Cell Structure: Subcellular Components — AP Biology

AP Biology · Unit 2: Cell Structure and Function · 14 min read

1. Structure and Function of Eukaryotic Organelles ★★☆☆☆ ⏱ 4 min

Eukaryotic subcellular components are categorized as non-membrane-bound or membrane-bound. Non-membrane-bound ribosomes are made of ribosomal RNA (rRNA) and protein, assembled into two subunits, and synthesize all cellular proteins, found in all living cells.

**Rough Endoplasmic Reticulum (rough ER):** Studded with attached ribosomes, modifies and packages newly synthesized proteins for transport.
**Smooth Endoplasmic Reticulum (smooth ER):** No attached ribosomes; functions in lipid synthesis, detoxification, and calcium ion storage.
**Golgi Complex:** Flattened membrane sacs (cisternae) with a cis face that receives vesicles and trans face that ships modified products to their final destination.
**Mitochondria:** Double-membrane organelle with inner membrane folded into cristae; carries out aerobic cellular respiration to produce ATP.
**Lysosomes:** Membrane-bound sacs of hydrolytic enzymes; digests macromolecules, recycles damaged organelles, and mediates apoptosis.
**Chloroplasts:** Double-membrane organelle with internal thylakoids stacked into grana; carries out photosynthesis in plants and algae.
**Central Vacuole:** Large membrane-bound plant cell sac; stores water, ions, 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.

First, identify the core requirements of the pancreatic acinar cell: it synthesizes large amounts of secretory proteins for export out of the cell.
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.
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.
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.

2. Prokaryotic vs Eukaryotic Subcellular Components ★★☆☆☆ ⏱ 3 min

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, and only have non-membrane-bound organelles, primarily 70S ribosomes. Eukaryotes have a membrane-bound nucleus storing linear chromosomes, plus full specialized membrane-bound organelles, with 80S ribosomes in the cytoplasm.

A key exception to this pattern is that mitochondria and chloroplasts in eukaryotic cells retain prokaryotic-like features: 70S ribosomes and circular DNA. This is core evidence for the endosymbiotic theory.

Exam tip: AP multiple-choice questions frequently test the 70S ribosome exception. Automatic assumption that 70S = prokaryote will lead to lost points.

3. Endosymbiotic Theory for Organelle Origin ★★★☆☆ ⏱ 4 min

Both mitochondria and chloroplasts have a double membrane: outer from host engulfment, inner from the original prokaryote membrane.
Both organelles have their own circular DNA, matching prokaryotic chromosome structure.
Both have 70S ribosomes, identical to prokaryotic ribosomes.
Both replicate independently of the host cell via binary fission, the same method prokaryotes use.

📐 Worked Example

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

First, recall the four key lines of evidence that support endosymbiosis for mitochondria and chloroplasts.
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.
Current evidence for nuclear origin points to infolding of the ancestral eukaryote's plasma membrane, not engulfment of a prokaryote.
Therefore, there is no supporting evidence from subcellular structure to support the claim, 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 earn points.

4. AP-Style Practice Worked Examples ★★★☆☆ ⏱ 3 min

📐 Worked Example

Different human cell types have very different functions, leading to predictable differences in the abundance of specific subcellular components.<br>(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.<br>(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.<br>(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.

(a) 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) 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) Smooth endoplasmic reticulum and 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.

📐 Worked Example

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.

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, therapeutic doses of chloramphenicol only affect the much larger population of bacterial 70S ribosomes, with minimal impact on human mitochondrial function.
Therefore, the patient’s concern that chloramphenicol will kill their own cells is largely unfounded. Chloramphenicol is selectively toxic to bacteria due to the difference in ribosome size between prokaryotes and eukaryotic cytoplasm.

Common Pitfalls

Why: Confuses endomembrane system organelles with non-membrane-bound organelles; forgets prokaryotes need to synthesize proteins

Why: Overgeneralizes from plant cells to all eukaryotes, forgets animal/fungal cells do not photosynthesize

Why: Confuses the role of attached ribosomes with the rough ER’s own function

Why: Overgeneralizes the fact that prokaryotes have no *membrane-bound* organelles

Why: Assumes one point is enough for full credit; forgets AP FRQs require multiple specific supporting points

Why: Mixes up the core functions of the two ER types

Cell Structure: Subcellular Components — AP Biology

1. Structure and Function of Eukaryotic Organelles ★★☆☆☆ ⏱ 4 min

2. Prokaryotic vs Eukaryotic Subcellular Components ★★☆☆☆ ⏱ 3 min

3. Endosymbiotic Theory for Organelle Origin ★★★☆☆ ⏱ 4 min

4. AP-Style Practice Worked Examples ★★★☆☆ ⏱ 3 min

Common Pitfalls

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