AP Biology · Unit 2 · Topic 2.1

The Secretory Route

A protein built by a ribosome on the rough ER has to be folded, modified, packaged and shipped. Follow one through the endomembrane system and see what each organelle does to it.

Learning objective 2.1.A Explain how the structure and function of subcellular components and organelles contribute to the function of cells.

Watch it work · a two-minute demo for teachers

Narrated walkthrough of the “Explore the cell” tab. Captions are on by default.

Every organelle is shaped for its job

Press any organelle to see what it is made of and what it does. Then press Send a protein and follow one through the endomembrane system, from the ribosome that builds it to the membrane it is delivered to.

endomembrane system not part of it bound ribosomes nucleoid cell wall the protein being shipped selected
Press an organelle to inspect it.
Selected—
Membrane—
Endomembrane system—
CED reference—

How the parts work together

The endomembrane system is a delivery network

The CED names seven things as the endomembrane system: . They work together to modify, package and transport polysaccharides, lipids and proteins. The route starts at a ribosome, which is not one of the seven; every stop after it is.

Press Send a protein and watch the order. Each stop does something specific, and the product only reaches the surface because every stop did its part.

What is deliberately not in it

Ribosomes are not membrane-bound at all — they are made of rRNA and protein. Mitochondria and chloroplasts are membrane-bound but are not part of the endomembrane network; they have their own double membranes and their own jobs.

That ribosomes are found in cells in all forms of life is why they are taken as evidence of the common ancestry of all known life.

Structure explains function, every time

Rough ER is rough because membrane-bound ribosomes stud it, which is why it is where protein synthesis happens. Smooth ER has none, and does detoxification and lipid synthesis instead.

A mitochondrion's inner membrane is highly convoluted. Those folds increase the area available, which lets ATP be synthesized more efficiently — the same surface area argument as Topic 2.2.

A lysosome's membrane is not incidental. It is what keeps hydrolytic enzymes enclosed and away from the rest of the cell.

Learning check

Answer these in your own words before you open the answers.

  1. Describe the path a newly made protein takes from the ribosome to the outside of the cell, naming what happens at each stop.
  2. Explain why rough ER and smooth ER have different functions, using their structural difference.
  3. Explain how the structure of a mitochondrion's inner membrane contributes to its function.
  4. A student claims mitochondria are part of the endomembrane system. Evaluate that claim.
Show answers

1. A ribosome synthesizes the protein from an mRNA sequence. It enters the rough ER, where it is folded. A transport vesicle carries it to the Golgi complex, which folds it correctly, chemically modifies it and packages it for trafficking. Another vesicle carries it to the plasma membrane, which it fuses with to deliver the protein.

2. Rough ER is studded with membrane-bound ribosomes, so it is where protein synthesis is carried out and it allows compartmentalization. Smooth ER has no ribosomes, and its functions are detoxification and lipid synthesis instead. The presence or absence of ribosomes is the structural difference that sets the different jobs.

3. The inner membrane is highly convoluted, forming folds. Those folds increase the membrane area available, which enables ATP to be synthesized more efficiently. The double membrane also provides separate compartments for the different metabolic reactions of aerobic cellular respiration.

4. The claim is wrong. Mitochondria are membrane-bound, but the endomembrane system is specifically the ER, Golgi, lysosomes, vacuoles and transport vesicles, the nuclear envelope and the plasma membrane. Mitochondria have their own double membrane and are not part of that network.