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Chlorophyll is a family of molecules that absorb light for photosynthesis. In chloroplasts, chlorophyll works with proteins in thylakoid membranes; absorbing light initiates energy capture rather than directly constructing a sugar. See OpenStax Biology 2e, 8.2: photosynthetic pigments.

In a chloroplast, light-dependent reactions take place at thylakoid membranes. Chlorophyll captures light; electron transfer helps establish an electrochemical gradient across the thylakoid membrane, enabling formation of ATP. The oxygenic photosynthesis pathway also forms NADPH and releases molecular oxygen from water. ATP and NADPH become available in the chloroplast stroma for the Calvin cycle. See Alberts et al.: chloroplast light reactions.

Antennae collect photons; reaction centres convert excitation into charge separation; electron flow then generates reducing power and a proton gradient for carbon assimilation. Chlorophyll and accessory pigments in light-harvesting complexes broaden light absorption and transfer excitation towards a photosynthetic reaction centre. Excitation-energy transfer between pigments is distinct from transferring an electron along the electron-transport chain. The reaction centre couples excitation to separation of an electron from its donor, creating an oxidizing partner and a reduced acceptor before the excitation is lost as heat or fluorescence.

In the thylakoid membrane, photosystem II uses light excitation of its reaction centre to drive electron transfer to acceptors. Its oxidized donor is regenerated using electrons extracted from water by the oxygen-evolving complex, which contains a manganese–calcium cluster. Four oxidizing equivalents permit the net reactionThe protons are released on the lumenal side. Electrons pass through plastoquinone, the cytochrome b6f complex and plastocyanin to photosystem I. Plastoquinone takes up protons on reduction and contributes to their net delivery into the lumen through the cytochrome-b6f pathway. The two photosystems operate in series because excitation at one centre alone does not bridge the full energetic span from water oxidation to efficient nicotinamide adenine dinucleotide phosphate reduction.

Excitation in photosystem I enables electron transfer to ferredoxin; ferredoxin-NADP reductase generates NADPH on the stromal side. For the four electrons corresponding to one oxygen molecule, the ideal minimum is eight photochemical excitations, four at each photosystem, yielding two NADPH. Real light requirements are higher because not every absorbed photon produces productive chemistry. The proton-motive force drives ATP synthase as protons return from the lumen to the chloroplast stroma. Counterion movements often allow a large pH component to accumulate without an equally large membrane voltage. In a classical test, an imposed acid-to-base transition in isolated chloroplast material supported ATP formation in darkness, separating the gradient's role from the immediate presence of light.^[ref] Appropriate controls compare transitions that produce a transmembrane gradient with treatments that dissipate it.

In the stroma, the Calvin cycle uses ATP and NADPH to reduce fixed carbon and regenerate the carbon acceptor. RuBisCO adds carbon dioxide to ribulose bisphosphate; subsequent phosphorylation and reduction produce triose phosphate. Fixation of three carbon dioxide molecules to one net triose phosphate requires nine ATP and six NADPH in the standard accounting, with most intermediate carbon recycled to regenerate the acceptor. “Light-independent” chemistry is not physiologically independent of light: energy supply, stromal conditions and enzyme regulation coordinate it with photochemistry.

Linear electron flow need not exactly match the ATP/NADPH demand. Cyclic electron flow around photosystem I increases proton-gradient-driven ATP production without net NADPH generation or water oxidation. Excess excitation is dangerous; non-photochemical quenching, changes in antenna allocation and photosystem repair limit damage. The chloroplast combines efficient collection with regulated conversion and protection, rather than maximizing photon capture regardless of metabolic demand.

A parasite can retain a photosynthetic ancestor's organelle because that organelle supplies essential metabolites after photosynthesis itself has been lost. Retained plastid genes or chlorophyll-pathway genes therefore need to be interpreted function by function. Their presence is evidence of evolutionary history and possible biochemical dependence, not proof that an animal parasite fixes carbon using light.

Many apicomplexan parasites possess an apicoplast, a reduced plastid derived from an algal endosymbiont. The surrounding membranes, plastid genome and imported nuclear-encoded proteins connect the organelle to an endosymbiotic ancestry. The nucleus must supply most of its working proteins, using targeting information to deliver them across the relevant membranes. A small organelle genome can therefore coexist with a much larger nuclear investment in plastid maintenance. Loss of photosystem genes does not imply loss of every ancestral plastid pathway.

Comparative evidence is strengthened by close relatives that still photosynthesize. Discovery of the photosynthetic alveolate Chromera velia close to the apicomplexan lineage made a photosynthetic ancestry more directly testable using phylogeny and organelle comparisons.^[ref] Such relationships reconstruct common ancestry; they do not establish that a modern parasite passed through the exact ecological conditions of the sampled relative. Independent gene losses, horizontal transfer and uneven sampling need consideration when inferring the ancestral state from one sequence.

Selection acts on the products that remain useful. The apicoplast can contain pathways for isoprenoid precursor biosynthesis, type II fatty acid synthesis, parts of heme metabolism and other functions, with the requirement varying by organism and life-cycle stage. A host may supply abundant carbon and energy while failing to supply a particular essential precursor at the correct place or rate. Retaining that precursor pathway can make the plastid indispensable even in a cell that has no chlorophyll or photosystems. Conversely, pathway loss becomes possible when another compartment or the host supplies an adequate replacement.

The clearest functional dissection is chemical rescue. Blood-stage Plasmodium falciparum could be maintained after loss of its apicoplast when supplied with isopentenyl diphosphate, identifying production of isoprenoid precursors as the essential organelle contribution under those culture conditions.^[ref] This is stronger evidence than inhibition by an antibiotic alone: a defined metabolite bypasses the need for the organelle. The conclusion is stage- and condition-specific. It does not mean that every apicomplexan needs only that product, or that the plastid is dispensable without supplementation. Loss of an organelle marker alone would also be insufficient without showing sustained propagation and loss of independent organelle functions.

An informative consequence is that some maintenance machinery is essential indirectly. plastid translation, genome replication, protein import and division may be needed because they sustain a single indispensable output. An inhibitor can therefore cause a delayed phenotype as an existing organelle and its products are diluted or inherited abnormally. To distinguish a direct metabolic block from loss of organelle biogenesis, follow growth over several generations, assay the pathway product and monitor organelle inheritance. Add the proposed rescue metabolite before a generalized lethal collapse, and establish that it can enter the relevant compartment.

The boundary of this explanation is visible in lineages that have lost the plastid. The compact genome of Cryptosporidium parvum lacked evidence for the characteristic plastid-related repertoire found in several other apicomplexans.^[ref] Such comparative loss supports the idea that plastid retention depends on remaining metabolic requirements rather than an unavoidable structural rule. Absence of a sequence from a draft assembly alone would be weak evidence; a stronger claim combines a high-quality genome, broad searches for imported proteins and independent cellular observations.

A more surprising case is corallicolids, coral-associated apicomplexans whose plastid genomes retain chlorophyll-biosynthesis genes despite lacking photosystem genes.^[ref] This separates retention of part of pigment metabolism from retention of photosynthesis as a complete energy-conversion process. Possible explanations include an unrecognized metabolic role for the retained enzymes, production of a useful tetrapyrrole-related intermediate, or constraints imposed by pathway interactions. The gene inventory alone does not choose among them. Nor should the ecological label “parasite” be inferred solely from occurrence inside an animal if the actual effect on host fitness has not been established.

To test residual photosynthetic capacity, look for assembled photosystems, pigment identity, light-driven charge separation, appropriate oxygen or electron-transfer signals and carbon fixation with proper dark controls. Host tissue and its other symbionts are major confounders in a coral sample: a bulk photosynthetic signal may come from a different organism. Isolate the relevant cells or obtain spatially resolved evidence, and distinguish the parasite's transcripts and proteins from host or symbiont contamination. A retained chlorophyll-pathway transcript demonstrates expression, not photosynthetic energy conservation.

To test why an individual retained pathway is selected, perturb it specifically, identify the biochemical product that changes and attempt a mechanistically appropriate rescue. A metabolite rescue can distinguish loss of a useful product from a structural role of the organelle; a localization-specific rescue can test whether the product must be generated locally. Evolutionary interpretation should then compare multiple lineages with different losses, rather than narrating every retained gene as an adaptation without functional evidence.

These names refer to several unrelated chemical classes: blood and bile pigments, a photosynthetic pigment family, an amino sulfonic acid, a disputed historical faecal extract, and storage polysaccharides or proteins. Similar colour or a historical name does not establish chemical identity.

Haematocrystallin denotes the crystallizable colouring substance of red blood, now identified with hemoglobin. It combines a globin protein with iron-containing haem groups. Within intact red cells it reversibly binds oxygen; oxygenated and deoxygenated forms have different absorption spectra. Crystals obtained from blood differ with species and preparation. This is not the same thing as a crystal formed from an old tissue haemorrhage. The word does not mean that all circulating blood normally consists of free pigment crystals.^[ref]

Haematoidin is the yellow-orange to reddish pigment characteristically encountered in old extravasated blood, sometimes as rhomboidal crystals or radiating clusters. It is associated with bilirubin and lacks the iron retained in haemosiderin. The historical inference that any reddish blood-derived crystal must be the same substance as haematocrystallin is therefore wrong. In a tissue preparation the local matrix and other constituents can complicate an exact chemical identification from appearance alone.^[ref]

Haematin is a dark iron-containing derivative of haem, obtained when the globin association and the iron's chemical environment are altered. In modern chemical usage, hematin is the ferric hydroxide form of the haem pigment; hemin is the corresponding chloride form. Historical preparations and terminology were less consistently separated. Neither should be equated with iron-free bilirubin, and a haem-derived crystal test is not the crystallization of intact hemoglobin.^[ref][ref]

Chlorophyll is the green pigment system of photosynthetic organisms, present in organized pigment-protein assemblies in chloroplasts in green plants. Modern chemistry distinguishes several chlorophylls: chlorophyll a, for example, has a magnesium-containing tetrapyrrolic ring system and a hydrophobic side chain. It absorbs light used in photosynthetic energy conversion. Extraction into alcohol can reveal the green pigment, but a leaf extract is a mixture rather than a chemically pure universal chlorophyll. Its green colour must not be attributed to iron in the same way as haem.^[ref]

Biliverdin is a green, open-chain tetrapyrrolic pigment. In modern haem catabolism, ring cleavage by haem oxygenase releases iron and forms biliverdin; reduction then produces bilirubin. The pigments contribute to changing colours of some resolving haemorrhages and to bile-pigment chemistry. This sequence concerns degradation of the haem group, not conversion of the entire globin protein directly into one green molecule.^[ref]

Taurin, now taurine, is a colourless crystalline amino sulfonic acid with the structure ${\mathrm{H}}_2\mathrm{NC}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_3\mathrm{H}$. It contains sulfur and nitrogen and can form conjugates with bile acids, including taurocholic acid. It is not itself a green bile pigment, an ordinary carboxylic amino acid, or a standard amino acid directly inserted into proteins by translation. The chemical classification and formula distinguish it from neighbouring names in the list.^[ref]

Excretin was the name William Marcet gave to a crystalline material extracted from human faeces. His contemporary account describes treatment with hot alcohol, removal of material with lime, extraction with ether, and formation of silky needle-like crystals; the material was insoluble in water, soluble in ether and hot alcohol, and melted in the low-to-mid 90s Celsius. His 1859 account reports carbon, hydrogen, oxygen and sulfur and gives an equivalent-weight formula based on an assumed sulfur content. These are historical analytical claims, not a modern authenticated molecular specification. Later work questioned the purity and chemical individuality of excretin, so that old name alone does not establish a definite modern structure.^[ref][ref]

Glycogen is an animal glucose-storage polysaccharide, particularly important in the liver and skeletal muscles. Its highly branched chains contain alpha-(1→4) links with alpha-(1→6) branch points. Preparations form opalescent dispersions and commonly show a reddish-brown iodine reaction rather than the intense blue associated with long amylose chains. Hydrolysis yields glucose. Liver glycogen can support the regulation of circulating glucose, whereas muscle glycogen mainly supports local muscle metabolism; a modern account must distinguish those physiological destinations.^[ref]

Gluten is the cohesive protein-rich material left when wheat dough is washed to remove much of its starch. It is not a single small molecule: modern analysis distinguishes gliadin and glutenin families whose associations contribute extensibility and elasticity. It is poorly soluble in water as a dough network, contains nitrogen, and on full hydrolysis supplies amino acids. Its nutritive role is that of a plant protein mixture, not a nitrogen-free carbohydrate.^[ref]

Legumin is a seed storage globulin associated particularly with peas and related legumes. A classical preparation extracts protein into suitable dilute salt solution and precipitates it under changed conditions. Modern legumins are 11S globulin assemblies, often hexamers formed from processed subunits. They are nitrogen-containing proteins, chemically distinct from starch despite occurring beside it in a seed. The older name vegetable casein indicates an analogy of precipitation or nutritive role; it does not establish identity with milk casein.^[ref]

The defensible core of William Odling's statement is a transfer of energy: sunlight helps build reduced food material, and its subsequent oxidation can supply animal heat and work. Here solar force means solar energy, and eremacausis means slow oxidation, rather than a special vital agent. The historical wording, from his 1866 lectures, must be qualified where it identifies the fuel with the substance of the working muscle or contrasts plant and animal metabolism too absolutely.^[ref]

In oxygenic photosynthesis, light absorbed by chlorophyll supports conversion of carbon dioxide and water into reduced organic material, with release of oxygen. An overall account, written for one glucose unit, isThis summarizes several stages, rather than describing a single elementary chemical reaction. The evolved oxygen comes from water. The product can furnish starch, other carbohydrates and precursors for fat and protein; the latter additionally requires a source of nitrogen. A human obtains these materials directly from plants or through animals that have eaten plants. Thus a grain meal and the flesh of a grain-fed animal can trace much of their chemical energy to the same original sunlight.^[ref]

The reverse overall balance for complete aerobic utilization of glucose isSimilarly a representative fatty acid, palmitic acid, has the complete oxidation balanceBoth equations conserve each chemical element. The relevant energy difference belongs to the complete set of reactants and products, including oxygen; merely breaking a bond does not supply net energy. In modern terms, controlled cellular respiration couples part of the available chemical free energy to ATP formation. ATP hydrolysis can then support muscle contraction, active transport and biosynthesis; some energy is transferred to heat. The process occurs through enzyme-mediated steps, without a flame, although complete combustion in a calorimeter can help measure the corresponding chemical energy change.

Glands and muscles continually replace constituents, and their proteins can be degraded. Amino acids can contribute carbon skeletons to energy metabolism, while their nitrogen is disposed of largely as urea. But ordinary muscle contraction does not require proportional destruction and rebuilding of the contractile protein at every stroke. Carbohydrates and fats are major fuels for muscle, and a gland can use those fuels to support secretion. The historical picture of burning the organ's own substance is therefore an inadequate general explanation.^[ref]

This correction was already an experimental issue before the exam. Edward Frankland's 1866 discussion compared the mechanical work of climbing with the energy that could be assigned to protein destruction estimated from urinary nitrogen. The latter was insufficient to account for all the work. His combustion measurements also distinguished the heat from burning a sample completely from the smaller amount available when nitrogenous material leaves the animal as still-combustible urea. Measuring external work, urinary products and calorific values gives the hypothesis a quantitative test; it is more informative than simply pointing out that an active animal gets warm.^[ref]

Finally, plants themselves carry out cellular respiration, and not every ecosystem obtains its input from sunlight: chemolithotrophy is an alternative. The plant-animal contrast is useful for following energy through an ordinary food chain, but it is not an exhaustive division of biochemical capabilities. Conservation of energy also requires the bookkeeping formfor an idealized interval in which $E_{\mathrm{food}}$ is the metabolizable food input, with energy lost in excreta already subtracted and other material-energy exchanges neglected. Here $W_{\mathrm{external}}$ is external work, $Q_{\mathrm{outward}}$ is net outward heat transfer, and $\Delta E_{\mathrm{body}}$ includes changes in stored chemical and other energy. The living body transforms energy; it does not create it.

Use a near-infrared/red ratio, or its normalized form, the normalized difference vegetation index:Useful bands are approximately $0.63$–$0.69\mu \mathrm{m}$ in red and $0.75$–$0.90\mu \mathrm{m}$ in near infrared, with exact ranges depending on the sensor. Chlorophyll absorbs strongly in red, whereas leaf structure scatters near infrared efficiently; healthy green vegetation therefore has a large near-infrared/red ratio and positive NDVI. Soil and water have different spectral contrasts.^[ref]

A single value is not a direct census of vegetation biomass or species. Soil background, canopy saturation, season, atmospheric effects and mixed pixels need consideration when mapping the distribution of vegetation.

Healthy green vegetation has strong visible absorption in the blue, around $0.43$–$0.47\mu \mathrm{m}$, and red, around $0.65$–$0.68\mu \mathrm{m}$, mainly from chlorophylls and other photosynthetic pigments. Weaker absorption around green wavelengths produces a relative reflectance maximum near $0.55\mu \mathrm{m}$ and the familiar green appearance. The red edge is the steep reflectance rise around $0.68$–$0.75\mu \mathrm{m}$. In the near-infrared, weak pigment absorption and multiple scattering at internal leaf tissue–air interfaces produce high reflectance, commonly a broad plateau. This plateau is primarily a structural scattering effect, rather than chlorophyll reflecting the light.^[ref]

Leaf water produces absorption troughs in the shortwave infrared, especially near $1.4$ and $1.9\mu \mathrm{m}$, with weaker features near $0.97$ and $1.2\mu \mathrm{m}$. Atmospheric water vapor also strongly absorbs near $1.4$ and $1.9\mu \mathrm{m}$, so satellite bands usually avoid the centres of those deep troughs. Drying, senescence, pigment loss and changes in canopy structure alter the spectrum; measured canopy reflectance also includes shadows and the soil background.

To highlight vegetation, assign the near-infrared band to the red display channel, red to green and green to blue: vigorously vegetated areas appear bright red in this conventional false-colour composite. Alternatively calculate the normalized difference vegetation index from atmospherically corrected, spatially registered red and near-infrared reflectances:Large positive values indicate a strong vegetation-like red/NIR contrast. For example, ${\rho }_{\mathrm{NIR}}=0.50$ and ${\rho }_{\mathrm{red}}=0.05$ give about $0.82$. Mask clouds, their shadows and invalid low-signal pixels. Bare soil usually has a smaller contrast, while water often has negative values; fixed universal thresholds are inappropriate, and dense vegetation can saturate the index. A band ratio improves contrast but is not a direct count of plants or a unique measure of physiological health.^[ref]

A thylakoid is a membrane-enclosed sac; its thylakoid membrane contains chlorophyll and light-dependent reactions machinery. A stack is a granum.^[ref]

The membrane and its enclosed fluid answer different location questions. A soluble protein in thylakoid lumen is inside the sac; a membrane protein is embedded in its boundary and can expose different parts on the two sides. Membrane topology and organelle protein import provide the reusable language for those distinctions.