The Cell as a Working System

All organisms are made of cells; the cell is the smallest unit of life; every cell comes from a pre-existing cell — an unbroken lineage back ~3.8 billion years.

Each lipid has a water-loving phosphate head and water-fearing fatty tails, so in water they self-arrange tails-inward, heads-outward. Thermodynamics builds it for free; the sheet is ~5 nm thick.

The membrane behaves like a two-dimensional fluid in which embedded proteins drift around like boats on a pond.

Channels (let specific ions flow down gradients), pumps (burn ATP to move ions uphill), and receptors (detect outside signals and change shape to pass the message in).

Stored concentration/charge differences can be released to do work — nerve impulses, nutrient import, and ATP synthesis. A neuron fires when channels open and the pump-built gradient collapses in a millisecond.

Prokaryotes: 1–5 µm, one open compartment, looped DNA in the cytoplasm. Eukaryotes: 10–100 µm, with internal membranes creating specialized compartments.

Nucleus archives DNA and exports RNA copies; ribosomes translate them; rough ER folds and quality-checks export/membrane proteins; Golgi adds sugar tags and sorts proteins into addressed vesicles.

Mitochondria (and chloroplasts) have their own small circular genome and a double membrane — relics of a free-living bacterium engulfed ~1.5–2 billion years ago.

Its digestive enzymes are bagged behind a membrane and only function at pH ~4.5, while cytoplasm sits near 7.2 — a double safety catch.

Actin filaments and microtubules give shape, pull chromosomes apart during division, and act as rails for motor proteins that walk cargo across the cell.