Photosynthesis

ATP Adenosine Triphosphate is the energy currency of the cell. It does not get destroyed (just like money), but cycles between 2 states. –ATP: 3 phosphates  high energy! –ADP: 2 phosphates  low energy!

Photosynthesis: Overview Photosynthesis is a process used by plant cells to convert light energy from the sun into chemical energy. 6CO 2 + 6H 2 O + light  C 6 H 12 O 6 + 6O 2 Photosynthesis combines carbon dioxide and water with light energy and creates sugar (glucose) and oxygen gas.

6CO 2 + 6H 2 O + light  C 6 H 12 O 6 + 6O 2 How do we know that this is the equation? –Van Helmont’s Experiment in the 1600s saw that water contributed to the creation of sugars. –Priestley (1700s) showed that plants produced something that could keep flames burning. –Ingenhousz showed that Priestley’s experiment only worked when plants are exposed to light Showed that H 2 O is required Showed that O 2 is a product Showed that light is a necessary!

Light Carbon Dioxide and water are converted into sugars and Oxygen gas in the presence of light. HOW??? Plants capture light energy through the use of a pigment called chlorophyll. 2 Types: chlorophyll a and chlorophyll b

Chlorophyll Only certain colors of light are absorbed by chlorophyll. Infact, chlorophyll absorbs red and blue light wavelengths best! Leaves of plants reflect green light, and chlorophyll absorbs poorly in green light.

Function of Chlorophyll In the chlorophyll molecule the light absorbed excites electrons in the molecule. It is these high-energy electrons that drive photosynthesis

Reactions of Photosynthesis Photosynthesis takes place in the chloroplasts of plant cells. –Thylakoid membrane forms structures called Grana –The area outside the thylakoid is known as the Stroma

Photosynthesis There are 2 parts of photosynthesis. –Light Reactions –Light-independent Reactions (Dark Reactions) (Calvin Cycle) Each take place in a different part of the chloroplast.

Light-Dependent Reactions In the thylakoid chlorophyll is arranged in the membrane by proteins into structures called Photosystems. As sunlight is absorbed by the photosystems, the excited electrons jump from chlorophyll to a carrier molecule called NADP + The purpose of the light reaction is to produce ATP and NADPH

Light-Dependent Reactions The purpose of the light reaction is to produce ATP and NADPH

Light-Dependent Reactions A. Light is absorbed in Photosystem II –The electrons become excited and passed down an electron transport chain to create energy. –These missing electrons must be replaced! –On the inside of the thylakoid water molecules are split into H + and O 2 and 2 electrons. –The electrons from the split water replace the excited electrons. –Hydrogen ions are released on the inside of the Thylakoid. Oxygen gas is released as a product!

Light-Dependent Reactions B. The high energy electrons are passed down Photosystem I. –Some of the energy in the excited electrons is used to PUMP H + ions into the inside of the Thylakoid from the Stroma. C. In Photosystem I the high energy electrons are combined with H + ions from the thylakoid and transferred to NADP + to create NADPH

Light-Dependent Reactions D. As electrons are passed, and NADPH is created, lots of H + ions are being pumped into the inside of the thylakoid  makes it positively charged and the outside (Stroma) negatively charged. This difference in charge is used to create ATP! E. Another protein pump called ATP Synthase transports the H + ions from the thylakoid to the Stroma like a water turbine, to create ATP. –ADP + Phosphate group  ATP

Light-Dependent Reactions Summary: –The Light-Dependent Reactions absorb light energy to excite electrons. The excited electrons are used: A. To pump H + ions into the Thylakoid of a Chlorplast B. To create NADPH To power ATP Synthase in creation of ATP

Light Independent Reactions The Calvin Cycle takes the energy products from the Light Reactions and uses them to synthesize sugar (glucose). AKA Calvin Cycle, Dark Reactions. These reactions do not require light energy to occur.

Light Independent Reactions Light reactions occur in the Thylakoid membrane. –Creates NADPH on the stroma side of Thylakoid –ATP Synthase creates ATP on the stromal side of the Thylakoid. Dark reactions occur in the Stroma of the chloroplast.

Light Independent Reactions Named after Melvin Calvin (hence the name Calvin Cycle). Converts Carbon Dioxide into sugar. Requires the energy of ATP and NADPH to power the cycle.

Rubisco Rubisco is the most abundant enzyme on the planet! It catalyzes the first reaction in the Calvin Cycle. It combines the incoming CO 2 with the five Carbon compound called Ribulose-bisphosphate.

Calvin Cycle

Light Independent Reactions A. 6CO 2 enters the cycle and through a chemical reaction is combined with 6 5-carbon compounds. –Keep track of your Carbon. 6* (CO 2 ) = 6 Carbons 6* (C5) = 30 Carbons 36 Carbon atoms total The result of the reaction is 12 molecules that contain 3 Carbon atoms each. 12*(C3) = 36 Carbons 36 Carbon atoms total

Light Independent Reactions B. From the 12 3-Carbon compounds, 2 leave and are used to make glucose. 36 Carbon atoms total 2 of the 3-Carbon molecules are used to create glucose (C 6 H 12 O 6 ) = 6 Carbons That leaves 30 Carbons. 10*(C3)= 30 Carbons. C. The remaining 10 3-Carbon compounds are rearranged back to the original 6 5-Carbon compounds the Calvin Cycle started with. 10*(C3) = 30 Carbons 6*(C5) = 30 Carbons

Light Independent Reactions The two 3-Carbon molecules are used to create glucose! The remaining ten 3-Carbon molecules are recycled, to keep the Calvin Cycle going. For each turn of the Calvin Cycle ONE glucose is made.

Summary

In the Light Reactions: –Water is split to generate electrons for the Photosystem II. The split water produces O 2 (waste product). H+ ions are pumped into the lumen of the Thylakoid to generate a charge across the membrane. –The excited electrons (via light) are passed along ultimately being picked up by NADP + to create NADPH. This occurs in Photosystem I. –The charge across the Thylakoid membrane is used by ATP Synthase to create ATP.

Summary In the Dark Reactions: –The ATP and NADPH created in the Light Reactions are used to fuel the Dark Reactions. –CO 2 is combined through a number of reactions with other carbon compounds to generate ONE glucose molecule. –The remaining carbon is recycled to keep the Dark Reactions going.