7. Why do leaves turn color in the fall? 8. How are photosystems I and II different? How are they related? 9. What is the source of energy for dark reactions? 10. Describe the C3 cycle. 11. What is the process called carbon fixation and why does it have that name? 12. Why is it important for RUBP to be regenerated? VII. NARRATION FOR PHOTOSYNTHESIS: TRANSFORMING LIGHT TO LIFE Huge sequoias rise out of the Earth, often climbing hundreds of feet into the sky and weighing hundreds of tons. Huge redwoods rise out of the Earth, often climbing hundreds of feet into the sky and weighing hundreds of tons. What is the process that creates these huge biological monoliths? Where does the matter that creates their mass come from? The earth from which they rise? The air that surrounds them? The rain that falls on the forest? And what provides the energy to build them, year by year, molecule by molecule? Scientific Experiments Slowly Reveal How Trees and Other Plants Grow Experiments by the 17th century Dutch physician Jan Van Helmont, indicated that the soil in which a tree is grown contributes little to its total mass. Van Helmont planted a seedling in a container, carefully weighing the soil in the container before planting the seedling. He then proceeded to care for the seedling for five years. After the five years he weighed both the seedling, which by now had become a small tree and the soil in the container. During the five years the mass of the tree had increased 75 kilograms, while the weight of the soil was nearly unchanged. The only substance that Van Helmont had added to the container had been water, so he hypothesized that it is water that provides for the increase in mass that occurs in trees and other plants as they grow. As we will see shortly Van Helmont was only partially correct. Nearly a hundred years later, scientist John Priestly performed an experiment that indicated the plants give off a gas essential for combustion, oxygen. Later, the Dutch scientist, Jan Ingenhousz noted that plants only give off oxygen only if exposed to light. Even later, scientists discovered that trees and all other forms of plant life, in fact all forms of life, were made up largely of molecules that contained carbon atoms. Where did all these carbon atoms come from, most soil has little or no carbon, and water certainly has none. The only likely source was the carbon in the gas carbon dioxide which is one the gases that makes up the Earth's atmosphere. Plants Grow as a Result of Photosynthesis As a result of the experiments by Van Helmont, Priestly, Ingenhousz and modern scientists a picture of the process by which trees and other plants obtain the matter and energy with which to sustain growth started to develop. Water and carbon dioxide from the atmosphere interact in a process powered by light energy that releases oxygen. In addition, the process produces other molecules made out of the carbon, hydrogen, and oxygen atoms provided by the water and carbon dioxide molecules. It is the molecules produced by the process that provide the energy and matter needed to fuel plant growth. The molecules produced by the process, scientists discovered, have the general chemical formula (CH 2 0) n, where n is the number of carbon atoms. As their chemical formula indicates these molecules are made out of carbon and water molecules, scientists named them carbohydrates, which literally means carbon plus water. One of these carbohydrate molecules, glucose, which has the chemical formula C 6 H 12 O 6, is one of the molecules most commonly formed by this process or
series of chemical reactions, scientists now call photosynthesis. The equation for photosynthesis is usually written as 6 CO 2 molecules plus 6 H 2 O molecules plus light energy yields one glucose molecule (C 6 H 12 O 6 ) plus 6 O 2 molecules. Photosynthesis Supports Most Life on Earth Nearly all life on Earth is supported, directly or indirectly by photosynthesis. Glucose molecules formed during photosynthesis are broken apart in the cells of plants and animals in a series of chemical reactions called cellular respiration, which you will learn about in more detail later in your study of biology. However, we will mention here, that cellular respiration reverses photosynthesis. The chemical equation for cellular respiration; 1 glucose (C 6 H 12 O 6 ) molecule plus 6 O 2 molecules yield 6 CO 2 molecules plus 6 H 2 O molecules plus a release of energy, reflects this. The chemical energy stored in glucose molecules can be released via cellular respiration either by the producing plant itself or by animals consuming the plant or it's fruits and nectars. Glucose, not consumed for energy by the plant producing it or by animals, is often joined in long molecular chains to form the material cellulose, which is the major structural component in the cell walls of plants, and thus an integral part of such major plant structures as tree trunks and branches. Let's now take a closer look at the process of photosynthesis. Photosynthesis: The Structure of Leaves and Chloroplasts As alluded to earlier photosynthesis is a process or series of reactions which use the energy of sunlight to convert the low energy reactants, water and carbon dioxide, into high energy glucose molecules and oxygen. In plants, photosynthesis takes place in cellular organelles called chloroplasts. As photosynthesis is dependent on light, the majority of photosynthesis takes place in parts of a plant with ample exposure to sunlight. As a result, in most land plants, the majority of chloroplasts are located in the cells of the plants leaves. The leaves of most plants are only a few layers of cells thick. The thin structure of leaves, that results, is important to the process of photosynthesis, insuring that light energy reaches even the leaves innermost cells. The upper and lower surfaces of leaves are covered with a layer of transparent cells called the epidermis. The epidermis cells are transparent, again, in order to allow light energy to reach the cells in the middle layers of the leaf where photosynthesis takes place. A transparent, waterproof, waxy layer called the cuticle covers the epidermis. This prevents one of the reactants critical to photosynthesis, water, from evaporating from the leaf, when weather conditions are hot and dry. Adjustable pores called stomata control the flow of the second major reactant, carbon dioxide, into the leaf and the flow of water and one of the products of photosynthesis, oxygen, out of the leaf. Between the epidermis and the cuticle that cover either side of the leaf there is an area called the mesophyll that simply means "middle of the leaf". The mesophyll is made up of a few layers of cells that contain the vast majority of a leaf's chloroplasts. Vascular bundles, or veins, supply water and minerals to the cells that make up the mesophyll and carry sugars that are produced in their chloroplasts to other parts of the plant. The leaf cells of plants usually contain a substantial number of chloroplasts in their cytoplasm. Chloroplasts are isolated from the cytoplasm of the cell by two membranes. Inside the membranes is a semifluid medium called the stroma. Contained within the stroma are disc-shaped, interconnected membranous sacs called thylakoids. The thylakoids in most chloroplasts are usually stacked one on top of the other, in stacks called grana.
The simple chemical equation for photosynthesis, that we saw earlier, hides the fact that dozens of chemical reactions, controlled by large molecules called enzymes, are necessary to complete the process of photosynthesis. The process of photosynthesis, however, can thought of as being made up of two different groups of reactions; light reactions that depend on light energy to power them and dark reactions which are powered by molecules formed during the light reactions. Photosynthesis: Light Reactions-An Overview The process of photosynthesis begins with the light reactions, which occur in the membranes of the thylakoid sacs, which are also often referred to as photosynthetic membranes. Chlorophyll and other pigment molecules in the photosynthetic membranes capture light energy and convert part of it into chemical energy, which is stored in energy carrier molecules, for later use in providing the energy for the dark reactions that occur in the stroma. Before going further let's take a closer look at the usual source of energy for light reactions, sunlight. Photosynthesis: Light Reactions-Chlorophyll and Other Pigments Capture Light Energy As you probably know, the sun emits a wide spectrum of electromagnetic radiation. On one end of the electromagnetic spectrum are very high-energy gamma rays, X-rays and ultraviolet waves. On the other end are longer wavelength, lower energy, infrared, micro, and radio waves. Visible light lies between these two ends of the electromagnetic spectrum and contains the wavelengths of electromagnetic radiation most useful to living organisms. What looks like white light reaching us from the sun is actually made up of many different colors corresponding to many different wavelengths. Visible light waves extend from the ultraviolet and blue range on one end, into green, yellow and red up to the edge of the infrared range on the other. Light striking a leaf can be reflected, transmitted (that is pass through the leaf) or absorbed. The light reflected or transmitted by a leaf produces it's color, while light that is absorbed heats up the leaf or drives biological processes, like photosynthesis. In biochemical terms pigments are any natural substance that occur in and color the tissues of an organism. Since chlorophyll, the key light-capturing molecule in photosynthetic membranes, is largely responsible for the green color of most plants it is often referred to as a pigment. Other molecules such as carotenoids and phycocyanins are also often present in photosynthetic membranes, capturing light energy and transferring it to chlorophyll molecules. Carotenoids and phycocyanins are referred to as accessory pigments. Each pigment or accessory pigment absorbs and reflects different wavelengths of light. Chlorophyll absorbs violet, blue and red light but reflects green light thereby giving leaves their green color. Carotenoids absorb blue and green light and reflect yellow and red, while phycocyanins absorb green and yellow and reflect blue or purple. During fall, it is the color of the carotenoids and phycocyanins made visible by the disappearance of chlorophyll that gives leave their dramatic yellow, orange, red, and purple colors. As all wavelengths of light are absorbed to some degree by either chlorophyll, carotenoids, or phycocyanins, all wavelengths of light can drive photosynthesis to some extent, but the peak wavelengths lie in the blue and red regions of the visible light spectrum. Let s now look at the cellular structures - photosystems I and II - that begin the process of converting light captured by pigments into chemical energy.
Photosynthesis: Light Reactions-Light Energy Captured by Pigments Powers Photosystems I and II Within photosynthetic membranes, chlorophyll, accessory pigments and electron carrier molecules form highly organized units called photosystems. The photosynthetic membrane of each thylakoid contains thousands of copies each of two different kinds of photosystems. Photosystem I and photosystem II. Both photosystems consist of two major parts, a light harvesting complex and an electron transport system. The lightharvesting complex of either photosystem contains roughly three hundred chlorophyll and accessory pigment molecules. These molecules absorb light and pass the energy to a specific chlorophyll molecule called the reaction center. The reaction center is located next to the electron transport system, which is a series of electron carriers embedded in the photosynthetic membrane. When the reaction centers chlorophyll molecule receives enough energy from other molecules in the light harvesting complex, one of it's electrons absorbs the energy and is ejected from the reaction centers chlorophyll molecule and over to the electron transport system. This energetic electron is passed from one carrier to another along the transport system. At some of the stops, the electron releases energy that is used to drive the synthesis of energy carrying ATP or NADPH molecules. The major difference between photosystem I and photosystem II is that photosystem II generates ATP molecules while photosystem I generates NADPH molecules. A photosystem II is always linked in series to a photosystem I. Let's take a look at how the two photosystems are linked and how energy carrying ATP and NADPH molecules are formed. Photosynthesis: Light Reactions-Photosystem II Produces ATP Molecules As we saw earlier, light energy is captured in photosystem II's light harvesting complex and the energy carried to the reaction center. After the reaction center receives a enough energy from the light-harvesting complex, an energized electron is ejected from the reaction center to the electron transport system of photosystem II. As the electron passes from molecule to molecule it releases energy that is used to pump positively charged hydrogen ions across the photosynthetic membrane. As a result the thylakoid compartment becomes positively charged while the surrounding stroma, outside the membrane, becomes negatively charged because of the loss of the positively charged hydrogen ions. This creates a source of energy that will be used later, as positively charged hydrogen ions flow back out of the thylakoid compartment through channel proteins due to their electrical attraction to the negatively charged stroma outside the membrane. Enzymes attached to the channel proteins use the energy created by the flow of hydrogen ions to create high-energy ATP molecules by adding a phosphate based molecules to ADP molecules. Photosynthesis: Light Reactions-Photosystem I Produces NADPH Molecules Meanwhile, light rays are also striking the light harvesting complex of photosystem I and the energy captured is causing an energized electron to jump from the photosystem I reaction center to the photosystem I electron transport system. The photosystem I reaction center immediately obtains a replacement for its lost electron in the form of the last electron down the photosystem II electron carrier. Photosystem I's high-energy electrons move through their own electron transport system, again pumping hydrogen ions inside the photosynthetic membrane. Eventually, the photosystem I electrons are taken up by "empty" electron carrying NADP+ molecules waiting at the end of the photosystem I electron transport. Each positively charged NADP molecules picks up two energetic electrons from the photosystem I electron transport and a hydrogen ion from the stroma to form an NADPH molecule.
Both the NADPH and ATP molecules formed by the two photosystems are water-soluble molecules, which dissolve in the chloroplasts stroma and remain there to be used later to supply the energy for the light independent dark reactions. Photosynthesis: Light Reactions- Photosystems I and II Electrons Come From H 2 O Molecules We have just seen the flow of electrons from the reaction center of photosystem II through photosystem II's electron transport system to the photosystem I reaction center where they replace the electrons the photosystem I reaction center has given up to the photosystem I electron transport system in order to pump hydrogen ions across the photosynthetic membrane and form NADPH molecules. In order for this flow of electrons to be sustained, photosystem II's reactions center must be continuously supplied with new electrons to replace the ones that it gives up. These replacement electrons come from water molecules. Photosystem II's reaction center captures electrons from H 2 O molecules from within the thylakoid compartment. As a result the H 2 O molecules break apart. The broken water molecules liberate hydrogen ions and oxygen atoms. The hydrogen ions increase the number of positively charged ions in the thylakoid compartment available to drive the synthesis of ATP molecules. The oxygen atoms combine to form molecules of oxygen gas, O 2. The oxygen may be retained by the plant for its own cellular respiration or given off to the atmosphere where it can be used by animals. Photosynthesis: Dark Reactions- The C3 Cycle The reactions we just looked at are called light reactions, because they require light energy to power them. The reactions that we are going to look at next, in which glucose or other molecules are actually synthesized, are referred to as dark reactions because they do not need light energy to power them as their energy is obtained from the energy stored in ATP and NADPH molecules formed during light reactions. At the beginning of the dark reactions, ATP and NADPH molecules synthesized during light reactions are dissolved in the stroma where the dark reactions take place. The synthesis of glucose and the capture of the carbon dioxide molecules critical to their formation occur in a set of dark reactions known both as the Calvin cycle, named after one of the discoverers of the cycle, and the C3 cycle because some of the important molecules in the cycle have 3 carbon atoms in them. We'll refer to it as the C3 cycle, as that name is both shorter and more descriptive. The C3 cycle requires four basic components; CO 2 from the atmosphere, a CO 2 capturing sugar (ribulose bisphosphate molecule or RUBP for short), enzymes to carry out and catalyze reactions, and energy in the form of ATP and NADPH molecules to power the reactions. The C3 cycle can be thought of as being divided into three parts; carbon fixation, phosphoglyceraldehyde and glucose synthesis, and RUBP replacement. Photosynthesis: Dark Reactions- The C3 Cycle- Carbon Fixation In order to form a six-carbon glucose molecule, six carbon atoms must be captured from carbon dioxide molecules. So as each C3 cycle begins six five-carbon RUBP molecules combine with six CO 2 molecules to form six extremely unstable six-carbon molecules. These molecules spontaneously react with six H 2 O
molecules and break apart to form twelve, three carbon molecules of phosphoglyceric acid or PGA for short. This stage is referred to as the carbon fixation stage, as during this stage gaseous inorganic C0 2 molecules are fixed into relatively stable, organic, PGA molecules. Photosynthesis: Dark Reactions-The C3 Cycle-Phosphoglyceraldehyde and Glucose Synthesis In the step after carbon fixation, phosphoglyceraldehyde and glucose synthesis, energy provided by twelve ATP and twelve NADPH molecules, is used in a series of enzyme-catalyzed reactions to synthesize twelve PGAL molecules from the twelve PGA molecules. PGAL is a versatile molecule that can be converted into a number of different molecules depending on the needs of the plant. Not only can two PGAL molecules, with three carbons each, combine to form a six carbon glucose molecule, the PGAL molecules may also be used to synthesize lipids, amino acids, or parts of nucleic acids. Thus the PGAL molecules synthesized in the C3 cycle provide the raw materials to produce almost everything needed by a cell. Photosynthesis: Dark Reactions-The C3 Cycle-RUBP Replacement In order for C3 cycles to continue on, in a continuous loop, however, the six RUBP molecules the C3 cycle started with must be regenerated. Thus, 10 out of the 12 PGAL molecules initially synthesized are used for the regeneration of RUBP molecules, meaning that only two of the twelve PGAL molecules are available to synthesize glucose or another molecules. Through a complex series of reactions requiring energy from 6 ATP molecules, the ten PGAL molecules unused in glucose synthesis, are converted back into six RUPB molecules, so that a new C3 cycle can begin. Review Let's now quickly review the process of photosynthesis from the capture of light energy to the formation of glucose and other molecules. During light reactions, which occur in the photosynthetic membranes of thylakoid sacs stacked inside chloroplasts, light energy is captured by two groups, of chlorophyll and other pigment molecules, that along with their accompanying electron transport systems, are referred to as photosystem I and photosystem II. The light energy captured by the pigment molecules of the photosystems is funneled into the reaction center of each photosystem. After enough energy is absorbed by the reaction centers, high-energy electrons jump from them to the adjacent electron transport system. Part of the energy of electrons traveling down the photosystem II electron transport system is used to pump positively charged hydrogen ions across the photosynthetic membrane into the thylakoid compartment. The flow of these hydrogen ions back out of the thylakoid compartment into the stroma is used by special enzymes to convert low energy ADP molecules into highly charged ATP molecules. At the end of the photosystem II electron transport system electrons jump to the photosystem I reaction center to replace the electrons released by that reaction center to its electron transport system. The electrons that were initially released by the photosystem II reaction center are replaced by electrons it captures from H 2 O molecules it breaks apart. The remaining hydrogen ions from the broken water molecule are used like the rest of the hydrogen ions stored in the thylakoid compartment to eventually power the building of ATP molecules. The oxygen from the broken water molecule is either used by the plant for cellular respiration or released to the atmosphere. The electrons that jump from the photosystem I reaction center to the photosystem I electron transport, like the electrons that travel down the photosystem II electron transport, pump hydrogen ions across
the photosynthetic membrane as they travel cross the electron transport system. Eventually, these electrons arrive at NADP+ molecules waiting at the end of the transport system. Two electrons from the electron transport and a hydrogen ion from the stroma join with the NADP+ molecule to form a NADPH molecule. The ATP and NADPH molecules formed by the photosystems in the light reactions are used to power the formation of glucose during the reactions of the C3 cycle, which are often referred to as the dark reactions. During the C3 cycle, six RUBP molecules react with six molecules of CO 2 to form six unstable, six carbon molecules. These six molecules react with six molecules of H 2 O to form 12 molecules of stable, three carbon PGA. The energy of 12 ATP molecules and two electrons and one hydrogen ion from each of 12 NADPH molecules are used to convert the 12 PGA molecules into 12 PGAL molecules. Two of the PGAL molecules are further processed into a six carbon glucose or other organic molecules such as glycerol, fatty acids, amino acids or the carbon skeletons of amino acids, depending on the needs of the plant. Finally, energy from six ATPs is used to rearrange the remaining 10 PGAL molecules back into 6 five-carbon RUPB molecules so that the cycle can start again. The depleted NADPH and ATP now NADP+ molecules and ADP molecules respectively return to the photosynthetic membranes to be recharged by the light reactions so the process, we call photosynthesis, can be repeated.