- The bacterium Vibrio cholerae produces a toxin that causes chloride ion channels in the intestinal epithelial cells to remain permanently open. This leads to a massive loss of chloride ions into the intestinal lumen. Which statement best explains the resulting severe diarrhea?
Answer: The high concentration of Cl⁻ in the lumen lowers its water potential, causing a large net movement of water out of the cells by osmosis.
The movement of chloride ions into the intestinal lumen dramatically increases the solute concentration there, which in turn lowers the water potential. Due to this osmotic gradient, water moves from the epithelial cells and underlying tissues into the lumen via osmosis, leading to watery diarrhea and dehydration. Water is not actively transported; its movement is passive and follows the solute gradient.
- The fusion of a vesicle with the plasma membrane during exocytosis and the fusion of a phagosome with a lysosome both depend on the ability of membranes to change shape, break, and reconnect. What property of the plasma membrane is essential for these events?
Answer: The fluidity of the phospholipid bilayer.
The fluid mosaic model describes the plasma membrane as a fluid structure where phospholipids and proteins can move relative to one another. This fluidity allows the membrane to be dynamic: it can curve to form vesicles, break, and fuse with other membranes without losing its integrity. Rigidity (A) would prevent this, while aquaporins (B) and impermeability (D) are properties of the membrane but not the key feature that allows for fusion and fission.
- Ouabain is a cardiac glycoside that specifically inhibits the Na⁺/K⁺-ATPase pump. If ouabain is applied to a neuron, which of the following effects would be observed over time?
Answer: The intracellular concentration of Na⁺ will increase and the intracellular concentration of K⁺ will decrease.
The Na⁺/K⁺ pump actively transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell. Inhibiting this pump will stop this process. As a result, Na⁺ that leaks in and K⁺ that leaks out will not be pumped back against their gradients. This will cause the intracellular Na⁺ concentration to gradually increase and the intracellular K⁺ concentration to gradually decrease, disrupting the resting membrane potential.
- Placing a plant tissue in a highly concentrated salt solution causes the cells to undergo plasmolysis. What is the correct sequence of events during this process?
Answer: Water leaves the cytoplasm and vacuole by osmosis, causing the plasma membrane to pull away from the cell wall.
The highly concentrated salt solution is hypertonic to the cell's cytoplasm. This causes a net movement of water out of the cell's large central vacuole and cytoplasm via osmosis. As the cell loses water, its volume decreases, and the flexible plasma membrane pulls away from the rigid cell wall. This process is called plasmolysis, and the cell is described as flaccid or plasmolyzed.
- A marine amoeba, which is isotonic with seawater, is transferred to a freshwater pond. To survive, it must actively pump water out using its contractile vacuole. A researcher adds a substance that specifically inhibits ATP synthesis in the amoeba. What is the most likely immediate consequence?
Answer: The amoeba will swell and likely burst due to the inability to power its contractile vacuole.
The amoeba is in a hypotonic environment (freshwater), causing water to constantly enter via osmosis. The contractile vacuole is an organelle that performs active transport to pump this excess water out, a process requiring ATP. Inhibiting ATP synthesis will disable the vacuole, leading to uncontrolled water influx, swelling, and eventual lysis (bursting).
- A red blood cell is placed in a solution of 0.3 M urea. Urea is a small, uncharged molecule that can slowly diffuse across the red blood cell membrane. Initially, the total solute concentration inside the cell is also approximately 0.3 M. What is the expected final outcome?
Answer: The cell will first shrink slightly, then swell and burst as urea diffuses in.
This question tests the difference between osmolarity and tonicity. Initially, the solution is isosmotic (same solute concentration). However, because urea is a penetrating solute, it will diffuse down its concentration gradient into the cell. As urea enters, the internal solute concentration rises above 0.3 M, making the cell hypertonic relative to the solution. This causes water to follow urea into the cell via osmosis, leading to swelling and lysis (bursting). The initial slight shrinkage (crenation) can occur as water initially moves out before significant urea has entered, but the dominant final effect is swelling and lysis.
- [HL] Acetylcholine is a neurotransmitter that binds to receptors on muscle cells. These receptors are ligand-gated ion channels. What happens when acetylcholine binds to its receptor?
Answer: The channel protein changes conformation, opening a pore that allows specific ions to diffuse across the membrane.
Ligand-gated channels are a type of membrane protein that opens or closes in response to the binding of a specific chemical messenger (the ligand), in this case, acetylcholine. The binding causes a conformational change in the protein structure, which opens the channel's pore. This allows ions (typically Na⁺ in this case) to flow across the membrane down their electrochemical gradient, a form of facilitated diffusion.
- Some organisms living in extremely cold environments adapt by altering their plasma membrane composition. Which adaptation would be most effective at maintaining membrane fluidity at low temperatures?
Answer: Increasing the proportion of unsaturated fatty acids and increasing the cholesterol content.
At low temperatures, membranes risk becoming too rigid. Unsaturated fatty acids have kinks in their tails, preventing tight packing and thus increasing fluidity. Cholesterol acts as a 'fluidity buffer'; at low temperatures, it prevents phospholipids from packing too closely together, thereby increasing fluidity. Therefore, increasing both unsaturated fatty acids and cholesterol is the most effective adaptation.
- A dialysis bag, permeable to water but not to sucrose, contains a 0.5 M sucrose solution. It is placed in a beaker containing a 0.2 M sucrose solution. What will be the net movement of water and the change in the bag's mass?
Answer: Water will move into the bag, and its mass will increase.
The solution inside the bag (0.5 M sucrose) is hypertonic to the solution outside (0.2 M sucrose). This means the water potential inside the bag is lower (more negative) than outside. By osmosis, there will be a net movement of water from the area of higher water potential (the beaker) to the area of lower water potential (the bag). This influx of water will cause the bag to swell and its mass to increase.
- [HL] The absorption of glucose from the gut into an epithelial cell is achieved by a Na⁺-glucose cotransporter. This protein moves one Na⁺ ion down its concentration gradient and one glucose molecule against its concentration gradient into the cell. How is this process best classified?
Answer: Secondary active transport, as it uses the electrochemical potential of Na⁺ established by a separate pump.
This is a classic example of secondary active transport (or cotransport). The transport of glucose against its gradient does not directly use ATP. Instead, it uses the kinetic energy stored in the Na⁺ concentration gradient. This Na⁺ gradient is maintained by the Na⁺/K⁺-ATPase pump (a primary active transporter) located elsewhere on the cell membrane, which does use ATP. Therefore, the glucose transport is indirectly powered by ATP.
- Based on the passage, what is the direct effect on water movement across the apical membrane of lung epithelial cells in a person with cystic fibrosis?
Answer: Water moves into the cells via osmosis, making the mucus layer outside the cells abnormally thick and sticky.
Normally, chloride ions are transported out of the cell, making the fluid outside (the mucus layer) hypertonic and drawing water out by osmosis. In cystic fibrosis, the inability to move Cl⁻ out of the cell means the mucus layer is less concentrated with solutes (less negative water potential) compared to the cell cytoplasm. Consequently, water does not move out sufficiently, and may even move in, resulting in a dehydrated, thick, and sticky mucus layer.
- A researcher is studying the transport of substance X into a cell. They observe that the rate of transport increases linearly with the concentration of X outside the cell. However, the rate of transport for substance Y increases with its external concentration but eventually reaches a maximum rate (saturates). What can be deduced about the transport mechanisms for X and Y?
Answer: X is transported by simple diffusion, and Y is transported by facilitated diffusion.
Simple diffusion rate is directly proportional to the concentration gradient, so it increases linearly without saturation. Facilitated diffusion and active transport both rely on a finite number of membrane protein transporters (channels or carriers). Once all transporters are occupied, the rate of transport reaches a maximum (Vmax), a phenomenon known as saturation. Since Y's transport saturates, it must be protein-mediated (facilitated diffusion or active transport). Since X's transport does not saturate, it is most likely simple diffusion.
- The secretion of insulin from pancreatic β-cells involves the packaging of insulin into vesicles which then fuse with the plasma membrane to release their contents. Which combination of processes is required for this to occur?
Answer: Exocytosis for release, which requires ATP for vesicle movement and fusion.
The release of large molecules like insulin is accomplished by exocytosis. This process involves vesicles, formed by the Golgi apparatus, moving to the plasma membrane and fusing with it. Both the movement of vesicles along the cytoskeleton and the fusion of the membranes are active processes that require energy in the form of ATP. It is not passive and does not involve carrier proteins for the insulin itself.
- Some cancer cells overexpress a type of protein called a multidrug resistance (MDR) transporter. This protein is an ATP-powered pump in the plasma membrane that expels a wide variety of chemotherapy drugs from the cell. How does this mechanism allow cancer cells to survive treatment?
Answer: It uses primary active transport to pump the drug out, keeping its intracellular concentration low.
The MDR transporter is described as an ATP-powered pump. This indicates it is a primary active transporter. It actively moves chemotherapy drugs, which have already entered the cell (likely by diffusion), out of the cell against a concentration gradient. By continuously pumping the drug out, the transporter maintains a low intracellular concentration, preventing the drug from reaching its target and killing the cell.
- A scientist creates artificial vesicles (liposomes) with a high internal concentration of K⁺ ions and a low internal concentration of Na⁺ ions, similar to a real cell. These vesicles are placed in a solution with high Na⁺ and low K⁺. If a drug is added that creates pores in the membrane permeable only to K⁺, what will occur?
Answer: K⁺ ions will flow out of the vesicle, making the interior of the vesicle negative relative to the exterior.
The vesicles have a high internal K⁺ concentration and are placed in a low K⁺ solution. This creates a strong concentration gradient for K⁺ to move out. Since the added pores are only permeable to K⁺, these positive ions will diffuse out of the vesicle, down their concentration gradient. This efflux of positive charge leaves behind unbalanced negative charges inside the vesicle, making the interior negative relative to the exterior, thus establishing a membrane potential.
- [HL] The resting potential of a neuron is primarily maintained by the Na⁺/K⁺ pump and K⁺ leak channels. The pump creates steep gradients for Na⁺ (high outside) and K⁺ (high inside). Why does the opening of K⁺ leak channels cause the membrane potential to become negative on the inside?
Answer: The diffusion of positive K⁺ ions out of the cell, down their concentration gradient, leaves a net negative charge inside.
The Na⁺/K⁺ pump establishes a high concentration of K⁺ inside the cell. The K⁺ leak channels are always open, allowing K⁺ ions to diffuse passively out of the cell, moving down their steep concentration gradient. As these positively charged ions leave the cell, they leave behind negatively charged proteins and anions that cannot cross the membrane, resulting in a net negative charge on the interior of the plasma membrane and establishing the negative resting potential.
- Why does the process of phagocytosis, such as a white blood cell engulfing a bacterium, require a significant amount of cellular energy (ATP)?
Answer: To fuel the movement of the cytoskeleton to extend pseudopods and enclose the bacterium.
Phagocytosis is a form of endocytosis that requires major reorganization of the cell's plasma membrane and underlying cytoskeleton. ATP is primarily required to power motor proteins that cause the actin cytoskeleton to polymerize and depolymerize, allowing the cell to extend pseudopods (cellular projections) that surround and engulf the large particle. This is a dynamic mechanical process, not a simple transport or synthesis event.
- Freshwater fish face the constant problem of water entering their bodies by osmosis. They do not drink water and excrete large volumes of dilute urine. What does this indicate about the cells of the fish relative to the freshwater environment?
Answer: The cells are hypertonic to the freshwater, causing a constant influx of water.
Freshwater is a hypotonic environment, meaning it has a very low solute concentration and high water potential. The cells and body fluids of a fish have a higher solute concentration, making them hypertonic relative to the freshwater. This osmotic gradient causes water to constantly move from the environment into the fish's body via osmosis, particularly across the gills. The physiological adaptations (not drinking, producing dilute urine) are to cope with this constant water influx.
- A cell requires the uptake of molecule Z. The process is observed to be highly specific for Z, requires ATP, and is not affected by changes in the membrane potential. Which transport mechanism is most likely responsible?
Answer: Receptor-mediated endocytosis.
Receptor-mediated endocytosis is highly specific because the substance (Z) must first bind to a specific receptor protein on the cell surface. The process of forming a vesicle to internalize the receptor-ligand complex requires ATP. Unlike ion pumps or cotransporters, this bulk transport mechanism is generally not directly affected by the membrane's electrical potential. The other options are incorrect: simple diffusion is not specific and uses no ATP; facilitated diffusion uses no ATP; secondary active transport would be affected by membrane potential if an ion gradient is used.
