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Stretch · Biology · Lesson 1

The Cell

About 30 trillion cells are holding your hand together, and not one is wide enough to see without a lens. The cell is the smallest unit that is alive on its own — and every single one came from a cell before it.
📜Cell Theory — Three Rules, Two Centuries
It took almost two hundred years and much better glass to get from “there are little boxes in cork” to the three statements every biology course still opens with.
1 All living things are made of one or more cells. 2 The cell is the basic unit of structure and function. 3 All cells arise from pre-existing cells. …and the road that got us there: 1665183818391855 Hooke slices cork, sees emptyboxes, names them cells Schleiden every plant tissueis built from cells Schwann bone, blood + nerve —animals too Virchow “omnis cellula e cellula”every cell from a cell
⚠️ Rule 3 is quietly enormous. It means every one of your ~30 trillion cells sits at the end of an unbroken chain of divisions running back billions of years, with no gaps. It also strands viruses outside the theory: genes and evolution, yes — but no membrane, no ribosomes, no metabolism, and no way to copy itself without hijacking a real cell.
🪓The Deepest Split in Biology
Every cell on Earth shares four things. After that, life forks: is the DNA loose in the cytoplasm, or sealed in a nucleus?
PROKARYOTE bacteria & archaea · 1–5 µm · always 1 cell DNA loose in the nucleoid — no compartment, one circular chromosome EUKARYOTE animals · plants · fungi · protists · 10–100 µm DNA sealed in a nucleus · many linear chromosomes plus a full set of membrane-bound organelles 🤝 EVERY cell on Earth has all four of these: a membranecytoplasmribosomesDNA holds it togetherthe inside fluidbuild the proteinscarries the instructions
Why compartments? Membranes let one cell run chemistries that would ruin each other. A lysosome holds digestive enzymes near pH 4.5 while the cytosol a few nanometres away stays close to neutral — and neither one destroys the other.
🏙️Tour the City: Eukaryotic Organelles
An organelle is literally a “little organ” — a structure with one specific job. Walk a eukaryotic cell like a working town.
1 2 3 4 5 6 7 8 1 Membrane the city wall — a fluid mosaic that decides who gets in 2 Nucleus city hall + archive. DNA never leaves — mRNA copies do 3 Ribosome factory floor: reads mRNA 3 letters at a time. No membrane, in every cell 4 Rough ER assembly line — folds and inspects proteins 5 Golgi the post office: tags, sorts and ships 6 Mitochondria power plant — food energy → ATP 7 Lysosome recycling plant 8 Cytoplasm — the streets everything travels through
🌿Same City, Three Extra Buildings
ANIMAL CELL no wall → free to change shape, crawl, engulf and contract PLANT CELL central vacuole (water + turgor pressure) rigid cellulose wall + full vacuole = a plant that stands up without a skeleton
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Plant-only: cell wall · chloroplasts · large central vacuole
Limp celery is a cell that has lost water to its surroundings. Stand it in fresh water and osmosis refills the vacuole in about an hour — the vacuole presses outward on the cellulose wall, and that pressure (turgor) is what “crisp” actually means. Fungi build walls too, but out of chitin, not cellulose. Animal cells have none.
plant wall = cellulosefungal wall = chitinanimal = no wall
📐Why Cells Stay Small: Surface vs Volume
A cell has to feed and drain its whole interior through its membrane. Grow too big and the arithmetic turns against you.
side = 1 surface = 6volume = 1 side = 2 surface = 24volume = 8 side = 3 surface = 54volume = 27 6 : 1 3 : 1 2 : 1 Surface grows with s² · volume grows with s³ · so the ratio = 6 ÷ s and always falls.
Structure predicts function — the most reliable habit of thought in all of cell biology. Folding a membrane inward cheats the arithmetic and buys back surface area: cristae in a mitochondrion mean more surface for making ATP, and stacked thylakoids in a chloroplast mean more surface for catching light.
🔬Magnification vs Resolution
Two different quantities, constantly confused. Magnification is how much bigger the image looks. Resolution is the smallest gap at which two points still read as two points — and light has a hard ceiling on it.
eyepiece 10× × objective 40× = 400× total magnification = eyepiece × objective …but pushing past ~1,000× only gives you a bigger blur. Here is why: ≈200 nm a light microscope CANNOT resolve here visible light is 400–700 nm wide a light microscope handles this fine ribosome~25 nm ✗ mitochondrion~1 µm ✓ (as a dot) bacterium1–5 µm ✓ eukaryote10–100 µm ✓ ⚡ Electron microscopes electron beams have far shorter wavelengths TEM = inside a slice · SEM = 3-D surface the specimen must be dead, in a vacuum 🏅 Super-resolution (Nobel 2014) fluorescence tricks now beat 200 nm — in cells that are still alive the edge of the visible was a tech problem
🦠The Power Plants Used to Be Alive
an ancestral cell meets a free-living bacterium it is engulfed — and never leaves The evidence (mitochondria + chloroplasts) ✔ two membranes, not one ✔ their own small circular DNA ✔ compact, prokaryote-style ribosomes ✔ reproduce by pinching in two → Lynn Margulis, 1967: endosymbiosis
🔑Key Terms
🔵CellThe smallest unit that can carry out all the processes of life on its own. Nothing smaller is alive by itself.
📜Cell theoryAll living things are made of cells · the cell is the basic unit of structure and function · all cells come from pre-existing cells.
⚙️OrganelleA specialised internal structure that does one job — a “little organ.” Most, not all, are wrapped in their own membrane.
🦠ProkaryoteNo membrane-bound nucleus; DNA coils loose in the nucleoid, usually one circular chromosome. Bacteria and archaea, 1–5 µm.
🧬EukaryoteDNA sealed in a nucleus plus a full set of membrane-bound organelles. Animals, plants, fungi, protists — usually 10–100 µm.
🏛️NucleusControl centre and archive: double membrane pierced by pores, holding the chromosomes and the nucleolus.
🔋MitochondriaRun cellular respiration, turning food energy into ATP. Inner membrane folded into cristae; carries its own DNA.
🏭RibosomeBuilds proteins by reading mRNA three letters at a time. No membrane — and present in every cell in every domain of life.
🧱MembraneA double sheet of phospholipids studded with proteins — the “fluid mosaic.” Selectively permeable: it decides what crosses.
🌱ChloroplastThe green organelle of photosynthesis. Chlorophyll on stacked thylakoids captures light; also carries its own DNA.
🌍Where You’ll See This in Real Life
💊Medicine & antibioticsAntibiotics work because prokaryotes and eukaryotes are built differently. Penicillin attacks peptidoglycan — a bacterial cell-wall molecule your cells never make. Tetracycline jams the compact bacterial ribosome and largely ignores your larger one. It is also why antibiotics do nothing against viruses.
🥬Farming & the produce aisleCrispness is turgor pressure. Growers, packers and those supermarket misters are all managing water so plant cells stay full and press against their walls. A limp stalk of celery is a cell that lost water — and osmosis reverses it in about an hour.
🍾Hooke named the cell in 1665 because cork’s empty compartments reminded him of cellulae — the bare little rooms where monks slept. He was drawing dead cell walls, and had no idea he had just named the basic unit of life.
👩‍🔬You inherit your mitochondrial DNA almost entirely from your mother, because sperm mitochondria are destroyed after fertilisation. Since it mutates at a fairly steady rate, geneticists use it to trace maternal ancestry back tens of thousands of years.
🔭The 200-nanometre light barrier was treated as a permanent law of physics for roughly a century. The 2014 Nobel Prize in Chemistry went to three scientists who found ways around it — super-resolution microscopy now images structures a few tens of nanometres across inside living cells.
📌Remember This
1Cell theory has three parts: all living things are made of one or more cells, the cell is life’s basic unit of structure and function, and every cell arises from a pre-existing cell.
2The deepest divide in cellular life is prokaryote vs eukaryote — DNA loose in the cytoplasm versus DNA sealed in a nucleus, with membrane-bound organelles that let one cell run many incompatible chemistries at once.
3Structure predicts function everywhere: folded cristae = more surface for ATP, stacked thylakoids = more surface for light, and a rigid wall plus a full vacuole = a plant that stands up without a skeleton.
🤔 Think about it
A virus has genes and it evolves — but no membrane of its own, no ribosomes, no metabolism. Does cell theory force us to call viruses non-living, or does it mostly reveal that “alive” is a boundary humans drew rather than one nature drew?
Mitochondria and chloroplasts look like captured bacteria that never left — and that merger seems to have happened at least twice. If cooperation this radical is a repeatable move rather than a fluke, what would you predict about complex cells that evolved somewhere other than Earth?
⭐ Remember: a cell is not a bag of parts, it is a city under a deadline. Every shape you see inside one — every fold, stack, pore and wall — is there because it solves a problem of surface, space or chemistry. Ask “what is this shape for?” and cell biology stops being memorisation.
✏️ ClickClass Anchor Chart · Stretch Biology 1 — The Cell: Life’s Smallest Unit
From ClickClass — hundreds of free printables at clickclassedu.com/printables