The cell is the basic unit of life, and the CLEP Biology exam expects you to match each structure to its function quickly. Because the Molecular and Cellular domain is about a third of the exam, organelle identification is high-yield.
All cells share a plasma membrane, cytoplasm, ribosomes, and DNA. The dividing line is the nucleus:
| Feature | Prokaryotic (bacteria, archaea) | Eukaryotic (plants, animals, fungi, protists) |
|---|---|---|
| True nucleus | No — DNA in a nucleoid region | Yes — membrane-bound nucleus |
| Membrane-bound organelles | Absent | Present (mitochondria, ER, etc.) |
| DNA shape | Usually one circular chromosome | Multiple linear chromosomes |
| Size | Smaller (~1–10 µm) | Larger (~10–100 µm) |
| Ribosomes | Present (smaller) | Present (larger) |
Note the trap: ribosomes occur in both cell types, because all cells make proteins. What prokaryotes lack is the membrane-bound nucleus and organelles.
Both are eukaryotic and share most organelles, but three structures separate them:
| Structure | Plant cell | Animal cell |
|---|---|---|
| Cell wall (cellulose) | Present | Absent |
| Chloroplasts | Present | Absent |
| Large central vacuole | Present | Absent (small vacuoles only) |
| Centrioles/lysosomes | Rare / less prominent | Common |
| Organelle | Function |
|---|---|
| Nucleus | Stores DNA; controls gene expression and cell activity |
| Nucleolus | Site of ribosome subunit assembly |
| Ribosomes | Protein synthesis (free or on rough ER) |
| Rough ER | Studded with ribosomes; makes and folds proteins |
| Smooth ER | Lipid synthesis, detoxification |
| Golgi apparatus | Modifies, sorts, and packages proteins for shipping |
| Mitochondria | Aerobic respiration; main ATP production |
| Chloroplasts | Photosynthesis (plants, algae) |
| Lysosomes | Digest wastes, worn organelles, engulfed material |
| Vacuoles | Storage of water, nutrients, wastes; turgor in plants |
| Cytoskeleton | Shape, support, and intracellular movement |
| Cell wall | Rigid support outside the membrane (plants, fungi, bacteria) |
A useful mental map: proteins are made on ribosomes/rough ER, modified and packaged by the Golgi, and shipped out — the secretory pathway. Cells that secrete a lot (e.g., gland cells) are rich in rough ER and Golgi.
[GRAPH: A eukaryotic cell cutaway. Central round nucleus with a dark nucleolus; rough ER (dotted with ribosomes) wrapping around it; nearby stacked Golgi discs; scattered oval mitochondria; and, for a plant version, green oval chloroplasts plus a large central vacuole bounded by a rigid cell wall.]
CLEP frequently asks items like: "A typical photosynthetic eukaryotic cell contains which of the following? I. Ribosomes II. Chloroplasts III. Mitochondria." A photosynthetic eukaryote (a plant or alga) has all three — it makes proteins (ribosomes), photosynthesizes (chloroplasts), and still respires to make ATP (mitochondria). The correct choice combines I, II, and III. Work these by checking each numeral independently, then picking the letter that lists exactly the true ones.
Mitochondria and chloroplasts are thought to have originated when an ancestral host cell engulfed free-living bacteria that were not digested but instead became permanent partners. Evidence: - Both organelles have their own circular DNA, similar to bacteria. - Both have a double membrane (the inner from the ancient bacterium, the outer from the host). - Both reproduce by dividing independently of the cell, and have their own bacteria-like ribosomes.
As a cell grows, its volume increases faster than its surface area (volume scales with the cube of radius, surface area with the square). A large cell therefore has too little membrane per unit of interior to exchange nutrients and wastes fast enough. This constraint keeps cells small and favors flat, folded, or elongated shapes that boost surface area.
Q1 — D (Lack membrane-bound organelles and a true nucleus). - Correct: The defining feature of prokaryotes is the absence of a membrane-bound nucleus and organelles; DNA sits in a nucleoid. - A) Mitochondria: prokaryotes have none. B) Membrane-bound nucleus: that describes eukaryotes. C) Chloroplasts: prokaryotes photosynthesize without them (e.g., cyanobacteria use membrane infoldings). E) Always multicellular: prokaryotes are typically single-celled. - Fix rule: Prokaryote = "before the nucleus" — no nucleus, no membrane-bound organelles.
Q2 — A (Ribosome). - Correct: Ribosomes read mRNA and assemble amino acids into proteins. - B) Golgi: modifies and ships proteins but does not synthesize them. C) Lysosome: digests material. D) Central vacuole: storage. E) Smooth ER: makes lipids, not proteins. - Fix rule: Ribosome = protein maker; the Golgi only finishes and ships.
Q3 — B (Mitochondrion). - Correct: Mitochondria carry out aerobic respiration, the cell's main ATP source. - A) Nucleus: stores DNA. C) Ribosome: makes protein. D) Golgi: packaging. E) Rough ER: protein folding/transport. - Fix rule: Mitochondrion = powerhouse (ATP); chloroplast captures energy, mitochondrion releases it.
Q4 — E (Cell wall). - Correct: A cellulose cell wall is present in plant cells but absent in animal cells. - A) Mitochondria, B) ribosomes, C) nucleus, D) plasma membrane: all shared by plant and animal cells, so none distinguishes them. - Fix rule: Plant-only trio = cell wall, chloroplasts, large central vacuole; everything else on this list is common to both.
Q5 — C (I, II, and III). - Correct: A photosynthetic eukaryote (plant/alga) has ribosomes (protein synthesis), chloroplasts (photosynthesis), and mitochondria (respiration) — all three. - A) I only and E) II only: omit organelles the cell clearly has. B) I and II only: forgets that photosynthetic cells still respire with mitochondria — the classic error. D) II and III only: drops ribosomes, which every cell has. - Fix rule: Photosynthetic cells still respire — they keep mitochondria; check each numeral separately.
Q6 — E (Rough ER and Golgi apparatus). - Correct: Secreted proteins are made on the rough ER and processed/packaged by the Golgi, so secretory cells are rich in both. - A) Chloroplasts: for photosynthesis, not enzyme secretion. B) Central vacuoles and C) cell walls: plant structures unrelated to secretion. D) Contractile vacuoles: pump out water in some protists, not enzyme export. - Fix rule: Heavy secretion → abundant rough ER + Golgi (the make-and-ship line).
Q7 — C (Lysosomes). - Correct: Lysosomes contain digestive enzymes that break down worn organelles and engulfed material. - A) Ribosomes: build, not break down. B) Smooth ER: lipid synthesis/detox. D) Chloroplasts: photosynthesis. E) Nucleolus: assembles ribosome parts. - Fix rule: Lysosome = cell's recycling/digestion center.
Q8 — D (Volume outpaces surface area). - Correct: Volume grows with the cube of size while surface area grows with the square, so a big cell has too little membrane per unit interior for efficient exchange. - A) Surface area faster than volume: the relationship reversed. B) Wall thickening: not the cause, and animal cells have no wall. C) Nucleus stops: unrelated to size limits. E) Ribosomes diluted: not the exchange-limiting factor. - Fix rule: Bigger cell → volume grows faster than surface area → exchange gets harder.
Q9 — E (Own circular DNA and bacteria-like division). - Correct: Mitochondria and chloroplasts have their own circular DNA and divide independently, pointing to a bacterial ancestry. - A) Only in prokaryotes: false; they are in eukaryotes. B) Single membrane: they have a double membrane. C) No genetic material: they do have DNA. D) Assembled by the nucleus alone: they are partly self-directed. - Fix rule: Endosymbiosis evidence = own DNA + double membrane + independent division.
Q10 — D (Plant cell). - Correct: A true nucleus plus a cellulose wall plus chloroplasts uniquely identifies a plant cell. - A) Animal cell: lacks wall and chloroplasts. B) Bacterium and E) prokaryote: have no true nucleus. C) Virus: not a cell at all. - Fix rule: Nucleus rules out prokaryotes; wall + chloroplasts then pins it to plant.
Q11 — A (Respire across the plasma membrane). - Correct: Prokaryotes run the electron transport reactions of respiration on their plasma membrane, doing without mitochondria. - B) Tiny mitochondria: prokaryotes have none. C) Do not respire: false; many are aerobic. D) Rely on chloroplasts: chloroplasts do photosynthesis, and most prokaryotes lack them. E) Import ATP: cells make their own. - Fix rule: No mitochondria? A prokaryote uses its plasma membrane for respiration.
Q12 — B (Flawed; ribosomes are in both cell types). - Correct: All cells synthesize proteins, so ribosomes occur in prokaryotes and eukaryotes alike. - A) and E): repeat the false "eukaryote/plant only" claim. C) No ribosomes in prokaryotes: false — protein synthesis requires them. D) Membrane-bound organelle: ribosomes are not membrane-bound. - Fix rule: Ribosomes are universal; the nucleus and other membrane organelles are what prokaryotes lack.
Q1 — D (Lack membrane-bound organelles and a true nucleus). - Correct: The defining feature of prokaryotes is the absence of a membrane-bound nucleus and organelles; DNA sits in a nucleoid. - A) Mitochondria: prokaryotes have none. B) Membrane-bound nucleus: that describes eukaryotes. C) Chloroplasts: prokaryotes photosynthesize without them (e.g., cyanobacteria use membrane infoldings). E) Always multicellular: prokaryotes are typically single-celled. - Fix rule: Prokaryote = "before the nucleus" — no nucleus, no membrane-bound organelles.
Q2 — A (Ribosome). - Correct: Ribosomes read mRNA and assemble amino acids into proteins. - B) Golgi: modifies and ships proteins but does not synthesize them. C) Lysosome: digests material. D) Central vacuole: storage. E) Smooth ER: makes lipids, not proteins. - Fix rule: Ribosome = protein maker; the Golgi only finishes and ships.
Q3 — B (Mitochondrion). - Correct: Mitochondria carry out aerobic respiration, the cell's main ATP source. - A) Nucleus: stores DNA. C) Ribosome: makes protein. D) Golgi: packaging. E) Rough ER: protein folding/transport. - Fix rule: Mitochondrion = powerhouse (ATP); chloroplast captures energy, mitochondrion releases it.
Q4 — E (Cell wall). - Correct: A cellulose cell wall is present in plant cells but absent in animal cells. - A) Mitochondria, B) ribosomes, C) nucleus, D) plasma membrane: all shared by plant and animal cells, so none distinguishes them. - Fix rule: Plant-only trio = cell wall, chloroplasts, large central vacuole; everything else on this list is common to both.
Q5 — C (I, II, and III). - Correct: A photosynthetic eukaryote (plant/alga) has ribosomes (protein synthesis), chloroplasts (photosynthesis), and mitochondria (respiration) — all three. - A) I only and E) II only: omit organelles the cell clearly has. B) I and II only: forgets that photosynthetic cells still respire with mitochondria — the classic error. D) II and III only: drops ribosomes, which every cell has. - Fix rule: Photosynthetic cells still respire — they keep mitochondria; check each numeral separately.
Q6 — E (Rough ER and Golgi apparatus). - Correct: Secreted proteins are made on the rough ER and processed/packaged by the Golgi, so secretory cells are rich in both. - A) Chloroplasts: for photosynthesis, not enzyme secretion. B) Central vacuoles and C) cell walls: plant structures unrelated to secretion. D) Contractile vacuoles: pump out water in some protists, not enzyme export. - Fix rule: Heavy secretion → abundant rough ER + Golgi (the make-and-ship line).
Q7 — C (Lysosomes). - Correct: Lysosomes contain digestive enzymes that break down worn organelles and engulfed material. - A) Ribosomes: build, not break down. B) Smooth ER: lipid synthesis/detox. D) Chloroplasts: photosynthesis. E) Nucleolus: assembles ribosome parts. - Fix rule: Lysosome = cell's recycling/digestion center.
Q8 — D (Volume outpaces surface area). - Correct: Volume grows with the cube of size while surface area grows with the square, so a big cell has too little membrane per unit interior for efficient exchange. - A) Surface area faster than volume: the relationship reversed. B) Wall thickening: not the cause, and animal cells have no wall. C) Nucleus stops: unrelated to size limits. E) Ribosomes diluted: not the exchange-limiting factor. - Fix rule: Bigger cell → volume grows faster than surface area → exchange gets harder.
Q9 — E (Own circular DNA and bacteria-like division). - Correct: Mitochondria and chloroplasts have their own circular DNA and divide independently, pointing to a bacterial ancestry. - A) Only in prokaryotes: false; they are in eukaryotes. B) Single membrane: they have a double membrane. C) No genetic material: they do have DNA. D) Assembled by the nucleus alone: they are partly self-directed. - Fix rule: Endosymbiosis evidence = own DNA + double membrane + independent division.
Q10 — D (Plant cell). - Correct: A true nucleus plus a cellulose wall plus chloroplasts uniquely identifies a plant cell. - A) Animal cell: lacks wall and chloroplasts. B) Bacterium and E) prokaryote: have no true nucleus. C) Virus: not a cell at all. - Fix rule: Nucleus rules out prokaryotes; wall + chloroplasts then pins it to plant.
Q11 — A (Respire across the plasma membrane). - Correct: Prokaryotes run the electron transport reactions of respiration on their plasma membrane, doing without mitochondria. - B) Tiny mitochondria: prokaryotes have none. C) Do not respire: false; many are aerobic. D) Rely on chloroplasts: chloroplasts do photosynthesis, and most prokaryotes lack them. E) Import ATP: cells make their own. - Fix rule: No mitochondria? A prokaryote uses its plasma membrane for respiration.
Q12 — B (Flawed; ribosomes are in both cell types). - Correct: All cells synthesize proteins, so ribosomes occur in prokaryotes and eukaryotes alike. - A) and E): repeat the false "eukaryote/plant only" claim. C) No ribosomes in prokaryotes: false — protein synthesis requires them. D) Membrane-bound organelle: ribosomes are not membrane-bound. - Fix rule: Ribosomes are universal; the nucleus and other membrane organelles are what prokaryotes lack.