Jan Baptista van Helmont- For the time of the Greeks, plants were thought to obtain their food from the soil, literally sucking it up with their roots. A Belgian doctor, Jan Baptista van Helmont (1580-1644) thought of a simple way to test this idea. HE planted a small willow tree in a pot of soil, after first weighting the tree and the soil. The tree grew in the pot for several years, during which time van Helmont added only water. At the end of 5 yrs., the tree was much larger, its weight having increased 74.4 kg. However, the soil in the pot weighted only 57 g less than it had 5 yrs. before. With this experiment, van Helmont demonstrated that the substance of the plant was not produced only from the soil. He incorrectly concluded, however, that the water he had been adding mainly accounted for the plant's increased biomass.
Joseph Priestly- On August 17th, 1771, Priestly put a living sprig of mint into air in which a wax candle had burnt out. On the 27th of the same month, Priestly found that another candle could be burned in this same air. Somehow, the vegetation seemed to have restored the air. Priestly found that while a mouse could not breathe candle-exhausted air, air "restored" by vegetation was not "at all inconvenient to a mouse." The key clue was that living vegetation adds something to the air.
Han Ingenhousz- How does vegetation "restore" air? 25 yrs. later, Dutch physician, Jan Ingenhouse solved the puzzle. He demonstrated that air was restored only in the presence of sunlight and only by a plant's green leaves, not by its roots. HE proposed that the green parts of the plant carry out a process that uses sunlight to split carbon dioxide into carbon and oxygen. He suggested that the oxygen atom combined with as oxygen gas into the air, while the carbon atom combined with water to form carbohydrates. Other research refined his conclusions, and by the end of the 19th century, the overall reaction for photosynthesis could be written as CO2 + H2O + light energy ïƒ (CH2O) + O2
F. F. Blackman- At the beginning of the twentieth century, the English plant physiologist F. F. Blackman (1866-1947) came to the startling conclusion that photosynthesis is in fact a multistage process, one portion of which uses light directly. Blackman measured the effects of different light intensities, carbon dioxide concentrations, and temperatures on photosynthesis. As long as light intensity was relatively low, he found photosynthesis could be accelerated by increasing the amount of light, but not by increasing the temperature or carbon dioxide concentration. At high light intensities, however, an increase in temperature or carbon dioxide concentration greatly accelerated photosynthesis. Blackman concluded that photosynthesis consists of an initial set of what he called "light" reactions, that are largely independent of temperature but depend on light, and a second set of "dark" reactions (more properly called light-independent reactions), that seed to be independent of light but limited by carbon dioxide. Do not be confused by Blackman's labels - the so-called "dark" reactions occur in the light (in fact, they require the products of the light-dependent reactions); his use of the word dark simply indicates that light is not directly involved in those reactions. Blackman found that increased temperature increased the rate of the light-independent reactions, but only up to about 35 degrees Celsius. Higher temperatures caused the rate to fall off rapidly. Because many plant enzymes begin to be denatured at 35 degrees Celsius, Blackman concluded that enzymes must carry out the light-independent reactions.
C. B. van Niel- In the 1930s, C.B. van Niel (1897-1985) working at the Hopkins Marine Station at Stanford, discovered that purple sulfur bacteria do not release oxygen during photosynthesis; instead, they convert hydrogen sulfide into globules of pure elemental sulfur that accumulate inside them. The process van Niel observed was: CO2 + 2H2S + light energy ïƒ (CH2O) + H2O + 2S. The striking parallel between this equation and Ingenhousz's equation led van Niel to propose that the generalized process of photosynthesis can be shown as: CO2 + 2H2A + light energy ïƒ (CH2O) + H2O + 2A. In this equation, the substance serves as an electron donor. In photosynthesis performed by green plants, H2A is water, whereas in purple sulfur bacteria, H2A is hydrogen sulfide. The product, A, comes from the splitting of H2A. Therefore, the oxygen produced during green plant photosynthesis results from splitting water, not carbon dioxide. When isotopes came into common use in the early 1950s, van Niel's revolutionary proposal was tested. Investigators examined photosynthesis in green plants supplied with water containing heavy oxygen (18O); they found that 18O label ended up in oxygen gas rather than in carbohydrate, just as van Niel had predicted: : CO2 + 2H218O + light energy ïƒ (CH2O) + H2O + 18O2
Robin Hill- in the 1950s, Robing Hill demonstrated that van Niel was right, light energy could be harvested and used in a reduction reaction. Chloroplasts isolated from leaf cells were able to reduce a dye and release oxygen in response to light. Later experiments showed that the electrons released from water were transferred to NADP+ and that illuminated chloroplasts deprived of Carbon dioxide accumulate ATP. If Carbon dioxide is introduced, neither ATP nor NADPH accumulate, and the Carbon dioxide is assimilated into organic molecules.
Define the following terms:
Light- Light is a form of electromagnetic energy conveniently thought of as a wave. The shorter the wavelength of light, the greater its energy. Visible light represents only a small part of the electromagnetic spectrum between 400 and 740 nm.
Photons- A particle of light, termed a photon, acts like a discrete bundle of energy. The energy content of a photon is inversely proportional to the wavelength of the light.
Light Wavelength- Light is a form of electromagnetic energy conveniently thought of as a wave. The shorter the wavelength of light, the greater its energy. Short wavelength light contains photons of higher energy than long-wavelength light.
Light Frequency- Visible light has wavelength in a range from about 380 nanometers to about 740 nm, with a frequency range of about 405 THz to 790 THz. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. Another property of an electromagnetic wave is its wavelength. The wavelength is inversely proportional to the frequency, so an electromagnetic wave with a higher frequency has a shorter wavelength, and vice-versa.
Define the term "a molecule's absorption spectrum" (4 pts)
Electrons occupy discrete energy levels in their orbits around atomic nuclei. The boost an electron into a different energy level requires just the right amount of energy. A specific atom, therefore, can absorb only certain photons of light-namely, those that correspond to the atom's available energy levels. AS a result, each molecule has a characteristic absorption spectrum, the range and efficiency of photons it is capable of absorbing.
Define the term "pigment'. What are the two general types of pigments used in plant photosynthesis? (3 pts)
Pigments are molecules that absorb light energy in the visible range. The two general types of pigments used in plant photosynthesis are chlorophylls and carotenoids.
What pigment can act directly to convert light energy into chemical energy? What is the general role of the other photosynthetic pigments? (2 pts)
-Chlorophyll can directly convert light energy into chemical energy.
Photosynthetic pigments other than chlorophyll mostly participate in the energy-transfer processes just as chlorophyll. They can also function to protect the photosynthetic reaction center from auto-oxidation. In non-photosynthesizing organisms they have been linked to oxidation-preventing mechanisms. They can also serve as free radical scavengers.
Describe the general chemical nature of the chlorophyll molecule and a carotenoid molecule (6 pts).
Chlorophyll molecules consist of a porphyrin head and a hydrocarbon tail that anchors the pigment molecule to hydrophobic regions of proteins embedded within the thylakoid membrane. The only difference between the two chlorophyll molecules is the substitution of an aldehyde group in chlorophyll b for a methyl group in chlorophyll a. Chlorophylls absorbs photons by means of an excitation process analogous to the photoelectric effect. These pigments contain a complex ring structure, called a porphyrin ring, with alternating single and double bonds. At the center of the ring is a magnesium atom. Carotenoids consist of carbon rings linked to chains with alternating single and double bonds. They can absorb photons with a wide range of energies, although they are not always highly efficient in transferring this energy. Carotenoids assist in photosynthesis by capturing energy form light composed of wavelengths that are not efficiently absorbed by chlorophylls. A typical carotenoid is beta-carotene, which contains two carbon rings connected by a chain of 18 carbon atoms with alternating single and double bonds. Splitting a molecule of beta-carotene into equal halves produced two molecules of vitamin A. Oxidation of vitamin A produced retinal, the pigment used in vertebrate vision. This connection explains why eating carrots, which are rich in beta-carotene, may enhance vision.
What are the four stages of the light reactions of photosynthesis? (8 pts.)
-The internal thylakoid membrane is highly organized and contains the structures involved in the light-dependent reaction. For this reason, the reactions are also referred to as the thylakoid reactions. The thylakoid reactions take place in four stages:
1. Primary photoevent. A photon of light is captured by a pigment. This primary photoevent excites an electron within the pigment.
2. Charge separation. This excitation energy is transferred to the reaction center, which transfers an energetic electron to an acceptor molecule, initiating electron transport.
3. Electron transport. The excited electrons are shuttled along a series of electron carrier molecules embedded within the photosynthetic membrane. Several of them react by transporting protons across the membrane, generating a proton gradient. Eventually the electrons are used to reduce a final acceptor, NADPH>
4. Chemiosmosis. The protons that accumulate on one side of the membrane now flow back across the membrane through ATP synthase where chemiosmotic synthesis of ATP takes place, just as it does in aerobic respiration.
These four processes make up the two stages of light-dependent reaction. Steps 1 through 3 represent the stage of capturing energy from light; step 4 is the stage of producing ATP.
Describe how the Emerson and Arnold experiment led to the discovery of photosystems (8 pts).
One way to study the role that pigments play in photosynthesis is to measure the correlation between the output of photosynthesis and the intensity of illumination-that is, how much photosynthesis is produced by how much light. Experiments on plants show that the output of photosynthesis increases linearly at low light intensities, but finally becomes saturated (no further increase) at high -intensity light. Saturation occurs because all of the light absorbing capacity of the plant is in use. This is Emerson and Arnold experiment. In the experiment Emerson and Arnold: Tested if at saturation all pigment molecules are in use, measured oxygen yield of Chlorella with microbursts of light, found If intensity of flashes increased, yield per flash increased to saturation, found that saturation achieved at one molecule of O2 per 2500 chlorophyll molecules, concluded that photons absorbed by groups of molecules not individual molecule, found that Clusters of chlorophyll and accessory pigments called photosystems and found that the reaction center of photosystem acts as energy sink, traps excitation energy. Emerson and Arnold observed individual reaction centers.
What are the two major components of a photosystem? (2pts)
A generalized photosystem contains an antenna complex and a reaction center.
Describe the two-photosystem method of obtaining ATP and reducing power (NADPH) commonly found in higher eukaryotic plants (12 pts.).
-In contrast to the sulfur bacteria, plants have two linked photo-systems. This overcomes the limitations of cyclic photophosphorylation by providing an alternative source of electrons from the oxidation of water. The oxidation of water also generates Oxygen, thus oxygenic photosynthesis. The noncyclic transfer of electrons also produces NADPH, which can be used in the biosynthesis of carbohydrates. One photosystem, called photosystem I, has an absorption peak of 700 nm, so its reaction center pigment is called P700. This photosystem functions in a way analogous to the photosystem found in the sulfur bacteria discussed earlier. The other photo-system, called photosystem II, had an absorption peak of 680 nm, so its reaction center pigment is called P680. This photosystem can generate an oxidation potential high enough to oxidize water. Working together, the two photosystems carry out a noncyclic transfer of electrons that is used generate both ATP and NADPH.
The photosystems were named I and II in the order of their discovery, and not in the order in which they operate in the light-dependent reaction. In plants and algae, the two photo-systems are specialized for different roles in the overall process of oxygenic photosynthesis. Photosystem I transfer electrons ultimately to NADP+, producing NADPH. The electrons lost from photosystem I are replaced by electrons from photosystem II. Photosystem II with its high oxidation potential can oxidize water to replace the electrons transferred to photosystem I. Thus there is an overall flow of electrons from water o NADPH.
Plants use photosystems II and I in series, first one and then the other, to produce both ATP and NADPH. This two-stage process is called noncyclic photophosphorylation because the path of the electrons is not a circle - the electrons ejected from the photosystems do not return to them, but rather end up in NADPH> The photosystems are replenished with electrons obtained by splitting water.
Photosystem II acts first. High-energy electrons generated by photosystem II are used to synthesize ATP and are then passed to photosystem I to drive the production of NADPH. For every pair of electrons obtained from a molecule of water, one molecule of NADPH and slightly more than one molecule of ATP are produced.
How does the light reactions stage of photosynthesis in most bacteria differ from that of eukaryotic plants? (5pts)
-In bacteria, a single photosystem is used that generates ATP via electron transport. This process then returns the electrons to the reaction center. For this reason it is called photophosphorylation. These systems do not evolve oxygen and are thus referred as to as anoxygenic photosynthesis. When a light-energized electron is ejected from the photosystem reaction center it returns to the photosystem via a cyclic path that produced ATP but not NADPH. In contrast to bacteria, plants have two lined photosystems. This overcomes the limitations of cyclic photophosphorylation by providing an alternative source of electrons from the oxidation of water. The oxidation of water also generates O2, thus oxygenic photosynthesis. The noncyclic transfer of electrons also produced NADPH, which can be used in the biosynthesis of carbohydrates.
Draw out the steps of the Calvin cycle. Make sure to give the structures of each molecule (look in cellular respiration chapter for help or go online) and the names of the first three enzymes involved in the cycle. (10 pts)
Define the term photorespiration and discuss how it relates to photosynthesis (5pts)
-Rubisco, the enzyme that catalyzes the key carbon-fixing reaction of photosynthesis, provides a decidedly suboptimal solution. This enzyme has a second enzymatic activity that interferes with carbon fixation, namely that of oxidizing RuBP. This this process, called photorespiration O2 is incorporated into RuBP, which undergoes additional reactions that actually release carbon dioxide. Hence, photorespiration releases carbon dioxide, essentially undoing carbon fixation. The carboxylation and oxidation of RuBP are catalyzed at the same active site on rubisco, and carbon dioxide and oxygen gas compete with each other at this site. Under normal conditions at 25 degrees Celsius, the rate of the carboxylation reaction is four times that of the oxidation reactions, meaning that 20% of photosynthetically fixed carbon is lost to photorespiration.
Compare and contrast the terms C3 and C4 photosynthesis (8 pts).
The C3 pathway uses the Calvin cycle to fix carbon. All reactions occur in mesophyll a cell using Carbon dioxide that diffuses in through stomata. The C4 pathway incorporates Carbon dioxide into a 4-carbon molecule of malate in mesophyll cell. This is transported to the bundle sheath cells where it is converted back into Carbon dioxide and pyruvate, creating a high level of Carbon dioxide. This allows efficient carbon fixation by the Calvin cycle.
The reduction in the yield of carbohydrate as a result of photorespiration is not trivial. C3 plants lose between 25% and 50% of their photosynthetically fixed carbon in this way. The rate depends largely on temperature. In C4 plants, the capture of Carbon dioxide occurs in one cell and the decarboxylation occurs in an adjacent cell. This represents a spatial solution to the problem of photorespiration.
This process if called the C4 pathway because the first molecule formed, oxaloacetate, contains four carbons. The oxaloacetate is converted to malate, which moves into bundle-sheath cells where it is decarboxylated back to Carbon dioxide and pyruvate. This produces a high level of Carbon dioxide in the bundle-sheath cells that can be fixed by the usual C3 Calvin cycle with little photorespiration. The pyruvate diffuses back into the mesophyll cells, where it is converted back to PEP to be used in another C4 fixation reaction.
The C4 pathway, although it overcomes the problems of photorespiration, does have a cost. The conversion of pyruvate back to PEP requires breaking to high-energy bonds in ATP. Thus each Carbon dioxide transported into the bundle-sheath cells cost the equivalent of two ATP. To produce a single glucose, this requires 12 additional ATP compared with the Calvin cycle alone. Despite this additional cost, C4 photosynthesis is advantageous in hot dry climates where photorespiration would remove more than half of the carbon fixed by the usual C3 pathway alone.
How the initial mode of carbon fixation does called "crassulacean acid metabolism" result in the reduction of photorespiration in succulent plants growing in hot regions? (5 pts)
In plants in hot regions, the stomata open during the night and close during the day. This pattern of stomatal opening and closing is the reverse of that in most plants. CAM plants initially fix carbon dioxide using PEP carboxylase to produce oxaloacetate. This oxaloacetate is often converted into other organic acids, depending on the particular CAM plant. These organic compounds accumulate during the night and are stored in the vacuole. Then during the day, when the stomata are closed, the organic acids are decarboxylated to yield high levels of carbon dioxide. These high levels of carbon dioxide drive the Calvin cycle and minimize photorespiration. Like C4 plants, CAMP plants use both C3 and C4 pathways. They differ in that they use both of these pathways in the same cell: the C4 pathway at night and the C3 pathway during the day> in C4 plants the two pathways occur in different cells.
Essay Questions:
Explain the reactions involving the use of light energy that occur in the thylakoids of the chloroplast (8pts)
Outline the effect of light intensity on the rate of photosynthesis. (2 pts)
- The internal thylakoid membrane is highly organized and contains the structures involved in the light-dependent reaction. For this reason, the reactions are also referred to as the thylakoid reactions. The thylakoid reactions take place in four stages: 1. Primary photo event. A photon of light is captured by a pigment. This primary photo event excites an electron within the pigment. 2. Charge separation. This excitation energy is transferred to the reaction center, which transfers an energetic electron to an acceptor molecule, initiating electron transport. 3. Electron transport. The excited electrons are shuttled along a series of electron carrier molecules embedded within the photosynthetic membrane. Several of them react by transporting protons across the membrane, generating a proton gradient. Eventually the electrons are used to reduce a final acceptor, NADPH. 4. Chemosmosis. The protons that accumulate on one side of the membrane now flow back across the membrane through ATP synthase where chemiosmosis synthesis of ATP takes place, just as it does in aerobic respiration. The rate of photosynthesis increases as light intensity increases. The photosynthetic rate reaches plateau at high light levels of carbon dioxide. Also as long as light intensity was relatively low, Blackman found that photosynthesis could be accelerated by increasing the amount of light, but not by increasing the temperature or carbon dioxide concentration. At high light intensities, however, an increase in temperature or carbon dioxide concentration greatly accelerated photosynthesis.
