Cells: The Unit of Life
The Cell Theory
The foundational claim of modern biology, agreed on by the mid-1800s:
- All living things are made of cells
- Cells are the basic unit of structure and function in life
- All cells come from other cells
A virus is an edge case. It has genetic material and proteins, but it cannot reproduce without hijacking a cell. Most biologists don't count viruses as alive; some do. The debate is old, mostly definitional, and not worth settling here.
Everything else, from bacteria to blue whales, is made of cells. A human has around 30 trillion.
Two Kinds of Cells
The biggest split in biology is between prokaryotes and eukaryotes.
Prokaryotes no nucleus; DNA floats in the cytoplasm
small (1 to 10 micrometres typically)
bacteria and archaea
Eukaryotes DNA packaged in a membrane-bound nucleus
larger (10 to 100+ micrometres)
protists, fungi, plants, animals
Eukaryotes are a more recent evolutionary development (about 2 billion years ago). They differ from prokaryotes in one enormous way: they have membrane-bound organelles, including a nucleus. That organisation enables the complexity that multicellular life requires.
Everything that is not a bacterium or an archaean is a eukaryote. You, a yeast, an oak tree, and a mushroom are closer relatives to each other than any of them is to a bacterium.
Size and Scale
Worth internalising a rough sense of cellular scale:
Atom ~0.1 nm
DNA helix diameter 2 nm
Protein (average) 5-10 nm
Ribosome 25 nm
Virus 20-300 nm
Bacterium 1-10 micrometres (1,000-10,000 nm)
Mitochondrion 1-2 micrometres
Human cell 10-30 micrometres
Human egg cell ~100 micrometres
Ostrich egg (cell) ~15 centimetres (outlier)
Neuron (length) up to 1 metre (extreme outlier)
Cells are tiny. A single millimetre cube of your skin holds a few hundred thousand cells. A single drop of blood holds millions.
The Prokaryotic Cell
A bacterium is a small bag of aqueous chemistry surrounded by a membrane and often a cell wall.
Inside a bacterium:
- Cell membrane: a lipid bilayer separating inside from outside. Selectively permeable
- Cell wall: outside the membrane, rigid, made of peptidoglycan in bacteria. Absent in animal cells
- Cytoplasm: the watery interior with dissolved molecules and organelles
- Nucleoid: the region where the circular DNA lives. Not membrane-bound
- Ribosomes: protein-making machines
- Plasmids: small extra circles of DNA, often carrying antibiotic resistance or other "extras". Hugely important in biotech because they're the vehicle for gene transfer
- Flagella or pili: propulsion or attachment structures, if present
That's most of what a bacterium is. Simple, fast-reproducing, staggeringly diverse as a group. E. coli can divide every 20 minutes under good conditions. Most antibiotics work by targeting structures bacteria have and animal cells don't.
The Eukaryotic Cell
Eukaryotes are more elaborate. Inside an animal cell:
Nucleus
A membrane-bound compartment holding the DNA. The nucleus has its own membrane (the nuclear envelope) with pores that regulate traffic. Inside is the chromatin (DNA wrapped around histone proteins) and the nucleolus, where ribosomes are assembled.
Mitochondria
The power plants. They convert nutrients into ATP, the molecule cells use as energy currency. Mitochondria are weird: they have their own small DNA genome, separate from the nuclear DNA, and they divide independently. The leading theory is that they were once free-living bacteria that a larger cell absorbed; the partnership worked and stuck. This is the endosymbiotic theory, and it's well-supported.
Mitochondrial DNA is inherited only from the mother, which makes it useful for tracing maternal ancestry.
Ribosomes
Protein-assembly machines. Some are free-floating; some are attached to the endoplasmic reticulum (see below). Both prokaryotes and eukaryotes have ribosomes, though the sizes differ slightly. Many antibiotics target bacterial ribosomes specifically.
Endoplasmic reticulum (ER)
A network of folded membranes inside the cell. Comes in two flavours:
- Rough ER: coated with ribosomes. Makes proteins destined for export or for membranes
- Smooth ER: no ribosomes. Handles lipid synthesis, detoxification, and calcium storage
Golgi apparatus
The shipping and sorting department. Proteins that leave the ER come here, get modified (often with sugars attached), and are sorted for their destinations. Named after Camillo Golgi, not a company.
Lysosomes
The recycling centres. Membrane-bound bags of digestive enzymes that break down old proteins, damaged organelles, and invaders. Leaky lysosomes cause problems (lysosomal storage diseases like Tay-Sachs).
Peroxisomes
Small organelles that break down specific molecules and neutralise some toxins.
Cytoskeleton
A network of protein filaments giving the cell shape and letting it move and divide. Three kinds:
- Microtubules (for transport, cell shape, the spindle during division)
- Actin filaments (for shape changes, movement, contraction in muscle)
- Intermediate filaments (for structural strength)
Cell membrane
A lipid bilayer. Studded with proteins that act as channels, pumps, receptors, and identity markers. The membrane is a major drug target; a huge fraction of pharmaceuticals act on membrane proteins.
Plant Cells (and Why They Differ)
Plant cells have everything animal cells have, plus:
- Cell wall: rigid, made of cellulose. Gives plants their shape
- Chloroplasts: do photosynthesis (converting light to chemical energy). Like mitochondria, they have their own DNA and appear to have been absorbed bacteria (cyanobacteria)
- Large central vacuole: a big membrane-bound storage compartment, often taking up most of the cell's volume
Cell Division
Cells come from cells. Two main ways:
Mitosis
One cell becomes two identical cells. This is how you grow, heal, and replace cells. The DNA is duplicated, the cell splits, and each daughter gets a full genome. Uncontrolled mitosis is cancer.
Meiosis
One cell becomes four cells, each with half the DNA. This is how sperm and eggs are made. Meiosis also shuffles the genetic material (recombination), which is where much genetic variation comes from.
Cell Types in a Multicellular Organism
A multicellular organism like a human has around 200 distinct cell types. All have the same DNA; they differ in which genes they express.
- Neurons: signal over long distances
- Muscle cells: contract
- Immune cells: defend
- Epithelial cells: line surfaces, absorb, secrete
- Red blood cells: carry oxygen (and are weird: they expel their nucleus before going to work)
- Stem cells: can divide and differentiate into other types
Cell type is determined by which genes are on and off. This is regulated by complex networks that chapter 4 touches on.
What This Means for Biotech
Almost every biotech tool interacts with cells:
- Drugs target proteins, often membrane-bound proteins
- Vaccines train immune cells to recognise pathogens
- Gene therapies get modified DNA into cells
- Cell therapies (CAR-T, stem-cell therapy) modify cells outside the body and return them
- Diagnostic tests look at cells or their products
Understanding what's happening inside a cell is usually the first step to understanding why a biotech tool does (or doesn't) work.
Common Pitfalls
"Bacteria are simple." Small, but not simple. A single E. coli runs thousands of genes, responds to its environment, and handles stress in sophisticated ways
"Eukaryotes are more evolved than bacteria." Bacteria have been evolving just as long. They're adapted to different niches. "More evolved" is not a meaningful phrase
"A cell is like a factory." Useful analogy; breaks down at the details. Factories don't randomly self-assemble, they don't have their parts competing with each other, and they don't replicate themselves from scratch every 20 minutes
"Cells are mostly water." Mostly. Around 70% water by mass. The remaining 30% is a crowded dense soup of proteins, nucleic acids, lipids, and sugars. "Bag of water" understates the complexity
"Organelles are discrete like Lego pieces." Less than they look. Organelles exchange material constantly, often through contact sites. The picture in the textbook is a simplification
Next Steps
Continue to 03-dna-rna-proteins.md for the information flow inside every cell.