Long before animal life appeared on the Earth, the plants were busy making food for them. By the wonders of a pigment known as chlorophyll, the plants took a simple gas called carbon dioxide and water, then borrowed energy from the sun and made a food called sugar. Sugar is a molecule consisting of carbon, oxygen and hydrogen held together by strong chemical bonds forged in the cell leaf, with ATP generated in the leaf using photons from the sun’s light.
Chloroplasts are organelles found in plant cells that conduct photosynthesis. They capture light energy and convert it to a form of ATP through photosynthesis, which occurs in several steps. Some of the light energy is stored in the form of ATP while most of it is used to remove electrons from water. These electrons are then used in the reactions that turn carbon dioxide into the organic compound know as sugar, actually sucrose. The plant achieves this by a sequence of reactions known as the Calvin cycle.
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The Calvin cycle. The fixation or reduction of carbon dioxide is a process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), making it a six-carbon sugar that yields two molecules of a three-carbon compound, called glycerate 3-phosphate (GP), also identified as 3-phosphoglycerate (PGA). GP plus ATP and NADPH from the light-dependent stages is reduced to glyceraldehyde 3-phosphate (G3P), a true three-carbon sugar, or triose. Five out of six molecules of the triose produced are used to regenerate RuBP so the process can continue. The other molecules of the triose phosphates that are not recycled condense to form hexose phosphates, namely sucrose, starch and cellulose. The sugars actually yield carbon skeletons that can be incorporated into other metabolic reactions for the production of amino acids and starch.
Photosynthesis and sugar
The leaf consists of an upper and lower epidermis with cells in between called mesophyll cells. Within the mesophyll cells are hundreds of structures called chloroplasts that contain chlorophyll packed into disc-shaped structures called grana. The grana are made up of stacks of single discs called thylakoids. The chlorophylls and other pigments, such as carotenoids, are in the outer layer of the thylakoids. Photons from sunlight hit the pigments and transfer high energy to electrons that are knocked loose from the chlorophyll and fly off to energize the complicated process of photosynthesis. This is the first part of the photosynthesis and the only part that requires light energy, thus it is often called the light reaction or light-dependant reaction. Water enters the process in the stroma where a special enzyme breaks it down to hydrogen and oxygen by photolysis of water. The hydrogen is further broken down into hydrogen ions and electrons. These electrons are returned to chlorophyll to be used again. The oxygen molecule is either used in the formation of other compounds or is returned to the air as molecular oxygen. After the thylakoids, the next process moves out to the stroma. The stroma is where enzymes take the carbon from carbon dioxide and combine it with hydrogen and oxygen to make simple carbohydrate molecules. This part of the process is generally referred to as the light-independent reactions or dark reactions.
What about the hydrogen ions? They pass out of the thylakoid space through special channels in the membrane, and the movement of these ions through the channels provides the energy to make ATP synthetase enzymes, where ATP molecules are made. A steady flow of electrons from the water, through the membrane and finally to NADPH produces an electric current that provides the energy for the making of ATP and NADPH. It is the energy in ATP and NADH that is used for the making of carbohydrates in stage two of photosynthesis, the carbon cycle.
The second stage of photosynthesis is sometimes referred to as the carbon-fixing process because carbon from carbon dioxide is fixed into the beginnings of simple sugar, also called carbohydrate molecules. In the stroma, different enzymes use the carbon dioxide molecules and hydrogen ions made during the light dependent phase to assemble sugar fragments that are only half of a glucose molecule (just three carbon atoms, instead of the six carbons in a complete glucose molecule). This is really the end of photosynthesis, but a sugar molecule has not yet been created. The three-carbon half-glucose molecule is pushed out of the chloroplast into the waiting arms of an eager enzyme that joins these little fragments into real six-carbon glucose molecules. The glucose molecules serve as building blocks for other carbohydrates, such as sucrose, lactose, ribose, cellulose and starch. Then when the leaf gets eaten, the glucose goes into animal cells where it can be used to make fats, oils, amino acids and proteins.
Bioenergetics and ATP
ATP is part of a more comprehensive topic called bioenergetics. When a student is first introduced the subject, it is either love or hate at first sight. In my case, initially I didn’t like it at all; perhaps it took me more than 30 years to love it. Keep in mind that every biochemical reaction in the body relates in some manner to the topic of bioenergetics. Energy is not an easy term to define since the only thing seen or felt is the transfer of energy. Wikipedia has a very good explanation for energy: “In physics, energy is a scalar physical quantity that describes the amount of work that can be performed by a force, an attribute of objects and systems that is subject to a conservation law. Eight different forms of energy exist to explain all known natural phenomena. These forms include (but are not limited to) kinetic, potential, thermal, gravitational, sound, light, elastic and electromagnetic energy. The forms of energy are often named after a related force.”
A key concept to remember is that any form of energy can be transformed into another form. For example, light can be transformed to electricity, and heat to kinetic energy, or motion, as in driving a car. In order to measure heat, a unit must be used, just as pounds or grams are used when measuring the weight of an object. In energy, the term “joule” is used. The definition of a joule is the work done to produce the power of one watt continuously for one second; or one watt second. First, one watt as the rate at which work is done is defined when an object is moved at a speed of one meter per second against a force of one newton. A newton is the amount of force required to accelerate a mass of one kilogram at a rate of one meter per second per second. Hang on a bit, and it will all be put together.
Let’s change now from basic terms to the real world of bioenergy. As learned earlier in the article, all energy comes from the sun, then into plants and made into sugars. Sugar can be taken and made into a sweet drink. For example, you drink an orange soda that contains 100 g of glucose, actually sucrose. The glucose is metabolized by the body, and the energy derived is converted to ATP. How much energy as ATP are you going to get from that amount of glucose? A little math will show you a surprising answer.
One hundred grams of sucrose is converted to 100 g of glucose. In the mitochondria, a single molecule of glucose will yield 36 molecules of ATP. The task at hand is to convert 100 g of glucose first to molecules of glucose then to molecules of ATP. A molecule weight of a substance in grams is called a mole, or m. Glucose has a molecular weight of 180 g/m, so divide 100/180 and get 0.55 m. Now, how many molecules of glucose is that? A mole of any substance will contain 6.02 x 1,023 molecules, which is very big numberd. Multiply 0.55 x 6.02 x 1,023 = 3.31 x 1,023 molecules in 0.55 m of glucose. Now each molecule of glucose makes 36 molecules of ATP. Multiply the two numbers 36 m x 3.31 x 1,023, resulting in 119.16 x 1,023 molecules of ATP, divided by 6.02 x 1,023 ( to get back to moles), with the answer of 19.8 moles of ATP. The weight of one mole of ATP is 507 grams, so multiplying 19.8 x 507 we get 10,038.6 grams of ATP or 10,039 kilograms of ATP from one soda. (To get pounds of ATP multiply 10,039 x 2.2 = 22,086 pounds.) Convert moles of ATP to kilocalories (kc) multiply by 7.3 kc/m will yield 7.3 kc x 19.8 m of ATP = 144.5 kc, or 604992.6 joules.
The energy in one joule will provide the energy to lift one pound to a height of nine inches. Therefore, if you want to lift an object weighing 10 pounds to counter height (36 inches), it would require about 40 joules, not a lot of energy. On the other hand, a piece of buttered toast contains 315 kilojoules, that is 315,000 joules! With that much energy you can jog six minutes, or walk fast for 10 minutes, or light a 60 watt bulb for 1-1/2 hours. One joule in everyday life is a very small quantity, so instead international nutritional calories are used. A heat calorie is a small quantity—it is the amount of heat required to raise a gram of water one degree centigrade, so a large quantity is needed. The international nutritional calorie is designated by a big C and is equal to 1,000 small heat calories. If a calorie is compared to joules, it would take 4,186.8 joules to equal one nutritional calorie (C).
One international nutritional calorie can supply enough energy to lift an adult human two stories high. Remember that one calorie equals 1,000 (heat) calories, which converts to 4,200 joules (rounded). One joule of energy is enough to lift 1/10 of a kilogram (about 1/4 pound) one meter in the air. One calorie, or 4,200 joules can lift a 70 kg (154 lb) person six meters into the air. Do you realize that if you eat one peanut, which contains 1.8 C, you will need to walk up a four-story building to burn up that peanut?