Membrane permeability Flashcards
How permeable is the plasma membrane?
Semi-permeable
What is a semi-permeable membrane?
A layer through which only allowed substances can pass
What types of molecules can pass across plasma membranes?
Hydrophobic molecules
Small uncharged polar molecules
What are some examples of hydrophobic molecules that pass across plasma membranes?
Oxygen
Carbon dioxide
What are some examples of small uncharged polar molecules that can pass across plasma membranes?
Water
Urea
What types of molecules cannot pass across plasma membranes?
Large uncharged polar molecules
Ions
What is an example of a large uncharged polar molecule that cannot pass across plasma membranes?
Glucose
What are the types of transport of molecules across plasma membranes?
Passive transport/active transport
Uniport/co-transport
What are the types of passive transport?
Simple diffusion
Facilitated diffusion
What is simple diffusion?
Molecule moving directly across plasma membrane down its concentration or electrical gradient
What shape does a graph of concentration/electrical gradient against rate of transport for simple diffusion look like?
Straight line
Is simple diffusion saturable?
No
What is facilitated diffusion?
Molecule moving across plasma membrane by transport protein down its concentration or electrical gradient
What are the types of transport proteins?
Carrier proteins
Channel proteins
How do carrier proteins transport molecules across the plasma membrane?
Ligand binds to protein
gives conformational change in protein
protein internalises ligand and releases it to other side of plasma membrane
What are the types of channel proteins?
Ligand-gated
Voltage-gated
Mechanically-gated
How do ligand-gated channels transport molecules across the plasma membrane?
Ligand binds to channel
channel opens up or closes
molecules can only move through open channel to the other side of the plasma membrane
How do voltage-gated channels transport molecules across the plasma membrane?
Changes in membrane potential
causes channel to open up
molecules move through it to the other side of the plasma membrane
What is an advantage of channel proteins over carrier proteins?
Work faster because they can transport more molecules across plasma membrane in one go
What determines the rate of passive transport?
How permeable the membrane is to the molecule, how many transporter proteins are present for that molecule
The difference in concentration on either side of the plasma membrane
What does a graph of a concentration against rate of transport for facilitated diffusion look like?
Rectangular hyperbola
What is Vmax?
The maximum transport rate in facilitated diffusion
What is Km?
The concentration that gives half of Vmax in facilitated diffusion
What is happening to the transporter proteins at Vmax?
They are all working as much as they can
cannot transport any more molecules any faster
What is active transport?
Transports molecules against their electrical or concentration gradient
using energy from ATP hydrolysis
What is deltaG for passive transport?
Negative
What is deltaG for active transport?
Positive
What determines whether a molecule is transported across a plasma membrane by passive or active transport?
Its concentration gradient
Its electrical gradient
What is uniport?
Transporter moves one molecule across plasma membrane at a time
What is co-transport?
Transporter moves two molecules across plasma membrane at a time
What are the types of co-transport?
Symport
Antiport
What is symport?
Transporter moves two molecules across plasma membrane in one direction at a time
What is antiport?
Transporter moves two molecules across plasma membrane in opposite directions at a time
What are the roles of transport of molecules across plasma membrane?
Maintaining ionic composition
Generating ion gradients for electrical excitability of nerves and muscle
Regulation of intracellular pH
Regulation of cell volume
Controlling concentrations of building blocks, fuels, waste products etc.
How do mechanically-gated ion channels transport molecules across the plasma membrane?
Plasma membrane deformation
causes the channel to open up
molecules move through it to the other side of the plasma membrane
