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Chapter 6

Chapter 6. Photosynthesis. HBIO – Tyska Holliston High School. THE DISCOVERY OF PHOTOSYNTHESIS. ___________________ : 1643 – After careful measurements of a plant’s water intake and mass increase, concludes that trees gain most of their mass from water

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Chapter 6

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  1. Chapter 6 Photosynthesis HBIO – Tyska Holliston High School

  2. THE DISCOVERY OF PHOTOSYNTHESIS • ___________________: 1643 – After careful measurements of a plant’s water intake and mass increase, concludes that trees gain most of their mass from water • ___________________:1771 – Using a bell jar, ca candle and a plant he finds that the plant releases oxygen • ___________________:1779 – finds that aquatic plants produce oxygen bubbles in the light but not in the dark; concludes that plants need sunlight to produce O2

  3. What is Photosynthesis? • ________% of the sun’s energy is captured by photosynthesis • Plants convert this energy into the energy found in organic compounds Photosynthesis involves a complex _____________________________________ in which the product of one reaction is consumed in the next reaction.

  4. 6-1 Capturing The Energy in Light What is energy? • ____________________________. Where does the energy that living things need come from? • ____________________: organisms that use light energy from the sun to make their own food (plants, bacteria, algae) • ____________________: obtain energy from foods they consume

  5. Cellular Respiration • Once the energy is stored in organic compounds (carbohydrates), cells can release this energy through another complex biochemical pathway called __________________________ • The products from photosynthesis (____________ & _____________) are the reactants used in cellular respiration. • The products from cellular respiration (_____________ & _____________) are reactants for photosynthesis

  6. The Stages of Photosynthesis There are two sets of reactions called: • _________________________: sunlight energy is absorbed + water  converted to make energy carriers ______________& _______________; ____________ is released • DARK REACTIONS (aka _____________________________): CO2 + ATP + NADPH ____________

  7. 2 Sets of Reactions in PHOTOSYNTHESIS SUN LIGHT Stroma Thylakoid membrane LIGHT REACTIONS CALVIN CYCLE NADP+, ADP CHLOROPLAST

  8. The Reactions of Photosynthesis leaf’s upper epidermis Photosynthetic Cells Chloroplast

  9. CHLOROPLAST thylakoid compartment LUMEN two outer membranes thylakoid membrane system inside stroma stroma

  10. THE OVERALL EQUATION FOR PHOTOSYNTHESIS light energy 6 H2O + 6CO2 6O2 + C2H12O6 Water Carbon Dioxide Oxygen Glucose enzymes

  11. LIGHT: waves of energy that travel through space • Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy • Visible light: appears white but is composed of an array of colors called the _______________________________

  12. LIGHT & PIGMENTS shortest wavelengths (most energetic) longest wavelengths (lowest energy) range of most radiation reaching Earth’s surface range of heat escaping from Earth’s surface gamma rays x rays ultraviolet radiation near-infrared radiation infrared radiation radio waves microwaves VISIBLE LIGHT 400 450 500 550 600 650 700 Wavelengths of light (nanometers) _____________: mixture of different wavelengths of light ______________________: molecules that absorbs the sun’s light; by absorbing certain colors, a pigment subtracts those colors from the visible spectrum…the light reflected is the color seen

  13. Chloroplast Pigments • main light-absorbing chemical in plants • Two main types that differ slightly in structure: • ______________________:absorbs less blue, more red, reflects green; involved in light reactions • _______________________: absorbs more blue, less red, reflects yellow-green; assists chlorophyll a in capturing light energy…considered an ACCESSORY PIGMENT

  14. Chlorophylls Main pigments in most photoautotrophs ABSORPTION SPECTRUM FOR CHLOROPHYLL a & b chlorophyll a Wavelength absorption (%) chlorophyll b Wavelength (nanometers)

  15. MOLECULAR STRUCTURE OF PIGMENTS • Light-catching part of molecule often has alternating single and double bonds • These bonds contain ____________________________ that are capable of being moved to higher energy levels by absorbing light

  16. Accessory Pigments What are they?Pigments also found in thylakoid membrane that absorb light in regions of spectrum where chlorophyll does not • ________________________________: reflect yellow, orange, browns • Anthocyanins: reflect red & violet • Phycobilins: reflects red

  17. Accessory Pigments Carotenoids, Phycobilins, Anthocyanins beta-carotene phycoerythrin (a phycobilin) percent of wavelengths absorbed wavelengths (nanometers)

  18. Why do leaves change color in the fall? • Chlorophyll is continuously made by bright green leaves during spring and summer. They are much more abundant so they mask the accessory pigments. • In the fall, chlorophyll synthesis slows down so accessory pigments show through giving the red, yellow & orange colors!

  19. Light-Dependent Reactions • PURPOSE: to take in sunlight energy + H2O and make _______________ + ___________________ (and O2 as a waste product) • LIGHT ABSORPTION: light hits pigments at _________________ & ____________________ this energy excites electrons  high energy electrons are transferred to _________________________________________(ETC)

  20. PHOTOSYSTEMS What are they? • Clusters of pigments embedded in the thylakoid membrane • PHOTOSYTEM I (PS I) • PHOTOSYSTEM II (PSII) What do they do? • Both involved in the light reactions by absorbing light energy • Absorbed energy excites electrons and passes these high-energy electrons to chlorophyll molecules

  21. ELECTRON TRANSPORT PHOTOSYSTEM II PHOTOSYSTEM I thylakoid compartment Primary electron acceptor Primary electron acceptor stroma Step 1: _____________ absorbs light energy  electrons in chlorophyll a energized Step 2: chlorophyll a is oxidized (loses high energy e-)  the __________________________________ is reduced (gains high energy e-)

  22. Step 3: Primary electron acceptor donates e- to _______________________________ (ETC) e- transferred from molecule to molecule  energy is used along the way to pump __________________ (__________ ions) into thylakoid lumen

  23. 5 Steps in the Light-Dependent Reactions Primary electron acceptor PHOTOSYSTEM I PHOTOSYSTEM II Primary electron acceptor sunlight sunlight STEP 2 STEP 4 STEP 1 STEP 3 H+ NADPH e- e- e- e- e- e- NADP + + H+ e- thylakoid compartment H2O H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O2 H+ Incr’d concentration of H+ thylakoid membrane Step 4: simultaneously, _______________ (PSI) absorbs light  re-energizes those electrons from PSII  high energy e- move to another primary electron acceptor ADP + Pi ATP stroma ATP Synthase H+

  24. 5 Steps in the Light-Dependent Reactions Primary electron acceptor PHOTOSYSTEM I PHOTOSYSTEM II Primary electron acceptor sunlight sunlight STEP 2 STEP 4 STEP 1 STEP 3 STEP 5 H+ NADPH e- e- e- e- e- e- NADP + + H+ e- thylakoid compartment H2O H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O2 H+ Incr’d concentration of H+ thylakoid membrane Step 5: primary electron acceptor from PSI donate e- to stroma  combine with H+ and NADP+  forms __________ What is NADP+?A molecule that will pick up high-energy e- + protons to form NADPH, an electron/energy carrier that delivers energy to chemical reactions ADP + Pi ATP stroma ATP Synthase H+

  25. RESTORING PHOTOSYSTEM II • Water molecules replace the electrons energized and passed on in PSII • A water-splitting enzyme breaks down water into protons, electrons, and oxygen: 2 H2O  _____ + _____ + ______

  26. RESTORING PHOTOSYSTEM II stroma H+ e- e- e- e- e- e- e- H+ H+ thylakoid compartment H2O H+ H+ H+ H+ H+ H+ H+ H+ H+ O2 thylakoid membrane • WATER IS _____________! • _____________ go to PSII • _____________ stay in thylakoid compartment • _____________ released into atmosphere

  27. CHEMIOSMOSIS: process of making ATP by the movement of protons across the thylakoid membrane ATP FORMATION in the Light-Dependent Reactions Stored potential energy _______________: a membrane carrier protein & an enzyme that adds phosphate to ADP forming ATP; reaction is driven by the energy from protons diffusing out to stroma ATP SYNTHASE _________ ADP + Pi chemical energy

  28. 5 Steps in the Light-Dependent Reactions Primary electron acceptor PHOTOSYSTEM I PHOTOSYSTEM II Primary electron acceptor sunlight sunlight STEP 2 STEP 4 STEP 1 STEP 3 STEP 5 H+ NADPH e- e- e- e- e- e- NADP + + H+ e- thylakoid compartment H2O H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O2 H+ Incr’d concentration of H+ thylakoid membrane ADP + Pi ATP stroma ATP Synthase CHEMIOSMOSIS H+

  29. REVIEW OF LIGHT-DEPENDENT REACTIONS H2O SECOND ETC e– e– ATP SYNTHASE NADPH FIRST ETC NADP+ ATP PHOTOSYSTEM II PHOTOSYSTEM I ADP + Pi

  30. Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP ADP + Pi LIGHT-DEPENDENT REACTIONS The CALVIN CYCLE NADPH NADP+ glucose P oxygen release new water

  31. 6-2 THE CALVIN CYCLE • Second set of reactions in photosynthesis • Takes place in the ___________________ • Uses _______________ + _______________ produced in light reactions & _____________ to produce___________ • Does not need light - can proceed in the dark • Named after Melvin Calvin – American (1911-1997) • Aka – light-independent or dark rxns Calvin - Won the Nobel Prize for his work on Photosynthesis

  32. Light-Independent Reactions THESE REACTIONS PROCEED IN THE CHLOROPLAST’S STROMA

  33. Overall reactants _____________ _____________ _____________ Overall products _____________ _____________ _____________ Calvin Cycle • Reaction pathway is cyclic • 3 main steps

  34. 6 CO2 (from the air) Carbon Fixation by the Calvin Cycle CARBON FIXATION STEP (1) CARBON FIXATION – ______________ is incorporated into (5-C)_____________(ribulose biphosphate) to form a 6-C molecule that splits into two 3-C molecules called _______________(phosphoglyceric acid) 6 RuBP unstable intermediate 12 PGA

  35. 6 CO2 (from the air) STEP (2) PGA is converted to PGAL – energy from _____________ and ___________ used to form _________________ (phosphate glyceraldehyde) ADP & NADP+ & P+ is formed which can be re-used in the light reactions CARBON FIXATION 6 RuBP unstable intermediate 12 PGA 12 ATP 12 NADPH 12 ADP 12 Pi 12 NADP+ 12 PGAL Products re-used in light rxns

  36. 6 CO2 (from the air) Calvin Cycle CARBON FIXATION 6 RuBP unstable intermediate STEP (3) PGAL forms ___________! This glucose can be converted to other organic compounds (proteins, lipids, carbohydrates) used by the plant 12 PGA 12 ATP 12 NADPH 12 ADP 12 Pi 12 NADP+ 12 PGAL 2 PGAL Pi glucose P

  37. CO2 (from the air) 6 Calvin Cycle CARBON FIXATION 6 6 RuBP unstable intermediate STEP (4) REGENERATION OF ____________ – energy from ATP is used rearrange most of the PGAL to form RuBP so that Calvin Cycle can continue to operate 12 PGA 6 ADP 12 ATP ATP 6 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL 2 PGAL Pi P glucose

  38. 6CO2 ATP Calvin- Benson Cycle 12 PGA 6 RuBP 12 6 ADP 12 ADP + 12 Pi Calvin-Benson cycle ATP 12 NADPH 4 Pi 12 NADP+ 12 PGAL 10 PGAL 1 Pi phosphorylated glucose 1

  39. General EquationPhotosynthesis ________ + _________ + light energy  _________ + ___________ • Water is split during light reactions • Yielding electrons, protons and oxygen as a byproduct • CH2O represents general formula for a carbohydrate; it is replaced in the equation by glucose (C6H12O6)

  40. OVERALL EQUATION FOR PHOTOSYNTHESIS light energy 6O2 + C2H12O6 6 H2O + 6CO2 Water Oxygen Carbon Dioxide Glucose enzymes Glucose in not always the actual product made from photosynthesis. It is used to emphasize the relationship between photosynthesis and cellular respiration. p.111

  41. sunlight Light- Dependent Reactions 12H2O 6O2 ATP ADP + Pi NADP+ NADPH Summary of Photosynthesis 6CO2 Calvin-Benson cycle 6 RuBP 12 PGAL Light- Independent Reactions 6H2O phosphorylated glucose end products (e.g., sucrose, starch, cellulose)

  42. ALTERNATIVE PATHWAYS • ________________: majority of plants that fix carbon through the Calvin Cycle (named from the 3-C PGA that is initially made) What about plants found in hot, dry climates? • Rapidly lose H2O through __________________ (small pores on leaves)  able to partially close stomata to reduce water loss • If stomata closed  less CO2 taken in and fixed + more O2 retained in plants  Calvin Cycle is inhibited!

  43. STOMATA Small pores located on the undersurface of leaves. OPEN: Water, CO2 and other gases can pass through and leave a plant CLOSED: Passage is restricted - Level of CO2 falls (Consumed in Calvin Cycle) Level of O2 rises - Light reactions split water molecules

  44. ALTERNATIVE PATHWAYS • ______________________: allows plants to fix CO2 when CO2 level is low and O2 level is high  produces a 4-C product (ex) corn, sugar cane, and crabgrass; lose ½ as much water • ______________________: plants in hot, dry climates that open stomata at night, close them during the day

  45. The CAM Pathway • Plants that use this pathway open their stomata at night when temperature is lower and close them during the day – opposite of other plants. • ______________: CAM plants take in CO2 and fix it into a variety of organic compounds. • _______________: CO2 is released from compounds and enters the Calvin cycle. • CAM plants grow slower • Ex.) Cactuses, pineapples, yuccas, some lilies and some orchids

  46. C4 Pathway • Certain plants (C4) fix CO2 into four-carbon compounds (oxaloacetate) • Use an enzyme (PEP carboxylase) to fix carbon • Compounds are then transported to the Calvin cycle

  47. Environmental Factors affectingRate of Photosynthesis • _____________________: • Light Intensity^, Rate of Photosynthesis^ up to a point where it levels off; all of the available electrons are excited. • _____________________: • Increasing levels of CO2 around a plant will stimulate and increase the rate of photosynthesis up to a plateau. • _____________________: • An increase in temp. accelerates the chemical reactions over a range; at a point it peaks and enzymes become unstable and ineffective. Stomata begin to close limiting water and CO2 entry into leaves; rate begins to decrease.

  48. Rate of Photosynthesis

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