Membrane Structure and Function Flashcards
Why are lipid bilayers highly impermeable to ions and most polar molecules?
- The ability of small molecules of cross a membrane is a function of its hydrophobicity
- Ions cannot cross membranes because of the energy cost of shedding their associated water molecules
Membranes performing different functions contain different kinds and amounts of proteins:
Plasma membranes: ~50% proteins
Internal membranes of mitochondria and chloroplast: ~75% proteins
What are some of the important cellular processes that membrane proteins are responsible for?
- Pumps
- Channels
- Receptors
- Enzymes
Membrane proteins make membranes selectivity permeable. Why?
Membrane proteins allow transport of molecules and information across the membrane.
- Membranes vary in protein content from as little as 18% to as much as 75%
What are integral membrane proteins?
Proteins that are embedded in the hydrocarbon chains of membrane lipids.
Released only when the membrane is disrupted.
What are the factors that determine whether a small molecule will cross a membrane?
1) The [gradient] of the molecule across the membrane
2) The molecule’s solubility in the hydrophobic environment of the membrane
By what process do lipophilic molecules cross the membrane?
Simple diffusion
By what processes can polar molecules cross the membrane?
- Passive transport
- Facilitated transport
- Active transport
Passive-transport systems called ion channels
Ion transport rates are 1000x faster than ion pumps
Voltage-gated channels
Open in response to changes in membrane potential
Ligand-gated channels
open in response to the binding of small molecules (ligands) such as neurotransmitters
What is passive transport?
Movement with the [gradient]
No added energy is needed
What is facilitated diffusion?
Diffusion through a protein channel
What is active transport?
Movement against the [gradient]
Additional energy is needed
What are primary transporters?
Hydrolyze ATP and transport molecules against the [gradient]
What are secondary transporters/cotransporters?
Couple transport of molecules with the [gradient] to transport of a molecule with its [gradient]
What is a symporter?
Transport molecules in the same direction
What is an antiporter?
Transport molecules in opposite directions
Na+ - K+ Pump (Na+ - Ka+ ATPase)
The hydrolysis of ATP by the pump provides the energy needed for the transport of 3Na+ ions out of the cell and 2K+ ions into the cell, generating the gradient
The structure of the potassium ion channel reveals the basis of specificity:
- The potassium channel selectively and rapidly transports K+ across the cell membrane. Larger ions are not transported because they are too big to enter the channel
- Smaller ions are excluded because they cannot interact with the selectivity filter. Such ions are small enough that the energy of desolvation cannot be compensated for by interactions with the selectivity filter.
