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Photosynthesis
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Last modified: 3 weeks, 5 days ago This chapter covers the water, carbon, and nitrogen cycle between abiotic resources and organic matter in the ecosystem, and how oxygen cycles through photosynthesis and respiration. 
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Photosynthesis
Lesson 4.1: Energy for Life: An Overview of PhotosynthesisLesson Objectives
IntroductionAll living things require an ongoing source of energy to do the work of life. You often see energy in action on a large scale: a whale breaches, apple blossoms swell and burst, a firefly glows, or an inky cap mushrooms overnight. However, energy works constantly to maintain life on a very small scale as well. Inside each cell of every organism, energy assembles chains of information and constructs cellular architecture. It moves tiny charged particles and giant protein molecules. Moreover, it builds and powers cell systems for awareness, response, and reproduction. All life’s work requires energy. Physics tells us that organized systems, such as living organisms, tend to disorder without a constant input of energy. You have direct, everyday experience with this law of nature: after a week of living in your room, you must spend energy in order to return it to its previous, ordered state. Tides and rain erode your sandcastles, so you must work to rebuild them. And your body, after a long hike or big game, must have more fuel to keep going. Living things show amazing complexity and intricate beauty, but if their source of energy fails, they suffer injury, illness, and eventually death. Physics also tells us that, although energy can be captured or transformed, it inevitably degrades, becoming heat, a less useful form of energy. This is why organisms require a constant input of energy; the work they must do uses up the energy they take in. Energy, unlike materials, cannot be recycled. The story of life is a story of energy flow – its capture, transformation, use for work, and loss as heat. Energy, the ability to do work, can take many forms: heat, nuclear, electrical, magnetic, light, and chemical energy. Life runs on chemical energy - the energy stored in covalent bonds between atoms in a molecule. Where do organisms get their chemical energy? That depends… How Do Organisms Get Energy? Autotrophs vs. HeterotrophsLiving organisms obtain chemical energy in one of two ways. Autotrophs, shown in Figure below, store chemical energy in carbohydrate food molecules they build themselves. Food is chemical energy stored in organic molecules. Food provides both the energy to do work and the carbon to build bodies. Because most autotrophs transform sunlight to make food, we call the process they use photosynthesis. Only three groups of organisms - plants, algae, and some bacteria - are capable of this life-giving energy transformation. Autotrophs make food for their own use, but they make enough to support other life as well. Almost all other organisms depend absolutely on these three groups for the food they produce. The producers, as autotrophs are also known, begin food chains which feed all life. Food chains will be discussed in the Principles of Ecology chapter. Heterotrophs cannot make their own food, so they must eat or absorb it. For this reason, heterotrophs are also known as consumers. Consumers include all animals and fungi and many protists and bacteria. They may consume autotrophs, or other heterotrophs or organic molecules from other organisms. Heterotrophs show great diversity and may appear far more fascinating than producers. But heterotrophs are limited by our utter dependence on those autotrophs which originally made our food. If plants, algae, and autotrophic bacteria vanished from earth, animals, fungi, and other heterotrophs would soon disappear as well. All life requires a constant input of energy. Only autotrophs can transform that ultimate, solar source into the chemical energy in food which powers life, as shown in Figure below. Photosynthetic autotrophs, which make food for more than 99% of the organisms on earth, include only three groups of organisms: plants such as the redwood tree (a), algae such as kelp (b), and certain bacteria like this (c). Photosynthesis provides over 99 percent of the energy supply for life on earth. A much smaller group of autotrophs - mostly bacteria in dark or low-oxygen environments - produce food using the chemical energy stored in inorganic molecules such as hydrogen sulfide, ammonia, or methane. While photosynthesis transforms light energy to chemical energy, this alternate method of making food transfers chemical energy from inorganic to organic molecules. It is therefore called chemosynthesis, and is characteristic of the tubeworms shown in Figure below. Some of the most recently discovered chemosynthetic bacteria inhabit deep ocean hot water vents or “black smokers.” There, they use the energy in gases from the Earth’s interior to produce food for a variety of unique heterotrophs: giant tube worms, blind shrimp, giant white crabs, and armored snails. Some scientists think that chemosynthesis may support life below the surface of Mars, Jupiter's moon, Europa, and other planets as well. Ecosystems based on chemosynthesis may seem rare and exotic, but they too illustrate the absolute dependence of heterotrophs on autotrophs for food. Food chains carry energy from producers (autotrophs) to consumers (heterotrophs). 99 percent of energy for life comes from the sun via photosynthesis. Note that only nutrients recycle. Energy must continue to flow into the system. Tubeworms deep in the Gulf of Mexico get their energy from chemosynthetic bacteria living within their tissues. No digestive systems needed! Photo: Charles Fisher Food and Other Energy-Carrying MoleculesYou know that the fish you had for lunch contained protein molecules. But do you know that the atoms in that protein could easily have formed the color in a dragonfly’s eye, the heart of a water flea, and the whiplike tail of a Euglena before they hit your plate as sleek fish muscle? As you learned above, food consists of organic (carbon-containing) molecules which store energy in the chemical bonds between their atoms. Organisms use the atoms of food molecules to build larger organic molecules including proteins, DNA, and fats and use the energy in food to power life processes. By breaking the bonds in food molecules, cells release energy to build new compounds. Although some energy dissipates as heat at each energy transfer, much of it is stored in the newly made molecules. Chemical bonds in organic molecules are a reservoir of the energy used to make them. Fueled by the energy from food molecules, cells can combine and recombine the elements of life to form thousands of different molecules. Both the energy (despite some loss) and the materials (despite being reorganized) pass from producer to consumer – perhaps from algal tails, to water flea hearts, to dragonfly eye colors, to fish muscle, to you! The process of photosynthesis, which usually begins the flow of energy through life, uses many different kinds of energy-carrying molecules to transform sunlight energy into chemical energy and build food. Some carrier molecules hold energy briefly, quickly shifting it like a hot potato to other molecules. This strategy allows energy to be released in small, controlled amounts. An example is chlorophyll, the green pigment present in most plants which helps convert solar energy to chemical energy. When a chlorophyll molecule absorbs light energy, electrons are excited and “jump” to a higher energy level. The excited electrons then bounce to a series of carrier molecules, losing a little energy at each step. Most of the “lost” energy powers some small cellular task, such as moving ions across a membrane or building up another molecule. Another short-term energy carrier important to photosynthesis, NADPH, holds chemical energy a bit longer but soon “spends” it to help to build sugar. Two of the most important energy-carrying molecules are glucose and ATP, adenosine triphosphate. These are nearly universal fuels throughout the living world and both are also key players in photosynthesis, as shown below. A molecule of glucose, which has the chemical formula C6H12O6, carries a packet of chemical energy just the right size for transport and uptake by cells. In your body, glucose is the “deliverable” form of energy, carried in your blood through capillaries to each of your 100 trillion cells. Glucose is also the carbohydrate produced by photosynthesis, and as such is the near-universal food for life. ATP molecules store smaller quantities of energy, but each releases just the right amount to actually do work within a cell. Muscle cell proteins, for example, pull each other with the energy released when bonds in ATP break open (discussed below). The process of photosynthesis also makes and uses ATP - for energy to build glucose! ATP, then, is the useable form of energy for your cells. Glucose is the energy-rich product of photosynthesis, a universal food for life. It is also the primary form in which your bloodstream delivers energy to every cell in your body. The six carbons are numbered. Why do we need both glucose and ATP? Why don’t plants just make ATP and be done with it? If energy were money, ATP would be a quarter. Enough money to operate a parking meter or washing machine. Glucose would be a dollar bill (or $10) – much easier to carry around in your wallet, but too large to do the actual work of paying for parking or washing. Just as we find several denominations of money useful, organisms need several “denominations” of energy – a smaller quantity for work within cells, and a larger quantity for stable storage, transport, and delivery to cells. Let’s take a closer look at a molecule of ATP. Although it carries less energy than glucose, its structure is more complex. “A” in ATP refers to the majority of the molecule – adenosine – a combination of a nitrogenous base and a five-carbon sugar. “T” and “P” indicate the three phosphates, linked by bonds which hold the energy actually used by cells. Usually, only the outermost bond breaks to release or spend energy for cellular work. An ATP molecule, shown below, is like a rechargeable battery: its energy can be used by the cell when it breaks apart into ADP (adenosine diphosphate) and phosphate, and then the “worn-out battery” ADP can be recharged using new energy to attach a new phosphate and rebuild ATP. The materials are recyclable, but recall that energy is not! How much energy does it cost to do your body’s work? A single cell uses about 10 million ATP molecules per second, and recycles all of its ATP molecules about every 20-30 seconds. A red arrow shows the bond between two phosphate groups in an ATP molecule. When this bond breaks, its chemical energy can do cellular work. The resulting ADP molecule is recycled when new energy attaches another phosphate, rebuilding ATP. Keep these energy-carrying molecules in mind as we look more carefully at the process which originally captures the energy to build them: photosynthesis. Recall that it provides nearly all of the food (energy and materials) for life. Actually, as you will see, we are indebted to photosynthesis for even more than just the energy and building blocks for life. Photosynthesis: The Most Important Chemical Reaction for Life on EarthWhat do pizza, campfires, dolphins, automobiles, and glaciers have in common? In the following section, you’ll learn that all five rely on photosynthesis, some in more ways than one. Photosynthesis is often considered the most important chemical reaction for life on earth. Let’s delve into how this process works and why we are so indebted to it. Photosynthesis involves a complex series of chemical reactions, each of which convert one substance to another. These reactions taken as a whole can be summarized in a single symbolic representation – as shown in the chemical equation below. We can substitute words for the chemical symbols. Then the equation appears as below. Like all chemical equations, this equation for photosynthesis shows reactants connected by plus signs on the left and products, also connected by plus signs, on the right. An arrow indicating the process or chemical change leads from the reactants to the products, and conditions necessary for the chemical reaction are written above the arrow. Note that the same kinds of atoms, and number of atoms, are found on both sides of the equation, but the kinds of compounds they form change. You use chemical reactions every time you cook or bake. You add together ingredients (the reactants), place them in specific conditions (often heat), and enjoy the results (the products). A recipe for chocolate chip cookies written in chemical equation form is shown below. Compare this familiar recipe to photosynthesis below. The equation shows that the “ingredients” for photosynthesis are carbon dioxide, water, and light energy. Plants, algae, and photosynthetic bacteria take in light from the sun, molecules of carbon dioxide from the air, and water molecules from their environment and combine these reactants to produce food (glucose). Of course, light, carbon dioxide, and water mix in the air even without plants. But they do not chemically change to make food without very specific necessary conditions which are found only in the cells of photosynthetic organisms. Necessary conditions include:
Within plant cells or algal cells, chloroplasts organize the enzymes, chlorophyll, and accessory pigment molecules necessary for photosynthesis. When the reactants meet inside chloroplasts, or the very similar cells of blue-green bacteria, chemical reactions combine them to form two products: energy-rich glucose molecules and molecules of oxygen gas. Photosynthetic organisms store the glucose (usually as starch) and release the oxygen gas into the atmosphere as waste. Let’s review the chemical equation for photosynthesis once more, this time at the level of atoms as in the equation below. Look closely at its primary purpose: storing energy in the chemical bonds of food molecules. The source of energy for food is sunlight energy. The source of carbon atoms for the food molecules is carbon dioxide from the air, and the source of hydrogen atoms is water. Inside the cells of plants, algae, and photosynthetic bacteria, chlorophyll, and enzymes use the light energy to rearrange the atoms of the reactants to form the products, molecules of glucose and oxygen gas. Light energy is thus transformed into chemical energy, stored in the bonds which bind six atoms each of carbon and oxygen to twelve atoms of hydrogen – forming a molecule of glucose. This energy rich carbohydrate molecule becomes food for the plants, algae, and bacteria themselves as well as for the heterotrophs which feed on them. One last detail: why do “6”s precede the CO2, H2O, and O2? Look carefully, and you will see that this “balances” the equation: the numbers of each kind of atom on each side of the arrow are equal. Six molecules each of CO2 and H2O make 1 molecule of glucose and 6 molecules of oxygen gas. Lesson SummaryAll organisms require a constant input of energy to do the work of life.
Life runs on chemical energy.
Organisms obtain chemical energy in one of two ways.
The process of photosynthesis produces more than 99% of all food for life, forming the foundation of most food chains.
All organisms use similar energy-carrying molecules for food and to carry out life processes.
The chemical equation below summarizes the many chemical reactions of photosynthesis.
In the process of photosynthesis, plants, algae, and blue green bacteria absorb sunlight energy and use it to change carbon dioxide and water into glucose and oxygen gas.
Review Questions
Further Reading / Supplemental Links
Vocabulary
Points to Consider
Lesson 4.2: Into the Chloroplast: How Photosynthesis WorksLesson Objectives
IntroductionLife requires photosynthesis for fuel and for the oxygen to burn that fuel. Since the Industrial Revolution (late 18th and early 19th centuries), we humans have relied on products of ancient photosynthesis for enormous quantities of fossil fuel energy. And, knowingly or not, we have also benefited from photosynthesis to remove the carbon dioxide produced when we burn those fuels. So it may not surprise you that biologists have studied this critical process in great detail. The goals of this lesson are:
You’ve learned that a single chemical reaction represents the overall process of photosynthesis as demonstrated in the equation below. Although photosynthesis may seem straightforward in this form, such simplicity is deceiving for two reasons. First, the equation above summarizes dozens of individual chemical reactions involving many intermediate compounds. And second, just discovering major players like CO2 and O2 was challenging, because our ordinary senses cannot detect these molecules in “thin air!” How do we know that the chemical reaction in photosynthesis really happens? Two famous historical experiments help us begin to understand this process. In the 17 century, Jan Van Helmont, a Flemish chemist, physiologist, and physician, weighed and potted a willow tree, showing that plants do not get food from the soil. In the 17th century, people who thought about it at all assumed that plants get their food from the soil. Many people, encouraged by sellers of “plant food,” still do. In 1638, Jan Baptist Van Helmont planted a 5 pound willow tree, like the one shown in Figure above, in a 200 pound tub of soil. After 5 years of watering the plant, he weighed both again. The willow had gained over 160 pounds, but the soil had lost only 2 ounces. Van Helmont concluded that plants do not get their materials from soil, and inferred that they grow using materials from water (which he did not measure). As you know now, he was half right. Although soil provides important nutrients to plants, it supplies neither the energy nor the vast majority of the materials to build the plant. We must excuse him, because no one in the 17th century knew that carbon atoms form the basis of life, or that they float around in air in the form of carbon dioxide. In the late 1770s, minister and natural philosopher Joseph Priestley burned a candle in a jar of air and observed that the candle burned out long before it ran out of wax. A similar experiment with a mouse resulted in the mouse’s death. Priestley suggested that animals, like candles, “injure” the air. Adding a mint plant, as shown in Figure below, however, “restored” the air which had been “injured” by the mouse or the candle. Only later, after many chemistry experiments, did Priestley publish his discovery of “dephlogisticated air.” But in his studies of mice, plants, and candles, he had shown that plants produce, and animals consume, oxygen gas. Joseph Priestly's bell jar experiment. During the 20th century, we learned that photosynthesis involves much more than just the three reactants, the three necessary conditions, and the two products shown in the equation. Using powerful microscopes, we’ve narrowed the process to one type of organelle within the plant – the chloroplast. In the next section, you will learn in more detail just how plants, algae, and photosynthetic bacteria make food for us all “from thin air.” First, let’s look at the organelle in which the drama of photosynthesis takes place and meet some of the key actors. For a detailed animation of the complete photosynthesis process, see http://vcell.ndsu.edu/animations/photosynthesis/first.htm. Chloroplasts: Theaters for PhotosynthesisIf you examine a single leaf of the aquatic plant Elodea, shown in Figure below, under a microscope, you will see within each cell dozens of small green ovals. These are chloroplasts, the organelles which conduct photosynthesis in plants and algae. Chloroplasts closely resemble some types of bacteria and even contain their own circular DNA and ribosomes. In fact, the endosymbiotic theory holds that chloroplasts were once independently living bacteria (prokaryotes). So when we say that photosynthesis occurs within chloroplasts, we speak not only of the organelles within plants and algae, but also of some bacteria – in other words, virtually all photosynthetic autotrophs. (above), like all plants and algae, consists of cells which contain organelles called chloroplasts (green ovals in the microphotograph below). If you look carefully at living cells through a microscope, you may see the chloroplasts moving slowly around the cell edges. The plant itself may not move, but this cyclosis hints at all the action within plant cells. Both chloroplasts and photosynthetic bacteria contain neat stacks (grana) of flattened sac-shaped membrane compartments (thylakoids), made in turn of elaborate and highly organized patterns of molecules which conduct photosynthesis, as shown in Figure below. In addition to enzymes, two basic types of molecules - pigments and electron carriers – are key players. The structure of a chloroplast shows how membrane and molecular architecture helps life processes. Stacks of oval compartments (thylakoids) arrange chlorophyll, accessory pigment molecules, and photosynthetic proteins to capture sunlight and allow a concentration of ions within the sacs. You can see the green color of the chlorophyll. You cannot see the electron carriers, sequenced within the sac membranes, but their arrangement helps harvest small amounts of energy from excited electrons. Pigment molecules, often arranged together with proteins in large, complex photosystems, absorb specific wavelengths of light energy and reflect others; therefore, they appear colored. The most common photosynthetic pigment is chlorophyll, which absorbs blue-violet and red wavelengths of light, and reflects green (Figure below and Figure below). Accessory pigments absorb other colors of light and then transfer the energy to chlorophyll. These include xanthophylls (yellow) and carotenoids (orange). Electron carrier molecules are usually arranged in electron transport chains (ETCs). These accept and pass along energy-carrying electrons in small steps (Figure below). In this way, they produce ATP and NADPH, which temporarily store chemical energy. Electrons in transport chains behave much like a ball bouncing down a set of stairs – a little energy is lost with each bounce. However, the energy “lost” at each step in an electron transport chain accomplishes a little bit of work, which eventually results in the synthesis of ATP. Now that you’ve met some of the key players and explored the theater, let’s put them together to see how the process unfolds. We will divide the process into two basic sets of reactions, known as the light reactions and the Calvin cycle, which uses carbon dioxide. As you study the details, refer frequently to the chemical equation of photosynthesis. In the first stage, you’ll discover how chloroplasts transform light energy, and why we owe our ability to breathe to plants! Photosynthesis Stage I: The Light Reactions: in which Chloroplasts Capture Sunlight Chemical Energy…Every second, the sun fuses over 600 million tons of hydrogen into 596 tons of helium, converting over 4 tons of helium (4.3 billion kg) into light and heat energy. Countless tiny packets of that light energy travel 93 million miles (150 million km) through space, and about 1% of the light which reaches the Earth’s surface participates in photosynthesis. Light is the source of energy for photosynthesis, and the first set of reactions which begin the process requires light – thus the name, Light Reactions, or Light-dependent Reactions. The pigment molecule, chlorophyll, appears green because its electrons absorb blue-violet and red light and reflect green, orange, and yellow light. This diagram shows that there are actually several different kinds of chlorophyll (a,b, and d shown here) in plants. Each kind of pigment absorbs specific wavelengths (colors) of light. Sunlight contains many different wavelengths, which you see when they separate into a rainbow. Not all colors of light are used to make food for life. Most plants, algae, and photosynthetic bacteria appear green because they reflect green wavelengths. Their pigments have absorbed the violet-blue and red wavelengths. The amount of photosynthesis depends on the wavelength of light available. When light strikes chlorophyll (or an accessory pigment) within the chloroplast, it energizes electrons within that molecule. These electrons jump up to higher energy levels; they have absorbed or captured, and now carry, that energy. High-energy electrons are “excited.” Who wouldn’t be excited to hold the energy for life? If you transfer spinach tissue onto a strip of paper and allow solvent to climb the paper, you can separate the pigment molecules. This technique for seeing molecules is known as chromatography (color-writing). The basic concept has many different applications in biochemistry. The images show two stages of a single chromatogram of spinach pigments. …And Change the Rules of Chemistry for Life!The excited electrons leave chlorophyll to participate in further reactions, leaving the chlorophyll “at a loss”; eventually they must be replaced. That replacement process also requires light, working with an enzyme complex to split water molecules. In this process of photolysis (“splitting by light”), H2O molecules are broken into hydrogen ions, electrons, and oxygen atoms. The electrons replace those originally lost from chlorophyll. Hydrogen ions and the high-energy electrons from chlorophyll will carry on the energy transformation drama after the Light Reactions are over. The oxygen atoms, however, form oxygen gas, which is a waste product of photosynthesis (Figure below). The oxygen given off supplies most of the oxygen in our atmosphere. Before photosynthesis evolved, Earth’s atmosphere lacked oxygen altogether, and this highly reactive gas was toxic to the many organisms living at the time. Something had to change! Most contemporary organisms rely on oxygen for efficient respiration. So plants don’t just “restore” the air, as Priestley suggested. They also had a major role in creating it! Photosynthesis has made the Earths atmosphere today very different from what it was 2-3 billion years ago, by giving off oxygen gas as waste. The table to the right shows the composition of todays atmosphere. On the left is an Apollo 17 photograph of Earth. To summarize, chloroplasts “capture” sunlight energy in two ways. Light “excites” electrons in pigment molecules, and light provides the energy to split water molecules, providing more electrons as well as hydrogen ions. Now let’s follow those excited electrons… How Do Chloroplasts Convert Light Energy to Chemical Energy?Excited electrons which have absorbed light energy are unstable. However, the highly organized electron carrier molecules embedded in chloroplast membranes order the flow of these electrons, directing them through electron transport chains (ETCs). At each transfer, small amounts of energy released by the electrons are captured and put to work or stored. Some is also lost as heat with each transfer, but overall the light reactions are extremely efficient at capturing light energy and transforming it to chemical energy. Two sequential transport chains harvest the energy of excited electrons, as shown in Figure below.
Membrane architecture: The large colored carrier molecules form electron transport chains which capture small amounts of energy from excited electrons in order to store it in ATP and NADPH. Follow the energy pathways: light electrons NADPH (blue line) and light electrons concentrated H ATP (red line). Note the intricate organization of the chloroplast. NADPH and ATP molecules now store the energy from excited electrons – energy which was originally sunlight – in chemical bonds. Thus chloroplasts, with their orderly arrangement of pigments, enzymes, and electron transport chains, transform light energy into chemical energy. The first stage of photosynthesis – light-dependent reactions or simply “light reactions” – is complete. Photosynthesis Stage II: The Calvin Cycle - Making Food “From Thin Air”You’ve learned that the first, light-dependent stage of photosynthesis uses two of the three reactants - water and light - and produces one of the products - oxygen gas (a waste product of this process). All three necessary conditions are required – chlorophyll pigments, the chloroplast “theater,” and enzyme catalysts. The first stage transforms light energy into chemical energy, stored to this point in molecules of ATP and NADPH. Look again at the overall equation below. What is left? Waiting in the wings is one more reactant – carbon dioxide, and yet to come is the star product which is food for all life – glucose. These key players perform in the second act of the photosynthesis drama, in which food is “made from thin air!” The second stage of photosynthesis can proceed without light, so its steps are sometimes called “light-independent” or “dark” reactions. Many biologists honor the scientist, Melvin Calvin, who won a 1961 Nobel Prize for working out this complex set of chemical reactions, naming it the Calvin Cycle. The Calvin Cycle has two parts. First carbon dioxide is "fixed." Then ATP and NADPH from the Light Reactions provide energy to combine the fixed carbons to make sugar. Carbon Dioxide is “Fixed”Why does carbon dioxide need to be fixed? Was it ever broken? Life on Earth is carbon-based. Organisms need not only energy but also carbon atoms for building bodies. For nearly all life, the ultimate source of carbon is carbon dioxide (CO2), an inorganic molecule. CO2, as you saw in Figure below, makes up .038% of the Earth’s atmosphere. Animals and most other heterotrophs cannot take in CO2 directly. They must eat other organisms or absorb organic molecules to get carbon. Only autotrophs can build low-energy inorganic CO2 into high-energy organic molecules like glucose. This process is carbon fixation. Stomata on the underside of leaves take in CO and release water and O. Guard cells close the stomata when water is scarce. Leaf cross-section (above) and stoma (below). Plants have evolved three pathways for carbon fixation. The most common pathway combines one molecule of CO2 with a 5-carbon sugar called ribulose biphosphate (RuBP). The enzyme which catalyzes this reaction (nicknamed RuBisCo) is the most abundant enzyme on earth! The resulting 6-carbon molecule is unstable, so it immediately splits into two 3-carbon molecules. The 3 carbons in the first stable molecule of this pathway give this largest group of plants the name “C-3.” Dry air, hot temperatures, and bright sunlight slow the C-3 pathway for carbon fixation. This is because stomata, tiny openings under the leaf which normally allow CO2 to enter and O2 to leave, must close to prevent loss of water vapor (Figure above). Closed stomata lead to a shortage of CO2. Two alternative pathways for carbon fixation demonstrate biochemical adaptations to differing environments. Plants such as corn solve the problem by using a separate compartment to fix CO2. Here CO2 combines with a 3-carbon molecule, resulting in a 4-carbon molecule. Because the first stable organic molecule has four carbons, this adaptation has the name C-4. Shuttled away from the initial fixation site, the 4-carbon molecule is actually broken back down into CO2, and when enough accumulates, Rubisco fixes it a second time! Compartmentalization allows efficient use of low concentrations of carbon dioxide in these specialized plants. Cacti and succulents such as the jade plant avoid water loss by fixing CO2 only at night. These plants close their stomata during the day and open them only in the cooler and more humid nighttime hours. Leaf structure differs slightly from that of C-4 plants, but the fixation pathways are similar. The family of plants in which this pathway was discovered gives the pathway its name, Crassulacean Acid Metabolism, or CAM (Figure below). All three carbon fixation pathways lead to the Calvin Cycle to build sugar. Even chemical reactions adapt to specific environments! Carbon fixation pathways vary among three groups. Temperate species (maple tree, left) use the C-3 pathway. C-4 species (corn, center) concentrate CO in a separate compartment to lessen water loss in hot bright climates. Desert plants (jade plant, right) fix CO only at night, closing stomata in the daytime to conserve water. How Does the Calvin Cycle Store Energy in Sugar?As Melvin Calvin discovered, carbon fixation is the first step of a cycle. Like an electron transport chain, the Calvin cycle, shown in Figure below, transfers energy in small, controlled steps. Each step pushes molecules uphill in terms of energy content. Recall that in the electron transfer chain, excited electrons lose energy to NADPH and ATP. In the Calvin Cycle, NADPH and ATP formed in the light reactions lose their stored chemical energy to build glucose. Use the diagram below to identify the major aspects of the process:
Overview of the Calvin Cycle Pathway. First, notice where carbon is fixed by the enzyme nicknamed Rubisco. In C-3, C-4, and CAM plants, CO2 enters the cycle by joining with 5-carbon ribulose bisphosphate to form a 6-carbon intermediate, which splits (so quickly that it isn’t even shown!) into two 3-carbon molecules. Now look for the points at which ATP and NADPH (made in the light reactions) add chemical energy (“Reduction” in the diagram) to the 3-carbon molecules. The resulting “half-sugars” can enter several different metabolic pathways. One recreates the original 5-carbon precursor, completing the cycle. A second combines two of the 3-carbon molecules to form glucose, universal fuel for life. The cycle begins and ends with the same molecule, but the process combines carbon and energy to build carbohydrates – food for life. So – how does photosynthesis store energy in sugar? Six “turns” of the Calvin cycle use chemical energy from ATP to combine six carbon atoms from six CO2 molecules with 12 “hot hydrogens” from NADPH. The result is one molecule of glucose, C6H12O6. Lesson SummaryThe single chemical equation below represents the overall process of photosynthesis as well as summarizes many individual chemical reactions that were understood only after hundreds of years of scientific exploration. Chloroplasts are the organelles where the process of photosynthesis takes place in plants and algae.
Photosynthesis consists of two groups of chemical reactions: the Light Reactions and the Calvin Cycle. Light Reactions transform energy from sunlight into chemical energy, and produce and release oxygen gas.
The Calvin Cycle uses the NADPH and ATP from the Light Reactions to “fix” carbon and produce glucose.
Summary Animations
Review Questions
Further Reading / Supplemental Links
Vocabulary
Points to Consider
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