Cells
Which of the following best explains why cells are constrained in how large they can grow?
(A) Larger cells contain too many chromosomes for the nucleus to manage.
(B) As a cell grows, its volume increases faster than its surface area, limiting substance exchange.
(C) Larger cells cannot produce enough phospholipids to build a functional plasma membrane.
(D) Small cells require more ATP per unit volume than large cells to maintain homeostasis.
Correct Answer: (B)
Explanation: Volume increases as a cubic function (r3) while surface area increases as a quadratic function (r2. As a cell grows larger, its surface area-to-volume ratio drops, making it impossible to move nutrients in and metabolic wastes out fast enough across the cell membrane.
$200 — Easy: Membrane Structure & Function
Question: Which of the following structural characteristics of the phospholipid bilayer is responsible for preventing large, polar molecules and ions from freely diffusing across the cell membrane?
(A) Hydrophilic glycerol and phosphate heads facing the aqueous cytosol and extracellular fluid
(B) Nonpolar, hydrophobic fatty acid tails packed in the interior of the membrane
(C) Peripheral membrane proteins attached loosely to the inner surface of the bilayer
(D) Glycoprotein carbohydrate chains protruding into the extracellular matrix
Correct Answer: (B)
Explanation: The interior of the lipid bilayer is composed of nonpolar, hydrophobic fatty acid tails. This creates a barrier that allows small nonpolar molecules (like O2 and CO2) to pass through easily, while blocking large or charged/polar substances.
Question:
Water potential measures the tendency of water to move from one area to another. In which direction will water always move naturally?
(A) From an area of lower water potential to an area of higher water potential.
(B) From an area of higher water potential to an area of lower water potential.
(C) From a hypertonic solution to a hypotonic solution.
(D) From an area with high solute concentration to an area with low solute concentration.
Correct Answer: (B)
Explanation: Water moves down its potential gradient from regions of higher water potential to regions of lower water potential.
Membrane proteins carry out several crucial jobs for the cell. Which of the following functions is primarily performed by glycoproteins protruding into the extracellular fluid?
(A) Hydrolyzing ATP to pump ions against a concentration gradient.
(B) Cell-cell recognition by serving as identification tags.
(C) Linking the cytoskeleton directly to the nuclear envelope.
(D) Catalyzing sequential metabolic steps in a linear pathway.
Correct Answer: (B)
Explanation: Glycoproteins have short carbohydrate chains attached to them that face the outside of the cell, functioning as specific molecular identification tags for cell-cell recognition.
Question:
Plasma membranes maintain a strict asymmetrical distribution of phospholipids between the inner and outer leaflets. Which of the following best describes how the negatively charged phospholipids (such as phosphatidylserine, PS) in the inner leaflet contribute to cellular function?
(A) They bind directly to extracellular glycolipids to trigger receptor-mediated endocytosis.
(B) They provide a specific binding surface for cytosolic proteins with basic domains, initiating intracellular signaling events.
(C) They decrease the thickness of the outer leaflet to allow rapid water transport via aquaporins.
(D) They form covalent disulfide bridges with extracellular antibodies to anchor the membrane to the ECM.
Correct Answer: (B)
Explanation: As noted in the slides, negatively charged lipid headgroups in the inner (cytoplasmic) leaflet provide a binding platform for signaling proteins with basic-rich domains, causing protein relocalization and initiating signaling cascades.
A cell with a mutation that prevents vesicles from fusing with the cis face of the Golgi apparatus would most directly experience a failure in which of the following cellular processes?
(A) Synthesis of ribosomal subunits inside the nucleolus
(B) Modification and sorting of proteins delivered from the rough endoplasmic reticulum
(C) Neutralization of hydrogen peroxide by catalase enzymes
(D) Generation of a proton gradient across the inner mitochondrial membrane
Correct Answer: (B)
Explanation: The cis face of the Golgi apparatus is the receiving side that merges with transport vesicles arriving from the rough ER. If vesicles cannot fuse at the cis face, newly synthesized proteins cannot enter the Golgi to undergo modification, sorting, and packaging.
Question: A student places two different chemical dyes into separate wells in an agar gel plate. Dye X has a molecular weight of 150 g/mol, while Dye Y has a molecular weight of 600 g/mol. After 2 hours, which observation and explanation regarding the diffusion rates is correct?
(A) Dye Y diffuses farther because larger molecules possess greater kinetic energy.
(B) Dye X diffuses farther because smaller molecules move faster through the matrix.
(C) Both dyes diffuse equal distances because rate depends only on agar concentration.
(D) Neither dye diffuses because movement across a solid medium requires ATP input.
Correct Answer: (B)
Explanation: The rate of passive diffusion is inversely related to molecular size. Smaller molecules (Dye X) travel faster and diffuse across a larger distance in a given time period compared to larger, heavier molecules (Dye Y).
What is the solute potential of pure water in an open beaker at sea level and room temperature?
(A) 0 bars
(B) -1 bars
(C) +1 bars
(D) +0.0831 bars
Correct Answer: (A)
Explanation: Pure water at atmospheric pressure has a solute potential of zero. Adding any solute lowers the solute potential, making it negative.
Question:
In 1925, Gorter and Grendel extracted lipids from red blood cells and spread them onto a Langmuir trough. They discovered that the surface area of the spread lipid film was approximately double the surface area of the intact red blood cells. What major conclusion did they draw?
(A) Plasma membranes consist of a single monolayer of amphipathic proteins.
(B) Membrane lipids are arranged as a phospholipid bilayer.
(C) Integral proteins span completely through the lipid matrix.
(D) Cholesterol acts as a temperature buffer inside animal cells.
Correct Answer: (B)
Explanation: The 2:1 ratio of lipid film surface area to cell surface area led Gorter and Grendel to correctly hypothesize that cell membranes are composed of a lipid bilayer.
Question:
Antibodies (immunoglobulins) are key immune proteins used in cell membrane experiments like Frye and Edidin's cell fusion study. Which structural feature of an antibody is responsible for recognizing and binding to a specific epitope on a membrane antigen?
(A) The constant region formed exclusively by cysteine disulfide bonds.
(B) The paratope located at the tip of the variable region on the "Y"-shaped protein.
(C) The hydrophobic transmembrane domain that anchors the heavy chain into the lipid bilayer.
(D) The polysaccharide carbohydrate tail attached to the light chain.
Correct Answer: (B)
Explanation: Each tip of the "Y" on an antibody contains a variable region called a paratope, which acts like a "lock" that specifically binds to a single epitope ("key") on an antigen.
Question: A toxicologist treats liver tissue with a chemical compound that inhibits metabolic reactions involving fatty acid oxidation and catalase production. Which organelle is primarily affected by this treatment?
(A) Lysosome
(B) Smooth Endoplasmic Reticulum
(C) Peroxisome
(D) Golgi Apparatus
Correct Answer: (C)
Explanation: Peroxisomes contain oxidative enzymes that break down fatty acids into smaller fuel molecules and utilize the enzyme catalase to safely convert harmful hydrogen peroxide ($H_2O_2$) byproducts into water.
Question:
A freshwater plant cell with an internal solute concentration of 0.2M is placed into a beaker containing a 0.8M sucrose solution. Which of the following describes the immediate direction of water movement and the resulting condition of the plant cell?
(A) Water moves into the cell; the cell becomes turgid.
(B) Water moves out of the cell; the cell undergoes plasmolysis.
(C) Water moves into the cell; the cell undergoes cytolysis and bursts.
(D) Water enters and leaves at equal rates; the cell remains in dynamic equilibrium.
Correct Answer: (B)
Explanation: The surrounding solution is hypertonic 0.8M relative to the interior of the cell 0.2M. Water moves passively down its concentration gradient (from higher water potential to lower water potential) out of the cell, causing the plasma membrane to pull away from the cell wall (plasmolysis).
Question:
What does Ψp represent, and how does it affect a plant cell?
(A) Ψp is solute potential; it decreases as more salt is dissolved in the cell.
(B) Ψp is osmotic potential; it causes water to leave the plant root.
(C) Ψp is pressure potential (turgor pressure); positive pressure increases the cell's water potential.
(D) Ψp is thermal potential; it drops when the surrounding temperature increases.
Correct Answer: (C)
Explanation: Pressure potential Ψp is the physical push exerted by the cell wall against the plasma membrane (turgor pressure). A positive pressure potential increases the overall water potential of the system.
Question:
Organisms regulate their membrane fluidity through a process called homeoviscous adaptation. If a plant or cold-blooded animal experiences a sudden drop in environmental temperature, how will its cell membranes adapt to maintain proper fluidity?
(A) Increase the proportion of saturated fatty acids with longer hydrocarbon chains.
(B) Increase the proportion of unsaturated fatty acids with double bonds (kinks).
(C) Remove cholesterol completely to allow phospholipids to crystallize into a gel.
(D) Translocate all peripheral proteins into the inner cytoplasmic leaflet.
Explanation: At low temperatures, membranes risk becoming rigid and gel-like. Cells respond by increasing unsaturated fatty acids; their double bonds create "kinks" that prevent tight packing, keeping the membrane fluid.
Question:
When the length of a protein's transmembrane domain (TMD) does not match the hydrophobic thickness of the surrounding lipid bilayer, a state called hydrophobic mismatch occurs. If a transmembrane protein's hydrophobic region is significantly longer than the thickness of the bilayer, how can the membrane lipids adapt to fulfill hydrophobic matching?
(A) Phospholipids activate flippases to move all saturated lipids to the outer leaflet.
(B) Phospholipids stretch their acyl chains to locally increase bilayer thickness.
(C) Phospholipids cleave their fatty acid tails using phospholipase enzymes.
(D) Phospholipids compress and disorder their acyl chains to make the bilayer thinner.
Correct Answer: (B)
Explanation: To minimize the exposed hydrophobic area when a protein TMD is too long, surrounding lipids can adapt by stretching their hydrocarbon acyl chains, thereby locally increasing effective bilayer thickness.
Question: Which of the following observations provides the strongest evidence supporting Lynn Margulis’s Endosymbiotic Theory regarding the origin of mitochondria?
(A) Mitochondria possess a single outer lipid bilayer identical in structure to the nuclear membrane.
(B) Mitochondria contain linear DNA sequences wrapped around eukaryotic histone proteins.
(C) Mitochondria replicate via binary fission and possess circular chromosomes similar to prokaryotes.
(D) Mitochondria synthesize proteins exclusively using eukaryotic cytosolic 80S ribosomes.
Correct Answer: (C)
Explanation: The Endosymbiotic Theory is supported by the fact that mitochondria and chloroplasts divide independently by binary fission, contain their own circular DNA (lacking histones), and possess 70S ribosomes—all characteristics shared with prokaryotes (bacteria).
Question:
The sodium-potassium pump (Na+/K+ATPase) actively transports 3 Na+ ions out of the cell for every 2 K+ ions pumped into the cell. Which of the following would immediately occur if ATP production in the cell were completely inhibited?
(A) Na+ and K+ ions would begin diffusing freely across the lipid bilayer down their concentration gradients.
(B) Active transport would halt, leading to a loss of the electrochemical gradient across the plasma membrane.
(C) The pump would reverse direction, moving 2 Na+ ions in and 3 K+ ions out using passive transport.
(D) Vesicular endocytosis would accelerate to compensate for the lost ion transport.
Correct Answer: (B)
Explanation: Active transport relies on energy supplied by ATP hydrolysis to move ions against their steep concentration gradients. Without ATP, the pump ceases to function, preventing the cell from maintaining its membrane potential.
Question:
DAILY DOUBLE! (Short-answer question — answer within 30 seconds)
A plant cell with a solute potential of -7.43 bars is placed into an open beaker containing a 0.10 NaCl solution with a total water potential of -4.95 bars.
Predict the initial direction of net water movement (into the cell or out of the cell).
Calculate the turgor pressure inside the plant cell at dynamic equilibrium (when net water movement stops).
Water moves INTO the cell (from the solution with higher water potential, -4.95 bars, into the cell with lower water potential, -7.43 bars
At equilibrium, Water pressure potential= 2.48 bars
WEEKLY TRIPPLE! In 1970, Frye and Edidin fused a human cell and a mouse cell to form a heterokaryon, labeling each cell's surface proteins with distinct fluorescent antibody tags.
Describe what happened to the fluorescent tags over time when the fused cell was kept at 37 degrees Celsius
State the primary conclusion this experiment provided regarding the Singer-Nicolson Fluid Mosaic Model.
Answer:
Observation: The human and mouse membrane proteins, initially restricted to separate halves of the fused cell, mixed together and redistributed evenly across the entire cell surface.
Conclusion: This provided direct experimental evidence that membrane proteins can move laterally within the plane of the lipid bilayer (proving membrane fluidity).
Question:
While lateral and rotational diffusion of membrane proteins happen rapidly, transverse diffusion (flip-flop across the bilayer) of integral membrane proteins is never observed. Why is protein flip-flop thermodynamically forbidden?
(A) Membrane proteins are tethered to extracellular glycolipids by covalent ionic bonds.
(B) Transverse movement would require passing large, highly polar or charged hydrophilic domains through the nonpolar, hydrophobic core of the bilayer.
(C) Flippase and scramblase enzymes actively hydrolyze ATP to prevent proteins from flipping.
(D) The actin-based cytoskeleton surrounds the outer leaflet, physically blocking protein movement.
Correct Answer: (B)
Explanation: Moving a protein across the membrane requires dragging its hydrophilic/charged regions through the low-dielectric, nonpolar hydrophobic interior of the lipid bilayer, which is thermodynamically unfavorable.
Question:Researchers pulse-label pancreatic acinar cells with radioactive amino acids for 5 minutes, then track the pathway of the labeled proteins over time (a pulse-chase experiment). Which of the following represents the correct sequence of organelles through which a secreted digestive enzyme will travel?
(A) Nucleolus -> Smooth ER -> Lysosome -> Plasma Membrane
(B) Free Ribosome -> Smooth ER -> Trans-Golgi -> Plasma Membrane
(C) Bound Ribosome -> Rough ER Lumen -> Golgi Apparatus -> Secretory Vesicle
(D) Rough ER -> Secretory Vesicle-> Cis-Golgi -> Endosome
Correct Answer: (C)
Explanation: Secreted proteins are synthesized by ribosomes attached to the Rough ER, threaded into the ER lumen, transported via vesicles to the Golgi apparatus for processing, and finally packaged into secretory vesicles that fuse with the plasma membrane to release the protein outside the cell.
Question:
White blood cells engulf large bacteria particles by extending their cell membrane around the bacteria to enclose them in a vesicle. This process is best classified as:
(A) Pinocytosis ("cell drinking")
(B) Phagocytosis ("cell eating")
(C) Exocytosis
(D) Receptor-mediated facilitated diffusion
Correct Answer: (B)
Explanation: Phagocytosis is a form of endocytosis used to bring large solid particles or whole cells (like bacteria) into the cell. Pinocytosis is used for taking in fluid/dissolved solutes.
Calculate the solute potential of a 0.3M sucrose solution in an open container at 27 degrees Celsius.
A) -0.67 bars
B) -2.24 bars
C) -7.48 bars
D) -14.96 bars
Correct Answer: (C)
Cholesterol plays a dual role as a "fluidity buffer" in animal plasma membranes. Which statement accurately describes how cholesterol affects membrane physical state across different temperatures?
(A) Above transition temperature Tm, it increases fluidity; below Tm, it decreases fluidity.
(B) Above transition temperature Tm , it constrains acyl chain motion to decrease fluidity; below Tm, it disrupts tight packing to increase fluidity.
(C) At high temperatures, it forces phospholipids to flip-flop across the bilayer at high rates.
(D) At low temperatures, it binds covalently to saturated fatty acid tails to form rigid
Correct Answer: (B)
Explanation: At high temperatures (above Tm), cholesterol restrains phospholipid movement to prevent the membrane from becoming too fluid. At low temperatures (below Tm), it prevents phospholipids from packing tightly together, preventing gelation.
What is Mr. Shakked's last name
Shakked.