Mysteries of the Membrane
A researcher creates an artificial phospholipid bilayer containing no transport proteins. O₂ and Na⁺ are added outside. After several minutes, this substance is more likely to appear inside
What is O₂?
A molecule moves from an area where it is highly concentrated to an area where it is less concentrated. Cellular ATP concentrations fall to nearly zero, but its movement continues. This direction of movement is described as moving this way relative to the concentration gradient.
What is down the concentration gradient?
A membrane separates two solutions and allows water, but not the dissolved solute, to cross. Water moves toward the solution containing the greater concentration of solute until the concentration difference is reduced. This movement of water is called this.
What is osmosis?
A chemical bond stores energy that can later become available during a reaction. Because this energy is stored rather than associated with movement, it belongs to this category.
What is potential energy?
A protein accelerates a chemical reaction, emerges from the reaction unchanged, and can catalyze the same reaction again. It therefore functions as this.
What is an enzyme?
Two membranes are maintained at the same temperature. Membrane A contains primarily saturated hydrocarbon tails, while membrane B contains more unsaturated tails containing kinks. The phospholipids in membrane B therefore have more freedom of movement than those in A. This membrane property is greater in membrane B.
What is membrane fluidity?
Glucose cannot cross a particular membrane efficiently on its own. After a specific membrane protein is added, glucose moves from high to low concentration without ATP consumption. This mechanism accounts for its movement.
What is facilitated diffusion?
An animal cell is transferred into a solution containing a greater concentration of nonpenetrating solute than its cytoplasm. Water leaves the cell and its volume decreases. Relative to the cell, the surrounding solution is described this way.
What is hypertonic?
A reaction begins with reactants possessing greater Gibbs free energy than its products. This reaction has this sign of ΔG.
What is negative?
A substrate repeatedly binds one particular region of an enzyme before being converted into product. A mutation altering this region dramatically reduces substrate binding. This region is called this.
What is the active site?
A membrane remains intact during a sudden decrease in environmental temperature. Analysis reveals that its phospholipids cannot pack as tightly together because many of their hydrocarbon tails contain double bonds. This type of phospholipid tail helps preserve membrane fluidity.
What is an unsaturated hydrocarbon tail?
A membrane protein moves an ion from an intracellular concentration of 5 mM to an extracellular concentration of 50 mM. ATP is hydrolyzed during transport. Because the ion is moving toward the region where it is already more concentrated, this process is classified as this form of transport.
What is active transport?
An animal cell placed in distilled water swells and eventually lyses. A plant cell exposed to the same solution takes in water but does not lyse. This structure generates resistance to the increasing volume of the plant cell.
What is the cell wall?
A reaction has ΔG < 0 but nevertheless occurs extremely slowly. Adding an enzyme greatly increases the reaction rate without altering the free energies of the reactants or products. The enzyme has lowered this energetic barrier.
What is activation energy?
An enzyme works efficiently at pH 7 but poorly at pH 2. The acidic environment disrupts hydrogen bonding and alters the shape of the region where substrate normally binds. This structural change explains the loss of enzyme activity.
What is a change in the active site's shape?
A researcher removes one membrane component without changing the phospholipid composition. The resulting membrane becomes excessively fluid at high temperatures but packs too tightly at low temperatures. This missing component normally buffers membrane fluidity at both temperature extremes.
What is cholesterol?
Researchers observe an ATP-dependent transporter repeatedly exporting three Na⁺ ions while importing two K⁺ ions during each transport cycle. These observations identify this membrane protein.
What is the sodium-potassium pump?
A membrane permeable only to water separates Solution A, containing 10% salt, from Solution B, containing 5% salt. Salt cannot cross the membrane. Net water movement will therefore occur in this direction.
What is from Solution B toward Solution A?
Protein synthesis requires an input of energy and is therefore endergonic. ATP hydrolysis releases energy and is exergonic. Cells link these processes so that the energetically favorable reaction drives the unfavorable one. This strategy is known as this.
What is energy coupling?
An inhibitor closely resembles the normal substrate of an enzyme and reduces product formation by occupying the same location the substrate normally binds. This mechanism is classified as this.
What is competitive inhibition?
Molecule X is small and uncharged, while molecule Y is large and charged. Neither molecule has a specific membrane transporter. X readily enters the cell while Y remains extracellular despite a large concentration gradient. This structural feature of the plasma membrane primarily produces the difference.
What is the hydrophobic interior of the phospholipid bilayer?
Molecule X enters cells through a membrane protein. ATP depletion does not stop its movement, but experimentally reversing its concentration gradient causes X to move in the opposite direction through the same protein. These two observations distinguish its transport from active transport and identify this mechanism.
What is facilitated diffusion?
Red blood cells placed into distilled water swell, while cells placed into a concentrated salt solution shrink. Researchers want an IV solution that produces neither response. Relative to the intracellular environment, the IV solution must have this tonicity.
What is isotonic?
During starvation, a cell increases reactions that break complex molecules into simpler products and release usable energy. When nutrients become abundant, it increases reactions that use energy to construct larger molecules from smaller components. These opposing sets of reactions are collectively classified as these two metabolic pathways.
What are catabolic and anabolic pathways?
A metabolic pathway converts A → B → C → D. When D becomes abundant, it binds to an enzyme responsible for an earlier step and reduces additional production of D. When D concentrations fall, pathway activity increases again. This regulatory mechanism prevents unnecessary product accumulation.
What is feedback inhibition?