Why the Grass Is Green—A Closer Look at Photosynthesis
“WHY is the grass green?” Perhaps you asked that question as a child. Were you satisfied with the answer? Children’s questions such as this one can be very profound. They can cause us to look more deeply at everyday things that we take for granted and reveal hidden wonders that we never suspected were there.
To understand why the grass is green, imagine something that may seem to have nothing to do with grass. Imagine, if you will, the perfect factory. The perfect factory would be quiet in operation and attractive to look at, wouldn’t it? Instead of polluting, the perfect factory would actually improve the environment by its very operation. Of course, it would produce something useful—indeed vital—to everyone. Such a factory would be solar powered, don’t you think? That way, it would not require an electric connection or deliveries of coal or oil to power it.
No doubt the perfect solar-powered factory would use solar panels far superior to man’s current technology. They would be highly efficient, inexpensive, and nonpolluting, both to make and to use. Although it would use the most advanced technology imaginable, the perfect factory would do so unobtrusively, without the unexpected glitches, breakdowns, or endless tweaking that cutting-edge technology seems to require these days. We would expect the perfect factory to be fully automated, requiring no human attention to operate. Indeed, it would be self-repairing, self-sustaining, and even self-duplicating.
Is the perfect factory just science fiction? A mere unattainable pipe dream? No, indeed, for the perfect factory is as real as the grass beneath your feet. As a matter of fact, it is the grass beneath your feet, along with the fern in your office and the tree outside your window. You see, the perfect factory is any green plant! Fueled by sunlight, green plants use carbon dioxide, water, and minerals to produce food, directly or indirectly, for almost all life on earth. In the process, they replenish the atmosphere, removing carbon dioxide and releasing pure oxygen.
All together, the earth’s green plants produce an estimated 150 billion to 400 billion tons of sugar every year—far more material than the combined output of all mankind’s iron, steel, automobile, and aerospace factories. They do this by using the energy from the sun to remove hydrogen atoms from water molecules and then attach those hydrogen atoms to carbon dioxide molecules from the air, turning the carbon dioxide into a carbohydrate known as sugar. This remarkable process is called photosynthesis. The plants can then use their new sugar molecules for energy or can combine them together into starch for food storage or into cellulose, the tough, stringy material that makes up plant fiber. Think of it! As it grew, that huge sequoia tree towering 300 feet [90 m] above you was made mostly out of thin air, one carbon-dioxide molecule and one water molecule at a time, in countless millions of microscopic ‘assembly lines’ called chloroplasts. But how?
Taking a Look at the “Engine”
Making a sequoia out of thin air (plus water and a few minerals) is truly amazing, but it is not magic. It is the result of intelligent design and technology far more sophisticated than any possessed by man. Little by little, scientists are prying the lid off the black box of photosynthesis to gaze in wonder at the supersophisticated biochemistry taking place within. Let’s take a peek with them at the “engine” responsible for almost all life on earth. Perhaps we will begin to get an answer to our question “Why is the grass green?”
Getting out our trusty microscope, let’s examine a typical leaf. To the naked eye, the whole leaf seems green, but that is an illusion. The individual plant cells that we see under the microscope are not so green after all. Instead, they are mostly transparent, but each contains perhaps 50 to 100 tiny green dots. These dots are the chloroplasts, where the light-sensitive green chlorophyll is found and where photosynthesis takes place. What is going on inside the chloroplasts?
The chloroplast is like a tiny bag with even smaller flattened bags called thylakoids inside it. Finally, we have located the green in the grass. Green chlorophyll molecules are embedded in the surface of the thylakoids, not at random, but in carefully organized assemblies called photosystems. There are two types of photosystems in most green plants, known as PSI (photosystem I) and PSII (photosystem II). The photosystems act like specialized production teams in a factory, each taking care of a specific series of steps in photosynthesis.
“Waste” That Is Not Wasted
As sunlight strikes the surface of the thylakoid, PSII arrays of chlorophyll molecules called light-harvesting complexes are waiting to snare it. These molecules are especially interested in absorbing red light of a specific wavelength. In different locations on the thylakoid, PSI arrays are on the lookout for light with a somewhat longer wavelength. Meanwhile, both chlorophyll and some other molecules, such as carotenoids, are absorbing blue and violet light.
So why is grass green? Of all the wavelengths falling on plants, only green light is useless to them, so it is simply reflected away to our waiting eyes and cameras. Think of it! The delicate greens of spring, like the deep emerald greens of summer, result from the wavelengths that plants do not appreciate but that we humans treasure! Unlike the pollution and the waste of man’s factories, this “waste” light is surely not wasted when we gaze upon a beautiful meadow or forest, refreshing our souls with the pleasing color of life.
Back in the chloroplast, in the PSII array, the energy from the red portion of the sunlight has been transferred to electrons in the chlorophyll molecules until, finally, an electron is so energized, or “excited,” that it jumps from the array altogether, into the arms of a waiting carrier molecule in the thylakoid membrane. Like a dancer being passed from partner to partner, the electron is passed from one carrier molecule to another as it gradually loses energy. When its energy is low enough, it can safely be used to replace an electron in the other photosystem, PSI.—See diagram 1.
Meanwhile, the PSII array is missing an electron, which makes it positively charged and hungry for an electron to replace the one it lost. Like a man who has just discovered that his pocket was picked, the area of PSII known as the oxygen-evolving complex is frantic. Where is an electron to be found? Aha! Loitering nearby is a hapless water molecule. It is in for a nasty surprise.
Ripping Apart Water Molecules
A water molecule consists of a relatively large oxygen atom and two smaller hydrogen atoms. The oxygen-evolving complex of PSII contains four ions of the metal manganese that remove the electrons from the hydrogen atoms in the water molecule. The result is that the water molecule is broken down into two positive hydrogen ions (protons), one oxygen atom, and two electrons. As more water molecules are dismembered, the oxygen atoms pair off as molecules of oxygen gas, which the plant returns to the air for our use. The hydrogen ions begin to accumulate inside the thylakoid “bag,” where they can be used by the plant, and the electrons are used to resupply the PSII complex, which is now ready to repeat the cycle many times per second.—See diagram 2.
Inside the thylakoid sac, the crowded hydrogen ions start looking for a way out. Not only are two hydrogen ions added each time a water molecule is broken down but other hydrogen ions are being enticed into the thylakoid sac by the PSII electrons as they are being passed over to the PSI complex. Pretty soon, the hydrogen ions are buzzing like angry bees in an overcrowded hive. How can they get out?
It turns out that the brilliant Designer of photosynthesis has supplied a revolving door with only one way out, in the form of a special enzyme used to make a very important cellular fuel called ATP (adenosine triphosphate). As the hydrogen ions force their way out the revolving door, they supply the energy needed to recharge spent ATP molecules. (See diagram 3.) ATP molecules are like tiny cellular batteries. They supply little bursts of energy, right on the spot, for all sorts of reactions in the cell. Later, these ATP molecules will be needed on the photosynthesis sugar assembly-line.
Besides ATP, another small molecule is vital for sugar assembly. It is called NADPH (a reduced form of nicotinamide adenine dinucleotide phosphate). NADPH molecules are like little delivery trucks, each carrying a hydrogen atom to a waiting enzyme that needs the hydrogen atom to help build a sugar molecule. Creating NADPH is the job of the PSI complex. While one photosystem (PSII) is busy ripping apart water molecules and using them to create ATP, the other photosystem (PSI) is absorbing light and ejecting electrons that are eventually used to create NADPH. Both the ATP and NADPH molecules are stored in the space outside the thylakoid for future use on the sugar assembly-line.
The Night Shift
Billions of tons of sugar are created each year by photosynthesis, and yet the light-powered reactions in photosynthesis do not actually make any sugar. All they make is ATP (“batteries”) and NADPH (“delivery trucks”). From this point, the enzymes in the stroma, or space outside the thylakoids, use the ATP and NADPH to make sugar. In fact, the plant can make sugar in complete darkness! You could compare the chloroplast to a factory with two crews (PSI and PSII) inside the thylakoids making batteries and delivery trucks (ATP and NADPH) to be used by a third crew (special enzymes) out in the stroma. (See diagram 4.) That third crew makes sugar by adding hydrogen atoms and carbon dioxide molecules in a precise sequence of chemical reactions using the enzymes in the stroma. All three crews can work during the day, and the sugar crew works a night shift as well, at least until the supplies of ATP and NADPH from the day shift are used up.
You could think of the stroma as a kind of cellular matchmaking agency, full of atoms and molecules that need to be “married” to each other but that will never work up the nerve on their own. Certain enzymes are like very pushy little matchmakers.* They are protein molecules with special shapes that allow them to grab onto just the right atoms or molecules for a particular reaction. They are not content merely to introduce future molecular marriage mates, however. The enzymes will not be satisfied until they see the marriage take place, so they grab the future couple and bring the reluctant mates into direct contact with each other, forcing the matrimony in a sort of biochemical shotgun wedding. After the ceremony, the enzymes release the new molecule and repeat the process, over and over. In the stroma the enzymes pass partially complete sugar molecules around with incredible speed, rearranging them, energizing them with ATP, adding carbon dioxide, attaching hydrogen, and, finally, sending off a three-carbon sugar to be further modified elsewhere in the cell into glucose and a host of variations.—See diagram 5.
Why Is the Grass Green?
Photosynthesis is far more than just a basic chemical reaction. It is a biochemical symphony of amazing complexity and subtlety. The book Life Processes of Plants puts it this way: “Photosynthesis is a remarkable, highly regulated process for harnessing the energy of the sun’s photons. The complex architecture of the plant and the incredibly intricate biochemical and genetic controls that regulate photosynthetic activity may be viewed as refinements of the basic process of trapping the photon and converting its energy into chemical form.”
In other words, to find out why the grass is green is to gaze in wonder at design and technology far superior to anything mankind has devised—self-regulating, self-maintaining, submicroscopic “machines” that operate at thousands, or even millions, of cycles per second (without noise, pollution, or ugliness), turning sunlight into sugar. To us it is to catch a glimpse of the mind of a designer and engineer par excellence—our Creator, Jehovah God. Think about it the next time you admire one of Jehovah’s beautiful, life-sustaining, perfect factories or the next time you just walk on that lovely green grass.
Some other types of enzymes are like pushy little divorce lawyers; their job is to split molecules apart.
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Inset photo: Colorpix, Godo-Foto
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How did photosynthesis make this tree grow?
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