8: Photosynthesis: Energy from the Sun

Identifying Photosynthetic Reactants and Products

· Photosynthesis, the biochemical process by which plants capture energy from sunlight and store it in carbohydrates, sugars, and starch, is the very basis of life on Earth.

· By the 1800s, scientists understood that there were three principal ingredients for photosynthesis: water, carbon dioxide, and light.

· There were two products: carbohydrates and oxygen. (See Figure 8.1.)

· The water, which comes primarily from the soil, is transported through the roots to the leaves.

· The CO₂ is taken in from the air through stomata, or pores, in the leaves.

· Light is an absolute necessity to produce oxygen and carbohydrates.

· By 1804, scientists summarized the overall chemical reaction of photosynthesis:

CO₂ + H₂O + light energy ® sugar + O₂

· More recently, using water and carbon dioxide labeled with radioactive isotopes of oxygen, it has been determined that the actual reaction is

6 CO₂ + 12 H₂O ® C₆H₁₂O₆ + 6 O₂ + 6 H₂O

· Water appears on both sides of the equation because water is both used as a reactant and released as a product. (See Figure 8.2.)

The Two Pathways of Photosynthesis: An Overview

· Photosynthesis occurs in the chloroplasts of green plant cells and, like metabolism, is a result of many steps—not just a single step.

· When looked at as a whole, it can be separated into two different pathways.

· The first pathway is called the light reaction and is driven by light energy that is captured by chlorophyll pigments. It produces ATP and the reduced electron carrier, NADPH + H⁺.

· The second pathway, called the Calvin–Benson cycle, does not use light directly. It uses ATP, NADPH + H⁺, and CO₂ to produce sugar.

· The light reactions are mediated by molecular assemblies called photosystems.

· These systems pass electrons from one molecule to another, and some of this flow is coupled to synthesis of ATP.

· The pathway is referred to as photophosphorylation. Both NADPH + H⁺ and ATP are produced by the light reactions.

· The Calvin–Benson cycle uses the energy stored in NADPH + H⁺ and ATP to fix CO₂ into carbohydrates.

· Note that although the Calvin–Benson cycle is sometimes referred to as the dark reactions, the cycle requires the products of the light reactions to run, and it stops in absence of light.

Properties of Light and Pigments

· Light is the source of the energy required to drive photosynthesis.

Light comes in packets called photons

· Visible light is part of the electromagnetic radiation spectrum. It comes in discrete packets called photons.

· Light also behaves as if it were a wave. The wavelength of light is the distance between two consecutive peaks of the wave. (See Figure 8.4.)

· Visible light fits into the overall electromagnetic spectrum between ultraviolet and infrared radiation. (See Figure 8.5.)

· Humans perceive light as having distinct colors. The colors relate to the wavelength of the light, as shown in Figure 8.5.

· The frequency of light is inversely related to its wavelength.

· The shorter the wavelength, the higher the frequency; the longer the wavelength, the lower the frequency.

· The amount of energy contained in a single photon is directly proportional to its frequency, and therefore inversely proportional to its wavelength.

· The shorter the wavelength, the higher the frequency, and the greater the energy of the photon.

· A photon of red light has a wavelength of 660 nm and has less energy than a photon of blue light, which has a wavelength of 430 nm.

· Brightness or intensity is a measure of the photons striking an area per unit time, such as one cm² per second. Light intensity is often expressed in energy units, such as calories, per square centimeter per second.

Absorption of a proton puts a pigment in an excited state

· When a photon and a pigment molecule meet, one of three things happens:

· The photon may bounce off the molecule. This is reflection.

· The photon may pass through the molecule. This is transmission.

· The photon may be absorbed by the molecule. This is excitation.

· If absorbed, the photon disappears, but the energy it possessed can be neither created nor destroyed and is therefore acquired by the molecule.

· The molecule is raised from its ground state to an excited state of higher energy.

· The difference between the excited and the ground state is precisely equal to the energy of the absorbed photon. (See Figure 8.6.)

· All molecules absorb electromagnetic radiation, but differ in the specific wavelengths absorbed.

· Molecules that absorb wavelengths in the visible range are called pigments.

· When a beam of white light shines on an object, and the object appears to be red in color, it is because it has absorbed all other colors from the white light except for the color red.

· In the case of chlorophyll, plants look green because they absorb green light less effectively than the other colors found in sunlight.

Light absorption and biological activity vary with wavelength

· A given type of molecule can absorb radiant energy of only certain wavelengths.

· If we plot the absorption of the compound as a function of wavelength, the result is an absorption spectrum. (See Figure 8.7.)

· Absorption spectra are good “fingerprints” of compounds. Sometimes an absorption spectrum contains enough information to enable us to identify an unknown compound.

· If absorption results in an activity of some sort, then a plot of the effectiveness of the light as a function of wavelength is called an action spectrum. Figure 8.8 shows the action spectrum of photosynthesis by Anacharis, a freshwater plant.

Photosynthesis uses chlorophylls and accessory pigments

· Chlorophylls are the most important pigments in photosynthesis. (See Figure 8.9.)

· Plants have two predominant chlorophylls: chlorophyll a and chlorophyll b. These molecules differ slightly in their structure.

· Both have a similar ring structure.

· In the center of the chlorophyll ring is a magnesium atom. At the peripheral location of the ring is a long hydrocarbon tail that can associate with the hydrophobic region of the thylakoid membrane.

· These chlorophylls absorb blue and red wavelengths, which are near the ends of the visible spectrum.

· Other accessory pigments absorb photons that are intermediate in energy between the red and blue wavelengths, and then transfer a portion of that energy to chlorophylls.

· Examples of accessory pigments are the carotenoids, such as b-carotene, which absorb photons in the blue and blue-green wavelength and appear deep yellow in color.

Light Reactions: Light Absorption

· A pigment molecule enters an excited state when it absorbs a photon.

· The excited state is unstable. One of two things will happen.

· The molecule might return to ground state, emitting much of the absorbed energy as fluorescence. When the molecule fluoresces, it emits a photon of a longer wavelength. (See Figure 8.10.)

· The second law of thermodynamics explains this—during an energy transfer, not all the energy is available to do work.

· The molecule might pass some of the absorbed energy to other pigment molecules.

· Pigments in the photosynthetic organisms arrange into energy-absorbing antenna systems. (See Figure 8.11.)

· In these systems, complex proteins hold the molecules in the correct orientation to enable the transfer of energy.

· The excitation is passed to the reaction center of the antenna complex.

· The pigment molecule in the center is always a molecule of chlorophyll a.

· There are other chlorophyll a molecules in the antenna, but they absorb light at shorter wavelengths.

Excited chlorophyll acts as a reducing agent

· The ground state chlorophyll, symbolized by Chl, is not much of a reducing agent, but excited Chl* is a good one.

· The reducing capability of Chl* is increased because excited molecules have electrons zipping around farther away from the nucleus. (See Figure 8.11.)

· Less tightly held, the electron is more likely to be passed on in a redox reaction to an oxidizing agent.

· Chl* can react with an oxidizing agent in a reaction such as Chl* + A ® Chl⁺ + A^–.

· This is the first biochemical consequence of light: Chlorophyll becomes a reducing agent.

· The electron is passed on to an oxidizing agent. Chlorophyll then becomes a positively charged ion, which is missing an electron.

· This redox reaction would not have occurred in the dark.

Electron Flow, Photophosphorylation, and Reductions

· The passing of an electron from chlorophyll to an electron acceptor begins an electron flow. (See Figure 8.12.)

· The electrons flow through a series of carriers, where redox reactions occur one after another, and the energy of these redox reactions is used to pump protons.

· The process is referred to as photophosphorylation.

· Another high-energy product that is generated is NADPH + H⁺. Similar to the NAD⁺ found in cellular respiration, NADP⁺ is nicotinamide adenine dinucleotide phosphate. This compound couples the photosynthetic pathways.

· There are two different systems for flow of electrons in photosynthesis.

· The noncyclic electron flow produces NADPH + H⁺ and ATP and also produces the oxygen that we find in the atmosphere. (See Figure 8.12.)

· Cyclic electron flow produces only ATP.

· All the oxygen in the atmosphere is generated by noncyclic photophosphorylation.

· When the excited electron is passed from chlorophyll P₆₈₀ (photosystem II), it leaves the chlorophyll molecule oxidized and charged, Chl⁺.

· The electrons to restore the chlorophyll come from water.

· H₂O ® 2 e^– + 2 H⁺ + 1/2 O₂. This “splitting” of water releases oxygen to the atmosphere.

· The oxidized chlorophyll a is reduced by an electron from a manganese-containing protein called protein Z.

· Protein Z regains its lost electron by oxidizing a water molecule.

· The protons (H⁺) generated are released into the thylakoid lumen, where they help to build the proton gradient.

Noncyclic electron flow produces ATP and NADPH

· Photons are absorbed by chlorophyll P₆₈₀ molecules.

· The P₆₈₀-excited electrons transfer to the electron transport chain.

· In photosystem I, P₇₀₀ absorbs photons.

· The P₇₀₀ electrons and free protons are used to reduce NADP⁺ to NADPH + H⁺. (See Figure 8.12.)

· The P₇₀₀ electrons are replaced by those flowing from P₆₈₀.

· In summary, noncyclic electron flow takes a molecule of water, four photons, one molecule each of NADP⁺ and ADP and one P_i to make one molecule each of NADPH + H⁺ and ATP and a half molecule of O₂.

Cyclic electron flow produces ATP but no NADPH

· Energy captured in the light reactions is stored in ATP and NADPH + H⁺ to drive the Calvin–Benson cycle, which uses more ATP than NADPH + H⁺.

· To keep balance, cyclic electron flow makes ATP without making NADPH + H⁺.

· “Cyclic” refers to the circular pathway of P₇₀₀ electrons.

· Water does not enter into the cyclic electron flow reactions, and no oxygen is released.

· In cyclic electron flow, photosystem I acts on its own, without photosystem II. (See Figure 8.13.)

· The P₇₀₀ molecule, photosystem I’s reaction center chlorophyll, starts at ground state.

· It absorbs a photon and becomes P₇₀₀*.

· The P₇₀₀* molecule reduces an oxidizing agent ferredoxin (Fd_red).

· In contrast to noncyclic photophosphorylation, where Fd_red reduces NADP⁺, Fd_red passes its electron to another oxidizing agent, plastoquinone (PQ).

· The PQ molecule passes the electron to the cytochrome complex.

· The electron continues down a redox chain, pumping protons as it goes.

· P₇₀₀⁺, the chlorophyll that lost an electron, gets it back again from the last reducing agent at the end of the chain.

· In summary, cyclic electron flow takes photons through chlorophyll molecules, passing excited electrons through a redox chain to produce ATP and some free energy as heat. Electron-deficit chlorophyll is restored and the process repeats.

Chemiosmosis is the source of ATP

· ATP molecules are produced by a mechanism similar to that described in Chapter 7 for mitochondria.

· The same type of chemiosmotic mechanism operates in photosynthesis, but is called photophosphorylation. (See Figure 8.14.)

· High-energy electrons move through a series of redox reactions, releasing energy that is used to transport protons across the membrane.

· Active proton transport results in the proton-motive force: a difference in pH and electric charge across the membrane.

· In chloroplasts, the electron carriers in the thylakoid membrane are oriented so as to move protons into the interior of the thylakoid, and the inside becomes acidic with respect to the outside.

· The ratio of H⁺ inside versus outside a thylakoid is usually 10,000 to 1, which is a difference of 4 pH units.

· This difference in pH leads to the diffusion of H⁺ out of the thylakoid through specific protein channels, ATP synthases.

· The ATP synthases couple the formation of ATP to proton diffusion back across the thylakoid membrane.

Photosynthetic pathways are the products of evolution

· The first photosynthetic organisms were probably anaerobic bacteria that used hydrogen sulfide rather than water for a source of electrons: CO₂ + 2 H₂S ® (CH₂O) + 2 S + H₂O.

· Many bacteria still use a system that releases sulfur rather than oxygen.

· Around 3 billion years ago, new pigments in some bacteria allowed the extraction of electrons from water, the reduction of NADP⁺, and the production of O₂ as a by-product.

· Over hundreds of millions of years, these cyanobacteria poured oxygen into the atmosphere, which made the evolution of cellular respiration possible and perhaps led to the great diversification of life we see on Earth today.

· The engulfment of cyanobacteria by larger cells may have resulted in a eukaryotic cell capable of photosynthesis

Making Sugar from CO₂: The Calvin–Benson Cycle

· The Calvin–Benson cycle is the second main pathway of photosynthesis.

· Most of the enzymes used to make sugar from CO₂ are found dissolved in the stroma of the chloroplast.

· The cycle created by these enzymes does not use sunlight directly; it uses the energy captured in ATP and NADPH + H⁺ in the light reactions.

· Thus the “dark” reactions require light indirectly and take place only in the presence of light.

Isotope labeling experiments reveal the steps of the Calvin–Benson cycle

· Radioactively labeled carbon in CO₂ was used to identify the products generated by photosynthesis in the unicellular green alga, Chlorella. (See Figure 8.15. The Instructor’s Resource CD-ROM includes micrographs of Chlorella and photographs of the experimental setup.)

· After a 30-second exposure to ¹⁴CO₂, the cells were killed and carbohydrates and amino acids were extracted.

· Many compounds, including monosaccharides and amino acids, contained ¹⁴C.

· The exposure time to ¹⁴CO₂ was shortened to just 3 seconds.

· Only one compound was found labeled with ¹⁴C.

· The compound was a three-carbon sugar called 3-phosphoglycerate (3PG).

· Other products of the cycle were found by increasing the length of time of exposure in a stepwise manner until the whole pathway was revealed.

· Finding a labeled 3-carbon sugar was unexpected. Additional research discovered that the real first product is a six-carbon sugar.

· The initial reaction of the Calvin–Benson cycle fixes one CO₂ into a five-carbon compound ribulose 1,5-bisphosphate (RuBP).

· An intermediate six-carbon compound forms, which is unstable and breaks down to form two three-carbon molecules of 3PG. This is the compound that Calvin discovered.

· The enzyme that catalyzes the fixation of carbon dioxide is ribulose bisphosphate carboxylase/oxygenase, called rubisco.

· Rubisco is the most abundant protein in the world, comprising 20% of all the protein in every plant leaf. (See Figure 8.16.)

The Calvin–Benson cycle is composed of three processes

· The Calvin–Benson cycle uses the ATP and NADPH + H⁺ from the light reactions to reduce CO₂ to carbohydrate.

· The Calvin–Benson cycle consists of three processes:

· Fixation of CO_2, by combination with RuBP (this is catalyzed by rubisco).

· Conversion of fixed CO₂ into carbohydrate (G3P). This step uses ATP and NADPH.

· Regeneration of the CO₂ acceptor RuBP by ATP.

· The end product of the cycle is glyceraldehyde 3-phosphate, G3P, which is a three-carbon sugar-phosphate, also called triose phosphate. (See Figure 8.17.)

· There are two fates for the G3P:

· One-third ends up as starch, which is stored in the chloroplast and serves as a source of glucose.

· Two-thirds is converted to the disaccharide sucrose, which is transported out of the leaf to other organs, where it is hydrolyzed into glucose and fructose to provide a source of energy.

· Most of the stored energy is released by glycolysis and cellular respiration by the plant itself, during growth, development, and reproduction.

· Some of the carbon of glucose becomes part of amino acids, lipids, and nucleic acids.

· Some of this stored energy is consumed by heterotrophs, where glycolysis and respiration release the stored energy. Refer to Figure 8.3 for an overview of the process of photosynthesis.

· The products of the Calvin–Benson cycle are vitally important to the Earth’s biosphere as they are the total energy yield from sunlight conversion by green plants.

Photorespiration and Its Evolutionary Consequences

· Rubisco is remarkably identical in all photosynthetic organisms.

· But Rubisco sometimes fixes O₂ instead of CO₂, lowering the overall rate of CO₂ fixation and limiting plant growth.

RuBP reacts with O₂ in photorespiration

· Rubisco is a carboxylase, adding CO₂ to an acceptor molecule, RuBP. It can also be an oxygenase, adding O₂ to RuBP.

· These two reactions compete with each other. When RuBP and oxygen react, one of the products is a two-carbon compound, glycolate: RuBP + O₂ ® glycolate.

· The glycolate diffuses into membrane-enclosed organelles called peroxisomes. (See Figure 8.18. The Instructor’s Resource CD-ROM includes a TEM of a peroxisome.)

· In the peroxisomes, a series of reactions converts glycolate to glycine.

· The glycine diffuses into the mitochondria, where two glycine molecules are converted into another amino acid, serine.

· This pathway is called photorespiration.

· It uses the ATP and NADPH produced in the light reaction.

· The net effect of photorespiration is to undo what the Calvin–Benson cycle accomplished.

· CO₂ is released instead of being fixed into a carbohydrate.

· In many plants, photorespiration reduces the amount of carbon fixed by 25%.

· Rubisco acts as an oxygenase if the CO₂ levels are very low and the oxygen levels are very high.

· Oxygen levels become very high when stomata are closed to prevent water loss (when the weather is hot and dry).

· When the stomata are closed during the day, CO₂ continues to be consumed and O₂ continues to be produced within the leaf. As the oxygen concentration increases, the oxygenase activity also increases.

· The oxygenase property of rubisco may be an artifact of a time when there was little atmospheric O₂.

Some plants have evolved systems to bypass photorespiration

· A solution to the problem of photorespiration has been achieved by some plants.

· The concentration of oxygen in the atmosphere is 21%, and the concentration of CO₂ is 0.036%.

· The rubisco enzyme prefers CO₂, but at very high oxygen concentrations, oxygenase activity begins.

· Many plants, called C₃ plants, first fix CO₂ in the three-carbon molecule for photorespiration, as already described.

· In corn, sugarcane, and other tropical grasses, chloroplasts with abundant rubisco are in layers in the interior of the leaf. (See Figure 8.19.)

· Like C₃ plants, these plants close stomata on hot, dry days, but their rate of photosynthesis neither falls off, nor does photorespiration occur. They are called C₄ plants.

· The C₄ plants perform normal Calvin–Benson cycles, but they have an additional early reaction that fixes CO₂ without losing carbon to photorespiration.

· The C₄ plants have two separate enzymes for CO₂ fixation in different chloroplasts, in two different locations in the leaf.

· One type, PEP carboxylase, is present in the mesophyll cells near the surface of the leaf. (See Figure 8.20. The Instructor’s Resource CD-ROM includes photographs of C₄ plants and a micrograph of a bundle sheath cell.)

· PEP carboxylase fixes CO₂ to a three-carbon acceptor compound, phosphoenol pyruvate, to form a four-carbon fixation product, oxaloacetate.

· PEP carboxylase does not have oxygenase activity.

· It fixes CO₂ even when the level of CO₂ is extremely low.

· The four-carbon compounds diffuse out of the mesophyll cells into the bundle sheath cells in the interior of the leaf.

· The chloroplasts of these cells contain abundant rubisco.

· The four-carbon compounds are decarboxylated, losing CO₂ and regenerating the three-carbon acceptors, which then diffuse back to the mesophyll cells.

· C₃ photosynthesis appears to have begun about 3.5 billion years ago.

· The C₄ plants appeared only about 12 million years ago.

· A hundred million years ago, the concentration of CO₂ was four times that of what it is now. As the CO₂ levels declined, more efficient C₄ plants would have had an advantage over C₃ counterparts.

· See Table 8.1 for a comparison of photosynthesis in C₃ and C₄ plants.

CAM plants also use PEP carboxylase

· Other plants besides C₄ species use PEP carboxylase to fix and accumulate CO₂ while their stomata are closed. (See Figure 8.21. The Instructor’s Resource CD-RM includes a photograph of rock dudleya, a CAM plant.)

· Such plants include succulents, such as cacti and pineapple, and also several other kinds of flowering plants.

· These plants conserve water by keeping stomata closed during the daylight hours and opening them at night.

· The CO₂ metabolism in these plants is called the crassulacean acid metabolism, or CAM, after the family of succulents in which it was discovered.

· In CAM plants, CO₂ is fixed initially in the mesophyll cells to form a four-carbon compound, oxaloacetate, which is then converted to malic acid.

· The fixation occurs during the night, when less water is lost through the open stomata.

· The stomata close during the day, and the accumulated malic acid is shipped to the chloroplast, where decarboxylation occurs to supply CO₂ for the operation of the Calvin–Benson cycle. (See Figure 8.21.)

· C₄ and CAM plants have evolved different solutions to the same problem.

· C₄ plants separate the fixation of four-carbon compounds spacially from the reactions of the Calvin–Benson cycle.

· CAM plants make the same separation temporally.

Metabolic Pathways in Plants

· Green plants are autotrophs and can synthesize all their molecules from three simple starting materials: CO₂, H₂O, and NH₄.

· Plants use ATP and NADPH for a number of processes besides the Calvin–Benson cycle, but ATP is also needed in organs that do not photosynthesize and also during times when the sun is not shining.

· To satisfy their need for ATP, plants, like all other organisms, carry out cellular respiration. Both aerobic respiration and fermentation can occur in plants, although respiration is more common.

· Cellular respiration takes place both in the dark and in the light.

· Photosynthesis and respiration are closely linked by the Calvin–Benson cycle.

· 3PG from the Calvin–Benson cycle can be converted into pyruvate, the end product of glycolysis.

· G3P from the Calvin–Benson cycle can be converted into hexose phosphates, such as glucose 1-phosphate, which can enter glycolysis.

· Once carbon skeletons from the Calvin–Benson cycle enter glycolysis and the citric acid cycle, they can be used anabolically to make lipids, proteins, and other carbohydrates. (See Figure 8.22.)

· Energy flows from sunlight to reduced carbon in photosynthesis to ATP in respiration.

· Energy can be stored in macromolecules such as polysaccharides, lipids, and proteins.

· For plants to grow, energy storage must exceed energy released. Stated another way, overall photosynthesis to fixed carbon must exceed respiration.

· The capture and movement of sun energy becomes the basis for ecological food chains.

The Instructor’s Resource CD-ROM includes photographs illustrating the early experiments into the Calvin–Benson cycle, a micrograph of a peroxisome, photographs of C₃, C₄, and CAM plants, and video microscopy of chloroplasts streaming in Elodea.