Chapter 8: Photosynthesis Converting Light Into Food

Students Will Be Able To: Describe how ATP/ADP and glucose are used to transfer energy. Distinguish between autotrophs and heterotrophs.

Energy needs of life All life needs a constant input of energy Heterotrophs (Animals) get their energy from “eating others” eat food = other organisms = organic molecules extract energy through respiration Autotrophs (Plants) “produce” their own energy (from “self”) convert energy of sunlight build organic molecules (CHO) from CO2 capture energy & synthesize sugars through photosynthesis, then extract through it respiration consumers producers

ATP Living economy Fueling the body’s economy Need an energy currency Eat & digest high energy organic molecules food = carbohydrates, lipids, proteins, nucleic acids capture released energy in a form the cell can use Need an energy currency a way to pass energy around short term energy storage molecule ATP

ATP Adenosine Triphosphate modified nucleotide nucleotide = adenine + ribose + Pi  AMP AMP + Pi  ADP ADP + Pi  ATP adding phosphates (phosphorylation) is endergonic high energy bonds

How does ATP store energy? ADP AMP ATP Each negative Phosphate more difficult to add a lot of stored energy in each bond most energy stored in 3rd Pi 3rd Pi is hardest group to keep bonded to molecule Bonding of negative Pi groups is unstable spring-loaded Pi groups “pop” off easily & release energy Instability of its P bonds makes ATP an excellent energy donor

How does ATP transfer energy? + ATP ADP ATP  ADP releases energy Fuel other reactions

A working muscle recycles over 10 million ATPs per second ATP / ADP cycle Can’t store ATP good energy donor, not good energy storage too reactive transfers Pi too easily only short term energy storage carbohydrates & fats are highly stable, though ATP cellular respiration 7.3 kcal/mole ADP Pi + A working muscle recycles over 10 million ATPs per second

Discuss If ATP is needed to power everything in the cell, but it doesn’t last long… Whereas sugars and lipids store a ton of energy and are very stable, but can’t be used to directly power cell organelles… …how can a cell survive? What must it do?

ATP and Glucose Most cells have only a small amount of ATP, enough to last for only a few seconds of activity. A single glucose molecule stores more than 90 times the chemical energy of an ATP.

Students Will Be Able To: Describe the purpose, location, reactants, and products of the Light Reactions, Calvin Cycle, and photosynthesis overall. Trace the pathway of energy through the photosynthetic reactions. List the steps of the Light Reactions, and state the overall process of the Calvin Cycle.

How do plants make energy & food? Plants use the energy from the sun to “charge up” temporary ATP to make sugars glucose, sucrose, cellulose, starch, & more sun ATP sugars

What do plants need to grow? The “factory” for making energy & sugars chloroplast Fuels sunlight carbon dioxide water The Helpers enzymes Make ATP! Make sugar! I can do it all… And no one even notices! sun CO2 ATP enzymes sugars H2O

Photosynthesis Converts light energy into chemical energy in glucose through complex series of reactions. Glucose = C6H12O6 What does a plant need in order to build glucose? Where can it get those from?

Using light & air to grow plants Photosynthesis In chloroplasts Capture sunlight Carry out the many reactions of photosynthesis

Plant structure Chloroplasts Thylakoid membrane contains ATP thylakoid Chloroplasts double membrane stroma fluid-filled interior thylakoid sacs grana = stacks of thylakoids Thylakoid membrane contains chlorophyll molecules electron transport chain ATP synthase H+ (proton) gradient built up within thylakoid sac outer membrane inner membrane granum stroma thylakoid

Light Light from the sun appears white, but it is a combination of all the colors (ROY G BIV) or the visible spectrum Light is measured in wavelengths. Smaller wavelengths (violet) have more energy. Larger wavelengths (red) have less energy.

Pigments Objects can reflect, transmit, or absorb light. Pigments are molecules that absorb light. Most pigments absorb some colors more than others. The color a pigment appears to the eye is actually the light being reflected Ex. A red shirt appears red because it absorbs all the colors of light except red, which bounces back to the eye.

Light: absorption spectra Photosynthesis gets energy by absorbing wavelengths of light chlorophyll a absorbs best in red & blue wavelengths & least in green accessory pigments with different structures absorb light of different wavelengths chlorophyll b, carotenoids, xanthophylls Why are plants green?

Building plants from sunlight & air Photosynthesis 2 separate processes ENERGY obtaining reactions collect sun energy use it to make ATP SUGAR building reactions take the ATP energy collect CO2 from air & H2O from ground use all to build sugars ATP H2O + CO2 sugars carbon dioxide CO2 water H2O sugars C6H12O6 +

Photosynthesis ATP ADP ENERGY building reactions sun Photosynthesis ENERGY building reactions ADP ATP SUGAR building reactions used immediately to synthesize sugars H2O sugar CO2

It’s not the Dark Reactions! Photosynthesis Light reactions “light-dependent reactions” energy conversion reactions convert solar energy to chemical energy Make electron acceptor “chemical batteries” ATP & NADPH Calvin cycle “light-independent reactions” sugar building reactions uses chemical energy from light rxns to reduce CO2 & synthesize glucose It’s not the Dark Reactions!

Photosystems of photosynthesis 2 photosystems in thylakoid membrane collections of chlorophyll molecules act as light-gathering molecules Photosystem II chlorophyll a P680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P700 = absorbs 700nm wavelength red light reaction center antenna pigments

ETC of Photosynthesis Photosystem II Photosystem I chlorophyll a chlorophyll b Photosystem I

Light Dependant Reactions

ETC of Photosynthesis Chloroplasts transform light energy into chemical energy of ATP use electron carrier NADPH generates O2

ETC of Photosynthesis Electron transport chain (ETC) uses light energy to produce ATP & NADPH go to Calvin cycle Photosystems absorb light photon, which excites an electron in that molecule excited electron passes from chlorophyll to “primary electron acceptor,” which passes it down ETC Eventually, used to synthesize NADPH NADP+ + H+ + e- -> NADPH Now need to replace electron in chlorophyll, so… enzyme extracts electrons from H2O & supplies them to chlorophyll This splits H2O O combines with another O to form O2 O2 released to atmosphere H+ used to power ATP synthase proton pump Enzyme which synthesizes ATP

  build stuff !! photosynthesis Light reactions Result: converted solar energy to chemical energy ATP NADPH What can we do now? ATP  energy  reducing power   build stuff !! photosynthesis

CO2 C6H12O6 How is that helpful? Want to make C6H12O6 synthesis How? From what? What raw materials are available? CO2 NADPH reduces CO2 carbon fixation NADP NADP C6H12O6

From CO2  C6H12O6 CO2 has very little chemical energy C6H12O6 contains a lot of chemical energy Synthesis = endergonic or exergonic? Reduction of CO2  C6H12O6 proceeds in many small uphill steps each catalyzed by a specific enzyme using energy stored in ATP & NADPH

From Light reactions to Calvin cycle Occurs in the stroma Requires energy from ATP and NADPH (from Light Rxns), and CO2 Stores that energy in food molecules—glucose stroma ATP thylakoid

From Light reactions to Calvin cycle Regenerates ADP and NADP+ which go back to the light-dependent reactions Does not require light stroma ATP thylakoid

The Calvin Cycle “Carbon fixation”: incorporation of inorganic carbon (from CO2 in this case) into organic compounds

Photosynthesis summary Light reactions produced ATP produced NADPH consumed H2O produced O2 as byproduct Calvin cycle consumed CO2 produced sugar regenerated ADP regenerated NADP ADP NADP

Light Reactions  produces ATP produces NADPH H2O ATP O2 light energy  + NADPH H2O sunlight produces ATP produces NADPH releases O2 as a waste product Energy Building Reactions NADPH ATP O2

Calvin Cycle  builds sugars uses ATP & NADPH recycles ADP & NADP CO2 C6H12O6  + NADP ATP NADPH ADP CO2 builds sugars uses ATP & NADPH recycles ADP & NADP back to make more ATP & NADPH ADP NADP Sugar Building Reactions NADPH ATP sugars

Putting it all together CO2 H2O C6H12O6 O2 light energy  + H2O CO2 sunlight ADP NADP Energy Building Reactions Sugar Building Reactions NADPH ATP O2 sugars

Photosynthesis Overview

Factors Affecting Photosynthesis Temperature Why might temperature affect photosynthesis? Light Availability of Water & Carbon Dioxide As light, water, and carbon dioxide concentration increases, the rate of photosynthesis increases and then reaches a plateau. Why do you think it plateaus?

How do we figure out something this complicated?! Thousands of scientists in many incremental steps, for instance: Van Helmont’s Experiment Measured water intake and weight increase, concluded that trees gain most of their mass from water. (Later experiments refuted this)

Investigating Photosynthesis Priestley’s Experiment Finds that the plant releases a substance that keeps the candle burning (oxygen).

Investigating Photosynthesis Ingenhousz’ Experiment Found that aquatic plants produce oxygen bubbles in the light but not in the dark. Concludes that plants need sunlight to produce oxygen.