Lecture 13: Transport Through Membranes I Flashcards
1
Q
Describe the transport properties of the plasma membrane
A
- Plasma membrane is semipermeable
- Semi-permeable to lipophilic molecules, move via diffusion e.g., steroid hormones
- Impermeable to hydrophilic/polar molecules - need a specific transport mechanism
- Membrane proteins facilitate the transport of specific molecules across the plasma membrane
2
Q
Describe membrane transporters
A
- Mostly polytopic, transmembrane, integral membrane proteins
- The activity and protein levels of specific transporters determines the ionic composition of the cell.
- Expression of specific transporters governs the biochemical/metabolic characteristics of a cell(can execute only those reactions whose substrates can be taken up).
- Alteration in expression levels may be one way of regulating metabolism.
3
Q
Give the extracellular, intracellular, and fold difference of Sodium, Calcium, Potassium, and Chlorine in mamallian cells.
A
- Sodium: 145, 12, 12
- Potassium: 4, 155, 0.026
- Calcium: 1, 0.0001, over 10,000
- Chlorine: 120, 4, 30
- Note: This number could greatly differ in cells, so it may not be worth memorizing beyond general fold difference. These are and aren’t static numbers since this is where it’s intentionally maintained, but it’s never exactly this in any given msec.
4
Q
How much ATP is utilized in brain signaling?
A
ATP consumption per gram per minute used for signaling in the brain is equal to the energy used by a human leg muscle running a marathon!!!!
5
Q
What are the two basic types of transport?
A
- Passive: molecules move down a concentration gradient by using energy stored in the gradient e.g., ion channels
- Active: molecules move against a gradient by using biochemical energy stored in ATP e.g., pumps/ATPases
6
Q
Describe the two types of passive transport
A
The energy independent movement can occur in 2 ways:
- Simple Diffusion: Unaided Molecules that are small, nonpolar and uncharged diffuse freely across the membrane via simple diffusion. The steeper the gradient the faster the diffusion
- Facilitated Diffusion: With Assitance of Proteins
- Molecules that are large and charged are unable to cross the membrane.
- Such molecules need a facilitator protein that can facilitate movement.
- Proteins function as channels or transporters
- Transmembrane polytopic proteins
- These greatly increase the rate of transport.
7
Q
What is the concept of free energy?
A
- Unequal distribution of molecules is associated with free energy.
- Energy is utilized to set up an ion gradient across a semipermeable membrane.
- Ion gradients are generated by membrane transporters
- These use free energy from ATP and store it as free energy of the gradient
- Energy is Minimum when the concentration across a semipermeable membrane is equal.
8
Q
What is the free energy of uncharged and charged molecules?
A
For Uncharged molecules
(DeltaG = 2.303 * RT log(c2/c1))
For Charged Molecules:
(Uncharged Equation) + ZFdeltaV
Where deltaV is membrane potential.
9
Q
Describe Active Transport
A
- The energy-dependent movement of molecules against a gradient. This can occur with 2 different types of proteins:
- P-Type ATPase:
- Sodium-Potassium ATPase
- Plasma Membrane Calcium ATPase (PMCA)
- Sarcoplasmic Reticulum Calcium ATPase (SERCA)
- Hydrogen-Potassium ATPase.
- ATP-binding Cassette Transporters:
- Multi-drug resistance protein
10
Q
Describe P-Type ATPases
A
- A total of 70 proteins in the human genome
- Integral Membrane Proteins
- Use a fundamental mechanism of action
- Use energy from ATP hydrolysis to drive transport against gradient
- ATP breaks into ADP and phosphate
- Transporter forms a covalent bond with the phosphate to form an enzyme-phosphate intermediate
- Phosphorylation on a conserved aspartate residue.
- Undergo drastic conformational changes that facilitates the transport of ions across the membrane.
11
Q
Describe the sodium-pump
A
- Sets up sodium and potassium gradients across the plasma membrane
- The stoichiometry of transport is 3 sodium ions and 2 potassium ions get moved for every 1 mol of ATP
- This helps to make neurons and muscle cells electrically excitable, controls cell volume, and drives the active transport of sugars and amino acids.
- 25-40% of the ATP used in the brain is used in the Sodium-Potassium pump. Makes sense since the brain’s all about dat neuron firing.
12
Q
What are some inhibitors of the sodium pump?
A
- Plant steroids can be potent inhibitors of the potassium pump.
- These include digitoxigenin and ouabain
- These inhibit dephosphorylation of E2P (dont’ really know what that is)
- These are known as cardiotronic steroids due to their strong effects on the heart.
- Digitalis is a mixture of cardiotronic steroids.
- It is used in the treatment of congestive heart failure.
13
Q
What is the mechanism of action of cardiotonic steroids?
A
- Cardiac glycosides such as ouabain and digitoxigenin inhibit potassium/sodium pump activity.
- These increase sodium levels in the cell
- They REDUCE the antiporter used in sodium-calcium exchange
- This results in an increase in intracellular calcium
- The elevated calcium increases the contractile force of the heart
- This is why it’s effective as a CHF drug.
14
Q
Describe the 2 types of calcium pumps
A
Kind of already went over this but…
- Plasma membrane Ca-ATPase (PMCA)
- Sarco2+ endoplasmic reticulum Ca-ATPase (SERCA)
15
Q
Describe PMCAs
A
- This protein has 10 transmembrane domains
- Molecular size of 140 kDa.
- Transports calcium from cytosol to the extracellular side
- Maintains 10,000 fold gradient across the PM
- Uses ATP to pump calcium uphill
- Stimulated by the calcium binding protein, calmodulin
- See Slide 26 and 27 for diagram
