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Photosynthesis
Photosynthesis & the Cycle of Life <ul><li>Captures the energy of light </li></ul><ul><ul><li>Chlorophyll  traps the light...
In the Beginning <ul><li>Life on Earth originated 3.5 to 4 billion years ago. </li></ul><ul><li>The atmosphere was compose...
Evolution of Photosynthesis <ul><li>Eventually, these food molecules grew scarce </li></ul><ul><li>Some organisms were abl...
Discovery of Photosynthesis <ul><li>Joseph Priestly - chemist and minister </li></ul><ul><ul><li>discovered that under an ...
Photosynthesis Reactions <ul><ul><li>6 CO 2  + 12 H 2 O     C 6 H 12 O 6    + 6 O 2 </li></ul></ul><ul><li>This is really...
Overview <ul><li>Light energy entering the plant splits the water into hydrogen and oxygen:  </li></ul><ul><li>H 2 O + lig...
The Nature of Light <ul><li>Light behaves as both a wave and a particle </li></ul><ul><ul><li>particles of energy are call...
Visible Light <ul><li>Visible light  is only part of the electromagnetic spectrum  </li></ul><ul><ul><li>Only 1% of light ...
Light Energy <ul><li>The energy in a light wave is related to frequency and wavelength </li></ul><ul><li>Different colors ...
Pigments <ul><li>How is light captured by living things? </li></ul><ul><li>Molecules, when struck by a wave or photon of l...
Absorption of Light <ul><li>Light energy comes in &quot;packets&quot; called  photons </li></ul><ul><li>Plants can utilize...
Absorption & Action Spectra <ul><li>An  absorption spectrum  for a pigment describes the wavelengths at which it can absor...
Absorption & Action Spectra
Chlorophyll <ul><li>Chlorophylls  are the green pigments in plants  </li></ul><ul><ul><li>Chlorophyll   a   </li></ul></ul...
Chlorophyll & Light <ul><li>When a  photon  strikes chlorophyll, the photon's energy is transferred to an electron in the ...
Structure of Chlorophyll
Photosystems <ul><li>Clusters of several hundred pigment molecules in the thylakoid membranes </li></ul><ul><li>Two types:...
Accessory Pigments <ul><li>Accessory pigments absorb light in other parts of spectrum & pass the energy to chlorophyll: </...
The Chloroplast <ul><li>The chloroplast is the organelle of photosynthesis.  </li></ul><ul><li>Resembles the mitochondrion...
Structures of Photosynthesis <ul><li>The compartments and membranes isolate different aspects of photosynthesis.  </li></u...
 
Materials for Photosynthesis <ul><li>CO 2  is the source of C & O used to make glucose   </li></ul><ul><li>H 2 O is the so...
The Light Reactions <ul><li>Use trapped energy to convert ADP to ATP, which stores energy for later use </li></ul><ul><li>...
Photosystems <ul><li>Non-cyclic photophosphorylation Involves two sets of pigments: </li></ul><ul><ul><li>Photosystem 1 (P...
Non-cyclic Photophosphorylation <ul><li>Energy enters the system when PS2 becomes excited by light.  </li></ul><ul><li>Ele...
The Redox Chain <ul><li>Each element in the pathway is reduced by the electrons </li></ul><ul><li>Each element then reduce...
 
Cyclic Photophosphorylation <ul><li>Sometimes an organism has all the reductive power (NADPH) needed to synthesize new car...
The Role of PS1 <ul><li>The role of PS1 seems contradictory </li></ul><ul><ul><li>In noncyclic phototphosphorylation PS1 w...
The Role of PS2 <ul><li>PS2 is better at grabbing electrons from water and transferring them to quinone (Q).  </li></ul><u...
Steps of the Light Reactions - 1 <ul><li>Chlorophyll   in the  grana  absorb  photons  of light </li></ul><ul><ul><li>ener...
Steps of the Light Reactions <ul><li>The excited electrons leave  chlorophyll a   </li></ul><ul><ul><li>They are transferr...
Steps of the Light Reactions - 2 <ul><li>Electrons lost from the  chlorophyll  are replaced by electrons from water molecu...
Steps of the Light Reactions - 3 <ul><li>The primary electron acceptor donates the electrons to the first of a series of m...
Steps of the Light Reactions - 4 <ul><li>Chemiosmosis </li></ul><ul><ul><li>Energy from the proton gradient created in the...
Steps of the Light Reactions - 5 <ul><li>Light is also absorbed by Photosystem I </li></ul><ul><ul><li>Electrons move from...
Steps of the Light Reactions - 6 <ul><li>The primary electron acceptor of Photosystem I donates electrons to a 2nd electro...
Summarizing the Light Reactions
The Dark Reactions <ul><li>The reduction of carbon dioxide to glucose, using the NADPH produced by the light reactions, is...
Products of The Calvin-Benson Cycle <ul><li>The cycle runs 6 times, each time incorporating a new carbon.  </li></ul><ul><...
Steps of the Dark Reactions - 1 <ul><li>CO 2  from the atmosphere combines with RDP in series of reactions which use ATP a...
Steps of the Dark Reactions - 2 <ul><li>PGA reacts with hydrogen from the light reactions to form PGAL  (phosphoglyceralde...
Steps of the Dark Reactions - 3 <ul><li>Some PGAL is combined to form glucose:  </li></ul><ul><ul><li>2 PGALs form one glu...
Rubisco <ul><li>The key enzyme in the Calvin Cycle catalyzes the transformation of the 5-C sugar, ribulose-5-phosphate and...
Abundance of Rubisco <ul><li>Rubisco accounts for 16% of the protein content of the chloroplast  </li></ul><ul><li>The mos...
Light Regulation of the Calvin Cycle <ul><li>The energy required for the Calvin Cycle, in the form of ATP and NADPH, comes...
Linkage of Late & Dark Reactions <ul><li>The pH of the stroma increases as protons are pumped out of it through the membra...
Linkage of Late & Dark Reactions (continued) <ul><li>This reduction breaks a disulphide bridge in thioredoxin.  </li></ul>...
Linkage of Late & Dark Reactions (continued) <ul><li>The reactions of the Calvin cycle stop when they run out of substrate...
Photosynthesis Summary
Alternative Pathways <ul><li>Many diverse environments </li></ul><ul><li>Led to different approaches to photosynthesis </l...
Adaptations to Hot Climates <ul><li>In hot, dry climates plants need special adaptations </li></ul><ul><ul><li>Water loss ...
Rubisco <ul><li>Rubisco is the most abundant enzyme on Earth.  </li></ul><ul><li>It is a very important  </li></ul><ul><li...
The Inefficiency of Rubisco <ul><li>When Rubisco uses oxygen as a substrate instead of carbon dioxide, it does not fix CO2...
Alternate Fate of Rubisco
The Effect of Temperature <ul><li>At 25°C, the rate of the carboxylase reaction is 4x that of the oxygenase reaction </li>...
Balancing CO 2  Input & Water Loss <ul><li>Plants in arid climates close the stomata  (pores) in their leaves when it is v...
The C4 Solution <ul><li>Plants living in these dry conditions have evolved a mechanism to make the CO 2  concentration ver...
The C4 Pathway <ul><li>The conventional pathway is called the C3 pathway  </li></ul><ul><ul><li>it involves only the 3-car...
Picturing the C4 Pathway
Chemistry of the C4 Pathway
Why Aren’t All Plants C4? <ul><li>Why don't the C4 plants out-compete the C3 plants, which are inefficient?  </li></ul><ul...
CAM Plants <ul><li>Another strategy used in hot or dry climates </li></ul><ul><ul><li>Prevents water loss </li></ul></ul><...
Comparing C4 & CAM Strategies
Lithotrophs <ul><li>Some  autotrophs  don’t use energy from sunlight </li></ul><ul><li>These bacteria derive reductive pow...
Chemosynthesis <ul><li>Chemosynthesis  oxidizes inorganic substances & transports electrons through the membrane </li></ul...
Methanogens <ul><li>The methanogens are a class of anaerobic bacteria.  </li></ul><ul><li>They derive energy by reducing C...
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Photosynthesis

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Transcript of "Photosynthesis"

  1. 1. Photosynthesis
  2. 2. Photosynthesis & the Cycle of Life <ul><li>Captures the energy of light </li></ul><ul><ul><li>Chlorophyll traps the light of the sun </li></ul></ul><ul><li>Overall reaction: </li></ul><ul><ul><li>6 CO 2 + 12 H 2 O  C 6 H 12 O 6  + 6 O 2 </li></ul></ul><ul><li>End product is glucose, a 6 carbon sugar </li></ul><ul><ul><li>the primary energy molecule for many living organisms </li></ul></ul><ul><li>Uses CO 2 & produces O 2 </li></ul>
  3. 3. In the Beginning <ul><li>Life on Earth originated 3.5 to 4 billion years ago. </li></ul><ul><li>The atmosphere was composed of methane, carbon dioxide, and water vapor. </li></ul><ul><li>The cooling water collected in pools, assimilating the nutrients from the rocks. </li></ul><ul><li>As water evaporated, the nutrients concentrated, forming a rich soup. </li></ul><ul><li>The first organisms used these molecules for food, breaking them down into water and carbon dioxide through respiration. </li></ul>
  4. 4. Evolution of Photosynthesis <ul><li>Eventually, these food molecules grew scarce </li></ul><ul><li>Some organisms were able (through random mutation) to use the sun's energy to synthesize large molecules from small molecules. </li></ul><ul><ul><li>This is the process of photosynthesis </li></ul></ul><ul><li>Organisms who create complex molecules this way are called autotrophs </li></ul><ul><ul><li>Autotrophs are found in the bacterial and in the plant kingdom. </li></ul></ul><ul><li>Produced O 2 as a byproduct, changing the atmosphere </li></ul>
  5. 5. Discovery of Photosynthesis <ul><li>Joseph Priestly - chemist and minister </li></ul><ul><ul><li>discovered that under an inverted jar, a candle would burn out quickly </li></ul></ul><ul><ul><li>found that a mouse could similarly &quot;injure&quot; air. </li></ul></ul><ul><ul><li>showed that the &quot;injured&quot; air could be restored by a plant. </li></ul></ul><ul><li>Jan Ingenhousz – 1778 - Austrian court physician </li></ul><ul><ul><li>repeated Priestly's experiments </li></ul></ul><ul><ul><li>discovered that it was the effect of sunlight on the plant that caused it to rescue the mouse </li></ul></ul><ul><li>Jean Senebier – 1796- a French pastor </li></ul><ul><ul><li>showed that CO 2 was the &quot;fixed&quot; or &quot;injured&quot; air and that it was taken up by plants in photosynthesis. </li></ul></ul><ul><li>Theodore de Saussure </li></ul><ul><ul><li>showed that the increased mass of a plant as it grows is not due only to CO 2 uptake, but also to water. </li></ul></ul>
  6. 6. Photosynthesis Reactions <ul><ul><li>6 CO 2 + 12 H 2 O  C 6 H 12 O 6  + 6 O 2 </li></ul></ul><ul><li>This is really a series of complex reactions </li></ul><ul><li>Involves 2 phases: </li></ul><ul><li>1. The Light Reactions </li></ul><ul><ul><li>use energy from sunlight </li></ul></ul><ul><li>2. The Dark Reactions </li></ul><ul><ul><li>fix carbon </li></ul></ul>
  7. 7. Overview <ul><li>Light energy entering the plant splits the water into hydrogen and oxygen: </li></ul><ul><li>H 2 O + light energy  ½ O 2 + 2H + + 2 electrons </li></ul><ul><li>Electrons travel through the membrane much like the electrons in oxidative phosphorylation </li></ul><ul><li>Their energy pumps protons through the membrane. </li></ul><ul><ul><li>This proton gradient can be used to synthesize ATP. </li></ul></ul><ul><li>The same electron reduces NADP + to NADPH. </li></ul><ul><ul><li>This molecule plays the same role in synthesis as does NAD + in respiration </li></ul></ul><ul><ul><li>a carrier of reductive power. </li></ul></ul><ul><li>This reductive power converts CO 2 to glucose </li></ul>
  8. 8. The Nature of Light <ul><li>Light behaves as both a wave and a particle </li></ul><ul><ul><li>particles of energy are called photons . </li></ul></ul><ul><li>As a wave. light has a wavelength (the distance from one peak of the wave to the next) and an amplitude (the distance the wave oscillates from its centerline). </li></ul><ul><ul><li>Different wavelengths of light have different characteristic energies and properties. </li></ul></ul><ul><li>Light travels at various speeds in different media, producing a frequency at which the wave travels. </li></ul>
  9. 9. Visible Light <ul><li>Visible light is only part of the electromagnetic spectrum </li></ul><ul><ul><li>Only 1% of light that reaches planet </li></ul></ul><ul><li>Visible white light is actually made of different colors </li></ul>
  10. 10. Light Energy <ul><li>The energy in a light wave is related to frequency and wavelength </li></ul><ul><li>Different colors have different wavelengths </li></ul><ul><ul><li>different wavelengths have different amounts of energy </li></ul></ul><ul><li>Short wavelengths have high energies and long wavelengths have lower energies. </li></ul><ul><ul><li>Violet light has 2x energy of red </li></ul></ul>
  11. 11. Pigments <ul><li>How is light captured by living things? </li></ul><ul><li>Molecules, when struck by a wave or photon of light, reflect some energy back </li></ul><ul><li>Absorb some of the energy, and thus enter into a higher energy or excited state. </li></ul><ul><li>Each molecule absorbs or reflects its own characteristic wavelengths of light. </li></ul><ul><li>Pigments = molecules that absorb wavelengths in the visible region of the spectrum </li></ul>
  12. 12. Absorption of Light <ul><li>Light energy comes in &quot;packets&quot; called photons </li></ul><ul><li>Plants can utilize energy only from absorbed wavelengths </li></ul><ul><li>The color we see is the color reflected; the rest is absorbed </li></ul>
  13. 13. Absorption & Action Spectra <ul><li>An absorption spectrum for a pigment describes the wavelengths at which it can absorb light and enter into an excited state. </li></ul><ul><li>An action spectrum describes the efficiency of a particular molecule at absorbing light </li></ul><ul><ul><li>Shows what wavelengths of light the molecule can trap to conduct photosynthesis. </li></ul></ul><ul><li>The action spectrum closely follows the absorption spectrum for a particular pigment </li></ul><ul><ul><li>the molecule has to be able to absorb light to enter into its excited state and pass the energy on. </li></ul></ul>
  14. 14. Absorption & Action Spectra
  15. 15. Chlorophyll <ul><li>Chlorophylls are the green pigments in plants </li></ul><ul><ul><li>Chlorophyll a </li></ul></ul><ul><ul><li>directly involved in the light reactions of photosynthesis </li></ul></ul><ul><ul><li>Chlorophyll b </li></ul></ul><ul><ul><li>assists chlorophyll a ; an accessory pigment </li></ul></ul><ul><li>Located in the membranes of the thylakoids inside chloroplasts </li></ul>
  16. 16. Chlorophyll & Light <ul><li>When a photon strikes chlorophyll, the photon's energy is transferred to an electron in the chlorophyll molecule </li></ul><ul><ul><li>energized electrons can't remain in this state </li></ul></ul><ul><ul><li>as the electron returns to its original energy level, it releases absorbed energy. </li></ul></ul>
  17. 17. Structure of Chlorophyll
  18. 18. Photosystems <ul><li>Clusters of several hundred pigment molecules in the thylakoid membranes </li></ul><ul><li>Two types: </li></ul><ul><ul><li>Photosystem I </li></ul></ul><ul><ul><li>Photosystem II </li></ul></ul><ul><li>Both are involved in the light reactions </li></ul>
  19. 19. Accessory Pigments <ul><li>Accessory pigments absorb light in other parts of spectrum & pass the energy to chlorophyll: </li></ul><ul><ul><li>Xanthophylls - yellow pigments </li></ul></ul><ul><ul><li>Carotenoids - orange pigments </li></ul></ul><ul><ul><li>Anthocyanins – red pigments </li></ul></ul>
  20. 20. The Chloroplast <ul><li>The chloroplast is the organelle of photosynthesis. </li></ul><ul><li>Resembles the mitochondrion </li></ul><ul><ul><li>Both are surrounded by a double membrane with an intermembrane space. </li></ul></ul><ul><ul><li>Both have their own DNA. </li></ul></ul><ul><ul><li>Both are involved in energy metabolism. </li></ul></ul><ul><ul><li>Both have membrane reticulations filling their inner space to increase the surface area on which reactions with membrane-bound proteins can take place. </li></ul></ul><ul><li>Has three membranes: inner, outer, and thylakoid </li></ul><ul><li>Has three compartments: stroma, thylakoid space, and inter-membrane space. </li></ul>
  21. 21. Structures of Photosynthesis <ul><li>The compartments and membranes isolate different aspects of photosynthesis. </li></ul><ul><li>Chloroplasts have a highly organized array of internal membranes called thylakoids </li></ul><ul><li>These form stacks of flattened structures = grana </li></ul><ul><ul><li>Photosynthesis begins in the grana </li></ul></ul><ul><ul><li>Pigments are embedded here </li></ul></ul><ul><li>Stroma are dark fluid-filled spaces between grana and the outer membrane </li></ul><ul><ul><li>contain enzymes, DNA, RNA, ribosomes </li></ul></ul><ul><li>Light reactions take place on the thylakoid membranes. </li></ul><ul><li>Dark reactions take place in the stroma. </li></ul>
  22. 23. Materials for Photosynthesis <ul><li>CO 2 is the source of C & O used to make glucose </li></ul><ul><li>H 2 O is the source of H </li></ul><ul><li>Oxygen from H 2 O is released into the air and produces O 2 in the atmosphere </li></ul><ul><ul><li>O 2 drives cellular respiration in living organisms </li></ul></ul>
  23. 24. The Light Reactions <ul><li>Use trapped energy to convert ADP to ATP, which stores energy for later use </li></ul><ul><li>Uses energy to split H 2 O to H and O </li></ul><ul><li>The reactions leading to the production of ATP and reduction of NADP+ are called the light reactions because they are initiated by splitting water by light energy. </li></ul>
  24. 25. Photosystems <ul><li>Non-cyclic photophosphorylation Involves two sets of pigments: </li></ul><ul><ul><li>Photosystem 1 (PS1) </li></ul></ul><ul><ul><li>Photosystem 2 (PS2) </li></ul></ul><ul><li>PS1 is better excited by light at about 700 nm </li></ul><ul><ul><li>sometimes called P-700 </li></ul></ul><ul><li>PS2 can’t use wavelengths longer than 680 nm </li></ul><ul><ul><li>sometimes called P-680. </li></ul></ul>
  25. 26. Non-cyclic Photophosphorylation <ul><li>Energy enters the system when PS2 becomes excited by light. </li></ul><ul><li>Electrons are shed by the excited PS2 (oxidation), which grabs electrons from water </li></ul><ul><ul><li>This produces a molecule of oxygen gas for every two waters split. </li></ul></ul><ul><li>PS2 thus returns it to its unexcited state (reduction) . </li></ul><ul><li>The electrons are passed through a chain of oxidation-reduction reactions. </li></ul>
  26. 27. The Redox Chain <ul><li>Each element in the pathway is reduced by the electrons </li></ul><ul><li>Each element then reduces its neighbor in the pathway by giving it the electrons </li></ul><ul><li>Thus each element is reoxidized and ready for the next electrons to pass through the photosystem. </li></ul><ul><li>PS2 passes on the energy to move the electron through the redox chain </li></ul><ul><ul><li>this pumps protons through the membrane to generate ATP. </li></ul></ul><ul><li>PS1 passes on the energy required to reduce NADPH. </li></ul>
  27. 29. Cyclic Photophosphorylation <ul><li>Sometimes an organism has all the reductive power (NADPH) needed to synthesize new carbon skeletons </li></ul><ul><ul><li>still needs ATP to power other activities in the chloroplast. </li></ul></ul><ul><li>Many bacteria can shut off PS2, allowing the production of ATP in the absence of glucose </li></ul><ul><li>A proton gradient is generated across the membrane using the mechanisms of photosynthesis. </li></ul><ul><li>This type of energy generation is called cyclic photophosphorylation. </li></ul>
  28. 30. The Role of PS1 <ul><li>The role of PS1 seems contradictory </li></ul><ul><ul><li>In noncyclic phototphosphorylation PS1 was responsible for NADPH production </li></ul></ul><ul><ul><li>In cyclic photophosphorylation it is needed for ATP production. </li></ul></ul><ul><li>PS1 is a good candidate for noncyclic photophosphorylation and for NADPH production. </li></ul><ul><ul><li>PS1 is good at transferring an electron, whether to NADP or to ferredoxin (fd). </li></ul></ul><ul><ul><li>It is a powerful reductant. </li></ul></ul>
  29. 31. The Role of PS2 <ul><li>PS2 is better at grabbing electrons from water and transferring them to quinone (Q). </li></ul><ul><ul><li>It is a good oxidant. </li></ul></ul><ul><li>The electron transferred is not derived from water, but from PS1 itself. </li></ul><ul><li>It therefore must be recycled to PS1. </li></ul>
  30. 32. Steps of the Light Reactions - 1 <ul><li>Chlorophyll in the grana absorb photons of light </li></ul><ul><ul><li>energy from the photons boosts electrons from the chlorophyll a molecules of Photosystem II to a higher energy level </li></ul></ul>
  31. 33. Steps of the Light Reactions <ul><li>The excited electrons leave chlorophyll a </li></ul><ul><ul><li>They are transferred to a molecule </li></ul></ul><ul><ul><li>in the thylakoid membrane: </li></ul></ul><ul><ul><li>the primary electron acceptor </li></ul></ul>
  32. 34. Steps of the Light Reactions - 2 <ul><li>Electrons lost from the chlorophyll are replaced by electrons from water molecules. </li></ul><ul><ul><li>This splits H 2 O into H ions & O 2 gas </li></ul></ul>
  33. 35. Steps of the Light Reactions - 3 <ul><li>The primary electron acceptor donates the electrons to the first of a series of molecules called the electron transport chain . </li></ul><ul><ul><li>energized electrons move from one molecule to another in the electron transport chain </li></ul></ul><ul><ul><li>each time a transfer is made, energy is released </li></ul></ul>
  34. 36. Steps of the Light Reactions - 4 <ul><li>Chemiosmosis </li></ul><ul><ul><li>Energy from the proton gradient created in the thylakoid membrane fuel ATP synthase, an enzyme </li></ul></ul><ul><ul><li>Energy released from electrons as they move down the electron transport chain is used to form ATP, combining ADP & phosphates in the stroma of chloroplasts </li></ul></ul><ul><li>Energy from the light reactions in the form of NADPH and ATP will fuel the dark reactions that follow </li></ul>
  35. 37. Steps of the Light Reactions - 5 <ul><li>Light is also absorbed by Photosystem I </li></ul><ul><ul><li>Electrons move from chlorophyll a molecules to another primary electron acceptor </li></ul></ul><ul><ul><li>These electrons are replaced by the electrons that passed through the electron transport chain in photosystem II </li></ul></ul>
  36. 38. Steps of the Light Reactions - 6 <ul><li>The primary electron acceptor of Photosystem I donates electrons to a 2nd electron transport chain. </li></ul><ul><ul><li>These electrons reduce NADP + to NADPH </li></ul></ul>
  37. 39. Summarizing the Light Reactions
  38. 40. The Dark Reactions <ul><li>The reduction of carbon dioxide to glucose, using the NADPH produced by the light reactions, is governed by the dark reactions </li></ul><ul><li>Also known as the Calvin Cycle for Melvin Calvin, described in 1950’s </li></ul><ul><ul><li>Requires several enzymes & produces several byproducts </li></ul></ul><ul><ul><li>Takes place in the stroma of the chloroplasts </li></ul></ul><ul><li>Fixes carbon from CO 2 to form glucose </li></ul><ul><li>Begins and ends with a five carbon sugar, RDP (ribulose diphosphate) </li></ul>
  39. 41. Products of The Calvin-Benson Cycle <ul><li>The cycle runs 6 times, each time incorporating a new carbon. </li></ul><ul><li>Ribulose is a five-carbon sugar and the gylceraldehydes are three-carbon sugars </li></ul><ul><li>Running through the cycle six times generates: </li></ul><ul><ul><li>6(5-carbon sugars) + 6(incorporated carbon dioxides) </li></ul></ul><ul><li>Those six CO 2 are reduced to glucose by the conversion of NADPH to NADP + . </li></ul><ul><li>Glucose serves as a building block to make polysaccharides, other monosaccharides, fats, amino acids, nucleotides, and all the molecules living things require. </li></ul>
  40. 42. Steps of the Dark Reactions - 1 <ul><li>CO 2 from the atmosphere combines with RDP in series of reactions which use ATP as an energy source. </li></ul><ul><li>This forms PGA (phosphoglyceric acid), a molecule with 3 carbons </li></ul>
  41. 43. Steps of the Dark Reactions - 2 <ul><li>PGA reacts with hydrogen from the light reactions to form PGAL (phosphoglyceraldehyde) </li></ul><ul><ul><li>Most of the PGAL formed is used to make more RDP. </li></ul></ul><ul><ul><li>RDP combines with more CO 2 and the cycle repeats. </li></ul></ul>
  42. 44. Steps of the Dark Reactions - 3 <ul><li>Some PGAL is combined to form glucose: </li></ul><ul><ul><li>2 PGALs form one glucose C 6 H 12 O 6 </li></ul></ul><ul><li>Excess glucose is stored as starch which can be used as needed </li></ul>
  43. 45. Rubisco <ul><li>The key enzyme in the Calvin Cycle catalyzes the transformation of the 5-C sugar, ribulose-5-phosphate and the single-C CO 2 to two 3-C 3-phosphoglycerates. </li></ul><ul><li>This reaction has a very high  G of -12.4 kcal/mol. </li></ul><ul><li>The enzyme is called ribulose-1-5-biphosphote carboxylase, or Rubisco . </li></ul>
  44. 46. Abundance of Rubisco <ul><li>Rubisco accounts for 16% of the protein content of the chloroplast </li></ul><ul><li>The most abundant protein on Earth. </li></ul><ul><li>Why is this protein so abundant? </li></ul><ul><ul><li>It is crucial to all life to have a source of carbon fixation </li></ul></ul><ul><ul><li>The enzyme is very inefficient </li></ul></ul>
  45. 47. Light Regulation of the Calvin Cycle <ul><li>The energy required for the Calvin Cycle, in the form of ATP and NADPH, comes from the light reactions. </li></ul><ul><li>The plant or photosynthesizing bacterium needs to tightly regulate the Calvin Cycle with photosynthesis. </li></ul><ul><ul><li>It would be wasteful to run the process using ATP generated for other plant metabolism. </li></ul></ul>
  46. 48. Linkage of Late & Dark Reactions <ul><li>The pH of the stroma increases as protons are pumped out of it through the membrane </li></ul><ul><ul><li>The enzymes of the Calvin Cycle function better at this higher pH. </li></ul></ul><ul><li>As the reduction potential of ferredoxin (fd) increases, it reduces a protein called thioredoxin . </li></ul>
  47. 49. Linkage of Late & Dark Reactions (continued) <ul><li>This reduction breaks a disulphide bridge in thioredoxin. </li></ul><ul><ul><li>The enzyme now has free cysteines that can compete for the the disulphide bonds in other enzymes. </li></ul></ul><ul><ul><li>Several enzymes of the Calvin Cycle are activated by the breaking of disulphide bridges. </li></ul></ul><ul><ul><li>So the activity of the light reactions is communicated to the dark reactions by an enzyme intermediate. </li></ul></ul>
  48. 50. Linkage of Late & Dark Reactions (continued) <ul><li>The reactions of the Calvin cycle stop when they run out of substrate </li></ul><ul><ul><li>As photosynthesis stops, there is no more ATP or NADPH in the stroma for the dark reactions to take place. </li></ul></ul><ul><li>The light reactions increase the permeability of the stromal membrane to cofactors such as Mg++ which are required for the Calvin </li></ul>
  49. 51. Photosynthesis Summary
  50. 52. Alternative Pathways <ul><li>Many diverse environments </li></ul><ul><li>Led to different approaches to photosynthesis </li></ul><ul><li>Some organisms forgo the use of light for energy production </li></ul><ul><li>Others modify photosynthetic pathways to make then more compatible with environmental conditions </li></ul><ul><li>All of these adaptations are variations on the same basic pathways of photosynthesis and respiration </li></ul>
  51. 53. Adaptations to Hot Climates <ul><li>In hot, dry climates plants need special adaptations </li></ul><ul><ul><li>Water loss through stomata which must open to exchange CO 2 for O 2 would be damaging </li></ul></ul><ul><li>Plants that fix carbon through the Calvin Cycle are C 3 plants </li></ul><ul><li>2 alternative pathways: </li></ul><ul><ul><li>C 4 pathway </li></ul></ul><ul><ul><li>CAM pathway </li></ul></ul>
  52. 54. Rubisco <ul><li>Rubisco is the most abundant enzyme on Earth. </li></ul><ul><li>It is a very important </li></ul><ul><li>It is believed that Rubisco is so abundant because of its inefficiencies. </li></ul><ul><li>Rubisco will sometimes recognize oxygen as a substrate instead of carbon dioxide . </li></ul>
  53. 55. The Inefficiency of Rubisco <ul><li>When Rubisco uses oxygen as a substrate instead of carbon dioxide, it does not fix CO2 into sugar </li></ul><ul><li>Instead, it creates phosphoglycolate, a nearly useless compound. </li></ul><ul><li>This wastes energy </li></ul><ul><li>This reaction, directly competes with the regular reaction at the same site on the enzyme. </li></ul><ul><li>The result is very detrimental to photosynthesis </li></ul>
  54. 56. Alternate Fate of Rubisco
  55. 57. The Effect of Temperature <ul><li>At 25°C, the rate of the carboxylase reaction is 4x that of the oxygenase reaction </li></ul><ul><ul><li>the plant is only about 20% inefficient. </li></ul></ul><ul><li>As temperature rises, the balance between O 2 and CO 2 in the air changes (due to changing solubility in the ocean) </li></ul><ul><li>The carboxylase reaction is less and less dominant. </li></ul><ul><li>Plants living in warm climates have to overcome this handicap </li></ul>
  56. 58. Balancing CO 2 Input & Water Loss <ul><li>Plants in arid climates close the stomata (pores) in their leaves when it is very dry </li></ul><ul><li>This creates a closed environment </li></ul><ul><li>As CO 2 is used up in photosynthesis, the relative concentration of O 2 increases </li></ul><ul><li>The oxygenase reaction begins to dominate. </li></ul>
  57. 59. The C4 Solution <ul><li>Plants living in these dry conditions have evolved a mechanism to make the CO 2 concentration very high in the immediate environment of Rubisco </li></ul><ul><li>This prevents the oxygenase reaction </li></ul><ul><li>This is called the C4 pathway because it involves a 4 carbon intermediate in the outer cells. </li></ul>
  58. 60. The C4 Pathway <ul><li>The conventional pathway is called the C3 pathway </li></ul><ul><ul><li>it involves only the 3-carbon sugars. </li></ul></ul><ul><li>In the C4 pathway, a 4-carbon intermediate brings a molecule of CO 2 into the bundle sheath cells </li></ul><ul><ul><li>it is dropped right next to the location of the Calvin Cycle. </li></ul></ul><ul><li>This ensures that the concentration of CO 2 at the site of Rubisco is very high, </li></ul><ul><li>Only the carboxylase reaction can take place. </li></ul><ul><li>The C4 pathway still uses the Calvin Cycle with its 3-carbon sugar intermediates </li></ul><ul><ul><li>it makes use of 4-carbon sugars to bring the carbon dioxide closer to the site of fixation. </li></ul></ul>
  59. 61. Picturing the C4 Pathway
  60. 62. Chemistry of the C4 Pathway
  61. 63. Why Aren’t All Plants C4? <ul><li>Why don't the C4 plants out-compete the C3 plants, which are inefficient? </li></ul><ul><li>It takes ATP to bring the CO 2 to the Rubisco. </li></ul><ul><li>In moderate temperatures, this energy burden outweighs the advantage of eliminating the 1 in 5 times that Rubisco binds O 2 instead of CO 2 . </li></ul><ul><li>In warmer climates the C4 plants win </li></ul>
  62. 64. CAM Plants <ul><li>Another strategy used in hot or dry climates </li></ul><ul><ul><li>Prevents water loss </li></ul></ul><ul><li>Plants open their stomata at night and close them during the day </li></ul><ul><li>Take in CO 2 at night and fix it in organic compounds </li></ul><ul><li>Later, release carbon from these compounds to enter the Calvin cycle </li></ul><ul><li>Steps of the photosynthetic pathway are separated in time </li></ul>
  63. 65. Comparing C4 & CAM Strategies
  64. 66. Lithotrophs <ul><li>Some autotrophs don’t use energy from sunlight </li></ul><ul><li>These bacteria derive reductive power by oxidizing compounds such as hydrogen gas, carbon monoxide, ammonia, nitrite, hydrosulphuric acid, sulphur, sulphate, or iron. </li></ul><ul><li>These organisms are called lithotrophs or “rock-eaters” </li></ul><ul><li>This process = Chemosynthesis </li></ul><ul><ul><li>oxidizing an inorganic substance and transporting electrons through the membrane </li></ul></ul>
  65. 67. Chemosynthesis <ul><li>Chemosynthesis oxidizes inorganic substances & transports electrons through the membrane </li></ul><ul><ul><li>like in oxidative phosphorylation and photosynthesis </li></ul></ul><ul><ul><li>(remember - oxidation is a loss of electrons, so this inorganic substance is the electron donor). </li></ul></ul><ul><li>This electron transport pumps protons through the membrane generating a proton gradient which can be used to form ATP. </li></ul><ul><li>These organisms make so much ATP that they can drive the electron transport chain backwards to generate NADH. </li></ul><ul><li>This NADH provides the reducing power needed to synthesize carbon structures from carbon dioxide. </li></ul>
  66. 68. Methanogens <ul><li>The methanogens are a class of anaerobic bacteria. </li></ul><ul><li>They derive energy by reducing CO 2 to methane </li></ul><ul><li>They use CO 2 as an energy source rather than treating it as an energy-depleted waste product. </li></ul><ul><li>The methanogens can oxidize hydrogen gas to directly reduce NAD+ to NADH, </li></ul><ul><li>Don’t have to waste energy making ATP through chemosynthesis and then driving it backwards through the electron transport chain. </li></ul><ul><li>These organisms still incorporate their carbon into the Krebs Cycle for processing into amino acids, nucleic acids, and sugars. </li></ul>
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