Photosynthesis

Oil and coal today provide about 90% of the energy needed to power trains, trucks, ships, airplanes, factories, computers, communication systems, and a multitude of electrically energized appliances. The energy within that oil and coal was originally captured from the sun by plants and algae growing millions of years ago and then transformed into fossil fuels by geological forces.

The energy needs of transportation, industry, and the modern household seem insignificant, however, when compared with the combined energy requirements of all living organisms. Every living cell requires energy just to remain alive, and more energy is needed for the cell to reproduce, grow, or do physical work as part of an organism. In addition, oxygen is vital to nearly all life in processes that release stored energy.

Photosynthesis, at least indirectly, is not only the principal means of keeping all forms of humanity functioning, but also the sole means of sustaining life at any level—except for a few bacteria that derive their energy from sulfur salts and other inorganic compounds. This unique manufacturing process of green plants furnishes raw material, energy, and oxygen. In photosynthesis, energy from the sun is harnessed and, with the aid of chlorophyll, is transformed from light energy to biochemical energy in the bonds between the atoms in a sugar molecule. Oxygen is given off as a by-product of the process.

It has been estimated that all of the world's green organisms (including those in the oceans) together produce between 100 billion and 200 billion metric tons (between 110 billion and 220 billion tons) of sugar each year. To visualize that much sugar, consider that it is enough to make about 300 quadrillion sugar cubes with a total volume exceeding that of two million Empire State Buildings.1

Much of the sugar produced by plants is converted to wood, fibers (such as cotton and linen), and other structural materials. The first products of photosynthesis may also be converted to disaccharides, polysaccharides such as starch, and other storage forms of carbohydrates. The digestive activities of living organisms break down the carbohydrates to smaller molecules.

Sugars produced by photosynthesis are also involved in the synthesis of amino acids for proteins and a host of other cell constituents. In fact, photosynthesis produces more than 94% of the dry weight substance of green organisms, with the remainder coming from the soil or dissolved matter.

The capacity of plants to meet our energy needs may well determine the ultimate size of human populations. In some heavily populated parts of the world, the food supply already is falling short of providing enough energy to sustain life, and starvation is widespread. Meanwhile, in the Western world, significant numbers of persons consume too much food and are spending large sums on weight reduction. We will, however, eventually approach a point at which human populations in general will need to stabilize, or even those in the most affluent areas could exceed the capacity of the plants to sustain them.

A great deal of photosynthesis occurs in organisms living in the oceans. It is estimated that between 40% and 50% of the oxygen in the atmosphere originates in oceans and lakes. Laboratory tests have shown, however, that pollutants—such as the PCBs (poly chlorinated biphenyls) used in electrical insulators—are capable, in concentrations as low as 20 parts per billion, of stopping many delicate algae from carrying on photosynthesis. The concentration of such substances in ocean waters at present is considerably less than one part per billion, but PCB concentrations of up to five or more parts per billion have already been reported in some estuaries. The use of PCBs and related chemicals has been curtailed in the United States, but other countries are still using them, and residues are still washing off into rivers and on into the oceans. It is important to make the world community fully aware of the dangers of allowing pollutant concentrations to build up to the point of creating significant problems that could ultimately adversely affect all life.

1. One quadrillion is 1,000,000,000,000,000. New York City's Empire State Building has 102 stories and is 381 meters (1,250 feet) tall.

Plant Metabolism 173

Note to the Reader Photosynthesis is undoubtedly the most important process on earth to life as we know it. It is also a complex process that can be summarized briefly or examined in detail. What follows is a discussion of the subject at three different levels: (1) The essence of photosynthesis; (2) a brief introduction to the major steps of photosynthesis; and (3) a closer look at photosynthesis. The process of respiration, which is discussed after photosynthesis, is treated in similar fashion. The third level, in particular, contains detail that either may or may not be discussed in your course.

1. The Essence of Photosynthesis

Cells need energy to do their work and reproduce. The energy for most of this cellular activity involves energy-storing molecules commonly known as ATP (adenosine triphosphate)? ATP doesn't last more than a few seconds and has to be produced constantly. Instead of ingesting food as animals do, plants can make ATP, using light as the source of energy, just like a solar-powered engine. If the light goes out, however, ATP production stops, and the cells could quickly die. In a solar-powered engine, this problem is avoided by using some of the generated electricity to charge batteries before the sun goes down. Plants also accumulate energy for later use by building sugar molecules for short-term energy storage or starch for longer-term energy storage.

The energy-storing process of photosynthesis takes place in chloroplasts and other parts of green organisms in the presence of light. The light energy stored in a simple sugar molecule is produced from carbon dioxide (CO2) present in the air and water (H2O) absorbed by the plant. When the carbon dioxide and water (H2O) are combined and ultimately a sugar molecule (C6H12O6, glucose) is produced in a chloroplast, oxygen gas (O2) is released as a by-product. The oxygen diffuses out into the atmosphere. The overall process can be depicted by the equation that follows. The equation should not, however, be taken literally because there are many intermediate steps to the process, and glucose is not the immediate first product of photosynthesis.

6 CO2 + 12 H2O + light energy chlorophyll carbon water enzymes^

dioxide

C6H12O6 + 6 O2 + 6 H2O glucose oxygen water

2. There are millions of ATP molecules in living cells. When an ATP molecule releases its energy, it gives up the terminal phosphate group in the row of three attached to its base and becomes ADP (adenosine diphosphate). An ADP molecule becomes an ATP molecule again when it regains its third phosphate group and stores energy. ATP is an important participant in many reactions involving the transfer of energy.

Photosynthesis takes place in chloroplasts (see Figs. 3.5 and 3.8) or in cells with membranes in which chlorophyll is embedded. The principals in the process are carbon dioxide, water, light, and chlorophyll; a brief examination of each follows.

Carbon Dioxide

Our atmosphere consists of approximately 78% nitrogen and about 21% oxygen. The remaining 1% is made up of a mixture of less common gases, including 0.036% (360 parts per million) carbon dioxide and a little hydrogen, helium, argon, and neon. The carbon dioxide in the air surrounding the leaves of plants reaches the chloroplasts in the mesophyll cells by diffusing through the stomata into the leaf interior. The carbon dioxide goes into solution in a thin film of water on the outside walls of cells. It then diffuses through the cell walls, across cell membranes, and into the cytoplasm where it finally reaches the chloroplasts.

The amount of carbon dioxide constantly being taken from the atmosphere during daylight hours by all green plants is enormous. Just four-tenths of a hectare (1 acre) of corn (10,000 plants) accumulates more than 2,500 kilograms (5,512 pounds) of carbon from the atmosphere during a growing season. Over 10 metric tons (11 tons) of carbon dioxide are needed to furnish this much carbon. It also has been calculated that the total present atmospheric supply of carbon dioxide (more than 2.2 billion metric tons or about 50 metric tons over each hectare of the earth's surface) would be completely used up in about 22 years if it were not constantly being replenished.

A large reservoir of carbon dioxide in the oceans has helped to maintain atmospheric carbon dioxide levels at about 0.036% throughout most of recorded history, but in the last 50 years, the percentage has begun to climb slightly. Plant and animal respiration, decomposition, natural fires, volcanoes, and similar sources replace carbon dioxide at roughly the same rate at which it is removed during photosynthesis. The use of fossil fuels, pollution, deforestation, and other human activities, however, have disrupted this cycle by adding excess carbon dioxide to the atmosphere.

Measurements of atmospheric carbon dioxide at the Mauna Loa research station on the big island of Hawaii have demonstrated this steady increase. Increased carbon dioxide levels have the potential to cause global increases in temperatures because, in the atmosphere, this gas acts much like a glass panel in a greenhouse by trapping solar radiation from the sun. Climatic models project that the earth's average atmospheric temperature will rise from 1.0°C to 3.5°C (1.8°F to 6.3°F) by the year 2100. Since the 1980s, global warming, presumably as a result of this increase, has become a major political and scientific issue. See the "Greenhouse Effect" in Chapter 25 for a discussion of this problem.

Although increases in carbon dioxide levels may enhance photosynthesis and increase food production, insects, bacteria, and viruses that proliferate with warmer temperatures can offset these potential gains. Under carefully

Chapter 10

monitored conditions, commercial greenhouses have pumped carbon dioxide through pipes placed over plant beds to supplement their natural supply and have found that fertilizing plants with this gas has increased yields by more than 20%. Indeed, this indicates that plants have the potential temporarily to limit elevated carbon dioxide levels in the atmosphere. However, recent evidence suggests that many plants develop fewer stomata when carbon dioxide levels increase, thereby adapting them to such changes and reducing photosynthetic efficiency. The abundance of photosyn-thetic organisms that may or may not proliferate under these conditions is a key factor affecting global warming.

Some scientists note that increased carbon dioxide levels will cause temperatures to rise globally, which, in turn, will result in longer growing seasons in middle and high latitudes and hence increase global photosynthesis. Nonetheless, these same temperature increases may accelerate plant and animal respiration and decomposition, which would add carbon dioxide to the environment. Respiration at higher temperatures may also diminish benefits to be anticipated from an increase in numbers of photosynthetic organisms during global warming. Through careful evaluation of global trends and physiological processes, scientists should be better able to predict what is in store for our planet's future and how to improve management of the environment.

Water

Less than 1% of all the water absorbed by plants is used in photosynthesis; most of the remainder is transpired or incorporated into cytoplasm, vacuoles, and other materials. The water used is the source of electrons involved in photosynthesis, and the oxygen released is a by-product, even though carbon dioxide also contains oxygen. This has been demonstrated by conducting photosynthetic experiments using either carbon dioxide or water containing isotopes of oxygen. When the isotope is used only in the water, it appears in the oxygen gas released. If, however, it is used only in the carbon dioxide, it is confined to the sugar and water produced and never appears in the oxygen gas, demonstrating clearly that the water is the sole source of the oxygen released.

If water is in short supply, it may indirectly become a limiting factor in photosynthesis; under such circumstances, the stomata usually close and sharply reduce the carbon dioxide supply.

Light

Light exhibits properties of both waves and particles. Energy reaches the earth from the sun in waves of different lengths, the longest waves being radio waves and the shortest being gamma rays. About 40% of the radiant energy we receive is in the form of visible light. If this visible light is passed through a glass prism, it splits up into its component colors. Reds are on the longer wavelength end and violets on the shorter wave end, with yellows, greens, and blues between (Fig. 10.2). Although nearly all of the visible light colors can

■66G red-orange boG

orange yellow green blue-green blue violet bGG oBG

Figure 10.2 Visible light that is passed through a prism is broken up into individual colors with wavelengths ranging from 390 nanometers (violet) to 780 nanometers (red).

be used in photosynthesis, those in the violet to blue and redorange to red wavelengths are used most extensively. Light in the green range is reflected. Leaves commonly absorb about 80% of the visible light that reaches them.

The intensity of light varies with the time of day, season of the year, altitude, latitude, and atmospheric conditions. On a clear summer day at noon in a temperate zone, sunlight reaches an intensity of about 2,000 |mols3 per square meter of surface per second. In contrast, consider that a room with fluorescent lighting produces only about 40 | mols per square meter of surface per second.

Plants vary considerably in the light intensities they need for photosynthesis to occur at optimal rates. Factors such as temperature and amount of carbon dioxide available can also be limiting (Fig. 10.3). For example, an increase in photosynthesis will not occur in some plants receiving more than 670 |mols of light per square meter unless supplemental carbon dioxide is provided. With supplements, however, rates of photosynthesis will continue to increase up to about 1,000 |mols. Herbaceous plants on a forest floor can survive with less than 2% of full daylight, and some mosses are reported to thrive on intensities as low as 0.05% to 0.01%. Most land plants that grow naturally in the open need at least 30% of full daylight to thrive. The optimal amount for some species of trees approaches full daylight, while shade plants often do well in 10% of full daylight.

Light that is too intense may change the way in which some of a cell's metabolism takes place. For example, higher light intensities and temperatures may change the

3. A |mol is a measurement of the number of photons (see p. 176) striking a specified area such as a leaf surface.

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