Apply Cell Structure - IB Biology

1. Which of the following provides the strongest evidence for the universal common ancestry of all life, based on cellular structure?
   Answer: The use of ribosomes to synthesize proteins using a nearly universal genetic code. The fact that organisms from all three domains of life (Bacteria, Archaea, Eukarya) use ribosomes (though differing slightly in size) to translate nucleic acid information into proteins based on the same codons is powerful evidence for a single origin of life. A is incorrect because the composition of cell walls differs significantly (cellulose, chitin, peptidoglycan), suggesting convergent evolution rather than common ancestry. C is a feature of eukaryotes only. D is strong evidence, but the universality of the genetic code and the machinery to read it (ribosomes) is considered even more fundamental evidence of a shared origin.
2. Paramecium is a single-celled eukaryote that lives in freshwater. It has a specialized organelle called a contractile vacuole, which actively pumps water out of the cell. What does the presence and function of this organelle imply about the cell's structure and its environment?
   Answer: The cytoplasm of the Paramecium is hypertonic to the surrounding freshwater environment. Freshwater is a hypotonic environment relative to the cell's cytoplasm. This means the cell's cytoplasm has a higher solute concentration (is hypertonic) than the outside. Consequently, water continuously enters the cell by osmosis. The contractile vacuole is an adaptation to actively expel this excess water to prevent the cell from lysing, as it lacks a cell wall. A is incorrect as Paramecium does not have a cell wall. C is incorrect as all cells have a plasma membrane. D states the opposite of the correct osmotic relationship.
3. A cell that is part of the lining of the small intestine is specialized for absorption. Which combination of structural features would best support this function?
   Answer: A high density of microvilli, numerous mitochondria, and abundant channel proteins. Absorption of nutrients is the key function. Microvilli vastly increase the surface area of the plasma membrane for absorption. Numerous mitochondria provide the ATP needed for the active transport of many nutrients. Abundant channel and carrier proteins are necessary to facilitate the movement of substances across the membrane. A is wrong because a smooth membrane has low surface area and animal cells don't have cell walls. C describes features more typical of a plant cell or one specialized for lipid synthesis. D describes features related to structure and protein synthesis, not absorption.
4. What is the most likely direct consequence for the cell's internal environment?
   Answer: An accumulation of damaged organelles and undigested macromolecules within the cytoplasm. The primary role of lysosomes is autophagy (breaking down old organelles) and digesting macromolecules. If the enzymes are not correctly delivered to the lysosomes, these waste products cannot be broken down and will accumulate within the cell, leading to cellular damage. A is incorrect as ATP synthesis occurs in mitochondria. B is incorrect as the enzymes are secreted out of the cell, not into the cytoplasm. D is incorrect for the same reason; the enzymes are not released into the cytoplasm to digest ribosomes.
5. How does the fundamental structure of a bacteriophage differ from the structure of the bacterium it infects?
   Answer: The bacteriophage lacks a plasma membrane and cytoplasm, which are present in the bacterium. Viruses, including bacteriophages, are non-cellular. They consist of genetic material (DNA or RNA) enclosed in a protein coat (capsid). They fundamentally lack the structures of a living cell, such as a plasma membrane, cytoplasm, and organelles like ribosomes. Bacteria are cells and possess all these features. A is incorrect as bacteria have DNA (and RNA). C is incorrect as viruses lack ribosomes entirely. D incorrectly swaps the structural components of the virus and bacterium.
6. Which structural observation would provide the strongest evidence to support this hypothesis over one of primary endosymbiosis?
   Answer: The chloroplast is surrounded by three or more membranes. Primary endosymbiosis involves a eukaryote engulfing a cyanobacterium, resulting in a chloroplast with two membranes (the cyanobacterium's plasma membrane and the host's vacuolar membrane). Secondary endosymbiosis involves a eukaryote engulfing another eukaryotic alga that already had a primary chloroplast. This results in a chloroplast surrounded by more than two membranes (typically three or four). Options A and D are evidence for endosymbiosis in general but do not distinguish between primary and secondary events. Option C is unrelated to the origin of the organelle.
7. A scientist observes a microorganism using an electron microscope. It has a cell wall, a plasma membrane, a nucleoid region with a circular chromosome, and no visible mitochondria. However, the cell performs aerobic respiration. Where are the components of the electron transport chain most likely located in this cell?
   Answer: Integrated into the plasma membrane, which is folded to increase surface area. The description matches a prokaryotic cell (nucleoid, no mitochondria). In prokaryotes that perform aerobic respiration, the enzymes and protein complexes of the electron transport chain are located in the plasma membrane. This membrane is often folded into structures called mesosomes to increase the surface area available for these reactions, analogous to the cristae of mitochondria in eukaryotes.
8. Some prokaryotes, such as cyanobacteria, are photosynthetic. How does the cellular structure of a cyanobacterium accommodate the process of photosynthesis without chloroplasts?
   Answer: Photosynthetic pigments and enzymes are located on internal extensions of the plasma membrane called thylakoids. Prokaryotes lack membrane-bound organelles like chloroplasts. In photosynthetic prokaryotes, the necessary pigments (like chlorophyll) and electron transport chains are located on extensive internal membrane systems that are infoldings of the plasma membrane. These are functionally analogous to the thylakoids in eukaryotic chloroplasts. The cell wall (A) is for structure, not photosynthesis. The cytoplasm (B) is the site of the Calvin cycle, but not the light-dependent reactions which require membranes. Chromatophore (D) is a term for a pigment-containing cell, not a prokaryotic organelle.
9. In what way does a striated muscle fibre challenge the concept that a cell is a single, simple structural unit?
   Answer: It contains multiple nuclei within a single, continuous cytoplasm and cell membrane. Striated muscle fibres are very large and are multi-nucleated (a syncytium). This challenges the simple idea of a cell having one nucleus. They are formed by the fusion of many individual muscle cells. A is incorrect; they are much larger, not smaller. C is incorrect; they have a plasma membrane (sarcolemma). D is incorrect; they are a specialized part of a multicellular organism, not self-sustaining.
10. A cell is found to have a high density of mitochondria with numerous, tightly packed cristae. Which of the following cells best fits this structural description?
    Answer: A sperm cell, in the midpiece region connecting the head and flagellum. A high density of mitochondria with many cristae indicates a very high rate of aerobic respiration to produce large amounts of ATP. The flagellum of a sperm cell requires a massive and constant supply of ATP to power its movement. These mitochondria are characteristically packed into the midpiece. Adipocytes (A) store energy but are not as metabolically active. Mature erythrocytes (C) and xylem vessels (D) are anucleated and lack mitochondria entirely.
11. A plant cell and an animal cell are placed in a hypotonic solution. The plant cell becomes turgid, while the animal cell lyses (bursts). Which structural difference is the primary reason for this different outcome?
    Answer: The presence of a rigid cellulose cell wall in the plant cell, which withstands the internal turgor pressure. The key difference is the cell wall. In a hypotonic solution, water enters both cells via osmosis. In the animal cell, this influx of water increases pressure on the plasma membrane until it ruptures. In the plant cell, the strong, rigid cellulose cell wall exerts an opposing pressure (turgor pressure), preventing the cell from swelling indefinitely and bursting. While the vacuole (A) swells, it is the wall that provides the ultimate structural resistance to lysis. Both cells have selectively permeable membranes (B). Photosynthesis (D) would not consume water fast enough to prevent lysis.
12. A scientist treats a culture of eukaryotic cells with a toxin that specifically causes the breakdown of the cytoskeletal component actin filaments (microfilaments). Which cellular process would be most directly disrupted?
    Answer: The formation of the cleavage furrow during cytokinesis in an animal cell. Actin filaments form a contractile ring that pinches the cell membrane inward, creating the cleavage furrow during animal cell cytokinesis. Disruption of actin would directly prevent this. Chromosome separation (A) is mediated by microtubules of the spindle apparatus. Vesicle transport (B) is primarily along microtubule tracks. Flagellar movement (D) is also driven by microtubules in a '9+2' arrangement. Therefore, cytokinesis is the process most dependent on actin filaments.
13. Which statement best explains why the surface area to volume ratio is a critical limiting factor for the maximum size of a single cell?
    Answer: As volume increases faster than surface area, the rate of diffusion across the membrane becomes insufficient to support the cell's metabolic needs. As a cell increases in size, its volume (proportional to the cube of its radius, r³) increases much faster than its surface area (proportional to the square of its radius, r²). The surface area (plasma membrane) is where exchange of nutrients, waste, and gases occurs. A decreasing SA:V ratio means the exchange surface becomes too small to service the metabolic needs of the much larger volume, limiting cell size. A is incorrect because the rates are different. B misidentifies the limiting factor. D describes a problem for very small cells (high SA:V ratio), not large ones.
14. A student is examining a cell under a microscope and observes that it lacks a nucleus but possesses a thick layer of peptidoglycan. They also note the presence of pili on the cell surface. Which deduction is most valid?
    Answer: The cell is a prokaryote from the domain Bacteria, potentially involved in conjugation. The lack of a nucleus indicates a prokaryote. The presence of peptidoglycan in the cell wall is the defining characteristic of the domain Bacteria (Archaean cell walls lack peptidoglycan). Pili are surface appendages often used for attachment or for transferring genetic material during conjugation, a process common in bacteria. A and D are incorrect because they are eukaryotes. B is incorrect because Archaea lack peptidoglycan.
15. What is the most likely immediate consequence of this mutation for the cell's primary function?
    Answer: Antibody proteins will accumulate in the cytoplasm instead of entering the endomembrane system for processing and secretion. The SRP is essential for targeting proteins destined for secretion to the rough ER. Without a functional SRP, the ribosomes translating antibody mRNA will remain free in the cytoplasm, and the complete antibody proteins will be released there. They will not enter the endomembrane pathway (ER -> Golgi -> vesicles) for modification and export. B is incorrect because the proteins never reach the Golgi. C is incorrect as translation itself is not blocked, only the targeting to the ER. D is an indirect effect, not the immediate consequence of the SRP failure.
16. The Golgi apparatus modifies, sorts, and packages proteins. A cell is genetically engineered to produce a protein that is normally found in the cytoplasm, but the engineering adds a signal sequence that directs it into the rough ER. What will be the ultimate fate of this protein?
    Answer: It will be processed by the Golgi apparatus and packaged into vesicles for secretion out of the cell. Once a protein enters the endomembrane system via the rough ER, it follows the default pathway unless it has other specific targeting signals. The default pathway is ER -> Golgi -> secretory vesicles -> exocytosis. Since the cytoplasmic protein was artificially given an ER signal sequence, it will enter this pathway and be secreted from the cell, even though that is not its normal location. There is no standard mechanism to return a protein from the ER to the cytoplasm (A). The protein will move from the ER to the Golgi (C). It enters the ER before it can be degraded in the cytoplasm (D).
17. A researcher identifies a cell with a cell wall containing chitin, a plasma membrane, 80S ribosomes, and membrane-bound organelles, but lacking chloroplasts. Which conclusion about the cell's identity and function is most justified by these structural features?
    Answer: It is a fungal cell, likely functioning as a heterotrophic decomposer, based on the chitinous cell wall and absence of chloroplasts. The presence of a cell wall with chitin is a defining characteristic of fungi. The 80S ribosomes and membrane-bound organelles confirm it is eukaryotic. The absence of chloroplasts indicates a heterotrophic lifestyle, consistent with a decomposer role. A is incorrect because plant cell walls contain cellulose, not chitin. B is incorrect because the cell is eukaryotic (80S ribosomes, organelles), not prokaryotic. D is incorrect because animal cells lack a cell wall.
18. The endosymbiotic theory proposes an origin for mitochondria and chloroplasts. Which structural feature of a modern eukaryotic cell provides the weakest evidence in support of this theory?
    Answer: Mitochondria are found within the cytoplasm of the eukaryotic cell. While it's true mitochondria are in the cytoplasm, this is a general feature of all organelles and does not specifically support the idea of an endosymbiotic origin. The other options are strong pieces of evidence: similarity in size to prokaryotes (A), possession of their own circular DNA like prokaryotes (B), and a double membrane consistent with engulfment via endocytosis (C) all strongly support the theory.
19. A cell in a plant leaf is actively photosynthesizing. A cell in the root of the same plant is actively absorbing mineral ions from the soil. Which structural comparison between these two specialized cells is most accurate?
    Answer: The root cell has a higher density of mitochondria than the leaf cell to provide ATP for active transport. Active transport of mineral ions into the root is an energy-intensive process that requires large amounts of ATP. Therefore, root cells will have numerous mitochondria to supply this energy via cellular respiration. Leaf cells also have mitochondria but rely on light for energy for photosynthesis. A is incorrect, support cells like xylem have thick walls, not necessarily photosynthetic cells. B is incorrect, root cells are not photosynthetic and lack chloroplasts. C is incorrect, both cells have large vacuoles for turgor, but this is not the most significant functional difference.
20. Which of the following cellular functions would be most immediately and severely compromised in a cell treated with this drug?
    Answer: The synthesis of enzymes essential for glycolysis in the cytoplasm. Free ribosomes synthesize proteins that function within the cytoplasm, such as the enzymes for glycolysis. The drug inhibits this process. Secreted proteins (A), lysosomal enzymes (B), and membrane proteins (D) are all synthesized on ribosomes attached to the rough endoplasmic reticulum, which the drug does not affect. Therefore, the synthesis of cytoplasmic enzymes would be the most directly impacted.

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