A historical fuel gas mixture made by destructive distillation of coal. Its composition depends on the feed and retort conditions; it is not synonymous with natural gas or pure methane.

Biological oxidation of reduced nitrogen, commonly from Ammonia through nitrite to nitrate.

The investigation of microorganisms relevant to human disease. Identification, susceptibility measurements and the clinical sampling context must be connected: recovering an organism can indicate infection, contamination or normal colonization depending on the evidence.

Growth and maintenance of bacteria under specified conditions. A culture can amplify initially rare cells, so its eventual composition reflects both the original sample and differences in growth under the chosen conditions.

A scientific instrument maker designs, constructs or improves scientific instruments, turning a proposed measurement principle into a working apparatus. Geometry, material choice, adjustment and readable scales can determine whether the intended measurement resolution and measurement calibration are achievable. The Fortin barometer illustrates why the design of a reference surface and a reading method belongs to the science of a measurement, as well as to its manufacture.

A metrizable space is separable exactly when it is Lindelöf. A countable dense subset gives a countable topological basis of balls with rational radii, hence the open-cover property. Conversely, choose countable subcovers of all radius-$1/n$ ball covers; their centers form a countable dense subset.

Every second-countable space that is also a T3 space is a metrizable space. First regularity of a topological space and the Lindelöf property yield a normal space: cover each of two disjoint closed sets by countably many open sets whose closures avoid the other, and subtract the first $n$ opposite closures from the $n$th neighborhood. The resulting unions are disjoint open neighborhoods. Urysohn's lemma then gives, for each pair of basic sets ${\overline{B}}_n\subseteq B_m$, a continuous function equal to one on the smaller closure and zero outside the larger set. There are countably many such pairs. The shrinking form of regularity of a topological space shows these functions generate the topology, and $T_1$ makes them separate points. Their coordinate map embeds the space into the countable cube, with compatible metric $d(x,y)={\sum }_j{2}^{-j}|f_j(x)-f_j(y)|$. The bounded tails prove compatibility with the product topology. Thus a countable base converts separation axioms into a numerical distance.

A naturally occurring material from which a useful constituent can be extracted under the relevant technical and economic conditions.

Conversion of an iron-bearing ore into metallic iron, for example by carbon monoxide reduction in a blast furnace.

Low-carbon worked iron, historically containing characteristic slag inclusions.

The nonmetallic melt or solid product separating gangue and other constituents from a metal during extraction or refining.

Steelmaking produces steel by controlling the carbon content, other alloying elements and unwanted constituents of iron. A blast furnace first produces carbon-rich pig iron; a basic oxygen furnace then refines the molten material by oxidation and composition adjustment. An electric arc furnace instead supplies heat electrically to melt a charge that can include recycled steel, direct reduced iron and pig iron.^[ref]

The distinction between iron extraction and steelmaking matters: converting an iron oxide to metallic iron and adjusting an existing iron-based alloy to the required composition are different tasks. The Bessemer process is an important historical example of refining pig iron by an air blast; modern basic oxygen steelmaking uses an oxygen supply rather than ordinary air.

Carbohydrate metabolism is the set of reactions that synthesize, interconvert, store and degrade carbohydrates. It connects dietary sugars and stored polymers with glycolysis, biosynthesis and glucose production. Different sugars can enter at different intermediates rather than all following identical initial reactions. Carbon tracing can distinguish storage from oxidation even when both begin with the same glucose supply. ^[ref]

Lipid metabolism includes synthesis, modification, transport, storage and breakdown of lipids. Stored triacylglycerols can release fatty acids through lipolysis, and subsequent oxidation can supply acetyl-CoA and reducing equivalents. Other lipids are assembled into membranes or signalling molecules. These distinct destinations explain why more lipid synthesis does not necessarily imply more energy storage. ^[ref]

Amino acid metabolism includes synthesis, interconversion and breakdown of amino acids. Their nitrogen and carbon skeletons can follow different routes: transamination transfers an amino group, while the remaining carbon can enter central metabolism. Excess nitrogen can be converted into an excreted form through processes such as the urea cycle. Availability of one essential amino acid can limit protein synthesis even when other carbon and nitrogen sources are abundant. ^[ref]

Biotransformation is chemical alteration of a substance by a biological system, usually through enzyme-catalysed reactions. Oxidation, reduction, hydrolysis or conjugation can change its solubility and subsequent fate. The change can reduce activity, increase activity or generate a reactive product, so transformation is not automatically detoxification. This principle applies to both endogenous compounds and substances acquired from the environment. ^[ref]

Ketogenesis produces ketone bodies from acetyl-CoA, prominently in liver mitochondria during conditions favouring fatty-acid oxidation. Exported ketone bodies carry fuel to other tissues, allowing carbon from lipids to support oxidative metabolism away from the liver. Increased production alone does not determine circulating concentration because tissue uptake and clearance also matter. The pathway is distinct from synthesis of glucose. ^[ref]

Glucose metabolism comprises the reactions that produce, use, store and interconvert glucose. Glycolysis supplies smaller carbon compounds and energy carriers, whereas glycogen synthesis stores glucose units and glycogenolysis remobilizes them. Carbon can also enter biosynthetic pathways instead of being fully oxidized. Tissue function and hormonal state determine the balance, explaining why glucose uptake and complete glucose oxidation are different measurements. ^[ref]

Uric acid is a nitrogen-containing product of purine breakdown; near physiological pH, much is present as urate. Its solubility depends on chemical conditions, so concentration alone does not completely specify crystallization behaviour. Species differ in subsequent breakdown and nitrogen excretion: humans retain urate as a major purine end product, whereas some other organisms metabolize it further. This connects nucleotide turnover with whole-organism waste handling. ^[ref]

Anabolism comprises metabolic processes that build cellular components from simpler precursors. It commonly consumes ATP and reducing power supplied by other reactions. Synthesis of proteins from amino acids is one example, while making the amino acids themselves is another. Anabolism and catabolism are coupled through shared intermediates and energy carriers, but are not generally exact reversals of one another. ^[ref]