Chapter Six: Photosynthesis 6-1 Capturing the Energy in Light 6-2 The Calvin Cycle

6-1 Capturing The Energy in Light I. Energy for Life Processes Solar NRG drives reactions of LIFE (no SUN, no NRG, no LIFE). (1) A Biochemical Pathway (e.g., photosynthesis—LIGHT and DARK) PRODUCT of 1 reaction is USED in next reaction.

Critical Thinking (1) A famous scientist once said that wherever in the universe life exists, some of those life-forms MUST be colored. Why would the scientist make such a statement?

PHOTOSYNTHESIS (2 STAGES, NOTE ON SIDE) (1) LIGHT RXNS: NRG captured from SUN; WATER split into (H+), (e-), and (O2). (2) Light NRG into Chemical NRG stored in ATP and NADPH. (3) CALVIN CYCLE: ATP and NADPH drive production of carbs using CO2.

II. Light Absorption in Chloroplasts (i.e., LIGHT rxns) Begins inside THYLAKOID, stacked to form GRANUM within chloroplast.

(2) Granum (10-12 per chloroplast) A stack of layered THYLAKOIDS. (3) Stroma (gelatinous matrix) Solution SURROUNDING the THYLAKOIDS.

(A) Light and Pigments Qu: HOW does a chloroplast absorb LIGHT?

(1) Visible Spectrum (ROYGBIV) WHITE light can be BROKEN down into a RANGE of wavelengths (colors). (2) Wavelength (distance between crests) Different colors = different wavelengths. (3) Pigments (clustered into PHOTOSYSTEMS) Absorb CERTAIN colors (wavelengths) more strongly than others.

(B) Chloroplast Pigments (different TYPES of pigments in plant cells) Absorb ORANGE-RED and BLUE-VIOLET colors BEST. (1) Chlorophylls (“a” & “b”) Pigments clumped into TWO PHOTOSYTEMS (PS I and PS II).

(2) Accessory Pigments (NOT directly involved in LIGHT rxns) HELP by capturing OTHER wavelengths-colors that chlorophylls cannot.

(3) Carotenoids (yellow, orange, and brown pigments) Absorb wavelengths of BLUE and GREEN light, NOT absorbed by chlorophyll.

III. Electron Transport Pigments grouped into CLUSTERS in THYLAKOID MEMBRANE. (NOTE: Each cluster of pigments is grouped into PS I or PS II)

LIGHT reactions BEGIN when accessory pigments in BOTH photosystems absorb sunlight, acquiring some ENERGY. In each photosystem, acquired energy is PASSED quickly to other pigments until finally a specific pair of chlorophyll a molecules are reached.

The BIOCHEMICAL pathway following this point to making NADPH can be summarized in FIVE steps.

(1) Step One… Sunlight energy forces e- to become energized in the chlorophyll a molecules of Photosystem II.

(2) Step Two… Excited e- leave chlorophyll a (oxidation), so reduction must follow  e- are next accepted by a PEA in thylakoid membrane (Primary e- Acceptor).

(3) Step Three… PEA donates e- to the ETC located in thylakoid membrane (ETC = Electron Transport Chain). As the e- pass THROUGH the transport chain, they RELEASE energy, of which gets USED to pump protons (H+) INSIDE the THYLAKOID.

(4) Step Four… At same time light is absorbed by PS II, light is also absorbed by PS I. Electrons move from chlorophyll a in photosystem I to another PEA. The e- that are LOST by these chlorophyll a are REPLACED by e- that have passed through the ETC from PS II.

(5) Step Five… PEA of PS I donates e- to A 2ND ETC. This chain brings e- to the side of thylakoid membrane that faces the STROMA. There the e- combine with a proton (H+) and NADP+ (NOTE: NADP+ accepts e-  this RXN causes NADP+ to become NADPH)

(A) Restoring Photosystem II In Step 4, e- from chlorophyll in PS II replaced the e- that LEAVES chlorophyll in PS I. If the e- from photosystem II were NOT replaced, both e- transport chains would STOP, preventing photosynthesis (stopping NRG flow). Replacement e- are provided by H2O molecules.

Enzyme inside thylakoid SPLITS the water into H+, e-, and oxygen as follows: 2H2O  4H+ + 4e- + O2 Splitting of water releases e- which replaces the e- that leaves PS II when it is illuminated. Protons (H+) made are left INSIDE the thylakoid, while oxygen diffuses OUT of the chloroplast, LEAVING the plant.

Critical Thinking (2) Explain how the LIGHT reactions would be affected if there were NO concentration gradient of protons across the thylakoid membrane?

IV. Chemiosmosis H+ is PUMPED OUT to STROMA  releases NRG!,  used to make ATP. NOTE: Chemiosmosis depends upon a CONCENTRATION GRADIENT of PROTONS (H+) across THYLAKOID MEMBRANE.

Some H+ are made as water is SPLIT inside thylakoid; other H+ are PUMPED INSIDE from outside stroma. The NRG required to PUMP these protons is supplied by the EXCITED e- as they pass along the ETC. Together, BOTH of these mechanisms work to build up a HIGHER [H+] INSIDE the thylakoid, and LOWER [H+] in the OUTSIDE STROMA.

(1) ATP Synthase (MAKES THE ATP) [H+] gradient = NRG used by ATP Synthase in membrane to make ATP for the plant cell.

IMPORTANT Note: ATP Synthase is a MULTIFUNCTIONAL protein: (1) Functions as a carrier protein (moving H+) (2) Functions as an enzyme (synthesizes ATP from ADP + P)

(2) Adenosine Diphosphate (ADP) Added to PO4 to make ATP by ATP Synthase. (NOTE: NRG that DRIVES this reaction is provided by MOVEMENT of H+ from the INSIDE of thylakoid to the OUTSIDE stroma. Together, ATP and NADPH both provide NRG for the SECOND set of RXNS in photosynthesis, known as the CALVIN CYCLE.

6-2 The Calvin Cycle I. Carbon Fixation by the Calvin Cycle Makes compounds using CO2 and NRG stored in ATP and NADPH.

NOTE: 3 STEPS occur within STROMA during this cycle. (1) Carbon Fixation (CO2 used to make carbs, lipids, & amino acids) Carbon atoms from CO2 are BONDED (fixed) into useful compounds. NOTE: 3 STEPS occur within STROMA during this cycle.

Critical Thinking (3) Why MIGHT the rate of photosynthesis increase then reach a plateau as the concentration of CARBON DIOXIDE around a plant increases?

Step 1: CO2 diffuses into STROMA from CYTOSOL of plant cell. Enzyme combines a CO2 with a 5-C carb called RuBP. Product is a 6-C that splits into a pair of 3-C of PGA.

Step 2: PGA changed into another 3-C molecule, PGAL, in a 2 part process. (a) Each PGA receives a P from an ATP. (b) Resulting compound receives a H+ from NADPH and releases a P, producing PGAL.

Step 3 (MAKING ORGANIC COMPOUNDS): Most PGAL is changed back into RuBP; Remaining PGAL becomes carb, lipid, or amino acids. NOTE: By REGENERATING the RuBP (that was used up in Step 1), the Calvin cycle is permitted to CONTINUE operating.

(A) Balance Sheet for Photosynthesis (3 ATP, 2 NADPH used each turn) 6 CO2 + 6 H2O + SOLAR NRG  C6H12O6 + 6 O2

II. Alternative Pathways Calvin cycle (C3 plants) is MOST common for C fixation, BUT OTHER pathways exist: C4 and CAM

(1) C3 Plants (OPEN stomata during the DAY) Fix carbon through Calvin cycle; named after the 3-C compound, PGA.

(2) Stomata (stoma) SMALL pores for CO2 ,O2 , & H2O gases, to enter AND leave a plant (commonly found on UNDERSIDE of leaves) NOTE: When stomata close, [CO2] drops and [O2] rises; BOTH of these INHIBIT carbon fixation—plants who AVOID opening stomata have ADAPTED to deal with this problem.

(A) The C4 Pathway (open PARTIALLY during the day) Can fix CO2 into a 4-C compound even when CO2 level is LOW and O2 level is HIGH. EX: C4 plants include corn and crabgrass, and LOSE only HALF the WATER as C3 plants when producing the SAME amount of CARBOHYDRATE.

(B) The CAM Pathway (similar to C3 and C4 pathways, BUT…) At NIGHT, CAM plants open stomata to take in and store CO2 ; during DAY (closed stomata), CO2 is let go to enter Calvin cycle. EX: Cacti, pineapples, and other species adapted to HOT, dry climates fix carbon through the CAM pathway.

Critical Thinking (4) Cactuses and other CAM plants are very efficient at carrying out photosynthesis while conserving water. Why aren’t they more common in environments where water is more plentiful?

III. Rate of Photosynthesis RATES will be INFLUENCED by 3 variables… SOLAR Intensity (Light Energy) [PEAK, then PLATEAU] [CO2] [PEAK, then PLATEAU] Temperature [PEAK then DROP]

Extra Slides AND Answers for Critical Thinking Questions (1) Without a concentration gradient, there would be no tendency for protons to move from the thylakoid into the stroma. As a result, ATP would not be made by ATP synthase. Also, there would be fewer protons in the stroma to combine with NADP and make NADPH. (2) Increasing the carbon dioxide concentration makes more of it available to enter the Calvin Cycle, thus accelerating photosynthesis. As the carbon dioxide levels rise still higher, the rate of photosynthesis begins to become limited by other components in the biochemical pathways, such as other chemical reactants or enzymes. (3) Most organisms on Earth depend on photosynthetic autotrophs, which capture light energy using colored pigment molecules. (4) Because CAM plants open their stomata at night, when temperatures are lower, they grow slowly. In places where water is plentiful, water conservation is less important and faster-growing C3 plants have an advantage.