DAT Review · Biology
Cell Structure and Organelles
On this page 7 sections
In 30 seconds
- Know every major organelle's structure and function — the DAT frequently asks "which organelle would be most abundant in a cell that secretes proteins?" (rough ER) or "which organelle detoxifies alcohol?" (smooth ER).
- Prokaryotes vs eukaryotes and plant vs animal cells are classic comparison questions — memorize the presence/absence tables.
- The endosymbiotic theory (mitochondria and chloroplasts have double membranes, circular DNA, and bacterial-sized ribosomes) is a guaranteed concept.
The college version
Core Review
The Nucleus
The nucleus is the most prominent organelle in a eukaryotic cell. It is surrounded by a double membrane called the nuclear envelope, which is studded with nuclear pores that regulate the passage of molecules (mRNA exits, transcription factors enter). Inside, chromatin — DNA wrapped around histone proteins — undergoes transcription. The nucleolus, a dense region within the nucleus, is the site of ribosomal RNA (rRNA) synthesis and ribosome subunit assembly. Think of the nucleus as the "control center" holding the master blueprint (DNA).
Ribosomes
Ribosomes are not membrane-bound; they consist of two subunits (large and small) made of rRNA and proteins. They translate mRNA into protein by reading codons and linking amino acids. Free ribosomes in the cytoplasm produce proteins destined for the cytosol, while bound ribosomes on the rough ER produce proteins destined for secretion, the plasma membrane, or lysosomes. A cell that secretes a lot of protein (e.g., pancreatic acinar cell) has abundant rough ER.
Endoplasmic Reticulum (ER)
The rough ER is studded with ribosomes and is a network of flattened sacs. It folds and chemically modifies newly synthesized proteins (e.g., glycosylation — adding carbohydrate groups). Proteins enter the ER lumen during synthesis and are subsequently packaged into transport vesicles.
The smooth ER lacks ribosomes. It synthesizes lipids (phospholipids, steroids), detoxifies drugs and poisons (abundant in liver cells), and stores calcium ions (critical in muscle cells — the sarcoplasmic reticulum is specialized smooth ER).
Golgi Apparatus
The Golgi is a stack of flattened membranous sacs (cisternae). It receives transport vesicles from the ER at its cis face, further modifies proteins (glycosylation, phosphorylation), sorts them, and packages them into vesicles that bud from the trans face. These vesicles can become lysosomes, secretory vesicles, or membrane components. Think of the Golgi as the "shipping and receiving" department.
Lysosomes and Peroxisomes
Lysosomes are membrane-bound sacs containing hydrolytic enzymes (low pH ~5) that digest macromolecules, old organelles (autophagy), and materials brought in by phagocytosis. They are critical in white blood cells and during apoptosis.
Peroxisomes contain oxidative enzymes (catalase, oxidases) that break down fatty acids via β-oxidation and detoxify hydrogen peroxide (H₂O₂ → H₂O + O₂). They are especially abundant in liver and kidney cells.
Mitochondria
Mitochondria are double-membraned organelles — the outer membrane is smooth, the inner membrane is highly folded into cristae (increasing surface area for the electron transport chain). The intermembrane space and matrix are separate compartments. Mitochondria perform cellular respiration: the Krebs cycle occurs in the matrix, and the ETC/chemiosmosis occurs on the inner membrane. They contain their own circular DNA and ribosomes (70S, like bacteria), supporting the endosymbiotic theory.
Chloroplasts (Plants Only)
Chloroplasts have three membrane systems: outer membrane, inner membrane, and the internal thylakoid membrane (stacked into grana). The stroma is the fluid surrounding the thylakoids. Light reactions occur on thylakoid membranes; the Calvin cycle occurs in the stroma. Like mitochondria, chloroplasts have their own circular DNA and 70S ribosomes — also endosymbiotic in origin.
The Cytoskeleton
Three types of fibers:
- Microfilaments (actin): smallest, involved in cell movement, muscle contraction, cytokinesis (cleavage furrow).
- Intermediate filaments: medium-sized, provide structural support; keratin, lamin are examples.
- Microtubules (tubulin): largest, hollow tubes; form the mitotic spindle, cilia, and flagella (9+2 arrangement). They serve as tracks for motor proteins (kinesin and dynein).
Prokaryotes vs. Eukaryotes
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| Nucleus | None; nucleoid region | True nucleus with envelope |
| DNA | Circular, single chromosome | Linear, multiple chromosomes |
| Membrane-bound organelles | Absent | Present |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) |
| Cell wall | Peptidoglycan (bacteria) | Cellulose (plants), chitin (fungi) |
| Size | 1–10 µm | 10–100 µm |
| Cell division | Binary fission | Mitosis/meiosis |
Plant vs. Animal Cells
| Feature | Plant | Animal |
|---|---|---|
| Cell wall | Present (cellulose) | Absent |
| Chloroplasts | Present | Absent |
| Central vacuole | Large, persistent | Small, temporary |
| Centrioles | Absent (higher plants) | Present |
| Lysosomes | Rare | Common |
| Plasmodesmata | Present | Absent |
Common Traps
- Ribosome size confusion: Eukaryotic ribosomes are 80S (not 70S — 70S is prokaryotic and organellar). The DAT will try to trick you.
- Lysosome vs. peroxisome: Both break things down, but lysosomes use acid hydrolases (low pH) and digest macromolecules/organelles, while peroxisomes neutralize H₂O₂ and oxidize fatty acids.
- Plant cells lack centrioles — but they still form a spindle. This is a classic trick question.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine your cell is a city. The nucleus is city hall — it holds the blueprints (DNA) for everything. Ribosomes are the factories that read the blueprints and build products (proteins). The rough ER is a factory with assembly-line workers (ribosomes attached), while the smooth ER is a chemical plant making fats and cleaning up toxins. The Golgi is the post office — it packages products, labels them, and ships them where they need to go. Lysosomes are the recycling center and garbage disposal. Mitochondria are the power plants burning fuel to make energy (ATP). The cytoskeleton is the road network — actin filaments are side streets, intermediate filaments are support beams, and microtubules are the highways that transport cargo. Plant cells have two extra features: chloroplasts (solar panels) and a big central vacuole (a giant water tower that keeps the cell plump).
Key takeaways
- Protein secretion pathway: Rough ER → vesicle → Golgi (cis→trans) → vesicle → plasma membrane. Know the order.
- Organelle abundance questions: a cell that detoxifies → abundant smooth ER; a secretory cell → abundant rough ER and Golgi; a phagocytic cell → abundant lysosomes; a muscle cell → abundant mitochondria.
- Endosymbiotic evidence: double membranes, circular DNA, 70S ribosomes, autonomous replication (binary fission), similar size to bacteria.
- Smooth ER functions: lipid synthesis, detoxification, Ca²⁺ storage (sarcoplasmic reticulum in muscle).
- Cytoskeleton: microtubules = spindle fibers (9+2 in cilia/flagella), actin = cleavage furrow in cytokinesis.
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
A pancreatic cell that produces and secretes large quantities of digestive enzymes would have abundant amounts of which organelle?
Show answer
The rough endoplasmic reticulum and Golgi apparatus. Since digestive enzymes are proteins destined for secretion, they are synthesized on ribosomes bound to the rough ER, then processed and packaged by the Golgi. A high concentration of these organelles supports the cell's secretory function.
Which organelle would an investigative pathologist expect to find in high concentration in liver cells (hepatocytes) given their role in drug metabolism?
Show answer
The smooth endoplasmic reticulum. The smooth ER is the primary site of detoxification and drug metabolism. Hepatocytes are rich in smooth ER to handle the continuous processing of toxins and metabolic waste products.
A researcher observes a single-celled organism with a cell wall containing peptidoglycan, circular DNA, and 70S ribosomes, but no membrane-bound organelles. Is it prokaryotic or eukaryotic?
Show answer
Prokaryotic. The peptidoglycan cell wall, circular chromosome, 70S ribosomes, and absence of membrane-bound organelles are all defining characteristics of prokaryotes. This description is consistent with a bacterium.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- Identify and describe the function of all major eukaryotic organelles: nucleus, ribosomes, rough and smooth ER, Golgi apparatus, lysosomes, peroxisomes, mitochondria, chloroplasts, and cytoskeletal elements.
- Compare and contrast prokaryotic and eukaryotic cells across at least five structural features.
- Distinguish plant cells from animal cells by the presence or absence of specific organelles.
- Explain the endosymbiotic theory and list the evidence supporting it.
Sources & references
- OpenStax Biology 2e, Chapter 4: "Cell Structure"
- NCBI Bookshelf, Molecular Biology of the Cell, 4th edition, Chapter 12: "Intracellular Compartments and Protein Sorting"
- NIH National Institute of General Medical Sciences: "Studying Cells"
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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