Photosynthesis

“ The almost magic process of photosynthesis combines sunlight, water and carbon dioxide to make sugar. Photosynthesis is the engine that drives life at the Earth’s surface.
We will make carbon dioxide measurements over the course of the next couple of hours. The detector has a small, metal chamber in the form of a clamp that fits around a leaf. We keep each leaf sample clamped for about 15 minutes while the device analyzes the leaf for carbon dioxide absorbed and water that evaporates as we wait. We don’t leave the carbon dioxide detector unattended for days at a time as we do with the logger. ”

“ But, aside from temperature reactions and space requirements, there is no law which has yet been discovered which determines the character of the energy-storage compound which any given species of plant will elaborate. The process of photosynthesis would seem to be identical in all cases, at least up to the point of the production of the first hexose sugar; but the transformation of glucose into other monosaccharides, disaccharides, and polysaccharides seems to be a matter which obeys no rule or law. ”

“ Living processes such as photosynthesis involve very specific changes in molecules and atoms; they are carried out by large, complex molecules that have very particular chemical properties allowing them to 'direct' change in their chemical environment that would not normally happen. ”

“ Under the influence of sunlight, this chlorophyll takes the carbon dioxide of the air, and, with water and certain minerals, makes starch, the raw material of plant construction. This process, called photosynthesis, goes on while the sun shines, and stops with the approach of darkness. The necessity of plenty of light cannot be overestimated.
In the manufacture of starch, oxygen occurs as a by-product. As the plant has no use for this element, it is breathed out from the surface of the leaves. From the standpoint of man, this makes plants atmospheric purifiers. ”

“ The energy of doing this work resides in the breakdown of complex chemical substances in internal combustion or respiration. The loss of energy must be restored by absorption and storage of energy from outside.
This is secured in green plants by photosynthesis, carbon being fixed for nutrition of the plant with the help of sunlight.
The carbon-assimilation of plants is of great theoretical interest as an example of the simplest type of assimilation. The plant absorbs the carbonic acid, CO2, and the rate of its intake therefore measures the rate of assimilation. ”

“ Since the energy of all living things—including man—is also made available by photosynthesis, and since most of the photosynthesis on earth is performed by algae afloat in the oceans, it is 29 apparent that this research is of more than academic interest. ”

“ Sharing their limestone cells are quite unrelated organisms, single-celled plants or algae. These plants possess the green of grass and forests, whose molecules create out of carbon dioxide and water through the energy of captured sunlight starches and sugars which are the fuel of animal life. This process of photosynthesis is the cornerstone of all life on earth. ”

“ If the organic compounds produced by plants be classified with reference to their uses in metabolism into the three groups known, respectively, as temporary foods, storage products, and permanent structures, it is clear that the carbohydrates which have been discussed in this chapter may fall into either one of the first two of these classes. There can be no doubt that the first products of photosynthesis, whichever ones they may be in different plants, may be directly used as temporary foods, to furnish the energy and material for the building up of permanent structures. ”

“ This they decompose, adding the carbon to compounds in their tissues and returning the oxygen to the air. It is found, however, that this process does not occur unless the plants are exposed to sunlight. The sunlight supplies the energy for overcoming the attraction between the atoms of oxygen and the atoms of carbon, while the plant itself serves as the instrument through which the sunlight acts. The energy for decomposing the carbon dioxide then comes from the sun, and through the decomposition of the carbon dioxide the sun's energy is stored—becomes potential. ”

“ The roots get some of it out of the earth, and the leaves get some out of the air. Nitrogen is what is left when the oxygen is burned out of the air. (See Air.)
A leaf is very thin. The sun can shine right through it, for the cell walls or skin is as transparent as glass. In some way sunlight mixes with the water and minerals from the roots and the oxygen, from the air, and makes green plant cells. The clear, unused part of the water is drawn away in vapor, and most of the oxygen is given back to the air for animals to breathe. The carbon is laid away in the plant cells. ”

“ Photosynthetic System Green plants contain chlorophyll which captures light energy thermodynamically required to convert carbon dioxide and water into carbohydrate which can subsequently be transformed into other foods such as protein and fat. During this process, carbon dioxide is consumed, and an approximately equal amount of oxygen gas is liberated. ”

“ For we must never forget that all plants mainly grow, not, as most people suppose, from the earth, but from the air. They are for the most part mere masses of carbon-compounds, and the carbon in them comes from the carbonic acid diffused through the atmosphere around, and is separated by the sunlight acting in the leaves. There it mixes with small quantities of hydrogen and nitrogen brought by the roots from soil and water; and the starches or other bodies thus formed are then conveyed by the sap to the places where they will be required in the economy of the plant system. ”

“ We have in these plants a power which appears special to them, in the possession of some mechanism for the construction of organic substance which differs essentially from the chlorophyll apparatus of green plants, and yet brings about substantially similar results. The steps by which this carbon dioxide is built up into a compound capable of being assimilated by the protoplasm of the cells are not known. The energy for the purpose appears to be supplied by the oxidation of the molecules containing nitrogen, so that it is dependent upon such oxidation taking place. ”

“ The energy of the solar radiations is absorbed by the chlorophyll of the leaves, and stored up in the organic carbohydrates formed by the synthesis of water and carbon. Chlorophyll has the peculiar property of reducing carbonic acid, and uniting the carbon with water in different proportions to form sugar and starch, whilst fats and vegetable albumens are also formed by an analogous reaction. All these complex bodies are stores of energy; the vital processes of oxydation do but liberate in the human body the energy which the chlorophyll of plants has absorbed from the solar rays. ”

“ Put more generally, we may now say that the process of carbon-dioxide assimilation in green leaves under the influence of light is a process of synthesis—photo-synthesis—resulting in the building up of a carbohydrate such as sugar, inulin or starch from the elements carbon, hydrogen and oxygen. ”

“ The vegetable matter has lost the principle of organic life, and, that gone, the tendency of all things being to be resolved into their most simple forms, a disunion of the elements commences: oxygen, hydrogen, and carbon pass off either in the gaseous state or as water, whilst some carbon is liberated in a very finely-divided condition, and enters slowly into combination with oxygen supplied by the water or the air. Hydrogenous compounds are at the same time formed, and, under all these circumstances, as in all other chemical phenomena, an alteration of temperature results. ”

“ A plant supplied with pure carbon, hydrogen, oxygen, and nitrogen, phosphorus, sulphur, and the like, would as infallibly die as the animal in his bath of smelling-salts, though it would be surrounded by all the constituents of protoplasm. Nor, indeed, need the process of simplification of vegetable food be carried so far as this, in order to arrive at the limit of the plant's thaumaturgy. Let water, carbonic acid, and all the other needful constituents be supplied except nitrogenous salts, and an ordinary plant will still be unable to manufacture protoplasm. ”

“ The soil and the atmosphere supply plants with water, composed of hydrogen and oxygen; air, consisting of nitrogen and oxygen; and carbonic acid, containing carbon and oxygen. They retain the hydrogen and the carbon, exhale the superfluous oxygen, and absorb little or no nitrogen. The essential character of vegetable life is the exhalation of oxygen, which is effected through the agency of light. Animals, on the contrary, derive their nourishment either directly or indirectly from plants. They get rid of the superfluous hydrogen and carbon, and accumulate nitrogen. ”

“ Green plants—those containing chlorophyll— manufacture their own starch from CO2 and H2O. These chlorophyll grains are the plant's chemical laboratories, and hundreds of thousands of them exist in every leaf. CO2 and a very little H2O enter the leaf from the air, H2O being also drawn up through the root and stem from the earth. In some unknown way in the leaf, light has the power of synthesizing these into starch and setting free O, which is returned to the atmosphere.6 CO2 + 5 H2O = C6H10O5 + 12 O. As no such change takes place in darkness, all green plants must have light. ”

“ They respire by means of their nitrogenous constituents; 2. They assimilate carbon by means of the carbonaceous matters organized in their tissues, i. e., the chlorophyll.
This act of respiration, as we have seen, becomes less apparent as the plant begins to assimilate carbon, but, in reality, it goes on uninterruptedly, being only masked by the increased activity of the other function. That this is the case may be proved experimentally.
In the first place, we know that plants cannot live in an atmosphere without oxygen. ”

“ Plants gain their nourishment by the absorption through their roots of certain substances from the soil, and by the decomposition, through their green portions, of a particular gas contained in the atmosphere—carbonic-acid gas. They decompose this gas into carbon, which they assimilate, and oxygen, which they reject. Now, this phenomenon, which is the vegetable's mode of respiration, can only be accomplished with the assistance of solar light. ”

“ The plant seizes the combined carbon and oxygen and tears them asunder, storing the carbon and letting the oxygen go free. By no special force, different in quality from other forces, do plants exercise this power—the real magician here is the sun. We have seen how heat is consumed in forcing asunder the atoms and molecules of solids and liquids, converting itself into potential energy, which reappears as heat when the attractions of the separated atoms are again allowed to come into play. ”

“ From the standpoint of the sum total of its activities, a green plant is essentially an absorber of solar energy and a synthetizer of organic substances. Each individual autotrophic plant takes up certain amounts of the anergic foods which are discussed in the preceding chapter and manufactures from them a great variety of complex organic compounds, using the energy of the sun's rays, absorbed by chlorophyll, as the source for the energy necessary to accomplish these synthetic reactions. ”

“ On the other hand he found, that the decomposition of carbon dioxide by the green parts in light is an occupation that is necessary to them, that plants die when they are deprived of it. The first clear insight into the chemical processes which accompany the decomposition of carbon dioxide in the interior of the plant was obtained by perceiving, that plants by appro printing a definite quantity of carbon make a much more than proportionate addition to their dry substance, and that this is due to the simultaneous fixation of the component parts of water. ”

“ Now, in order that a plant may grow, its four organic constituents must be absorbed by it, and that this absorption may take place, it is essential that they be presented to it in suitable forms. A seed may be planted in pure carbon, and supplied with unlimited quantities of hydrogen, nitrogen, oxygen, and inorganic substances, and it will not germinate; and a plant, when placed in similar circumstances, shows no disposition to increase, but rapidly languishes and dies. ”

“ The difference between the photosynthetic and electrolysis-chemosynthetic systems is the way electrical energy is made available to the organisms. In the photosynthetic system, electrical energy is converted to light which the algae or plants transform into chemical energy. In the chemosynthetic process, electrical energy is transformed into the chemical energy of hydrogen gas which is used by the bacteria. Both organisms use the chemical energy available to them to synthesize cell material with similar degrees of efficiency. ”

“ On plants, however, the solar rays that produce in us the impression of yellow, are the immediate agents of those molecular changes through which are hourly accumulated the materials for further growth. Experiments have shown that when the Sun shines on living leaves, they begin to exhale oxygen and to accumulate carbon and hydrogen—results which are traced to the decomposition, by the solar rays, of the carbonic acid and water absorbed. ”

“ The first process sets energy free, the other process stores it up. Animals give off carbon dioxide from their lungs or gills, and plants give off oxygen from their leaves. But plants need some energy in growing, so oxidation also occurs in plants, but to a far less extent than in animals. At night, because of the absence of sunlight, no deoxidation is taking place 6in the plant, but oxidation and growth continue; so at night the plant actually breathes out some carbon dioxide. The deepest part of the lungs contains the most carbon dioxide. ”