Cells and Basic Biology: 34-Question Lesson Test

Answer: B

Correct answer: Living things are made of one or more cells. Cell theory rests on three ideas: all living things are made of cells, the cell is the basic unit of structure and function, and all cells come from preexisting cells. Notice that the correct statement allows for both single-celled organisms such as bacteria and many-celled ones such as humans. Watch for absolute words like every, exactly, or only; they usually signal a distractor, because chloroplasts appear only in some cells and new cells come from cell division, not from nonliving chemicals.

• A: Chloroplasts occur in certain photosynthetic eukaryotic cells, not every cell.
• B (correct): Correct: Cell theory identifies cells as basic units of living organisms; new cells arise from preexisting cells.
• C: Multicellular organisms have many cells.
• D: Classical cell theory states cells arise from existing cells.

Answer: B

Correct answer: Ribosome. Ribosomes read the codons on messenger RNA and link the matching amino acids with peptide bonds, a process called translation. They are found free in the cytoplasm and attached to rough ER, and even bacteria have them, since every cell must make proteins. The common trap is the nucleus: it holds the DNA recipe, but it does not do the assembling. The Golgi apparatus comes later, modifying and shipping proteins that ribosomes have already built.

• A: The nucleus usually stores the genetic instructions.
• B (correct): Correct: Ribosomes translate RNA instructions and assemble proteins.
• C: A central vacuole mainly helps storage and water balance in plants.
• D: The Golgi processes and sorts products rather than assembling the polypeptide.

Answer: A

Correct answer: Bacterium lacks a membrane-bound nucleus. The word prokaryote means before nucleus: a bacterium's DNA sits in a region called the nucleoid with no membrane around it, while an animal cell, a eukaryote, encloses its DNA in a nuclear envelope. To reason to the answer, recall what every cell needs to stay alive: DNA for instructions, ribosomes for proteins, and a plasma membrane as a boundary. Any option claiming bacteria lack one of these universal parts must be wrong.

• A (correct): Correct: Bacteria are prokaryotes: their DNA is not within a membrane-bound nucleus.
• B: Bacteria contain genetic material.
• C: Bacteria synthesize proteins using ribosomes.
• D: Bacteria have plasma membranes.

Answer: B

Correct answer: Chloroplast. Chloroplasts contain chlorophyll, the green pigment that absorbs light, and they use that energy to convert carbon dioxide and water into sugar while releasing oxygen. The green color of a leaf is a direct clue pointing to them. The tempting mix-up is the mitochondrion, which also handles energy but in the opposite direction, releasing energy from sugar during cellular respiration. A memory hook: chloroplasts make the food, and mitochondria burn it.

• A: Lysosomes digest and recycle material.
• B (correct): Correct: Chloroplasts capture light energy and support carbohydrate formation.
• C: Mitochondria are associated with cellular respiration.
• D: Rough ER handles many proteins.

Answer: D

Correct answer: Controlling the movement of materials into and out of the cell. The plasma membrane is a phospholipid bilayer with embedded proteins, and it is selectively permeable: small nonpolar molecules slip through, while ions and larger molecules need channels or carriers. This lets the cell keep a stable internal environment. Do not confuse it with the nuclear envelope, which is a double membrane around the DNA, or with the cell wall, a rigid outer layer that only some cells have. ATP production belongs mainly to mitochondria.

• A: That describes the nuclear envelope around the nucleus, not the plasma membrane at the cell’s outer boundary.
• B: A cell wall is separate from the plasma membrane and may be absent.
• C: Major aerobic ATP-production pathways are associated with mitochondria in eukaryotes.
• D (correct): Correct: A selectively permeable membrane regulates exchange between cell and surroundings.

Answer: A

Correct answer: Cellulose cell wall. Plants lack skeletons, so each cell is surrounded by a stiff wall made largely of cellulose, which keeps the cell from bursting when it fills with water and helps the plant stand upright. To answer questions like this, separate features shared by all eukaryotes, such as a nucleus, mitochondria, and a plasma membrane, from plant-specific extras: the cell wall, chloroplasts, and a large central vacuole. A common error is thinking plants have walls instead of membranes; they have both.

• A (correct): Correct: Typical plant cells have a cellulose-rich wall outside the plasma membrane.
• B: Both plant and animal cells have membranes.
• C: Both typical plant and animal cells are eukaryotic and have a nucleus.
• D: Both typically contain mitochondria.

Answer: C

Correct answer: ATP. ATP, adenosine triphosphate, stores energy in the bonds linking its phosphate groups. When the last phosphate is removed, ATP becomes ADP and releases energy that powers muscle contraction, active transport, and building molecules. Cells then recharge ADP back to ATP using energy from food. Think of ATP as rechargeable cash for the cell. DNA is a tempting distractor because it is also a famous three-letter molecule, but it stores genetic information, not usable energy.

• A: Chlorophyll is a light-absorbing pigment.
• B: Cellulose is structural plant wall material.
• C (correct): Correct: ATP transfers usable energy to cellular work, including transport and movement.
• D: DNA chiefly stores hereditary information.

Answer: C

Correct answer: Cell -> tissue -> organ -> organ system. Biological organization builds upward: similar cells working together form a tissue, such as muscle tissue; several tissue types combine into an organ, such as the heart; and organs that cooperate on a task form an organ system, such as the circulatory system. Start from the smallest living unit, the cell, and ask what each level is made of. Mistakes usually come from placing organ before tissue, but an organ cannot exist without tissues to build it.

• A: This puts a tissue above an organ system before a cell.
• B: This reverses the order of basic levels.
• C (correct): Correct: Individual cells cooperate to form tissues; tissues form organs; organs cooperate in systems.
• D: This begins at a larger level and then returns to smaller ones.

Answer: B

Correct answer: Ribosome on rough ER -> Golgi -> secretory vesicle -> cell membrane. This route is called the secretory pathway. Proteins meant for export are built by ribosomes on rough ER and threaded into the ER, where folding begins. Transport vesicles carry them to the Golgi apparatus, which modifies, sorts, and packages them. Secretory vesicles then fuse with the plasma membrane and release the hormone outside by exocytosis. Think of it as factory, finishing room, shipping box, and loading dock. Wrong sequences include structures such as chloroplasts or mitochondria that play no part in export.

• A: Mitochondria and nuclei are not a linear secretory delivery system.
• B (correct): Correct: Many secreted proteins are synthesized by ribosomes associated with rough ER, then processed/sorted through Golgi and exported in vesicles.
• C: The proposed sequence confuses storage, translation and photosynthesis.
• D: Chloroplasts and cell walls are not the typical route for protein secretion.

Answer: A

Correct answer: Smooth endoplasmic reticulum. The ER is a network of internal membranes. Rough ER is studded with ribosomes and helps make proteins, while smooth ER lacks ribosomes and holds the enzymes that build lipids such as phospholipids and steroids. Liver cells have abundant smooth ER because they also use it to detoxify drugs and alcohol. Ribosomes make proteins, the nucleolus assembles ribosomes, and lysosomes break material down rather than build it.

• A (correct): Correct: Smooth ER is especially linked to lipid synthesis and detoxification in relevant cells.
• B: Ribosomes synthesize proteins rather than lipids.
• C: Nucleolus is connected with ribosome assembly in the nucleus.
• D: Lysosomes use digestive enzymes to break down worn-out parts and other material; they do not build lipids.

Answer: A

Correct answer: Chloroplasts support photosynthesis, and mitochondria support aerobic respiration. Plants are not exempt from respiration. Chloroplasts turn light energy into chemical energy stored in sugar, but the cell still needs ATP for daily work, and mitochondria release energy from that sugar through aerobic respiration, day and night. Photosynthesis and respiration are partner processes: one stores energy, the other releases it. The common misconception is that plants photosynthesize instead of respiring, which would leave root cells and plants at night with no energy supply.

• A (correct): Correct: Green plant cells can both capture light energy in chloroplasts and use mitochondria in aerobic cellular respiration.
• B: Photosynthesis requires water as a reactant.
• C: Plants have mitochondria and need respiration.
• D: Mitochondria occur in plant cells as well as animal cells.

Answer: C

Correct answer: Out of the cell, so the cell tends to shrink. In osmosis, water crosses a membrane toward the side with the higher concentration of dissolved particles, which is also the side with the lower water concentration. An outside solution with more solute is called hypertonic, so water leaves the cell and the cell shrivels, a change called crenation in red blood cells. The usual mistake is reversing the direction; remember that water moves to dilute the saltier side. This is why drinking seawater dehydrates you instead of hydrating you.

• A: A membrane generally allows water movement; zero net flux applies only at osmotic balance.
• B: Water tends toward the side of higher effective solute, which here is outside.
• C (correct): Correct: A hypertonic outside solution draws water outward by osmosis, under the stated permeability assumptions.
• D: Solute need not pass through for osmosis to occur.

Answer: B

Correct answer: Facilitated diffusion. Two clues decide this: movement goes down the concentration gradient, and no ATP is spent, so the process is passive. Because a protein channel or carrier helps the molecule through, it is facilitated rather than simple diffusion. Glucose and many ions enter cells this way because they cannot easily pass through the lipid bilayer by themselves. Active transport also uses membrane proteins, which makes it tempting, but it moves substances against the gradient and requires energy.

• A: Binary fission is prokaryotic cell division.
• B (correct): Correct: Protein-assisted downhill transport is facilitated diffusion and is passive.
• C: Exocytosis exports vesicle contents.
• D: Active uphill transport requires an energy source.

Answer: D

Correct answer: The wall provides external support while the membrane regulates exchange. A plant cell has two boundary layers with different jobs. The cell wall, built mostly of cellulose, is rigid and fairly porous; it gives shape and resists bursting but does not choose what enters. Just inside it, the plasma membrane, a lipid bilayer with proteins, decides which substances pass. A useful image is a wall around a garden with a guarded gate inside it. A common error is swapping their materials: walls are cellulose, membranes are lipids.

• A: The plasma membrane is lipid-rich, and the plant cell wall contains cellulose.
• B: Animal cells normally do not have walls.
• C: Plant cells have both structures and membranes regulate exchange.
• D (correct): Correct: The rigid plant wall lies outside a selectively permeable plasma membrane, which performs transport regulation.

Answer: D

Correct answer: It decreases from 6:1 to 3:1. Surface area grows with the square of the side length, while volume grows with its cube. Doubling the side multiplies area by 4 and volume by 8, so the ratio falls by half. This explains why cells stay small: a cell takes in nutrients and removes wastes through its surface, but its needs grow with its volume. Large cells would not have enough membrane to serve their interiors. The tempting error is assuming the ratio rises because area gets larger.

• A: The numerical direction is reversed.
• B: The area quadruples but the volume rises eightfold.
• C: Surface-area-to-volume ratio is size dependent.
• D (correct): Correct: A 1-unit cube has area 6 and volume 1 (6:1); a 2-unit cube has area 24 and volume 8 (3:1).

Answer: C

Correct answer: Mitosis. Before a body cell divides, each chromosome is copied into two identical sister chromatids. During mitosis, spindle fibers pull the sister chromatids apart so each new nucleus receives a complete, identical set of chromosomes. Cytokinesis then splits the cytoplasm to finish forming two daughter cells. Mitosis is used for growth and repair, while meiosis makes sex cells with half the chromosome number. Osmosis may sound similar, but it describes only water movement.

• A: Secretion exports material but does not segregate chromosomes.
• B: Photosynthesis uses light to produce organic molecules.
• C (correct): Correct: Mitosis segregates the nuclear chromosomes in dividing eukaryotic cells; cytokinesis divides the cell contents.
• D: Osmosis concerns water transport.

Answer: D

Correct answer: It is likely prokaryotic. Ribosomes, DNA, and a plasma membrane are found in every cell, so they show that the specimen is a living cell but not which kind. The deciding clue is what is missing: no nucleus or other membrane-bound organelles, which is the defining trait of prokaryotes such as bacteria and archaea. Viruses are a tempting answer, but they lack ribosomes and a membrane and cannot make proteins on their own, so they are not cells.

• A: Typical leaf cells have eukaryotic organelles.
• B: Viruses lack the usual independent cellular machinery.
• C: Ribosomes can make proteins.
• D (correct): Correct: The combination of genetic material, ribosomes and an absent nucleus fits a prokaryotic cell.

Answer: D

Correct answer: Carbon dioxide and water. The simplified equation is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Carbon dioxide supplies the carbon atoms for sugar, and water is split to supply hydrogen and electrons, which releases oxygen as a byproduct. A useful check is that photosynthesis and aerobic respiration are reverse processes: the products of one are the reactants of the other. That is why glucose and oxygen, the outputs of photosynthesis, are a tempting but wrong choice.

• A: Oxygen is a product of oxygenic photosynthesis, not its carbon source.
• B: Glucose and oxygen are classic aerobic-respiration inputs.
• C: These are not the standard reactant pair for oxygenic photosynthesis.
• D (correct): Correct: Photosynthesis uses carbon dioxide and water, powered by light, to make carbohydrates and release oxygen.

Answer: A

Correct answer: Some plant cells are nonphotosynthetic and need not have functioning chloroplasts. Plant cells specialize. Leaf cells exposed to light carry many chloroplasts, but root cells grow underground where light cannot reach, so they lack working chloroplasts and depend on sugar transported from the leaves. They still have a nucleus, mitochondria, and a cell wall, so they are fully living plant cells. The misconception is that every plant cell must be green; chloroplasts are a feature of photosynthetic tissue, not a requirement for being a plant cell.

• A (correct): Correct: Many non-green plant cells, including roots, do not carry out photosynthesis yet remain living plant cells.
• B: Roots contain many living cells that get sugar made in the leaves, so chloroplasts are not required for life.
• C: Many plant cells lack chloroplasts, and lacking them alone does not make a cell a prokaryote.
• D: Living plant cells keep their hereditary information in the nucleus, whether or not they have chloroplasts.

Answer: D

Correct answer: Vacuoles lose water, lowering turgor pressure. A plant cell's large central vacuole holds water and presses the cell membrane against the wall, creating turgor pressure, much like air inflating a tire. That pressure keeps soft stems and leaves upright. In a concentrated outside solution, water leaves by osmosis, the vacuole shrinks, and the cells go limp; severe loss pulls the membrane away from the wall, called plasmolysis. Watch the direction: gaining water raises turgor, so wilting must mean water loss.

• A: Ribosomes synthesize proteins and do not become walls.
• B: Organelles do not transform into one another this way.
• C: Taking in water would make the cells firmer, but here the cells are losing water to the stronger solution.
• D (correct): Correct: Water loss reduces vacuole volume and turgor pressure, making tissues less rigid.

Answer: A

Correct answer: contains multiple tissue types working together to perform a function. The heart includes muscle, connective, nervous, epithelial and other tissues coordinated as an organ.

• A (correct): Correct: The heart includes muscle, connective, nervous, epithelial and other tissues coordinated as an organ.
• B: Cardiac muscle is only one part; the heart also has connective, nervous and epithelial tissues, which makes it an organ.
• C: The heart is built from cells organized into several tissues.
• D: It is much larger and more complex than a cell.

Answer: C

Correct answer: It increases reaction rate by lowering activation energy. Every reaction must clear an energy barrier, the activation energy, before reactants can become products. An enzyme binds its substrate at the active site and positions it so the barrier is lower, letting the reaction happen much faster at body temperature. The enzyme leaves unchanged and can be used again. Enzymes speed up reactions that are already possible but cannot make an impossible reaction happen or create energy, and they are not fuel that gets used up.

• A: Catalysts do not remove the need for reactants and favorable energy conditions.
• B: An enzyme is not ordinarily used up as its reaction proceeds.
• C (correct): Correct: Enzymes act as catalysts and lower activation energy without being consumed in the overall reaction.
• D: Enzymes do not create energy from nothing.

Answer: A

Correct answer: endocytosis. Endocytosis means bringing material into the cell: the membrane folds around the substance and pinches off as a vesicle. When the cargo is a large solid particle, such as a bacterium, the specific form is called phagocytosis, or cell eating, and it is how white blood cells destroy invaders. Its opposite, exocytosis, uses vesicles to export material. Simple diffusion is tempting because it also moves substances inward, but it works only for small molecules passing through the bilayer.

• A (correct): Correct: The cell membrane encloses material and takes it in by vesicle formation, a form of endocytosis.
• B: Mitosis concerns nuclear division.
• C: Osmosis is water movement across a membrane.
• D: Large particles cannot enter by simple diffusion through the bilayer.

Answer: C

Correct answer: It includes cytosol and structures outside the nucleus but within the membrane. Think of the cytoplasm as everything between the plasma membrane and the nucleus. It includes the cytosol, a water-based gel, and the organelles suspended in it, such as mitochondria, ribosomes, and the ER. Many reactions happen here, including glycolysis, the first stage of glucose breakdown, and protein synthesis on free ribosomes. A frequent confusion is between cytoplasm and cytosol: cytosol is only the fluid, while cytoplasm is the fluid plus the structures within it.

• A: Many reactions, such as the first steps of glucose breakdown, take place in the cytoplasm.
• B: The nucleus is a separate compartment.
• C (correct): Correct: The cytoplasm comprises the cell interior outside the nucleus, including cytosol and various cellular structures.
• D: That describes a plant cell wall, which is outside the cell membrane rather than inside it.

Answer: B

Correct answer: Aerobic cellular respiration. Photosynthesis needs light, but cellular respiration does not, so plants respire all the time, day and night. In darkness, the plant breaks down sugar it stored during the day, often as starch, and its mitochondria use oxygen to convert that sugar's energy into ATP. The key clue in the question is that oxygen and sugar are the inputs. Any choice relying on light-dependent reactions cannot explain energy production in the dark.

• A: Light-dependent reactions cannot account for production in darkness.
• B (correct): Correct: Aerobic respiration can continue in darkness; photosynthetic light reactions need light.
• C: Mitosis is not a general ATP-production route.
• D: Osmosis moves water and does not directly convert sugar energy to ATP.

Answer: D

Correct answer: Bacterial reproduction by splitting — binary fission; ordinary eukaryotic body-cell division — mitosis. Bacteria have a single circular chromosome and no nucleus, so they divide by binary fission: the DNA is copied, the copies separate, and the cell pinches in two. Eukaryotic body cells have many chromosomes inside a nucleus, so they need the more elaborate process of mitosis to sort them evenly. Meiosis is reserved for making eukaryotic sex cells. A good check is to ask whether the organism even has a nucleus; mitosis requires one.

• A: Bacteria generally do not use eukaryotic meiosis; skin repair is not photosynthesis.
• B: Bacteria lack nuclear mitosis and skin cells are eukaryotic.
• C: Endocytosis transports bulk material, not whole-cell division.
• D (correct): Correct: Bacteria commonly reproduce by binary fission, while eukaryotic body-cell division involves mitosis.

Answer: A

Correct answer: Ribosomes. Protein production happens in two steps: transcription copies a gene into mRNA in the nucleus, and translation reads that mRNA to build a polypeptide. Because mRNA levels are normal, transcription is working, so the failure lies at the translation step, which takes place on ribosomes. This logic of locating the broken step is useful in many questions. The Golgi is tempting because it handles proteins, but it acts only after a polypeptide already exists.

• A (correct): Correct: Translation builds polypeptide chains on ribosomes; making mRNA does not itself produce a protein.
• B: Vacuoles are mainly storage and homeostasis structures.
• C: Golgi sorts and modifies products after they are synthesized.
• D: A wall provides structural support, not polypeptide synthesis.

Answer: C

Correct answer: X swells; Y shrinks. Compare solute outside with solute inside, then send water toward the side with more solute. Around X the solution is hypotonic, so water enters and the cell swells; an animal cell lacks a wall and may even burst. Around Y the solution is hypertonic, so water leaves and the cell shrinks. Hypo means under, as in less solute outside. A common mistake is reversing the direction, as if water followed the lower solute concentration.

• A: Different tonicity can change cell volume.
• B: This reverses the osmotic predictions.
• C (correct): Correct: Outside X is hypotonic so water tends to enter; outside Y is hypertonic so water tends to leave.
• D: Water follows osmotic conditions, not a fixed outward direction.

Answer: C

Correct answer: The pump keeps moving ions inward even though this is against their concentration gradient. Uphill movement requiring cellular energy is the hallmark of active transport.

• A: Moving down a gradient through a channel needs no cellular energy, so it is passive transport.
• B: The described water movement is osmosis.
• C (correct): Uphill movement against a gradient, powered by cellular energy, is the hallmark of active transport.
• D: Gas movement down a gradient through the bilayer is simple diffusion, a passive process.

Answer: D

Correct answer: They could be nonphotosynthetic eukaryotic cells, such as certain fungal or plant cells. Mitochondria and a nucleus show the cells are eukaryotic, which immediately rules out bacteria. A cell wall narrows the list to plants, fungi, or some protists, since animal cells lack walls. Missing chloroplasts does not rule out plants, because root cells lack them, and fungi never have chloroplasts but get energy by absorbing food. Defensible conclusions use words like could, while choices that use must or definitely overreach the evidence.

• A: Typical animal cells have no walls.
• B: Bacteria lack mitochondria and membrane-bound nuclei, so these features point away from bacteria.
• C: Many living cells, such as fungal and animal cells, get energy from food molecules instead of from photosynthesis.
• D (correct): Correct: Nucleus and mitochondria support eukaryotic classification; walls can occur in fungi and plants without chloroplasts.

Answer: A

Correct answer: Chloroplasts store light energy in carbohydrate, and mitochondria make ATP through respiration. Photosynthesis and respiration are distinct processes that may coexist in a green plant cell.

• A (correct): Photosynthesis and cellular respiration are distinct processes that both run in a green plant cell.
• B: Mitochondria work alongside chloroplasts in green plant cells and carry out respiration day and night.
• C: Carbon dioxide provides carbon in photosynthesis.
• D: Plant cells carry out cellular respiration, and in the dark it is their main source of ATP.

Answer: C

Correct answer: Surface area ×4 and volume ×8, so the surface-area-to-volume ratio halves. For similar shapes, area scales with length squared and volume with length cubed: radius ×2 -> area ×4 and volume ×8.

• A: This reverses the square and cube factors; volume grows faster than area as a cell gets bigger.
• B: Both area and volume increase when radius increases.
• C (correct): For similar shapes, area scales with length squared and volume with length cubed, so doubling the radius gives area ×4 and volume ×8.
• D: Linear doubling does not produce only twofold $\frac{\mathrm{area}}{\mathrm{volume}}$ changes.

Answer: B

Correct answer: Cell grows and copies DNA in interphase -> mitosis separates chromosomes -> cytokinesis divides the cell. DNA is copied before mitosis during interphase; nuclear segregation occurs in mitosis; cytokinesis usually follows or overlaps.

• A: Ribosome assembly and osmosis are different processes.
• B (correct): DNA is copied during interphase before mitosis, mitosis separates the chromosomes, and cytokinesis then splits the cell.
• C: Replication must come before mitosis so that each daughter cell receives a full copy of the chromosomes.
• D: Photosynthesis does not separate chromosomes and DNA synthesis precedes mitosis.

Answer: B

Correct answer: It is eukaryotic, but being single-celled does not identify its species. A membrane-bound nucleus is the defining feature of eukaryotes, so seeing one is enough to classify the cell. Many eukaryotes are single-celled, including amoebas, paramecia, and yeasts, so one cell does not mean bacterium. Beyond that, the image does not tell you the species or whether the organism photosynthesizes; that would require more evidence such as chloroplasts. The trap is assuming all unicellular life is prokaryotic, which confuses cell number with cell type.

• A: Many protists and yeasts are single-celled eukaryotes.
• B (correct): Correct: A membrane-bound nucleus supports eukaryotic classification, and some eukaryotes are unicellular.
• C: One cell does not make a multicellular animal.
• D: Presence of a nucleus does not prove photosynthetic capacity.

Answer: A

Correct answer: A virus, which is not a cell at all. Every true cell has a membrane, cytoplasm, DNA and ribosomes. This particle has no ribosomes or cytoplasm and must use a host cell's machinery to reproduce, so it is a virus, not a prokaryotic or eukaryotic cell.

• A (correct): Genetic material in a protein coat with no ribosomes, cytoplasm or metabolism of its own is the description of a virus.
• B: Prokaryotes are true cells with a membrane, cytoplasm and ribosomes; lacking a nucleus does not make something a prokaryote.
• C: Bacteria have their own ribosomes and cytoplasm; this particle has neither, and antibiotics do not cure viral infections.
• D: Every cell and virus carries genetic material; a eukaryote also needs a membrane-bound nucleus and organelles.

Answer: C

Correct answer: Meiosis. Look for the clue that the chromosome number is cut in half, from 46 to 23. Halving the chromosome number to make egg or sperm cells is the job of meiosis, which takes place only in reproductive tissues.

• A: Mitosis makes two identical daughter cells with the same chromosome number, used for growth and repair.
• B: Binary fission is how prokaryotes such as bacteria split; human cells have a nucleus and do not divide this way.
• C (correct): Meiosis occurs in reproductive tissue and produces cells with half the original chromosome number.
• D: Cytokinesis only splits the cytoplasm at the end of division; it does not halve the chromosome number.

Answer: B

Correct answer: Worn-out organelles and engulfed particles build up instead of being broken down. Lysosomes are sacs of digestive enzymes that break down worn-out organelles, food particles and engulfed bacteria so the pieces can be reused. If they fail, that material piles up inside the cell.

• A: DNA copying happens in the nucleus during interphase and does not depend on lysosomes.
• B (correct): Lysosomes hold digestive enzymes that break down old cell parts and swallowed material, so without them this debris accumulates.
• C: ATP is made mainly by mitochondria; lysosomes digest materials rather than release energy.
• D: Ribosomes link amino acids into proteins; lysosomes break molecules down rather than build them.

Answer: D

Correct answer: It doubles, raising the surface-area-to-volume ratio from 1:1 to 2:1. Large cube: surface area

µm² and volume

µm³, a 1:1 ratio. Each small cube has area

µm², so 8 cubes give 432 µm² for the same 216 µm³, a 2:1 ratio. This is why smaller cells exchange materials more easily.

• A: Cutting a cube exposes new faces, so surface area grows even though the total volume stays at 216 µm³.
• B: Each small cube has only one fourth of the large cube's surface area, so 8 of them give 2 times the area, not 8 times.
• C: This reverses the direction; smaller cubes have more surface relative to volume, so the ratio rises.
• D (correct): One large cube has 216 µm² of surface; eight small cubes have 8 × 54 = 432 µm², twice as much for the same 216 µm³ of volume.

Answer: C

Correct answer: Peroxisome. Peroxisomes are small sacs that break down fatty acids and certain toxins. Their reactions produce hydrogen peroxide, which an enzyme inside the peroxisome quickly converts to water and oxygen.

• A: Lysosomes digest worn-out parts and engulfed particles with acidic enzymes; they are not named for hydrogen peroxide.
• B: The Golgi modifies, sorts and packages proteins and lipids; it does not break down fatty acids.
• C (correct): Peroxisomes break down fatty acids and neutralize toxins, and an enzyme inside them turns hydrogen peroxide into water and oxygen.
• D: Smooth ER makes lipids and detoxifies drugs, but the hydrogen peroxide breakdown happens in peroxisomes.

Answer: D

Correct answer: Water keeps entering by osmosis, so the cell swells and may burst. Water follows solute. Because the cytoplasm has more solute than the fresh pond water, water constantly enters the protist by osmosis. The contractile vacuole normally collects and expels this extra water; without it, the cell swells and may burst.

• A: This reverses the direction; water moves toward the side with more solute, which is the inside of the cell.
• B: The pond has less solute than the cell, and in osmosis it is mainly water, not solute, that moves.
• C: The stem says the cell has more solute than the pond, so the solutions are not isotonic and water keeps entering.
• D (correct): The pond water is hypotonic to the cell, so water moves in constantly; without the vacuole pumping it out, the cell overfills.

Answer: D

Correct answer: Nucleolus. The nucleus is wrapped in the nuclear envelope and holds the cell's chromosomes. Within it sits the nucleolus, a dense region where ribosome parts are assembled before they leave through the nuclear pores.

• A: The nuclear envelope is the double membrane surrounding the nucleus, not a region inside it.
• B: A nucleoid is the region holding the DNA of a prokaryote, which has no nucleus at all.
• C: A chromosome is a packaged DNA molecule carrying genes, not the site where ribosome parts are made.
• D (correct): The nucleolus is the dense spot inside the nucleus where ribosome parts are put together.

Answer: A

Correct answer: The sodium-potassium pump moving sodium out and potassium in. Ask which process moves a substance against its gradient. Diffusion, osmosis and facilitated diffusion all run downhill and need no ATP. The sodium-potassium pump pushes ions uphill using ATP, so it is the process that stops when ATP production fails.

• A (correct): The pump moves ions against their gradients, which is active transport and requires ATP, so it stops when ATP runs out.
• B: Oxygen crosses by simple diffusion down its gradient, which uses no cell energy.
• C: Osmosis is passive; water moves toward higher solute without the cell spending ATP.
• D: Movement down a gradient through a carrier is facilitated diffusion, which needs no ATP.

Answer: B

Correct answer: Muscle tissue. Animals have four main tissue types: epithelial tissue covers and lines, connective tissue supports and binds, muscle tissue contracts, and nervous tissue carries signals.

• A: Epithelial tissue covers body surfaces and lines organs and cavities.
• B (correct): Muscle tissue is made of cells that shorten, or contract, to produce movement.
• C: Connective tissue, such as bone and tendon, supports and binds other tissues.
• D: Nervous tissue carries signals; it can trigger contraction but does not contract itself.

Answer: B

Correct answer: The cell likely has ribosomes that are too small to see with a light microscope. Reason only from what the microscope can actually show. A light microscope reveals nuclei and membranes, but ribosomes require an electron microscope. Since all cells have ribosomes, the safe conclusion is that they are present but too small to see.

• A: Ribosomes are one of the universal features of cells; not seeing them under a light microscope does not mean they are missing.
• B (correct): Every cell has ribosomes, and they are visible only with an electron microscope, so their absence in this view says nothing.
• C: A visible membrane-bound nucleus marks the cell as eukaryotic, which rules out a prokaryote.
• D: Ribosome parts are assembled in the nucleolus, but working ribosomes sit in the cytosol and on the rough ER.

Answer: C

Correct answer: Proteins. An enzyme is a biological catalyst that is usually a protein. Do not confuse the enzyme with its substrate: amylase acts on starch, a carbohydrate, but amylase itself is a protein.

• A: Starch, the substrate amylase acts on, is a carbohydrate, but the enzyme itself is not.
• B: Lipids such as fats and phospholipids store energy and form membranes; they do not usually act as enzymes.
• C (correct): Most enzymes, including amylase, are proteins: chains of amino acids folded into a shape with an active site.
• D: DNA and RNA store and carry genetic information; the enzyme is coded by a gene but is not a nucleic acid.

Answer: A

Correct answer: During differentiation, each cell type switches on some genes and switches off others. Nearly all of a person's body cells contain the same DNA. Through differentiation, each cell turns certain genes on and others off, so a nerve cell builds long extensions for signaling while a skin cell builds a protective covering.

• A (correct): Differentiation lets cells with identical DNA use different sets of genes and take on specialized forms.
• B: Almost every body cell keeps the same full set of DNA; specialization comes from which genes are used, not from losing genes.
• C: Human body cells carry the same 46 chromosomes; the chromosome number does not explain specialization.
• D: All of a person's body cells descend from one fertilized egg by cell division, which is why their DNA matches.