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Peptidoglycan is a mesh of linked sugar chains cross-linked by short chains of amino acids in the cell walls of most bacteria. Its absence from the cell walls of archaea helps distinguish envelope chemistry without confusing either cell wall with a cell membrane. See OpenStax Microbiology, 3.3: cell walls and peptidoglycan.

A phosphodiester bond connects two groups through a phosphate diester. In a nucleic acid backbone, the bridging phosphate is connected through oxygen to the $3^{\prime }$ carbon position of one sugar and the $5^{\prime }$ carbon position of another. These covalent bonds connect successive nucleotides along a strand; they are distinct from the hydrogen bonds between paired nucleobases.

Photosynthesis captures light energy and transforms it into chemical forms that a biological cell can use. In plants, algae and cyanobacteria, this supports carbon fixation: incorporation of carbon from carbon dioxide into chemical compounds used to build living material. Capturing light and incorporating carbon are distinct processes coupled through metabolism. See OpenStax Biology 2e, 8.1: overview of photosynthesis.

For oxygenic photosynthesis, a simplified net equation writes a six-carbon sugar, glucose, as the representative product:This is net stoichiometry, not a one-step mechanism. A worked bookkeeping check counts six carbon chemical atoms, twelve hydrogen chemical atoms, and eighteen oxygen chemical atoms on each side: the product side has six oxygen chemical atoms in the sugar and twelve in molecular oxygen. The light label supplies energy, not additional chemical atoms.

Crucially, the released molecular oxygen comes from water, not carbon dioxide. Cancelling water made and consumed in different steps hides this distinction in the net equation. The Calvin cycle supplies a net three-carbon triose phosphate, such as glyceraldehyde 3-phosphate, for subsequent sugar synthesis; it does not directly export glucose. See OpenStax Biology 2e, 8.2: light-dependent reactions and OpenStax Biology 2e, 8.3: using light energy to make organic molecules.

The useful achievement is connecting incoming light to both usable energy and new biological material. ATP is an immediate energy currency; NADPH supplies Electrons for synthesis; carbon dioxide supplies the carbon incorporated into carbohydrates. These roles are complementary. Conservation of energy still holds: photosynthesis transforms energy, never creates it, and some absorbed energy is dissipated as heat. A plant also carries out cellular respiration, including in biological cells that perform photosynthesis. Gas exchange reflects both processes. Oxygenic photosynthesis remains central to interpreting terrestrial molecular oxygen as an atmospheric biosignature gas. See OpenStax Biology 2e, 8.3: photosynthesis and respiration.

Ribonucleic acid, or RNA, is a nucleotide polymer whose nucleotide sequence can both carry information and organize a working molecular shape. Its backbone contains alternating ribose sugars and phosphate groups connected through covalent bonds. Its usual nucleobases are adenine A, uracil U, guanine G, and cytosine C. Thus RNA uses U where DNA uses T; its ribose also differs chemically from DNA's deoxyribose. See NHGRI: RNA.

In transcription, an enzyme uses one DNA strand as a template for RNA. For example, the DNA template $3^{\prime }\text{-}\mathrm{TACGG}\text{-}{5}^{\prime }$ specifies $5^{\prime }\text{-}\mathrm{AUGCC}\text{-}{3}^{\prime }$ in RNA. The base pairing rules now include A–U rather than A–T, while G–C remains. This RNA matches the other DNA strand's $5^{\prime }$-to-$3^{\prime }$ lettering after T is replaced by U. Identifying the template strand matters: replacing T by U in an arbitrarily chosen DNA strand is not a general method for deriving the RNA product. See Alberts et al., Molecular Biology of the Cell: from DNA to RNA.

RNA is often a single strand, but single-stranded does not mean straight or entirely unpaired. Complementary regions within one RNA molecule can undergo base pairing, bringing distant positions together. Consider the illustrative twelve-nucleotide nucleotide sequence $5^{\prime }\text{-}\mathrm{GCAUUUUUAUGC}\text{-}{3}^{\prime }$. Pair positions 1 with 12, 2 with 11, 3 with 10, and 4 with 9. The resulting stem-loop has four base pairs: G–C, C–G, A–U, U–A. Positions 5–8 remain unpaired in the loop. Both arms belong to the same continuous backbone, and the paired arms have the directions of antiparallel nucleic acid strands.

The diagram proposes a possible pairing pattern, not a prediction that this short RNA must adopt that shape under every condition. Actual RNA folding depends on additional interactions, temperature, ions, and associated proteins; the full three-dimensional arrangement contains more information than the drawn base pairs. The example's purpose is to show how a linear nucleotide sequence can specify contacts within itself.

Different RNA roles make this flexibility useful. Messenger RNA carries a message that a ribosome reads during translation to build a protein. Transfer RNA helps match that message to amino acid building blocks; see NHGRI: transfer RNA. Ribosomal RNA forms structural and catalytic parts of the ribosome. Other RNA participates in gene regulatory networks, changing how strongly genes are expressed. Many RNA molecules therefore function without being translated into proteins; for some genes, functional RNA is the product itself.

Most strikingly, some folded RNA acts as an enzyme: these catalytic RNA molecules are ribozymes. The ribosome's bond-forming active site is built from RNA, although the complete ribosome also contains proteins. Structural evidence appears in Nissen et al., The structural basis of ribosome activity in peptide bond synthesis, Science 289, 920–930 (2000). RNA therefore connects a readable record with physical recognition, regulation, and catalysis: the same kind of polymer can be a message, an adaptor, or part of molecular machinery.

In chemical usage, sugars commonly means small carbohydrates, including single-unit monosaccharides and two-unit disaccharides. Glucose, ribose, and deoxyribose are monosaccharides. Cellulose is instead a polysaccharide assembled from many glucose units. The distinction counts chemically linked units; it does not classify every carbohydrate by sweetness or biological role. See Clark, Douglas and Choi, Biology 2e, 3.2: carbohydrates.

A useful first description of a monosaccharide asks how many carbon chemical atoms it contains and what its open-chain structure is. Ribose is an aldopentose, a five-carbon aldehyde sugar; glucose is an aldohexose, with six carbon chemical atoms. A Fischer projection then records the stereochemistry, information missing from the molecular formula. See McMurry, Organic Chemistry, 25.1: carbohydrate classification.

A triose phosphate is a three-carbon sugar carrying a phosphate group. Glyceraldehyde 3-phosphate is one example. The Calvin cycle supplies a net triose phosphate for further carbohydrate synthesis. ^[ref].

Can a gene from Schizosaccharomyces pombe replace a missing function in Saccharomyces cerevisiae? A modern laboratory project tested functional complementation of methionine synthesis. For each tested gene deletion, the investigators compared plasmids carrying the corresponding Saccharomyces cerevisiae gene, a candidate Schizosaccharomyces pombe gene, and an unrelated gene. The first was a positive scientific control; the last was a negative scientific control. The paper describes this design rather than publishing a complete set of individual growth measurements. ^[ref]

The recipient biological strains also lacked the ability to make uracil. The pYES2.1/V5-His plasmids carried URA3, a gene restoring that ability, and used the GAL1 gene promoter to control the inserted gene. Initial plates lacked uracil but supplied methionine: this selected biological colonies containing plasmids without requiring successful methionine synthesis. Subsequent plates lacking methionine tested the missing function, comparing galactose induction with glucose conditions. Thus selecting for a plasmid and testing its functional contribution were separate operations. ^[ref]

The apparatus can be simple and informative. In the associated protocol, replica plating uses a block covered with velveteen to transfer small samples of biological colonies from a master plate onto plates with different nutrient conditions. Orientation marks preserve correspondence between locations; gentle contact limits the transferred material. The specified replicas omit uracil and methionine with either inducing sugar or glucose, and include a fresh glucose plate supplying methionine. The readout is growth at corresponding locations, not a direct measurement of enzyme activity. ^[ref]

The scientific controls answer different objections. Growth on the methionine-supplied replica checks survival and transfer. Rescue by the native gene checks that this biological strain and assay can reveal functional complementation. The unrelated gene tests whether carrying a plasmid alone is sufficient. Comparing induction conditions checks dependence on gene expression, although changing the sugar also changes metabolism: it is not an isolated change of gene expression in every other respect.

If the candidate gene restores growth while those comparisons behave as intended, it supplies a function sufficient for the tested growth requirement in this recipient. Failure to rescue is less decisive: the candidate protein might not be produced, might be altered by its attached tag, or might not work in this cellular context. The investigators therefore also used Western blots to examine protein production. A detected protein still does not establish correct protein folding or catalytic activity. ^[ref]