One genome. Two hundred cell types. How a single instruction set produces nerves, muscles and skin.
Every sub-topic below feeds at least one of these questions.
What are the roles of stem cells in multicellular organisms?
How are differentiated cells adapted to their specialized functions?
The required syllabus content for B2.3, in order. Each card is one lesson-sized checkpoint.
Production of unspecialized cells following fertilization and their development into specialized cells by differentiation
Properties of stem cells
Location and function of stem cell niches in adult humans
Differences between totipotent, pluripotent and multipotent stem cells
Cell size as an aspect of specialization
Surface area-to-volume ratios and constraints on cell size
Every differentiated cell in the body has the same DNA. Differences are about which genes are expressed.
A new individual starts as a single cell — the zygote, formed when sperm and egg fuse. The zygote divides repeatedly. Some of those daughter cells differentiate into the specialised cells that make up tissues. All differentiated cells carry the same genome; the differences arise because each cell type expresses a different subset of genes.
Inside an early embryo, signalling molecules called morphogens diffuse out from source cells. They form concentration gradients across tissues. Cells respond to the local morphogen concentration by switching specific genes on or off — which is how identical cells in the same embryo end up becoming nerve, muscle, skin, gut or bone.
All stem cells share two key properties: they can divide endlessly, and they can differentiate along multiple pathways.
Adult stem cells live in specialised tissue locations called niches, which maintain them and trigger their proliferation when needed.
Houses hematopoietic stem cells — multipotent cells that differentiate into every type of blood cell: red blood cells, all five white blood cell types, and platelets. Continuously replenishes the blood throughout life.
Contain multiple stem cell populations (epithelial, melanocyte, mesenchymal). They self-renew, drive hair growth cycles, replace skin cells, and contribute to wound healing.
Stem cell capability shrinks as development progresses.
Any cell type, including extra-embryonic tissues (placenta), and can form an entire embryo. Only the zygote and the first few divisions after fertilisation are totipotent.
Can differentiate into any of the body's 200+ cell types — but cannot form a complete embryo on its own. Embryonic stem cells from the blastocyst inner cell mass are pluripotent.
Can differentiate into a limited range of related cell types. Adult stem cells are multipotent — e.g. hematopoietic stem cells make all blood cells but not muscle or nerve.
Specialisation comes with size diversity — cells in the same body span four orders of magnitude.
Smallest human cells; streamlined for motility.
Small enough to squeeze through capillaries.
Largest cell by volume; carries cytoplasm and reserves.
Sciatic neurons run from spinal cord to toe.
Striated muscle fibres are multinucleate cells formed by fusion of many embryonic precursors — they can be centimetres long and 10–100 µm wide.
As cells grow, volume increases faster than surface area. Exchange across the surface can't keep up with the demands of the volume — so cells divide.
Think of a cube with side L. Surface area is 6L². Volume is L³. As L grows, volume grows faster than surface area, and the surface-to-volume ratio falls.
Surface area = the plasma membrane = the rate at which materials and heat can be exchanged with the environment. Volume = the cytoplasm = the rate of metabolism. When metabolism outpaces exchange, the cell can't sustain itself. Cells divide before they reach that point — keeping S:V high enough to support metabolism.
Real cells aren't cubes. But the ratio relationship works the same way for any shape — modelling cells with cubes (or playdough!) lets students see the scaling effect with simple arithmetic.
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Adaptations to increase surface area-to-volume ratios of cells
Adaptations of type I and type II pneumocytes in alveoli
Adaptations of cardiac muscle cells and striated muscle fibres
Adaptations of sperm and egg cells
Cells specialised for exchange evolve shapes that maximise surface area without increasing volume — flattening, microvilli, invagination.
Red blood cells are flat and dimpled rather than spherical. The biconcave shape maximises surface area for oxygen exchange per unit volume. Also makes the cell flexible enough to squeeze through capillaries.
Cells lining the proximal convoluted tubule of the nephron have densely packed microvilli on their luminal surface. The "brush border" massively increases surface area for reabsorption of glucose, amino acids, ions and water from the filtrate.
The alveolar epithelium contains two specialised cell types with different jobs. Each is adapted for its specific function — gas diffusion or surfactant production.
Extremely flat (often <0.1 µm thick) and elongated. Cover ~95% of the alveolar surface. Adaptation: thinness minimises the diffusion distance for O₂ and CO₂ across the alveolar wall.
Cuboidal cells, contain many secretory vesicles called lamellar bodies. Adaptation: lamellar bodies discharge surfactant (a phospholipid-protein mix) into the alveolar lumen. Surfactant reduces surface tension, prevents alveolar collapse, and creates a thin liquid film for gas dissolution.
Both have myofibrils. They differ in branching, length, and number of nuclei — adaptations to very different functions.
Cells are branched, joined to neighbours by intercalated discs containing gap junctions. The branching+gap-junction network allows electrical impulses to spread rapidly throughout the heart, synchronising contractions. Typically one nucleus per cell.
Long unbranched fibres formed by fusion of many embryonic muscle cells. Multinucleate — many nuclei distributed along each fibre — to support gene expression across the very long cell. Up to several centimetres long.
It's debatable. Because muscle fibres form by fusion of multiple cells, they don't fit the standard "one cell, one nucleus" picture. Some biologists consider them syncytia rather than cells. The IB treats them as atypical cells.
Sperm and ova are both haploid gametes — but they're optimised for completely different jobs.
Three regions:
Head contains the haploid nucleus and the acrosome (a cap of hydrolytic enzymes for penetrating the egg's outer layers).
Midpiece packed with mitochondria — provides ATP for swimming.
Flagellum — the tail — propels the sperm through the female reproductive tract.
Contains the haploid nucleus, plus extensive cytoplasm with lipid droplets to fuel the embryo's early divisions. Surrounded by the zona pellucida (a glycoprotein matrix). The cytoplasm holds cortical granules that release after fertilisation, hardening the zona pellucida and blocking polyspermy (multiple sperm entries).
If you can't define one of these in a sentence, that's where to revise next.
“What are the advantages of small size and large size in biological systems?”
“How do cells become differentiated?”