concentrations of the two compartments will occur because of the equal rates of diffusion of solute molecules in both directions between the two compartments.
endocytosis
This is when regions of the plasma membrane invaginate and pinch off to form vesicles that enclose a small volume of exracellular material.
mediated transport system
This kind of transport of molecules or ions across a membrane involves binding the transported solute to a transporter protein in the membrane. Changes in the conformation of the transporter move the binding site to the opposite side of the membrane, where the solute dissociates from the protein.
net flux
This occurs between two compartments and it always proceeds from higher to lower concentrations.
net flux
This occurs between two compartments at any instant and is the difference between the two one-way fluxes.
exocytosis
This occurs when intracellular vesicles fuse with the plasma membrane,
exocytosis
This provides a means of adding components to the plasma membrane and a route by which membrane impermeable molecules, such as proteins the cell synthesizes, can be released into the extracellular fluid.
osmotic pressure
This represents the amount of pressure that would have to be applied to a solution to prevent the net flow of water into the solution.
Na+K+ATPase pump
This transporter is present in all cells. It moves Na+ ions from intracellular to extracellular fluid, and K+ ions in the opposite direction. In both cases, the movements of the ions are against their respective concentration gradients.
secondary active transport
This used the binding of ions to the transporter to drive the secondary-transport process.
active transport
This uses energy to move a substance uphill across a membrane against the substance's concentration gradient. It requires a substance to bind to the transporter in the membrane. They are pumps. It shows specificity and saturation.
Primary active transport
This uses the phosphorylation of the transporter by ATP to drive the transport process.
secondary active transport
This uses the stored energy of an electrochemical gradient to move both an ion and a second solute across a plasma membrane.
secondary active transport
Transporters that mediate this type of transport have two binding sites, one for an ion and another for the cotransported molecule.
temperature, the mass of the molecule, surface area, the medium through which the molecules are moving
What additional factors does the magnitude of the net flux depend on?
oxygen, CO2, fatty acids, and steroid hormones
What are examples of nonpolar molecules that diffuse rapidly through the lipid portions of membranes?
K+ and various organic solutes
What are the major effectively non penetrating solutes in the intracellular fluid?
fluid endocytosis, phagocytosis, receptor-mediated endocytosis
What are the three classes of endocytosis?
through the extracellular spaces between the cells-the paracellular pathway and through the cell, across both the luminal and basolateral membranes as well as the cell's cytoplasm, the transcellular pathway.
What are the two pathways that molecules can cross an epithelial layer of cells?
facilitated diffusion and active transport
What are the two types of mediated transport?
The extent to which the transporter binding sites are saturated, the number of transporters in the membrane determines the flux at any level of saturation, and the rate at which the conformational change in the transport protein occurs
What are there factors that determine the magnitude of solute flux through a mediated-transport system?
Oxygen, nutrients, and other molecules enter and leave the smallest blood vessels (capillaries) by simple diffusion. Also, the movement of many substances across plasma membranes and organelle membranes occurs by simple diffusion.
What are two examples of simple diffusion in the body?
primary active transport and secondary active transport
What are two ways of coupling energy to transporters?
barriers
What do membranes act like to slow the diffusion of molecules across their surfaces?
chemical specificity, affinity, and saturation
What do the binding sites on mediated transporters exhibit?
the movement of ions across the membrane
What does an electrical force of the membrane potential influence?
the ability of cells to actively transport a substance between the fluid on one side of the cell and the fluid on the opposite side
What does the difference between the permeability and transport characteristics of the apical and basolateral plasma membranes in epithelial cells result in?
The concentration gradient and the membrane potential
What does the diffusion of ions across a membrane depend on?
the concentration difference and the electrical difference (the membrane potential)
What does the direction and magnitude of ion fluxes across membranes depend on?
the solute concentration and the affinity of the transporters for the solute
What does the extent to which the transporter binding sites are saturated depend on?
the degree of transport saturation, the number of transporters in the membrane, and the rate at which the conformational change in the transporter occurs
What does the magnitude of the flux through a mediated transport system depend on?
It causes water to move by osmosis in the same direction as the transported Na+
What happens when the active transport of Na through an epithelium increases the osmolarity on one side of the cell and decreases it on the other,
Na+ and Cl-
What ions are the major effectively non penetrating solutes in the extracellular fluid?
they can show selectivity for the type of ion or ions that can diffuse through them
What is an important characteristic of ion channels?
They are in a continuous state of movement or vibration.
What is one of the fundamental physical features of molecules of any substance, whether solid, liquid, or gas?
Binding of K+ results in dephosphorylation of the transporter. This returns the transporter to its original conformation, resulting in reduced affinity of the K+ binding sites and increased affinity of the Na+ binding sites. K+ is therefore released into the intracellular fluid, allowing new molecules of Na+ (and ATP) to be bound at the intracellular surface.
What is the fifth step in the Na+K+ATPase pump?
Initially, the transporter, with an associated molecule of ATP, binds three Na+ ions at high-affinity sites on the intracellular surface of the protein. Two binding sites also exist for K+, but at this stage they are in a low-affinity state and therefore do not bind intracellular K+.
What is the first step in the Na+K+ATPase pump?
The new conformation of the transporter results in an increased affinity of the two binding sites for K+, allowing two molecules of K+ to bind to the transporter on the extracellular surface.
What is the fourth step in the Na+K+ATPase pump?
its chemical composition, namely the hydrophobic interior of its lipid bilayer
What is the major factor limiting diffusion across a membrane?
Binding of Na+ results in activation of an inherent ATPase activity of the transporter protein, causing phosphorylation of the cytosolic surface of the transporter and releasing a molecule of ADP.
What is the second step in the Na+K+ATPase pump?
the channel diameter, the charged and polar surfaces of the protein subunits that form the channel walls and electrically attract or repel the ions, and on the number of water molecules associated with the ions (so-called waters of hydration)
What is the selectivity of ion channels based on?
Phosphorylation results in a conformational change of the transporter, exposing the bound sodium ions to the extracellular fluid and, at the same time, reducing the affinity of the binding sites for Na+. The Na+ ions are released from their binding sites.
What is the third step in the Na+K+ATPase pump?
Net water movement
What occurs from a region of lower osmotic pressure to one of higher osmotic pressure?
They undergo too many collisions
What prevents individual molecules from traveling very far in a straight line?
They will distribute uniformly throughout a container.
What would happen eventually to a random thermal motion of molecules in a liquid or gas?
osmotic pressure
When a solution containing solutes is separated from pure water by a semipermeable membrane (a membrane permeable to water but not to solutes), the pressure that must be applied to the solution to prevent the net flow of water into it is known as this.
type 1 diabetes mellitus
When insulin is not available, muscle and adipose cells cannot efficiently transport glucose into their cells because fewer GLUTs exist in the plasma membranes of those cells. This contributes to the accumulation of glucose in the extracellular fluid.
over the short distances between the blood, interstitial fluid, and intracellular fluid.
Where does diffusion provide movement?
because they can dissolve in the non polar regions of the membrane occupied by the fatty acid chains of the membrane phospholipids.
Why do non polar molecules have large permeability constants?
Although the movement of individual molecules is random, the net flux is always greater from regions of higher concentration to regions of lower concentration.
Why do we say that substances move "downhill" by diffusion?
the greater the surface area between two regions, the greater the space available for diffusion and, therefore, the faster the net flux
Why does the magnitude of the net flux depend on surface area?
the more elevated the temperature, the greater the speed of molecular movement and the faster the net flux
Why does the magnitude of the net flux depend on temperature?
large molecules such as proteins have a greater mass and lower speed than smaller molecules such as glucose and, consequently, have a slower net flux
Why does the magnitude of the net flux depend on the mass of the molecule?
molecules diffuse more rapidly in air than in water
Why does the magnitude of the net flux depend on the medium through which the molecules are moving?
It is one of the key mechanisms by which cells maintain homeostasis.
Why is simple diffusion important overall?
It is the net rate of material transfer from one location to another.
Why is the net flux the most important component in diffusion?
because glucose is metabolized in the cytosol to glucose 6-phosphate pretty fast
Why isn't there as much glucose inside the cell as there is outside the cell?
mechanically gated channels
physically deforming (stretching) the membrane may affect the conformation of some channel proteins
nonpenetrating solutes
they do not penetrate through the lipid bilayer.
