1.5 Membrane transport Flashcards
What is special about the lipid bilayer?
- fairly permeable to a few unchanged molecules such as O2 and CO2
- hydrophobic and retards diffusion of hydrophilic, polar or non ionic compounds across the membrane
- developed the means to move hydrophilic compounds across their membranes
What are some membrane proteins?
- lipids (cholesterol - steroid)
- peripheral and integral proteins
- glycoproteins
What is facilitated diffusion?
Membrane proteins transporting ions and small molecules that do not pass readily by passive diffusion
What are the two types of membrane transport?
Passive
Active - requires an input of free energy to function
What is passive transport?
AKA: accelerated diffusion
- move molecules. faster so bring about an equilibrium faster
- does not generate a concentration gradient, can only dissipate the gradient
- equilibrium is energetically favoured
What is active transport?
- movement of a solute across a membrane against its concentration gradient
- requires ATP
What are pores and channels?
Passive transport can be facilitated by transporters that provide passageways across the membrane of the right size and environment for a particular compound to cross
What are porins?
Form aqueous channels and accelerate the passive diffusion of small hydrophilic molecules across the membrane
What determines solute selectivity of a porin?
Characteristics of the amino acid side chains at the entrance and interior lining of the pore as well as the size of the opening
- positively charged regions at the mouth of the pore and at the construction site makes the pore specific for small anions
What is the difference between leakage and gated channels?
Leakage channels continously allow a certain type of ions to pass through whereas gated channels only allow certain ions to pass through after a conformational change takes place
Why are ion channels more complex than porins?
Generally requiring more than one subunit to form a membrane passageway
What causes channel like transporters to open?
Ligand binding to the transporter
Changes in membrane potential, pH
Covalent modification by a cellular enzyme
What happens to a blocked gate after stimulation?
Opens by structural changes that move a polypeptide segment out of the channel or by a concerted conformational rotation of helices that open the pore
What are bind and release conformational transporters?
Bind molecules very selectively and change their structure to allow them to pass to the other side of the membrane
- can be classified as either uniport, symport or antiport
What is a uniport transporter?
Transport protein found in liver cells
- shuttle glucose between the liver and the bloodstream
- moves only one solute
- passive or with the concentration gradient
- dissipation of gradient therefore higher glucose concentration
What is a symport transporter?
Not passive - potential energy of a steep gradient os dissipated and used to drive movement of another molecule against concentration gradient
- i.e., Na+-glucose in the renal epithelial cells of the kidney
What is a antiport transporter?
AKA: Secondary active transport
- one solute moves along its electrochemical gradient and results in another to move against its electrochemical gradient
- i.e., Na+/H+ in kidney tubules
How does the Na+/K+ pump work?
Free energy of ATP hydrolysis is used to drive the movement of Na+ and K+ against their concentration gradients, maintaining a source of potential energy
- Na+ always pumped out
- K+ always pumped in
What is the function of the Na+/K+ pump?
Responsible for maintaining the membrane potential so important for neural cell function
