Why Fresh Curd Always Needs a Spoonful of Old Curd

A bacterium, a banyan tree and a human being look nothing alike, yet all are built from cells. Anything smaller than a complete cell — a bit of membrane, a strand of DNA — cannot live independently.

This part covers cell theory, the cell as the unit of life, prokaryotic and eukaryotic cells, and the structures of a prokaryotic cell.

**Schleiden and Schwann's cell theory stated that all plants and animals are made of cells and products of cells; it could not explain how new cells form, so Virchow added that new cells arise only from pre-existing cells by division — Omnis cellula-e cellula — completing the modern cell theory.

Modern cell theory:

- All living organisms are made of cells and products of cells
- All cells arise from pre-existing cells

Why the addition mattered. The original theory described what bodies are made of, but left open how a body gets new cells. The added principle explained growth and reproduction through cell division and ruled out cells appearing from non-living matter.

An everyday example. To set fresh curd at home, you add a spoonful of old curd to warm milk — the new curd's bacteria all come from bacteria already present, just as every cell comes from an earlier cell.

The substance. "Products of cells" matters** — materials such as plant cell walls and bone matrix are not cells, yet they are part of living bodies.

The cell is the smallest unit that can carry out all life processes independently, and cells vary widely in size, shape and number because each is shaped by the job it performs.

Independent life. A single-celled organism such as Amoeba feeds, respires, excretes and reproduces on its own — nothing less than a whole cell can do this.

Size:

- Mycoplasma — among the smallest cells, about $0.3$ micrometres
- Bacteria — about $3$ to $5$ micrometres
- Human red blood cell — about $7$ micrometres across
- Nerve cells — among the longest cells
- The egg of an ostrich — the largest isolated single cell

Shape and function:

- Red blood cells — biconcave discs, giving a large surface for carrying oxygen
- Nerve cells — long and branched, to carry signals over distances
- White blood cells — amoeboid, changing shape to squeeze through vessels and engulf microbes
- Columnar and cuboidal cells — lining and secreting surfaces

An everyday example. A red blood cell is shaped like a coin pressed thin in the middle, which lets it bend through narrow capillaries.

The substance. Bigger organisms are not built from bigger cells — they have more cells.

Prokaryotic cells have no membrane-bound nucleus or membrane-bound organelles, carry naked circular DNA and 70S ribosomes, while eukaryotic cells have a nucleus enclosed by a nuclear envelope, many membrane-bound organelles, DNA organised with histone proteins into chromosomes, and 80S ribosomes in the cytoplasm.

Point-by-point comparison:

- Nucleus: no nuclear membrane — DNA lies in a region called the nucleoid — versus a true nucleus with a double membrane
- Membrane-bound organelles: absent, apart from membrane infoldings such as mesosomes, versus present — endoplasmic reticulum, Golgi apparatus, mitochondria, plastids
- Ribosomes: 70S versus 80S (though mitochondria and chloroplasts contain 70S ribosomes)
- Genetic material: a single circular DNA without histones, often with extra small DNA called plasmids, versus linear DNA wound with histones into chromosomes
- Cell division: binary fission versus mitosis and meiosis

Examples. Prokaryotes: bacteria, cyanobacteria, mycoplasma. Eukaryotes: protists, fungi, plants and animals.

An everyday example. An open-plan office, where everyone works in one hall, is like a prokaryote; an office divided into separate cabins is like a eukaryote with its compartments.

The substance. Plasmids can carry genes such as antibiotic resistance, and they are used as carriers in genetic engineering.

A prokaryotic cell is wrapped in a three-layered envelope — glycocalyx, cell wall and plasma membrane — and contains mesosomes that increase membrane surface, 70S ribosomes for protein synthesis, inclusion bodies that store reserves, and surface structures: flagella for movement and pili and fimbriae for attachment.

Cell envelope:

- Glycocalyx — outermost; a loose slime layer in some bacteria or a thick, tough capsule in others
- Cell wall — gives shape and strong support, preventing the cell from bursting
- Plasma membrane — selectively permeable, like that of eukaryotes

Bacteria that take up the Gram stain are Gram positive; those that do not are Gram negative.

Inside the cell:

- Mesosomes — infoldings of the plasma membrane forming vesicles, tubules and lamellae; they help form the cell wall, replicate and distribute DNA, and aid respiration and secretion
- 70S ribosomes — made of 50S and 30S subunits; several on one messenger RNA form a polysome
- Inclusion bodies — unbounded stores such as phosphate granules, cyanophycean granules and glycogen granules; gas vacuoles in some photosynthetic bacteria

On the surface:

- Flagella — filament, hook and basal body; for movement
- Pili — tubular protein structures
- Fimbriae — small bristle-like fibres that attach bacteria to rocks in streams and to host tissues

An everyday example. Plaque on teeth builds up as bacteria cling to the tooth surface and to one another using surface structures and sticky coatings.

The substance. S units measure how fast particles settle and do not simply add — which is why $50$S and $30$S subunits make a $70$S ribosome, not an $80$S one.