Membrane Mechanics
A phospholipid has one region attracted to water and another that avoids water. What term describes a molecule with both properties?
What is amphipathic?
A small molecule moves directly through the lipid bilayer from high concentration to low concentration without ATP.
What is simple diffusion?
Substance X is at a concentration of 8 mM outside a cell and 2 mM inside. If X can freely cross the membrane, what is its initial net direction of movement?
Into the cell.
During osmosis, what substance is moving across the selectively permeable membrane?
Water.
A cell surrounds and engulfs a solid particle that is far too large to pass through a membrane protein.
What is phagocytosis?
A membrane protein is completely embedded in and spans the phospholipid bilayer. How would it be classified?
What is an integral membrane protein?
A substance moves down its concentration gradient through a membrane protein without using ATP.
What is facilitated diffusion?
Substance A is more concentrated outside the cell while substance B is more concentrated inside. How many concentration gradients exist?
Two.
A cell and its surrounding solution have equal effective solute concentrations. What term describes their relationship?
Isotonic.
A cell takes in extracellular fluid and dissolved substances by forming small vesicles.
What is pinocytosis?
If the hydrophobic tails of phospholipids suddenly faced the extracellular fluid while the hydrophilic heads pointed inward, what basic property of the membrane would be violated?
The hydrophilic regions should interact with water while hydrophobic regions are sheltered from it.
Simple diffusion and facilitated diffusion differ in one major structural requirement. What is it?
Facilitated diffusion requires a transport protein; simple diffusion does not.
At equilibrium, molecules continue crossing a membrane in both directions. What has actually become zero?
Net movement.
A cell is placed in a hypertonic solution. Predict the net direction of water movement and the change in cell volume.
Water moves out and the cell shrinks.
A secretory vesicle fuses with the plasma membrane and releases its contents outside the cell.
What is exocytosis?
A membrane protein has a carbohydrate attached to it. What membrane structure have you created?
What is a glycoprotein?
A molecule moves from an area of low concentration to an area of high concentration. What type of transport is required?
What is active transport?
Diffusion begins rapidly but slows with time even though molecules are still moving. What changed?
The concentration gradient became smaller.
A cell swells after being placed in a new solution. Assuming osmosis caused the change, how would you describe the solution relative to the cell?
Hypotonic.
Small molecules leave a cell rapidly at first, but their net movement gradually slows. No ATP or membrane proteins are involved. Identify the process and explain why it slows.
Simple diffusion; the concentration gradient decreases over time.
A membrane becomes too rigid at low temperatures. What general membrane property must the cell preserve in order for membrane proteins and transport processes to continue functioning normally?
What is membrane fluidity?
The downhill movement of one substance is used to drive another substance uphill against its gradient.
What is cotransport?
A substance has a large concentration gradient across a membrane but never crosses it. What is the most likely explanation?
The membrane is impermeable to that substance.
Two sides of a membrane contain equal total amounts of solute, but some solutes can cross the membrane and others cannot. Why might water still redistribute?
Because membrane permeability matters; permeable and impermeable solutes do not affect the final distribution in the same way.
A cell must move Na⁺ out even though Na⁺ concentration is already greater outside than inside. What type of transport is required, and why?
Active transport, because Na⁺ is being moved against its concentration gradient and energy is required.