AP BiologyUnit 2: Cells2.1
AP Biology · Unit 2 · Topic 2.1

Cell Structure and Function

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Proteins for the surface or for export never skip a stop: rough ER → Golgi → vesicle → membrane.

Membranes split a eukaryotic cell into compartments, each with its own enzymes and conditions. That is how a cell can build proteins, digest old parts, and make ATP all at once without the jobs wrecking each other.

Key terms

  • Ribosome — makes proteins in prokaryotes and eukaryotes alike. No membrane. In eukaryotes, free in the cytosol or bound to the ER.
  • Rough ER — membranes studded with ribosomes. Proteins headed for the plasma membrane, lysosomes, or export are threaded inside as they are made.
  • Smooth ER — ER without ribosomes. It makes lipids and detoxifies drugs; liver cells build more of it when a drug keeps coming.
  • Golgi complex — modifies proteins from the ER, sorts them, and ships each one in a vesicle to its destination.
  • Lysosome — a membrane sac of digestive enzymes that work best at acidic pH. It breaks down engulfed material and worn-out organelles.
  • Mitochondrion — two membranes. The inner one folds into cristae, which carry the electron transport chain and ATP synthase.

How it works

  1. A protein bound for the surface or export starts on a ribosome in the cytosol. A signal at the front of the chain sends the ribosome to the rough ER.
  2. The growing chain is threaded into the rough ER. There it folds and can pick up its first sugars.
  3. A vesicle buds off the ER and carries the protein to the Golgi complex, which modifies it further and sorts it by destination.
  4. A vesicle from the Golgi fuses with the plasma membrane. A secreted protein is released outside the cell.
  5. A membrane protein stays in the membrane. The side that faced the inside of the ER now faces outside the cell, sugars and all.

Why it matters

A membrane is more than a wall. It sets conditions: a lysosome keeps its interior acidic and its enzymes packed near their targets, away from the rest of the cell. Membrane area also sets capacity. The electron transport chain sits in the inner mitochondrial membrane, so more cristae means more capacity to use oxygen and make ATP. Chloroplasts use the same trick with stacked thylakoid membranes.

Exam traps

TrapActually
Proteins for the cell surface are made on free ribosomes, or start in the smooth ER.Their ribosomes dock on the rough ER. The route is rough ER → Golgi → vesicle → plasma membrane.
The nucleolus makes proteins.The nucleolus makes ribosomal RNA and builds ribosome subunits. Proteins are made by ribosomes out in the cytoplasm.
More mitochondria always means more respiration.Capacity depends on total inner-membrane area: how many mitochondria, times how much membrane each one folds in.
A liver cell clears a drug by digesting it in lysosomes.Smooth ER enzymes modify the drug to make it more water-soluble, so it can be excreted. Heavy use expands the smooth ER.
Worked example

Pancreatic cells are given radioactive amino acids for 3 minutes, then switched to unlabeled ones. In untreated cells, the label is in the rough ER at 3 minutes, the Golgi at 20 minutes, and secretory vesicles at 90 minutes. A second batch gets a drug that stops vesicles from budding off the ER. Where is the label at 90 minutes, and is the protein secreted?

  1. The label tags only proteins made during the 3-minute pulse, so it tracks one batch along its route.
  2. The route is rough ER → vesicle → Golgi → vesicle → outside. The first hand-off, ER to Golgi, needs a vesicle.
  3. With budding blocked, the batch can't reach the Golgi, so it stays where it was made.

The label stays in the rough ER, and the protein is not secreted.

Compartments pass proteins forward only in vesicles, so blocking the first vesicle step stalls everything after it. The near-miss is the Golgi: students remember it does the packaging and expect the pile-up there, but this drug acts one step earlier.

Now try it on real exam-style questions

Ten questions on Cell Structure and Function, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 2.1 Cell Structure and Function

Which organelle uses hydrolytic enzymes to digest worn-out organelles and engulfed food particles?

  1. A Mitochondrion
  2. B Golgi complex
  3. C Smooth ER
  4. D Lysosome
Show the answer

D — Lysosomes are membrane sacs of hydrolytic enzymes that break macromolecules into monomers the cell can reuse. Mitochondria also break down fuel molecules, but they do it to release energy as ATP, not to digest organelles or engulfed particles.

Question 2 · 2.1 Cell Structure and Function

What is the main role of the Golgi complex in a eukaryotic cell?

  1. A Modifying, sorting, and packaging proteins
  2. B Linking amino acids to build new proteins
  3. C Digesting worn-out organelles with enzymes
  4. D Building phospholipids for new membranes
Show the answer

A — The Golgi receives proteins from the rough ER, modifies them (for example, by changing their sugar groups), and ships them in vesicles to their destinations. It works on proteins that are already made; ribosomes are what link amino acids together.

Question 3 · 2.1 Cell Structure and Function

Plant cell walls are built mainly from which polysaccharide?

  1. A Starch
  2. B Cellulose
  3. C Chitin
  4. D Peptidoglycan
Show the answer

B — Plant cell walls are made mostly of cellulose, whose long glucose chains bundle into strong fibers. Starch is also a glucose polymer made by plants, but it is a stored fuel rather than a structural material.