Passive Transport Flashcards
What is a pure phospholipid bilayer only permeable to?
- H20
- Small hydrophobic molecules
- Small uncharged molecules
What are the 2 forms of passive transport?
- Simple diffusion
2. Facilitated diffusion
What characterises passive transport?
- No metabolic energy required
2. Net movement is awn concentration gradient
What molecules can cross the bilayer via simple diffusion?
Small molecules (gases, hydrophobic molecules such as O2, CO2, NO, urea)
What does a large difference in concentration cause?
A faster rate of diffusion
What does facilitation diffusion involve?
- Integral membrane proteins that help molecules cross bilayer
- Proteins are specific to molecules being transported
What does a lower Km value result in for facilitated diffusion?
Higher affinity of transporter for molecule
What are the 4 examples of facilitated diffusion?
- Ionophore
- Ion channels
- Glucose transport
- Aquaporins
What is job of ionophores?
Tranport ions across membranes
What are ionophores produced by?
Bacteria as antibiotics to discharge ion gradients of target cells
What are the 2 types of ionophores?
- Carriers
2. Channel-forming
What are carrier ionophores?
- Hydrophobic molecules that carry ion in their core
- Shield it from hydrophobic membrane environment
What are channel-forming ionophores?
- Membrane-spanning hydrophobic proteins that form a hydrophilic channel
- Allows ions to pass freely through cell membrane
What is Valinomycin an example of?
Carrier ionophore
What is Gramicidin A an example of?
Channel-forming ionophore
Describe Valinomycin
- Macrocyclic molecule
- Specific to K+ ions
- Effective as antibiotic
How does Valinomycin transport K+?
- K+ binds to interior
2. It takes K+ from one side of membrane to other down concentration gradient
How is Valinomycin effective as antibiotic?
- Differential K and Na gradient on either side of membrane
- Destroying electrochemical gradient by allowing free movement of K+ ions across membrane –> KILLS CELL
Describe Gramicidin A
- Hydrophilic exterior and hydrophilic pore down middle which spans membrane
- Made up of alternating L- and D- amino acids (highly unusual as most proteins made exclusively of L- amino acids)
How is Gramicidin A effective as antibiotic?
Allows Na+ and K+ to move readily through membrane and destroy electrochemical gradient ]
1st antibiotic used clinically (1939)
What is job of ion channels?
Allows rapid and gated passage of anions (-) and cations (+) across membrane
Describe meaning of ‘gated’ channels
Ion channels normally closed by opened by specific stimulus
- Voltage-gated channe;s
- Ca2+ activated channels
- Non-selective cation channels (TRP)
Describe voltage-gated channels
Change in voltage leads to passage membrane (Ca2+, K+, Na+ channels)
Describe Ca2+ activated channels
Ca2+ activated K+ channels
Describe non-selective cation channels
Activated by diverse stimuli (e.g. mechanical) and important for detecting taste (marmite) and touch sensations
What are ion channels important for?
- Maintaining osmotic balance
- Signal transduction
- Nerve impulses
How is glucose transported into and out of cell?
Glucose transporter (GLUT1) moves glucose across membrane
Can move glucose either way (depends on concentrations)
Describe GLUT1
- Integral membrane protein
- 12 transmembrane alpha-helices
- Central pore through which glucose can move
Describe transport of glucose into cell
- Glucose binds to transporter
- Induces conformational change in transporter
- Glucose molecule moved into cytosol by diffusion
- Transporter resets
What happens to glucose inside cytosol?
It is phosphorylated by hexokinase –> converted into glucose-6-phosphate (G-6-P)
How is glucose stopped from moving back out of cell?
G-6-P is not a substrate for glucose transporter
What is glucose transport specific for?
For D-isomer of glucose
Has higher affinity (lower Km) for glucose than other sugars
What are aquaporins?
Water channel proteins required for bulk flow of H20 across cell membranes
Rapid transfer of water down concentration gradient
Describe aquaporins
- Tetramer with 4 pores through which H20 can pass
- Identical 28 kDa subunits (each subunit containing 6 transmembrane alpha-helices)
Where are aquaporins abundant?
Abundant in erythrocytes and kidney cells
