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CHAPTER 10

CHAPTER 10. PHOTOSYNTHESIS. INTRODUCTION. PHOTOSYNTHESIS- conversion of solar energy from the sun to chemical energy stored in sugar and other organic molecules - an organism obtains the organic compounds it needs for energy and carbon skeletons by one of two ways:

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CHAPTER 10

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  1. CHAPTER 10 PHOTOSYNTHESIS

  2. INTRODUCTION PHOTOSYNTHESIS- conversion of solar energy from the sun to chemical energy stored in sugar and other organic molecules - an organism obtains the organic compounds it needs for energy and carbon skeletons by one of two ways: Autotrophic nutrition/ Heterotrophic nutrition

  3. AUTOTROPHS Autotrophs can sustain themselves without eating other organisms or substances that come from other organisms - produce organic molecules from CO2 and other inorganic raw materials that come from the environment - they are the main sources of organic compounds for all other organisms  called PRODUCERS of the biosphere

  4. - plants are autotrophs  PHOTOAUTOTROPHS- organisms that use light as a source of energy to synthesize organic compounds - photosynthesis also occurs in algae and some prokaryotes

  5. HETEROTROPHS Heterotrophs are unable to make their own food, so they live on compounds produced by other organisms - they are the biosphere’s CONSUMERS - most common form is when an animal eats plants or other animals - some consume the remains of dead organisms DECOMPOSERS

  6. - almost all heterotrophs are completely dependent on photoautotrophs for food and oxygen

  7. CHLOROPLASTS All green parts of a plant have chloroplasts, but the main sites of photosynthesis are leaves - Color of the leaf is from CHLOROPHYLL- green pigment found inside the chloroplast - light energy absorbed by chlorophyll drives the synthesis of food molecules in the chloroplast

  8. SOME RELATED TERMS - Chloroplasts are found mainly in the cells of the MESOPHYLL- interior tissue in leaf - CO2 enters the leaf and oxygen exits by way of microscopic pores called STOMATA - 2 membranes enclose the STROMA- the fluid inside the chloroplast - inside the inner membrane is another membrane system called THYLAKOIDS

  9. - in some places the thylakoid sacs are stacked in columns called GRANA - chlorophyll is found within the thylakoid membranes

  10. SUMMARY EQUATION 6 CO2 + 6 H2O+ Light energy C6H12O6 + 6 O2 The overall chemical change is the reverse of the one that occurs in cellular respiration - but plants do not make food by simply reversing respiration

  11. SPLITTING OF WATER It was originally thought that the oxygen given off by plants came from CO2 and not water - chloroplasts split water into hydrogen and oxygen - original idea challenged by C.B. van Niel in the 1930s - studying photosynthesis in bacteria

  12. - 20 years later van Niel’s hypothesis was confirmed when scientists used an isotope to trace the fate of oxygen atoms during photosynthesis

  13. REDOX REACTIONS In respiration, energy is released from sugar when electrons are transported by carriers to oxygen - electrons lose potential energy as they are pulled down the chain, and mitochondria use that energy to make ATP Photosynthesis REVERSES the flow of electrons

  14. - water is split and electrons (along with H+) are transferred from water to CO2, reducing it to sugar - electrons increase in potential energy as they move from water to sugar

  15. OVERVIEW OF LIGHT REACTIONS AND CALVIN CYCLE Photosynthesis actually has 2 stages- the LIGHT REACTIONS and the CALVIN CYCLE LIGHT REACTIONS - steps of photosynthesis that convert solar energy to chemical energy - light energy that is absorbed by chlorophyll drives a transfer of electrons and hydrogen from water to an acceptor called NADP+, which stores the electrons

  16. - water is split, and O2 is given off - NADP+ is related to NAD+ (in respiration) - light reactions use solar power to reduce NADP+ to NADPH by adding a pair of electrons and an H+ - the light reactions generate ATP by the addition of a phosphate to ADP  PHOSPHORYLATION

  17. - the light reactions produce no sugar CALVIN CYCLE - named for Melvin Calvin - incorporates CO2 from the air into organic molecules present in the chloroplast > CARBON FIXATION - then reduces fixed carbon to carbohydrate by the addition of electrons

  18. - reducing is provided by NADPH - Calvin cycle makes sugar, but can only do so with the NADPH and ATP made in the light reactions - sometimes called the dark reactions, or the light-independent reactions because steps do NOT require light directly

  19. The light reactions take place in the THYLAKOIDS The dark reactions take place in the STROMA

  20. LIGHT Light travels in waves of energy  WATER WAVELENGTH- distance between crests of electromagnetic waves - range from very small (gamma rays) to very large (radio waves) ELECTROMAGNETIC SPECTRUM- entire range of radiation VISIBLE LIGHT- radiation that is most important to life (380 to 750 nm) - detected as colors

  21. Sometimes light behaves like it is made up of particles - these are called PHOTONS - act like objects in that each has a fixed amount of energy - this energy is inversely related to the wavelength of the light (shorter wavelength = greater energy of photon) - visible light drives photosynthesis

  22. PIGMENTS PIGMENT- a substance that absorbs visible light - different pigments absorb different wavelengths of light - wavelengths that are absorbed disappear - wavelengths that are transmitted are the ones we see - if a pigment absorbs all colors, it appears black

  23. Wavelength can be measured with a SPECTROPHOTOMETER - directs beams of light through a solution of a pigment and measures the fraction of the light TRANSMITTED at each wavelength ABSORPTION SPECTRUM- a graph plotting a pigment’s light absorption

  24. http://www.biology.lsu.edu/introbio/tutorial/Spec/spectrophotometer.JPGhttp://www.biology.lsu.edu/introbio/tutorial/Spec/spectrophotometer.JPG

  25. - blue and red light work best for photosynthesis - green is least effective

  26. ACCESSORY PIGMENTS Chlorophyll a is not the only important pigment in chloroplasts - only chlorophyll a can participate directly in the light reactions - other pigments capture light energy and transfer it to chlorophyll a - CHLOROPHYLL b- slightly different structure gives it a different color: Chlorophyll a = blue-green Chlorophyll b = yellow-green

  27. Other accessory pigments such as CAROTENOIDS are various shades of yellow and orange - broaden the spectrum of colors used for photosynthesis - function in photoprotection- absorb excess light energy that would damage chlorophyll

  28. “EXCITING” CHLOROPHYLL When a pigment molecule absorbs a photon, one of the molecule’s electrons is elevated to a higher energy level - the pigment molecule is then said to be “excited” - only photons of specific wavelengths are absorbed - the photons are absorbed by clusters of pigment molecules in the thylakoid membrane

  29. The electron cannot remain in this excited state for long - it is unstable - usually, when pigments absorb light, their excited electrons drop to a lower energy level, releasing energy as heat * what makes the top of an automobile so hot on a sunny day

  30. PHOTOSYSTEMS Inside the thylakoid membrane, chlorophyll is organized along with proteins and other organic molecules into PHOTOSYSTEMS - has a light-gathering complex made up of chlorophyll a, chlorophyll b, and carotenoids - enables the photosystem to harvest more light over more of the spectrum

  31. When any pigment molecule absorbs a photon: - energy is transferred from pigment to pigment until it reaches a particular chlorophyll a - only this chlorophyll a molecule is located in the REACTION CENTER- where the first light-driven reaction of photosynthesis takes place

  32. Along with the chlorophyll a molecule in the reaction center is the PRIMARY ELECTRON ACCEPTOR - In a Redox reaction, chlorophyll a in the reaction center loses one of its electrons to the primary electron acceptor - this happens when light excites the electron to a higher energy level - the electron acceptor “catches” the excited electron before it can return to its ground state

  33. Each photosystem functions in the chloroplast as a light-harvesting unit - the transfer of electrons from chlorophyll to the primary electron center is the first step of the light reactions

  34. 2 PHOTOSYSTEMS There are 2 types of photosystems found in the thylakoid membrane: PHOTOSYSTEM I & PHOTOSYSTEM II - each has a unique reaction center - the reaction center of photosystem I is called P700 because it most effectively absorbs light of wavelength 700 nm

  35. - the reaction center of photosystem II is called P680 - the 2 pigments are identical chlorophyll a molecules, but they are associated with different proteins- affects light-absorbing properties NEXT: HOW THE 2 PHOTOSYSTEMS WORK TOGETHER

  36. NONCYCLIC ELECTRON FLOW The key to making NADPH and ATP (needed for dark reactions) is a flow of electrons through the photosystems and other molecules in the thylakoid membrane There are 2 possible routes for electron flow: Cyclic and Noncyclic

  37. STEPS OF NONCYCLIC ELECTRON FLOW - most common route 1. Photosystem II absorbs light and an electron moves to a higher energy level, becoming excited - this electron is captured by the primary electron acceptor - chlorophyll is oxidized; electron “hole” must be filled

  38. 2. An enzyme extracts electrons from water and supplies them to photosytem II, replacing the lost electrons - this reaction splits water into 2 H atoms and an O atom - the O immediately combines with another O to form O2 (released)

  39. 3. Each excited electron passes from the primary electron acceptor of photosystem II to photosystem I by an electron transport chain - very similar to the one in cellular respiration 4. As electrons “fall” down the chain the energy that they release is harnessed by the thylakoid membrane to make ATP - called photophosphorylation (specifically noncyclic photophosphorylation)

  40. - this ATP will provide chemical energy to make sugar in the dark reactions 5. When electrons reach the bottom of the electron transport chain, they fill a “hole” in the reaction center of photosystem I - this hole is created when light energy drives an electron to the primary acceptor of photosystem I

  41. 6. The primary electron acceptor of photosystem I gives the electrons to a second electron transport chain - this chain transmits electrons to ferredoxin (Fd), an iron-containing protein - an enzyme called NADP+ reductase transfers electrons from Fd to NADP+ - this becomes NADPH, the other molecule needed in the dark reactions

  42. CYCLIC ELECTRON FLOW In CYCLIC ELECTRON FLOW, photosystem I is used, but not photosystem II - the electrons cycle back from ferredoxin (Fd) to the cytochrome complex, and then to the reaction center of photosystem I - NADPH is not made, and oxygen is not released - ATP is still made!

  43. - this is called CYCLIC PHOTOPHOSPHORYLATION Why is cyclic electron flow needed? - Noncyclic electron flow makes equal amounts of NADPH and ATP - the Calvin cycle (dark reactions) use more ATP than NADPH - cyclic flow makes up the difference by only producing ATP

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