chapter 12 cell and molec

membrane transport proteins

membrane transport proteins

span the lipid bilayer and provide passageways across the membrane for certain substances

transporters

class of membrane proteins that shift small organic molecules or inorganic ion from one side of the membrane to the other

transfer only those molecules that fit into the specific binding sites on the protein (great specificity)

channels

class of membrane proteins that form tiny hydrophilic pores across the membrane through which substances can pass by diffusion

ion channels

ion selective and gates

four types: voltage-gated, ligand gated (extracellular), ligand gated (intracellular) and mechanically gated

membrane potential

electrical imbalances that generate a voltage difference across the membrane

allows cells to drive the transport of certain metabolites and provides excitable cells with the means to communicate with their neighbors

resting membrane potential

when a cell is “unstimulated”, which holds the voltage difference across the cell-membrane steady

usually neg bc the interior of a cell is more negatively-charged than the exterior

electrochemical gradient

net driving force for the direction of flow of solutes that combines voltage and concentration

(this is bc membrane potential can drive passive transport in a certain direction for any substance with an electrical charge and charged molecules move down the concentration gradient )

if the voltage and concentration work in the same direction

the electrochemical gradient is high

if the voltage and concentration work in different directions

the elctrochemical gradient is low

osmosis

movement of water down a concentration gradient from high concentration to a low concentration

aquaporins

specialized channels that facilitate the movement of water

osmorality

total concentration of solute particles inside the cell

glucose transporter

adopts different conformations and switches between them spontaneously

pumps

carry out active transport

gradient-driven pumps

link the uphill transport of one solute across a membrane to the downhill transport of another

atp-driven pumps

use energy released by the hydrolysis of ATP to drive uphill transport

light-driven pump

found mainly in bacterial cells and use sunlight to drive uphill transport

Na+ pump

ATP-driven, moves Na+ out and K+ in, drives coupled pumps in animal cells

• uses energy from ATP hydrolysis

• fuels protein conformational changes that allow for the exchange of ions

• phosphate group leaves ATP and is moved into the pump itself

Ca2+ pump

ATP driven, removes Ca 2+ from cytosol

• Ca2+ can bind to proteins in the cell and alter their activities so it must be removed from the cell

• Ca2+ pumps return to their normal conformation w/o binding or transporting any other ion

• driven by the phosphorylation of ATP

symports

use the flow of Na+ from a high concentration to a low concentration to also allow glucose into the cell against its concentration gradient

antiports

transfer solutes in different directions

H+ pumps

pump H+ out of the cell which sets up an electrochemical proton gradient and creates an acidic pH (drives coupled pumps in bacteria)

H+ driven symport

actively transports H+ out of the cytosol into the organelle to help keep the pH neutral

K+ leak channels

allow K+ to move freely across the membrane

• K+ flows out the cell which causes a change in the voltage (or membrane potential)

• due to an imbalance of charge, K+ stops moving out of the cell so the cell reaches a state of equilbrium

patch clamp recording

measures the current flowing through a single channel molecule and detects conformational changes

“all-or-nothing”

describes activity of ion channels so when they switch randomly between the open and closed channels the bias is greatly changed

voltage-gated ion channels

openness is controlled by membrane potential

• changes in the membrane potential are detected through voltage sensors

• membrane potential can chnage which can activate/inactivate certain ion channels

ligand-gated channel

opening is controlled by the binding of a molecule to the channel

mechanically gated channel

opening is controlled by mechanical force applied to the channel

neuron

nerve cell

axon

one long extension from a neuron which conducts electrical signals away from the cell body toward distant targets

dendrites

several shorter branching extensions off the axon which reduce signals from the axons of other neurons

nerve terminal

branches off the far end of the axon which allows the neuron’s message to be passed to many target cells

action potential

carries a message w/o weakening from one end to another

synapses

junctions where signals are transmitted to the target cells

neurotransmitter

electrical signals are converted to transmit the message across a gap

synaptic vesicles

membrane-enclosed nerve terminals where neurotransmitters are stored

transmitter-gated ion channels

subclass of ligand-gated ion channels and their function is to convert the chemical signal carried by a neurotransmitter back into an electrical signal