Term
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Definition
| rate a substance moves across a membrane per unit time |
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Term
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Definition
net movement of molecules across a membrane per unit time
diffusional equilibrium is reached at the point at which net flux equals zero |
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Term
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Definition
chemical gradient
electrical gradient
electrochemical gradient |
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Term
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Definition
also known as the concentration gradient
molecules will move from an area of high to low concentration (typically described as a molecule moving down or along a concentration gradient) |
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Term
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Definition
| results from the electrical charges of ions |
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Term
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Definition
sum total of chemical and electrical gradients
physiologically, the electrochemical gradient is the ultimate driving force determining the movement of molecules across cell membranes |
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Term
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Definition
requires no external energy
simple diffusion
facilitated diffusion |
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Term
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Definition
involves the random movement of molecules down a concentration gradient
related to the following:
--magnitude of driving force (chemical gradient)
--permeability of the membrane
--surface area
lipophilic molecules (because the cell membrane is lipophilic throughout the majority of its width, lipophilic molecules move by simple diffusion directly through the membrane.
hydrophilic molecules--small hydrophilic molecules (ions) utilize channels formed by membrane proteins to pass across the cell membrane.
--number of channels present will affect rate of flux
--number of channels opened or closed will affect the rate of flux
--because ions are charged, the electrical gradient present will also affect the movement of these molecules |
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Term
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Definition
permeability is affected by
--lipid solubility (lipids are more permeable)
--size and shape (smaller molecules are more permeable)
--temperature (increased temperature equals increased permeability)
--membrane thickness (thin membranes are more permeable than thick)
Surface area of the membrane (greater surface area increases diffusion)
concentration gradient (greater gradient increases diffusion)
Ficks Law |
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Term
| Facilitated (carrier mediated) diffusion |
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Definition
involves the passive movement of molecules across membranes utilizing carrier molecules which mediate the movement of a molecule across a membrane
carrier molecules involved in passive transport have the following characteristics
--transmembrane proteins which bind molecules on one side and transfer to the other side
--undergo spontaneous conformational change (external energy not required)
--can transport in both directions (determined by concentration gradient
--rate of facilitated diffusion limited by number of carriers and concentration gradient
--demonstrate specificity, affinity, saturation |
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Term
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Definition
any molecule that is bound to a protein surface with a non-covalent bond
the site to which the ligand binds is called the binding site
bond is usually the result of electrical attraction between oppositely charged ionic or polar groups or weaker attractions due to van der waals forces
the force of these particular attractions weakens as the distance between the protein and ligand increases
in order to have a good bond, it is important for the ligand and protein to have a very close proximity. This is achieved by the ligand and its receptor having complementary binding sites. |
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Term
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Definition
the ability of a protein binding site to bind a specific ligand
some binding sites are very specific and will only bind to one ligand. others are less specific and may potentially bind several different ligands.
primarily related to the shape of the binding site |
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Term
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Definition
the strength of attraction between a ligand and protein
high affinity binding sites hold on tightly to a ligand; ones in which the ligand is weakly bound is said to have a low affinity binding site. |
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Term
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Definition
refers to the fraction of total binding sites that are occupied at any given time
saturation depends on:
--the concentration of unbound ligand in the solution (the greater the concentration of unbout ligand, the more likely it is that the ligand will bind an unoccupied binding site)
--the affinity of the binding site for the ligand (if a binding site has high affinity for the ligand, then even low ligand concentration will result in a relatively high saturation. Once ligand is bound, it is not as likely to let loose. The higher the affinity, the less concentration of ligand is required to obtain the same level of saturation.)
If a carrier is 100% saturated, further increase in ligand concentration will not affect the rate of transport. |
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Term
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Definition
occurs when multiple ligands are able to bind at the same binding site, resulting in the binding of one of the ligands being reduced.
competiton occurs between ligands, not between binding sites |
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Term
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Definition
utilizes "outside" energy to move substances against a gradient
two types:
--primary active transport
--secondary active transport |
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Term
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Definition
affected by:
--number of carriers present
--percent saturation of the carriers
--speed with which the pumps are able to transport substances |
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Term
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Definition
directly uses ATP as primary energy source to transport substances
primary active transporters are commonly referred to as 'pumps'
these transmembrane proteins have both an enzymatic and transporter function
the enzymatic process involves the hydrolyzing of ATP to ADP, which provides energy to power the carrier
these transporters always transport against a concentraton gradient
Na-K ATPase pump is the most notable primary active transporter
--transports 3 Na ions out of the cell and, at the same time, brings 2 K ions into the cell |
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Term
| secondary active transport |
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Definition
couples an energy releasing process to an energy requiring process
as one substance moves passively down its concentration gradient, energy is "released." This energy is harnessed and utilized in the transport of another substance against its gradient
the movement of Na+ down its concentration gradient (Na+ moving into the cell) is typically utilized to provide the energy for secondary active transport
cotransport involves the movement of a molecule in the same direction. This will result in movement of a molecule inot the cell when the movement of Na+ is providing the energy. (these carriers may be called symports)
counter transport involves the movement of a molecule in the opposite direction. This will result in the movement of a molecule out of the cell when the movement of Na+ is providing the energy. (these carriers may also be called antiports) |
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Definition
| diffusion of water across a membrane caused by differences in solute concentration (diffuses down its concentration gradient) |
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Definition
| amount of pressure required to prevent the movement of water |
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Term
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Definition
these solute particles, when present in solution, will freely move across a membrane and reach equilibrium. Because penetrating solute does not create an osmotic gradient, penetrating solutes will have no effect on the net movement of water.
ex: urea |
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Term
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Definition
these solute particles do not freely move across a membrane. In our cells, the ions Na+ and K+ are considered the major non-penetrating ions. They are transported across the membrane by the Na/K pump, but never reach equilibrium. In this case, because of an unequal distribution volume of solute, an osmotic gradient is created. For this reason, non-penetrating solutes will cause the movement of water.
Na+ is the single most important ion determining the effective osmolarity of our intra and extracellular fluid |
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Term
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Definition
describes the total solute concentration of a solution without regard to wheter the solute is penetrating or non-penetrating.
Not a particularly useful term with regards to human physiology |
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Term
effective osmolarity
tonicity |
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Definition
osmolarity as it relates to the non-penetrating solute
helps to determine movement of water
effective osmolarity of our intracellular fluid is ~280 mOsm |
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