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Term Paper on Photosynthesis
Term Paper Contents:
- Term Paper on the Introduction to Photosynthesis
- Term Paper on the Nature of Light
- Term Paper on the Chlorophyll and Other Pigments found in Photosynthetic Organisms
- Term Paper on the Photosynthetic Membranes: The Thylakoid
- Term Paper on the Stages of Photosynthesis
- Term Paper on the Products of Photosynthesis
Term Paper # 1. Introduction to Photosynthesis:
Photosynthesis is the physico-chemical process by which producers of nature such as plants, algae and photosynthetic bacteria use light energy to drive the synthesis of organic compounds. This results in the release of molecular oxygen and the removal of carbon dioxide from the atmosphere that is used to synthesize carbohydrates.
Photosynthesis provides the energy and reduces carbon required for the survival of virtually all life on our planet, as well as the molecular oxygen necessary for the survival of oxygen consuming organisms. Each year more than 10% of the total atmospheric carbon dioxide is reduced to carbohydrate by photosynthetic organisms.
The energy that drives photosynthesis originates in the center of the sun, where mass is converted to heat by the fusion of hydrogen. A small fraction of the visible light incident on the earth is absorbed by plants. Through a series of energy transducing reactions, photosynthetic organisms are able to transform light energy into chemical free energy in a stable form that can last for hundreds of millions of years (e.g., fossil fuels).
The overall process of photosynthesis is little bit complex, to produce a sugar molecule such as sucrose. The plants require nearly 30 distinct proteins that work within a complicated membrane structure. In plants, these energy factories are called chloroplasts, which collect energy from the sun and use carbon dioxide and water in the process called photosynthesis to produce sugars.
Animals can make use of the sugars provided by the plants in their own cellular energy factories, the mitochondria. The mitochondria produce a versatile energy currency in the form of adenosine triphosphate (ATP), the high-energy molecule. These molecules store enough immediately available energy to allow plants and animals to their necessary work.
Joseph Priestley (1770s), an English chemist and clergyman showed that plants release oxygen, by his experiment placing a sprig of mint in the chamber and burning a candle in that closed vessel until the flame went out and after several days he showed that the candle could burn again. Later Jan Ingenhousz, a Dutch physician, demonstrated that sunlight was necessary for photosynthesis and that only the green parts of plants could release oxygen.
During this period Jean Senebier, a Swiss botanist and naturalist, discovered that CO2 is required for photosynthesis and Nicolas- de Saussure, a Swiss chemist and plant physiologist, showed that water is essential for photosynthesis. In 1845 Julius Robert von Mayer, a German physician and physicist proposed that photosynthetic organisms convert light energy into chemical free energy (Fig. 3.38).
Over 300 years ago, the English physicist Sir Isaac Newton (1642-1727) separated visible light into a spectrum of colours by letting it pass through a prism. Then by passing the light through a second prism, he recombined the colours, producing white light once again. By this experiment, Newton showed that white light is actually made up of a number of different colours, ranging from violet at one end of the spectrum to red at the other.
Their separation is possible because light of different colours is bent at different angles in passing through the prism. Newton believed that light was a stream of particles (or, as he termed them, “corpuscles”), in part, because of its tendency to travel in a straight line.
In the nineteenth century, through the genius of James Clerk Maxwell, it became known that what we experience as light is in truth a very small part of a vast continuous spectrum of radiation, the electromagnetic spectrum. As, Maxwell showed, all the radiations included in this spectrum act as if they travel in waves.
The wavelengths-that is, the distances from one peak to the next- range from those of gamma rays, which are measured in nanometers, to those of low frequency radio waves, which are measured in kilometers. Within the spectrum of visible light, red light has the longest wavelength, violet the shortest. Another feature that these radiations have in common is that, in a vacuum, they all travel at the same speed-300,000 kilometers per second.
By 1900, it had become clear, however, that the wave model of light was not adequate. The key observation, a very simple one, was made in 1888: When a zinc plate is exposed to ultraviolet light, it acquires a positive charge.
The metal, it was soon deduced, becomes positively charged because the light energy dislodges electrons, forcing them out of the metal atoms. Subsequently, it was discovered that this photoelectric effect, as it is called, can be produced in all metals.
Every metal has a critical wavelength for the effect; the light (visible or invisible) must be of that wavelength or a shorter wavelength for the effect to occur. With some metals, such as sodium, potassium, and selenium, the critical wavelength is within the spectrum of visible light, and as a consequence, visible light striking the metal can set up a moving stream of electrons (such a stream is an electric current).
Burglar alarms, exposure meters, television cameras, and the electric eyes that open the doors for you at supermarkets or airline terminals all operate on this principle of turning light energy into electrical energy.
Term Paper # 2. The Nature of Light:
Now here is a problem. The wave model of light would lead you to predict that the brighter the light-that is, the stronger the beam-the greater the force with which the electrons would be dislodged. But as we have already seen, whether or not light can eject the electrons of a particular metal depends not on the brightness of the light but on its wavelength.
A very weak beam of the critical wavelength or a shorter wavelength is effective, while a stronger beam of a longer wavelength is not. Furthermore, as was shown in 1902, increasing the brightness of the light increases the number of electrons dislodged but not the velocity at which they are ejected from the metal. To increase the velocity, one must use a shorter wavelength of light.
Nor is it necessary for energy to be accumulated in the metal. With even a dim beam of a critical wavelength, electrons may be emitted the instant the light hits the metal.
To explain such phenomena, the particle model of light was resurrected by Albert Einstein in 1905. According to this model, light is composed of particles of energy called photons. The energy of a photon is not the same for all kinds of light but is, in fact, inversely proportional to the wavelength-the longer the wavelength, the lower the energy. Photons of violet light, for example, have almost twice the energy of photons of red light, the longest visible wavelength.
The wave model of light permits physicists to describe certain aspects of its behaviour mathematically, and the photon model permits another set of mathematical calculations and predictions. These two models are no longer regarded as opposed to one another; rather, they are complementary, in the sense that both-or a totally new model-are required for a complete description of the phenomenon we know as light.
The Fitness of Light:
Light, as Maxwell showed, is only a tiny band in a continuous spectrum. From the physicist’s point of view, the difference between radiations we can see and radiations we cannot see-so dramatic to the human eye-is only a few nanometers of wavelength. Why does this particular small group of radiations, rather than some other, make the leaves grow and the flowers burst forth, cause the mating of fireflies and palolo worms, and when reflecting off the surface of the moon, excite the imagination of poets and lovers?
Why is it that this tiny portion of the electromagnetic spectrum is responsible for vision, for the rhythmic, day-night regulation of many biological activities, for the bending of plants toward the light, and also for photosynthesis, on which all life depends? Is it an amazing coincidence that all these biological activities are dependent on these same wavelengths?
George Wald of Harvard, one of the greatest living experts on the subject of light and life, says no. He thinks that if life exists elsewhere in the universe, it is probably dependent on this same fragment of the vast spectrum. Wald bases this conjecture on two points.
First, living things, as we have seen, are composed of large, complicated molecules held in special configurations and relationships to one another by hydrogen bonds and other weak bonds. Radiation of even slightly higher energies than the energy of violet light breaks these bonds and so disrupts the structure and function of the molecules. Radiations with wavelengths less than 200 nanometers drive electrons out of atoms.
Light of wavelengths longer than those of the visible band is absorbed by water, which makes up the great bulk of all living things on earth. When this light reaches molecules, its lower energies cause them to increase their motion (increasing heat) but do not trigger changes in their electron configurations.
Only those radiations within the range of visible light have the property of exciting molecules-that is, of moving electrons into higher-energy orbitals and so of producing chemical and, ultimately, biological changes. The second reason for the visible band of the electromagnetic spectrum being “chosen” by living things is that it, above all, is what is available. Most of the radiation reaching the earth from the sun is within this range.
Higher-energy wavelengths are screened out by the oxygen and ozone high in the atmosphere. Much infrared radiation is screened out by water vapor and carbon dioxide before it reaches the earth’s surface. This is an example of what has been termed “the fitness of the environment”; the suitability of the environment for life and that of life for the physical world are exquisitely interrelated. If they were not, life could not, of course, exist.
Term Paper # 3. Chlorophyll and Other Pigments found in Photosynthetic Organisms:
In order for light energy to be used by living systems, it must first be absorbed. A pigment is any substance that absorbs light. Some pigments absorb all wavelengths of light and so appear black. Some absorb only certain wavelengths, transmitting or reflecting the wavelengths they do not absorb.
Chlorophyll, the pigment that makes leaves green, absorbs light in the violet and blue wavelengths and also in the red; because it reflects green light, it appears green. Different pigments absorb light energy at different wavelengths. The absorption pattern of a pigment is known as the absorption spectrum of that substance.
Different groups of plants and algae use various pigments in photosynthesis. There are several different kinds of chlorophyll that vary slightly in their molecular structure. In plants, chlorophyll a is the pigment directly involved in the transformation of light energy to chemical energy.
Most photosynthetic cells also contain a second type of chlorophyll- in plants, it is chlorophyll b and a representative of another group of pigments called the carotenoids. One of the carotenoids found in plants is beta-carotene. The carotenoids are red, orange, or yellow pigments. In the green leaf, their colour is masked by the chlorophylls, which are more abundant.
In some tissues, however, such as those of a ripe tomato or the root of a carrot plant, the carotenoid colours predominate, as they do also when leaf cells stop synthesizing chlorophyll in the fall.
The other chlorophylls and the carotenoids are able to absorb light at wavelengths different from those absorbed by chlorophyll a and apparently can pass the energy on to chlorophyll a, thus extending the range of light available for photosynthesis. Whether or not a particular pigment can cause a chemical reaction depends not only on its structure but also on its relationship with neighboring molecules.
An action spectrum defines the relative effectiveness (per number of incident photons) of different wavelengths of light for light-requiring processes, such as photosynthesis, flowering, and phototropism (the bending of a plant toward light). Similarity between the absorption spectrum of a pigment and the action spectrum of a process is considered evidence that that particular pigment is responsible for that particular process. When pigments absorb light, electrons are boosted to a higher energy level.
Three of the possible consequences are:
(1) The energy may be dissipated as heat;
(2) It may be re-emitted immediately as light energy of a longer wavelength, a phenomenon known as fluorescence;
(3) The energy may cause a chemical reaction, as happens in photosynthesis.
If chlorophyll molecules are isolated in a test tube and light is permitted to strike them, they fluoresce. In other words, the molecules absorb light energy, and the electrons are momentarily raised to a higher energy level and then fall back again to a lower one.
As they fall to a lower energy level, they release much of this energy as light. None of the light absorbed by isolated chlorophyll molecules is converted to any form of energy useful to living systems. Chlorophyll can convert light energy to chemical energy only when it is associated with certain proteins and embedded in a specialised membrane.
Term Paper # 4. Photosynthetic Membranes: The Thylakoid:
The structural unit of photosynthesis is the thylakoid, which usually takes the form of a flattened sac or vesicle. In the photosynthetic prokaryotes, thylakoids may form a part of the cell membrane, or they may occur singly in the cytoplasm or, as in the blue-green algae, they may be part of an elaborate internal membrane structure.
In eukaryotes, the thylakoids form a part of the internal membrane structure of specialised organelles, the chloroplasts. The alga Chlamydomonas, for instance, has a single very large chloroplast; the cell of a leaf characteristically has 40 to 50 chloroplasts, and there are often 500,000 chloroplasts per square millimeter of leaf surface.
Chloroplasts, like mitochondria, are surrounded by two outer membranes. The interior of the chloroplast is filled with a dense solution, the stroma, which (like the matrix of the mitochondrion) is different in composition from the material surrounding the organelles in the cytoplasm.
With the light microscope under high power, it is possible to see little spots of green within the chloroplasts of leaves. The early microscopists called these green specks grana (“grains”), and this term is still in use.
Under the electron microscope, it can be seen that the grana are stacks of thylakoids. Some of the thylakoid membranes have extensions that interconnect the grana through the stroma that separates them.
All the thylakoids in a chloroplast are oriented parallel to each other. Thus the whole chloroplast can swing toward the light, simultaneously aiming all of the millions of pigment molecules for optimum reception, as if they were a giant electromagnetic antenna (which, of course, they are).
Term Paper # 5. Stages of Photosynthesis:
It was discovered about 200 years ago that light is required for the process we now call photosynthesis. It is now known that photosynthesis actually takes place in two stages, only one of which requires light. Evidence for this two-stage mechanism was first presented in 1905 by the English plant physiologist F.F. Blackman, as the result of experiments in which he measured the rate of photosynthesis under varying conditions. Blackman first plotted the rate of photosynthesis at various light intensities.
In dim to moderate light, increasing the light intensity increased the rate of photosynthesis, but at higher intensities, a further increase in light intensity had no effect. He then studied the combined effects of light and temperature on photosynthesis. In dim light, an increase in temperature had no effect.
However, Blackman found that if he increased the light and also increased the temperature, the rate of photosynthesis was greatly accelerated. As the temperature increased above 30°C, the rate of photosynthesis slowed and finally the process ceased.
On the basis of these experiments, Blackman concluded that more than one set of reactions was involved in photosynthesis. First, there was a group of light dependent reactions that were temperature- independent.
The rate of these reactions could be accelerated in the dim-to-moderate light range by increasing the amount of light, but it was not accelerated by increases in temperature. Second, there was a group of reactions that were dependent not on light but rather on temperature. Both sets of reactions seemed to be required for the process of photosynthesis.
Increasing the rate of only one set of reactions increased the rate of the entire process only to the point at which the second set of reactions began to hold back the first (that is, it became rate-limiting). Then it was necessary to increase the rate of the second set of reactions in order for the first to proceed unimpeded.
Photosynthesis was thus shown both a light-dependent stage, the so called “light reactions,” and a light-independent stage, the “dark reactions.” It is important to keep in mind that the dark reactions do not necessarily take place in the dark. They simply do not require light as such. (However, they do require the products of the light reactions.)
The “dark reactions” increased in rate as the temperature was increased, but only up to about 30°C, after which the rate began to decrease. From this evidence it was concluded that these reactions were controlled by enzymes, since this is the way enzymes are expected to respond to temperature. This conclusion has since proved to be correct.
In the first stage of photosynthesis-the light reactions-light energy is used to form ATP from ADP and to reduce electron carrier molecules. In the second stage of photosynthesis-the dark reactions-the energy products of the first stage are used to reduce carbon from carbon dioxide to a simple sugar, thus converting the chemical energy of the carrier molecules to forms suitable for transport and storage and, at the same time, forming a carbon skeleton on which other organic molecules can be built. This binding of CO2 into organic compounds is known as the fixation of carbon.
A. The Light Reactions:
In the thylakoids, chlorophyll and other molecules are, according to the present model, packed into photosynthetic units. Each unit contains from 250 to 400 molecules of pigment, which serve as light-trapping antennae. Once a quantum of energy is absorbed by one of the antenna pigments, it is bounced around (like a hot potato) until it reaches a special form of chlorophyll a, which is the reaction center.
When this particular chlorophyll molecule absorbs the energy, an electron is boosted to a higher energy level from which it is transferred to another molecule, an electron acceptor. The chlorophyll molecule is thus oxidised (minus an electron) and positively charged.
According to present evidence, there are two different kinds of photosynthetic units, each forming part of a different photosystem. In Photosystem I, the reactive chlorophyll a molecule is known as P700 because one of the peaks of its absorption spectrum is at 700 nanometers, a slightly longer wavelength than the usual chlorophyll a peak.
When P700 (P is for pigment) is oxidised, it bleaches, which is how it was detected. No one has managed to isolate pure P700 Recent evidence indicates that P700 is not an unusual kind of chlorophyll but rather a dimer (“two-part”) of two chlorophyll a molecules that has unusual properties because of its association with special proteins in the membrane and its position in relation to other molecules.
Photosystem II also contains a specialised chlorophyll a molecule, which passes its electron on to a different electron acceptor. The reactive chlorophyll a molecule of Photosystem II is P680.
Model of the Light Reactions:
The two photochemical systems probably evolved separately, with Photosystem I coming first. Photosystem I can operate independently. In general, however, the two systems work together simultaneously and continuously, as shown in Figure 5.9.
According to this model, light energy enters Photosystem II where it is trapped by the reactive chlorophyll molecule P680. An electron is boosted to a higher energy level from which it is transferred to an electron acceptor molecule. The electrons then pass downhill along an electron transport chain to Photosystem I.
As the electrons pass along this transport chain, ATP is formed from ADP, probably via the same mechanism by which ATP is formed along the electron transport chain of the mitochondrion. This process is known as photophosphorylotion.
Other events are taking place simultaneously:
1. The P680 chlorophyll molecule, as a result of having lost its electron, is avidly seeking a replacement. It finds it in the water molecule, which is thus dissociated into protons and oxygen gas.
2. Light energy is trapped in the reactive chlorophyll molecule (P700) of Photosystem I. The molecule is oxidised, and an electron is passed to a primary electron acceptor from which it goes downhill to NADP+.
3. The electron removed from the P700 molecule is replaced by the electron from Photosystem II.
Thus in the light there is a continuous flow of electrons from water to Photosystem II to Photosystem I to NADP+.
The energy harvest from these steps is represented by an ATP molecule (whose formation releases a water molecule) and NADPH, which then becomes the chief source of energy for the reduction of carbon. To generate one molecule of NADPH, four photons must be absorbed, two by Photosystem II and two by Photosystem I.
In the words of Nobel laureate Albert Szent-Gyorgyi: “What drives life is … a little electric current, kept up by the sunshine.”
Cyclic Electron Flow:
There is also some evidence that Photosystem I can work independently. When this occurs, no NADPH is formed. In this process, called cyclic electron flow, electrons are boosted from P700 to an electron acceptor and from there pass downhill through a series of intermediates, including cytochromes, back into the reactive molecule. ATP is produced in the course of this passage. It is believed that the most primitive photosynthetic mechanisms did, indeed, work in this way, and this is apparently the way in which some prokaryotes carry out photosynthesis.
Also, eukaryotic cells are able to synthesize ATP by cyclic electron flow in the absence of NADP+. However, O2 is produced and carbon is reduced only if both systems are in operation.
There are three current hypotheses for the mechanism of oxidative phosphorylation. These same three hypotheses also apply to photophosphorylation, and in the case of this latter process, there is much evidence supporting the chemiosmotic model.
When isolated chloroplasts are illuminated, they absorb protons from the medium in which they are suspended; when the light is turned off and electron flow stops, the protons slowly return from the chloroplasts to the medium.
Also, and most impressive, is the demonstration that an artificially produced proton gradient across the inner membranes of a chloroplast can drive the phosphorylation of ATP in the dark. The chemiosmotic model of photophosphorylation is shown in Figure 5.10.
Summary of the Light Reactions:
The reactions that we have just described are the “light reactions” of photosynthesis. In -the course of these reactions, as we saw, light energy is converted to electrical energy-the flow of electrons-and the electrical energy is converted to chemical energy stored in the bonds of NADPH and ATP.
B. The Dark Reactions:
In the second stage of photosynthesis, the energy generated by the light reactions is I used to reduce carbon. Carbon is available to photosynthetic cells in the form of carbon dioxide. In algae, such as those seen in Figure 5.11, the carbon dioxide is dissolved in the surrounding water. In plants, carbon dioxide reaches the photosynthetic cells through specialised openings in leaves and green stems, called stomata.
The Calvin Cycle: The Three-Carbon Pathway:
The reduction of carbon takes place in the stroma in a cycle called the Calvin cycle (named after its discoverer, Melvin Calvin). The Calvin cycle is analogous to the Krebs cycle in that, in each turn of the cycle, the starting compound is again regenerated. The starting (and ending) compound is a five-carbon sugar with two phosphates attached, ribulose diphosphate (RuDP).
The cycle begins when carbon dioxide enters the cycle and is bound to RuDP, which then splits to form two molecules of phosphoglycerate, or PGA. (Each PGA molecule contains three carbon atoms, hence the name, the three-carbon pathway.)
The enzyme catalyzing these crucial reactions (RuDP carboxylase) is very abundant in chloroplasts, making up more than 15 percent of the total chloroplast protein. (It is said to be the most abundant protein in the world.) This enzyme is located on the surface of the thylakoid membranes.
The complete cycle is diagrammed in Figure 5.12. As in the Krebs cycle, each step is regulated by a specific enzyme. At each full turn of the cycle, a molecule of carbon dioxide enters the cycle, is reduced, and a molecule of RuDP is regenerated.
Six revolutions of the cycle, with the introduction of six atoms of carbon, are necessary to produce a six-carbon sugar, such as glucose.
The overall equation is:
6RuDP + 6CO2 + 18ATP + 12NADPH + 12H+ + 12H2O → 6RuDP + glucose + 18Pi + 18ADP + 12NADP+
The immediate product of the cycle itself is glyceraldehyde phosphate. This same sugar-phosphate is formed when the fructose diphosphate molecule is split at the fourth step in glycolysis.
The Four-Carbon Photosynthetic Pathway:
The Calvin cycle is not the only carbon fixation pathway used in the dark reactions. In some plants, the first product of CO2 fixation to be detected is not the three carbon molecule phosphoglycerate, as it is in the Calvin cycle.
It is the four-carbon compound oxaloacetic acid. (Oxaloacetic acid is also an intermediate in the Krebs cycle.) Plants that utilise this pathway, also known as the Hatch-Slack pathway, are commonly called C4, or four-carbon plants, as distinct from the C3 plants that use only the Calvin cycle.
The oxaloacetic acid is formed when carbon dioxide is bound to a compound known as phosphoenolpyruvate (PEP). This reaction is catalysed by the enzyme PEP carboxylase. The oxaloacetic acid is then reduced to malic acid or converted (with the addition of an amino group) to aspartic acid. These steps take place in mesophyll cells.
The next step is a surprise: The malic acid (or aspartic acid, depending on the species) is transported to bundle-sheath cells where it is decarboxylated to yield CO2 and pyruvic acid. The CO2 then enters the Calvin cycle.
One might well ask why C4 plants should have evolved such a seemingly clumsy and energetically expensive method of providing carbon dioxide to the Calvin cycle. This question can be answered only by considering the function of the leaf as a whole.
Carbon dioxide is not continuously available to the photosynthesizing cells. It enters the leaf by way of the stomata, specialised pores that open and closes depending on, among other things, water stress.
PEP carboxylase has a higher CO2 affinity than does RuDP carboxylase, so it keeps the CO2 concentration lower within the leaf (that is, it fixes carbon dioxide faster, at low levels).
This maximises the gradient of carbon dioxide between the cells and the outside air. A higher gradient means the leaf will trap a larger fraction of the passing stream of carbon dioxide.
If the stomata must be closed much of the time, as in a hot, dry climate, the plant with C4 metabolism will take up more carbon dioxide with each gasp (so to speak) than the plant that has only C3 metabolism. Hence, it is at a distinct advantage in drought-ridden areas.
The list of plants known to utilise the four-carbon pathway has grown to over 100 genera, at least a dozen of which have both C3 and C4 species. This pathway undoubtedly has arisen many times independently in the course of evolution. Sugarcane, corn, and sorghum are among the best-known C4 plants.
Perhaps the most familiar example of the competitive capacity of C4 plants is seen in lawns in the summertime. In most parts of the United States, lawns consist mainly of C3 grasses such as Kentucky bluegrass and creeping bent.
In the summer, these dark green, fine-leaved grasses are often overwhelmed by rapidly growing crabgrass, which, to the dismay of the suburbanite, disfigures the lawn as its yellowish-green, broader-leaved plants slowly take over. Crabgrass, you will not be surprised to hear, is a C4 plant.
Term Paper # 6. The Products of Photosynthesis:
The three-carbon sugar produced by the Calvin cycle may seem an insignificant reward, both for all the enzymatic activity on the part of the cell and for our own intellectual stress. However, this molecule and those derived from it provide – (1) the energy source for all living systems, and (2) the basic carbon skeleton for all organic molecules. Carbon has been fixed-that is, it has been brought from the inorganic world into the organic one.
Molecules of glyceraldehyde phosphate may flow into a variety of different metabolic pathways, depending on the activities and requirements of the cell. Often they are built up to glucose or fructose, following a sequence that is in many of its steps the reverse of the glycolysis sequence. (At some steps, the reactions are simply reversed and the enzymes are the same: Other steps-the highly exergonic ones of the downhill sequence-are bypassed.) Plant cells use these six-carbon sugars to make starch and cellulose for their own 3 purposes and sucrose for export.
Animal cells store them as glycogen. All cells use sugars, including glyceraldehyde phosphate and glucose, as the starting point for the manufacture of other carbohydrates, fats and other lipids, and, with the addition of nitrogen, amino acids and nitrogenous bases. Finally, the carbon fixed in photosynthesis is the source of ATP energy for heterotrophic cells.