What the Cell

DP Biology · links across the course

Nature of Science

How biological knowledge is built and tested, with one real example for each subtopic.

SL path

Unit 1 Body Systems

Subtopic	Aspect	Where it comes into play
B3.1 (Animal)	Measurements	Spirometers turn breathing into numbers, but results vary with age, height, effort and the device, so a single reading needs careful interpretation.
B3.2 (Animal)	Evidence	In 1628 William Harvey argued that blood circulates, using measurements and reasoning. He could not see capillaries; Malpighi observed them decades later.
C2.2	Experiments	Hodgkin and Huxley recorded nerve impulses from inside squid giant axons, because the large size made measurement possible.
C3.1	Experiments	In 1902 Bayliss and Starling showed that a chemical, secretin, could trigger pancreatic secretion with nerves cut, revealing control by hormones.
D3.3	Global impact of science	Isolating insulin in 1921 turned type 1 diabetes from a fatal disease into a treatable one, saving millions of lives.
C3.2	Observations	In the 1840s Ignaz Semmelweis noticed higher death rates where doctors came straight from autopsies, and found that handwashing reduced them.
D3.1 (Human)	Global impact of science	Research on reproductive hormones led to contraception and, in 1978, the first baby born through IVF, changing many people's lives.

Unit 2 Plant Systems

Subtopic	Aspect	Where it comes into play
B3.1 (Plant)	Evidence	Researchers found that stomatal density falls as carbon dioxide rises, so fossil leaves can be used to estimate past atmospheres.
B3.2 (Plant)	Hypotheses	Dixon and Joly proposed the cohesion-tension theory in 1894. It was hard to test directly, because inserting instruments disturbs the water under tension.
D3.1 (Plant)	Hypotheses	Darwin saw an orchid with a very long nectar tube and predicted a moth with a matching tongue. It was described decades later.

Unit 3 Genetics

Subtopic	Aspect	Where it comes into play
D1.1	Experiments	Meselson and Stahl used heavy nitrogen in 1958 to separate three models of replication, supporting the semi-conservative one.
D1.2	Experiments	Nirenberg and Matthaei cracked the first codon in 1961, using a synthetic RNA made only of uracil.
D1.3	Experiments	In 1943 Luria and Delbruck showed that bacterial mutations arise before selection, and are not a response to it.
D2.1	Observations	New staining methods let Walther Flemming describe mitosis in 1882, showing how technique can open up what is visible.
D3.2	Patterns and trends	Mendel's large counts of pea plants revealed ratios that small samples would have hidden.

Unit 4 Biomolecules

Subtopic	Aspect	Where it comes into play
A1.1	Falsification	In the late 1960s, scientists reported polywater, an unusual form of water. It was later traced to contamination.
A1.2	Models	Watson and Crick built their double helix model using Rosalind Franklin's X-ray data, which she had not given them directly.
B1.1	Evidence	Advice about saturated fats has shifted as studies improved, partly because diet is hard to measure and to control.
B1.2	Measurements	Frederick Sanger's sequencing of insulin in the 1950s showed that every protein has a precise order of amino acids.
C1.1	Models	Fischer's lock-and-key model of 1894 was refined by Koshland's induced-fit model in 1958, as evidence showed that enzymes change shape.
C1.2	Experiments	In the 1780s Lavoisier and Laplace compared the heat from a guinea pig with burning charcoal, linking respiration to combustion.
C1.3	Experiments	Van Helmont grew a willow in weighed soil and concluded that it gained its mass from water. His data were good, but he missed the role of air.

Unit 5 Cells

Subtopic	Aspect	Where it comes into play
A2.2	Observations	Hooke and van Leeuwenhoek's microscopes revealed a world nobody had seen, and each improvement in microscopy has changed what biologists can ask.
B2.1	Falsification	The Davson-Danielli protein sandwich model was replaced by the Singer-Nicolson fluid mosaic model in 1972, as new evidence contradicted the older one.
B2.2	Evidence	Electron microscopy and cell fractionation, developed by Claude, Palade and de Duve, together revealed organelles and what they do.
B2.3	Global impact of science	Stem cell research promises new treatments. In 2006 Yamanaka's team reprogrammed adult cells, easing some objections to using embryos.
D2.3	Experiments	Measuring mass change in tissue lets us infer water movement across membranes, even though we never see the molecules move.

Unit 6 Biodiversity

Subtopic	Aspect	Where it comes into play
A3.1	Observations	For decades, textbooks said that humans have 48 chromosomes. Better techniques let Tjio and Levan count 46 in 1956.
A4.1	Evidence	Evolution is supported by independent evidence from fossils, anatomy, selective breeding and molecular sequences.
A4.2	Global impact of science	Conservation science informs international policy, yet resources are limited, so scientists and governments must choose what to protect.
D4.1	Observations	Peter and Rosemary Grant measured Galapagos finches for decades, and directly observed beak size changing after droughts.
B4.2	Experiments	Gause's experiments on Paramecium in 1934 showed that two similar competitors could not stably coexist, supporting competitive exclusion.

Unit 7 Ecosystems

Subtopic	Aspect	Where it comes into play
B4.1	Patterns and trends	Mapping climate against vegetation reveals biome patterns that help to explain where organisms live.
C4.1	Measurements	Sampling lets us estimate populations that cannot be counted, with the assumptions and uncertainty stated.
C4.2	Measurements	Charles Keeling began measuring carbon dioxide at Mauna Loa in 1958. His careful, continuous record became a key piece of climate evidence.
D4.2	Experiments	In 1966 Robert Paine removed a sea star from an area of shore and saw diversity fall, leading to the idea of keystone species.
D4.3	Science as a shared endeavour	Arrhenius calculated the warming effect of carbon dioxide in 1896. Today the IPCC brings thousands of scientists together to assess the evidence.

AHL path

Unit H1 Body Systems

Subtopic	Aspect	Where it comes into play
B3.3	Science as a shared endeavour	In 1954 two independent teams, led by Andrew Huxley and Hugh Huxley, published the sliding filament idea together.
C2.1	Experiments	Earl Sutherland's work on epinephrine in the 1950s revealed second messengers, showing how a signal outside a cell can act inside it.
B3.1	Measurements	In 1904 Bohr, Hasselbalch and Krogh measured how carbon dioxide alters haemoglobin's oxygen binding. Only one name stayed attached to the effect.
B3.2	Measurements	In the early 1900s, Einthoven's string galvanometer recorded the heart's electrical activity, leading to the modern ECG.
C2.2	Experiments	In 1921 Otto Loewi showed chemical transmission using two frog hearts. He later said that the idea came to him in a dream.
D3.3	Global impact of science	Willem Kolff built an early dialysis machine in the 1940s, and went on to save lives by applying physical principles to kidney failure.
D3.1	Evidence	Observational studies suggested that hormone replacement therapy protected the heart. A large randomised trial reported in 2002 challenged this.

Unit H2 Plant Systems

Subtopic	Aspect	Where it comes into play
C3.1	Experiments	Charles Darwin and his son Francis covered shoot tips to study bending towards light. In 1928 Frits Went isolated the signal, later named auxin.
D2.3	Measurements	Water potential cannot be seen directly, but Scholander's pressure chamber of the 1960s made it possible to measure tension in plants.

Unit H3 Genetics

Subtopic	Aspect	Where it comes into play
D2.2	Evidence	Studies of people conceived during the Dutch Hunger Winter of 1944-45 linked famine to health effects decades later, with evidence of lasting epigenetic marks.
D1.1	Experiments	In the 1960s, Reiji and Tsuneko Okazaki found short DNA fragments, which explained how a strand can be copied against its direction.
D1.2	Falsification	In 1977 Roberts and Sharp found that genes can be split. This overturned the simple one gene, one polypeptide picture.
D1.3	Global impact of science	Doudna and Charpentier developed CRISPR-Cas9 editing. In 2018 He Jiankui's announcement of edited babies drew worldwide condemnation.
D2.1	Global impact of science	Cells taken from Henrietta Lacks in 1951, without her knowledge, became the HeLa line, which has supported countless discoveries about cell division.
D3.2	Patterns and trends	Morgan's fruit fly results did not fit independent assortment. In 1913 his student Sturtevant used this to make the first genetic map.

Unit H4 Biomolecules

Subtopic	Aspect	Where it comes into play
A1.1	Measurements	The Rosetta mission measured the hydrogen isotopes in a comet's water, and found a ratio unlike Earth's, adding evidence to the debate on where our water came from.
A1.2	Experiments	Avery's team pointed to DNA as the genetic material in 1944, but many scientists were only convinced after Hershey and Chase's 1952 experiment.
B1.2	Models	Anfinsen's work suggested that sequence determines shape. In 2020 AlphaFold predicted many protein structures with high accuracy.
C1.1	Models	In 1913 Leonor Michaelis and Maud Menten described enzyme kinetics with an equation that is still in use.
C1.2	Hypotheses	Peter Mitchell's chemiosmotic theory of 1961 was doubted by many. It later gained strong support, and he received a Nobel Prize in 1978.
C1.3	Experiments	Melvin Calvin's team traced radioactive carbon through algae to map the steps of carbon fixation.

Unit H5 Cells

Subtopic	Aspect	Where it comes into play
A2.1	Experiments	In 1953 Miller and Urey made amino acids from simple gases, showing that a step in life's origin was possible, though not that it happened that way.
A2.3	Evidence	In the 1890s Ivanovsky and Beijerinck found that a disease agent passed through filters that stopped bacteria, before anyone could see a virus.
A2.2	Evidence	Lynn Margulis's endosymbiosis theory faced strong resistance when published in 1967. Later genetic evidence strongly supported it.
B2.1	Experiments	In 1970 Frye and Edidin fused mouse and human cells, and saw their membrane proteins mix, giving strong evidence that membranes are fluid.
B2.2	Observations	Camillo Golgi described his apparatus in 1898, but many thought it was a staining artefact until electron microscopes confirmed it.
B2.3	Observations	Some early microscopists believed they saw tiny humans inside sperm, an idea that shows how expectation can colour observation.

Unit H6 Biodiversity

Subtopic	Aspect	Where it comes into play
A3.2	Evidence	In 1977 Carl Woese used rRNA sequences to propose a third domain, Archaea. The idea met strong initial resistance.
A3.1	Science as a shared endeavour	DNA barcoding relies on shared international databases, where researchers across the world compare sequences.
A4.1	Observations	The Grants observed a new finch lineage arise after a hybridization event on Daphne Major, within a few generations.
D4.1	Models	The Hardy-Weinberg principle describes a population that does not really exist, yet it is useful for detecting when evolution is occurring.

Unit H7 Ecosystems

Subtopic	Aspect	Where it comes into play
D4.2	Hypotheses	In the early 1900s, Clements saw succession as ordered and predictable, while Gleason emphasised chance. Each view still shapes ecology.
D4.3	Science as a shared endeavour	Phenology uses records from scientists, the public and even centuries-old documents, such as Kyoto's cherry blossom dates.