Cellular Organelles and Compartmentalization
A mutation prevents a cell from attaching ribosomes to its endoplasmic reticulum. The cell can still make cytosolic proteins normally, but proteins destined for secretion decrease dramatically.
What structure has been disrupted?
Rough endoplasmic reticulum.
Rank these from most likely to cross the phospholipid bilayer directly to least likely:
O₂, Na⁺, CO₂, large polar protein
Answer: O₂/CO₂ → Na⁺ → large polar protein.
O₂ and CO₂ are small, hydrophobic molecules and cross readily. Na⁺ is charged and requires transport proteins. A large polar protein cannot freely cross the bilayer.
A membrane protein moves a substance from a region containing 2 mM of that substance to a region containing 45 mM.
What type of transport must be occurring?
Answer: Active transport because the substance is moving against its concentration gradient.
Two spherical cells have radii of 1 µm and 4 µm.
Which cell has the larger surface-area-to-volume ratio?
Answer: The 1 µm cell.
Cell A has a water potential of −2.7 bars.
Cell B has a water potential of −6.1 bars.
In which direction will water move?
Answer: From Cell A to Cell B because water moves from higher water potential to lower water potential.
A eukaryotic cell contains normal ribosomes, a functional nucleus, and healthy mitochondria. However, proteins synthesized for export leave the ER but are never properly modified or packaged into transport vesicles.
Which organelle is most likely defective?
The Golgi complex.
A transmembrane protein contains both hydrophobic and hydrophilic amino acid regions.
Where would you predict each region to be located?
Answer: Hydrophobic regions would interact with the hydrophobic interior of the phospholipid bilayer, while hydrophilic regions would face the aqueous cytosol or extracellular environment.
A toxin completely prevents ATP from being used by a cell.
Which would most likely continue initially?
A. Na⁺/K⁺ pumping
B. Exocytosis
C. Facilitated diffusion
D. Endocytosis
Answer: C. Facilitated diffusion
A spherical cell doubles its radius.
Without calculating the exact surface area or volume, predict what happens to its surface-area-to-volume ratio.
Answer: It decreases by half because SA:V for a sphere is inversely proportional to radius.
A plant cell has:
Ψs=−7.8 bars
Ψp= +2.3 bar
Determine its total water potential.
−7.8+2.3=−5.5 bars
Researchers discover an organelle containing hydrolytic enzymes. A mutation causes those enzymes to become inactive while leaving the membrane surrounding the organelle intact.
Predict two cellular consequences.
Answer: Macromolecules and damaged cell components would accumulate because the lysosome could no longer digest them, and the cell's ability to destroy invading pathogens or participate normally in cellular breakdown would decrease.
A membrane remains intact but all glycoproteins and glycolipids are experimentally removed.
Which cellular function would be most directly impaired?
Answer: Cell recognition.
A cell contains:
Explain why the Na⁺/K⁺ pump must use energy.
Answer: It transports Na⁺ from its lower intracellular concentration toward the higher extracellular concentration and K⁺ from its lower extracellular concentration toward the higher intracellular concentration. Both movements oppose concentration gradients.
Three artificial cells have identical volumes but different membrane folding.
Cell A has no folds.
Cell B has moderate folds.
Cell C has extensive folds.
Predict which would have the highest potential rate of membrane exchange per unit volume.
Answer: Cell C because membrane folding increases surface area without proportionally increasing cell volume.
Two solutions are at the same temperature and pressure.
Solution A: 0.30 M glucose
Solution B: 0.20 M NaCl
Assume NaCl completely dissociates.
Which solution has the lower solute potential?
Answer: Solution B.
Glucose: iC=1(0.30)=0.30)
NaCl: (iC=2(0.20)=0.40)
The greater effective particle concentration makes the NaCl solution's solute potential more negative.
A drug selectively destroys the inner membranes of mitochondria without damaging the mitochondrial outer membrane or matrix enzymes.
Explain why ATP production would decrease even though enzymes of the citric acid cycle remain functional.
Answer: Destroying the folded inner membrane greatly reduces the membrane surface available for processes involved in ATP production and prevents the mitochondrion from properly maintaining the proton gradient required for ATP production.
Scientists gradually increase the extracellular concentration of molecule X.
At first, the rate at which X enters the cell rises rapidly. Eventually, increasing X concentration causes almost no further increase in transport rate.
What transport mechanism is most consistent with the results, and why?
Answer: Facilitated diffusion. Transport proteins are required, so eventually all available proteins become occupied and the transport rate reaches saturation.
One complete cycle of the Na⁺/K⁺ pump exports three Na⁺ ions and imports two K⁺ ions.
After 1,200 cycles, determine:
Answer:
Students create agar cubes containing an indicator and place them into vinegar.
Cube X: 1 cm per side
Cube Y: 2 cm per side
Cube Z: 4 cm per side
After the same amount of time, which cube should have the largest percentage of its volume penetrated by vinegar?
Explain.
Answer: Cube X. Smaller cells have larger surface-area-to-volume ratios, providing more exchange surface relative to the amount of internal material.
A 0.20 M CaCl₂ solution is at 27°C and is open to the atmosphere.
Assume complete dissociation.
Calculate its water potential.
−15.0 bars
Scientists discover an unknown membrane-bound organelle with:
Construct the evolutionary explanation most strongly supported by these observations.
Answer: The organelle probably evolved from a formerly free-living prokaryotic cell that was engulfed by another cell and eventually became permanently incorporated into the host through endosymbiosis.
Molecule A and molecule B both move into a cell down their concentration gradients.
When membrane transport proteins are chemically blocked:
Neither process requires ATP.
Identify the transport mechanism of each molecule and explain the evidence.
Answer: A uses simple diffusion because blocking membrane proteins does not affect it. B uses facilitated diffusion because it moves down its gradient without ATP but depends on membrane transport proteins.
A researcher finds that a chemical does not directly block the Na⁺/K⁺ pump but causes cellular ATP concentrations to collapse.
Predict what will eventually happen to Na⁺ and K⁺ concentration gradients.
Answer: The gradients will progressively disappear because the pump will no longer have sufficient energy to move Na⁺ and K⁺ against their concentration gradients. Passive movement will gradually reduce the differences between the inside and outside of the cell.
Cell A is a sphere with radius 2 units.
Cell B is another sphere with radius 6 units.
Determine the ratio:
Cell A has three times the SA:V ratio of Cell B.
A plant cell has:
Ψs=−11.0 bars
and initially:
Ψp= +4.0 bar
It is placed into a solution with:
Ψ=−9.0 bars
Predict:
Ψp=+2 bars