AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-1)
Unit 2 · Topic 2.1
2.1 Cell Structure and Function
A cell is full of specialized parts, and each one's structure fits its job. Ribosomes build proteins in every living cell, the endomembrane system makes, modifies and ships molecules, and mitochondria and chloroplasts handle energy. Knowing what each part does lets you predict what goes wrong when one fails.
Key terms
- ribosome
- endomembrane system
- Golgi complex
- lysosome
- mitochondrion
- chloroplast
Ribosomes: found in every cell
Ribosomes are the protein-building machines of the cell. Each one is made of ribosomal RNA (rRNA) and proteins, arranged in two subunits. Ribosomes read the instructions in messenger RNA (mRNA) and link amino acids in the order it specifies (6.4).
Ribosomes have no membrane, and they're found in every living cell: bacteria, archaea and eukaryotes. Because all known life shares them, ribosomes are evidence of common ancestry.
In eukaryotes, free ribosomes float in the cytosol and make proteins that work there (plus some proteins for the nucleus, mitochondria and chloroplasts). Bound ribosomes sit on the rough ER and make proteins that will be sent out of the cell, placed in a membrane or delivered to organelles such as lysosomes.
The endomembrane system
The endomembrane system is a group of membranes and organelles that work together to make, modify, package and transport proteins, lipids and polysaccharides. It includes the nuclear envelope, the endoplasmic reticulum (ER), the Golgi complex, lysosomes, vacuoles, transport vesicles (small membrane bubbles) and the plasma membrane. Parts of it are connected directly, and others pass material along in vesicles.
- Rough ER: a network of membrane sacs studded with ribosomes. Proteins made there are threaded into the ER, where they begin to fold and get modified. The ER also helps the cell keep its shape and moves materials around inside the cell.
- Smooth ER: ER without ribosomes. It makes lipids, such as the phospholipids for new membranes, and helps detoxify harmful substances.
- Golgi complex: a stack of flattened membrane sacs. It receives products from the ER, finishes folding and chemically modifying them (for example, by attaching sugar chains to proteins), then sorts and packages them into vesicles addressed to the right destination.
- Lysosomes: membrane sacs filled with hydrolytic enzymes, which break molecules down by hydrolysis (1.3). They digest food taken in by the cell, recycle worn-out organelles, and help carry out apoptosis, the cell's own planned self-destruction.
- Vacuoles: membrane sacs with many jobs. Plant cells usually have one large central vacuole that stores water and nutrients and pushes outward to keep the cell firm (turgor pressure). Animal cells have smaller, more numerous vacuoles that store materials.
Following a secreted protein
A protein that a cell exports, such as a digestive enzyme or an antibody, takes this route: a ribosome on the rough ER builds it → it enters the ER, where folding begins → a transport vesicle carries it to the Golgi → the Golgi modifies and packages it → a secretory vesicle carries it to the plasma membrane → the vesicle fuses with the membrane and releases the protein outside (exocytosis, 2.5).
Mitochondria and chloroplasts
Mitochondria are where most of cellular respiration happens (3.5). Each has a double membrane. The outer membrane is smooth. The inner membrane is folded into many ridges called cristae, which greatly increase its surface area. More membrane area means room for more of the proteins that make ATP, so the cell can make ATP more efficiently. The double membrane also creates separate compartments, the space between the membranes and the fluid inside (the matrix), where different reactions happen.
Chloroplasts, found in plant cells and in algae, are where photosynthesis happens (3.4). They also have a double membrane. Inside are stacks of flattened sacs called thylakoids, surrounded by fluid called the stroma.
You don't need to know the specialized smooth ER functions of particular cell types, or which specific lipids and enzymes the Golgi makes or packages.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Tracking a labeled protein
Pancreatic cells that secrete digestive enzymes are given radioactive amino acids for a few minutes, then switched to unlabeled amino acids. Researchers track where the radioactivity appears over time. Predict the order in which the label shows up in these locations: Golgi complex, outside the cell, rough ER, secretory vesicles.
Show the solutionHide the solution
- Step 1: The label enters proteins as they're built, and secreted proteins are built by ribosomes on the rough ER, so the rough ER is first.
- Step 2: From the ER, transport vesicles carry the proteins to the Golgi complex for modification and packaging, so the Golgi is second.
- Step 3: The Golgi packages the finished enzymes into secretory vesicles, so those are third.
- Step 4: The vesicles fuse with the plasma membrane and release the enzymes, so outside the cell is last.
Answer: Rough ER → Golgi complex → secretory vesicles → outside the cell.
- Example 2
Predicting the effect of a broken organelle
In Tay-Sachs disease, one hydrolytic enzyme in lysosomes doesn't work, so a particular lipid isn't broken down in nerve cells. Predict what happens inside the affected cells.
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- Step 1: Recall the job: lysosomal enzymes hydrolyze molecules so their parts can be recycled.
- Step 2: Apply the defect: the lipid that this enzyme normally breaks down can't be digested.
- Step 3: Predict the result: the lipid builds up inside the lysosomes, which swell and interfere with the cell's normal work.
- Step 4: Connect to the organism: nerve cells are damaged over time, which causes the disease's symptoms.
Answer: The undigested lipid accumulates in the lysosomes, which swell and disrupt the nerve cells' function.
Common mistakes
- Saying ribosomes are part of the endomembrane system or have a membrane. They're not membrane-bound; they just attach to the rough ER.
- Saying only animal cells have mitochondria, or that plants have chloroplasts instead of mitochondria. Plant cells have both.
- Mixing up the ER and Golgi order. Products go from the ER to the Golgi, not the other way around.
- Describing the folded inner mitochondrial membrane without its purpose. Say that folding increases surface area for making ATP.
On the exam
- Expect questions that ask you to explain how an organelle's structure fits its function, such as cristae and surface area or the double membrane and separate compartments.
- Pathway questions ask you to trace a secreted or membrane protein from ribosome to destination. Name each organelle in order.
Connected topics
Videos
Check yourself
4 questions on 2.1 Cell Structure and Function. Pick an answer to see if you got it, and why.
Cells in the pancreas produce and secrete large amounts of digestive enzymes, which are proteins. Compared with cells that secrete little protein, these cells would be expected to have more of which of the following?
| Time after labeling (min) | Rough ER (% of label) | Golgi complex (% of label) | Secretory vesicles (% of label) |
|---|---|---|---|
| 5 | 85 | 10 | 5 |
| 20 | 40 | 50 | 10 |
| 40 | 15 | 45 | 40 |
| 90 | 5 | 15 | 80 |
Experimental data: cells that secrete a protein were given radioactive amino acids for 3 minutes, then washed and given unlabeled amino acids. The table shows where the radioactive label was found over time.
Which of the following best describes the pathway of the labeled protein shown by the data?
A drug that prevents vesicles from leaving the Golgi complex is added at the start of the experiment. Which result would be expected at 90 minutes?
In some inherited disorders, a single hydrolytic enzyme normally found in lysosomes is missing. Which of the following is the most likely result in affected cells?
0 of 4 answered