Organelles and the Endomembrane System
General Biology · Section 1.5 · 18 study cards
Eukaryotic organelles, the endomembrane pathway in order, the cytoskeleton, cell surfaces, and the endosymbiotic theory.
Practice this set → Spaced repetition, card by card. No account needed.
Method
Group organelles by job, not alphabetically
Four functional groups cover almost everything: information storage and ribosome production in the nucleus and nucleolus; synthesis and processing along the endomembrane system; energy conversion in mitochondria and chloroplasts; and structure and movement in the cytoskeleton and cell surface. Placing each organelle in a group makes the comparisons far easier than memorising an unordered list.
Follow one protein through the pathway
- Translation begins on a free ribosome; a signal sequence appears and directs the ribosome to the rough ER.
- The polypeptide enters the ER lumen, folds and is glycosylated.
- A transport vesicle buds off and fuses with the cis face of the Golgi.
- The Golgi edits the carbohydrate chains and tags the product for its destination.
- A vesicle leaves the trans face and travels to the plasma membrane, a lysosome or another compartment.
- For secretion, the vesicle fuses with the plasma membrane and releases its contents by exocytosis.
Rehearse this path in both directions. Exam questions frequently ask what happens if one step is blocked, and the answer is always that everything downstream fails while everything upstream accumulates.
Use membrane count as a diagnostic
One membrane is the endomembrane norm: ER, Golgi, lysosomes, vacuoles and vesicles. Two membranes flag the nucleus, mitochondria and chloroplasts, and in the latter two the second membrane is the fingerprint of endosymbiosis. When a question gives you a membrane count, it is usually pointing at this.
Treat endosymbiosis as an argument, not a fact to recite
State the claim, then list the evidence, then say what each piece of evidence rules out. Circular DNA and 70S ribosomes point to a prokaryotic ancestry; independent division points to descent from a self-replicating cell; the double membrane points to engulfment by vesicle. An answer that lists evidence without linking each item to the claim scores poorly.
Definitions and theorems
Worked example
A pancreatic cell secretes the protein hormone insulin. A drug is applied that specifically blocks vesicle budding from the trans face of the Golgi apparatus. (i) Name, in order, the compartments the insulin precursor would normally pass through. (ii) Predict what accumulates and what is depleted. (iii) State one experiment using a radioactive label that would confirm your prediction.
Insulin is a secreted protein, so its synthesis begins on a ribosome that is directed to the rough ER by a signal sequence on the growing chain.
Normal route: rough ER lumen, transport vesicle, cis face of the Golgi, through the Golgi cisternae, trans face, secretory vesicle, fusion with the plasma membrane, exocytosis into the blood.
The drug blocks the step between the trans Golgi and the secretory vesicle, so everything upstream of that point continues to operate.
Prediction: insulin accumulates inside the Golgi, which swells, and back-pressure causes accumulation in the ER as well. No new secretory vesicles form.
Depleted: secretory vesicles at the plasma membrane and extracellular insulin. Blood insulin concentration falls even though synthesis continues.
Experiment: supply a pulse of radioactively labelled amino acids, then chase with unlabelled ones, and follow the label by autoradiography at successive time points.
Expected result: in untreated cells the label appears sequentially in the ER, then the Golgi, then secretory vesicles, then outside the cell. In treated cells the label reaches the Golgi and stops there, confirming the block is at the trans face.
Common mistakes
- Including mitochondria and chloroplasts in the endomembrane system. They have membranes, so students add them in. They are excluded because they do not exchange material with the system by vesicles and are not continuous with it. Peroxisomes are excluded for the same reason.
- Confusing lysosomes with peroxisomes. Lysosomes carry hydrolytic enzymes at pH 5 and digest macromolecules; peroxisomes carry oxidative enzymes, generate hydrogen peroxide and destroy it with catalase. Different chemistry, different origin, and only one of them is part of the endomembrane system.
- Saying free and bound ribosomes are different kinds of ribosome. They are interchangeable and identical. What differs is the destination of the protein being made, determined by whether a signal sequence appears on the emerging polypeptide.
- Listing endosymbiosis evidence without the double membrane logic. Students recite circular DNA and 70S ribosomes but omit the two membranes, which is the structural signature of engulfment: the inner from the bacterium, the outer from the host vesicle. Include it and say what it implies.
- Mixing up cilia with bacterial flagella. The eukaryotic 9 plus 2 structure with dynein motors is unrelated to the rotating protein filament of a bacterial flagellum. Naming the 9 plus 2 pattern for a bacterium, or forgetting that the basal body is 9 plus 0, costs marks on structure questions.
Practice it
Reading the method is not the same as being able to recall it under pressure. This set drills 18 cards one at a time and schedules each card separately, so the ones you keep missing come back sooner.