Organelles as a System

After a meal, the sugar level in your blood rises, and small cells in your pancreas called beta cells release insulin, a protein hormone. Each beta cell keeps insulin packed in tiny sacs, ready to send out. The instructions for insulin are stored in the cell’s nucleus, but the hormone leaves at the cell’s outer edge. How does it get from one to the other, and what do the other cell parts do while it travels?

Explore: follow a protein shipment and switch parts off

In the protein shipping route you follow one shipment of insulin through a model beta cell, one step at a time, from the instructions in the nucleus to release at the cell membrane. You can switch off any one cell part and run the route again. A meter shows the ATP supply from the mitochondria.

The orange dot is the insulin shipment. Dashed lines show ATP from the mitochondria.

Switch off one cell part

none (all on) nucleus ribosomes and rough ER Golgi apparatus mitochondria lysosomes

Next step Reset

ATP supply from the mitochondria (relative units, not to scale)

In this sketch the shipment moves one stop at a time, only one part is off at once, and a part on the route that is off stops the shipment before its step, and with the mitochondria off it stops at the rough ER. Rule to keep: a secreted protein goes from the nucleus to a ribosome on the rough ER, by vesicle to the Golgi, and out through the membrane, with mitochondria supplying ATP. This is a sketch of the logic, not a measurement.

Try this.

1. Run the route with every part switched on. Before you start, predict how many cell parts the shipment passes through. Then count.
2. Predict where the shipment will stop if you switch off the Golgi apparatus. Then test it.
3. Switch the Golgi back on and switch off the mitochondria. Predict what happens to the shipment, then run it.
4. Switch off the lysosomes instead. Predict whether insulin still leaves the cell, and what might build up if the cell ran this way for days.

Explain: the parts of a cell and their jobs

An organelle is a structure inside a cell that does a particular job; most are wrapped in their own membrane. The Virginia list names eleven cell structures. The table gives each one’s job for a typical plant cell and a typical animal cell.

Structure	Main job	Plant cells	Animal cells
Cell membrane	Thin outer layer that controls what enters and leaves the cell	Yes	Yes
Cell wall	Stiff layer outside the membrane that supports the cell	Yes	No
Chloroplast	Carries out photosynthesis, storing light energy in sugar	In green cells, such as leaf cells	No
Cytoplasm	Everything inside the membrane but outside the nucleus; many reactions happen here	Yes	Yes
Nucleus	Holds most of the cell’s DNA, its instructions	Yes	Yes
Ribosome	Builds proteins	Yes	Yes
Endoplasmic reticulum (ER)	Folded membranes; rough ER, dotted with ribosomes, folds new proteins for export, and smooth ER makes lipids	Yes	Yes
Golgi apparatus	Sorts and packs proteins and sends them on	Yes	Yes
Lysosome	Sac of digestive enzymes that breaks down worn-out cell parts	Usually not; the vacuole does similar digesting	Yes
Mitochondria	Release energy from sugar and store it in ATP	Yes	Yes
Vacuole	Sac that stores water and other materials	One large central vacuole that presses on the wall and keeps the cell firm	Small ones, in some cells

Two cautions. Plant cells have mitochondria as well as chloroplasts. And not every plant cell has chloroplasts: a root cell underground is not green.

Explain: following insulin through the cell

A list of parts does not show how they work together. Follow insulin instead.

1. Nucleus. The gene for insulin is a stretch of DNA. The cell copies it into messenger RNA, which leaves the nucleus. You can read how in that lesson.
2. Ribosome on the rough ER. A ribosome reads the messenger RNA and builds a protein chain. The chain threads into the ER, where it folds. In a beta cell this chain is an insulin precursor, a longer protein that is cut into insulin later.
3. To the Golgi. A vesicle, a small bubble of membrane that carries materials, buds off the ER and delivers the protein to the Golgi apparatus.
4. Golgi and storage. The Golgi sorts the protein and packs it into new vesicles. Insulin is stored in these until it is needed.
5. Out through the membrane. When blood sugar rises, more of these vesicles fuse with the cell membrane and release insulin outside the cell. Beta cells also release a smaller amount between meals.

Building a protein chain and moving vesicles both use energy. ATP is a small molecule that carries energy in a form cell parts can use; that lesson shows how mitochondria make it. In a 1968 experiment on pancreas cells, chemicals that blocked the mitochondria’s ATP making also stopped proteins from moving from the rough ER to the Golgi.

A factory is a useful model. The nucleus is the office that keeps the blueprints, ribosomes are the assembly line, the ER and Golgi are finishing and shipping, vesicles are delivery trucks and mitochondria are the power plant. The model breaks in three places. No manager gives orders; each step follows from chemistry. A power plant does not create energy from nothing, and neither do mitochondria; they move energy from sugar into ATP. And a cell’s parts move around and touch one another all the time.

Explain: cells with and without a nucleus

A prokaryotic cell has no nucleus and none of the membrane-bound organelles in this lesson; bacteria are prokaryotes. A eukaryotic cell has a nucleus and membrane-bound organelles; plants, animals and fungi are eukaryotes.

Feature	Prokaryotic cell	Eukaryotic cell
DNA	In an open region called the nucleoid	Inside a nucleus
Membrane-bound organelles such as ER, Golgi and mitochondria	None	Yes
Cell membrane, cytoplasm, ribosomes	Yes	Yes
Cell wall	Often	Plants and fungi, not animals

A single bacterium still carries out every life process with this simpler set of parts.

Eukaryotic cells with different jobs hold different amounts of each organelle. In one study of mouse muscles, mitochondrial content differed nearly tenfold between muscle types. In one study, normal Arabidopsis leaf cells held 121 chloroplasts each.

Elaborate: tracking a protein and counting organelles

Jamieson and Palade marked newly made proteins in slices of guinea pig pancreas with a 3-minute radioactive pulse. Then they counted radioactive grains over each cell part in electron microscope pictures.

Minutes of chase after the pulse	Cell part	Share of counted grains in that part (%)
0	Rough ER	86.3
7	Golgi vesicles	43.0
17	Condensing vacuoles (forming storage granules)	19.5
37	Condensing vacuoles	48.5
57	Storage (zymogen) granules	32.9
117	Storage (zymogen) granules	58.6

Source: Jamieson and Palade, Journal of Cell Biology, 1967, paper II, Table II. These were the pancreas cells that make digestive enzymes, not beta cells. Only some times and parts from the full table are shown, so the rest of each time’s grains were in other parts.

Cells measured	What was measured	Value
Heart muscle cells, normal mice (3 mice)	Mitochondria, % of cell volume	30.2 ± 1.1
Heart muscle cells, GENA348 disease-model mice	Mitochondria, % of cell volume	41.1 ± 2.7
Liver, 5 normal rats	Liver-cell mitochondria, % of whole liver volume	18
Arabidopsis leaf cells, normal plants	Chloroplasts per cell	121
Arabidopsis leaf cells, arc5 mutant	Chloroplasts per cell	13

Sources: Rajab and colleagues, Scientific Reports, 2022 (six-month-old mice); Weibel and colleagues, Journal of Cell Biology, 1969; Pyke and Leech, Plant Physiology, 1994.

1. In the first table, put the cell parts in the order the label reached them.
2. The pulse lasted 3 minutes. How many minutes after the pulse began were the grains counted in the 7-minute row?
3. Why is comparing 30.2% in heart cells with 18% in liver not a fair, matched comparison?

Design problem

Do leaves grown in brighter light have more chloroplasts per cell? Plan an investigation with a light microscope. Name your variables, constants, control, sample size and safety steps.

One way to do it

• Independent variable: light level. Grow plants of one species from the same seed batch, some in bright light and some in dim light.
• Dependent variable: chloroplasts counted per cell in a thin leaf sample under the microscope.
• Constants: species, plant age, leaf position, watering, temperature, magnification and the counting rule.
• Control: the dim-light plants are the comparison group for the bright-light plants.
• Sample size and trials: several plants per group, several leaves per plant, and at least 20 cells per leaf. Report the mean and the spread.
• Fairness: a partner labels the slides with codes, so the counter does not know the group.
• Limit: one focal plane hides chloroplasts above and below it, so record that you counted one plane.
• Safety: handle slides and cover slips carefully and report broken glass.

Evaluate: check your thinking

Insulin’s instructions stay in the nucleus as DNA, and a messenger RNA copy carries them to ribosomes on the rough ER. The new protein folds in the ER, travels by vesicle to the Golgi, is packed into storage vesicles, and leaves when a vesicle fuses with the cell membrane. Mitochondria supply the ATP that powers the work along the way.

Science practices you used: using a model and naming its limits, reading data tables with means and their spread, doing a time calculation from data, and planning a fair test with a large enough sample.