Simple Diffusion
What is simple diffusion?
Unassisted net movement of a solute from a region where its concentration is higher to a region where its concentration is lower.
What is facilitated diffusion?
Protein-mediated (transport proteins) movement down the gradient. It is exergonic because solute diffuses in the direction dictated by the concentration gradient (uncharged molecules) or electrochemical gradient (for ions) with no input of energy needed.
OR
Movement of polar molecules and ions down a concentration gradient, specificity for the particular substrate transported, the ability to be saturated at high levels of substrate, and sensitivity to specific inhibitors of transport.
What is active transport?
Solutes are moved away from thermodynamic equilibrium (up concentration gradient, or against electrochemical potential).It does require an input of energy.
What main factors affect diffusion of solutes?
Size
*lipid bilayers are more permeable to smaller molecules than larger ones (H2O, O, CO2)
*Molecular weight of up to about 100 (ethanol and glycerol)
Polarity
* lipid bilayers are more permeable to nonpolar molecules and less permeable to polar molecules
*steroid hormones estrogen and testosterone are largely nonpolar and can diffuse (with molecular weights of 370 and 288)
* a simple measure of polarity of solute is partition coefficient- ratio of its solubility in an organic solvent to its solubility in water
-amino acids incorporated in proteins degree of polarity is determined by side chain
-more nonpolar (polar) or hydrophobic = the higher its partition coefficient and its ability to dissolve in the membrane (low partition coefficients)
Charge
* association of ions with water molecules to form shells of hydration dramatically restricts ion transport across membranes
- animal = sodium ions ; other cells = protons
What is a hypertonic solution?
A solution with a higher solute concentration than that inside a cell. Water molecules move out of cell, dehydrating it.
What can it transport? (Hint: hydrophobic interior)
It can only transport gases, nonpolar (small) molecules, very small polar molecules (water, glycerol, and ethanol).
What is the difference between simple and facilitated diffusion?
Facilitated diffusion can become saturated at high solute concentrations because there are a limited number of transport proteins. Graph is hyperbolic (similar to enzyme vs substrate!).
Example: transport of Glucose (concentration of glucose is higher in blood than cell) so it is exergonic; however glucose is too large and too polar so transport protein is required.
What are the three major functions of active transport?
ACTIVE TRANSPORT ESTABLISHES A DIFFERENCE IN SOLUTE CONCENTRATION AND/OR ELECTRICAL POTENTIAL ACROSS MEMBRANES= STEADY STATE
1. Makes the uptake of essential nutrients possible (even if concentration is already high inside)
2. Make the removal of waste and secretory products possible (even if concentration is greater outside)
3. Allows cell to maintain constant, nonequilibrium intracellular concentrations of specific inorganic ions, notably K+, Na+, Ca2+, and H+
Is simple diffusion is exergonic or endergonic? What is the relationship between the inward flux of the solute across the membrane and the concentration gradient of the solute, with no saturations at high concentrations?
It is always an exergonic process. LINEAR, NONSATURATING RELATIONSHIP BETWEEN THE DIFFUSION RATE AND THE CONCENTRATION GRADIENT!
What is a hypotonic solution?
A solution with a solute concentration lower than that inside a cell. Water molecules diffuse into the cell, increasing internal pressure.
What does diffusion always tend to? Is it a spontaneous process? Is energy/transport proteins required?
Diffusion always moving to equilibrium (as well as minimum free energy/ decreasing free energy in accordance with the second law of thermodynamics). No energy is required and no transport protein is needed. It is a spontaneous reaction!
What are carrier proteins?
Integral proteins (transporters and permeases) bind one or more solute molecules on one side of the membrane and then undergo a conformation change that transfers the solute to the other side of the membrane.
-Alternate between two conformation changes
-analogous to enzymes in their specificity and kinectics
Simple and facilitated diffusion are both nondirectional. What is active transport? (Hint: What are the two types?)
Active transport does have directionality.
1. Direct active transport: transport system coupled to an exergonic chemical reaction, hydrolysis of ATP. It drives outward transport of protons (H+) and establishes electrochemical potential (transported by ATPases or ATPase pumps).
-P-type: pumps only cations. V-type or F-type: class pump- proton pumps (H+). ABC- type: superfamily drug/ nutrient exchange moves everything else
2. Indirect active transport ( ALSO KNOWN AS SECONDARY ACTIVE TRANSPORT): transport system coupled of a solute S and ions (ex: protons). The exergonic inward movement of protons provides energy to move transported solute (S) against its concentration gradient or electrochemical potential. Ions move exergonically as second solute moves endergonic.
Carrier proteins are like enzymes. Give an example of the specificity of carrier proteins.
A carrier protein is needed to facilitate the diffusion of glucose intro erythrocytes. Protein recognizes only glucose (and sometimes even galactose and mannose). It is also stereospecific and only binds to d- but not 1-isomer of these sugars. This is competitive inhibition and can reduce rate of glucose transport in the presence of other transportable sugars.
What is an isotonic solution?
A solution with the same solute concentration as the cell. No net movement of water in either direction.
What is the driving force for diffusion?
Entropy! (concentrations need to equalize on both sides of the membrane)
What are channel proteins?
Integral proteins that form hydrophilic channels through the membrane that allow the passage of solutes without a major change in the conformation of the protein (pores- like found in bacteria, mitochondria, and chloroplast). Pores are formed by transmembrane proteins called porins.
Direct Active transport example and indirect active transport example.
Direct Active transport: the Na+/K+ pump maintains electrochemical ion gradients.
Indirect active transport: sodium symport drives the uptake of glucose
What is the process when a protein transports a single solute across the membrane? What about 2 solutes, simultaneously (stops if one stops)? What if two solutes are moving in opposite directions?
1. Uniport (transported by the uniporter)
2. Coupled transport, also known as symport and cotransport, (transported by symporters)
-Antiport, or countertransport, ( transported by antiporters)
Are most cells hypertonic or hypotonic?
Most cells are hypertonic because of high concentrations of ions and small organic molecules required for normal cellular functions and to the large numbers of macromolecules dissolved in the cytosol. Most molecules are charged and this contributes to the intracellular osmolarity. Water will move inward causing cells to swell.
What is osmosis?
Osmosis is the diffusion of water across a selectively permeable membrane. Because some solutes cannot cross cell membranes by diffusion, water tends to move across membranes in response to differences in solute concentrations on the two sides of the membranes (difference in osmolarity- total solute concentration).
Why do solutes move more rapid through ion channels?
Conformational changes are not necessary.
Energetics of Transport give the two factors.
1. For uncharged solutes, the delta G of transport depends only on concentration gradient.
Keq for uncharged solutes always equals 1 at equilibrium. Which means that delta G is always 0.
R = 1.987 cal/mol-k
T= K= celsius +273
EX: uptake of lactose
2. For charged solutes, the delta G of transport depends on the electrochemical potential.
EX: uptake of chloride ions
Explain Glucose transporter (a uniport) in Erythrocyte. How is glucose kept low in the cell?
READILY REVERSIBLE: Glucose transport (GLUT), carrier protein, allows glucose to enter cell through lipid bilayer.
(1) molecule of glucose collides and binds to a GLUT1 in T1 conformational- (2) with glucose bound, GLUT1 shifts to T2 conformation- (3) conformational change allows the release of glucose molecule to interior of cell- (4) after release of glucose, GLUT1 returns to its original conformation (HIGH TO LOW CONCENTRATION)
Glucose is kept low in the cell because when glucose enters the cell, it becomes phosphorylated by hexokinase. This process locks glucose in cell because there is no transport protein for glucose with phosphate. Equilibrium cannot be reach and glucose can continue to be imported.
How do plant cells cope with an influx of water? What about our cells?
Plant cells become firm from the turgor pressure (if not then plasmolysis-plasma membrane pulls away from cell wall- will occur). Our cells continuously and actively pumping out inorganic ions. This reduces intracellular osmolarity and minimizes differences in solute concentration between cell and its surroundings.
EXAMPLE: Animals move sodium ions (Na+/K+ pump)!