1.3 MEMBRANE PROTEINS Flashcards
What is the cell membrane made up of?
A bilayer of phospholipids and proteins
Why is the membrane described as semi-permeable?
It only allows certain molecules to enter/exit the cell
What does the phospholipid bilayer act as a barrier to?
Ions and most uncharged polar molecules
Give two examples of small molecules that can pass through the phospholipid bilayer?
Oxygen and Carbon Dioxide
What types of proteins control ion concentration and concentration gradients?
Transmembrane proteins such as channels and transporters
Why do different cell types/compartments have different channel and transporter proteins?
To allow them to perform specialised functions
What type of transport are channel proteins involved in?
Passive transport
Give an example of a channel protein.
Aquaporin
What is the benefit of having gated channel proteins?
Allows for control over the rate of diffusion
What are ligand-gated channels?
Channels controlled by signal molecules
What are voltage-gated channels?
Channels controlled by changes in ion concentrations
What is the purpose of transporter proteins?
To change conformation in order to transport molecules across a membrane
Give an example of a facilitated transporter protein.
Glucose symport
Do facilitated transporter proteins require energy?
No because they are a passive form of transport
Give an example of an active transporter protein.
Na/KATPase
What provides the energy for the conformational change in active transporter proteins?
Energy from hydrolysis of ATP
What is hydrolysis?
Energy stored in high-energy bond is released
What is the purpose of signal transduction?
To convert an extracellular chemical signal to a specific intracellular response
Give four possible intracellular responses brought about by signal transduction.
Activation of a G protein, change in uptake/secretion of molecules, rearrangement of cytoskeleton, activation of proteins which regulate gene transcription
What is a G protein?
A molecular switch involved in transmitting extracellular signals within the cell
What does the sodium potassium pump do?
Transports ions against a steep concentration gradient
What is the sodium potassium pump’s energy source?
ATP
What does the pump have an affinity for inside the cell?
Sodium ions
What happens after ions bind to the pump?
Phosphorylation by ATP causes a conformational change
What is the result of the conformational change of the pump?
Its ion affinity changes and the ions previously bound are released on the other side of the membrane
What does the pump have an affinity for outside the cell?
Potassium ions
What is the result of dephosphorylation of the pump?
Conformational change
What is the ratio of sodium:potassium transported by the pump?
3:2 (3 sodium ions out, 2 potassium ions in)
What is another name for the sodium potassium pump?
Na/KATPase
Give three functions of Na/KATPase
Generation of ion gradient for glucose symport in small intestine, generation/maintenance of ion gradient for resting potential in neurons, generation of ion gradient in kidney tubules
Up to what % of human metabolic rate does maintenance of ion gradient by Na/KATPase account for?
Up to 25%
What is nerve transmission?
A wave of depolarisation of the resting potential of a neuron
What is resting potential?
The membrane potential of a neuron that is not transmitting signals
What stimulates a neuron?
A signal molecule
Give an example of a signal molecule.
Neurotransmitter
How does the signal molecule stimulate the neuron?
Triggers the opening of a ligand-gated ion channel at a synapse
What is the result of the ligand-gated ion channel opening?
If sufficient ion movement occurs, voltage-gated ion channels open
What is the result of voltage-gated ion channels opening?
The effect travels along the length of the nerve in a wave of depolarisation
What happens once the wave of depolarisation has passed?
The channel proteins close and others open to restore the resting potential
How is the resting potential restored?
Ions move in the opposite direction (compared to the wave of depolarisation)
