Quiz: Cell Structure and Function — 27 questions

1. Which feature best identifies a prokaryotic cell?

A. Linear chromosomes enclosed within a nucleus

B. Membrane-bound organelles surrounded by cytosol

C. Circular DNA located in a nucleoid region

D. Nuclear DNA organized inside a membrane-bound compartment

Answer: (C)

Explanation

Prokaryotic cells contain circular DNA in a nucleoid region and lack membrane-bound organelles. A membrane-bound nucleus and linear chromosomes are characteristic of eukaryotic cells instead.

2. What distinguishes a eukaryotic cell from a prokaryotic cell?

A. It lacks ribosomes and a plasma membrane

B. It contains circular DNA within a nucleoid region

C. It stores genetic material as plasmids in the cytosol

D. It contains membrane-bound organelles and a nucleus

Answer: (D)

Explanation

Eukaryotic cells contain membrane-bound organelles and linear chromosomes enclosed within a nucleus. Prokaryotic cells have a nucleoid region rather than a membrane-bound nucleus.

3. Which set of structures is present in both prokaryotic and eukaryotic cells?

A. Circular chromosomes, plasmids, nuclei, and ribosomes

B. Golgi complexes, lysosomes, ribosomes, and cytosol

C. A nucleus, mitochondria, chloroplasts, and a plasma membrane

D. Genetic material, ribosomes, cytosol, and a plasma membrane

Answer: (D)

Explanation

Both major cell types contain genetic material, ribosomes, cytosol, and a plasma membrane. Nuclei, mitochondria, chloroplasts, and Golgi complexes are membrane-bound structures found in eukaryotic cells.

4. A cell contains a circular chromosome in a nucleoid region and several plasmids; which description best matches its genetic organization?

A. It has chromosomes packaged within membrane-bound organelles

B. It has prokaryotic DNA organization

C. It has linear chromosomes inside a nucleus

D. It has eukaryotic nuclear DNA organization

Answer: (B)

Explanation

A circular chromosome in a nucleoid, potentially accompanied by plasmids, is characteristic of prokaryotic DNA organization. Eukaryotic DNA is generally arranged as linear chromosomes inside a membrane-bound nucleus.

5. What is the primary function of a ribosome?

A. Assembling amino acids into polypeptide chains

B. Modifying proteins and packaging them into vesicles

C. Synthesizing lipids and detoxifying harmful substances

D. Digesting worn-out organelles and invading microbes

Answer: (A)

Explanation

Ribosomes are made of proteins and ribosomal RNA and assemble amino acids into polypeptide chains during translation. Protein modification and vesicle packaging are functions of the Golgi complex.

6. Which comparison correctly distinguishes rough and smooth endoplasmic reticulum?

A. Rough ER supports lipid synthesis, whereas smooth ER has ribosomes for protein synthesis

B. Rough ER contains hydrolytic enzymes, whereas smooth ER modifies proteins into their final forms

C. Rough ER has ribosomes for protein synthesis, whereas smooth ER supports lipid synthesis and detoxification

D. Rough ER packages proteins into vesicles, whereas smooth ER digests cellular waste

Answer: (C)

Explanation

Rough ER contains membrane-bound ribosomes and is associated with protein synthesis, while smooth ER lacks ribosomes and functions in lipid synthesis and detoxification. Vesicle packaging is associated with the Golgi complex, not the endoplasmic reticulum.

7. What happens to many proteins as they pass through the Golgi complex?

A. They are translated from amino acids on free ribosomes

B. They are modified into final forms and packaged into transport vesicles

C. They are broken down by hydrolytic enzymes in the cytosol

D. They are converted into lipids and used for detoxification

Answer: (B)

Explanation

The Golgi complex modifies proteins into their final conformation and packages them into vesicles for transport. Translation occurs at ribosomes, while lipid synthesis and detoxification are associated with smooth ER.

8. Which structure-function pairing correctly describes a mitochondrion?

A. A folded inner membrane and enzyme-containing matrix support energy production

B. Ribosomes attached to membranes synthesize proteins for cellular export

C. Hydrolytic enzymes digest macromolecules within a membrane-bound vesicle

D. Thylakoids stacked into grana support light-dependent photosynthesis

Answer: (A)

Explanation

Mitochondria have a smooth outer membrane, a folded inner membrane, and an enzyme-containing matrix where Krebs-cycle reactions occur. Thylakoids and grana belong to chloroplasts, whereas hydrolytic digestion is a lysosomal function.

9. What does the endosymbiosis hypothesis propose about the origins of mitochondria and chloroplasts?

A. They developed from folded regions of the plasma membrane

B. They evolved from free-living prokaryotes engulfed by larger prokaryotes

C. They arose when lysosomes fused with preexisting cellular compartments

D. They formed when eukaryotic nuclei divided into separate organelles

Answer: (B)

Explanation

The hypothesis proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by larger prokaryotes and later became interdependent. It therefore explains these organelles through engulfment rather than nuclear division or lysosomal fusion.

10. Which observation provides evidence for the endosymbiotic origin of mitochondria and chloroplasts?

A. They contain linear chromosomes enclosed within the eukaryotic nucleus

B. They reproduce through mitosis like the surrounding eukaryotic cell

C. They contain circular DNA and ribosomes resembling those of prokaryotes

D. They lack genetic material and depend entirely on nuclear ribosomes

Answer: (C)

Explanation

Circular DNA and prokaryote-like ribosomes support the idea that mitochondria and chloroplasts descended from engulfed prokaryotes. Linear nuclear chromosomes and mitosis describe features of eukaryotic cells rather than evidence for organelle ancestry.

11. How does compartmentalization improve the efficiency of a eukaryotic cell?

A. It removes the need for genetic material in organelles

B. It separates metabolic enzymes and reduces harmful cross-reactions

C. It prevents organelles from carrying out specialized chemical processes

D. It makes all metabolic reactions occur in one shared space

Answer: (B)

Explanation

Compartmentalization places enzymes for different metabolic processes in separate regions, helping prevent harmful cross-reactions. It therefore supports specialized activities rather than combining all reactions in one space.

12. For a spherical cell with radius $r$, which expression gives its surface-area-to-volume ratio?

A. $\frac{3}{r}$

B. $\frac{3}{4\pi r^2}$

C. $\frac{4\pi r^2}{3}$

D. $\frac{4}{3}\pi r^3$

Answer: (A)

Explanation

Dividing the surface area $4\pi r^2$ by the volume $\frac{4}{3}\pi r^3$ simplifies to $\frac{3}{r}$. The expression $\frac{4}{3}\pi r^3$ represents volume rather than a ratio.

13. Why does a larger cell generally exchange nutrients and wastes less efficiently with its environment?

A. Its membrane becomes less permeable as the cytoplasm expands.

B. Its internal molecules lose their concentration gradients during growth.

C. Its phospholipid bilayer contains fewer transport proteins per unit of volume.

D. Its surface-area-to-volume ratio decreases as cell size increases.

Answer: (D)

Explanation

As cell size increases, volume grows faster than surface area, lowering the surface-area-to-volume ratio available for exchange. A larger cell does not necessarily become less permeable or lose its concentration gradients merely because it grows.

14. Which feature best characterizes the fluid mosaic model of the plasma membrane?

A. Phospholipids form a rigid layer that restricts protein movement.

B. Carbohydrate chains create a solid barrier around the cell.

C. Membrane components move within a flexible phospholipid surface.

D. Membrane proteins remain fixed between immobile lipid molecules.

Answer: (C)

Explanation

The fluid mosaic model describes mobile phospholipids, proteins, glycoproteins, glycolipids, and steroids within the membrane surface. A rigid membrane model would conflict with this mobility.

15. How are phospholipids arranged in the plasma membrane bilayer?

A. Hydrophobic heads face aqueous regions, while hydrophilic tails face inward.

B. The phospholipids form a single layer with all tails facing extracellular fluid.

C. Both heads and tails face the cytoplasm because water is inside the cell.

D. Hydrophilic heads face aqueous regions, while hydrophobic tails face inward.

Answer: (D)

Explanation

Hydrophilic phosphate heads interact with water on the extracellular and cytoplasmic sides, while hydrophobic tails avoid water in the bilayer interior. Reversing these orientations would place the water-avoiding tails in aqueous environments.

16. Which membrane components are most directly associated with cell recognition?

A. Phospholipid tails and membrane steroids

B. Glycoproteins and glycolipids

C. Transport channels and cytoplasmic enzymes

D. Hydrophilic phosphate heads and ATP molecules

Answer: (B)

Explanation

Carbohydrate-containing glycoproteins and glycolipids help cells recognize one another. Transport proteins instead move substances or perform signaling, anchoring, and catalytic roles.

17. Which substance would cross a phospholipid bilayer most readily without a membrane protein?

A. A large protein dissolved in cytoplasm

B. A large polar molecule such as glucose

C. A charged ion such as $Na^+$

D. A small hydrophobic molecule such as $O_2$

Answer: (D)

Explanation

Small hydrophobic molecules such as $O_2$ can pass through the bilayer's hydrophobic interior without assistance. Large polar molecules, ions, and proteins require membrane proteins or vesicular transport.

18. Which process is an example of passive transport?

A. Movement of a solute from high concentration to low concentration without ATP use

B. Movement of water into a vesicle by membrane engulfment

C. Movement of a solute from low concentration to high concentration using ATP

D. Movement of sodium out of a cell through an ATP-powered pump

Answer: (A)

Explanation

Passive transport moves molecules down their concentration gradient without energy input. Movement against a gradient, including sodium export by the pump, requires energy.

19. Which statement correctly distinguishes osmosis from facilitated diffusion?

A. Osmosis expels macromolecules, whereas facilitated diffusion imports them in vesicles.

B. Osmosis moves water, whereas facilitated diffusion moves polar solutes or ions through proteins.

C. Osmosis moves ions, whereas facilitated diffusion moves water through the lipid bilayer.

D. Osmosis requires ATP, whereas facilitated diffusion moves solutes against gradients.

Answer: (B)

Explanation

Osmosis is the diffusion of water across a membrane, while facilitated diffusion is protein-assisted passive movement of polar molecules or ions. Neither process requires ATP or vesicle formation.

20. What movement does the Na+/K+ pump perform during one transport cycle?

A. Three $Na^+$ ions leave and two $K^+$ ions enter using ATP.

B. Equal numbers of $Na^+$ and $K^+$ ions move down their gradients without ATP.

C. Two $Na^+$ ions leave and three $K^+$ ions enter using ATP.

D. Three $K^+$ ions leave and two $Na^+$ ions enter without ATP.

Answer: (A)

Explanation

The Na+/K+ pump hydrolyzes ATP to move three sodium ions out of the cell and two potassium ions into it against their concentration gradients. The reverse ion arrangement describes a different transport pattern and does not match the pump's stated action.

21. What does water potential predict in a solution?

A. The pressure exerted by solutes alone

B. The total number of dissolved particles

C. The direction in which water will move

D. The temperature at which water freezes

Answer: (C)

Explanation

Water potential represents the potential energy of water and predicts the direction of water movement. Osmolarity measures total dissolved solute concentration, while pressure and freezing point describe different properties.

22. If two solutions are separated by a selectively permeable membrane, in which direction will water generally move?

A. From the solution with lower water potential to the one with higher water potential

B. From the solution with higher solute concentration to the one with lower solute concentration

C. From the solution with greater pressure potential to the one with lower solute concentration

D. From the solution with higher water potential to the one with lower water potential

Answer: (D)

Explanation

Water moves down its water-potential gradient, from higher water potential, usually hypotonic, toward lower water potential, usually hypertonic. A higher solute concentration generally corresponds to lower rather than higher water potential.

23. Which equation represents total water potential?

A. $\psi=\psi_s\times\psi_p$

B. $\psi=\psi_s+\psi_p$

C. $\psi=\frac{\psi_s}{\psi_p}$

D. $\psi=\psi_s-\psi_p$

Answer: (B)

Explanation

Total water potential is the sum of solute potential and pressure potential. Solute potential alone does not include the pressure contribution, so subtraction, multiplication, and division do not represent the stated relationship.

24. A solution is prepared with a solute concentration of $0.20\,\text{M}$, an ionization constant of $2$, a pressure constant of $0.00831\,\text{L}\cdot\text{MPa}\,\text{mol}^{-1}\cdot\text{K}^{-1}$, and a temperature of $300\,\text{K}$. What is its solute potential?

A. $0.997\,\text{MPa}$

B. $-0.499\,\text{MPa}$

C. $-0.00499\,\text{MPa}$

D. $-0.997\,\text{MPa}$

Answer: (D)

Explanation

Using $\psi_s=-iCRT$ gives $\psi_s=-(2)(0.20)(0.00831)(300)\approx-0.997\,\text{MPa}$. Omitting the ionization factor would produce approximately $-0.499\,\text{MPa}$, which underestimates the effect of the dissolved particles.

25. What does osmolarity measure?

A. The total concentration of solutes in a solution

B. The potential energy stored by water in a solution

C. The pressure potential generated by a cell wall

D. The rate at which water crosses a membrane

Answer: (A)

Explanation

Osmolarity is the total concentration of solutes in a solution. Water potential instead describes the potential energy of water and helps predict its movement.

26. Why does a freshwater paramecium use a contractile vacuole?

A. Solutes enter because the surrounding freshwater has higher osmolarity than the cell interior

B. Water leaves because its interior has lower solute concentration and higher water potential than the freshwater

C. Water enters because its interior has higher solute concentration and lower water potential than the freshwater

D. The vacuole adds solutes to the cell so that water movement toward the interior increases

Answer: (C)

Explanation

A freshwater paramecium has a more concentrated interior and lower water potential than its environment, so water enters and must be expelled by the contractile vacuole. The contrasting description applies to water loss rather than the freshwater paramecium’s balance problem.

27. How do saltwater-fish cells maintain water balance in seawater?

A. The fish takes in freshwater, releases water through its kidneys, and stores seawater salts

B. The fish avoids seawater, loses water from its cells, and absorbs solute through its gills

C. The fish allows water to enter its cells and removes excess water through contractile vacuoles

D. The fish drinks seawater, retains water, and excretes excess solute through specialized organs

Answer: (D)

Explanation

Saltwater-fish cells have lower solute concentration and higher water potential than seawater, so the fish drinks seawater, retains water, and removes excess solute through specialized salt-secreting organs. Contractile vacuoles are associated with expelling excess water in freshwater organisms, not saltwater fish.