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.

• 6CO2 + 6H2O + light C6H12O6 + 6O2

• Photosynthesis combines carbon dioxide and water with light energy and creates sugar (glucose) and oxygen gas.

6CO2 + 6H2O + light C6H12O6 + 6O2

• 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 H2O is required

Showed that O2 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 O2 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 CO2 with the five Carbon compound called Ribulose-bisphosphate.

Calvin Cycle

Light Independent Reactions

• A. 6CO2 enters the cycle and through a chemical reaction is combined with 6 5-carbon compounds. – Keep track of your Carbon.

• 6* (CO2) = 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

(C6H12O6) = 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

Summary

• In the Light Reactions:– Water is split to generate electrons for the

Photosystem II. • The split water produces O2 (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.

– CO2 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.