Cell কী?

Example 29.3 (How small is small?)

A typical cell of your body measures about a fiftieth of a millimetre. Line up $50$ of them and they span one millimetre — a pencil dot could cover the parade. That is why nobody suspected cells for most of human history: the building blocks of life are smaller than the eye’s smallest visible speck.

Example 29.5 (Two classics under the lens)

Two observations any school microscope can repeat: a shred of the thin skin inside an onion shows neat rows of brick-like cells, wall against wall; a gentle scrape from the inside of a cheek shows scattered, rounder cells — yours. Plant and animal, side by side, both cellular.

Example 29.8 (From one to trillions)

The fertilized egg divides into $2$ cells, then $4$, $8$, $16$ … Repeated division, over months and years, built the tens of thousands of billions of cells reading this sentence. Every cell of you descends from that first one.

Example 2.3 (One cell or many)

A yeast, a paramecium or a bacterium is a single cell that does everything — feeds, moves, divides. An oak or a human is a cooperative of cells of many kinds, each doing a share of the work: a human body contains some $3 \times 10^{13}$ cells, of roughly two hundred kinds. Between the two lie colonies and simple multicellular organisms, but there is no living thing built of anything other than cells.

Example 2.7 (Cheek cells)

A cheek cell drawn $24\,\mathrm{mm}$ wide from a $\times 400$ image is $24/400 = 0.06\,\mathrm{mm} = 60\,\text{µ}\mathrm{m}$ across, and its nucleus, drawn $3\,\mathrm{mm}$, is about $8\,\text{µ}\mathrm{m}$. A red blood cell is $7\,\text{µ}\mathrm{m}$; the bacteria of the mouth, $1\,\text{µ}\mathrm{m}$ long, appear as barely resolved dots at the same magnification.

Example 2.10 (Reading a bacterium)

An electron micrograph of the gut bacterium Escherichia coli shows a rod $2\,\text{µ}\mathrm{m}$ long and $0.5\,\text{µ}\mathrm{m}$ wide: a wall, a membrane just inside it, a granular cytoplasm packed with ribosomes, and a paler central region where the single circular DNA molecule lies. No nucleus, no mitochondria. Yet the cell feeds, grows and divides every twenty minutes in a rich broth: everything the cell theory demands, in a thousandth of the volume of a cheek cell.

Example 5.3 (One cell, many cells)

A bacterium, a yeast, an amoeba are whole organisms of one cell, doing everything — feeding, moving, dividing — within a few micrometres. A human is some $3 \times 10^{13}$ cells of two hundred kinds, none of which could live alone in a pond, each of which descends by unbroken divisions from one fertilised egg. The rabbit of Chapter 2 and the oak of Chapter 3 are the same theory applied twice.

Example 5.7 (Why the eukaryotic cell is compartmented)

A cell of $20\,\text{µ}\mathrm{m}$ has a thousand times the volume of a bacterium but only a hundred times its membrane surface (Proposition 1.6). Membranes are where a cell carries its energy-transducing chains and its transport systems; the eukaryote recovers surface by folding membranes inside itself — the inner membrane of a mitochondrion, the sheets of the endoplasmic reticulum — and gains, besides, separate chambers in which incompatible chemistries (digestion in the lysosome, synthesis in the cytosol) can run side by side.

Example 5.11 (A bacterium and a mitochondrion)

Under the light microscope at $1000\times$, an E. coli cell is a rod $2\,\mathrm{mm}$ long on the retina’s scale, with no visible interior; a mitochondrion is a dot at the limit of resolution. In a TEM section the same bacterium shows its two membranes, its wall, its nucleoid as a pale fibrous region and its ribosomes as dark grains; the mitochondrion shows its double membrane and the folds of the inner one. Neither is alive.