Cell Structure: Subcellular Components — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Structure and function of ribosomes, endoplasmic reticulum, Golgi complex, mitochondria, lysosomes, vacuoles, chloroplasts, prokaryotic vs eukaryotic subcellular components, and endosymbiotic theory evidence.

You should already know: Cell theory fundamentals, basic differences between prokaryotic and eukaryotic cells, basic structure of biological macromolecules.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Cell Structure: Subcellular Components?

Cell Structure: Subcellular Components is the study of specialized functional structures (collectively called organelles) found inside all prokaryotic and eukaryotic cells. This topic forms the core foundation of Unit 2: Cell Structure and Function, which accounts for 10–13% of the total score on the AP Biology exam. Subcellular components range from small non-membrane-bound structures like ribosomes to large, complex membrane-bound organelles that compartmentalize specific metabolic reactions in eukaryotic cells. On the exam, this topic appears in both multiple-choice (MCQ) and free-response (FRQ) sections: it is often tested directly as identification or structure-function matching, and indirectly as prerequisite knowledge for questions on cell compartmentalization, membrane transport, cellular respiration, and photosynthesis. Unlike basic memorization of organelle names, the AP exam prioritizes connecting organelle structure to its functional role in the cell, and supporting the endosymbiotic theory with evidence from subcellular structure.

2. Structure and Function of Eukaryotic Organelles

Eukaryotic cells contain a wide range of subcellular components, categorized as either non-membrane-bound or membrane-bound. Non-membrane-bound ribosomes are made of ribosomal RNA (rRNA) and protein, assembled into two subunits, and their core function is synthesizing all proteins for the cell. Ribosomes are found in every living cell, prokaryotic and eukaryotic. Membrane-bound organelles form specialized compartments for specific metabolic reactions:

• Rough endoplasmic reticulum (rough ER): Studded with attached ribosomes, it modifies and packages newly synthesized proteins for transport to the Golgi complex.
• Smooth endoplasmic reticulum (smooth ER): No attached ribosomes, it functions in lipid synthesis, detoxification of harmful compounds, and calcium ion storage.
• Golgi complex: Flattened membrane sacs called cisternae, with a cis face that receives transport vesicles and a trans face that ships modified products to their final destination.
• Mitochondria: Double-membrane organelle with the inner membrane folded into cristae, it carries out aerobic cellular respiration to produce ATP.
• Lysosomes: Membrane-bound sacs of hydrolytic enzymes, they digest macromolecules, recycle damaged organelles, and participate in apoptosis.
• Chloroplasts: Double-membrane organelle with internal thylakoids stacked into grana, they carry out photosynthesis in plants and algae.
• Central vacuole: Large membrane-bound sac in plant cells that stores water, ions, and nutrients, and maintains turgor pressure.

Worked Example

Pancreatic acinar cells secrete large amounts of digestive enzymes (which are proteins) into the small intestine. Which two organelles would you expect to be far more abundant in pancreatic acinar cells than in a cell that produces only cytoplasmic proteins? Justify your selection.

1. First, identify the core requirements of the pancreatic acinar cell: it synthesizes large amounts of secretory proteins for export out of the cell.
2. Recall that secretory proteins are synthesized by ribosomes attached to the rough ER, which processes the newly made protein. So rough ER must be highly abundant to support high volumes of secretory protein synthesis and processing.
3. After processing in the rough ER, secretory proteins are transported to the Golgi complex, which sorts, modifies, and packages them into secretory vesicles for export. So the Golgi complex must also be highly abundant.
4. Other organelles (like smooth ER or mitochondria) support general cell function but are not specifically overrepresented for protein secretion, so the two key organelles are rough ER and Golgi complex.

Exam tip: Always link your prediction of organelle abundance directly to the cell’s specific primary function. AP exam graders require a clear functional connection, not just a correct organelle name, to award points.

3. Prokaryotic vs Eukaryotic Subcellular Components

The most fundamental difference between prokaryotic and eukaryotic cells is the presence of membrane-bound organelles in eukaryotes, and their absence in prokaryotes. Prokaryotes (bacteria and archaea) store their circular chromosomal DNA in an unenclosed nucleoid region, rather than a membrane-bound nucleus. They only have non-membrane-bound organelles, primarily 70S ribosomes (smaller than eukaryotic ribosomes) for protein synthesis. Eukaryotes (animals, plants, fungi, protists) have a membrane-bound nucleus that stores linear chromosomes, plus a full set of specialized membrane-bound organelles. A key exception to this general pattern is that mitochondria and chloroplasts in eukaryotic cells have retained prokaryotic-like subcellular features: 70S ribosomes and circular DNA. This exception is a core piece of evidence for the endosymbiotic theory, which we cover in the next section.

Worked Example

A researcher isolates a pure fraction of 70S ribosomes from a cell lysate and claims the original cell must be prokaryotic. Is this claim necessarily true? Justify your answer.

1. Recall the general rule: prokaryotic ribosomes are 70S, while eukaryotic cytoplasmic ribosomes are 80S.
2. Recall the exception: eukaryotic mitochondria and chloroplasts are descended from prokaryotes, so they have their own 70S ribosomes for protein synthesis inside the organelle.
3. The 70S ribosome fraction could easily have been isolated from the mitochondria or chloroplasts of a eukaryotic cell, rather than from a whole prokaryotic cell.
4. Therefore, the claim that the original cell must be prokaryotic is not necessarily true.

Exam tip: AP multiple-choice questions frequently test the 70S ribosome exception. If you automatically assume 70S = prokaryote, you will miss these questions.

4. Endosymbiotic Theory for Organelle Origin

Endosymbiotic theory is the widely accepted model for the origin of mitochondria and chloroplasts in eukaryotic cells. The theory states that these organelles evolved from free-living prokaryotes that were engulfed by a larger ancestral eukaryotic cell, and eventually evolved into permanent, mutually dependent endosymbionts. The theory is supported by multiple lines of evidence from subcellular structure:

1. Both mitochondria and chloroplasts have a double membrane: the outer membrane is derived from the host cell’s plasma membrane during engulfment, and the inner membrane is the original plasma membrane of the engulfed prokaryote.
2. Both organelles have their own circular DNA, matching the chromosome structure of prokaryotes.
3. Both have 70S ribosomes, identical to prokaryotic ribosomes.
4. Both replicate independently of the host cell via binary fission, the same reproduction method used by prokaryotes.

No other membrane-bound eukaryotic organelles have all these features.

Worked Example

A student claims that the nucleus of eukaryotic cells originated via endosymbiosis. Evaluate this claim using evidence from subcellular structure.

1. First, recall the four key lines of evidence that support endosymbiosis for mitochondria and chloroplasts.
2. Compare the nucleus’s structure to these requirements: while the nucleus has a double membrane (the nuclear envelope), it does not have independent 70S ribosomes or circular prokaryotic-like DNA. The nucleus also cannot replicate independently of the endomembrane system.
3. Current evidence for nuclear origin instead points to infolding of the plasma membrane in the ancestral eukaryote, not engulfment of a prokaryote.
4. Therefore, there is no supporting evidence from subcellular structure to support the claim that the nucleus originated via endosymbiosis, so the claim is unsupported.

Exam tip: On FRQ questions asking for evidence for endosymbiosis, always name at least two specific subcellular features to earn full credit. Vague claims like "it has prokaryotic characteristics" will not get you points.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Claims that ribosomes are membrane-bound organelles found only in eukaryotes. Why: Confuses endomembrane system organelles with non-membrane-bound organelles; forgets prokaryotes need to synthesize proteins. Correct move: Always categorize ribosomes as non-membrane-bound and universal to all prokaryotic and eukaryotic cells.
• Wrong move: Claims that all eukaryotic cells contain both mitochondria and chloroplasts. Why: Overgeneralizes from plant cells to all eukaryotes, forgets animal/fungal cells do not photosynthesize. Correct move: Remember that all aerobic eukaryotic cells have mitochondria, but only photosynthetic eukaryotes have chloroplasts.
• Wrong move: Assigns core protein synthesis function to the rough ER. Why: Confuses the role of attached ribosomes with the rough ER’s own function. Correct move: Always state that ribosomes (not rough ER) carry out protein synthesis; rough ER modifies and packages proteins made by attached ribosomes.
• Wrong move: Justifies a claim with "prokaryotes have no organelles at all". Why: Overgeneralizes the fact that prokaryotes have no membrane-bound organelles. Correct move: Always specify that prokaryotes lack membrane-bound organelles, but do have non-membrane-bound organelles like ribosomes.
• Wrong move: Provides only one piece of evidence for endosymbiotic theory when the question asks for supporting evidence. Why: Assumes one point is enough for full credit; forgets AP FRQs require multiple specific supporting points. Correct move: Memorize three specific evidence points (double membrane, circular DNA, 70S ribosomes) to draw from for any endosymbiosis question.
• Wrong move: Claims the smooth ER synthesizes proteins and the rough ER synthesizes lipids. Why: Mixes up the core functions of the two ER types. Correct move: Use the mnemonic "Rough for Proteins, Smooth for Lipids" to avoid this swap.

6. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

A researcher observes an unknown cell under an electron microscope and records the following features: 70S ribosomes, a cell wall, circular DNA, and no membrane-bound organelles. Which of the following best classifies this cell? A) A eukaryotic animal cell B) A prokaryotic bacterial cell C) A eukaryotic plant leaf cell D) A eukaryotic photosynthetic algal cell

Worked Solution: First, we can eliminate all eukaryotic cell options because the cell explicitly has no membrane-bound organelles, a defining trait of prokaryotes. All eukaryotic cells (options A, C, D) have a membrane-bound nucleus and other membrane-bound organelles, so these are incorrect. The observed features all match the standard subcellular composition of a prokaryotic bacterial cell. The correct answer is B.

Question 2 (Free Response)

Different human cell types have very different functions, leading to predictable differences in the abundance of specific subcellular components. (a) Identify one organelle that you would expect to be more abundant in heart muscle cells than in most other human cell types. Justify your answer. (b) Identify one organelle that you would expect to be more abundant in liver cells than in most other human cell types. Justify your answer. (c) Identify two organelles that you would expect to be more abundant in ovarian cells that produce the steroid hormone estrogen than in most other cell types. Justify your answer.

Worked Solution: (a) The organelle that is more abundant in heart muscle cells is mitochondria. Heart muscle contracts almost continuously to pump blood, requiring a very large supply of ATP. Mitochondria produce ATP via aerobic cellular respiration, so higher ATP demand requires more mitochondria. (b) The organelle that is more abundant in liver cells is the smooth endoplasmic reticulum. A core function of the liver is detoxifying harmful compounds that enter the bloodstream, and smooth ER is responsible for detoxification of organic compounds. This high demand for detoxification requires more smooth ER than other cell types. (c) The two organelles are smooth endoplasmic reticulum and the Golgi complex. Estrogen is a steroid lipid, which is synthesized by the smooth ER. After synthesis, estrogen must be packaged into secretory vesicles for secretion out of the ovarian cell by the Golgi complex. High levels of steroid hormone production and secretion require elevated abundance of both organelles.

Question 3 (Application / Real-World Style)

The antibiotic chloramphenicol inhibits protein synthesis by binding specifically to 70S ribosomes, and does not affect 80S ribosomes. A patient is prescribed chloramphenicol to treat a bacterial (prokaryotic) infection, and asks if the drug will kill their own human cells. Evaluate the patient’s concern using your knowledge of subcellular component structure.

Worked Solution: First, bacterial cells are prokaryotic, so all of their ribosomes are 70S, meaning chloramphenicol will block their protein synthesis and kill the bacteria. Human cells are eukaryotic, so all cytoplasmic ribosomes are 80S, which are not affected by chloramphenicol. While human mitochondria do have 70S ribosomes, the therapeutic dose of chloramphenicol is low enough that it only affects the much larger population of 70S ribosomes in bacterial cells, with minimal impact on human mitochondrial function. Therefore, the patient’s concern that chloramphenicol will kill their own human cells is largely unfounded. This means chloramphenicol is selectively toxic to bacterial cells due to the difference in ribosome size between prokaryotes and eukaryotic cytoplasm.

7. Quick Reference Cheatsheet

8. What's Next

This topic is the non-negotiable foundation for the rest of Unit 2: Cell Structure and Function. Next, you will study cell compartmentalization, which explains how membrane-bound organelles increase metabolic efficiency by separating incompatible reactions, and how the structure of each subcellular component supports compartmentalization. Without mastering the structure and function of each subcellular component, you will not be able to explain the benefits of compartmentalization or connect organelle dysfunction to cellular disease, which are common FRQ topics. Later in the course, this topic is also critical for understanding cellular respiration (which occurs in mitochondria) and photosynthesis (which occurs in chloroplasts), which together account for ~12-16% of the total AP Biology exam score.

• Cell Compartmentalization
• Cell Size
• Cellular Respiration
• Photosynthesis