Photosynthesis BMZ116B Study Guide
•	Life on Earth is __________  powered.
•	The  __________  of plants use a process called  __________  to capture  __________  energy from the  __________  and convert it to  __________  energy stored in sugars and other organic molecules.
A. The Process That Feeds the Biosphere
1. Plants and other autotrophs are the producers of the biosphere.
•	Photosynthesis nourishes almost all the living world directly or indirectly.
•	All organisms use organic compounds for  __________  and for  __________  __________.
•	Organisms obtain organic compounds by one of two major modes: __________  nutrition or  __________  nutrition.
•	__________  produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment.
•	Autotrophs are the ultimate sources of  __________  compounds for all heterotrophic organisms.
•	Autotrophs are the  __________  of the biosphere.
•	Autotrophs can be separated by the source of energy that drives their metabolism.
•	__________  use light as a source of energy to synthesize organic compounds.
•	__________  occurs in plants, algae, some other protists, and some prokaryotes.
•	__________  harvest energy from oxidizing  __________  substances, such as sulfur and ammonia.
•	Chemoautotrophy is unique to  __________ .
•	__________  live on organic compounds produced by other organisms.
•	These organisms are the __________  of the biosphere.
•	The most obvious type of heterotrophs feeds on other  __________ .
•	Animals feed this way.
•	Other heterotrophs  __________  and feed on  __________  organisms or on  __________  litter, like feces and fallen leaves.
•	Most fungi and many prokaryotes get their nourishment this way.
•	Almost all  __________  are completely dependent on  __________  for food and for  __________ , a by-product of photosynthesis.
2. Photosynthesis converts __________  energy to the  __________  energy of food.
•	All green parts of a plant have  __________ .
•	However, the  __________  are the major site of photosynthesis for most plants.
•	There are about half a  __________  chloroplasts per square millimeter of leaf surface.
•	The color of a leaf comes from  __________ , the green pigment in the chloroplasts.
•	Chlorophyll plays an important role in the  __________  of light energy during photosynthesis.
•	Chloroplasts are found mainly in  __________  cells forming the tissues in the interior of the leaf.
•	__________  exits and  __________  enters the leaf through microscopic pores called  __________  in the leaf.
•	Veins deliver  __________  from the roots and carry off  __________  from mesophyll cells to nonphotosynthetic areas of the plant.
•	A typical mesophyll cell has 30–40 chloroplasts, each about 2–4 microns by 4–7 microns long.
•	Each chloroplast has two membranes around a central aqueous space, the  __________ .
•	In the stroma is an elaborate system of interconnected membranous sacs, the  __________ .
•	The interior of the thylakoids forms another compartment, the  __________   __________ .
•	Thylakoids may be stacked into columns called  __________ .
•	Chlorophyll is located in the  __________ .
•	Photosynthetic  __________  lack chloroplasts.
•	Their photosynthetic membranes arise from infolded regions of the  __________  __________ , folded in a manner similar to the thylakoid membranes of chloroplasts.
B. The Pathways of Photosynthesis
1. Evidence that chloroplasts  __________  water molecules enabled researchers to track atoms through photosynthesis.
•	Powered by light, the green parts of plants produce  __________  compounds and  __________  from  __________  and  __________ .
•	The equation describing the process of photosynthesis is:
•	6CO2 + 12H2O +  __________  energy  C6H12O6 + 6O2+ 6H2O
•	C6H12O6 is  __________ .
•	Water appears on both sides of the equation because 12 molecules of water are  __________ , and 6 molecules are newly  __________  during photosynthesis.
•	We can simplify the equation by showing only the net consumption of water:
•	6CO2 + 6H2O + light energy  C6H12O6 + 6O2
•	The overall chemical change during photosynthesis is the reverse of  __________  __________ .
•	In its simplest possible form: CO2 + H2O + light energy  [CH2O] + O2
•	[CH2O] represents the general formula for a  __________ .
•	One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from  __________ , not  __________ .
•	Before the 1930s, the prevailing hypothesis was that photosynthesis split carbon dioxide and then added water to the carbon:
•	Step 1: CO2  C + O2
•	Step 2: C + H2O  CH2O
•	C. B. van Niel challenged this hypothesis.
•	In the bacteria that he was studying, hydrogen sulfide (H2S), not  __________ , is used in photosynthesis.
•	These bacteria produce yellow globules of sulfur as a waste, rather than  __________ .
•	Van Niel proposed this chemical equation for photosynthesis in sulfur bacteria:
•	CO2 + 2H2S  [CH2O] + H2O + 2S
•	He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis:
•	CO2 + 2H2O  [CH2O] + H2O + O2
•	Thus, van Niel hypothesized that plants split  __________  as a source of  __________  from hydrogen atoms, releasing  __________  as a byproduct.
•	Other scientists confirmed van Niel’s hypothesis twenty years later.
•	They used 18O, a heavy isotope, as a  __________ .
•	They could label either C18O2 or H218O.
•	They found that the 18O label only appeared in the  __________  produced in photosynthesis when  __________  was the source of the tracer.
•	Hydrogen extracted from water is incorporated into  __________ , and  __________  is released to the atmosphere (where it can be used in  __________ ).
•	Photosynthesis is a  __________  reaction.
•	It reverses the direction of electron flow in  __________ .
•	Water is  __________  and electrons transferred with H+ from water to  __________ , reducing it to sugar.
•	Because the electrons increase in potential energy as they move from water to sugar, the process requires  __________ .
•	The energy boost is provided by  __________ .
2. Here is a preview of the two stages of photosynthesis.
•	Photosynthesis is two processes, each with multiple stages.
•	The  __________  __________  (photo) convert solar energy to chemical energy.
•	The  __________  __________  (synthesis) uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar.
•	In the light reactions, light  __________  absorbed by  __________  in the  __________  drives the transfer of  __________  and  __________  from  __________  to  __________  (nicotinamide adenine dinucleotide phosphate), forming  __________ .
•	NADPH, an electron acceptor, provides reducing power via energized  __________  to the Calvin cycle.
•	__________  is split in the process, and  __________ is released as a by-product.
•	The light reaction also generates  __________  using chemiosmosis, in a process called  __________ .
•	Thus  __________  energy is initially converted to  __________  energy in the form of two compounds: NADPH and ATP.
•	The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many of its steps in the 1940s.
•	The cycle begins with the incorporation of CO2 into organic molecules, a process known as  __________  __________ .
•	The fixed carbon is reduced with electrons provided by  __________ .
•	__________  from the light reactions also powers parts of the Calvin cycle.
•	Thus, it is the Calvin cycle that makes  __________ , but only with the help of ATP and NADPH from the  __________  __________ .
•	The metabolic steps of the Calvin cycle are sometimes referred to as the  __________  __________ , because none of the steps requires light directly.
•	Nevertheless, the Calvin cycle in most plants occurs during daylight, because that is when the light reactions can provide the  __________  and  __________  the Calvin cycle requires.
•	While the light reactions occur at the  __________ , the Calvin cycle occurs in the  __________ .
3. The  __________  reactions convert solar  __________  to the chemical  __________  of ATP and NADPH.
•	The thylakoids convert light energy into the chemical energy of  __________  and  __________ .
•	Light is a form of  __________  radiation.
•	Like other forms of electromagnetic energy, light travels in rhythmic  __________ .
•	The distance between crests of electromagnetic waves is called the  __________ .
•	Wavelengths of electromagnetic radiation range from less than a nanometer (gamma rays) to more than a kilometer (radio waves).
•	The entire range of electromagnetic radiation is the electromagnetic  __________ .
•	The most important segment for life is a narrow band between 380 to 750 nm, the band of  __________  __________ .
•	While light travels as a wave, many of its properties are those of a discrete particle, the  __________ .
•	Photons are not tangible objects, but they do have fixed  __________  of energy.
•	The amount of energy packaged in a photon is inversely related to its  __________ .
•	Photons with shorter wavelengths pack  __________  energy.
•	While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only  __________  __________  to pass in significant quantities.
•	Visible light is the radiation that drives  __________ .
•	When light meets matter, it may be reflected, transmitted, or absorbed.
•	Different pigments  __________  photons of different wavelengths, and the wavelengths that are  __________  disappear.
•	A leaf looks green because chlorophyll, the dominant pigment, absorbs  __________  and  __________  light, while transmitting and reflecting  __________  light.
•	A  __________  measures the ability of a pigment to absorb various wavelengths of light.
•	It beams narrow wavelengths of light through a solution containing the pigment and measures the fraction of light transmitted at each wavelength.
•	An  __________  __________  plots a pigment’s light absorption versus wavelength.
•	The light reaction can perform work with those wavelengths of light that are  __________ .
•	There are several pigments in the  __________  that differ in their absorption spectra.
•	__________ , the dominant pigment, absorbs best in the red and violet-blue wavelengths and least in the green.
•	Other pigments with different structures have different  __________  spectra.
•	Collectively, these photosynthetic pigments determine an overall  __________  spectrum for photosynthesis.
•	An action spectrum measures changes in some measure of  __________  __________  (for example, O2 release) as the  __________  is varied.
•	The action spectrum of photosynthesis was first demonstrated in 1883 in an elegant experiment performed by Thomas Engelmann.
•	In this experiment, different segments of a filamentous alga were exposed to different  __________  of light.
•	Areas receiving wavelengths favorable to photosynthesis produced excess  __________ .
•	Engelmann used the abundance of  __________  bacteria that clustered along the alga at different segments as a measure of O2 production.
•	The  __________  spectrum of photosynthesis does not match exactly the  __________  spectrum of any one photosynthetic pigment, including chlorophyll a.
•	Only chlorophyll a participates directly in the light reaction, but accessory photosynthetic pigments absorb  __________  and transfer  __________  to chlorophyll a.
•	__________ , with a slightly different structure than chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.
•	__________  can funnel the energy from other wavelengths to chlorophyll a and also participate in  __________  against excessive light.
•	These compounds absorb and dissipate excessive light energy that would otherwise damage  __________ .
•	They also interact with oxygen to form reactive oxidative molecules that could damage the cell.
•	When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an  __________  with more  __________  energy.
•	The electron moves from its  __________  state to an  __________  state.
•	The only photons that a molecule can  __________  are those whose energy matches exactly the energy  __________  between the ground state and excited state of this electron.
•	Because this energy difference varies among atoms and molecules, a particular compound absorbs only photons corresponding to specific  __________ .
•	Thus, each pigment has a unique  __________  spectrum.
•	Excited electrons are  __________ .
•	Generally, they drop to their  __________  state in a billionth of a second, releasing  __________  energy.
•	Some pigments, including chlorophyll, can also release a photon of light in a process called  __________ .
•	If a solution of chlorophyll isolated from chloroplasts is illuminated, it will  __________  and give off heat.
•	Chlorophyll excited by absorption of light energy produces very different results in an  __________  chloroplast than it does in isolation.
•	In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into  __________ .
•	A photosystem is composed of a  __________  center surrounded by a  __________  complex.
•	Each light-harvesting complex consists of  __________  molecules (which may include chlorophyll a, chlorophyll b, and carotenoid molecules) bound to particular  __________ .
•	Together, these light-harvesting complexes act like light-gathering “____________________ ” for the __________  center.
•	When any antenna molecule absorbs a  __________ , it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the __________  center.
•	At the reaction center is a  __________  __________  acceptor, which accepts an excited electron from the reaction center chlorophyll a.
•	The solar-powered transfer of an  __________  from a special chlorophyll a molecule to the primary electron acceptor is the first step of the  __________  reactions.
•	Each  __________ —reaction-center chlorophyll and primary electron acceptor surrounded by an antenna complex—functions in the chloroplast as a light-harvesting unit.
•	There are two types of photosystems in the thylakoid membrane.
•	Photosystem I (PS I) has a reaction center chlorophyll a that has an absorption peak at  __________  nm.
•	Photosystem II (PS II) has a reaction center chlorophyll a that has an absorption peak at  __________  nm.
•	The differences between these reaction centers (and their absorption spectra) lie not in the chlorophyll molecules, but in the  __________  associated with each reaction center.
•	These two photosystems work together to use light energy to generate  __________  and  __________ .
•	During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic.
•	__________  electron flow, the predominant route, produces both ATP and NADPH.
1.	Photosystem  __________  absorbs a photon of light. One of the electrons of P680 is excited to a higher energy state.
2.	This electron is captured by the primary electron acceptor, leaving the reaction center  __________ .
3.	An enzyme extracts electrons from  __________  and supplies them to the oxidized reaction center. This reaction splits water into two  __________  ions and an  __________  atom that combines with another oxygen atom to form O2.
4.	Photoexcited electrons pass along an  __________  __________  __________  before ending up at an oxidized photosystem  __________  reaction center.
5.	As these electrons “fall” to a lower energy level, their  __________  is harnessed to produce ATP.
6.	Meanwhile, light energy has excited an electron of PS  __________ ’s P700 reaction center. The photoexcited electron was captured by PS I’s primary electron acceptor, creating an electron “hole” in P700. This hole is filled by an electron that reaches the bottom of the electron transport chain from PS __________ .
7.	Photoexcited electrons are passed from PS I’s  __________  __________  __________  down a second  __________  __________  __________  through the protein ferredoxin (Fd).
8.	The enzyme NADP+ reductase transfers  __________  from Fd to  __________ . __________  electrons are required for NADP+’s reduction to NADPH. NADPH will carry the  __________  power of these high-energy electrons to the  __________  __________ .
•	The light reactions use the solar power of  __________  absorbed by both photosystem I and photosystem II to provide chemical  __________  in the form of __________  and reducing power in the form of the electrons carried by  __________ .
•	Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, __________  electron flow.
•	Excited electrons cycle from their reaction center to a primary acceptor, along an  __________  __________  __________ , and return to the  __________  P700 chlorophyll.
•	As electrons flow along the electron transport chain, they generate ATP by  __________  photophosphorylation.
•	There is no production of  __________  and no release of  __________ .
•	What is the function of cyclic electron flow?
•	Noncyclic electron flow produces  __________  and  __________  in roughly equal quantities.
•	However, the Calvin cycle consumes more  __________  than  __________ .
•	__________  electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the  __________  __________ .
•	Chloroplasts and mitochondria generate ATP by the same mechanism: __________ .
•	In both organelles, an  __________  __________  __________  pumps __________  across a __________  as __________  are passed along a series of increasingly electronegative carriers.
•	This transforms  __________  energy to a  __________ - __________  force in the form of an H+  __________  across the membrane.
•	ATP  __________  molecules harness the proton-motive force to generate  __________  as  __________  diffuses back across the membrane.
•	Some of the electron carriers, including the cytochromes, are very similar in  __________  and  __________ .
•	The ATP __________  complexes of the two organelles are also very similar.
•	There are differences between  __________  phosphorylation in mitochondria and  __________phosphorylation in chloroplasts.
•	Mitochondria transfer  __________  energy from food molecules to ATP; chloroplasts transform  __________  energy into the chemical energy of ATP.
•	The  __________  organization of chemiosmosis also differs in the two organelles.
•	The  __________  membrane of the mitochondrion pumps  __________  from the mitochondrial  __________  out to the  __________  space. The  __________  membrane of the chloroplast pumps  __________  from the  __________  into the  __________  space inside the  __________ .
•	The thylakoid membrane makes ATP as the hydrogen ions diffuse down their concentration gradient from the  __________  space back to the  __________  through ATP  __________  complexes, whose catalytic knobs are on the  __________  side of the membrane.
•	The proton  __________ , or pH  __________ , across the thylakoid membrane is substantial.
•	When chloroplasts are illuminated, the pH in the thylakoid space drops to about 5 and the pH in the stroma increases to about 8, a  __________ fold different in H+ concentration.
•	The light-reaction “machinery” produces ATP and NADPH on the  __________  side of the thylakoid.
•	Noncyclic electron flow pushes electrons from water, where they have __________  potential energy, to NADPH, where they have __________  potential energy.
•	This process also produces  __________ and  __________  as a by-product.
4. The Calvin cycle uses  __________  and  __________  to convert  __________  to __________ .
•	The Calvin cycle regenerates its  __________  material after molecules enter and leave the cycle.
•	The Calvin cycle is  __________ , using energy to build sugar from smaller molecules.
•	Carbon enters the cycle as  __________  and leaves as  __________ .
•	The cycle spends the energy of  __________  and the reducing power of electrons carried by  __________ to make sugar.
•	The actual sugar product of the Calvin cycle is not glucose, but a  __________ -carbon sugar, glyceraldehyde-3-phosphate (G3P).
•	Each turn of the Calvin cycle fixes  __________  carbon.
•	For the net synthesis of one G3P molecule, the cycle must take place  __________  times, fixing three molecules of  __________ .
•	To make one glucose molecule requires  __________  cycles and the fixation of six  __________  molecules.
•	The Calvin cycle has  __________  phases.
Phase 1: __________   __________
•	In the  __________  __________  phase, each CO2 molecule is attached to a  __________ -carbon sugar, ribulose bisphosphate (RuBP).
•	This is catalyzed by RuBP  __________  or  __________ .
•	Rubisco is the most abundant  __________  in chloroplasts and probably the most abundant __________  on Earth.
•	The six-carbon intermediate is  __________  and splits in half to form  __________  molecules of 3-phosphoglycerate for each CO2.
Phase 2: __________ 
•	During  __________ , each 3-phosphoglycerate receives another phosphate group from  __________ to form 1,3-bisphosphoglycerate.
•	A pair of electrons from  __________  reduces each 1,3-bisphosphoglycerate to G3P.
•	The  __________  reduce a carboxyl group to the aldehyde group of G3P, which stores more potential energy.
•	If our goal was the net production of one G3P, we would start with  __________ CO2 (3C) and  __________  RuBP (15C).
•	After fixation and reduction, we would have  __________  molecules of G3P (18C).
•	One of these six G3P (3C) is a net gain of  __________ .
•	This molecule can exit the  __________  and be used by the plant cell.
Phase 3: __________ 
•	The other five G3P (15C) remain in the cycle to  __________  three RuBP. In a complex series of reactions, the carbon skeletons of  __________  molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate  __________  molecules of RuBP.
•	For the net synthesis of one G3P molecule, the Calvin cycle consumes  __________  ATP and  __________  NADPH.
•	The light reactions regenerate  __________  and  __________ .
•	The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including  __________  and other carbohydrates.
5. Alternative mechanisms of carbon fixation have evolved in hot, arid climates.
•	One of the major problems facing terrestrial plants is  __________ .
•	At times, solutions to this problem require tradeoffs with other metabolic processes, especially photosynthesis.
•	The stomata are not only the major route for gas exchange (CO2 in and O2 out), but also for the evaporative loss of  __________ .
•	On hot, dry days, plants  __________  their stomata to conserve __________ . This causes problems for photosynthesis.
•	In most plants (__________  plants), initial fixation of CO2 occurs via  __________ , forming a  __________ -carbon compound, 3-phosphoglycerate.
•	C3 plants include rice, wheat, and soybeans.
•	When their stomata partially close on a hot, dry day, __________  levels drop as  __________  is consumed in the  __________  cycle.
•	At the same time, __________ levels rise as the light reaction converts light to chemical energy.
•	While rubisco normally accepts CO2, when the O2:CO2 ratio  __________  (on a hot, dry day with closed stomata), rubisco can add  __________  to RuBP.
•	When rubisco adds O2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called  __________ .
•	The  __________ -carbon fragment is exported from the chloroplast and degraded to CO2 by  __________  and  __________ .
•	Unlike normal respiration, this process produces no  __________ .
•	In fact, photorespiration  __________  ATP.
•	Unlike photosynthesis, photorespiration does not produce  __________  molecules.
•	In fact, photorespiration  __________  photosynthetic output by siphoning organic material from the  __________  cycle.
•	A hypothesis for the existence of photorespiration is that it is  __________  baggage.
•	When rubisco first evolved, the atmosphere had far less __________  and more __________  than it does today.
•	The inability of the __________  __________ of  rubisco to exclude O2 would have made little difference.
•	Today it does make a difference.
•	__________  can drain away as much as 50% of the carbon fixed by the Calvin cycle on a hot, dry day.
•	Certain plant species have evolved alternate modes of carbon fixation to minimize photorespiration.
•	__________  plants first fix CO2 in a four-carbon compound.
•	Several thousand plants, including sugarcane and corn, use this pathway.
•	A unique  __________  anatomy is correlated with the mechanism of C4 photosynthesis.
•	In C4 plants, there are two distinct types of photosynthetic cells: __________ - __________  cells and  __________  cells.
•	__________ - __________  cells are arranged into tightly packed sheaths around the veins of the leaf.
•	__________  cells are more loosely arranged cells located between the bundle sheath and the leaf surface.
•	The Calvin cycle is confined to the  __________  of the bundle-sheath cells.
•	However, the cycle is preceded by the incorporation of __________  into organic molecules in the mesophyll.
•	The key enzyme, phosphoenolpyruvate carboxylase, adds  __________  to phosphoenolpyruvate (PEP) to form oxaloacetate.
•	__________   __________  has a very high affinity for CO2 and can fix CO2 efficiently when  __________  cannot (i.e., on hot, dry days when the stomata are closed).
•	The  __________  cells pump these four-carbon compounds into  __________ - __________  cells.
•	The bundle-sheath cells strip a carbon from the  __________ -carbon compound as CO2, and return the  __________ -carbon remainder to the mesophyll cells.
•	The bundle-sheath cells then use  __________  to start the Calvin cycle with an abundant supply of  __________ .
•	In effect, the  __________  cells pump  __________  into the  __________ - __________  cells, keeping  __________  levels high enough for  __________  to accept  __________  and not  __________ .
•	C4 photosynthesis minimizes  __________  and enhances sugar production.
•	C4 plants thrive in  __________  regions with intense  __________ .
•	A second strategy to minimize photorespiration is found in  __________  plants, cacti, pineapples, and several other plant families.
•	These plants are known as  __________  plants for crassulacean acid metabolism.
•	They  __________  their stomata during the night and  __________  them during the day.
•	Temperatures are typically  __________  at night, and humidity is  __________ .
•	During the night, these plants fix  __________  into a variety of organic acids in  __________  cells.
•	During the day, the light reactions supply  __________ and  __________  to the Calvin cycle, and  __________  is released from the organic acids.
•	Both C4 and CAM plants add CO2 into organic intermediates before it enters the  __________  cycle.
•	In C4 plants, carbon fixation and the Calvin cycle are  __________  separated.
•	In CAM plants, carbon fixation and the Calvin cycle are __________ separated.
•	Both eventually use the  __________  cycle to make sugar from carbon dioxide.
6. Here is a review of the importance of photosynthesis.
•	In photosynthesis, the energy that enters the chloroplasts as  __________  becomes stored as chemical  __________  in organic compounds.
•	Sugar made in the chloroplasts supplies the entire plant with  __________  energy and  __________  skeletons to synthesize all the major organic molecules of cells.
•	About 50% of the organic material is consumed as fuel for cellular  __________  in plant mitochondria.
•	Carbohydrate in the form of the disaccharide  __________  travels via the veins to nonphotosynthetic cells.
•	There, it provides fuel for  __________  and the raw materials for  __________  pathways, including synthesis of proteins and lipids and formation of the extracellular polysaccharide  __________ .
•	Cellulose, the main ingredient of  __________  __________ , is the most abundant organic molecule in the plant, and probably on the surface of the planet.
•	Plants also store excess sugar by synthesis of  __________ .
•	Starch is stored in  __________  and in storage cells in roots, tubers, seeds, and fruits.
•	__________ , including humans, may completely or partially consume plants for fuel and raw materials.
•	On a global scale, __________  is the most important process on Earth.
•	It is responsible for the presence of __________  in our atmosphere.
•	Each year, photosynthesis synthesizes 160 billion metric tons of  __________ .