Ch 6Cell: The Unit of Life — Class 11 Botany, concept-first.

The word cell traces back to the Latin cella, meaning a small chamber or compartment. It entered biology when Robert Hooke used the term around 1662 to describe the tiny box-like compartments he saw i…

The idea that living things are built of small, discrete units took shape gradually over two millennia.

A microscope is the indispensable tool of cell biology, because almost everything this chapter describes is far too small to see with the unaided eye.

The bright field microscope is the ordinary compound microscope used in almost every school and college laboratory, and it is the workhorse instrument for routine cell observation.

The dark field microscope, invented by Richard Zsigmondy in 1905, solves a very specific viewing problem: instead of showing a bright specimen against a bright background (which can wash out fine deta…

The phase contrast microscope, invented by Frits Zernike in 1935, is built on the same basic optical principles as an ordinary light microscope but adds one clever modification that lets it reveal det…

The electron microscope, first built by Ernst Ruska in 1931 and substantially advanced by Gerd Binnig and Heinrich Rohrer in 1981, replaced visible light with a beam of accelerated electrons as its il…

In 1833, the German botanist Matthias Schleiden and the German zoologist Theodor Schwann independently studied plant and animal tissue and arrived at the same striking conclusion: every plant and ever…

No scientific generalisation in biology is completely without exception, and cell theory is no different. Viruses, along with the related viroids and prions, are the acknowledged exceptions.

The cell doctrine (sometimes called the cell principle) is essentially a checklist that gathers together the core statements that, taken as a set, make up the modern cell theory.

Long before anyone had identified individual organelles, biologists were already convinced that a cell's living material - whatever it was made of - deserved a name and a theory of its own.

Cells are dramatically variable, both in size and in shape, and that variability tracks both the organism they belong to and the specific job the cell has to do within that organism.

Once cells are accepted as the basic unit of every organism, the next natural question is how to sort cells into meaningful categories.

The word 'prokaryote' comes directly from Greek - pro meaning primitive and karyon meaning nucleus - and that etymology is a fair description of the group's defining feature.

In 1966, the scientist J. D. Dodge and his co-workers described an unusual group of organisms that did not fit cleanly into either the prokaryote or eukaryote category, and proposed a distinct third g…

'Eukaryote' comes from the Greek Eu, meaning true, and karyon, nucleus - and a eukaryotic cell is defined precisely by having a true nucleus, with its DNA properly enclosed inside a nuclear membrane.

Having established that eukaryotic cells are internally complex, the chapter now compares two very familiar eukaryotic cell types side by side: the animal cell and the plant cell.

Comparing an animal cell and a plant cell side by side is a useful way to see both what eukaryotic cells share and where they genuinely diverge.

Protoplasm is the collective name for all of the genuinely living material inside a cell, everything that sits within the boundary drawn by the plasma membrane.

The cell wall is the outermost, protective covering of a cell, and it was, historically, the very first cell structure ever seen - Robert Hooke was looking at cork cell walls, not living cells, when h…

The cell membrane - also called the cell surface membrane or plasma membrane - is the thin boundary that holds in a cell's cytoplasmic content (its cytosol); it is remarkably thin, well under 10 nanom…

With the outer boundary and general contents of the cell established, the chapter now turns to the individual working parts inside it - the cell organelles.

The endomembrane system is a useful organising concept rather than a single physical structure: it is the collective name given to the group of internal membranes within a eukaryotic cell that are fun…

The endoplasmic reticulum, or ER, is the largest of all the cell's internal membrane systems, and it was named by K. R. Porter in 1948.

In 1898, the Italian scientist Camillo Golgi, examining nerve cells under the microscope, saw a net-like tangle of fine fibrils sitting close to the nucleus; this structure was later named the Golgi b…

Mitochondria were first seen by A. Kolliker in 1880, initially named 'Bioplasts' by Richard Altmann in 1894, and finally given their current name, mitochondria, by Carl Benda in 1897-98.

The word 'plastid' comes from the Greek Platikas, meaning formed or moulded, and was introduced into biology by A. F. W. Schimper in 1885.

Chloroplasts are the organelle that makes green plants green, and they are found across green algae and higher plants.

Ribosomes were first observed as small, dense particles or granules under the electron microscope by George Palade in 1953.

Lysosomes were discovered by Christian de Duve in 1953, and their nickname - 'suicidal bags' - hints at exactly what makes them potentially dangerous to the cell that contains them.

Peroxisomes were formally identified and named as a distinct kind of organelle by Christian de Duve, the same scientist who discovered lysosomes, in 1967.

The glyoxysome was discovered by Harry Beevers in 1961. Like the peroxisome, it is a single-membrane-bound sub-cellular organelle, but its enzyme content is different and more specialised: a glyoxysom…

Microbodies is a broad, umbrella term used for the many enzyme-carrying, membrane-enclosed vesicles found scattered through the cytoplasm of eukaryotic cells.

A sphaerosome, as its name suggests, is spherical in shape and — like the microbodies covered just before it (6.6.11) — is bounded by a single unit membrane, not a double envelope.

A centriole is one of the very few organelles in this chapter that has no surrounding membrane at all - it is described as a non-membranous organelle.

In plant cells, vacuoles are typically large, fluid-filled sacs, each bounded by its own single unit membrane called the tonoplast.

The nucleus is, in every meaningful sense, the control centre of a eukaryotic cell: it is the single largest organelle in the cell, and it is the structure that both controls the cell's overall activi…

Chromosomes were first reported inside eukaryotic cells by Eduard Strasburger in 1875, though the actual term 'chromosome' was only coined thirteen years later, by Wilhelm Waldeyer in 1888.

Motility is a defining feature of many cells, and this section covers the whip-like structures that make it possible: flagella.

Bacterial flagella are long, helical, whip-like appendages that give many bacteria their motility, and they are considerably thinner than the flagella or cilia found in eukaryotic cells.

A eukaryotic flagellum is a considerably more elaborate structure than its bacterial counterpart, and it is powered completely differently.

Cilia are, in essence, a shorter, more numerous version of the same basic structure as a flagellum. The plural form is cilia, and the singular is cilium; unlike the flagellum, which typically occurs s…

The final section of the chapter steps back from cell structure itself and looks instead at the practical laboratory workflow used to actually study cells under a microscope - covering, in turn, how a…

Before any specimen can be examined under a microscope, it has to be mounted onto a slide, and the exact mounting method depends on what part of the specimen actually needs to be seen.

Once a specimen has actually been observed under the microscope, that observation needs to be recorded somehow so it can be studied or compared later, and there are two standard ways of doing this.

Staining is one of the most important practical techniques in cell biology, because it is often the only way to actually see different components of a cell clearly at all - each part of the cell has a…

This closing Evaluation set of ten items checks understanding across the full sweep of the chapter: it opens with five objective-type questions covering ribosome subunit cohesion, the use of rRNA sequ…

Sample papers and previous-year board questions for this subject.