6-2: Calvin Cycle

What comes next?  In the second set of reactions of photosynthesis, plants use the energy that was stored in ATP and NADPH during the Light Reactions to produce organic compounds in the form of SUGARS  These organic compounds are then consumed by autotrophs + heterotrophs alike for energy  The most common way that plants produce organic compounds is called the Calvin Cycle

Carbon Fixation  Calvin cycle is a series of enzyme- assisted chemical reactions that produces a 3-C sugar  CO 2 “fixed” into organic compounds  Known as Carbon Fixation  3 CO 2 molecules must enter to make 3-C sugar  Occurs within the stroma of chloroplast

Steps of Calvin Cycle 1.CO 2 diffuses into the stroma from cytosol. An enzyme combines CO 2 + RuBP (5-C molecule). The 6-C molecules immediately splits into 2 3-C molecules called 3-PGA

Let’s STEP 1

Steps of Calvin Cycle 2.3-PGA is converted into another 3-C molecule called G3P in two parts.  First: 3-PGA receives a phosphate group from ATP  Second: receives a proton (H + ) from NADPH, which releases the phosphate group  ADP, NADP +, and phosphate can go back to the Light Reactions to make more ATP + NADPH

Let’s STEP 2

Steps of Calvin Cycle 3.One of the G3P molecules leaves Calvin Cycle and is used to make organic compounds (a carbohydrate) that is stored for later use 4.The remaining G3P molecules are converted back to RuBP through addition of phosphate groups from ATP molecules. RuBP then starts the Calvin Cycle over again.

Let’s STEP 3

Calvin Cycle Misc.  Pathway named for American scientist Melvin Calvin  Calvin Cycle is most common pathway for carbon fixation to take place  C 3 plants  Plants that fix carbon exclusively through the Calvin Cycle  C 3 = (C-C-C) = 3-C compound initially formed in process  Examples: wheat, barley, potatoes

Alternative Pathways  Under hot + dry climates, plants rapidly lose water to the air  Stomata (or stoma) – a small pore usually located on the underside of a leaf  Used in gas exchange

Alternative Pathways (cont.)  Plants can partially close their stomata to reduce the water loss  Major passageways through which CO 2 enters and O 2 leaves a plant  Low CO 2 levels and high O 2 levels inhibit the Calvin Cycle from working   Plants must find a way to deal with those conditions

C 4 Pathways  C 4 plants  Plants that fix CO 2 into a 4-C compound  During hottest part of day, stomata are partially  During hottest part of day, stomata are partially closed  Certain cells of C 4 plants have enzymes that fix CO 2 when CO 2 levels are down + O 2 levels are up  Compounds transported to other cells where they can enter the Calvin Cycle to produce carbohydrates  Examples: corn, sugar cane, crabgrass  C 4 plants do not lose as much water as C 3 plants do – Why?   Stomata are partially closed during day

CAM Pathway  CAM plants include pineapples + cacti   At night, the stomata open   During the day, the stomata are closed  CO 2 that is taken in at night is fixed into a variety of organic compounds  During the day, CO 2 is released by the compounds and enters the Calvin Cycle  CAM plants grow slowly  Lose less water than either C 3 + C 4 plants

Summary of Photosynthesis  Two Stages:  Light Reactions  AKA – Light-Dependent Reactions, Light Harvesting Reactions  Calvin Cycle  AKA – Light-Independent Reactions, Dark Reactions  Ongoing cycle - Biochemical pathways  Simplest Equation for Photosynthesis  CO 2 + H 2 O + light energy(CH 2 O) + O 2  Glucose is not a direct product of photosynthesis!!!

Factors that Affect Photosynthesis  Light Intensity  More light stimulates more electrons in Photosystems of ETC  Light reactions happen more rapidly  Peaks at a point where there are no more electrons to excite

Factors that Affect Photosynthesis  Carbon Dioxide  More CO 2 allows “fixing” in Calvin Cycle to take place faster but reaches a saturation point as well

Rate of Photosynthesis  Temperature  Increases initially but declines at peak  Why?  Because at some higher temperatures, enzymes stop working + become ineffective  Also, the stomata close limiting water loss + CO 2 entry

Calvin Cycle  Video Recap Video Recap Video Recap

The Photosynthesis Song  It’s Catchy! It’s Catchy! It’s Catchy!