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AP Biology 6CO2 + 6H2O → C6H12O6 + 6O2

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Presentation on theme: "AP Biology 6CO2 + 6H2O → C6H12O6 + 6O2"— Presentation transcript:

1 AP Biology 6CO2 + 6H2O → C6H12O6 + 6O2
Photosynthesis: Reaction is the opposite of glycolysis 6CO2 + 6H2O → C6H12O6 + 6O2

2 Photosynthesis in nature
Autotrophs: biotic producers; photoautotrophs; chemoautotrophs; obtains organic food without eating other organisms Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by-products (includes decomposers) Copy table from textbook on autotrophs/heterotrophs – pg 539

3 The chloroplast Sites of photosynthesis
Occurs in specialized plant cell (mesophyll) Gas is exchanged in openings called stomata Chloroplast is organelle responsible for photosynthess Double membrane Contains thylakoids, grana, stroma Light is absorbed in pigments called chlorophyll

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5 Photosynthesis 2 Reactions Light Reactions Calvin Cycle

6 Photosynthesis Light Reactions – Light energy is converted to chemical energy to split hydrogen from water. As the name implies, you MUST have light for this reaction Light is first absorbed by the chloroplasts Takes place in the grana of the chloroplasts (the coin-like stacks of sacs thylakoids).

7 Light absorption Chloroplasts only absorb a portion of the sun
Sunlight is composed of white light which contains all the colors (example: prism) The chloroplasts contain pigments that only absorb certain “colors” of light Chlorophyll a: absorbs mostly “red” light Chlorophyll does not really absorb much “green” light so this light is reflected (why many plants appear green) Carotenoids: absorb mostly “yellow”, “orange” and “brown” light In the fall, many leaves lose their carotenoids and that is why they take on the fall colors because that light is reflected.

8 Purpose of the light reaction
The purpose of the light reaction is to capture light energy and then convert it to usable chemical energy Chemical energy is stored in the ATP molecule and in NADPH molecules As a result of the light reaction, O2 is released

9 PURPOSE OF CALVIN CYCLE (C3 PATHWAY)
Calvin cycle is the process by which atmospheric CO2 is taken in by the plant and utilized to make the high energy glucose molecule In order to produce glucose, CO2 must be incorporated into an organic compound, carbon fixation In the first step, CO2 diffuses into the stroma (again, inside the chloroplast) from the surrounding cytosol of the cell The carbon dioxide then goes through a series of “cycles” to create organic compounds

10 Photosynthesis: an overview
Redox process H2O is split, e- (along w/ H+) are transferred to CO2, reducing it to sugar Light reactions: Uses 2 photosystems Creates ATP and NADPH Calvin cycle: Fixes CO2 to create larger (energy rich) organic compounds Uses ATP and NADPH from light reactions

11 Light energy to chemical energy
To convert the sunlight to chemical energy (usable by the plant and other organisms), a redox reaction must occur in two: photosystems : a cluster of pigment molecules and proteins Photosystem I – where electrons from photosystem II continue to move electrons to reduce a molecule Photosystem II – where the light energy is initially used to oxidize a molecule (release an electron)

12 Photosystems Light harvesting units of the thylakoid membrane
Composed mainly of protein and pigment antenna complexes Antenna pigment molecules are struck by photons Energy is passed to reaction centers (redox location) Excited e- from chlorophyll is trapped by a primary e- acceptor

13 Photosystem II Photosystem II (P680):
photons excite chlorophyll, e- to an acceptor e- are replaced by splitting of H2O (release of O2) e-’s travel to Photosystem II down an electron transport chain (cytochromes) as e- fall, ADP ---> ATP (photophosphorylation)

14 START WITH PHOTOSYSTEM II
Accessory pigments absorb light energy and transfer it to chlorophyll a molecules As a result of the light energy, electrons are released from the chlorophyll a molecule in an oxidation reaction The free electrons from the chlorophyll a molecule are then “accepted” by a protein called a primary electron acceptor which reduces the molecule The electrons then move from one molecule to another in a series of events called the electron transport chain Think of this as a water wheel with the electrons being the water and it fuels a pump

15 Photosystem II The electron transport chain uses its energy to pump H+ into the thylakoid membrane This creates a high concentration of H+ inside the thylakoid Think of adding more and more air to a balloon. The more you add, the more pressure is inside the balloon. This “pressure” created by a high concentration of H+ is used to create ATP

16 Photosystem II ADP + Phosphate + Energy  ATP
The high pressure created by the H+ pump is “released” through a process called chemiosmosis An enzyme, called ATP synthase, is coupled to the release of the H+ ions This enzyme uses the energy from the H+ pump to add a phosphate molecule to ADP to form the energy rich molecule ATP ADP + Phosphate + Energy  ATP

17 Photosystem II 2H2O  4H+ + 4e- + O2
Electrons from photosystem II can add to electrons used in photosystem I (kind of a reserve supply) If the electrons from photosystem II run out, the light reaction stops NOTE: another source of e-: there is an enzyme in the thylakoid membrane that splits water molecules to provide electrons for the electron transport chain and H+ for chemiosmosis 2H2O  4H+ + 4e- + O2 Note: This is how a plant releases O2 so we can breathe

18 Photosystem I Photosystem I (P700):
‘fallen’ e- replace excited e- to primary e- acceptor 2nd ETC (NADP+ reductase) transfers e- to NADP+  NADPH (...to Calvin cycle…) Photosystems produce equal amounts of ATP and NADPH

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20 Photosystem I Accessory pigments absorb light energy and transfer it to chlorophyll a molecule (just like in photosystem II) simultaneously with photosystem II As a result of the light energy, electrons are released from the chlorophyll a molecule in an oxidation reaction The free electrons from the chlorophyll a molecule are then “accepted” by a protein called a primary electron acceptor which reduces the molecule NOTE: So far this is just like photosystem II (it’s the same process)

21 Photosystem I The electrons then move from one molecule to another in a second electron transport chain This electron transport chain ends on the side of the thylakoid membrane that faces the stroma (a solution that surrounds the grana) At this point the electron is used to combine with H+ and NADP+ to form NADPH (a high energy molecule) NADP+ + H+ + 2e- + Energy NADPH (reduction) NADPH is then used in the Calvin cycle

22 Summary Light is absorbed by pigments in the grana of the thylakoid (inside a chloroplast) The pigment chlorophyll releases electrons to move through an electron transport chain Photosystem II creates a H+ pump that creates an uneven distribution of H+ on each side of the thylakoid membrane ATP synthase uses the “pressure” create from this H+ pump to create ATP Photosystem I uses the electron transport to combine with H+ and NADP+ to form NADPH

23 Calvin cycle Calvin Cycle – ATP and NADPH from the light reactions are used along with CO2 to form a simple organic compound (made of carbon) Takes place in the stroma of the chloroplasts (the liquid filling). Byproducts are C6H12O6 (glucose, sort of), ADP, and NADP+ (which return to the light reactions).

24 The Calvin cycle 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P) Phases: 1- Carbon fixation~ each CO2 is attached to RuBP (rubisco enzyme) 2- Reduction~ electrons from NADPH reduces to G3P; ATP used up 3- Regeneration~ G3P rearranged to RuBP; ATP used; cycle continues

25 The Calvin cycle With the aid of an enzyme, CO2 is combined with a 5-C molecule called ribulose-bisphosphate (RuBP) to form 2 molecules called 3-phosphoglycerate (3- PGA) C organic compound CO2 + RuBP  2 3-PGA The 3-PGA is converted to another molecule called glyceraldehyde 3-phospate (G3P) 2 3-PGA  2 G3P (3 carbon organic compound) This process uses up one ATP and one NADPH created in the light cycle

26 The Calvin cycle One molecule of G3P leaves the Calvin cycle and is used to make organic compounds (carbohydrates) The other molecule of G3P remains in the Calvin cycle to make more RuBP Summary: Now the CO2 is fixed into an organic compound that can be used by the cell to make either energy or structural molecules

27 Calvin Cycle IMPORTANT NOTE: Glucose is not directly created by photosynthesis The G3P molecule is the one created This acts as a precursor to MANY organic molecules Glucose is emphasized because it is most important to us Breakdown of photosynthesis: 6CO2 + 6H2O + energy  C6H12O6 + 6O2 H2O is used in the light cycle to provide electrons for electron transport chain CO2 is absorbed from atmosphere for Calvin cycle O2 is produced as a by-product in light cycle Glucose is eventually created from G3P molecules

28 Calvin Cycle, net synthesis
For each G3P (and for 3 CO2) Consumption of 9 ATP’s & 6 NADPH (light reactions regenerate these molecules) G3P can then be used by the plant to make glucose and other organic compounds

29 O2 H2O ATP CO2 CHLOROPLAST Sunlight NADP+ ADP NADPH Glucose
Photosystem I & II H2O ATP NADP+ ADP NADPH CO2 Calvin cycle Glucose CHLOROPLAST

30 Cyclic electron flow Alternative cycle when ATP is deficient
Photosystem II used but not I; produces ATP but no NADPH Why? The Calvin cycle consumes more ATP than NADPH……. Cyclic photophosphorylation

31 Alternative carbon fixation methods, I
Photorespiration: hot/dry days; stomata close; CO2 decrease, O2 increase in leaves; O2 added to rubisco; no ATP or food generated Two Solutions….. 1- C4 plants: 2 photosynthetic cells, bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll; new 4C molecule releases CO2 (grasses)

32 Alternative carbon fixation methods, II
2- CAM plants: open stomata during night, close during day (crassulacean acid metabolism); cacti, pineapples, etc.

33 Factors that affect photosynthesis
Light intensity In general, the higher the light intensity, the faster the light cycle Eventually all electrons are being used and you hit a maximum rate for photosynthesis Carbon dioxide levels Works like light intensity: increased CO2 levels increase the rate of photosynthesis When the maximum level of CO2 is used, the rate does not increase further Temperature Increasing temperature increases the rate of reactions and photosynthesis At a point, the heat denatures (or breaks apart) the enzymes needed for photosynthesis and the rate decreases

34 A review of photosynthesis

35 CHLOROPLAST MITOCHONDRION Sunlight Heat O2 ATP H2O ATP NAD+ NADH CO2
Photo- system II Photo- System I Electron Transport System ATP H2O ATP NADP+ ADP NADPH NAD+ NADH CO2 Calvin Citric Acid Cycle ATP Glycolysis Cycle ATP Glucose Pyruvate CHLOROPLAST MITOCHONDRION


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