WHY STUDY PHOTOSYNTHESIS?

•Photosynthesis is arguably the most important biological process on earth.

• By liberating oxygen and consuming carbon dioxide, it has transformed the world into the hospitable environment we know today.

•. Directly or indirectly, photosynthesis fills all of our food requirements and many of our needs for fiber and building materials.

•The energy stored in petroleum, natural gas and coal all came from the sun via photosynthesis, as does the energy in firewood, which is a major fuel in many parts of the world.

•. This being the case, scientific research into photosynthesis is vitally important.

• If we can understand and control the intricacies of the photosynthetic process, we can learn how to increase crop yields of food, fiber, wood, and fuel, and how to better use our lands.

•

. The energy-harvesting secrets of plants can be adapted to man-made systems which provide new, efficient ways to collect and use solar energy.

The light and carbon reactions of photosynthesis in chloroplasts of vascular plants

Photosynthesis

• Photo means ‘light’ and synthesis means ‘to make’

• Process in which plants convert carbon dioxide and water into sugars using solar energy

• Occurs in chloroplast

• ENERGY TRANSDUCTION AND THE CHEMIOSMOTIC SYNTHESIS OF ATP

• It has been known for many years that the three principal energy-transducing membrane systems (in bacteria, chloroplasts, and mitochondria) were able to link electron transport with the synthesis of ATP. The mechanism, however, was not understood until Peter Mitchell proposed his chemiosmotic hypothesis in 1961.

• Although not readily accepted by many biochemists in the beginning, Mitchell's hypothesis is now firmly supported by experimental results.

Respiration is an Oxidation-Reduction process

– Loss of electrons from one substance = oxidation.

– Addition of electrons to a substance = reduction.

– Oxidizing agent - accepts electrons.

– Reducing agent - gives up electrons.

• Oxygen - very strong oxidizing agent (hence: “oxidizing” or “oxidation”)

Redox reactions

E.g. Na + Cl -> Na⁺ + Cl^-

• In honor of his pioneering work, Mitchell was awarded the Nobel prize for chemistry in 1978.

• Mitchell's hypothesis is based on two fundamental requirements. First, energy-transducing membranes are impermeable to H+.

• Second, electron carriers are organized asymmetrically in the membrane.

• The result is that, in addition to transporting electrons, some carrier so serve to translocate protons across the membrane against a proton gradient.

• The effect of these proton pumps is to conserve some of the free energy electron transport as an unequal or nonequilibrium distribution of protons across the membrane.

• In plants, chloroplasts and mitochondria are the main energy-transducing organelles.

• The biochemical mechanism by which ATP is synthesized is directly related to the specific compartments that exist in each of these organelles.

• The structure and development of chloroplasts have been studied extensively.

• As a result, we recognize four major structural regions or compartments:

• (1) a pair of outer limiting membranes, collectively known as the envelope,

• (2) an unstructured background matrix or stroma,

• (3) a highly structured internal system of membranes, called thylakoids, and (4) the intrathylakoid space, or lumen

• These membranes are 5.0 to 7.5 nm thick and are separated by a 10 nm intermembrane space.

• Because the inner envelope membrane is selectively permeable, the envelope also serves to isolate the chloroplast and regulate the exchange of metabolites between the chloroplast and the cytosol that surrounds it.

• The envelope encloses the stroma, a predominantly protein solution.

Key points:

• Protons are translocated across the membrane, from the matrix to the intermembrane space

• Electrons are transported along the membrane, through a series of protein carriers

• Oxygen is the terminal electron acceptor, combining with electrons and H⁺ ions to produce water

Key points:

4. As NADH delivers more H⁺ and electrons into the ETS, the proton gradient increases, with H⁺ building up outside the inner mitochondrial membrane, and OH^- inside the membrane.

• The stroma contains all of the enzymes responsible for photosynthetic carbon reduction, including ribulose-1,5-bisphosphate carboxylase/oxygenase, generally referred to by the acronym rubisco

• Rubisco, which accounts for fully half of the total chloroplast protein, is no doubt the world's single most abundant protein.

• In addition to rubisco and other enzymes involved in carbon reduction, the stroma contains enzymes for a variety of other metabolic pathways as well as DNA, RNA, and the necessary machinery for transcription and translation.

• Embedded within the stroma is a complex system of membranes often referred to as lamellae , individual pairs of parallel membranes that appear to be joined at the end, a configuration that in cross-section gives the membranes the appearance of a flattened sack, or thylakoid (Gr., sacklike).

• The thylakoid membranes contain the chlorophyll and carotenoid pigments and are the site of the light-dependent, energy-conserving reactions of photosynthesis.

• The interior space of the thylakoid is known as the lumen.

• The lumen is the site of water oxidation and, consequently, the source of oxygen evolved in photosynthesis.

• Otherwise it functions primarily as a reservoir for protons that are pumped across the thylakoid membrane during electron transport and that are used to drive ATP synthesis.

• CHLOROPLASTS AND MITOCHONDRIA SYNTHESIZE ATP BY CHEMIOSMOSIS

• In chloroplasts the protons are pumped across the thylakoid membrane, from the stroma into the lumen.

• The difference in proton concentration across the membrane may be quite large-as much as three or four orders of magnitude.

• Since protons carry a positive charge, a proton concentration also contributes to an electrical potential gradient across the membrane.

• Together the membrane potential difference plus the proton gradient constitute a proton motive force.

• In order to pump protons into the lumen against a proton motive force of this magnitude, a large amount of energy is required.

• The direction of the proton motive force also favors the return of protons to the stroma, but the low proton conductance of the thylakoid membrane will not allow the protons to simply diffuse back.

• In fact, the return of protons to the stroma is restricted to highly specific, protein-lined channels that extend through the membrane and that are a part of the ATP synthesizing enzyme, ATP synthase.

• When the electron-transport complexes and the ATP-synthesizing complex are both operating, a proton circuit is established in chloroplasts as well as mitochondria.

• In chloroplasts, the photosynthetic electron-transport complex pumps the protons from the stroma into the lumen and thus establishes the proton gradient.

• At the same time, the ATP synthase allows the protons to return to the stroma.

• Some of the free energy of electron transport is initially conserved in the proton gradient. As the energy-rich proton gradient collapses , that conserved energy is available to drive the synthesis of ATP.

• The light-dependent synthesis of ATP by chemiosmosis in the chloroplast is called photophosphorylation.

• In mitochondria, the respiratory electron-transport complex pumps protons from the matrix to the intermembrane space (IMS) to establish a proton gradient.

• The potential energy of this proton gradient is consumed by the mitochondrial ATP synthase to synthesize ATP.

• The chemiosmotic synthesis of ATP by mitochondria is called oxidative phosphorylation.

• Could a proton gradient be driving ATP synthesis?

• To test this idea, Jagendorf and colleagues devised a simple but very elegant experiment (Fig. 2.8B).

• Washed thylakoids were allowed to equilibrate with an acid solution (pH 4) in the dark.

• At equilibrium, the concentration of protons in the lumen would be the same as in the surrounding medium.

• LIGHT CAN BE CHARACTERIZED AS A WAVE PHENOMENON

• The propagation of light through space is characterized by regular and repetitive changes, or waves, in its electrical and magnetic properties

• . Electromagnetic radiation actually consists of two waves-one electrical and one magnetic-that oscillate at 900 to each other and to the direction of propagation (Fig. 3.2).

• The wave properties of light may be characterized by either wavelength or frequency.

• The distance in space between wave crests is known as the wavelength and is represented by the Greek letter lambda (X).

• Biologists commonly express wavelengths in units of nanometers (nm), where 1 nm = 10-9 m.

• Frequency, represented by the Greek letter nu (v), is the number of wave crests, or cycles, passing a point in space in one second.

• Frequency is thus related to wavelength in the following way:

• v = CA (3.1)

• where c is the speed of light (3 X 108 m s-').

• Biologists most commonly use wavelength to describe light and other forms of radiation, although frequency is useful in certain situations.

• Wavelengths of primary interest to photobiologists fall into three distinct ranges: ultraviolet, visible, and infrared

• 4 LIGHT ENERGY CAN INTERACT WITH MATTER

• For light to be used by plants, it must first be absorbed.

• Therefore, any photobiological phenomenon requires the participation of a molecule that absorbs light.

• Such a molecule may be defined as a pigment.

• Plants contain a variety of pigments that are prominent visual features and important physiological components of virtually all plants.

• The characteristic green color of leaves, for example, is due to a family of pigments known as the chlorophylls.

• Chlorophyll absorbs the light energy used in photosynthesis.

• The pleasing colors of floral petals are due to the anthocyanin pigments that serve to attract insects as pollen vectors.

• Other pigments, such as phytochrome, are present in quantities too small to be visible but nonetheless serve important roles in plant morphogenesis.

• What actually happens when a pigment molecule absorbs light?

• Absorption of light by a pigment molecule is a rapid, photophysical, electronic event, occurring within a femtosecond (fs = 10-15 s).

• In accordance with the First Law of Thermodynamics the energy of the absorbed photon is transferred to an electron in the pigment molecule during that extremely short period of time.

• The energy of the electron is thus elevated from a low energy level, the ground state, to a higher energy level known as the excited, or singlet, state.

• This change in energy level is illustrated graphically in Figure 3.3.

• Like photons, the energy states of electrons are also quantized, that is, an electron can exist in only one of a series of discrete energy levels.

• A photon can be absorbed only if its energy content matches the energy required to raise the energy of the electron to one of the higher, allowable energy states.

• An excited molecule has a very short lifetime (on the order of a nanosecond, or 10^-9 s) and, in the absence of any chemical interaction with other molecules in its environment, it must rid itself of any excess energy and return to the ground state.

• Dissipation of excess energy may be accomplished in several ways.

• 1. Thermal deactivation occurs when a molecule loses excitation energy as heat.

• The electron will very quickly drop or relax to the lowest excited singlet state.

• The excess energy is given off as heat to its environment.

• If the electron then returns to the ground state, that energy will also be dissipated as heat.

• 2. Fluorescence is the emission of a photon of light as an electron relaxes from the first singlet excited to ground state.

• Since the rate of relaxation through fluorescence is much slower than the rate of relaxation through thermal deactivation, fluorescence emission occurs only as a consequence of relaxation from the first excited singlet state .

• Consequently, the emitted photon has a lower energy content, and a longer wavelength, than the exciting photon.

• In the case of the photosynthetic pigment chlorophyll, for example, peak fluorescent emission falls to the long wavelength side of the red absorption band .

• 3. Energy may be transferred between pigment molecules by what is known as inductive resonance or radiationless transfer.

• 4. The molecule may revert to another type of excited state, called the triplet state

• The absorption light by a molecule. (A) An energy level diagram depicting the various possible transitions when light is absorbed. A nonexcited molecule is said to be in the ground state (So). Upon absorption of light of wavelength of either X1 or X2, a molecule can undergo an electronic transition (solid arrows) to a singlet excited state represented by either Sl or S2, respectively

• CAROTENOIDS ACCOUNT FOR THE AUTUMN COLORS

• Carotenoids comprise a family of orange and yellow pigments present in most photosynthetic organisms.

• Found in large quantity in roots of carrot and tomato fruit, carotenoid pigments are also prominent in green leaves.

• In the fall of the year, the chlorophyll pigments are degraded and the more stable carotenoid pigments account for the brilliant orange and yellow colors so characteristic of autumn foliage.

The Splitting of Water

• Chloroplasts split water into

– Hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

• The Reductive Pentose Phosphate Pathway (RPPP, aka Photosynthetic Carbon Reduction Pathway, aka Calvin cycle (after Mel Calvin, in whose lab at UC-Berkeley this pathway was elucidated following the availability of 14C; aka Benson-Calvin cycle

• (to recognize the contributions of a senior postdoctoral associate, A. A. “Andy” Benson), and on occasion, the Calvin-Benson-Bassham cycle

• (to recognize also the contributions of James “Al” Bassham, another member of the research team) is the pathway by which inorganic carbon is fixed photosynthetically.

• THE PHOTOSYNTHETIC CARBON REDUCTION (PCR) CYCLE

• Now that we understand the processes involved in the control of CO2 entry into a leaf, we will now examine in some detail the biochemical mechanisms by which chloroplasts fix this CO2 and convert it to stable phosphorylated carbon intermediates.

• Subsequently, we will examine specific adaptations in the mechanisms by which C4 and crassulacean acid metabolism (CAM) plants fix CO2 and discuss how the specific biochemical adaptations are related to the conservation of water.

• THE PCR CYCLE REDUCES CO2 TO PRODUCE A THREE-CARBON SUGAR

• The pathway by which all photosynthetic eukaryotic organisms ultimately incorporate CO2 into carbohydrate is known as carbon fixation or the photosynthetic carbon reduction (PCR) cycle.

• Mapping the complex sequence of reactions involving the formation of organic carbon and its conversion to complex carbohydrates represented a major advance in plant biochemistry.

• The solution to the puzzle of carbon fixation was made possible largely because of two technological advances in the late 1940s and early 1950s: the discovery of radioactive carbon-14 (14C) and the development of paper chromatography as an analytical tool

• In order to trace the path of carbon it was necessary to provide CO2 that was labeled in some way so that its fate in the cell could be followed.

• Early experiments were conducted with 11C but its short halflife' (about 22 minutes) required that complex organic analyses be completed rather quickly before the radioactivity disappeared.

• The availability of 14C with its much longer half-life (5730 years) clearly solved the time problem!

• The technique of paper chromatography made it possible to separate complex mixtures of sugars and other small molecules.

• The PCR cycle can be divided into three primary stages :

• (1) carboxylation which fixes the CO2 in the presence of the five-carbon acceptor molecule, ribulose bisphosphate (RuBP), and converts it into two molecules of a three-carbon acid;

• (2) reduction, which consumes the ATP and NADPH produced by photosynthetic electron transport to convert the three carbon acid to triose phosphate; and

• (3) regeneration, which consumes additional ATP to convert some of the triose phosphate back into RuBP to ensure the capacity for the continuous fixation of CO2.

• 'A half-life is the time required for one-half of the material to decay.

The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration

THE CARBOXYLATION REACTION FIXES THE CO2

• Calvin's strategy for unraveling the path of carbon in photosynthesis was conceptually very straightforward: identify the first stable organic product formed following uptake of radiolabeled CO₂ .

• In order to achieve this, cultures of the photosynthetic green alga Chlorella were first allowed to establish a steady rate of photosynthesis. ¹⁴C0₂ was then introduced and photosynthesis continued for various periods of times before the cells were dropped rapidly into boiling methanol.

• The hot methanol served two functions: it denatured the enzymes, thus preventing any further metabolism, while at the same time extracting the sugars for subsequent chromatographic analysis.

• When the time of photosynthesis in the presence of ¹⁴C0₂ was reduced to as little as two seconds, most of the radioactivity was found in a three-carbon acid, 3-phosphoglycerate (3-PGA).

• Thus 3-PGA appeared to be the first stable product of photosynthesis.

• Other sugars that accumulated the label later in time were probably derived from 3-PGA.

• Because Calvin's group determined that the first product was a three carbon molecule, the PCR cycle is commonly referred to as the C3 cycle.

• The next step was to determine what molecule served as the acceptor-the molecule to which CO2 was added in order to make the three-carbon product.

• Systematic degradation of 3-PGA demonstrated that the ¹⁴C label was predominantly in the carboxyl carbon.

• A two-carbon acceptor molecule would be logical, but the search was long and futile.

• No two-carbon molecule could be found.

• Instead, Calvin recognized that the acceptor was the five-carbon keto sugar, ribulose-l,5-bisphosphate (RuBP).

• This turned out to be the key to the entire puzzle.

• The reaction is a carboxylation in which CO2 is added to RuBP, forming a six-carbon intermediate (Fig. 5.9).

• The intermediate, which is transient and unstable, remains bound to the enzyme and is quickly hydrolyzed to two molecules of 3-PGA.

• The carboxylation reaction is catalyzed by the enzyme ribulose-1,5-bisphosphate carboxylaseoxygenase, or Rubisco.

• 2 Rubisco is without doubt the most abundant protein in the world, accounting for approximately 50 percent of the soluble protein in most leaves.

Rubisco

• Ribulose bisphosphate carboxylase oxygenase

• (fixes CO2 & O2)

• Enzyme in Calvin Cycle (1^st step)

• Most abundant protein on Earth

– Ca. 25% total leaf protein

• The enzyme also has a high affinity for CO2 that, together with its high concentration in the chloroplast stroma, ensures rapid carboxylation at the normally low atmospheric concentrations of CO2.

• Thus, the reaction catalyzed by Rubisco maintains the CO2 concentration gradient (dc/dx) between the internal air spaces of a leaf and the ambient air to ensure a constant supply of this substrate for the PCR cycle.

• In the current literature, Rubisco is sometimes referred to as RuBPCO.

• ATP AND NADPH ARE CONSUMED IN THE PCR CYCLE

• If the equilibrium constant of the reaction favors carboxylation with such a high negative free energy change, where is the need for an input of energy from the light reactions of photosynthesis?

• Energy is required at two points: first for the reduction of 3-PGA and second for regeneration of the RuBP acceptor molecule.

• Each of these requirements will be discussed in turn.

• The Calvin cycle

• Reduction of 3-PGA.

• In order for the chloroplast to continue to take up C02, two conditions must be met.

• First, the product molecules (3-PGA)¹ must be continually removed and, second, provisions must be made to maintain an adequate supply of the acceptor molecule (RuBP⁵).

• Both require energy in the form of ATP² and NADPH³.

• The 3-PGA¹ is removed by reduction to the triose phosphate, glyceraldehyde-3-phosphate⁴ (PGAL).

• The Calvin cycle

• This is a two-stpe reaction in which 3-PGA is first phosphorylated to 1,3-bisphosphoglycerate⁶, which is then reduced to glyceraldehyde-3-phosphate⁴ (G3P).

• Both the ATP² and the NADPH³ required in these two steps are products of the light reactions and together represent one of two sites of energy input.

• The resulting triose sugar-phosphate, G3P⁷, is available for export to the cytoplasm, probably after conversion to dihydroxyacetone phosphate (DHAP) .

• The Calvin cycle

• Regeneration of RuBP. –heavy stuff….

• In order to maintain the process of CO2 reduction, it is necessary to ensure a continuing supply of the acceptor molecule, RuBP.

• This is accomplished by a series of reactions involving 4-, 5-, 6-, and 7-carbon sugars (Figs. 5.11, 5.12).

• These reactions include the condensation of a 6-carbon fructose-phosphate with a triose-phosphate to form a 5-carbon sugar and a 4carbon sugar.

• Another triose joins with the 4-carbon sugar to produce a 7-carbon sugar.

• When the 7-carbon sugar is combined with a third triose-phosphate, the result is two more 5-carbon sugars.

• All of the five-carbon sugar can be isomerized to form ribulose-5-phosphate (Ru5P). Ru5P can, in turn, be phosphorylated to regenerate the required ribulose1,5-bisphosphate.

• The net effect of these reactions is to recycle the carbon from five out of every six G3P molecules, thus regenerating three RuBP molecules to replace those used in the earlier carboxylation reactions.

Rubisco

• Ribulose bisphosphate carboxylase oxygenase

• (fixes CO2 & O2)

• Enzyme in Calvin Cycle (1^st step)

• Most abundant protein on Earth

– Ca. 25% total leaf protein

The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration

Carbon mobilization in vascular plants

• WHY PHOTORESPIRATION?

• In normal air (21% 02), the rate of photorespiration in sunflower leaves is about 17 percent of gross photosynthesis.

• Every photorespired C02 however, requires an input of two molecules of 02 (Fig. 5.12).

• The true rate of oxygenation is therefore about 34 percent and the ratio of carboxylation to oxygenation is about 3 to 1 (1.00/0.34).

• This experimental value agrees with similar values calculated for several species based on the known characteristic of purified Rubisco.

• The ratio of carboxylation to oxygenation depends, however, on the relative levels of 02 and CO2 since both gases compete for binding at the active site on Rubisco

• As the concentration of 02 declines, the relative level of carboxylation increases until, at zero 02, photorespiration is also zero.

• On the other hand, increases in the relative level of 02 (or decrease in C02) shifts the balance in favor of oxygenation.

• An increase in temperature will also favor oxygenation, since as the temperature increases the solubility of gases in water declines, but 02 solubility is less affected than CO2

• . Thus 02 will inhibit photosynthesis, measured by net CO2 reduction, in plants that photorespire.

• There is also an energy cost associated with photorespiration and the glycolate pathway.

• Not only is the amount of ATP and NAD(P)H expended in the glycolate pathway following oxygenation (5 ATP + 3 NADPH) greater than that expended for the reduction of one CO2 in the PCR cycle (3 ATP + 2 NADPH), but there is also a net loss of carbon.

• On the surface, then, photorespiration appears to be a costly and inefficient process with respect to both energy and carbon acquisition.

• It is logical to ask, as many have, why should the plant indulge in such an apparently wasteful process?

• This question is not easily answered, although several ideas have been put forward.

• One has it that the oxygenase function of Rubisco is inescapable.

• Rubisco evolved at a time when the atmosphere contained large amounts of CO2 but little oxygen.

• Under these conditions, an inability to discriminate between the two gases would have had little significance to the survival of the organism.

• Both CO2 and 02 react with the enzyme at the same active site, and oxygenation requires activation by CO2 just as carboxylation does.

• It is believed that oxygen began to accumulate in the atmosphere primarily due to photosynthetic activity, but by the time the atmospheric content of 02 had increased to significant proportions, the bifunctional nature of the enzyme had been established without recourse.

• In a sense, C3 plants were the architect of their own problem-generating the oxygen that functions as a competitive inhibitor of carbon reduction.

• By this view, then, the oxygenase function is an evolutionary "hangover" that has no useful role.

• Clearly, any inefficiencies resulting from photorespiration in C3 plants are apparently not severe.

• There is no evidence that selection pressures have caused evolution of a form of Rubisco with lower affinity for 02.

• While most agree that oxygenation is an unavoidable consequence of evolution, many have argued that plants have capitalized on this apparent evolutionary deficiency by turning it into a useful, if not essential, metabolic sequence.

• The glycolate pathway, for example, undoubtedly serves a scavenger function.

• For each two turns of the cycle, two molecules of phosphoglycolate are formed by oxygenation.

• Of these four carbon atoms, one is lost as CO2 and three are returned to the chloroplast.

• The glycolate pathway thus recovers 75 percent of the carbon that would otherwise be lost as glycolate.

• The salvage role alone may be sufficient justification for the complex glycolate cycle.

• There is also the possibility that some of the intermediates, serine and glycine, for example, are of use in other biosynthetic pathways, although this possibility is still subject to some debate.

• Recently, strong experimental support has been provided for the thesis that photorespiration could also function as a sort of safety valve in situations that require dissipation of excess excitation energy.

• For example, a significant decline in the photosynthetic capacity of leaves irradiated in the absence of CO, and 02 has been reported.

• Injury is prevented, however, if sufficient 02 is present to permit photorespiration to occur.

• Apparently the 02 consumed by photorespiration is sufficient to protect the plant from photo-oxidative damage by permitting continued operation of the electron transport system.

• This could be of considerable ecological value under conditions of high light and limited CO2 supply, for example, when the stomata are closed due to moisture stress.

Photorespiration

• When rubisco “fixes” O2, not CO2

• Lose 1/2 C as CO2; costs 2.5 extra ATP

• Take up O2

• Only occurs in light

• Occurs 1 out of 4 reactions under today’s atmospheric [CO2]

• Rate increases with temperature

Types of photosynthesis

• C3

– The majority of plants

• C4

– CO₂ temporarily stored as 4-C organic acids resulting in more more efficient C exchange rate

_– Advantage in high light, high temperature, low CO₂

– Many grasses and crops (e.g., corn, sorghum, millet, sugar cane)

• CAM

– Stomata open during night

– Advantage in arid climates

– Many succulents (e.g., cacti, euphorbs, bromeliades, agaves)

CO2 effects on photosynthesis

• C4 > C3 at low CO2

• But, C3 > C4 at high CO2

The C₄ photosynthetic carbon cycle

• THE C4 SYNDROME: ANOTHER BIOCHEMICAL MECHANISM TO ASSIMILATE CO2

• Plants that incorporate carbon solely through the PCR or Calvin cycle are generally known as C3 plants because the first product to incorporate ¹⁴CO2 is the three-carbon acid PGA.

• Certain other groups, known as C4 plants, are distinguished by the fact that the first product is a four-carbon acid oxaloacetate (OAA).

• These C4 plants also exhibit a number of specific anatomical, physiological, and biochemical characteristics that constitute the C4 syndrome.

• One particular anatomical feature characteristic of most C4 leaves is the presence of two distinct photosynthetic tissues (Fig. 5.21).

• In C4 leaves the vascular bundles are quite close together and each bundle is surrounded by a tightly fitted layer of cells called the bundle sheath.

• Between the vascular bundles and adjacent to the air spaces of the leaf are the more loosely arranged mesophyll cells.

• This distinction between mesophyll and bundle sheath photosynthetic cells, called Kranz anatomy , plays a major role in the C4 syndrome.

• C4 plants are generally of tropical or subtropical origin representing nearly 1500 species spread through at least 18 different angiosperm families (3 monocots, 15 dicots).

• Interestingly, no one family has been found to express the C4 syndrome exclusively-all 18 families contain both C3 and C4 representatives.

• This suggests that the C4 cycle has arisen rather recently in evolution of angiosperms and in a number of diverse taxa at different times.

• Under conditions of high fluence (light)rates and high temperature (30° to 40°C) the photosynthetic rate of C4 species may be two to three times greater than that of C3 species.

• They appear to be better equipped to withstand drought and are able to maintain active photosynthesis under conditions of water, stress that would lead to stomatal closure and consequent reduction of CO2 uptake by C3 species.

• All of these features appear to be a consequence of the C02concentrating capacity of C4 plants and the resulting suppression of photorespiratory CO2 loss.

Crassulacean acid metabolism (CAM) (Part 1)

Crassulacean acid metabolism (CAM) (Part 2)

Concentration of atmospheric CO₂ from 420,000 years ago to the present (Part 1)

Concentration of atmospheric CO₂ from 420,000 years ago to the present (Part 2)

Global Environmental Change & Photosynthesis:
C3 vs. C4 vs. CAM

• Increasing CO2

• Increasing chronic and acute temperatures

• Increasing N (vs. decreasing C:N from increasing CO2)

• Changes in water

• CRASSULACEAN ACID METABOLISM (CAM): AN ADAPTATION TO LIFE IN THE DESERT

• Another CO2 concentrating mechanism is crassulacean acid metabolism (CAM)-so named because it was originally studied most extensively in the family Crassulaceae.

• This specialized pattern of photosynthesis has now been found in some 23 different families of flowering plants (including the Cactaceae and Euphorbiaceae), one family of ferns (the Polypodiaceae), and in the primitive plant Welwitschia.

•

• Like C4 plants, however, most families, with the exception of Crassulaceae and Cactaceae, are not exclusively CAM.

• Most families will have C3 representatives as well and some are known to contain all three photosynthetic patterns; C3, C4, and CAM.

• One of the most striking features of CAM plants is an inverted stomatal cycle-the stomata open mainly during the nighttime hours and are usually closed during the day.

• This means that CO2 uptake also occurs mainly at night.

• In addition, CAM plants are characterized by an accumulation of malate at night and its subsequent depletion during daylight hours and storage carbohydrate levels that fluctuate inversely with malate levels

• . Nocturnal stomatal opening supports a carboxylation reaction producing C4 acids that are stored in the large, watery vacuole (Fig 5.26).

• Accumulation of the organic acids leads to a marked acidification of these cells at night.

• The acids are subsequently decarboxylated during daylight hours and the resulting CO2 is fixed by the PCR cycle.

• . IS CAM A VARIATION OF THE C4 SYNDROME?

• CAM and C4 plants share certain similarities, but there are significant differences.

• A comparison between CAM and the C4 cycle is unavoidable since they both use cytoplasmic PEPcase to form C4 acids from PEP and bicarbonate, and in both cases the acids are subsequently decarboxylated to provide CO2 for the PCR cycle.

• However, there are two significant differences.

The first is that the C4 cycle requires a specialized anatomy by which C4 carboxylation is spatially separated from the C3 PCR cycle-in CAM both occur in the same cells but are separated in time.

• Second, in CAM there is no closed cycle of carbon intermediates as there is in C4 plants.

• Thus CAM is cyclic in time only.

• Since CAM occurs in the more primitive ferns and Welwitschia whereas the C4 cycle is found only in angiosperms, it appears that CAM preceded C4 photosynthesis in evolutionary time..

• CAM PLANTS ARE PARTICULARLY SUITED TO DRY HABITATS

• As mentioned above, CAM represents a particularly significant adaptation to exceptionally dry habitats

• . Many CAM plants are true desert plants, growing in shallow, sandy soils with little available water.

• Nocturnal opening of the stomata allows for CO2 uptake during periods when conditions leading to evaporative water loss are at a minimum.

• Then, during the daylight hours when the stomata are closed to reduce water loss, photosynthesis can proceed by using the reservoir of stored CO2.

• The CAM pathway is similar to the C₄ pathway

• This interpretation is supported by the transpiration ratio for CAM plants, in the range of 50 to 100, which is substantially lower than that for either C3 or C4 plants.

• There is a price to be paid, however. Rates for daily carbon assimilation by CAM plants are only about one-half those of C3 plants and one-third those of C4.

• CAM plants can be expected to grow more slowly under conditions of adequate moisture.

•

• On the other hand, CO2 uptake by CAM plants will continue under conditions of water stress that would cause complete cessation of photosynthesis in C3 plants and severely restrict carbon uptake by C4 plants.

• CAM plants enjoy the further advantage of being able to retain and reassimilate respired C02, thus preventing loss of carbon and helping to maintain a favorable dry weight through extended periods of severe drought.

Photosynthetic N-use efficiency

• C4 plants need (have) less leaf N than C3

• Photosynthesis higher per unit N in C4

• Humans are increasing global N, which benefits C3 more than C4

• Increasing CO2 decreases leaf N content, more in C3 than C4

Photosynthetic water-use efficiency

• C4 plants use less water than C3

• (cause stomates open less)

• Water availability may increase or decrease in the future.

Predicting the future for plants

• How will increases in CO2, N, and chronic and acute heat stress affect photosynthesis?

• Who will win or lose? C3? C4?

• How will pollution (eg, ozone) interact?

Sucrose synthesis