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Photosynthesis 2: The Calvin Cycle & Control Big Questions Why is the Calvin Cycle necessary? How do the products of the light reactions contribute to.

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Presentation on theme: "Photosynthesis 2: The Calvin Cycle & Control Big Questions Why is the Calvin Cycle necessary? How do the products of the light reactions contribute to."— Presentation transcript:

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2 Photosynthesis 2: The Calvin Cycle & Control

3 Big Questions Why is the Calvin Cycle necessary? How do the products of the light reactions contribute to the function of the Calvin cycle? Why have some plants had to adapt photosynthesis to the constraints of their environment?

4 The Calvin Cycle Whoops! Wrong Calvin… 1950s | 1961

5 Remember what it means to be a plant… Need to produce all organic molecules necessary for growth – carbohydrates, lipids, proteins, nucleic acids Need to store chemical energy (ATP) produced from light reactions – in a more stable form – that can be moved around plant – saved for a rainy day + water + energy → glucose + oxygen carbon dioxide 6CO 2 6H 2 O C 6 H 12 O 6 6O 2 light energy → +++

6 Light reactions Convert solar energy to chemical energy – ATP – NADPH What can we do now? → energy → reducing power → → build stuff !! photosynthesis ATP

7 How is that helpful? Want to make C 6 H 12 O 6 – synthesis – How? From what? What raw materials are available? CO 2 C 6 H 12 O 6 NADPH NADP reduces CO 2 carbon fixation

8 From CO 2 → C 6 H 12 O 6 CO 2 has very little chemical energy – fully oxidized C 6 H 12 O 6 contains a lot of chemical energy – highly reduced Synthesis = endergonic process – put in a lot of energy Reduction of CO 2 → C 6 H 12 O 6 proceeds in many small uphill steps – each catalyzed by a specific enzyme – using energy stored in ATP & NADPH

9 From Light reactions to Calvin cycle Calvin cycle – chloroplast stroma Need products of light reactions to drive synthesis reactions – ATP – NADPH stroma thylakoid ATP

10 Calvin cycle

11 glycolysis glucose C-C-C-C-C-C fructose-1,6bP P-C-C-C-C-C-C-P DHAP P-C-C-C G3P C-C-C-P pyruvate C-C-C ATP 2 ADP 2 ATP 4 ADP 4 NAD + 2 2 Remember G3P? glyceraldehyde 3-phosphate Photosynthesis

12 To G3P and Beyond! Glyceraldehyde-3-P – end product of Calvin cycle – energy rich 3 carbon sugar – “C3 photosynthesis” G3P is an important intermediate G3P → → glucose → → carbohydrates → → lipids → → phospholipids, fats, waxes → → amino acids → → proteins → → nucleic acids → → DNA, RNA To G3P and beyond!

13 AP Biology RuBisCo Enzyme which fixes carbon from air – ribulose bisphosphate carboxylase – the most important enzyme in the world! it makes life out of air! – definitely the most abundant enzyme I’m green with envy! It’s not easy being green!

14 Accounting The accounting is complicated – 3 turns of Calvin cycle = 1 G3P – 3 CO 2 → 1 G3P (3C) – 6 turns of Calvin cycle = 1 C 6 H 12 O 6 (6C) – 6 CO 2 → 1 C 6 H 12 O 6 (6C) – 18 ATP + 12 NADPH → 1 C 6 H 12 O 6 – any ATP left over from light reactions will be used elsewhere by the cell

15 starch, sucrose, cellulose & more 1C CO 2 Calvin cycle 5C RuBP 3C RuBisCo 1. Carbon fixation 2. Reduction 3. Regeneration of RuBP ribulose bisphosphate ribulose bisphosphate carboxylase 6 NADP 6 NADPH 6 ADP 6 ATP 3 ADP 3 ATP used to make glucose 3C G3P glyceraldehyde-3-P CCCCC CCCCC CCCCC 6C CCCCCC CCCCCC CCCCCC CCC CCC CCC CCC CCC CCC PGA phosphoglycerate CCC CCC CCC CCC CCC CCC CCC C C C CCC == | H | H | H | H | H | H CCC –– 5C

16 Light Reactions O2O2 H2OH2O Energy Building Reactions ATP  produces ATP  produces NADPH  releases O 2 as a waste product sunlight H2OH2O ATP O2O2 light energy → +++ NADPH

17 Calvin Cycle sugars CO 2 Sugar Building Reactions ADP  builds sugars  uses ATP & NADPH  recycles ADP & NADP  back to make more ATP & NADPH ATP NADPHNADP CO 2 C 6 H 12 O 6 → +++ NADPATP + NADPHADP

18 Putting it all together CO 2 H2OH2OC 6 H 12 O 6 O2O2 light energy → +++ Sugar Building Reactions Energy Building Reactions Plants make both:  energy  ATP & NADPH  sugars sunlight O2O2 H2OH2O sugars CO 2 ADP ATP NADPHNADP

19 Energy cycle

20 Summary of photosynthesis Where did the CO 2 come from? Where did the CO 2 go? Where did the H 2 O come from? Where did the H 2 O go? Where did the energy come from? What’s the energy used for? What will the C 6 H 12 O 6 be used for? Where did the O 2 come from? Where will the O 2 go? What else is involved…not listed in this equation? 6CO 2 6H 2 OC 6 H 12 O 6 6O 2 light energy → +++

21 Variations on the Theme

22 Leaf anatomy

23 Remember The Needs of Plants! Plants need to take in: water (from soil) nutrients (from soil) CO 2 (from atmosphere) Plants need to release: water vapor (through leaves) O 2 (through leaves)

24 Controlling water loss from leaves Hot or dry days – stomates close to conserve water – guard cells gain H 2 O = stomates open lose H 2 O = stomates close – adaptation to living on land, but… – creates PROBLEMS!

25 When stomates close… Closed stomates lead to… – O 2 build up → from light reactions – CO 2 is depleted → in Calvin cycle causes problems in Calvin Cycle The best laid schemes of mice and men… and plants!

26 Inefficiency of RuBisCo: CO 2 vs O 2 RuBisCo in Calvin cycle – carbon fixation enzyme normally bonds C to RuBP CO 2 is the optimal substrate reduction of RuBP building sugars – when O 2 concentration is high RuBisCo bonds O to RuBP O 2 is a competitive substrate oxidation of RuBP breakdown sugars photosynthesisphotorespiration

27 6C unstable intermediate 1C CO 2 Calvin cycle when CO 2 is abundant 5C RuBP 3C PGA ADP ATP 3C NADP NADPH ADP ATP G3P to make glucose 3C G3P 5C RuBisCo C3 plants

28 Calvin cycle when O 2 is high 5C RuBP 3C2C to mitochondria ––––––– lost as CO 2 without making ATP photorespiration O2O2 Hey Dude, are you high on oxygen! RuBisCo It’s so sad to see a good enzyme, go BAD!

29 Impact of Photorespiration Oxidation of RuBP – short circuit of Calvin cycle – loss of carbons to CO 2 can lose 50% of carbons fixed by Calvin cycle – reduces production of photosynthesis no ATP (energy) produced no C 6 H 12 O 6 (food) produced – if photorespiration could be reduced, plant would become 50% more efficient strong selection pressure to evolve alternative carbon fixation systems

30 Reducing photorespiration Separate carbon fixation from Calvin cycle – C4 plants PHYSICALLY separate carbon fixation from Calvin cycle – different cells fix carbon vs. where Calvin cycle occurs (different leaf structure) – PEP carboxylase – CAM plants TEMPORALLY separate carbon fixation from Calvin cycle fix carbon during night, Calvin cycle during day

31 C4 plants A better way to capture CO 2 – 1st step before Calvin cycle, fix carbon with enzyme PEP carboxylase store as 4C compound – adaptation to hot, dry climates have to close stomates a lot different leaf anatomy – sugar cane, corn, other grasses… sugar cane corn

32 AP Biology Comparative anatomy C3C4 Location, location,location! PHYSICALLY separate C fixation from Calvin cycle

33 C4 Leaf Biochemistry Up Close: Photosynthesis across 2 different cells.

34 CAM ( Crassulacean Acid Metabolism ) plants  Adaptation to hot, dry climates  separate carbon fixation from Calvin cycle by TIME  close stomates during day  open stomates during night  at night: open stomates & fix carbon in 4C “storage” compounds  in day: release CO 2 from 4C acids to Calvin cycle  increases concentration of CO 2 in cells  succulents, some cacti, pineapple It’s all in the timing!

35 CAM plants succulents cacti pineapple

36 CAM Plant Biochemistry: Photosynthesis at 2 times of day

37 C4 vs CAM Summary C4 plants separate 2 steps of C fixation anatomically in 2 different cells CAM plants separate 2 steps of C fixation temporally = 2 different times night vs. day solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge

38 Why the C3 problem? Possibly evolutionary baggage – Rubisco evolved in high CO 2 atmosphere there wasn’t strong selection against active site of Rubisco accepting both CO 2 & O 2 Today it makes a difference – 21% O 2 vs. 0.03% CO 2 – photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day – strong selection pressure to evolve better way to fix carbon & minimize photorespiration We’ve all got baggage!

39 Supporting a biosphere On global scale, photosynthesis is the most important process for the continuation of life on Earth – each year photosynthesis… captures 121 billion tons of CO 2 synthesizes 160 billion tons of carbohydrate – heterotrophs are dependent on plants as food source for fuel & raw materials

40 The poetic perspective… All the solid material of every plant was built by sunlight out of thin air All the solid material of every animal was built from plant material Then all the plants, cats, dogs, elephants & people … are really particles of air woven together by strands of sunlight! sun air

41 If plants can do it… You can learn it! Ask Questions!!

42 Review Questions

43 1.The final product of the Calvin Cycle is A.Carbon dioxide B.Fructose C.Glucose D.G3P E.Oxygen

44 2. Which of the following is true of the Calvin Cycle A.It is controlled by enzymes in the stroma B.It takes place in the thylakoid disks of the inner chloroplast membrane C.Carbon dioxide is a product D.It is an ATP-independent process E.One cycle consumes four molecules of PGAL

45 3.If a toxin was administered to a plant that prevented the action of ribulose bisphosphate carboxylase, which of the following steps of the Calvin cycle would be most directly affected? A.Regeneration of RUBP B.Donation of phosphates from ATP to Calvin cycle intermediary compounds C.The initial fixation of carbon dioxide D.Oxidation of NADPH E.Production of Glucose.

46 4. In an experiment studying photosynthesis performed during the day, you provide a plant with radioactive carbon ( 14 C) dioxide as a metabolic tracer. The 14 C is incorporated first into oxaloacetic acid. The plant is best characterized as a A.C 4 plant. B.C 3 plant. C.CAM plant. D.heterotroph. E.chemoautotroph.

47 The following questions refer to the following choices: A.C3 plants B.C4 plants C. CAM Plants A.All plants 5.Use a temporal separation to reduce photorespiration 6.Do not have any adaptations to reduce photorespiration 7.Carry out carbon fixation by rubisco 8.Use a spatial separation to reduce photorespiration 9.Carry out aerobic cellular respiration

48 10.Keeping It Straight! Compare aerobic cellular respiration to photosynthesis (you will have 10 minutes). Write down as many similarities and differences as you can think of. The person with the most wins a prize!


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