AP Biology · Free-response skill

Read a model: how a cell ships a protein

The move. Read a model of a cell process, describe what it shows in order, and predict the effect of blocking one step, tracing the cause through the model.

How to do it

  1. Read the model one step at a time. For each step, note the structure, what happens there and where the product goes next.
  2. To describe, use the model's own labels and order. Do not add structures the model does not show.
  3. To predict the effect of a block, find the step that is blocked. Everything before it can still happen; everything after it cannot. Say what stops, then say where things pile up.
  4. To explain a prediction, trace the cause through the model step by step and name the step that is blocked or the structure that does the work.

Practice prompt Try it

The table models how a fictional gland cell makes an enzyme and releases it outside the cell. The cell releases large amounts of this enzyme.

Model of enzyme release in a fictional gland cell

StepStructureWhat happens
1Ribosomes on the rough ERThe enzyme's amino acid chain is built and threaded into the rough ER
2Rough ERThe enzyme folds and is packed into transport vesicles
3Golgi apparatusThe enzyme gets sugar groups added and is packed into secretory vesicles
4Secretory vesiclesThe vesicles carry the enzyme to the plasma membrane
5Plasma membraneThe vesicles fuse with the membrane and the enzyme is released outside the cell

Using the model, respond to each part.

  1. List, in order, the structures the enzyme passes through from where it is built to where it leaves the cell.
  2. A drug stops vesicles from fusing with the plasma membrane. Predict what happens to the amount of enzyme released outside the cell.
  3. For the same drug, predict where the enzyme builds up inside the cell, and explain your prediction using the steps of the model.
  4. Predict whether this gland cell has more or less rough ER than a cell that releases no proteins, and explain your prediction using the model.

Write each part, then check yourself against the checklist. Nothing you type here is saved or graded.

Check yourself and see a model answer

Checklist

Our own checklist of what a full answer does, one line per point. Tick each one your response earns.

0 of 6 points

Model answer, annotated

(a) The enzyme is built by ribosomes on the rough ER and passes into the rough ER, then goes in transport vesicles to the Golgi apparatus, then in secretory vesicles to the plasma membrane, where it leaves the cell.

  • Point 1 Names every structure in the model's order, from the rough ER to the plasma membrane.

(b) The amount of enzyme released outside the cell will fall, and release will stop if no vesicles can fuse.

  • Point 2 Gives the direction of the change: release falls and stops.

(c) The enzyme will build up inside secretory vesicles near the plasma membrane. Steps 1 to 4 still work, so the enzyme is still made, processed in the Golgi and carried to the membrane in vesicles. But step 5 is blocked, so the vesicles cannot fuse with the membrane, and the enzyme stays trapped inside them.

  • Point 3 Says where the enzyme piles up: in secretory vesicles near the membrane.
  • Point 4 Traces the model: steps 1 to 4 still happen, step 5 is blocked, so the vesicles cannot empty outside.

(d) The gland cell should have more rough ER. In the model, ribosomes on the rough ER build the enzyme and the rough ER folds it (steps 1 and 2), and this cell releases large amounts of the enzyme, so it needs much more rough ER than a cell that releases no proteins.

  • Point 5 Makes the prediction first: more rough ER.
  • Point 6 Links the rough ER's job in steps 1 and 2 to the large amount of enzyme this cell releases.