Lecture 13

Membrane potential

(electrical, like a battery) Difference in electric potential between interior and exterior of a cell. Requires energy (usu. ATP) to establish Can involve movement with or against a concentration gradient

coupled transporters

Also called secondary active transport, energy is used to transport molecules across a membrane, but no direct coupling of ATP; relies on electrochemical gradient

membrane transport proteins

Also called transporter membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane. integral transmembrane proteins, may assist in movement of substances by facilitated or active diffusion

Neurons: voltage-gated ion channels responding to and creating membrane potentials

An electrical stimulus depolarizes the local membrane by activating adjacent (local) voltage-gated Na+ channels, Channels in the immediate area open until membrane depolarized (Na+ flows into cell), Channel inactivated briefly!!! Membrane repolarizes immediately voltage-gated K+ channels activated by depolarization (K+ flows outside) Ion concentrations re-established more slowly by the Na+-K+ pump Depolarization propagated along axon because inactivated channels are on the back side and potentially active channels are on the front leading side.

active transporter: coupled, ATP-driven, light-driven

...

Channel pores are usually gated, they open transiently, but they are

1000 times more efficient than transporters for conducting molecules across the bilayer.

Antiporter

2 species of solutes are pumped in opposite directions across a membrane. 1 species is going from high to low, which provides energy to drive the transport of other species from low to high concentration

Asymmetric metabolite distribution: diffusion versus transport

Diffusion Yes: small hydrophobic and (some*) small uncharged polar molecules Mostly no*: large uncharged polar molecules and ions

Symporter

Downhill movement of 1 species from high to low to move other species uphill from low to high concentration. Both molecules are transported in same direction

Specific needs for small molecules in different parts of the cell

Each cell membrane has a characteristic set of protein transporters: They transport small molecules that normally do not move freely across membranes. Related to the specialized function internal to the membrane Each transporter is usu. highly selective for a type of molecule

Membrane potential: Charge differences across a membrane

Each ion has an internal and external concentration, contributes to the overall charge distribution across the membrane Positive ions moving across a membrane (right to left) through a channel create a charge imbalance (+), also called a membrane potential (6,000 K+ in 1 um2 create a 100 mV potential) Charge imbalance can be negated by movement of another positive ion (Na+) in the opposite direction

Molecules can move "along" (with) their gradients across membranes

Each molecule responds to its concentration gradient and moves Charged molecules respond to the electrical gradient (membrane potential), greatest charge difference at the membrane interface

Neuron

Electrically excitable cell that processes and transmits info through electrical and chemical signals.

Active transport needs energy

Energy-driven transporters, commonly called pumps move molecules (yellow squares) against gradients. There are various sources of energy (red highlight): Coupled (uses energy of other molecules moving down the gradient), ATP-driven (ATP hydrolyzed to ADP, energy released), Light-driven (electromagnetic radiation captured by electrons)

Ion channels: Patch clamp recording

Gating activity is like a light switch: opened or closed by membrane potential, ligand binding or mechanical force Measure open/closed activity by a patch-clamp recording where you create an electrical circuit with the channel as a switch

Active transport: Glucose uptake from gut against a concentration gradient

Gut lumen-orange-low levels of glucose Epithelial cell-gray-high levels of glucose Dark blue circles: sodium-high levels in gut, low levels in cells Light blue hexagons: glucose Green circle-glucose-sodium symporter Unidirectional transport of glucose into cells using cotransport of sodium

Ion channels: passive but selective transport

Integral membrane proteins that span the bilayer Ion selectivity created by narrow channels and multiple ionic interactions that discriminate one ion from another Gated: opened or closed by membrane potential, ligand binding or mechanical force

Patch clamp recording

Lab technique in electrophysilogy that allows study of single or multiple ion channels in cells. Especially useful in study of excitable cells such as neurons, cardiomyocytes, muscle fibers and pancreatic beta cells.

osmotic pressure

Minimum pressure needed to be applied to solution to prevent inward flow of water across semipermeable membrane. Tendency of a solution to take in water by osmosis

Stress-gated channel

Open pores in response to mechanical deformation of a neuron's plasma membrane. Opening of ion channel depolarizes the afferent neuron producing an action potential with sufficient depolarization. Channels open in response to prokaryotic model and mammalian hair cell model Detect vibraction, pressure, strectch, touch, sounds, tastes, smell, heat, volume and vision

Passive transport

Movements of substances across cell membranes, doesn't require input of chemical energy, driven by growth of entropy of system. Rate depends on permeability of membrane

Passive transport: "go with the flow"

Passive transporters are selective for the small molecules they move across the membrane No energy required for transporter activity because the energy for transport is provided by the electrochemical (or pH) gradient GRADIENTS ARE STOREHOUSES OF ENERGY THAT CAN POWER CELLULAR PROCESSES

Transporters: active or passive

Passive: transporters and channels Requires no direct energy, Relies on concentration gradient Active: transporters Requires energy (usu. ATP), Involves movement against a concentration gradient

Nernst equation

Relates the reduction potential of a half-cell (or total voltage of full cell) at any point in time in the standard electrode potential, temperature, activity, and reaction quotient of the underlying reactions and species used. When reactions quotient = equilibirum constant of reaction at given temp, equilibrium voltage = 0

electrochemical gradient

Separation of oppositely charged ions by membrane, generated by actively transporting one or more ions: Na/Ka pumps, protons, pump. Battery

osmosis

Spontaneous net movement of solvent molecules through a partially permeable membrane into region of high solute. Direction tends to equalize solute concentrations on 2 sides. Primary means of water being transported into and out of cells

Ligand-gated channel

Transmembrane ion channel that opens to allow ions ( Na+, K+, Ca2+, or Cl-- to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter. Composed to 2 domains: Transmembrane domain - ion pore Extracellular domain - ligand binding location

Voltage-gated channel

Transmembrane ion channels that are activated by changes in electrical membrane potential near the channel. Allow a rapid and coordinated depolarization in response to triggering voltage change.

Chemical gradient

higher concentration of an ion on one side of the membrane Ions will move from higher to lower concentration

Transporters: can use a gradient for one molecule to drive the transport of another

Uniport: a transporter that moves only one type of molecule across the membrane Coupled: active transporters that use the gradient of one molecule to transport another molecule against a gradient. Symport: movement of both ions in the same direction Antiport: movement of ions in opposite directions

Active Transport

movement of substance across cell membrane against gradient Requires cellular energy

Channel proteins form aqueous pores across the lipid bilayer that

allow passive transport of selected molecules.

The osmotic gradients, or the ion concentration gradients and the electric fields

comprise the electrochemical gradient across membranes that are essential for cell function.

Neurons are highly specialized cell types that use

membrane potentials and voltage-gated Na+ channels to propagate action potential signals.

Other gradients

pH, water gradient

ion channel

pore-forming membrane proteins. Establishes resting membrane potential, shaping actions potentials and other electrical signals by gating flow of ions across cell membrane, controlling flow of ions across secretory and epithelial cells and regulations cell volume

Passive transport involves movement of water, ions or charged molecules along their

respective gradients by specific transporters and channel proteins.

Action Potential

short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory.

Active transport involves specific transporters moving a molecule against

the electrochemical gradient in processes requiring chemical or light energy or coupling with molecules moving along the electrochemical gradient.

Membrane potentials are created by

the unequal distribution of electrical charge across the lipid bilayer.

The internal ion composition of a cell is

very different from the external ion concentration.