Process of Photosynthesis

Objectives Explain why almost all organisms depend on photosynthesis Describe what happens during the light- dependent reactions Describe what happens during the light- independent reactions Distinguish between C3, C4 and CAM plants Identify factors that affect the rate at which photosynthesis occurs

THE LIGHT-DEPENDENT REACTIONS: An Overview Takes place inside of the thylakoids Chlorophyll traps the light energy Makes NADPH and ATP which get used in the Calvin cycle Splits water and gives off oxygen Oxygen (O2) is a waste product

Photosystems Are clusters of chlorophyll and proteins They are surrounded by accessory pigments, which are essential to the light- dependent reactions Photosystems absorb sunlight and generate high-energy electrons that are then passed to a series of electron carriers embedded in the thylakoid membrane

Steps of the Light-Dependent Reactions Light energy excites electrons in chlorophyll a molecules of photosystem II The electrons are shot out of photosystem II down the electron transport chain Protons are moved into the thylakoids as a result of the energy loss

Steps of the Light-Dependent Reactions Photosystem I then absorbs light and shoots out electrons. At the same time, water is split into protons, electrons & oxygen. The released electrons then change NADP+ into NADPH which is used in the Calvin Cycle ATP is synthesized when protons are pumped through ATP synthase from inside the thylakoid to the stroma

Splitting of Water

LIGHT REACTIONS

LIGHT REACTIONS Chemiosmosis: The synthesis of ATP Photosystem I Photosystem II Chemiosmosis: The synthesis of ATP

The Light-Independent Reactions AKA the Calvin Cycle Is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar from carbon dioxide It was discovered by Melvin Calvin, an American biochemist. Carbon atoms from CO2 are “fixed” into organic compounds in a process known as carbon fixation It is the most common pathway for carbon fixation!

The Light-Independent Reactions Continued Uses ATP and NADPH from the light reactions It requires three CO2 molecules and occurs in the stroma of the chloroplast It makes glucose Also known as the C3 pathway, dark reactions & the Calvin-Benson cycle reactions

Steps of the Calvin Cycle An enzyme combines CO2, from the stroma, with a 5-carbon molecule called ribulose bisphosphate (RuBP) forming an unstable 6- carbon molecule This unstable 6-carbon molecule immediately breaks down into 2 three-carbon molecules known as 3-phosphoglycerate (3-PGA) Then the 3-PGA molecules are converted into another 3-carbon molecule called glyceraldehyde 3-phosphate (G3P) while using 6 ATPs and 6 NADPHs for energy

Steps of the Calvin Cycle One of the G3P molecules leaves the Calvin cycle and is used to make carbohydrates The remaining G3P molecule is converted back into RuBP using 3 more ATPs RuBP molecules then enter the Calvin cycle again

Light-Independent Reactions

Photosynthesis Equation Made in the light reactions Used in the Calvin cycle Light Energy 6 CO2 C6H12O6 6 O2 6 H2O Glucose Oxygen Carbon Dioxide Water Made in the Calvin cycle Used in the light reactions

Alternative Pathways Are necessary for plants that live in hot, dry climates where water is easily evaporated from the stomata of leaves Stomata are pores located underneath plant leaves through which O2 & CO2 enter and leave

C4 Pathway Enables certain plants to fix CO2 into 4- carbon compounds During the hottest part of the day, C4 plants have their stomata partially closed They use an enzyme to fix CO2 into a 4-carbon compounds even when CO2 levels are low Examples: corn, sugar cane, & crab grass

CAM Pathway Cactuses, pineapples, & jade plants use this pathway CAM is the abbreviation for ___________________________ These plants open their stomata at night and close them during the day CAM plants take in CO2 at night and fix it into several organic compounds Then during the day, CAM plants run the Calvin cycle

Factors that Affect Photosynthesis ___________: as light intensity increases the rate of the photosynthesis increases ________________: an increased level of CO2 also increases the rate of photosynthesis ___________: increases photosynthesis in a certain range

Summary of Photosynthesis Happens in ________ stages The light reactions happen in the ____________ membrane The Calvin cycle happens in the ________ The light reactions use _______, & ________ and produce _________, __________, & ____________. The Calvin cycle uses _________, _________, & _________ and produces ________.

Concept Map Photosynthesis includes use take place in takes place in Light reactions Calvin cycle use take place in takes place in uses Thylakoid membranes Stroma NADPH ATP Light energy from sun of to produce to produce Glucose ATP NADPH O2 Chloroplasts

WHAT HAPPENS TO THE GLUCOSE? BUT HOW?