Photosynthesis 2 The Calvin Cycle Control Big Questions

Photosynthesis 2: The Calvin Cycle & Control

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?

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 carbon + water + energy →glucose + oxygen dioxide 6 CO 2 + 6 H 2 O + light → C 6 H 12 O 6 + 6 O 2 energy

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

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

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

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

Calvin cycle

The Calvin Cycle • Each turn of the Calvin cycle fixes one carbon. • For the net synthesis of one G 3 P molecule, the cycle must take place three times, fixing three molecules of CO 2. • To make one glucose molecules would require six cycles and the fixation of six CO 2 molecules.

Part 1 of the Calvin Cycle • In the carbon fixation phase, each CO 2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (Ru. BP). – This is catalyzed by Ru. BP carboxylase or rubisco (an enzyme). – The six-carbon intermediate splits in half to form two molecules of 3 -phosphoglycerate per CO 2.

Part 1

Part 2 of the Calvin Cycle • During reduction (the addition of electrons to a substance), each 3 -phosphoglycerate receives another phosphate group from ATP to form 1, 3 bisphoglycerate. • A pair of electrons from NADPH reduces each 1, 3 bisphoglycerate to G 3 P.

Part 2

Part 3 of the Calvin Cycle • In the last phase, regeneration of the CO 2 acceptor (Ru. BP), these five G 3 P molecules are rearranged to form 3 Ru. BP molecules. • To do this, the cycle must spend three more molecules of ATP (one per Ru. BP) to complete the cycle and prepare for the next.

Remember G 3 P? glycolysis glucose C-C-C-C 2 ATP 2 ADP fructose-1, 6 b. P P-C-C-C-P DHAP P-C-C-C G 3 P glyceraldehyde 3 -phosphate C-C-C-P 2 NAD+ 2 4 ADP Photosynthesis pyruvate C-C-C 4 ATP

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

Ru. Bis. Co • 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! AP Biology It’s not easy being green!

Accounting • The accounting is complicated – 3 turns of Calvin cycle = 1 G 3 P – 3 CO 2 → 1 G 3 P (3 C) – 6 turns of Calvin cycle = 1 C 6 H 12 O 6 (6 C) – 6 CO 2 → 1 C 6 H 12 O 6 (6 C) – 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

Light Reactions H 2 O + light → ATP + NADPH + O 2 energy H 2 O sunlight Energy Building Reactions NADPH ATP O 2 § produces ATP § produces NADPH § releases O 2 as a waste product

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

Putting it all together light CO 2 + H 2 O + energy → C 6 H 12 O 6 + O 2 H 2 O CO 2 sunlight ADP Energy NADP Building Reactions Sugar Building Reactions NADPH ATP O 2 sugars Plants make both: §energy §ATP & NADPH §sugars

Summary of photosynthesis 6 CO 2 + 6 H 2 O + light → C 6 H 12 O 6 + 6 O 2 energy • • • 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?

Leaf anatomy

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)

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!

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

Inefficiency of Ru. Bis. Co: CO 2 vs O 2 • Ru. Bis. Co in Calvin cycle – carbon fixation enzyme • normally bonds C to Ru. BP • CO 2 is the optimal substrate • reduction of Ru. BP • building sugars – when O 2 concentration is high • Ru. Bis. Co bonds O to Ru. BP • O 2 is a competitive substrate • oxidation of Ru. BP • breakdown sugars photosynthesis photorespiration

Calvin cycle when CO 2 is abundant 1 C ATP Ru. BP ADP G 3 P to make glucose CO 2 5 C Ru. Bis. Co 6 C unstable intermediate 5 C G 3 P 3 C C 3 plants 3 C PGA NADPH ATP NADP 3 C

Calvin cycle when O 2 is high to mitochondria ––––––– lost as CO 2 without making ATP O 2 Ru. BP Hey Dude, are you high on oxygen! It’s so sad to see a good enzyme, go BAD! 5 C Ru. Bis. Co 2 C 3 C photorespiration

Photorespiration • In most plants initial fixation of CO 2 occurs via rubisco and results in a three-carbon compound, 3 phosphoglycerate. • When their stomata are closed on a hot, dry day, CO 2 levels drop as CO 2 is consumed in the Calvin cycle. • At the same time, O 2 levels rise as the light reaction converts light to chemical energy. • While rubisco normally accepts CO 2, when the O 2/CO 2 ratio increases (on a hot, dry day with closed stomata), rubisco can add O 2 to Ru. BP.

• When rubisco adds O 2 to Ru. BP, Ru. BP 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 CO 2 by mitochondria and peroxisomes. – Unlike normal respiration, this process produces no ATP, nor additional organic molecules. • Photorespiration decreases photosynthetic output by taking organic material from the Calvin cycle. • 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.

Impact of Photorespiration • Oxidation of Ru. BP – 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

Reducing photorespiration • Separate carbon fixation from Calvin cycle – C 4 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

Working of the C 4 • After entering through stomata, CO 2 diffuses into a mesophyll cell. • Instead the CO 2 is inserted into a 3 -carbon compound (C 3) called phosphoenolpyruvic acid (PEP) forming the 4 -carbon compound oxaloacetic acid (C 4). • Oxaloacetic acid is converted into malic acid or aspartic acid (both have 4 carbons), which is transported (by plasmodesmata) into a bundle sheath cell. – Bundle sheath cells are deep in the leaf so atmospheric oxygen cannot diffuse easily to them;

• These features keep oxygen levels low. • Here the 4 -carbon compound is broken down into carbon dioxide, which enters the Calvin cycle to form sugars and starch.

Comparative anatomy Location, location! C 3 C 4 PHYSICALLY separate C fixation from Calvin cycle AP Biology

C 4 Leaf Biochemistry Up Close: Photosynthesis across 2 different cells.

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

CAM plants cacti succulents pineapple

CAM Plant Biochemistry: Photosynthesis at 2 times of day

C 4 vs CAM Summary solves CO 2 / O 2 gas exchange vs. H 2 O loss challenge C 4 plants CAM plants separate 2 steps of C fixation anatomically in 2 different cells separate 2 steps of C fixation temporally = 2 different times night vs. day

Why the C 3 problem? We’ve all got baggage! • 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

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

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 air sun Then all the plants, cats, dogs, elephants & people … are really particles of air woven together by strands of sunlight!

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

Concept Check 1. What is photorespiration? 2. What environmental conditions favor photorespiration? 3. Why do these conditions favor photorespiration? 4. How does a C 4 plant solve the photorespiration problem? 5. How does a CAM plant solve the photorespiration problem?

Review Questions

1. The final product of the Calvin Cycle is A. B. C. D. E. Carbon dioxide Fructose Glucose G 3 P Oxygen

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

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.

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. B. C. D. E. C 4 plant. C 3 plant. CAM plant. heterotroph. chemoautotroph.

The following questions refer to the following choices: A. C 3 plants B. C 4 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

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!