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Photosynthesis: Life from Light and Air. Energy. Free energy NECESSARY for life Used to grow, maintain organization, reproduce Offsets increased entropy, maintains order Energy input must exceed free energy lost to entropy

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energy
Energy

Free energy NECESSARY for life

Used to grow, maintain organization, reproduce

Offsets increased entropy, maintains order

Energy input must exceed free energy lost to entropy

If more free energy is expended than acquired -> loss of mass -> eventual death of organism

energy1
Energy

All organisms capture and store (in organic molecules) free energy for use in biological processes

Autotrophs (Auto = self)

Produce organic energy storage molecules themselves

Photoautotrophs (Photosynthesis) capture energy from sunlight

Chemoautotrophs (Chemosynthesis) capture energy from inorganic molecules (hydrogen sulfide, ammonia)

Heterotrophs (Hetero = different)

Acquire organic energy storage molecules from other organisms

common chemical themes
Common Chemical “Themes”
  • Coupled reactions
    • An energetically unfavorable reaction (endergonic) can be driven (“powered”) by coupling it mechanistically with an energetically favorable one (exergonic)
      • The energy released by the exergonic rxn is used to drive the endergonic rxn, rather than being lost as heat
        • Analogy: instead of burning a can of fuel, couple it with a car engine to drive a car!
common chemical themes1
Common Chemical “Themes”
  • Redox reactions (oxidation-reduction reactions)
    • Oxidation = loss of electrons. Reduction = gain in electrons. (Think charge. If you gain an electron, your charge is “reduced,” becomes more negative.)
common chemical themes2
Common Chemical “Themes”
  • We can see coupling within redox reactions.
    • Ex: cellular respiration. Glucose is oxidized, electrons are transferred out as it’s converted to CO2. Oxygen is reduced, is gains those electrons and is converted to water.

oxidation

C6H12O6 + 6O2 -> 6CO2 + 6H2O

reduction

common chemical themes3
Common Chemical “Themes”
  • How is energy “moved” and “released” in photosynthesis and respiration?
    • Electron donors and receptors
      • Electron donor in a reaction = gives away electrons.
      • Electron receptor = acquires electrons.
      • Discussion: is an electron donor oxidized or reduced in the course of a reaction?
common chemical themes4
Common Chemical “Themes”
  • Electron donors and acceptors
    • Electrons (sometimes via an atom/ion – phosphate acceptors are also electron acceptors!) can be thought of as our principal energy “units”
      • If you’ve accepted electrons, you now have options in forming or breaking new bonds
        • And remember: energy is the ability to do work. Chemical energy is the ability to change bond configuration!
    • Electron transfer = energy transfer
don t forget this
DON’T FORGET THIS!
  • In both photosynthesis and respiration:
  • Electron transfer = Energy transfer!
  • Electron carriers = energy carriers!

Electrons = Energy

discussion
Discussion
  • So what does oxidation/reduction have to do with energy? How could a cell use a redox reaction to manage energy?
common chemical themes5
Common Chemical “Themes”
  • This electron transfer = energy transfer also connects back to…
    • Making bonds requires free energy
    • Bonds store free energy
    • Breaking bonds releases free energy
slide12
ATP

Adenosine triphosphate is a basic energy transfer molecule. ATP = electron donor ADP = electron acceptor

The phosphate groups are highly charged and electron-rich (i.e. energy rich), and when broken off by hydrolysis release large amounts of energy. (ΔG = -7.3 kcal/mole)

how does atp store energy

O–

O–

O–

O–

O–

O–

O–

O–

P

P

P

P

P

P

P

P

–O

–O

–O

O–

O–

O–

–O

–O

–O

O–

O–

O–

–O

–O

O–

O–

O

O

O

O

O

O

O

O

How does ATP store energy?
  • 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

AMP

ADP

ATP

Instability of its P bonds makes ATP an excellent energy donor

how does atp transfer energy

O–

O–

O–

O–

P

P

P

P

–O

O–

–O

O–

–O

–O

O–

O–

O

O

O

O

How does ATP transfer energy?
  • ATP  ADP
    • releases energy
  • This is ATP hydrolysis (being hydrolysis, it does consume a molecule of water)

7.3kcal/

mole

+

ADP

ATP

how atp does work
How ATP does work

With the help of enzymes, the cell can couple the energy release of ATP hydrolysis with an endergonic process by transferring the 3rd phosphate group from ATP to another molecule.

The recipient of the phosphate is phosphorylated and becomes a more reactive intermediate

ATP is like an electron donor,

but with phosphate

atp adp cycle

+

Pi

ATP / ADP cycle

ATP

  • 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 long term energy storage

cellularrespiration

7.3 kcal/mole

ADP

A working muscle recycles over 10 million ATPs per second

regeneration of atp
Regeneration of ATP

An organism at work (alive) uses ATP continuously, but ATP is a renewable resource and can be regenerated from ADP by adding a phosphate (Pi)

The energy required to phosphorylate ADP comes from the break down (catabolism) of molecules in the cell

Very quick processA working muscle cell regenerates ALL its ATP in under 1 minute (10 million molecules/second)

discussion1
Discussion
  • Suppose I use ATP hydrolysis to power the synthesis of molecule B from two molecule As. Is this a coupled reaction? How?
  • Redox? Where’s the oxidation vs. the reduction?

ATP + 2MolA -> ADP + MolB

photosynthesis
Photosynthesis
  • OVERALL GOAL: synthesize

an energy-rich monosaccharide

to serve as an energy-storage molecule

  • INTERMEDIATES
    • Light reactions: capture energy from sunlight, temporarily store in ATP and NADPH
    • Calvin cycle: use the energy from ATP and NADPH to power simple sugar anabolism
  • Takes place in chloroplasts
overview
Overview
  • We will do this in two parts.
  • 1) Make ATP
    • ADP and Pi are always floating around, but the enzyme that makes ATP needs protons and electrons in order for it to work
  • 2) Use energy from ATP to make glucose
    • Build glucose from CO2, protons, and electrons
plant structure

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

outer membrane

inner membrane

stroma

thylakoid

granum

chloroplast

Plant structure

ATP

thylakoid

  • Chloroplasts
    • double membrane
    • stroma
      • fluid-filled interior
    • thylakoid sacs
      • stacks of grana
  • Thylakoid membrane contains
    • chlorophyll molecules
    • electron transport chain
    • ATP synthase
      • H+ gradient built up within thylakoid sac
step 1 capture light energy
Step 1: capture light energy
  • Chlorophylls & other pigments
    • embedded in thylakoid membrane
    • arranged in a “photosystem”
      • collection of molecules
a look at light
A Look at Light
  • The spectrum of color

V

I

B

G

Y

O

R

light absorption spectra
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 areplants green?

photosystems of photosynthesis
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

reactioncenter

antennapigments

“a” - “earlier letter” absorbs the “earlier frequency” and

comes first yet is labeled “second!” Argh!

slide26

Photosystem II

Photosystem I

ETC of Photosynthesis

chlorophyll a

chlorophyll b

slide27

e

e

e

e

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

ETC of Photosynthesis

sun

sun

O

to Calvin Cycle

split H2O

ATP

etc of photosynthesis

http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swfhttp://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/photosynthetic_electron_transport_and_atp_synthesis.html

ETC of Photosynthesis
  • Electron transport chain (ETC) uses light energy to produce
    • ATP & NADPH -> go to Calvin cycle
  • PSII absorbs light photon, which excites an electron in that molecule
    • excited electron passes from chlorophyll to “primary electron acceptor,” which passes it down ETC, providing energy for moving H+ into thylakoid interior for ATP synthesis
  • PSI absorbs light photon, which excites an electron in that molecule
    • Electron passes further down ETC, used with a proton to synthesize NADPH
      • NADP+ + H+ + e- -> NADPH
  • Now need to replace electrons 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
the atp synthase enzyme complex chemiosmosis

H+

H+

H+

H+

H+

H+

H+

H+

ADP + Pi

H+

http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter39/proton_pump.html

The ATP Synthase Enzyme Complex - Chemiosmosis

photosynthesis

respiration

sunlight

breakdown of C6H12O6

  • moves the electrons
  • runs the pump
  • pumps the protons
  • builds the gradient
  • drives the flow of protons through ATP synthase
  • bonds Pi to ADP
  • generates the ATP

ATP

experimental evidence

Experiment 1

Experiment 2

light

energy

light

energy

light

energy

6CO2

6CO2

6CO2

+

+

+

6H2O

6H2O

6H2O

+

+

+

+

+

+

6O2

6O2

6O2

C6H12O6

C6H12O6

C6H12O6

Experimental evidence
  • Where did the O2 come from?
    • radioactive tracer = O18

Showed O2 came from H2O not CO2 = plants split H2O!

discussion2
Discussion

Where did the energy come from? Where does it go? Trace its pathway.

Where did the electrons come from?

Where did the H2O come from?

Where did the O2 come from?

Where did the O2 go?

Where did the H+ come from?

Where did the ATP come from?

Where did the NADPH come from?

remember what it means to be a plant

+ water + energy  glucose + oxygen

carbon

dioxide

light

energy

6CO2

+

6H2O

+

+

6O2

C6H12O6

Remember what it means to be a plant…
  • Need to produce all organic molecules necessary for growth
    • carbohydrates, lipids, proteins, nucleic acids
  • Need to store chemical energy (ATP) produced from light reactions
    • in a more stable form that can be moved around plant
    • saved for a rainy day
light reactions
Light reactions
  • Convert solar energy to chemical energy
    • ATP
    • NADPH
  • What can we do now?

ATP

 energy

 reducing power/H

  build stuff !!

photosynthesis

how is that helpful

C6H12O6

NADP

How is that helpful?
  • Want to make C6H12O6
    • synthesis
    • How? From what? What raw materials are available?

CO2

carbon fixation = taking inorganic carbon and “fixing” it in a biomolecule

NADPH

reduces CO2

NADP

from co 2 c 6 h 12 o 6
From CO2 C6H12O6
  • CO2 has very little chemical energy
    • fully oxidized
  • C6H12O6contains a lot of chemical energy
    • highly reduced
  • Synthesis = endergonic process
    • put in a lot of energy
  • Reduction of CO2C6H12O6proceeds in many small uphill steps
    • each catalyzed by a specific enzyme
    • using energy stored in ATP & NADPH
from light reactions to calvin cycle

stroma

thylakoid

From Light reactions to Calvin cycle
  • Calvin cycle
    • chloroplast stroma
  • Need products of light reactions to drive synthesis reactions
    • ATP
    • NADPH

ATP

slide38

5C

1C

3C

3C

CO2

C

5C

C

3 ATP

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

3 ADP

3C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

6C

=

=

C

C

C

6 ATP

6 NADPH

H

H

H

H

H

H

|

|

|

|

|

|

C

C

C

6 NADP

6 ADP

C

C

Calvin cycle

C

1. Carbon fixation

3. Regenerationof RuBP

RuBP

RuBisCo

ribulose bisphosphate

starch,sucrose,cellulose& more

ribulose bisphosphate carboxylase

used

to makeglucose

glyceraldehyde-3-P

PGA

G3P

phosphoglycerate

2. Reduction

to g3p and beyond
To G3P and Beyond!
  • Glyceraldehyde-3-P
    • end product of Calvin cycle
    • energy rich 3 carbon sugar
  • G3Pis an important intermediate
  • G3P  glucose   carbohydrates

  lipids  phospholipids,fats, waxes

  amino acids  proteins

  nucleic acids  DNA, RNA

rubisco
RuBisCo
  • Enzyme which fixes carbon from air
    • ribulose bisphosphate carboxylase
    • Very important enzyme to all life!
      • it makes life out of air!
    • definitely the most abundant enzyme

It’s not easy being green!

accounting
Accounting
  • The accounting is complicated
    • 3 turns of Calvin cycle = 1G3P
    • 3 CO2 1G3P (3C)
    • 6 turnsof Calvin cycle = 1C6H12O6(6C)
    • 6 CO2 1C6H12O6(6C)
    • 18 ATP+ 12 NADPH 1C6H12O6
    • anyATPleft over from light reactions will be used elsewhere by the cell
photosynthesis summary

NADP

ADP

Photosynthesis summary
  • Light reactions
    • produced ATP
    • produced NADPH
    • consumed H2O
    • produced O2as byproduct
  • Calvin cycle
    • consumed CO2
    • produced G3P (sugar)
    • regenerated ADP
    • regenerated NADP
light reactions1

light

energy

H2O

+

+

+

O2

ATP

NADPH

sunlight

Light Reactions

H2O

  • produces ATP
  • produces NADPH
  • releases O2 as a waste product

Energy Building

Reactions

NADPH

ATP

O2

calvin cycle

CO2

+

+

+

+

ATP

NADPH

C6H12O6

ADP

NADP

Calvin Cycle
  • builds sugars
  • uses ATP & NADPH
  • recycles ADP & NADP
    • back to make more ATP & NADPH

CO2

ADP

NADP

SugarBuilding

Reactions

NADPH

ATP

sugars

putting it all together

light

energy

CO2

+

H2O

+

C6H12O6

+

O2

sunlight

Putting it all together

H2O

CO2

ADP

NADP

SugarBuilding

Reactions

Energy Building

Reactions

NADPH

ATP

sugars

O2

energy cycle

sun

light

energy

CO2

+

H2O

+

+

O2

C6H12O6

glucose

H2O

ATP

energy

+

O2

+

CO2

+

H2O

C6H12O6

Energy cycle

Photosynthesis

plants

CO2

O2

animals, plants

Cellular Respiration

ATP

discussion3

light

energy

6CO2

+

6H2O

+

C6H12O6

+

6O2

Discussion
  • Where did the CO2 come from?
  • Where did the CO2 go?
  • Where did the H2O come from?
  • Where did the H2O go?
  • Where did the energy come from?
  • What’s the energy used for?
  • What will the C6H12O6be used for?
  • Where did the O2 come from?
  • Where will the O2 go?
  • What else is involved…not listed in this equation?