Teaching Materials eneral Biology KPC 8101) Chapter 3 ... file• Difference Catabolic pathway and Anabolic pathways

Teaching Materials General Biology (KPC 8101)Chapter 3: Physiology3.1. The Basic Principles of Metabolism3.2. Photosynthesis

Scope of Physiology:3.1. The Basic Principles of Metabolism3.2. Photosynthesis3.3. Respiration

Teaching Materials General Biology (KPC 8101)Chapter 3: Physiology3.1. The Basic Principles of Metabolism

Reference: this slides from ...• AP Biology PowerPoints Biology 6th edition By Neil A. Campbell, Jane B. Reece,

Lawrence G. MitchellCopyright Pearson/Benjamin Cummings (http://biologyjunction.com/ap_powerpoints.htm)Access 12nd December 2013 by Nursal• You can also read 7th edition and 8th edition

http://biologyjunction.com/ap_powerpoints.htm

Teaching Materials General Biology (KPC 8101)Chapter 3: Physiology3.1. The Basic Principles of Metabolism

Scope of this topics:• Explain metabolism, energy, and life• Difference Catabolic pathway and Anabolic pathways• Explain the energy transformations of life by two laws

of thermodynamics• Explain the ATP as source of energy that powers

cellular workTo understanding this topics, read the slide below !!

• The totality of an organism’s chemical reactions is called metabolism.

• A cell’s metabolism is an elaborate road map of the chemical reactions in that cell.

• Metabolic pathways alter molecules in a series of steps.

(http://biologyjunction.com/ap_powerpoints.htm)12nd December 2013

1. The chemistry of life is organized into metabolic pathways

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings


• Enzymes selectively accelerate each step.• The activity of enzymes is regulated to maintain an

appropriate balance of supply and demand.

• Catabolic pathways release energy by breaking down complex molecules to simpler compounds.• This energy is stored in organic molecules until it needs

to do work in the cell.

• Anabolic pathways consume energy to build complicated molecules from simpler compounds.

• The energy released by catabolic pathways is used to drive anabolic pathways.


• Energy is the capacity to do work - to move matter against opposing forces.• Energy is also used to rearrange matter.

• Kinetic energy is the energy of motion.• Objects in motion, photons, and heat are examples.

• Potential energy is the energy that matter possesses because of its location or structure.• Chemical energy is a form of potential energy in

molecules because of the arrangement of atoms.

2. Organisms transform energy


• Cellular respiration and other catabolic pathways unleash energy stored in sugar and other complex molecules.

• This energy is available for cellular work.• The chemical energy stored on these organic

molecules was derived primarily from light energy by plants during photosynthesis.

• A central property of living organisms is the ability to transform energy.


• Thermodynamics is the study of energy transformations.

• In this field, the term system indicates the matter under study and the surroundings are everything outside the system.

• A closed system, like liquid in a thermos, is isolated from its surroundings.

• In an open system energy (and often matter) can be transferred between the system and surroundings.

3. The energy transformations of life are subject to two laws of thermodynamics


• Organisms are open systems.• They absorb energy - light or chemical energy in organic

molecules - and release heat and metabolic waste products.

• The first law of thermodynamics states that energy can be transferred and transformed, but it cannot be created or destroyed.• Plants transform light to chemical energy; they do not

produce energy.


• The second law of thermodynamics states that every energy transformation must make the universe more disordered.

• Entropy is a quantity used as a measure of disorder, or randomness.

• The more random a collection of matter, the greater its entropy

• While order can increase locally, there is an unstoppable trend toward randomization of the universe.

• Much of the increased entropy of the universe takes the form of increasing heat which is the energy of random molecular motion.Heat is energy in its most random state.

• Combining the two laws, the quantity of energy is constant, but the quality is not.


• Living organisms, ordered structures of matter, do not violate the second law of thermodynamics.

• Organisms are open systems and take in organized energy like light or organic molecules and replace them with less ordered forms, especially heat.

• An increase in complexity, whether of an organism as it develops or through the evolution of more complex organisms, is also consistent with the second law as long as the total entropy of the universe, the system and its surroundings, increases. • Organisms are islands of low entropy in an increasingly

random universe.


• Spontaneous processes are those that can occur without outside help.• The processes can be harnessed to perform work.

• Nonspontaneous processes are those that can only occur if energy is added to a system.

• Spontaneous processes increase the stability of a system and nonspontaneous processes decrease stability.

4. Organisms live at the expense of free energy


• The concept of free energy provides a criterion for measuring spontaneity of a system.

• Free energy is the portions of a system’s energy that is able to perform work when temperature is uniform throughout the system.


Fig. 6.5

• The free energy (G) in a system is related to the total energy (H) and its entropy (S) by this relationship:• G = H - TS, where T is temperature in Kelvin units.

• Increases in temperature amplify the entropy term.

• Not all the energy in a system is available for work because the entropy component must be subtracted from the maximum capacity.

• What remains is free energy.


• Free energy can be thought of as a measure of the stability of a system.• Systems that are high in free energy - compressed

springs, separated charges - are unstable and tend to move toward a more stable state, one with less free energy.

• Systems that tend to change spontaneously are those that have high energy, low entropy, or both.

• In any spontaneous process, the free energy of a system decreases.


• We can represent this change in free energy from the start of a process until its finish by:• delta G = G final state - G starting state

• Or delta G = delta H - T delta S

• For a system to be spontaneous, the system must either give up energy (decrease in H), give up order (decrease in S), or both.• Delta G must be negative.

• The greater the decrease in free energy, the greater the maximum amount of work that a spontaneous process can perform.

• Nature runs “downhill.”Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• A system at equilibrium is at maximum stability.• In a chemical reaction at equilibrium, the rate of forward

and backward reactions are equal and there is no change in the concentration of products or reactants.

• At equilibrium delta G = 0 and the system can do no work.

• Movements away from equilibrium are nonspontaneous and require the addition of energy from an outside energy source (the surroundings).


• Chemical reactions can be classified as either exergonic or endergonic based on free energy.

• An exergonic reaction proceeds with a net release of free energy and delta G is negative.


Fig. 6.6a

• The magnitude of delta G for an exergonic reaction is the maximum amount of work the reaction can perform.• For the overall reaction of cellular respiration:

• C6H12O6 + 6O2 -> 6CO2 + 6H2O

• delta G = -686 kcal/mol

• Through this reaction 686 kcal have been made available to do work in the cell.

• The products have 686 kcal less energy than the reactants.


• An endergonic reaction is one that absorbs free energy from its surroundings.• Endergonic reactions store energy,

• delta G is positive, and

• reactions are nonspontaneous.


Fig. 6.6b

• If cellular respiration releases 686 kcal, then photosynthesis, the reverse reaction, must require an equivalent investment of energy.• Delta G = + 686 kcal / mol.

• Photosynthesis is steeply endergonic, powered by the absorption of light energy.


• Sunlight provides a daily source of free energy for the photosynthetic organisms in the environment.

• Nonphotosynthetic organisms depend on a transfer of free energy from photosynthetic organisms in the form of organic molecules.


• A cell does three main kinds of work:• Mechanical work, beating of cilia, contraction of muscle

cells, and movement of chromosomes.

• Transport work, pumping substances across membranes against the direction of spontaneous movement.

• Chemical work, driving endergonic reactions such as the synthesis of polymers from monomers.

• In most cases, the immediate source of energy that powers cellular work is ATP.

5. ATP powers cellular work by coupling exergonic reactions to endergonic reactions


• ATP (adenosine triphosphate) is a type of nucleotide consisting of the nitrogenous base adenine, the sugar ribose, and a chain of three phosphate groups.


Fig. 6.8a

• The bonds between phosphate groups can be broken by hydrolysis.• Hydrolysis of the end phosphate group forms adenosine

diphosphate [ATP -> ADP + Pi] and releases 7.3 kcal of energy per mole of ATP under standard conditions.

• In the cell delta G is about -13 kcal/mol.


Fig. 6.8b

• While the phosphate bonds of ATP are sometimes referred to as high-energy phosphate bonds, these are actually fairly weak covalent bonds.

• They are unstable, however, and their hydrolysis yields energy because the products are more stable.

• The phosphate bonds are weak because each of the three phosphate groups has a negative charge.

• Their repulsion contributes to the instability of this region of the ATP molecule.


• In the cell the energy from the hydrolysis of ATP is coupled directly to endergonic processes by transferring the phosphate group to another molecule.• This molecule is now phosphorylated.

• This molecule is now more reactive.



Fig. 6.9 The energy released by the hydrolysis of ATP is harnessed to the endergonic reaction that synthesizes glutamine from glutamic acid through the transfer of a phosphate group from ATP.

• ATP is a renewable resource that is continually regenerated by adding a phosphate group to ADP.• The energy to support renewal comes from catabolic

reactions in the cell.

• In a working muscle cell the entire pool of ATP is recycled once each minute, over 10 million ATP consumed and regenerated per second per cell.

• Regeneration, an endergonic process, requires an investment of energy: delta G = 7.3 kcal/mol.


Fig. 6.10

Teaching Materials General Biology (KPC 8101)Chapter 3: Physiology3.2. Photosynthesis

Scope of Physiology:3.1. The Basic Principles of Metabolism3.2. Photosynthesis3.3. Respiration


Reference: this slides from ...• AP Biology PowerPoints Biology 6th edition By Neil A. Campbell, Jane B. Reece,

Lawrence G. MitchellCopyright Pearson/Benjamin Cummings (http://biologyjunction.com/ap_powerpoints.htm)Access 12nd December 2013 by Nursal• You can also read 7th edition and 8th edition



Scope of this topics:• Importance of photosynthesis for all living world• Different kind of Autotrophs and Heterotrophs organism• Chloroplasts are the sites of photosynthesis in plants• Explain the light function in the photosynthesis process• Explain cyclic and noncyclic photophosphorylation• Explain the steps of carbon fixation in Kelvin Cycle• Comparison of C3, C4 and CAM plants in fixation of CO2

To understanding this topics, read the slide below !!

CHAPTER 10 PHOTOSYNTHESIS


Section A: Photosynthesis in Nature

1. Plants and other autotrophs are the producers of the biosphere2. Chloroplasts are the site of photosynthesis in plants

• Life on Earth is solar powered.

• The chloroplasts of plants use a process called photosynthesis to capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules.

Introduction


• Photosynthesis nourishes almost all of the living world directly or indirectly.• All organisms require organic compounds for energy and

for carbon skeletons.

• Autotrophs produce their organic molecules from CO2 and other inorganic raw materials obtained from the environment.• Autotrophs are the ultimate sources of organic compounds

for all nonautotrophic organisms.

• Autotrophs are the producers of the biosphere.

1. Plants and other autotrophs are the producers of the biosphere


• Autotrophs can be separated by the source of energy that drives their metabolism.• Photoautotrophs use light as the energy source.

• Photosynthesis occurs in plants, algae, some other protists, and some prokaryotes.

• Chemoautotrophs harvest energy from oxidizing inorganic substances, including sulfur and ammonia.

• Chemoautotrophy is unique to bacteria.


Fig. 9.1

• Heterotrophs live on organic compounds produced by other organisms.• These organisms are the consumers of the biosphere.

• The most obvious type of heterotrophs feed on plants and other animals.

• Other heterotrophs decompose and feed on dead organisms and on organic litter, like feces and fallen leaves.

• Almost all heterotrophs are completely dependent on photoautotrophs for food and for oxygen, a byproduct of photosynthesis.


• Any green part of a plant has chloroplasts.• However, the leaves are the major site of

photosynthesis for most plants.• There are about half a million chloroplasts per square

millimeter of leaf surface.

• The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.• Chlorophyll plays an important role in the absorption of

light energy during photosynthesis.

2. Chloroplasts are the sites of photosynthesis in plants


• Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf.

• O2 exits and CO2 enters the leaf through microscopic pores, stomata, in the leaf.

• Veins deliver water from the roots and carry off sugar from mesophyll cells to other plant areas.


Fig. 10.2

• 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 stroma.

• In the stroma aremembranous sacs, the thylakoids.• These have an internal

aqueous space, the thylakoid lumen or thylakoid space.

• Thylakoids may be stacked into columns called grana.


Fig. 10.2



Section A1: The Pathways of Photosynthesis1. Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis2. The light reaction and the Calvin cycle cooperate in converting light energy to the chemical energy of food: an overview3. The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look

• Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and H2O.

• Using glucose as our target product, the equation describing the net process of photosynthesis is:• 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2

• In reality, photosynthesis adds one CO2 at a time:

• CO2 + H2O + light energy -> CH2O + O2

• CH2O represents the general formula for a sugar.

1. Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis


• One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2.

• Before the 1930s, the prevailing hypothesis was that photosynthesis occurred in two steps:

• Step 1: CO2 -> C + O2 and Step 2: C + H2O -> CH2O

• C.B. van Niel challenged this hypothesis.

• In the bacteria that he was studying, hydrogen sulfide (H2S), not water, is used in photosynthesis.

• They produce yellow globules of sulfur as a waste.

• Van Niel proposed this reaction:

• CO2 + 2H2S -> CH2O + H2O + 2SCopyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• He generalized this idea and applied it to plants, proposing this reaction for their photosynthesis.• CO2 + 2H2O -> CH2O + H2O + O2

• Other scientists confirmed van Niel’s hypothesis.• They used 18O, a heavy isotope, as a tracer.

• They could label either CO2 or H2O.

• They found that the 18O label only appeared if water was the source of the tracer.

• Essentially, hydrogen extracted from water is incorporated into sugar and the oxygen released to the atmosphere (where it will be used in respiration).


• Photosynthesis is a redox reaction.• It reverses the direction of electron flow in respiration.

• Water is split and electrons transferred with H+ from water to CO2, reducing it to sugar.

• Polar covalent bonds (unequal sharing) are converted to nonpolar covalent bonds (equal sharing).

• Light boosts the potential energy of electrons as they move from water to sugar.


Fig. 10.3

• Photosynthesis is two processes, each with multiple stages.

• The light reactions convert solar energy to chemical energy.

• The Calvin cycle incorporates CO2 from the atmosphere into an organic molecule and uses energy from the light reaction to reduce the new carbon piece to sugar.

2. The light reactions and the Calvin cycle cooperate in converting light energy to chemical energy of food: an overview


• In the light reaction light energy absorbed by chlorophyll in the thylakoids drives the transfer of electrons and hydrogen from water to NADP+ (nicotinamide adenine dinucleotide phosphate), forming NADPH.• NADPH, an electron acceptor, provides energized

electrons, reducing power, to the Calvin cycle.

• The light reaction also generates ATP by photophosphorylation for the Calvin cycle.



Fig. 10.4

• The Calvin cycle is named for Melvin Calvin who, with his colleagues, worked out many of its steps in the 1940s.

• It begins with the incorporation of CO2 into an organic molecule via carbon fixation.

• This new piece of carbon backbone is reduced with electrons provided by NADPH.

• ATP from the light reaction also powers parts of the Calvin cycle.

• While the light reactions occur at the thylakoids, the Calvin cycle occurs in the stroma.


• The thylakoids convert light energy into the chemical energy of ATP and NADPH.

• Light, like other form of electromagnetic energy, travels in rhythmic waves.

• The distance between crests of electromagnetic waves is called the wavelength.• Wavelengths of electromagnetic radiation range from less

than a nanometer (gamma rays) to over a kilometer (radio waves).

3. The light reactions convert solar energy to the chemical energy of ATP and NADPH: a closer look


• The entire range of electromagnetic radiation is the electromagnetic spectrum.

• The most important segment for life is a narrow band between 380 to 750 nm, visible light.


Fig. 10.5

• While light travels as a wave, many of its properties are those of a discrete particle, the photon.• Photons are not tangible objects, but they do have fixed

quantities of energy.

• The amount of energy packaged in a photon is inversely related to its wavelength.• Photons with shorter wavelengths pack more energy.

• While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to pass in significant quantities.


• When light meets matter, it may be reflected, transmitted, or absorbed.• Different pigments absorb photons of different

wavelengths.

• A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.


Fig. 10.6

• A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light.• It beams narrow wavelengths of light through a solution

containing a pigment and measures the fraction of light transmitted at each wavelength.

• An absorption spectrum plots a pigment’s light absorption versus wavelength.


Fig. 10.7

• The light reaction can perform work with those wavelengths of light that are absorbed.

• In the thylakoid are several pigments that differ in their absorption spectrum.• Chlorophyll a, the dominant pigment, absorbs best in

the red and blue wavelengths, and least in the green.

• Other pigments with different structures have different absorption spectra.


Fig. 10.8a

• Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis.• An action spectrum measures changes in some measure

of photosynthetic activity (for example, O2 release) as the wavelength is varied.


Fig. 10.8b

• The action spectrum of photosynthesis was first demonstrated in 1883 by an elegant experiment by Thomas Engelmann.• In this experiment, different segments of a filamentous

alga were exposed to different wavelengths of light.

• Areas receiving wavelengths favorable to photosynthesis should produce excess O2.

• Engelmann used the abundance of aerobicbacteria clustered along the alga as a measure of O2 production.


Fig. 10.8c

• The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment, including chlorophyll a.

• Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.• Chlorophyll b, with a slightly different structure than

chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

• Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.


• When a molecule absorbs a photon, one of that molecule’s electrons is elevated to an orbital with more potential energy.• The electron moves from its ground state to an excited

state.

• The only photons that a molecule can absorb are those whose energy matches exactly the energy difference 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 wavelengths.

• Thus, each pigment has a unique absorption spectrum. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Photons are absorbed by clusters of pigment molecules in the thylakoid membranes.

• The energy of the photon is converted to the potential energy of an electron raised from its ground state to an excited state.• In chlorophyll a and b, it is an electron from magnesium

in the porphyrin ring that is excited.



Fig. 10.9

• Excited electrons are unstable.• Generally, they drop to their ground state in a

billionth of a second, releasing heat energy.

• Some pigments, including chlorophyll, release a photon of light, in a process called fluorescence, as well as heat.


Fig. 10.10

• In the thylakoid membrane, chlorophyll is organized along with proteins and smaller organic molecules into photosystems.

• A photosystem acts like a light-gathering “antenna complex” consisting of a few hundred chlorophyll a, chlorophyll b,and carotenoidmolecules.


Fig. 10.11

• When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule, the reaction center.

• At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.• This starts the light reactions.

• Each photosystem - 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.

• Photosystem I has a reaction center chlorophyll, the P700 center, that has an absorption peak at 700nm.

• Photosystem II has a reaction center with a peak at 680nm.• The differences between these reaction centers (and

their absorption spectra) lie not in the chlorophyll molecules, but in the proteins associated with each reaction center.

• These two photosystems work together to use light energy to generate ATP and NADPH.


• During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic.

• Noncyclic electron flow, the predominant route, produces both ATP and NADPH.

1. When photosystem II absorbs light, an excited electron is captured by the primary electron acceptor, leaving the reaction center oxidized.

2. An enzyme extracts electrons from water and supplies them to the oxidized reaction center.• This reaction splits water into two hydrogen ions and an

oxygen atom which combines with another to form O2.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

3. Photoexcited electrons pass along an electron transport chain before ending up at an oxidized photosystem I reaction center.

4. As these electrons pass along the transport chain, their energy is harnessed to produce ATP.• The mechanism of noncyclic photophosphorylation is

similar to the process on oxidative phosphorylation.



Fig. 10.12

5. At the bottom of this electron transport chain, the electrons fill an electron “hole” in an oxidized P700 center.

6. This hole is created when photons excite electrons on the photosystem I complex. • The excited electrons are captured by a second primary

electron acceptor which transmits them to a second electron transport chain.

• Ultimately, these electrons are passed from the transport chain to NADP+, creating NADPH.

• NADPH will carry the reducing power of these high-energy electrons to the Calvin cycle.



Fig. 10.13

• The light reactions use the solar power of photons absorbed by both photosystem I and photosystem II to provide chemical energy in the form of ATP and reducing power in the form of the electrons carried by NADPH.

• Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow.• Excited electrons cycle from their reaction center to a

primary acceptor, along an electron transport chain, and returns to the oxidized P700 chlorophyll.

• As electrons flow along the electron transport chain, they generate ATP by cyclic photophosphorylation.


• Noncyclic electron flow produces ATP and NADPH in roughly equal quantities.

• However, the Calvin cycle consumes more ATP than NADPH.

• Cyclic electron flow allows the chloroplast to generate enough surplus ATP to satisfy the higher demand for ATP in the Calvin cycle.


• Chloroplasts and mitochondria generate ATP by the same mechanism: chemiosmosis.• An electron transport chain pumps protons across a

membrane as electrons are passed along a series of more electronegative carriers.

• This builds the proton-motive force in the form of an H+ gradient across the membrane.

• ATP synthase molecules harness the proton-motive force to generate ATP as H+ diffuses back across the membrane.

• Mitochondria transfer chemical energy from food molecules to ATP and chloroplasts transform light energy into the chemical energy of ATP.



Fig. 10.14

• The proton gradient, or pH gradient, across the thylakoid membrane is substantial.• When illuminated, the pH in the thylakoid space drops

to about 5 and the pH in the stroma increases to about 8, a thousandfold different in H+ concentration.

• The light-reaction “machinery” produces ATP and NADPH on the stroma side of the thylakoid.



Fig. 10.16

• Noncyclic electron flow pushes electrons from water, where they are at low potential energy, to NADPH, where they have high potential energy.• This process also produces ATP.

• Oxygen is a byproduct.

• Cyclic electron flow converts light energy to chemical energy in the form of ATP.




Section A2: The Pathways of Photosynthesis

4. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look5. Alternative mechanisms of carbon fixation have evolved in hot, arid climates6. Photosynthesis is the biosphere’s metabolic foundation: a review

• The Calvin cycle regenerates its starting material after molecules enter and leave the cycle.

• CO2 enters the cycle and leaves as sugar.

• The cycle spends the energy of ATP and the reducing power of electrons carried by NADPH to make the sugar.

• The actual sugar product of the Calvin cycle is not glucose, but a three-carbon sugar, glyceraldehyde-3-phosphate (G3P).

4. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar: a closer look


• Each turn of the Calvin cycle fixes one carbon.

• For the net synthesis of one G3P molecule, the cycle must take place three times, fixing three molecules of CO2.

• To make one glucose molecule would require six cycles and the fixation of six CO2 molecules.


• The Calvin cycle has three phases.

• In the carbon fixation phase, each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP).• This is catalyzed by RuBP carboxylase or rubisco.

• The six-carbon intermediate splits in half to form two molecules of 3-phosphoglycerate per CO2.



Fig. 10.17.1

• During reduction, each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3-bisphosphoglycerate.

• A pair of electrons from NADPH reduces each 1,3-bisphosphoglycerate to G3P.• The electrons reduce a carboxyl group to a carbonyl

group.



Fig. 10.17.2

• If our goal was to produce one G3P net, we would start with 3 CO2 (3C) and three RuBP (15C).

• After fixation and reduction we would have six molecules of G3P (18C). • One of these six G3P (3C) is a net gain of carbohydrate.

• This molecule can exit the cycle to be used by the plant cell.

• The other five (15C) must remain in the cycle to regenerate three RuBP.


• In the last phase, regeneration of the CO2 acceptor (RuBP), these five G3P molecules are rearranged to form 3 RuBP molecules.

• To do this, the cycle must spend three more molecules of ATP (one per RuBP) to complete the cycle and prepare for the next.



Fig. 10.17.3

• For the net synthesis of one G3P molecule, the Calvin recycle consumes nine ATP and six NAPDH.• It “costs” three ATP and two NADPH per CO2.

• The G3P from the Calvin cycle is the starting material for metabolic pathways that synthesize other organic compounds, including glucose and other carbohydrates.


• One of the major problems facing terrestrial plants is dehydration.

• At times, solutions to this problem conflict 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 water.

• On hot, dry days plants close the stomata to conserve water, but this causes problems for photosynthesis.

5. Alternative mechanisms of carbon fixation have evolved in hot, arid climates


• In most plants (C3 plants) initial fixation of CO2 occurs via rubisco and results in a three-carbon compound, 3-phosphoglycerate.• These plants include rice, wheat, and soybeans.

• When their stomata are closed on a hot, dry day, CO2 levels drop as CO2 is consumed in the Calvin cycle.

• At the same time, O2 levels rise as the light reaction converts light to chemical energy.

• While rubisco normally accepts CO2, when the O2/CO2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O2 to RuBP.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• When rubisco adds O2 to RuBP, RuBP splits into a three-carbon piece and a two-carbon piece in a process called photorespiration.• The two-carbon fragment is exported from the

chloroplast and degraded to CO2 by mitochondria and peroxisomes.

• Unlike normal respiration, this process produces no ATP, nor additional organic molecules.

• Photorespiration decreases photosynthetic output by siphoning organic material from the Calvin cycle.


• A hypothesis for the existence of photorespiraton (a inexact requirement for CO2 versus O2 by rubisco) is that it is evolutionary baggage.

• When rubisco first evolved, the atmosphere had far less O2 and more CO2 than it does today.

• The inability of the active site of rubisco to exclude O2 would have made little difference.

• Today it does make a difference. • Photorespiration 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.


• The C4 plants fix CO2 first in a four-carbon compound.• Several thousand plants, including sugercane and corn,

use this pathway.

• In C4 plants, mesophyll cells incorporate CO2 into organic molecules.• The key enzyme, phosphoenolpyruvate carboxylase,

adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetetate.

• PEP carboxylase has a very high affinity for CO2 and can fix CO2 efficiently when rubisco cannot, i.e. on hot, dry days when the stomata are closed.


• The mesophyll cells pump these four-carbon compounds into bundle-sheath cells.• The bundle-sheath cells strip a carbon, as CO2, from the

four-carbon compound and return the three-carbon remainder to the mesophyll cells.

• The bundle-sheath cells then use rubisco to start the Calvin cycle with an abundant supply of CO2.



Fig. 10.18

• In effect, the mesophyll cells pump CO2 into the bundle sheath cells, keeping CO2 levels high enough for rubisco to accept CO2 and not O2.

• C4 photosynthesis minimizes photorespiration and enhances sugar production.

• C4 plants thrive in hot regions with intense sunlight.


• A second strategy to minimize photorespiration is found in succulent plants, cacti, pineapples, and several other plant families.• These plants, known as CAM plants for crassulacean

acid metabolism (CAM), open stomata during the night and close them during the day.

• Temperatures are typically lower at night and humidity is higher.

• During the night, these plants fix CO2 into a variety of organic acids in mesophyll cells.

• During the day, the light reactions supply ATP and NADPH to the Calvin cycle and CO2 is released from the organic acids.


• Both C4 and CAM plants add CO2 into organic intermediates before it enters the Calvin cycle.• In C4 plants, carbon fixation and the Calvin cycle are

spatially separated.

• In CAM plants, carbon fixation and the Calvin cycle are temporally separated.

• Both eventually use the Calvin cycle to incorporate light energy into the production of sugar.



Fig. 10.19

• In photosynthesis, the energy that enters the chloroplasts as sunlight becomes stored as chemical energy in organic compounds.

6. Photosynthesis is the biosphere’s metabolic foundation: a review


Fig. 10.20

• Sugar made in the chloroplasts supplies the entire plant with chemical energy and carbon skeletons to synthesize all the major organic molecules of cells.• About 50% of the organic material is consumed as fuel

for cellular respiration in plant mitochondria.

• Carbohydrate in the form of the disaccharide sucrose travels via the veins to nonphotosynthetic cells.

• There, it provides fuel for respiration and the raw materials for anabolic pathways including synthesis of proteins and lipids and building the extracellular polysaccharide cellulose.


• Plants also store excess sugar by synthesizing starch.• Some is stored as starch in chloroplasts or in storage cells in roots,

tubers, seeds, and fruits.

• Heterotrophs, including humans, may completely or partially consume plants for fuel and raw materials.

• On a global scale, photosynthesis is the most important process to the welfare of life on Earth.• Each year, photosynthesis synthesizes 160 billion metric tons of

carbohydrate per year.


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Teaching Materials eneral Biology KPC 8101) Chapter 3 ... file• Difference Catabolic pathway and Anabolic pathways