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Aerobic respiration uses molecular oxygen as the final electron acceptor. For complete oxidation of glucose, the net material balance isThis records the overall conversion of glucose, molecular oxygen, carbon dioxide and water; it is not one enzyme-catalyzed step, and it omits the separate ATP accounting. For an original stoichiometry check, each side contains six carbon, twelve hydrogen and eighteen oxygen chemical atoms. If exactly $1\mathrm{mmol}$ of glucose undergoes this complete conversion, the equation requires $6\mathrm{mmol}$ of molecular oxygen and yields $6\mathrm{mmol}$ each of carbon dioxide and net water. This is an equation-based calculation, not an observed seed measurement.

The molecular oxygen is reduced to water, rather than being the source of the carbon dioxide's carbon. A formal redox electron balance, using acidic half-reaction notation, makes the roles explicit:Adding and cancelling gives the net equation above. Both half-reactions conserve chemical atoms and electric charge. The $24$ electrons balance a net transfer; they are carried through cellular intermediates, rather than released as a pool of free electrons. The final respiratory reduction can be written ${\mathrm{O}}_2+4{\mathrm{H}}^{+}+4e^{-}⟶2{\mathrm{H}}_2\mathrm{O}$. These equations are bookkeeping, not a tracing experiment that assigns every product oxygen chemical atom to its original molecule.^[ref]

Alanine is an amino acid represented by Ala or A in protein sequence notation. GCU is one of its codons in the standard genetic code, and supplies the second residue in the worked gene expression example. ^[ref]

Anaerobic respiration uses a respiratory electron transport chain with a final electron acceptor other than molecular oxygen. External acceptors can include sulfate, depending on the organism. “External” distinguishes a supplied terminal acceptor from the fuel-derived organic intermediates used in fermentation; it does not mean that the reduction must take place outside the biological cell. In fermentation, regeneration of carriers such as NADH to their oxidized form permits glycolysis to continue without a respiratory electron transport chain. Both routes can operate without molecular oxygen, but they are different mechanisms.^[ref]

The gametophyte inside an angiosperm's ovule, containing an ovum and, in the common arrangement, a central biological cell with two polar cell nuclei.^[ref]

Hexokinase provides a concrete connection between sugar chemistry and energy coupling: it transfers a terminal phosphate group from ATP to glucose, forming a phosphorylated sugar and adenosine diphosphate. The enzyme accelerates the transfer, while consuming ATP changes the net chemical reaction being carried out. The donated group reaches the substrate through the catalytic route.^[ref]

For a symbolic teaching model, write $S$ for an unphosphorylated sugar, $S\text{-}P$ for its phosphorylated form, and $P_i$ for inorganic phosphate. These are labels for chemical states, not complete elemental formulas. Using one consistent reference convention at fixed pH and temperature, stipulate the following molar changes in Gibbs free energy:All three numbers are chosen illustrative values, not measured properties of glucose or a particular enzyme. The prime marks a biochemical reference convention with pH held fixed; explicit proton and metal-binding bookkeeping is suppressed consistently. The first two lines are a thermodynamic decomposition: cancelling water and inorganic phosphate gives the net transfer. They do not assert that the reaction mechanism first releases free inorganic phosphate and then recaptures it.

Because Gibbs free energy is a state function, consistently defined reaction changes add. A working reaction mechanism must also connect the desired conversion to ATP consumption; independently allowing ATP hydrolysis beside the sugar does not establish energy coupling. The general distinction is developed under energy coupling. ATP-dependent phosphorylation of a membrane protein can likewise connect a chemical change to active transport.^[ref]

Actual Gibbs free energy changes depend on thermodynamic activities. Even a favourable reference sum can be overcome by accumulation of reaction products or depletion of ATP; it supplies no prediction of the reaction rate. A living biological cell also regenerates ATP. Sustained operation therefore requires a connected metabolism, not an inexhaustible stock of a special molecule.

The biological species concept groups naturally interbreeding biological populations with viable, fertile offspring, distinguished from other such groups by reproductive isolation. An operational test examines mating, fertilization, offspring survival and offspring reproduction, rather than only whether two adults can be paired. Sexual reproduction is its scope: fossils and organisms studied only through asexual reproduction cannot supply the same mating evidence. ^[ref]

For an original hypothetical example, imagine two separated biological populations that reproduce successfully after contact. The separation alone did not demonstrate reproductive isolation. Conversely, failure to mate in one unfamiliar chamber could reflect the assay conditions. Field observations of gene flow and tests under suitable conditions address the biological question more directly. A successful laboratory pairing does not establish frequent interbreeding in nature.

Section of Biology 2e by Mary Ann Clark and others, whose whole-book authors are Mary Ann Clark, Matthew Douglas and Jung Choi. No separately credited section author is asserted.

Online text: openstax.org/books/biology-2e/pages/30-6-plant-sensory-systems-and-responses

Cited passages: Biology 2e, §30.6, The Phytochrome System and the Red/Far-Red Response, Figure 30.38: interconversion of Pr and Pfr and cellular signaling.

Section of Biology 2e by Mary Ann Clark and others, whose whole-book authors are Mary Ann Clark, Matthew Douglas and Jung Choi. No separately credited section author is asserted.

Online text: openstax.org/books/biology-2e/pages/32-2-pollination-and-fertilization

Cited passages: Biology 2e, §32.2, Development of the Seed, Figure 32.20; endospermic and non-endospermic seeds, pea cotyledons and early reserve dependence.

Section of Biology 2e by Mary Ann Clark and others. Whole-book authors: Mary Ann Clark, Matthew Douglas and Jung Choi; no separate section byline is established. OpenStax publishes and hosts the text. The book was published on 28 March 2018; no separate section revision date is established.

Online text: openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis

Inspected locators: Basic Photosynthetic Structures, Figure 8.6; The Two Parts of Photosynthesis, Figure 8.7; citation information.

Online text: openstax.org/books/biology-2e/pages/10-1-cell-division

Section of Biology 2e by Mary Ann Clark and others, whose whole-book authors are Mary Ann Clark, Matthew Douglas and Jung Choi. OpenStax publishes the book; no separate section author is credited. The citation panel gives whole-book publication on 28 March 2018, rather than a section revision date.

Cited passages: §10.1, Genomic DNA; Eukaryotic Chromosomal Structure and Compaction, Figure 10.4 and final paragraph; citation information identifying the authors and publication date.

A female reproductive unit of a flower, comprising a stigma, a style and an ovary. A flower may have several carpels, including fused ones.^[ref]

Cell differentiation is the acquisition of a specialized cell state, involving changes in structure, function and gene expression. A neuron and a red blood cell illustrate different outcomes: signaling and oxygen transport. Different functions usually do not require a different inherited DNA sequence for each function. The shared-genome principle concerns nucleated precursors and most nucleated descendants: a mature mammalian red blood cell has lost its cell nucleus and mitochondria, so it is not an example of two mature biological cells both retaining the same nuclear DNA. ^[ref]

The puzzle is how a shared set of genes supports different jobs. Gene expression changes which RNA and proteins a biological cell produces, and in what amounts. A transcription factor can regulate several genes; different combinations of regulators can interpret the same signal differently. Stable regulatory patterns can persist through the cell cycle, providing memory of developmental history. Specialization therefore usually changes use of inherited information, rather than replacing the genome wholesale. ^[ref]

A small counting example separates sequence from activity. Suppose the same three toy genes are present in every nucleated biological cell, and independently assign each gene an on/off state. There are $2^3=8$ possible expression profiles with no DNA replacement. The figure shows only two: H and A on versus H and B on. Real gene expression is graded and coordinated, so this count illustrates possible descriptions, not a prediction of eight actual cell types. The interesting consequence is that a stable cell state can depend on how a shared record is used.

Stem cells combine self-renewal with production of differentiated descendants; progenitor cells generally have more restricted developmental options and renewal capacity. Neither term means that a biological cell must give rise to every cell type. Terminal differentiation denotes advanced specialization within a lineage, often accompanied by exit from the cell cycle. Cell fate concerns the developmental outcome of a biological cell or its descendants, whereas cell state describes its current properties. A temporary change in gene expression after signal transduction is not, by itself, proof of a new cell fate. ^[ref]

Cellular respiration transfers energy from chemical fuels into usable forms including ATP. Oxidation of the fuel is coupled to reduction of an electron acceptor; the energy transfer can support ATP synthesis. Plants carry out cellular respiration as well as photosynthesis. Their biological cells need usable energy in darkness too, and a seed can draw on stored fuel during germination.^[ref][ref]

Follow three different accounts: carbon moving from fuel to products, electrons moving to an electron acceptor, and energy becoming available for cellular work. They are connected, but an oxygen uptake measurement alone does not identify the fuel or count the ATP made. The pea seed respirometry experiment makes this distinction visible with simple apparatus.

The carbon sequence is oxaloacetate $(4)$ plus an acetyl group $(2)$ to citrate $(6)$, then isocitrate $(6)$, alpha-ketoglutarate $(5)$, succinyl-CoA $(4)$, succinate $(4)$, fumarate $(4)$, malate $(4)$ and back to oxaloacetate $(4)$. The numbers count the cycling carbon skeleton, excluding coenzyme A. Two carbon dioxide releases accompany the changes $6\to 5\to 4$.^[ref]

Per acetyl group: three NADH, one FADH2 equivalent, one GTP or ATP by substrate-level phosphorylation, and two carbon dioxide. Succinate dehydrogenase retains FADH2 and transfers its electrons to ubiquinone, forming ubiquinol. The FADH2 equivalent and ubiquinol count the same electron pair.^[ref]

A useful check adds the stepwise changes. The carbon count is $4+2-1-1=4$; the NADH count is $1+1+1=3$. Each NADH carries two electrons, and the succinate dehydrogenase route supplies another pair, so one turn exports $3\times 2+2=8$ electrons. This arithmetic is an accounting identity, not an ATP yield calculation. Converting it to ATP requires the separate energy coupling and transport assumptions discussed under oxidative phosphorylation.

For a chosen ideal conversion of $5\mu \mathrm{mol}$ of acetyl groups, multiplication gives $15\mu \mathrm{mol}$ of NADH, $5\mu \mathrm{mol}$ of ubiquinol, $5\mu \mathrm{mol}$ of GTP and $10\mu \mathrm{mol}$ of carbon dioxide. The electron output is $40\mu \mathrm{mol}$. If all these electrons subsequently reach oxygen with no competing sinks, four electrons per oxygen molecule require $10\mu \mathrm{mol}$ of oxygen. That is consumption by the connected electron transport chain; it is not an oxygen term in the cycle's own substrate ledger.

No cycle step directly consumes molecular oxygen. In ordinary aerobic mitochondrial operation, however, the mitochondrial respiratory chain must reoxidize NADH and ubiquinol to restore the oxidized electron carriers. Without that outlet, a finite oxidized-carrier pool cannot sustain continued turnover. Other electron acceptors and other forms of metabolism require their own analysis.^[ref]

The pyruvate dehydrogenase complex acts upstream: one three-carbon pyruvate gives one two-carbon acetyl group, one carbon dioxide and one NADH. These are additional to the cycle ledger.^[ref] Consequently, processing one pyruvate through both stages gives three carbon dioxide and four NADH, while the succinate dehydrogenase contribution and the substrate-level phosphorylation contribution each remain one. Counting the upstream step inside the cycle would obscure where the extra carbon dioxide and NADH arise.

A codon is a three-nucleotide unit read in a specified reading frame during biological translation. For example, GCU specifies alanine in the standard genetic code. Its letters are read $5^{\prime }\to 3^{\prime }$.

For an original example, choose the messenger RNA segment $5^{\prime }\text{-}\mathrm{AUG}\mathrm{GCU}\mathrm{UUC}\mathrm{UGA}\text{-}{3}^{\prime }$ and stipulate initiation at the displayed AUG in a suitable eukaryotic context. Under the standard genetic code, its first three codons specify methionine, alanine and phenylalanine; the fourth is a stop codon. The resulting peptide before processing is N–methionine–alanine–phenylalanine–C. Twelve nucleotides therefore specify three amino acid residues and termination. The displayed segment omits the surrounding initiation signals; its nucleotide sequence alone is not a complete expression system.

A codon belongs to an oriented nucleotide sequence: GCU and UCG are different ordered triplets. Four possible nucleobases, adenine, cytosine, guanine and uracil, allow $4\times 4\times 4=64$ triplets because each position can be chosen independently when enumerating possible sequences. This is a counting argument, not a claim that nucleobases occur independently in real RNA. Sixty-one triplets are sense codons, meaning amino acid assignments, and UAA, UAG and UGA are stop codons in the standard genetic code.^[ref]

The useful unit is the ordered triplet together with its reading frame and genetic code, rather than three letters viewed in isolation. That combination lets DNA sequencing data support predictions about protein sequences after identifying the relevant messenger RNA. The Nirenberg and Matthaei experiment illustrates how the correspondence became experimentally accessible.

Degeneracy of the genetic code means that distinct codons can specify the same amino acid. For example, UUU and UUC both specify phenylalanine in the standard genetic code.^[ref] In the original AUG GCU UUC UGA example, changing UUC to UUU preserves the predicted amino acid order. This illustrates redundancy of assignment, not equality of every possible effect on gene expression.

Degeneracy of the genetic code differs from ambiguity: multiple inputs share one output, whereas an ambiguous assignment would give a single codon more than one meaning in the same specified decoding context. The standard genetic code table has one listed assignment per triplet. Alternative genetic codes, including those used in some mitochondria, and engineered decoding contexts must be specified separately; the standard table is not a universal statement about all organelles or synthetic systems.

An electron carrier cycles between oxidation and reduction. Ubiquinone carries two electrons, while cytochrome c carries one at a time.^[ref]

For example, transferring the two electrons supplied by one NADH requires two one-electron deliveries by cytochrome c. It does not require two permanently consumed cytochrome c molecules: an available electron carrier can unload, return and reload. Electron carriers recycle material while the net redox conversion supplies free energy.

An electron transport chain transfers electrons from an electron donor through electron carriers to a terminal electron acceptor. In the mitochondrial respiratory chain, some transfers drive protons across the inner mitochondrial membrane; their return through ATP synthase can drive ATP formation.^[ref]

The useful separation is between an electron route and a proton route. The former ends in a redox chemical reaction; the latter changes an electrochemical gradient across a membrane. Energy coupling connects them without making electrons and protons interchangeable. An oxygen readout consequently need not be an ATP readout.

This article focuses on the mitochondrial respiratory chain. Bacteria can use other terminal electron acceptors in anaerobic respiration. In chloroplast light-dependent reactions, light supplies energy and linear electron transport connects water oxidation to NADP+ reduction, releasing oxygen. Thus neither oxygen consumption nor the mitochondrial arrangement defines every electron transport chain.^[ref]

A nutritive plant tissue distinct from the plant embryo; it may remain in the mature seed or be depleted during development.^[ref]

A flower is an angiosperm's reproductive structure. Stamens produce pollen; carpels enclose ovules. Petals and sepals surround these organs in a complete flower, but some flowers lack one or more groups, including either reproductive group.^[ref]

Read the drawing from outside inward, then trace two routes: where pollen is released, and where an ovule is enclosed. This reveals why catching pollen and forming a seed require different structures. A detached petal cannot substitute for a stigma simply because both belong to the same flower.