Published on Agron 501 (https://courses.agron.iastate.edu/agron501)

Home > Lesson 4

Lesson 4: Photosynthesis - ProcessesLesson 4: Photosynthesis - ProcessesDeveloped by: By R. Shibles, D. Muenchrath and A. D. Knapp

About 95% of plant dry matter is organic material produced by photosynthesis, a process unique togreen plants and a few other organisms—algae for example. Photosynthesis is responsible, directly orindirectly, forall the food and feed humans and other animals need to live. In fact, when you consider thesource of fossil fuels, photosynthesis accounts for much of the energy we use today.

Through photosynthesis (literally "building by use of light") plants construct themselves from carbondioxide and water using solar energy to drive the process. The net reaction, where (CH2O) representscarbohydrate (a hydrated carbon), is written:

Crop physiologists consider photosynthesis as three integrated processes:

1. The light reactions—solar energy is captured and converted to chemical energy,2. A diffusion process—CO2 moves from the atmosphere through the leaf into the chloroplast, and3. A two step assimilation process—CO2 in the chloroplast is fixed and the resulting compound is

reduced to form carbohydrate using the chemical energy from captured sunlight.

Because photosynthesis is fundamental to yield, crop management techniques are designed tominimize any stress that would limit photosynthesis. In contrast, some herbicide families function byinterfering with photosynthetic processes.

ObjectivesObjectives

1. Describe the mechanism of solar energy capture and conversion to chemical energy inchloroplasts.

2. Explain the mechanisms of carbon diffusion to the site of carboxylation.3. Understand carbon assimilation and carbohydrate production by chloroplasts.4. Describe photorespiration and its impact on C3 and C4 plants.5. Distinguish mechanistically between the three major types of CO2 assimilation: C3, C4, and

CAM.6. Identify how and why the three photosynthetic types differ in terms of productivity and ecological

adaption.

Before you begin, listen to the instructor's comments on the key concepts in this Module.

Equation 4.1

Agron 501 Module 4 CommentsAgron 501 Module 4 CommentsAgron 501 Module 4 Comments

The Light ReactionsThe Light Reactions

The Nature of Solar EnergyThe Nature of Solar Energy

Solar energy is transmitted in waves as bundles of energy called photons, or quanta. Light capable ofenergizing photosynthesis is called photosynthetically active radiation (PAR) and encompasseswavelengths in the visible spectrum between 380 and 720 nm (Fig. 4.1). Because it occurs in the visibleportion of the solar spectrum, PAR often is referred to casually as ‘light’. The spectrum for theperception of light by the human eye, however, differs from that of PAR for photosynthetic activity.

The quantity of PAR available for photosynthesis at a leaf surface is the Photosynthetic Photon FluxDensity (PPFD). Some refer to this value as the "light intensity."

Fig. 4.1 PAR and visible light portions of the electromagnetic spectrum.

Table 4.1 Comparative Solar Radiation Terminologies RadiationName

Wavelengthsencompassed, λ

QuantityName

Units ofQuantity

Maximumflux*

light 400-700 nm illuminance lux 14,000

PAR 380-720 nm PPFDµmolphotons m-2s-1

2200

radiantenergy 0.1nm - 1.0m irradiance W m-2 950

* at solar noon on a clear, summer day

Chloroplast Organization and StructureChloroplast Organization and StructurePhotosynthesis occurs in a cellular organelle, the chloroplast (Fig. 4.2). Inside the chloroplast is aliquid matrix, the stroma. Suspended within the stroma is a system of flattened, sac-like membranousstructures, the thylakoids. An individual thylakoid consists of two membranes enclosing an internalspace, the lumen. Thylakoids occur in stacked groups, called grana, or as single stromalthylakoids that connect grana. The capture of PAR and its conversion to chemical energy,i.e. transduction, occurs within the thylakoids.

The interactive activity Study Question 4.1 can only be completed online.

Fig. 4.2 Photomicrograph of a sunflower chloroplast. Courtesy of Dr. Harry T.Horner, Department of Botany, Iowa State University, Ames, Iowa, U.S.A.

PAR Capture and Transduction to Chemical EnergyPAR Capture and Transduction to Chemical EnergyLight energy is captured by various chlorophyll pigments and through the actions of photosystem I andphotosystem II electrons and protons are stripped from the hydrogen atoms of water and separated.The electrons and protons so formed are separated by the thylakoid membrane to generate anelectrochemical gradient that allows for the formation of two compounds. NADPH provides reducingequivalents for the reduction of CO2 to CH2O. And ATP, a well known currency of energy in biologicalsystems, is generated by proton mediated ATPases. Since the formation of ATP, in this case, isbasically driven by light energy its formation is called photophosphorylation. Both the NADPH and ATPaccumulate in the stroma of the chloroplast.

The photosynthetic process of photophosphorylation occurs in two energy capture and conversionsystems: Photosystem I (PSI) stationed in stromal thylakoids, and Photosystem II (PSII) located ingranal thylakoids.(Fig. 4.3) The spatial arrangement of PSI and PSII may be important for optimumdistribution of photons between the two photosystems. Efficiency of energy conversion is greatest whenthe light absorbed by PSII and PSI is balanced so that both photosystems can function.

Fig. 4.3 Schematic of a chloroplast withits thylakoids: granal (stacked) and stromal (unstacked area).

(Adapted from Buchanan et al., 2000)

Fig. 4.4 Close up diagram showing detail of chloroplast thylakoid systems. (Adaptedfrom Buchanan et al., 2000)

The photosystems are linked by a series of electron carriers that form an 'electron transport chain'.These carrier molecules in the thylakoid membrane accept and release electrons based on theirrelative redox potentials. The arrangement of the photosystems and electron transport chain areconceptualized as the "Z-scheme" in the figure below.

Keep the organization of this Z-scheme in mind as we examine the sequence of reactions involving lightharvest and energy conversion. Note that the sequence begins with PSII. (Yes, you read that correctly.Light is first captured by PSII; the number designations of Photosystems I and II are based on theirorder of discovery, rather than the order of their roles in PAR capture).

The interactive activity Try This! Redox Reactions can only be completed online.

The interactive activity Path of Electron and Proton Flow can only be completed online.

The interactive activity Study Question 4.2 can only be completed online.

Z-schemeZ-scheme

"Z-scheme" electron transport chain of PSII and PSI. Scale on left shows energypotential corresponding to the midpoint of carrier redox potential.

PigmentsPigmentsPigment molecules absorb light energy (photons), to provide energy for the formation of NADPH andATP. The pigments are contained in the chloroplast light-harvesting complexes (LHC-II and I). Two mainclasses of pigments are involved in the capture of light energy: chlorophyll and carotenoids.

Chlorophyll—Chlorophyll a and b absorb blue and red light. Chlorophyll reflects green light,giving plants their green color. These pigments are the main molecules involved in the capture ofPAR.Carotenoids—Carotenes and xanthophylls are yellow to orange pigments. They participateas accessory pigments in the absorption of blue light. Some carotenoid molecules absorbphotons and donate excited electrons to chlorophyll or relay excited electrons between thephotosystems. Carotenoids also have a role in the assembly of light-harvesting complexes andalso they protect chlorophyll from photo-oxidative damage by attenuating or quenching excessiveexcitation energy.

Fig. 4.5 Absorption spectra of chlorophylls (A) and carotenoids (B).

Photon absorption converts the pigment from its lowest-energy or ground state to an excited state,causing one of the pigment's electrons to shift its molecular orbit. (Recall that electrons rotate or orbitaround the atom's nucleus.) The excited electron can return to its more stable ground state via severalmechanisms (Buchanan et al., 2000; Hillier and Babcock, 2001; Ort, 2001). Each mechanism releasesthe energy in a different form as the electron returns to ground state.

Photochemistry—The electron from the excited molecule is lost to an electron acceptor molecule,reducing that acceptor molecule.

This mechanism transduces light energy into chemical products. Thus, excitation of the pigment initiatesthe photophosphyration process.

Relaxation—Excitation energy is released simply as heat during a non-radiative decay.Carotenoids are involved in thermal energy dissipation (Ort, 2001).Fluorescence—Energy is released as light; the light is emitted in a wavelength slightly longer(lower energy) than that of the absorbed light.Energy Transfer—This mechanism transfer the energy from one molecule to another, such asfrom one pigment molecule to another. This is an important and efficient means of energy transferbetween antenna and reaction centers. Other names for this mechanism are 'inductiveresonance' and 'radiationless transfer'.

Fig. 4.6 Chlorophyll (Chl) and accessory pigments (Acc) capture light energy. Accessory pigments transfer energy to chlorophyll.

Fig. 4.7 Photochemistry of pigment photon absorption and energy transfer.

What happens when there is excessive light or more energy than the plant can use? Ordinarily, energyis dissipated by combinations of photochemistry, relaxation, fluorescence, and energy transfermechanisms. Some plants are also able to reduce the amount of light incident on the leaf by alteringleaf display and/or chloroplast movement, and thus, reduce photon absorption. However, when theamount of energy absorbed exceeds the energy that the pigments are able to transfer via the usualmechanisms, the photosynthetic apparatus can be damaged—Photosystem II is particularly vulnerable.

When light energy is excessive, the pH gradient between the lumen and stroma sides of the thylakoidmembrane increases (low lumen pH). This change in pH gradient signals a state of excessive energyand may induce a conformational change in carotenoids within the PSII antenna (Ort, 2001). Theconformation change, in turn, may promote thermal dissipation of energy, quenching the excessiveenergy and protecting chlorophyll in the light-harvesting complex (LHC-II).

When chlorophyll is in an excessive excitation state, that excited state can last longer and generatedamaging molecules, such as excited oxygen molecules and free radicals. These reactive moleculescan damage thylakoid membranes, chlorophyll, enzymes, and other molecules, disrupting or interferingwith photosynthetic processes. Carotenoids can accept energy from excited chlorophyll and thus, helpprevent the formation of oxygen free radicals. Peroxidases and repair mechanisms also enable theplant to rid itself of reactive molecules and repair photo-induced damage.

Herbicides are commonly designed to disrupt the light reactions, resulting in eventual plant death.Some herbicides target the biosynthetic pathways involved in the production of carotenoids. Withoutsufficient carotenoid molecule to provide photo-protection, thylakoid membranes and photosyntheticapparata are damaged. Some herbicides block the electron transport chain of the Z-scheme,interrupting energy transfer. Others autooxidize, forming reactive oxygen species, or free radicals whichdisrupt membranes. Still other herbicides inhibit photosynthesis by binding to a protein in the thylakoidmembrane, stopping ATP and NADPH production and CO2 fixation.

Fig. 4.8 Diagram of chlorophyll excitation and energy dissipation. Some energy is also lost as heat.

Carbon Dioxide DiffusionCarbon Dioxide Diffusion

Leaf AnatomyLeaf Anatomy

Carbon dioxide (CO2) diffuses from the atmosphere into the leaf and to the site of assimilation within theleaf. Re-familiarize yourself with the anatomy of a leaf. In the following activity, click on the terms to viewtheir definitions. Then, click on the boxed area in the graphic for additional terms and definitions.

The interactive activity Soybean Leaf Anatomy can only be completed online.

The Leaf Diffusion PathwayThe Leaf Diffusion PathwayGas movement through a leaf presents a unique analytical problem. The pathway involves diffusionthrough different substances (e.g., air, cell sap), passage through pores (stomata) of varying size, andin the case of CO2, absorption may be enzyme assisted. Crop physiologists have dealt with thiscomplex circumstance by assuming gas movement is analogous to the flow of electricity, describedmathematically by Ohm's Law. Ohm's Law states that flux (gas flow) is directly proportional to thepotential gradient (concentration in the case of gas diffusion) and inversely proportional to resistance ofthe path. In the gas diffusion analogy, flow can be characterized by a series of resistances through theair, stomatal pores and cell sap. In the particular case of CO2, diffusion for photosynthesis, the simplestformula describing flux is:

A, assimilation, is the net flux or flow rate of CO2 into the leaf as measured during photosynthesis;usually A is expressed on a leaf surface (one side) basis in units of µmol CO2 m-2 s-1. Ca refers to theCO2 concentration of ambient air— air that is external to the leaf. Cstr represents that CO2 in thechloroplast stroma. The CO2 transport resistances are partitioned into stomatal resistance, rs , whichis determined largely by the density and porosity of stomata; and cellular resistance, rc (sometimescalled mesophyll resistance), the resistance internal to the mesophyll cell. A third resistance, boundarylayer resistance, ra, is related to diffusion in air external to the leaf; because it is inseparable from rs byconventional measurement techniques and, in most cases, ra is many times smaller than rs, so ra isignored. Some find it more straightforward to think of the movement of CO2 in termsof conductance rather than one of resistance.

The interactive activity Study Question 4.3 can only be completed online.

Equation 4.2

Fig. 4.9

Measuring the Photosynthetics Characteristics of aMeasuring the Photosynthetics Characteristics of aLeafLeafModern instrument systems use infrared gas analyzers (IRGA) to measure photosynthetic ratesbecause CO2 absorbs infrared light very strongly. The IRGA continuously samples the air flowing froman enclosed, transparent leaf chamber, and compares it to a 'reference sample' taken from the same airstream just before it enters the leaf chamber. Using differential infrared (IR) light absorption, theinstrument 'senses' the decline in CO2 and build-up of water vapor across the chamber by comparingchamber air with reference air. Given these changes of gas concentrations, plus measurements of theleaf and air temperatures, and a value entered by the operator for the leaf surface area, the instrumentcalculates and displays A; transpiration (Tr); Ca; the leaf internal air CO2 concentration (Ci); and rs.

Equation 4.2 requires a value for Cstr to calculate A. This value cannot be measured and must beestimated. A common estimate of Cstr is approximately 0.5 Ca. But this relationship will vary withphotosynthetic rate, stomatal resistance, and photosynthetic type.

Cellular resistance to CO2 diffusion is the most difficult to characterize and essentially impossible tomeasure directly. So it usually is calculated as the residual resistance by subtracting rs from totalresistance obtained from Equation 4.2. The stomatal resistance value given by the instrument iscalculated for water vapor flux. The rate of water vapor loss (Transpiration, Tr) is estimated by theinstrument from a formula similar to that for CO2 flux. The flux equation for water vapor (Equation 4.4)differs from that for photosynthesis in only one respect—there is presumed to be no cellular resistanceto water movement. Why? Because the cell surfaces bordering intercellular leaf space are consideredto be saturated with water, so that there is no component of cellular resistance, only stomatal resistancein water vapor diffusion.

Fig. 4.10 Schematic representation of typical infrared gas analysis system used formeasurement of gas exchange characteristics of leaves.

Equation 4.3 This equation requires a value for Cstr to calculate A.

Where,

Tr is the transpiration rate;Wi is the internal concentration of water vapor;Wa is atmospheric humidity; and

rsH20 is the stomatal resistance to water vapor diffusion.

Stomatal resistance for CO2 flux, rsH20, differs, in absolute value, from that for water vapor flux by theratio of diffusion coefficients in air for the two gases, 1.6, which is a physical property of the molecules.

By subtraction, then, we arrive at cellular resistance to CO2 diffusion, rc.

Cellular resistance, as estimated by subtraction, probably should more properly be called residualresistance, and should not be considered a true transport resistance because carboxylation(CO2 fixation) may be the major component of cellular resistance.

In plotting A vs. Ci, the slope of the linear portion of the relationship is the carboxylation efficiency, 1/rc.

Equation 4.4

Equation 4.5

Equation 4.6

Fig. 4.11 Assimilation, A, as a function ofinternal CO2concentration, Ci, and

carboxylation efficiency, 1/rc, the linearportion of A-Ci curve.

Carbon Dioxide Assimilation and Sugar ProductionCarbon Dioxide Assimilation and Sugar ProductionCarbon dioxide assimilation occurs by three different but related mechanisms in green plants. C3photosynthesis is the most common among crop plants. The two other mechanisms, C4photosynthesis and Crassulacean Acid Metabolism (CAM), represent ecological adaptations thatimprove photosynthetic productivity under stressful conditions. This diversity allows for breadth ofecological adaptation and also is responsible for some, though not all, of the species differences in cropproductivity.

Study Tip: Be able to describe and explain the similarities and differences among C3, C4, and CAMphotosynthetic pathways. Know the main steps and key compound and enzyme names. Thebiochemistry of these pathways is presented to assist you in understanding the processes involved.However, you are not expected to memorize chemical structures.

C3 PhotosynthesisC3 Photosynthesis

C3 photosynthesis gets its name from the first product of CO2 fixation—a three-carbon (3-C) compound, 3-phosphoglycerate or PGA.

CO2 is assimilated in the Photosynthetic Carbon Reduction (PCR) or Calvin Cycle. The PCR cycle hasthr ee phases:

1. Carboxylation—CO2 is fixed into a five carbon sugar (5-C), ribulose-1,5-bisphosphate (RuBP)by the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase, (Rubisco) giving two PGAs;

Carboxylation PhaseCarboxylation Phase

CO2 is assimilated in the carboxylation phase of the PCR cycle. In the first step, oneCO2 molecule is added to RuBP via a carboxylation reaction.

Note that the final step of these reactions is the enzymatic break down of the six carbonintermediate formed by the carboxylation RUBP into two(2) three carbon compounds.These carbon compounds are called phosphoglycerate. Or, 3-phosphoglycerate afterremembering the carbons with respect to their 3 carbon structure.

CO2 is fixed into RuBP by Rubisco forming a six carbon (6-C) intermediate, which isunstable and immediately hydrolyzes to give two molecules of PGA. Phosphate

groups, PO32-, are shown as . Each C atom is numbered to help you track their fates.

2. Reduction—PGA is reduced using NADPH and an ATP to another three carbon (3-C)compound, glyceraldehyde 3-phosphate (GAP), a sugar phosphate molecule used forbiosynthesis and energy and in the regeneration phase of the PCR cycle.

3. Regeneration— RuBP is regenerated through a series of enzyme-mediated reactions, usingGAP molecules, water, and ATP.

Carboxylation itself does not require input of energy, but the reduction and regeneration phases do. Thelatter two phases are driven by the chemical energy (NADPH, ATP) generated by the light reactions.

3-phosphoglycerate after remembering the carbons with respect to their 3 carbonstructure.

Reduction PhaseReduction Phase

Each of the 2PGAs are then reduced to make two glyceraldehyde-3-phosphates (GAP)because they contain 3 carbons and a phosphorous group, they are called triosephosphate (TP). So, the carboxylation and reduction reactions yield six carbohydratecarbon atoms, the two TPs, in place of the initial five carbons of RuBP. NOTE howeverthat only one "new" carbon has been added.

The GAP molecules form PART of the chloroplast TP pool. These TP can be usedseveral different ways depending on the plant/cell carbon balance. Three common usesfor the TPs are:

1. to regenerate RUBP to keep the carboxylation/reduction reactions going.2. TPs can be combined to feed into the hexose (C carbon) phosphate pool. This is

necessary in order to support starch synthesis.3. TPs, perhaps after conversion to dihydroxyacetone phosphate (DHAP), can also

be exported from the chloroplast to the cytoplasm via a TP/Pi antiporter.

You should have deduced by now that it takes 3 carboxylation/reduction reactions toadd 3 "new" carbons to the triose pool.

Both PGA molecules produced by the RuBP carboxylase reaction are reduced to forma molecule of glyceraldehyde 3-phosphate (GAP). Thus, two ATP and

two NADPH2 per CO2 molecule fixed are used in the reduction reaction. Note, thereduction reaction for only one of the PGA molecules is shown here.

Each turn of the PCR cycle:

1. Fixes one CO2;2. Generates one-sixth of a hexose (6-C) sugar molecule;3. Regenerates one RuBP molecule (containing five C atoms); and4. Consumes four hydrogen (H+) atoms.

Three CO2 molecules need to be fixed to yield one "new" GAP molecule. A total of 6 GPA moleculesare involved in these 3 cycles. Five GAP molecules are used to regenerate RuBP (recall that RuBP is a5-C molecule), plus one GAP does into the triose phosphate pool in the stroma. GAP can move into thehexase phosphate and or be converted into dihydroxyacetone phosphate (DHAP) and exported to thecell cytoplasm for use in hexose formation. So again, the assimilation of three molecules ofCO2 produces a net synthesis of a single three-carbon sugar phosphate molecule, GAP.

The interactive activity Study Question 4.5 can only be completed online.

In summary, the PCR cycle occurs in the stroma of the chloroplast, deriving energy from the lightreactions—photophosphorylation—to assimilate CO2 into carbohydrates (sugar).

Fig. 4.12 The PCR cycle has three phases: carboxylation, reduction, and regeneration.(Adapted from Buchanan, 2000).

The interactive activity Try This! PCR Cycle - Animation can only be completed online.

What is the fate of the TP generated in the PCR cycle?

Sucrose Synthesis—Most of the TP is shuttled from the chloroplast into the cell cytoplasm viathe TP-Pi transporter located in the chloroplast membrane. In the cytoplasm, TP is assembledinto sucrose, a 12-C sugar, which is loaded into the leaf phloem and exported to growing tissues(Lesson 12). However, not all of the TP exits the chloroplast.RuBP Regeneration—As you know, some of the TP must be used to regenerate theCO2 acceptor molecule, RuBP.Starch Synthesis—Some of the TP is also used to produce starch. During the daytime,chloroplasts continually produce starch. During the night, the starch is hydrolyzed back to TP,exported from the chloroplast into the cytoplasm, and assembled into sucrose for export from theleaf to support nighttime plant growth. Thus, another major function of the chloroplast is tosynthesize and store starch.

The interactive activity Study Question 4.6 can only be completed online.

Fig. 4.13 The Photosynthetic Carbon Reduction (PCR) cycle of CO2 assimilation.Keep in mind the three phases of the PCR cycle: carboxylation, reduction, and

regeneration.

PhotorespirationPhotorespirationThe Process

Remember that the name of the enzyme that catalyzes the carboxylation ribulose 1,5 - bisphosphatecarboxylase - oxygenase (RUBISCO) is so named because in addition to the carboxylation reaction, theenzyme can also act as an oxygenase reaction. This reaction and those that follow, characterize what iscalled photorespiration. Photorespiration, refers to light-stimulated evolution of CO2. Photorespiration isnot a true respiration because ATP is not generated as it is in 'dark respiration'.

Photorespiration occurs because Rubisco is not specific for CO2 fixation. About 33% of the time,Rubisco will fix an oxygen molecule instead of CO2. When this happens, one molecule of a toxiccompound, 2-phosphoglycolate (PG), is produced.

Because the plant cannot utilize PG, it must rid itself of the compound metabolically. The process ofbreaking down PG involves shuttling compounds between three organelles and ultimately leads to therelease of CO2. The rate of photorespiratory CO2 loss is estimated between 18 and 27% of thephotosynthetic rate. Until recently, photorespiration was believed to have no useful function; but newresearch suggests that photorespiration may serve several beneficial purposes.

Fig. 4.14 Oxygenation of RuBP

At 25oC and 350 PPM (Parts Per Million) CO2, Rubisco will average a ratio of 1 O2 to 34 CO2 fixed.The fixation of O2 obviously does not result in carbon gain, and the PG produced is metabolizedthrough a mechanism that results in additional loss of CO2 (1 CO2 for every 2 PG produced). The netconsequence of photorespiratory O2 fixation is a 30% reduction in net carbon gain, with the fixation ofO2 instead of CO2 accounting for two-thirds of the reduction. The other one-third is a consequence ofthe loss of CO2 upon decarboxylation of glycine.

As temperature increases above 25oC, photorespiration increases because O2 solubility in cell sapincreases faster than does that of CO2. At temperatures cooler than 25oC, inhibition by O2 is

Fig. 4.15 PG is formed when Rubisco fixes O2 instead of CO2. PG is shuttled fromthe chloroplast to the peroxisome to mitochondrion, where the CO2 is released

PPM (Parts Per Million)PPM (Parts Per Million)

Parts per million, or ppm, is rarely used now in science writing because it was not included inthe international system of scientific units (SI Units). The reason is that ppm has no realmeaning unless one specifies whether it is volume/volume, mass/volume, or mass/mass. Oneneeds to use actual units for the meaning to be clear. So, 350 ppm CO2 in air is more properlywritten 350 µl CO2 L-1 air.

proportionately less. But, temperature effects on photorespiration can be masked in whole leaves bydirect temperature effects onphotosynthetic enzymes. For example, increasing temperature from 15oCto 25oC usually increases net carbon gain in C3 types because Rubisco cycling is accelerated relativeto photorespiration.

COCO22-- O O22 Competitiveness for Rubisco Competitiveness for Rubisco

The CO2-O2 relationship with Rubisco consists of molecules of each gas competing for the samebinding site on the enzyme. The enzyme having a much stronger affinity for CO2 than for O2. (Oxygenis 600-times more abundant in the atmosphere than is CO2, yet on average is fixed only 30% of thetime.) Since the beginning of the industrial revolution, the CO2 concentration of the atmosphere hasincreased about 25%, and with continued burning of fossil fuels at an accelerating rate, it continues torise rapidly.

In the normal atmosphere of 21% O2, the CO2 compensation concentration, G, is about 50 to 80 µL ofCO2 L-1of air. This is the concentration of CO2 at which CO2 fixation by photosynthesis balances theevolution of CO2 resulting from photorespiration and respiration. Below a concentration of 2%, oxygen isnot fixed, there is no photorespiration, and G is zero or nearly so. As oxygen increases above21%, G increases and photosynthesis declines. Normal atmospheric concentration of CO2 is about 350µL L-1 air (350 ppm).

Fig. 4.16 Graphic representation of the result ofCO2 and O2competition for the active site on Rubisco in C3

photosynthesis. G, which is the CO2 compensation atmosphericconcentration, is a function of O2 concentration. The red Xmarks

the G for each given O2 percentage.

C4 PhotosynthesisC4 PhotosynthesisC4 Leaf Anatomy

C4 photosynthesis is so named because the first product of CO2 fixation is a four-carbon (4-C)acid,oxaloacetic acid (OAA). C4 photosynthesis was discovered in sugarcane in the mid-1960's, abouta decade and a half after Calvin elucidated the PCR Cycle. Not long afterward, it was noted thatphotorespiration was not detectable in C4 plants.

Some C4 plants feature a unique leaf anatomy, called Kranz anatomy, which facilitates and spatialseparation of biochemical functions in photosynthesis. Kranz means "wreath-like" in German; inC4 plants a sheath of large cells surround the vascular bundles, giving a wreath-like appearance incross-section (Fig. 4.17). Although not all plants exhibit the Kranz anatomy, it helps to illustrate theCO2 concentrating mechanism and thus we will base our discussion of C4 photosynthesis around it.

Bundle sheath cells are dense in chloroplasts, contain a lot of starch, and havenumerous plasmodesmatal connections with adjacent mesophyll cells. Where a bundle sheath exists

in C3 plants, it usually is much reduced in volume and has few or no chloroplasts.

C4 Carbon Assimilation Cycle

In C4 plants, the PCR Cycle occurs in the bundle sheath cells, but the initial fixation of carbon occurs inmesophyll cells. In mesophyll cells, HCO3

-, bicarbonate, in the cytoplasm is incorporated into a C

Fig. 4.17 Comparative leaf anatomy of C3 (top) and C4 (bottom) plants

compound,phosphoenolpyruvate (PEP), by the enzyme phosphoenolpyruvate carboxylase(PEPCase) to form a 4-C acid, oxalacetic acid (OAA) — hence the name C4 photosynthesis.

The interactive activity Try This! C4 Metabolism can only be completed online.

Mesophyll chloroplasts perform the first energy-requiring step, the conversion of OAA to malate, andthey also use energy to regenerate the CO2 acceptor, PEP. Bundle sheath chloroplasts perform PCR

C4 EquilibriumC4 Equilibrium

HCO3-. Remember, in liquids CO2 exists in equilibrium with bicarbonate.

Fig. 4.18 Initial CO2 fixation reaction in C4 photosynthesis.

Fig. 4.19 Schematic showing locations of C4 photosyntheticreactions. PEPCase fixation of CO2 occurs in mesophyll cells, and the PCR cycle

occurs in bundle sheath cells.

Cycle metabolism, including fixation of CO2 released upon malate decarboxylation and reduction ofPGA to triose phosphate (TP).

The interactive activity Study Question 4.7 can only be completed online.

C4 Plants Avoid COC4 Plants Avoid CO22 Loss by Photorespiration. Loss by Photorespiration.

By combining more efficient system for CO2 fixation with a unique anatomical structure, C4 plants avoidthe fixation of O2 by Rubisco. C4 metabolism essentially pumps CO2 into the small volume of the

bundle sheath, raising the CO2 concentration around Rubisco many times higher than in the palisademesophyll cells of a C3 plant. This process essentially denies O2 access to Rubisco. Although the

enzymes for photorespiration are present in C4 plants, photorespiratory CO2 loss does not occur insignificant amounts because of the high concentration of CO2 in the vicinity of the active site of Rubisco,

and the potential to re-fix photorespired CO2 in the surrounding bundle sheath cells. Thus, C4sgenerally have higher photosynthetic rates than C3s. As a consequence, C4 plants are more

photosynthetically efficient at ambient CO2 concentrations and high light intensities.

Photosynthesis and Plant AdaptationPhotosynthesis and Plant Adaptation

C3 plants are marginally adapted to hot climates that often experience water deficit. Coping with heatand occasional water stresses, plus the 30% loss in carbon acquisition due to photorespiration,

threatens their existence in these climates. Some ecologists theorize that C4 plants evolved as anadaptation to stressful climates. C4 plants have better water-use efficiency and use nitrogen more

efficiently in photosynthesis than C3s do.

Water-Use EfficiencyWater-Use Efficiency

Water-use efficiency is calculated as dry matter gain per unit water used. The greater water-use efficiency of C4s is not due to their using less water per se. Because C4s do

not photorespire they have greater dry matter gains per unit of water used than do C3s.

Hot environments tend to be nitrogen poor. Substantial quantities of nitrogen are “tied-up”in photorespiratory cycling in C3 plants. Lacking this, C4 plants are much more efficient in the

use of nitrogen for photosynthesis.

C3-C4 IntermediatesC3-C4 Intermediates

For the most part, crop plants are either C3 or C4. In a few genera, both C3 and C4 species occur. Oneof these, Panicum, is a genus of tropical to temperate grasses. Not only are there distinct C3 and

C4 Panicumspecies, there also is a species in which the leaves exhibit both C3 and C4 characteristics— a C3-C4 intermediate. However, these plants are mostly curiosities; they have little agronomic

worth.

The interactive activity Study Question 4.8 can only be completed online.

Breeding for Improved PhotosynthesisBreeding for Improved Photosynthesis

This raises the question: Would it be practical to breed a C4 soybean, or rice, or wheat?

Lack of occurrence of C4 species within the many genera of crop plants has precluded breedingC4photosynthesis into traditional C3 crops. Some genera, Triticum (wheat), for example, have beensearched without success for the occurrence of C4 variants. Mutagenesis to produce a C4 type insoybean (Glycine) and perhaps in other genera, too, has been tried; but again without success.

Recently, genes for C4 photosynthesis have been incorporated successfully into rice (Oryza sativa) aC3 species using transgenic approaches. Transformants expressing PEP carboxylase have slightlyhigher rates of photosynthesis and decreased sensitivity to O2 inhibition of photosynthesis (a test for

photorespiration). Interestingly, the reason for these improvements was a decrease in stomatalresistance, which apparently resulted in higher concentrations of CO2 at the site of carboxylation in the

mesophyll chloroplasts.

Photorespiratory cycling

Discussion Topic 4.1Discussion Topic 4.1

What is photorespiration? How is it different from metabolic respiration?

Outline the pros and cons of photorespiration. It is a good or bad thing for plants?Discuss why you think plant ‘tolerate’ a Rubisco enzyme that fixes both CO2 and O2. Consider the potential benefits/risks of incorporating a C4 photosynthetic pathway into

a C3 plant such as rice. Many millions are being spent in pursuit of this goal.

Complete this assignment in Canvas. Be sure to check the course calendar for due dates.

Crassulacean Acid Metabolism (CAM)Crassulacean Acid Metabolism (CAM)Crassulacean Acid Metabolism (CAM) was named for the plant family in which it was discovered, theCrassulaceae, but it also occurs in many other "succulents". CAM plants exhibit a kind ofC4 photosynthesis in which fixation into C4 acids and assimilation via the PCR Cycle are separatedtemporally, rather than spatially as with C4s. CAM plants can open their stomata at night and assimilateCO2 into C4 acids, mostly malate, via PEPCase. The malate is stored in vacuoles. In daytime, themalate is released from vacuoles, decarboxylated in the cytoplasm, and the CO2 is re-fixed via Rubiscoin mesophyll chloroplasts.

The interactive activity Try This! CAM Diurnal Pattern can only be completed online.

Another unique aspect of CAM photosynthesis is that the PEP needed for accepting HCO3- is

generated, in species like pineapple, from vacuolar sugars, which are accumulated in daytime. Finally,CAM differs from C4 in yet a third way. In C4 plants, PEPCase is a light-activated enzyme—obviously,that is not true for CAM plants.

CAM plants are particularly adapted to desert environments. The rapid decarboxylation of malate indaytime raises the intercellular CO2 to a high level, causing stomata to close, thus conserving water.Some species are obligate CAM, but many, perhaps most, actually are facultative C3-CAM. Duringtransition periods, early morning and late afternoon, when shifting between day and night metabolism,their stomata are open and CO2 may be assimilated directly via Rubisco.

Fig. 4.20 CAM photosynthetic metabolism.

CAM Water ConservationCAM Water Conservation

CAM water conservation. The ability to open stomata and assimilate CO2 at nightwhen transpirational demand is low, and then close their stomata and metabolize the“stored-CO2” of malate in daytime when sunlight energy is available but transpirationaldemand is great, confers high water-use efficiency and adaptivity of CAM plants to arid andsemiarid environments. Through this mechanism, CAM plants are able to cope withconditions that C3 and C4 plants would find extremely stressful, if not intolerable. Crops thatutilize CAM generally are less productive than C3 types, but their unique photosyntheticsystem adapts them to regions where C3 types would be unproductive, even if they couldexist. Though CAM crops are not among the food or feed staples that are valued for energyand protein, many are highly valued as ornamentals or for dietary variation.

Discussion Topic 4.2Discussion Topic 4.2

Give your location and list three crops commonly produced in your region. Indicate whethereach utilizes C3, C4, or CAM photosynthetic metabolism.

Complete this assignment in Canvas. Be sure to check the course calendar for due dates.

SummarySummaryUnderstanding the processes involved in photosynthesis is fundamental to understanding cropproduction and management. Photosynthesis can be separated into three processes:

1. The light reactions—solar energy is captured and converted to chemical energy,2. A diffusion process—carbon dioxide moves from the atmosphere through the leaf and into the

chloroplast; and3. A two-step assimilation process—carbon dioxide in the chloroplast is fixed and the resulting

compound is reduced, using the chemical energy from captured sunlight, to form carbohydrate.

The interactive activity Study Question 4.9 can only be completed online.

Lesson 4 ReflectionLesson 4 Reflection

1. In your own words, write a short summary (< 150 words) for this lesson.2. What is the most valuable concept you learned from the lesson? Why is this concept

valuable to you?3. What concepts in the lesson are still unclear/the least clear to you?

Complete this assignment in Canvas. Be sure to check the course calendar for due dates.

ReferencesReferencesBuchanan, B. B., W. Gruissem, R. L. Jones. 2000. Biochemistry & Molecular Biology of Plants.American Society of Plant Physiologists, Rockville, MD.

Hillier, W., and G. T. Babcock. 2001. Photosynthetic reaction centers. Plant Physiol. 125:33-37.

Ort, D. R. 2001. When there is too much light. Plant Physiol. 125:29-32.