PHOTOSYNTHESIS:  HARVESTING  LIGHT  ENERGY    

u Autotrophs  are  organisms  that  create  their  own  energy  molecules  from  inorganic  molecules  –  Plants  are  autotrophs  since  they  convert  sunlight  into  sugar  molecules  

u Heterotrophs  are  organisms  that  consume  other  organisms  (dead  or  alive)  to  meet  their  energy  needs  

Energy  and  Life  

Importance  of  Photosynthesis    •  photosynthesis  –  uses  the  energy  in  sunlight  to  convert  carbon  dioxide  to  sugars  

•  provides  usable  chemical  energy  for  life  on  earth,  carbon  building  blocks,  and  oxygen  

•  food,  clothing,  and  shelter    

Chemical  Energy  

u For  most  organisms,  adenosine  triphosphate  (ATP)  is  the  principle  energy  molecule  –  Contains  one  adenine  molecule,  one  ribose  (sugar)  molecule  and  three  phosphate  groups  

u Adenosine  diphosphate  (ADP)  has  one  less  phosphate  group  than  ATP  and  is  the  breakdown  product  of  ATP  

Chemical  Energy  

u  ATP  is  the  basic  energy  source  for  most  cells  –  AcDve  transport  across  membranes  –  Movement  within  the  cell  

u  ATP  has  a  very  short  life  span  in  cells  and  thus  is  not  ideal  for  energy  storage  

u  Cells  store  energy  in  the  form  of  carbohydrates,  lipids  or  protein  –  One  glucose  molecules  contains  90  Dmes  the  stored  energy  in  a  molecule  of  ATP  

Photosynthesis  u  Historical investigations of photosynthesis

–  Van Helmont (1600’s) determined that plant growth was not due to soil…he thought it was due to water

–  Joseph Priestly (1700’s) determined that a substance in the atmosphere was required for combustion and that plants produce this substance

–  Jan Ingenhousz (1700’s) determined the substance produced by plants only occurred when sunlight was present

u  These investigations indicated that plants utilize CO2 & H2O to grow and produce O2 as a by-product

Origin  of  Photosynthetic  Organisms    

• At  first  it  is  thought  that  only  heterotrophs  existed  (heterotroph  hypothesis)  that  fed  on  an  organic  soup.    As  the  soup  was  depleted  so  did  the  heterotroph  organisms.    However,  some  of  the  heterotrophs  were  now  capable  of  using  light  energy,  they  used  the  light  energy  and  began  to  provide  food  for  themselves  and  others.    These  were  simple  bacteria.  •  Today  similar  bacteria  exist,  and  it  is  thought  that  those  simple  bacteria  became  a  part  of  the  eukaryoDc  cell  

Photosynthesis  

u To  carry  out  PS  plants  need  chlorophyll  –  Chlorophyll  is  a  pigment  that  absorbs  light  –  Chlorophyll  a  and  chlorophyll  b  are  the  two  principle  types  found  in  plants  

Photosynthesis  

u Chlorophyll  a  absorbs  light  in  the  red  and  violet  region  of  the  light  spectrum  

u Chlorophyll  b  absorbs  light  in  the  red  and  blue  region  of  the  light  spectrum  

u In  both  forms  of  chlorophyll,  light  energy  raises  the  energy  levels  in  electrons…these  high-­‐energy  electrons  make  PS  work  

Photosynthesis  u  In photosynthetic eukaryotes, PS takes place in chloroplasts – Outer membrane – regulates substances into and out of organelle

– Thylakoids: saclike membranes that are stacked into grana which increase membrane surface area u Contain pigments and proteins that capture the energy in sunlight

u  Stroma: region outside the grana– surrounds the thylakoids, contains enzymes as well as its own DNA and RNA, and ribosomes – evidence of bacterial origin for chloroplasts

u  Light dependent reactions take place in the thylakoids – Energy fixing reactions

u  Light independent reactions (Calvin cycle) take place in the stroma – Carbon fixing reactions

OVERVIEW  OF  PS  

Chloroplast  Structure      Thylakoids  •  Thylakoids    •  Grana  –  stacked  thylakoids  oriented  to  have  maximum  sunlight  absorpDon  •  Light  absorbing  pigments  are  embedded  in  the  thylakoids  •  Chlorophyll  a  &  Chlorophyll  b      •  Both  absorb  light  in  the  violet/blue  and  orange/red  regions  and  reflect  green  •  accessory    pigments  are  available,  their  absorbed  energy  is  given  to  special  chlorophyll  a  •  cartenoids  

Chloroplast Anatomy

Chloroplast  >

~  contains  chlorophyll;    makes  plants  green  

~  performs  Photosynthesis  

~  makes  Glucose  to  harness  Energy

~  found  only  in  plant  cells

THE  PROCESS  OF  PHOTOSYNTHESIS    An  Overview  

• major  steps  in  energy  conversion  •  absorpDon  of  light  energy  •  conversion    of  light  energy  into  chemical  energy  •  storage  of  chemical  energy  in  sugars  •  light  reacDons  and  the  Calvin  cycle                      (Calvin-­‐Benson  cycle  or  photosyntheDc  carbon  reducDon  cycle)  •  EquaDon:    3  CO2    +    3  H2O  à    C3H6O3    +    3  O2  

Photosynthetic Pigment Absorptions

Photosynthesis

Two Phases of Photosynthesis

1.) Light-Dependent Reactions (Light reactions)

Light-Dependent Reactions

~ occurs in thylakoid membrane of chloroplast

~ needs light to start process

~ chlorophyll absorbs light, and water is split into O2 gas and H+

~ photolysis - process of water being split by light

~ H+ ions have a positive charge that powers the formation of ATP

~ final products = Oxygen, NADPH, and ATP

~ Light Reactions provide the ENERGY for next step (Calvin Cycle)

Photosynthetic ET and ATP synthase

The  Light  Reactions    •  Light  reac+ons  convert  visible  light  into  chemical  energy  that  powers  sugar  produc+on  • Chlorophyll  absorbs  light,  water  molecules  split,  chemical  energy  stored  • Photosystem  I  •  connected  by  electron  transport  system  • work  together  to  absorb,  concentrate,  and  transfer  light  energy  to  special  chlorophyll  a  • Photosystem  II  

The  Light  Reactions    Continued  •  chlorophyll  a    -­‐  reacDon  center  •  in  each  photosystem  the  reacDon  center  loses  energy–rich  electrons  which  are  carried  on  the  electron  transport  • PSI  –  loses  electrons  but  they  are  replaced  by  electrons  from  PSII  • PSII  –  loses  electrons  but  they  are  replaced  by  electrons  removed  from  water  •  EquaDon  –  H2O  à    2  H+    +    2e+    +    ½  O2  

The  Light  Reactions    Continued  •  at  end  of  flow  the  electrons  from  water  combine  with  a  hydrogen  carrier,  NADP+  -­‐  nicoDnamide  adenine  dinucleoDde  phosphate  •  forms  NADPH  –  which  transports  the  electrons  needed  for  the  Calvin  cycle  and  other  biosyntheDc  reacDons  in  the  chloroplast  •  as  they  flow  some  of  the  energy  is  used  in  acDve  transport  of  protons  across  the  thylakoid  membrane  •  high  []  of  protons    •  high  []  of  protons  now  diffuse  down  steep  []  gradient  through  ATP  synthtase  forming  ATP  +  P  

The  Light  Reactions    Continued  •  The  first  two  events  of  photosynthesis  occur  •  AbsorpDon  of  light  •  Conversion  of  light  energy  into  chemical  energy  •  Three  products  formed:    oxygen  gas,  ATP,  and  NADPH  

 Light  Reactions  

The  Calvin  Cycle    • powered  by  the  light  reac+ons    •  Series  of  reacDons  in  which  carbon  dioxide  is  combined  with  the  hydrogen  split  from  water  in  the  light  reacDons  •  Takes  place  in  the  stroma  `    • Products  of  the  Calvin  Cycle  (G3P)  •  The  3  carbon  sugar  produced  can  be  used  in  many  biosynthesis  reacDons  •  sucrose  made  in  cytosol,  and  starch  made  in  chloroplasts  

The Calvin Cycle (continued) For each fixed carbon atom, 9 ATP equivalents are required; thus 54 ATP are needed to make one glucose molecule and to complete the cycle

Even though there is an expenditure of energy in the Calvin Cycle, recall that one glucose molecule contains 90 times the stored energy contained in a molecule of ATP

2.) Light-Independent Reactions (Dark Reactions AKA Calvin Cycle)

~ occurs in stroma of the chloroplast

~ happens with and without light

~ Uses energy, ATP and NADPH, from light reactions to make Glucose

Photosynthesis

Calvin cycle

Photosynthesis Song

I Need a Little Light (Photosynthesis Song) by Ross Durand

Do Now

Photosynthesis Song

1.  “I need a little light so I can store energy. I need a light like so I can make ATP”

2.  “I’m sitting here in this chloroplast feeling green the whole day through.”

3.  “The ATP and NADPH are going to make a cycle go round.”

Use your notes on Photosynthesis to explain the following lyrics. Please write in complete sentences.

I need a little light so I can store energy. I need a little light so I can make ATP. I need a little light; I’m gunna make some food. It’s out of sight, and I need a little light. I’m sitting here in this chloroplast feeling green the whole day through. You add a little thylakoid just a-waiting for something to do. I’m going to move some electrons through a chain And pump some Hydrogen in But its been dark all night, and you can see the state I’m in. So I need a little light so I can store energy. I need a little light so I can make ATP. I need a little light; I’m gunna make some food. It’s out of sight, and I need a little light.

When the electrons leave that chain They’re going to join NADP And that high concentration of hydrogen Is gunna walk outside of me It’s gunna go through a special protein, It’s gunna kick some ATP out But that’s what moving these elections is really all a-bout So I need a little light so I can store energy. I need a little light so I can make ATP. I need a little light; I’m gunna make some food. It’s out of sight, and I need a little light. The ATP and NADPH are going to make a cycle go round. At the end of the Calvin cycle, glucose can be found. You gotta put in 6 pieces of CO2 to make that precious food. And use up the products of the thylakoid - darn this systems good

So I need a little light so I can store energy. I need a little light so I can make ATP. I need a little light; I’m gunna make some food. It’s out of sight, and I need a little light. There’s one other thing I need besides light – it’s good old water. And see that chlorophyll, that loses its electrons, needs to get some back - and that’s what the water does But the whole reason that electron moved is because of the light So I need a little light so I can store energy. I need a little light so I can make ATP. I need a little light; I’m gunna make some food. It’s out of sight, and I need a little light.

Photosynthesis  Summary  

u PS  is  the  process  plants  use  to  turn  the  energy  in  sunlight  into  chemical  energy  

u Plants  are  autotrophs  u There  are  two  major  steps  in  PS:  

–  Light  reacDons  =  fix  energy  –  Light  Independent  ReacDons  (Dark  reacDons)  =  fix  carbon  into  G3P  

Photosynthesis  Summary  

u The  light  reacDons  have  two  parts:  –  PS  system  II:  occurs  in  thylakoids,  absorbs  light  (P680),  creates  high  energy  electrons  and  H+,  forms  ATP,  splits  water  and  releases  O2  

–  PS  I:  occurs  in  thylakoids,  absorbs  light  (P700),  makes  the  electron  carrier  NADPH  

   

Photosynthesis  Summary  

u The  dark  reacDons  occur  as  the  Calvin  Cycle  –  CO2  is  removed  from  the  air  – One  of  every  three  carbon  atoms  fixed  in  the  Calvin  cycle  is  used  to  make  glucose…the  rest  are  used  to  replenish  chemicals  used  in  the  cycle  

– Nine  ATP  equivalents  are  required  to  fix  each  carbon  to  complete  the  cycle  

 Rate  of  Photosynthesis    •  environment  effects  photosynthesis  •  rate  –  acDvity  per  unit  of  Dme  •  measure  CO2    absorbed,  or  O2      given  off  •  four  environmental  factors  •  light  intensity  -­‐  levels  off  at  certain  point,  can  determine  what  plant  grows  there  and  then  what  ecosystem  can  exist  •  Temperature  -­‐  opDmum  temperature  is  necessary,  or  freezing  or  denaturing  of  Calvin  cycle  enzymes  may  occur  •  []  of  CO2  -­‐  as  CO2    increases  so  does  the  rate  of  photosynthesis,  to  a  certain  point  before  leveling  off  •  []  of  O2-­‐  an  increase  of  O2  reduces  the  rate  of  photosynthesis  •  the  environmental  factors  play  together,  some  are  opDmum  while  others  are  minimum  so  they  have  a  combined  effect  •  limiDng  factors  

Photorespiration  and  Photosynthesis  • PhotorespiraDon  –  light-­‐dependent  process  involving  the  uptake  of  oxygen  and  release  of  carbon  dioxide  that  occurs  along  with  photosynthesis  in  all  plants  • CounterproducDve  to  photosynthesis  • Prevents  incorporaDon  of  CO2  in  the  Calvin  cycle  and  results  in  the  loss  of  previously  incorporated  CO2  • When  rubisco  binds  with  oxygen  in  the  chloroplast,  no  new  carbon  dioxide  is  fixed,  and  RuBP  is  converted  to  Carbon  dioxide  • Photosynthesis  slows,  photorespiraDon  interferes  with  Calvin  cycle  

 

Special  Adaptations    • C-­‐4      plants,  carbon  dioxide  is  first  incorporated  in  a  4-­‐carbon  acid  (that’s  where  the  name  comes  from)  •  Low  CO2,  high  light  and  temp  •  sugarcane,  corn,  crabgrass  •  special  features  keep  CO2  low  in  an  area  so  it  diffuses  quickly  keeping  photosynthesis  running  • And  CAM  plants  (crassulacean  acid  metabolism)  –  open  stomates  at  night,  desert  plants  –  cactus  and  jade  plants