Membrane transport Flashcards
Why are membrane transport proteins important?
Important for the maintenance of homeostasis in the internal and the external environment by allowing the movement of molecules across the plasma membrane of the cell to establish equilibrium.
What is the role of the plasma membrane of the cell?
The plasma membrane is an effective barrier that protects the intracellular environment of the cell, by separating it from the external environment, and also facilitated the movement of certain molecules into and out of the cell.
Describe the composition of the cells plasma membrane
The cells plasma membrane is composed of a phospholipid bilayer which is inter dispersed with cholesterol molecules and glycoproteins.
What is the role of cholesterol in the plasma membrane?
Cholesterol has a role in ensuring plasma membrane rigidity and structure. Cholesterol reduces the fluidity of the phospholipid membrane of cells.
Describe the components of phospholipids
Phospholipids are composed of a hydrophilic (‘water-loving’) head and a hydrophobic (‘water-repelling’) tail. Phospholipids are amphipathic as they have both hydrophobic and hydrophilic characteristics. Phospholipids naturally form a bilayer or droplets in an aqueous environment, with their heads facing outwards and their tails facing inwards.
What is flux?
Flux is the net movement of ions across a membrane. Ions move in both directions to reach an equilibrium, but the net movement of the ions is one direction is known as flux.
What are the 4 types of membrane transport?
- Simple diffusion: across the membrane bilayer
- Simple Diffusion: through an aqueous channel.
- Facilitated Diffusion: binding of a protein to a ‘carrier’.
- Active Transport: movement against a concentration gradient.
What is simple diffusion across the membrane bilayer?
Simple Diffusion is the passive, non-mediated movement of simple molecules across the membrane eg. Oxygen molecules
Movement down concentration gradient.
Non-selective as any small molecule is able to pass through the membrane.
What is simple diffusion through an aqueous channel?
Simple diffusion through an aqueous channel is the passive, non-mediated movement of ions through the membrane eg. Sodium ions. Channels are simply holes in the membrane; the molecule doesn’t bind to the channel, and simply passes through it.
Movement down concentration gradient.
What is facilitated diffusion?
Facilitated Diffusion is the passive, facilitated movement of molecules through the membrane, eg. Glucose. The molecule binds to the protein carrier in order to pass through the membrane. The pore is specific to that molecule, otherwise many molecules would able to pass through it and enter or leave the membranes.
Movement down concentration gradient.
What is active transport?
Active transport is the mediated transport of molecules, involving energy, usually in the form of ATP. Movement against concentration gradient.
What are the 2 types of active transport?
- Primary Active Transport
2. Secondary Active Transport
What is osmosis?
Osmosis is the movement of water across a partially permeable membrane, down an osmotic, water potential gradient.
What factors affect the rate of passive diffusion (flux)?
- Steepness of concentration gradient (Dm)
- Temperature
- Surface area
- Membrane thickness
- Molecular weight
- Molecular membrane permeability
Describe the patch clamp test
Suction is applied to a cell through a polished glass micropipette which is wired as an electrode. Current (ion movement) is measured. Different ion channels can be opened and closed, depending on the voltage applied to the cell.
What are the features of ion channels that make them adapted to their function?
Ion channels are highly selective and transport molecules very fast. Accessible from both sides simultaneously, however the net flux is down a concentration/ voltage gradient. Ion channels oscillate between open and closed to regulate the movement of ions into and out of the cell.
How do ion channels work?
Ion channels are highly selective to ion charge and size. Ion channels use the fact that ions exist in a hydrated form in an aqueous solution to differentiate ion channels.
How do sodium ion channels differentiate between sodium and potassium ions?
Sodium and potassium exist in a hydrated form. Hydrated potassium ions are too large to pass through sodium ion channels, so differentiate for sodium ions only.
How do potassium ion channels differentiate between sodium and potassium ions?
Potassium ion channels strip water molecules from potassium and sodium ions in their hydrated form. Hydrated potassium ions are large enough to interact with the carbonyl oxygen on the potassium ion channel, allowing their movement through the channel. Sodium ions are too small to interact with the carbonyl oxygen of the channel, so do not pass through.
What are the factors that affect passive diffusion through ion channels?
- Steepness of concentration gradient (Dm)
- Temperature
- Number of channel proteins
- Permeability/affinity
What are aquaporins?
Aquaporins act as a hydrophilic (“water-loving”) pore through a hydrophobic (“water-repellent”) plasma membrane, which allows for more rapid transport of water molecules than simple diffusion.
Why would specific channels for large molecules result in extremely low specificity?
Channels that are large enough for a specific molecule like glucose would also be large enough for smaller molecules like ions. Transport mediated diffusion is therefore used to transport large molecules into and out of the cell.
How to transport proteins work in facilitated diffusion?
Solute (ie. the large molecule) binds to the “binding site” on either side of the plasma membrane. Binding causes a conformational change that brings the molecule across the membrane where it is then “released”.
What are the 3 classes of carrier protein involved in facilitated diffusion?
- Uniporter
- Symporter
- Antiporter
