AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-9)
Unit 2 · Topic 2.9
2.9 Cell Compartmentalization
Eukaryotic cells are divided by internal membranes into compartments, each with its own set of enzymes and conditions. Separating reactions keeps them from interfering with each other, and folded membranes add surface area where reactions happen. This division of labor is one big reason eukaryotic cells can be large and complex.
Key terms
- compartmentalization
- organelle
- endomembrane system
- surface area
- eukaryotic cell
What compartmentalization means
Compartmentalization means dividing a cell into separate spaces with membranes. In a eukaryotic cell, the nucleus, ER, Golgi, lysosomes, vacuoles, mitochondria and chloroplasts are all membrane-bound organelles, each with a different internal environment and its own enzymes.
Each compartment can run specific metabolic processes and enzyme reactions. DNA replication and transcription happen in the nucleus. Most of cellular respiration happens in mitochondria. Digestion of large molecules happens in lysosomes.
The nucleus shows the idea well. Because the nuclear envelope keeps DNA apart from the ribosomes in the cytosol, a eukaryotic cell can make and edit its RNA in the nucleus before any of it is used to build proteins (6.3).
Benefit 1: keeping reactions from interfering
Some reactions would cancel or harm each other if they happened in the same place. Membranes keep them apart, so the cell can run building reactions and breaking-down reactions at the same time.
Lysosomes are a great example. They hold hydrolytic enzymes that would digest the cell's own proteins and nucleic acids if they were loose in the cytosol. The lysosome membrane keeps them contained, and the inside of a lysosome is kept acidic (around pH 5) while the cytosol is close to neutral (around pH 7.2). Lysosomal enzymes work best at that low pH, so they work well inside the lysosome.
Compartments also concentrate the enzymes and substrates for one pathway in a small space. That makes them more likely to meet, which speeds up the pathway.
Benefit 2: more surface area for reactions
Many important reactions happen on membranes, carried out by proteins embedded in them. More membrane means room for more of these proteins.
Folded internal membranes pack a huge area into a small volume. The cristae of the inner mitochondrial membrane hold the electron transport chain and ATP synthase (3.5). The thylakoid membranes in chloroplasts hold the photosystems (3.4). The extensive membranes of the ER provide space for making proteins and lipids. In each case, folding raises the surface area available for reactions without making the organelle much bigger.
Compartments make gradients possible
A closed membrane can hold different concentrations on its two sides. Mitochondria and chloroplasts depend on this: they pump H⁺ into one compartment to build a proton gradient, then let the protons flow back through ATP synthase to make ATP. Without a sealed compartment, there could be no gradient. This is a clear case of structure (a closed, folded membrane) fitting function (making ATP).
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Explaining a safety feature
Sometimes a lysosome leaks a small amount of its enzymes into the cytosol, yet the cell usually isn't badly damaged. Using what you know about compartments and enzymes, explain why.
Show the solutionHide the solution
- Step 1: Recall the conditions: the inside of a lysosome is acidic (about pH 5), and the cytosol is close to neutral (about pH 7.2).
- Step 2: Recall how enzymes respond to pH: each enzyme has an optimal pH, and activity drops outside that range (3.2).
- Step 3: Apply: lysosomal enzymes work best at acidic pH, so they're much less active in the near-neutral cytosol.
- Step 4: Conclude: a small leak causes little digestion of the cell's own molecules. The different pH of the two compartments acts as a backup safeguard.
Answer: Lysosomal enzymes work best at the low pH inside lysosomes and are much less active at the near-neutral pH of the cytosol, so a small leak does little damage.
- Example 2
Two benefits, not one (classic trap)
A question asks: 'Explain two ways that the inner membrane of the mitochondrion helps the cell make ATP.' A student writes: 'It is folded, so it is bigger.' What's missing?
Show the solutionHide the solution
- Step 1: The answer names a structure (folding) but not the result. 'Bigger' should be 'increases surface area,' and it needs a link to function: more area holds more electron transport chains and ATP synthase proteins, so more ATP can be made.
- Step 2: The question asks for two ways. A second, separate benefit is compartmentalization.
- Step 3: The inner membrane separates the matrix from the intermembrane space, so protons can be pumped into the intermembrane space to build a gradient.
- Step 4: The gradient drives ATP synthase, so the separate compartment is what makes the gradient, and the ATP synthesis, possible.
Answer: (1) Its folds increase surface area for more electron transport chains and ATP synthase; (2) it encloses a separate compartment so a proton gradient can be built and used to make ATP.
Common mistakes
- Saying compartmentalization makes a cell 'more organized' without explaining how. Name the benefit: separating competing reactions, concentrating enzymes, or increasing surface area.
- Saying folds make an organelle bigger. Folds increase the membrane's surface area without increasing the organelle's volume much.
- Thinking only mitochondria and chloroplasts are compartments. Every membrane-bound organelle is one.
On the exam
- Expect 'explain how this structure supports this function' questions about folded membranes or separate compartments. A complete answer names the structure, what it does physically (separates or increases area), and how that helps the process.
- Compartmentalization links to energy questions in Unit 3, because proton gradients only exist across sealed membranes.
Connected topics
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Check yourself
4 questions on 2.9 Cell Compartmentalization. Pick an answer to see if you got it, and why.
Lysosomal enzymes work best at about pH 5, while the cytosol is about pH 7.2. Which of the following best explains how compartmentalization benefits the cell in this case?
The inner membrane of a mitochondrion is highly folded. Which of the following is the best explanation for this structure?
In animal cells, fatty acids are built in the cytosol but broken down inside mitochondria. Which of the following best explains an advantage of carrying out these two processes in different compartments?
The thylakoid membrane encloses a small inner space within the chloroplast. Which of the following best explains why this compartment is important for making ATP in the light?
0 of 4 answered