AP Biology · Unit 2 · Topic 2.9
Run the same reactions in one open space and in separate compartments, and the difference in what gets made is the answer.
Learning objectives 2.9.A Describe the membrane-bound structures of the eukaryotic cell. 2.9.B Explain how internal membranes and membrane-bound organelles contribute to compartmentalization of eukaryotic cell functions.
Narrated walkthrough of the “Run the experiment” tab. Captions are on by default.
A cell holds a pool of substrate. One enzyme turns it into the product the cell needs; a second enzyme destroys it. Both enzymes are equally fast. The only thing you can change is how much of the substrate is shut away with the wanted enzyme.
Product against the fraction of substrate enclosed. Nothing else changes: same substrate, same enzymes, same time.
At 0% enclosed, the compartment model behaves exactly like an open cytosol — the two runs are identical. That is what makes the comparison mean something: the only difference between 0% and 100% is the compartment.
Reactions that happen on a membrane are limited by how much membrane there is. Internal membranes are closed sacs (the ER, the Golgi complex, vesicles, the inner membrane of a mitochondrion): each one encloses a compartment, and both of its faces add reaction surface while the cell stays exactly the same size.
In Topic 2.2 a cell that needed more exchange surface folded its plasma membrane. Here a cell that needs more reaction surface folds its internal membranes. Same geometry, same reason: area can be increased freely, volume cannot.
A membrane does not only keep molecules in. It lets a compartment hold a concentration — of protons, of enzymes, of anything — that is nothing like the cytosol around it.
Each claim below is one advantage the CED names. For each claim, pick the result from this tool that is evidence for it, then check. Four of the seven statements are true but are not evidence for any of the three.
Answer these in your own words before opening the answers.
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