Cell organisation and organelles
Introduit| English | Sens |
|---|---|
| ribosome/ˈriːbəʊsəʊm/ | A cellular structure that joins amino acids into a polypeptide using information from messenger RNA. |
| mitochondrion | mitochondrion |
| nucleus/ˈnjuːklɪəs/ | A membrane-bound organelle in eukaryotic cells that contains most of the genetic material. |
| surface area/ˈsɜːfɪs ˈeərɪə/ | The total area of the outer surfaces of a three-dimensional object. |
| rough endoplasmic reticulum/rʌf ˌendəʊˈplæzmɪk reˈtɪkjʊləm/ | rough endoplasmic reticulum |
| Golgi/ˈɡɒlɡɪ/ | Golgi |
| lysosome/ˈlaɪsəʊsəʊm/ | lysosome |
| storage disease | storage disease |
A factory of compartments
- A cell is not one bag of chemicals. It is a factory of rooms, and each room keeps its own pH, enzymes and surface.
- Medicine reads a cell like an engineer reads a plant diagram: what lives where, and what breaks when one room fails.
The size ladder
- A ribosome 核糖体 is 25 nm, a mitochondrion 线粒体 2 µm, the nucleus 细胞核 6 µm and the whole cell 20 µm.
- That is three orders of magnitude. Function follows size: anything the whole cell must do fast needs a membrane to do it on.
Surface against volume
- Transport scales with surface area 表面积, $4\pi r^{2}$. Demand (protein synthesis, ATP use) scales with volume, $\tfrac{4}{3}\pi r^{3}$.
- Divide: $SA/V = 3/r$. Double the radius and every unit of cytoplasm gets half the membrane service.
Doubling a cell's radius changes its surface-to-volume ratio how?
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Area grows as r², volume as r³, so SA/V = 3/r: doubling r halves the ratio. This is the argument that caps cell size.
Worked example: why big cells are rare
- A 20 µm liver cell ($r = 10$ µm): $SA/V = 3/10 = 0.3$ per µm. A 200 µm oocyte: $3/100 = 0.03$ per µm, ten times worse.
- So large cells fold their membranes: the ER packs surface into the cytoplasm, and the gut lining is villi, not a flat sheet.
A spherical cell is 60 µm across. What is its surface-to-volume ratio, in "per µm"?
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r = 30 µm, so SA/V = 3/30 = 0.1 per µm: a third of a 20 µm cell's 0.3.
Put these in order of size, smallest first.
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The ladder spans 20 µm / 25 nm = 800: a ribosome is as small against a cell as a cell is against a 1.6 cm marble.
One room, one job
- The rough endoplasmic reticulum 糙面内质网 is studded with ribosomes and makes proteins for export. The Golgi 高尔基体 sorts and glycosylates them.
- Lysosomes 溶酶体 keep pH near 4.8 to take cargo apart; mitochondria make ATP. Counts vary: a liver cell has about 1000 mitochondria, a resting lymphocyte few.
Match each organelle to its job.
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Each room has the enzymes and conditions for one job, so a failure points to one room.
Two metres in a nucleus
- The genome is 6.4 × 10⁹ base pairs at 0.34 nm each: 2.2 m of DNA inside a 6 µm nucleus.
- That length is about 4 × 10⁵ times the nucleus's width, so the DNA must be packed in stages (unit 1.3 shows how).
2.2 m of DNA sits in a 6 µm nucleus. How many times the nucleus's width is the DNA's length, in units of 10⁵?
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2.2 m = 2.2 × 10⁶ µm, and 2.2 × 10⁶ / 6 ≈ 3.7 × 10⁵: a few-hundred-thousand-fold linear compression.
Worked example: the lysosome that fails
- A (fictional) patient with slowly worsening muscle weakness lacks one lysosomal hydrolase. The substrate cannot be taken apart at pH 4.8.
- It builds up, the lysosome swells and the cell fails to recycle. One missing enzyme in one room is one storage disease 贮积病: the organelle map is the diagnosis map.
Why does one missing lysosomal hydrolase cause disease?
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Lysosomes take cargo apart at pH 4.8; one missing enzyme means one undigested substrate, which is one storage disease.
Three common slips
- "The cytoplasm is well mixed." Rooms differ in pH, calcium and redox state by design.
- Fixed organelle counts. Numbers follow demand: a busy liver cell and a resting lymphocyte differ widely.
- Forgetting the membrane. It is where the cell meets the world, and its area sets the cell's size limit.
The cytoplasm is one well-mixed compartment with the same pH everywhere.
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The cytosol is near pH 7.2 while lysosomes sit near 4.8; calcium and redox state also differ between rooms by design.
Recap
- Organelles are reaction rooms on a 25 nm to 20 µm ladder.
- $SA/V = 3/r$ caps cell size and explains folded membranes.
- 2.2 m of DNA fits in a 6 µm nucleus; one failed room is one disease.