CH 6PHOTOSYNTHESIS

6-1: THE LIGHT REACTIONS

ENERGY CONVERSION: LIGHT TO CHEMICAL

Light energy absorbed by pigments in photosystems on thylakoid membrane

PS II first, then PS I1. Sunlight energizes electrons at PSII2. e- picked up by Primary Electron Acceptor Electrons lost are replaced from water H2O splitting enzyme splits water

H2O H+ + e- + O2

ENERGY CONVERSION: LIGHT TO CHEMICAL

3. e- move down electron transport chain (ETC) to PS I

e- lose energy Energy used to create H+ gradient in

thylakoid 4. Light re-energizes e- at PSI5. energized e- picked up by NADP+ to make NADPH

CARRIER MOLECULE Compound that can accept a

pair of high energy electrons and transfer them along with most of their energy to another molecule

Ex.) NADP+

Q: WHAT DOES THIS DO? NADP+ NADPH A: this traps sunlight in chemical

form

ENERGY CONVERSION: LIGHT TO CHEMICAL

Light rxn creates H+ gradient inside thylakoid

Concentration higher inside thylakoid Lower in stroma

H+ move through ATP synthase to generate ATP

WHAT’S THE POINT OF THE LIGHT REACTION?

To make some ATP To make NADPH

Which will be used to make sugar

WHY ALL THE ACRONYMS??? NADP+ = nicotinamide adenine dinucleotide phosphate RuBP = ribulose biphosphate 3-PGA= 3-Phosphoglyceric acid G3P = glyceraldehyde 3-phosphate

THE CALVIN CYCLE These reactions don’t require

light, therefore these reactions are called

Light – independent reactions

THE CALVIN CYCLE 2nd set of reactions in photosynthesis In stroma Plants use energy stored in ATP and

NADPH to make sugars

CARBON FIXATION Incorporation of CO2 into organic compounds 3CO2 must enter cycle to make a 3Carbon sugar

Need 3 C to make a 3-carbon sugar. How many 3-carbon sugars are needed to make

a sugar like glucose?Glucose is C6H12O6. So we need two 3-carbon sugars to make one 6-carbon sugar like

glucose.So… it takes two cycles through the Calvin

cycle to produce one 6-carbon sugar.

WHAT OTHER 6-CARBON SUGARS ARE THERE?

Fructose Galactose

These sugars all share the chemical formula C6H12O6.

CALVIN CYCLE1. CO2 combines with Ribulose

biphosphate (RuBP), a 5C molecule(What is 1 + 5?)

-Resulting 6C molecule is unstable and breaks in half (6/2=?) forming a 3C molecule 3-PGA2. 3-PGA converted into G3P using

energy from NADPH and ATP

CALVIN CYCLE3. One G3P leaves cycle-is used to make organic compounds like glucose4. Remaining G3P molecules are converted back into RuBP using ATP

ALTERNATIVE PATHWAYS Plants that live in hot & dry places fix

carbon in different ways Plants lose water rapidly

Water lost through pores on leaves called Stomata

CO2 and O2 also move through these pores

ALTERNATIVE PATHWAYS C4 pathway Stomata stays closed during hot parts

of day Corn, sugar cane, crab grass (tropical

environments) CAM pathway Stomata only open at night Pineapple, cactus

FACTORS THAT INFLUENCE PHOTOSYNTHESIS

Light intensity- increases rate of photosynthesis up to a point

CO2 levels- increases rate up to a point

Temperature- increases rate to a point, then drops Necessary proteins are

destroyed at high temps