Question: What are the roles that molecules play of chemical substances of food?
About the article "Biology", section "Energy".

Candidate passages:
The survival of a living organism depends on the continuous input of energy.
Chemical reactions that are responsible for its structure and function are tuned to extract energy from substances that act as its food and transform them to help form new cells and sustain them.
In this process, molecules of chemical substances that constitute food play two roles; first, they contain energy that can be transformed for biological chemical reactions; second, they develop new molecular structures made up of biomolecules.
The organisms responsible for the introduction of energy into an ecosystem are known as producers or autotrophs.
Nearly all of these organisms originally draw energy from the sun.
Plants and other phototrophs use solar energy via a process known as photosynthesis to convert raw materials into organic molecules, such as ATP, whose bonds can be broken to release energy.
A few ecosystems, however, depend entirely on energy extracted by chemotrophs from methane, sulfides, or other non-luminal energy sources.
Some of the captured energy is used to produce biomass to sustain life and provide energy for growth and development.
The majority of the rest of this energy is lost as heat and waste molecules.
The most important processes for converting the energy trapped in chemical substances into energy useful to sustain life are metabolism and cellular respiration.
Major philosophical questions arise as soon as one attempts to define chemistry and what it studies.
Atoms and molecules are often assumed to be the fundamental units of chemical theory, but traditional descriptions of molecular structure and chemical bonding fail to account for the properties of many substances, including metals and metal complexes and aromaticity.
Additionally, chemists frequently use non-existent chemical entities like resonance structures to explain the structure and reactions of different substances; these explanatory tools use the language and graphical representations of molecules to describe the behavior of chemicals and chemical reactions that in reality do not behave as straightforward molecules.
Some chemists and philosophers of chemistry prefer to think of substances, rather than microstructures, as the fundamental units of study in chemistry.
There is not always a one-to-one correspondence between the two methods of classifying substances.
For example, many rocks exist as mineral complexes composed of multiple ions that do not occur in fixed proportions or spatial relationships to one another.
A related philosophical problem is whether chemistry is the study of substances or reactions.
Atoms, even in a solid, are in perpetual motion and under the right conditions many chemicals react spontaneously to form new products.
A variety of environmental variables contribute to a substance's properties, including temperature and pressure, proximity to other molecules and the presence of a magnetic field.
As Schummer puts it, "Substance philosophers define a chemical reaction by the change of certain substances, whereas process philosophers define a substance by its characteristic chemical reactions.
"Philosophers of chemistry discuss issues of symmetry and chirality in nature.
Organic (i.e., carbon-based) molecules are those most often chiral.
Amino acids, nucleic acids and sugars, all of which are found exclusively as a single enantiomer in organisms, are the basic chemical units of life.
Chemists, biochemists, and biologists alike debate the origins of this homochirality.
Philosophers debate facts regarding the origin of this phenomenon, namely whether it emerged contingently, amid a lifeless racemic environment or if other processes were at play.
Some speculate that answers can only be found in comparison to extraterrestrial life, if it is ever found.
Other philosophers question whether there exists a bias toward assumptions of nature as symmetrical, thereby causing resistance to any evidence to the contrary.
One of the most topical issues is determining to what extent physics, specifically, quantum mechanics, explains chemical phenomena.
Can chemistry, in fact, be reduced to physics as has been assumed by many, or are there inexplicable gaps?
Some authors Roald Hoffmann have recently suggested that a number of difficulties exist in the reductionist program with concepts like aromaticity, pH, reactivity, nucleophilicity, for example.
The noted philosopher of science, Karl Popper, among others, predicted as much. 
Chemical properties.
The majority of these compounds are solubles molecules but the smaller molecules can be volatiles.
Many natural phenols present chirality within their molecule.
An example of such molecules is catechin.
Cavicularin is an unusual macrocycle because it was the first compound isolated from nature displaying optical activity due to the presence of planar chirality and axial chirality.
Natural phenols chemically interact with many other substances.
Stacking, a chemical property of molecules with aromaticity, is seen occurring between phenolic molecules.
When studied in mass spectrometry, phenols easily form adduct ions with halogens.
They can also interact with the food matrices or with different forms of silica (mesoporous silica, fumed silica or silica-based sol gels). 
Factors affecting reaction rate.
Nature of the reactants.
Depending upon what substances are reacting, the reaction rate varies.
Acid/base reactions, the formation of salts, and ion exchange are fast reactions.
When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow.
Nature and strength of bonds in reactant molecules greatly influence the rate of its transformation into products. 
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions.
Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds.
Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or complexes held together by coordinate covalent bonds.
Pure chemical elements are not considered chemical compounds, even if they consist of molecules that contain only multiple atoms of a single element (such as H2, S8, etc.
), which are called diatomic molecules or polyatomic molecules.