Biology · Introductory biology · Worked example
Predict a cell’s organelles from its job
Three human cells have different jobs. A pancreatic cell makes digestive enzymes, which are proteins, and releases them into the gut. A heart muscle cell contracts about once a second for a lifetime. A liver cell breaks down drugs and alcohol. Which organelles should each have in abundance, and why?
Follow the enzymes out of the pancreatic cell
Digestive enzymes are proteins made for export. Ribosomes on the rough ER build them, the Golgi apparatus modifies and sorts them, and vesicles carry them to the surface. A cell that exports a great deal of protein needs plenty of all three.
Find the energy for the heart cell
Each contraction uses ATP, and a heart cell contracts without rest. Mitochondria make most of a cell’s ATP, so heart muscle cells are packed with them, lined up beside the fibers that contract.
Match the liver cell to its chemistry
Many of the enzymes that break down drugs sit in the membranes of the smooth ER. A liver cell that clears drugs from the blood therefore has a great deal of smooth ER.
Check against the shared plan
All three are animal cells with the same kinds of organelles. They differ in the amount of each, which is what “structure fits function” means. None has a cell wall or chloroplasts.
Result
The pancreatic cell has abundant rough ER, Golgi apparatus and secretory vesicles, because it makes and exports protein. The heart muscle cell has abundant mitochondria, because it needs a steady supply of ATP. The liver cell has abundant smooth ER, where many drug-breaking enzymes work.
Your turn
1. One like the example
A plasma cell, a kind of white blood cell, releases large amounts of antibody, a protein, into the blood. Which organelles should it have in abundance?
Show the answer and explanation
Rough ER, Golgi apparatus and secretory vesicles.
Antibodies are proteins made for export, so they follow the same path as digestive enzymes. Ribosomes on the rough ER build them, the Golgi apparatus finishes and sorts them, and vesicles release them at the surface.
2. A variation
A sperm cell swims by beating its tail, which is driven by motor proteins that use ATP. Where would you expect its mitochondria, and why?
Show the answer and explanation
Packed around the base of the tail, in the midpiece.
The tail uses ATP every time it beats, and ATP does not travel far quickly. Mitochondria packed right where the tail begins supply it where it is spent.
3. Somewhere new
In a leaf, the cells of the surface layer are clear and have almost no chloroplasts, while the cells just beneath them are packed with chloroplasts. Explain the difference in terms of structure and function.
Show the answer and explanation
The surface cells protect the leaf and let light through, and the cells beneath carry out photosynthesis, so the chloroplasts are where the light is used.
Chloroplasts in the surface layer would absorb light before it reached the cells below. A clear surface passes the light on to the chloroplast-filled cells, where photosynthesis happens. As in animal cells, each cell holds the organelles its job needs.
Keep exploring
A secretory animal cell opens in Biology with three stretches of rough ER, two Golgi stacks and four secretory vesicles. Trace a protein from a ribosome to the plasma membrane.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.
- Milo and Phillips, Cell Biology by the Numbers — How big is a human cell?
- Milo and Phillips, Cell Biology by the Numbers — How big are mitochondria?
- Milo and Phillips, Cell Biology by the Numbers — How big are nuclei?
- Milo and Phillips, Cell Biology by the Numbers — How big is a budding yeast cell?
- OpenStax Biology 2e — Prokaryotic cells
- OpenStax Biology 2e — Eukaryotic cells
- OpenStax Biology 2e — The endomembrane system and proteins
Study it, then try it
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