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PHOTOSYNTHESIS


 
 

Lecture 4:  
PHOTOSYNTHESIS 
 
 
Life��s grand device 

By

Edgar Moctezuma


 
 

TODAY�� 

  • Photosynthesis
    1. Intro
    2. Properties of light and pigments
    3. Chloroplast structure and function
    4. Light reactions
    5. ��Dark�� or Carbon reactions
    6. Summary and conclusions
 
 
  •   Respiration
    1. Energy and food chains
    2. Carbon Cycle
 
 

I.  Introduction to photosynthesis 

  • From the Greek  

   PHOTO = produced by light

    SYNTHESIS = a whole made of parts put 
      together. 

Definition PHOTOSYNTHESIS is the process whereby plants, algae, some bacteria,  
use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO2 and water).


 
 

WHY IS PHOTOSYNTHESIS SO IMPORTANT? 

PHOTOSYNTHESIS is one of the most important biological process on earth!

  •   Provides the oxygen we breathe
  •   Consumes much of the CO2
  •   Food
  •   Energy
  •   Fibers and materials
 
 

GENERAL FORMULA FOR PHOTOSYNTHESIS  

                      light

6 CO2 + 12 H2O  --------->  C6H12O6 + 6 O2 + 6 H2O

                  pigments, enzymes 



  •   Oxygen on earth allowed for the evolution of aerobic respiration and higher life-forms.
  •   Respiration: extracting energy from compounds (sugars) 
 

C6H12O6 +  O2    6 CO2  +  ATP


 
 
  1. PROPERTIES OF LIGHT 
    Virtually all life depends on it!
 
  • Light moves in waves, in energy units called PHOTONS
 
  •   Energy of a PHOTON inversely proportional

   to its wavelength 
 

  •   Visible light (between UV and IR) occurs in 

    a spectrum of colors


 
 

Visible light contains just the right amount of energy for biological reactions


 
 

Light is absorbed by pigments 

  • The primary pigment for photosynthesis is chlorophyll a
  • It absorbs blue and red light, not green (green light is reflected back!)
 

Absorption spectrum

of chlorophyll a


 
 
  • Absorption spectrum of chlorophyll a: BLUE & RED
  • Action spectrum of photosynthesis closely matches absorption spectrum of

    chlorophyll a, but not perfectly (due to accessory pigments) 


 
 

Accessory pigments like chlorophyll b and carotenoids (beta-carotene, lycopene):

  •   absorb light at different wavelengths,  (extending the absorption range)
  •   help transfer some energy to chlorophyll a
  •   protect cell from harmful byproducts
 
 

Chlorophyll a is the primary photosynthetic pigment that drives photosynthesis. 

Accessory pigments absorb at

different wavelengths,

extending the range of light

useful for photosynthesis.


 
 

Where does photosynthesis occur?


 
 

The plant cell


 
 

III.  Chloroplast structure and function: solar chemical factory


 
 

Chloroplast structure 

  • Football shaped
  • Double membrane
  • Stroma
  • Thylakoid  
    membrane
  • Grana (stacks)
  • Lumen  
    (inside thylakoid)
 

stroma 

Grana 

thylakoids 

lumen


 
 

Inside a Chloroplast 

  • Remember:  Structure correlates to function!
 
 

Overview of photosynthesis:  
Note: The Light and ��Dark��or Carbon reactions happen at different sites in the chloroplast 

LIGHT REACTIONS

(Thylakoids) 

��DARK�� or CARBON

REACTIONS

(Stroma) 

light 

ATP 

NADPH 

(ENERGY) 

H2O 

O2

(OXYGEN GAS) 

CO2 

C6H12O6

(GLUCOSE)


 
 

IV.   The Light Reactions 

    1.  Light dependent 

2.  Occur in the thylakoid membrane of chloroplast 

4.  Use light energy (photons) to generate two chemical energy compounds: ATP & NADPH 

3.  Water is split into oxygen gas (O2) and H+

 


 
 

Chemical energy compounds made in the light reactions 

   ADP   +   Pi  +  Energy     ATP

adenosine             inorganic                                                 adenosine

diphosphate          phosphate                                               triphosphate 

   NADP+  + 2e-  + H+   NADPH 

Nicotinamide adenin dinucleotide phosphate


 
 

Sequence of events in the Light Reactions 

STROMA 

LUMEN (inside thylakoid) 

PS II 

PS I 

ATPS 

e- 

2 H2

O2  +  4 H+ 

(gas)   (protons) 

NADP+ + H+ 

NADPH 

ADP + Pi 

ATP 

H+


 
 

Summary of the Light reactions 

  2 H2O  +  2  NADP+  +  3 ADP  +  3 Pi 

O2  +  2 NADPH  +  3 ATP  +  4 e-  +  2 H+ 

(gas) 

Light reactions: Chemical energy compounds

are made from light energy, water is split into

O2 and protons


 
 

V.  The��Dark�� or Carbon Reactions 

    1.  Light independent (can occur in light or dark; some enzymes require activation by light) 

2.  Occur in the stroma of chloroplasts 

3.  Use the chemical energy produced in Light Reactions (ATP; NADPH) to reduce CO2 to carbohydrate (sugar). 
 

  1. CO2 is converted to sugar by entering the

      Calvin Cycle


 
 

CO2 

RuBP

Ribulose bisphosphate 

rubisco 

3-PGA

3-phosphoglycerate 

ATP 

ADP 

NADPH 

NADP+

   Pi 

ATP  

ADP 

carboxylation 

reduction 

regeneration 

GAP

Glyceraldehyde 3-phos. 

sugars 

The Calvin Cycle 

  •   Named for M. Calvin
 
  •   3 phases, 13 steps
 
  •   CO2 goes 6 cycles

    to produce 1 glucose


 
 

The Calvin Cycle 

  • CO2 enters the Calvin Cycle
 
  •   First product is a 3-carbon molecule: 3-PGA   (phosphoglyceric acid).  That��s why it��s also called C-3 cycle.
 
  •   Enzyme RUBISCO (ribulose bisphosphate carboxylase/oxygenase) is the main enzyme that catalyzes the first reactions of the Calvin Cycle.
  •   RUBISCO: Is the most abundant protein on earth!
 
 

Most plants use the Calvin Cycle to

Convert CO2 into sugars. 

These plants are called C-3 plants


 
 

Summary of Carbon Reactions 

6 CO2 + 18 ATP + 12 NADPH + 12 H2

C6H12O6 + 18 ADP + 18 Pi + 12 NADP+  

+ 6 H2O + 6 O2 

glucose 

Carbon reactions:  Use CO2 and chemical energy (ATP &

NADPH) to produce sugars by means of the Calvin Cycle


 
 

Limitations on Photosynthesis 

  • Photosynthesis is not perfect in C-3 plants,

        it is only 1 - 4 % efficient 

  •   Low efficiency due to photorespiration
 
  •   Photorespiration occurs when internal CO2

      concentration becomes too low (drought);

      rubisco begins fixing oxygen.


 
 

C-4 plants are more efficient 

  •   C-4 plants first product is a 4-carbon molecule
  •   The C-4 plants (sugar cane, corn, etc.), are more  efficient than C-3 plants – they grow in  hotter climates with more light.
 
  • For example, sugar cane��s  
    photosynthetic efficiency is 7%
  •   C-4 plants have a different leaf anatomy

 


 
 

C-3 vs. C-4 leaf anatomy 

Net venation                                           Parallel venation


 
 

VI.  Summary of Photosynthesis: 

  1. Light energy absorbed by chlorophyll a drives the reactions of photosynthesis.
 

2.  Converts light energy into chemical energy to make organic compounds. 

3.  CO2 and H2O used to produce   C6H12O6 (glucose) and O2 (gas).

 


 
 

4.  Light Reactions occur in thylakoids of the chloroplasts; ATP and NADPH are formed; water is split to O2 (gas) and protons. 

5.  Carbon Reactions occur in stroma – Calvin Cycle fixes CO2  to produce C6H12O6 (glucose). 

6.  Low efficiency, about 1- 4% in C-3 plants. 

7.  Nevertheless, PHOTOSYNTHESIS is still the most important biological process on earth!

 


 
 

 
Importance of photosynthesis and the impact that it has in all our lives. 
 
Without photosynthesis, virtually all plants and animals would become extinct.


 
 

Respiration, Energy & Carbon Cycle 

  • Energy
  • Virtually all organisms require energy of food for:
  • Making chemicals  
    (proteins, carbs, etc.)
  • Movement
  • Cell division
  • Heat, electricity and light production
  • The way living organisms obtain energy is through 
    Cell respiration 
 
 

RESPIRATION 

  • Process of making energy of food available in the cell��
  • Involves breaking down
      • Complicated molecules  into simple molecules

         (C6H12O6, sugars)   (CO2, water)

 
 

RESPIRATION 

The energy held by complicated molecules is held temporarily as ATP (energy currency) 

C6H12O6 + 6 O2     6CO2 + 6 H2O + 36 ATP

(glucose)      (energy) 
 

Respiration occurs mainly in

Mitochondria and Cytoplasm


 
 

Stages of Respiration 

Cellular Respiration has three main stages:

  • Glycolysis
  • Krebs Cycle
  • Electron transport system

 


 
 

3 Stages of cellular respiration 

  • Glycolysis: Splitting of glucose – 2 net ATP generated
  • Krebs Cycle: Energy of glucose molecule is harvested as ATP (2) – it occurs in the mitochondria (matrix) 
  • Electron Transport System: also happens in the mitochondria, more ATP are generated (32).  
  • For each glucose molecule, total ATP = 36 
  • Only 39% efficient, rest is lost as heat.
 
 

Photosynthesis     Respiration 

  • Reaction:  CO2+H2O+sunC6H12O6+O2+H2O      C6H12O6+O2CO2+H2O+36ATP
  • Reactants: Carbon dioxide, water, sun      Glucose, oxygen
  • Products:    Glucose       Energy
  • By-products:  Oxygen       Carbon dioxide, water
  • Cellular location:  Chloroplasts   Cytoplasm, mitochondria
  • Energetics:  Requires energy      Releases energy
  • Chemical paths: Light reactions &       Glycolysis, Krebs cycle      
          Calvin cycle              & Electron Transport Syst.
  • Summary: Sugar synthesized using      Energy released from      energy from the sun                 sugar breakdown
 

Chapter 4: Table 4.1, p. 63


 
 

Photosynthesis and respiration 

  • Photosynthesis and respiration are complimentary reactions��
 
 

PHOTOSYNTHESIS                     RESPIRATION      

CO2 + H2O  O+ SUGARS           SUGARS + O2  H2O + CO2  

PLANTS, ALGAE, BACTERIA 
 

MOST LIVING ORGANISMS 

H2

H2

O2 

O2 

CO2 

CO2 

SUGARS 

Sunlight

energy 

USEFUL CHEMICAL

ENERGY (ATP)


 
 

ENERGY: ability to do work 

Newton��s First Law of Thermodynamics: 
 

��Energy cannot be created or destroyed, it can only be transformed from one form to another�� 

  • Once a cell has used energy to do work, it cannot be used again by any organism.
 

(1701)


 
 

ENERGY 

ENERGY FLOW IS LINEAR

Sun  Earth Producers 1o consumers  2o consum

           heat            resp, heat resp, heat    resp, heat 
 

Energy flows into ecosystem from the sun

Energy travels in a straight line by way of

food chains.


 
 

ENERGY 

However, much energy is lost as heat along the way – as a result of respiration.

    Approximately 90% energy is lost on each step!  

  • Newton��s Second Law of

    Thermodynamics: 

    ��In any transfer of energy  there is always a loss of useful energy to the system, usually in the form of heat��


 
 

Food Chains 

  • (Not referring to SHOPPERS, SAFEWAY or GIANT !!!)
  • Food chains demonstrate linear nature of energy
  • Producers are the base of the food chain, they include photosynthetic organisms like:
      • Plants
      • Algae
      • Certain bacteria
 
 

Food chains 

  • Primary consumers – all plant  
    eaters (herbivores).
  • Secondary consumers – 
     

   Eat primary consumers,  
(carnivores)


 
 

Food chains 

  • Decomposers – obtain energy by breaking down remaining organic material of the other members of the food chain.
  • Fungi and bacteria.
 
 

Matter 

  • All important elements move in cycles;
 

          Environment  Organisms 

Cycles called biogeochemical cycles:

          Water cycle

          Carbon cycle

          Nitrogen cycle


 
 

The Carbon Cycle 

  • Carbon from the atmosphere (CO2) enters the biosphere by way of plants!
    • CO2 used in photosynthesis
    • Carbon moves into food chain
  • Carbon is released to the physical environment by respiration
    • Release CO2 during respiration
    • Amount CO2 fixed in photosynthesis = the amount released by respiration
 
 

Carbon Cycle 

  • Carbon moves from atmosphere to plants to animals and back to atmosphere.
 
 

��Look deep into nature,  
and then you will  
understand everything  
better.�� 
             Albert Einstein


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