Movement Across Membranes (Lecture 16) Flashcards
What are membranes composed of?
What are the main structural components of biological membranes?
What describes membrane properties?
Cells with different functions have membranes with what?
What is important for cell function? And now is it determined?
What spans the lipid bilayer?
- Membranes are composed of lipid bilayers
- Amphipathic phospholipids are the main structural components of biological membranes
- Fluid Mosaic Model describes membrane properties
- Cells with different functions have membranes with different protein compositions
- (e.g.mitochondriainnermembranevsneuronmyelinsheath)
- membrane fluidity is important for cell function and is determined by temperature and lipid composition
- Integral/transmembrane proteins span the lipid bilayer
Integral membrane proteins have what kind of roles?
Integral membrane proteins have diverse roles
What is a transmembrane domain (TMD)
(4 points)
- A transmembrane domain (TMD) is a hydrophobic (uncharged) peptide sequence that spans the PM
- TMDs permanently insert proteins into the PM
- TMDs are a-helical
- TMDs foster protein-protein interactions
Movement of substances across cell membranes
What can/cannot cross the membrane?
- small, uncharged molecules cross membranes relatively easily (O2, CO2,
NO, H2O)
- many molecules do not pass freely through lipid bilayers
- large/polar/charged compounds cannot easily cross lipid bilayers
- specific mechanisms are required for the controlled transport of many substances across membranes
What are the 4 basic mechanisms for moving molecules across membranes?
- Simple diffusion
- Diffusion through a channel
- Carrier-mediated diffusion
- Active Transport
IT REQUIRES ENERGY
What is the passive movement of substances?
Passive movement of substances across cell membranes from higher to lower concentrations and does not require energy
Passive movement of substances across cell membranes from higher to lower concentrations and does not require energy
Explain simple diffusion
- Simple diffusion
- Very small molecules
- Uncharged
-
Down a concentration gradient (flow is ‘downhill’)
- e.g. O2, CO2, H2O (osmosis)
Passive movement of substances across cell membranes from higher to lower concentrations and does not require energy
What are aquaporins?
H2O can also move through specific water channels: Aquaporins
H2O moves through aquaporin channels in “single file”
Passive movement of substances across cell membranes from higher to lower concentrations and does not require energy
Explain diffusion through a channel
- Diffusion through a channel
- Small, charged molecules (ions)
- Down a concentration gradient (flow is ‘downhill’)
- e.g. Na+, K+, Ca2+, Cl-
What are Ion Channels?
(6 points)
- Ion Channels are integral membrane proteins that form an aqueous pore
- channels are selective, allowing only particular types of ions to pass
- ions move down concentration gradients
- channels often are ‘gated’ (can be open or closed)
- Ion channels have an important role in nerve cell function
- Changes in Na+ and K+ are critical for neurons to transmit signals
2 Types of Gated Channels
What is the 1st type?
1) Voltage-gated channels (e.g. Na+, K+ channels)
➡ Channel responds to changes in charge across the membrane
2 Types of Gated Channels
What is the 2nd type?
What are the three steps of this gated channel?
2) Ligand-gated channels (e.g. acetylcholine)
➡ Channel responds to binding of a specific molecule (a ‘ligand’)
- acetylcholine is a neurotransmitter
- binding of a ligand causes a conformational change in the structure of the receptor/channel.
1. Neurotransmitter binds
2. Channel opens
3. Ions flow across membrane
A tale of 2 toxins
What are the two toxins?
1) Tetrodotoxin (TTX)
→ Na+ channel blocker
→ potent neurotoxin
2) Curare
→ acetylcholine receptor antagonist
- competitive antagonist of the acetylcholine receptor
Passive movement of substances across cell membranes from higher to lower concentrations and does not require energy
Explain facilitated diffusion
- Compound binds specifically to an integral membrane protein called a ‘facilitative transporter’
- Change in transporter conformation allows the compound to be released on the other side of the membrane
- Compound moves down a concentration gradient
What is a glucose transporter?
Glucose transporter: example of facilitated diffusion
e. g., most animal cells import glucose from the blood into cells down a concentration gradient
i. e. [glucose] inside the cell is lower than outside cell
What is needed to move substances from lower concentration to higher concentrations?
Energy
e. g., cells of the small intestine & kidney need to transport glucose from low [gluc] outside the cell to very high [gluc] inside the cell
i. e., “uphill”
What is a Na+ Glucose Symporter?
The Na+ glucose symporter uses the higher concentration of Na+ to drive glucose against a concentration gradient
How does the [Na+](and other ions) get high in the first place?
According to the diagram sodium is more concentrated outside the cell than inside the cell
Potassium is more concentrated inside the cell than outside the cell
What is active transport?
- Compound to be transported binds specifically to an integral membrane protein called an ‘active transporter’
- Change in transporter conformation allows the compound to be released on the other side of the membrane
- Compound moves against a concentration gradient
- Requires input of ENERGY
The Na+/K+ ATPase maintains cellular [Na+] and [K+] using ATP what is this an example of?
Active Transport!
Active transport uses a Sodium-Potassium Pump
