Special Notes for Crop Science. September 22-26, 2003

It is very important that you physically write these notes rather than print and read them. Copy one day's notes at a time.

Be sure to read the photosynthesis section in your textbook.

 

Tuesday, September 23

Photosynthesis  II

** As you know, the light reaction captures and converts light energy. The "dark" reaction utilizes this energy trapped in ATP to produce glucose from CO2 . The energy from the sun is captured in a Glucose molecule.

 

  1. Looking at Ps in excruciating detail
    1. Light Rxn (Hill reaction)
      1. Light Energy Capture
bulletInside the chloroplasts are Photosystem II and Photosystem I. The Photosystems are conglomerations of pigments (chlorophyll) . The pigments absorb energy and transfer it to the reaction center of the photosystems.Picture

 

bulletLet's examine the transfer of energy from photons into the chloroplasts in three steps:
        1. The transfer of photon energy to the Reaction Centers of P.S. I and II.  In this step, electrons in the Reaction Centers gain so much energy that they break free and are transferred to an acceptor molecule. The Photosystems become electron deficient.
        2. The electrons lost from the reaction center of PS I are replaced.  They are replenished by the electrons that were lost from PS II. They arrive there after being transferred through a chain of molecules
        3. The electrons lost from the reaction center of PS II are also replaced. They are replaced by  electrons from a water molecule that splits.

 

    View animations of each of the steps covered above.

    		Step 1. 
    Step 2.
    Step 3.


      1. Energy Transfer to ATP. Once the energy in the photons is transferred into the electrons in the Photosystems, it still must be captured in ATP to be of use to the plant. Two processes which produce ATP are Photophosphorylation and Chemiosmosis.

 

        1. Photophosphorylation = The production of ATP due to the flow of electrons through the photosystems. In the previous section, you saw how electrons were passed from one molecule to another. While this occurs, some of the energy from the electrons is siphoned off to form ATP. 
bullet ADP + energy + P ----> ATP. The energy is trapped in the phosphorous bond. The complete reaction is shown below.
bullet Click here to view an animation of Photophophorylation (Flash Clip #4).
        1. Along with ATP production, NADPH2 is also produced during photophosphorylation.
4 ADP + P + 2 H2O ----> 4 ATP + O2 + 4 H+ .      Then, the 4 H+ ---> trapped in 2 NADPH2
bullet

  See Animation of NADPH2 production (Flash Clip #5) 

bulletBoth the ATP and NADPH2 will later be used in the Dark Reaction.

 

        1. Chemiosmosis.  The production of ATP through the movement of H+ ions through a membrane.
bullet Remember that water molecules split and release their H+ ions. This causes the concentration of H+ ions to increase in the Stroma. The H+ want to move from the high concentration inside the stroma to the lower concentration outside the stroma (diffusion). The theory is that as the H+ ions diffuse through a port (opening) in the membrane, energy is extracted.  The port acts as a hydroelectric 'turbine'. 
bullet Once again ADP + energy + P ----> ATP. The energy is trapped in the phosphorous bond.  Once the H+ ions pass through the port, they combine with NADP to produce the NADPH2. (see above)

            Click here to view an animation of Chemiosmosis (Flash Clip #6).

Continuing with a detailed look at Photosynthesis, we come to the "dark" reaction.

  1. The Dark Reaction (Calvin Cycle).    
bullet Note that this does NOT usually occur at night!
  1. From the light Rxn.
    1. As you remeber, the the Light rxn produced H+ (trapped in NADPH2) And energy (trapped in ATP). 
4 ADP + P + 2 H2O ----> 4 ATP + O2 + 4 H+ .      Then, the 4 H+ ---> trapped in 2 NADPH2

 

      1. The next step
        bullet Carbon dioxide (CO2) is assimilated into a 5-carbon sugar to produce 2  3-carbon sugar chains

CO2    +Picture   -->     C-C-C     +    C-C-C

 

bulletIn this step, the ATP and NADPH2 are used.
bulletThis is the the first step in the Calvin Cycle. In order to produce a new glucose molecule, however, a total of 6 CO2 molecules must be assimilated. Therefore, the above formula should be modified to: 
6 CO2  (6 Carbons)  +  6  5-Carbon sugars (30 carbons)  --> 
12  3-Carbon sugars  (36 Carbons)

 

      1. The Calvin Cycle.
        2. The Calvin Cycle begins with the first step discussed above.   12  3-Carbon sugars were produced...
        bullet2 of the 3-Carbon sugars will be reformed (or transformed) into a 6-Carbon sugar. This is glucose! 
        bulletThe other 10 sugars will also go through a transformation and will eventually become 6  5-Carbon sugars.
        bulletThis completes the cycle. The newly reformed 5-Carbon sugars can again combine with CO2. 

You need to learn this  simplified Calvin Cycle.

Calvin Cycle. Extremely simplified
Picture

Summary: For every 6 molecules of CO2 assimilated, 1 glucose molecule is formed.

Click here to see a more detailed Calvin Cycle. While you do not need to learn this version, please examine it carefully.

           

           

Stop Here!

Review the questions for this section. 

Continue Thursday

 

Thursday, September 25

 

      1. CO2 Assimilation. Plants use one of three general mechanisms to assimilate  CO2 in the dark reaction.
        1. The C-3 mechanism - This is what we just covered. 
          2.   
          bulletIt is called C-3 because the first products of the assimilation are 3-carbon chains.
          bulletThis mechanism is used by most crop plants and by most of the plants in the temperate regions (here). E.g. Wheat, rice, soybean, potato, bermuda, etc.
        2. The C-4 mechanism -Corn and other tropical grasses.
          1.  The C-4 process breaks assimilation into 2 steps. 
            1. In the mesophyll cells, the CO2 combines with a 3-Carbon sugar   to produce a 4-Carbon sugar. 
            2. The 4-Carbon sugar is transported into a Bundle Sheath cell and releases the Carbon for the Calvin Cycle.
C-4 anatomy.  
Picture
  1. Therefore, The CO2 assimilation is physically separated from the Calvin Cycle. They each occur in different cells.
          bulletIn this case the first products are 4-carbon sugars (C-4 !).
          bulletThis provides a means of concentrating CO2 in high O2 environments.
          bulletAssimilates well in high sunlight.
          bulletMany tropical plants use this system. e.g. corn
  1. Crassulacean Acid Metabolism (CAM) -Desert plants.
    1. The CAM plants use a modified  C-4 mechanism. There are 2 differences: 
      bulletFirst, The CO2 assimilation occurs at night (stomates open) then the Calvin Cycle occurs in the day (stomates closed). The CO2 assimilation is temporally separated from the Calvin Cycle. They each occur at different times of the day.
      bulletSecond, CO2 assimilation and the Calvin Cycle occur in the same cell. Therefore there is no special C-4 anatomy (bundle sheath cells).
      bulletThese plants can save water, because the stomates are closed in the daytime when it is hot and open at night when it is cool.

Now We Begin a new Topic:

Environmental Factors in Plant Development 

 

Environmental Factors in Plant Development 2/99

The FOUR most important Environmental Factors are Light, Temperatures, Water and Soil

  1. Light  
    1. general 
      1. Part of the electromagnetic spectrum emitted by the sun 
      2. Transmitted through space in energy packets called Photons.
      3. The ultimate source of most all energy used on earth
      4. Energy contained in the light is converted to chemical energy trapped in carbon bonds during photosynthesis 
      5. Allows animal life to exist!

       

    2. 3 Characteristics of light
  1. Quality -spectrum, colors
bulletPlants look green because they absorb red and blue light, reflecting the green.
  1. Intensity -the amount of energy/area or photons cm-2.
bulletMore intensity = more production
  1. Duration -Hours of sunlight per day
  1. Temperatures
  1. General Effects of on Plants
  1. Controls rate of development due to the effect on chemical rxns
  2. Can control number of days to maturity (heat units in corn)
  1. Specific effects on plants
  1. Cold
  1. Sometimes required for plants to enter into reproductive growth (vernalization)
  2. Some seeds require cold to break dormancy
  3. Improves tuber production (potatoes)
  4. Freezing disrupts cells
  1. Heat
  1. Causes dehydration
  2. High temps destroy proteins
  1. Water
  1. Functions
    1. Medium for Biochemical reactions (rxns), where they occur.
    2. a part of many rxns
    3. Stabilizes temperatures
    4. Provides ‘bulk’ or substance to the cells
    5. Medium for transport of nutrients
  1. Physical Movement of water and other liquids
    1. Mass Flow -the liquid moves together in bulk. Usually in response to gravity
      1. Active Transport = Plant energy is used to cause movement (phloem)
      2. Passive Transport = no plant energy is used (gravity, transpiration)
    2. Molecular movement -liquid moves molecule at a time
      1. Diffusion = movement from high concentration to low concentration
      2. Osmosis = diffusion, with liquid passing through semi-permeable membrane
Demonstration of osmosis. Note that the flow of water is into the tube, through the membrane. This is because the water is moving from where it is highly concentrated (pure water), to where it is less concentrated (sugar solution)

Picture
bullet

The above demonstration is analogous to what occurs in a plant. The cells are similar to the tube. They have sugars and minerals which make the cell water less concentrated. The xylem, contains relatively pure water (high concentration). So water flows out of the xylem and into the cells.

 

  1. Plasmolysis = when water is pulled out of the cell, causing it to collapse. This will occur if the soil water has salts in it or too much fertilizer.
  1. Imbibition, the 3rd type of molecular movement. = Water is attracted electrically. "Adhesion" 
    bulletThis is how seeds often take in water.
  1. Movement of water through plants
  1. Transpiration = Evaporation of water from plants (leaves) through the stomates
  1. Causes water movement through the plants from the roots.
  1. Benefits:
  1. Cools the plant
  1. Mineral transport

 
 Note: Regardless of the benefits, transpiration is merely a consequence of photosynthesis. Because the stomates are open, water vapor escapes. Plants do not transpire on purpose. This is similar to humans. When we breathe, water vapor escapes. It is not our intention to lose water through our mouths.
  1. Process
    1. The water that evaporates from the leaves comes from the cells in the mesophyll of the leaves. (Diffusion).
  1. These cells then have a lower water concentration. So they pull water out of the xylem (osmosis)
  1. IF the xylem is unable to resupply the cells as fast as they lose water, the leaves wilt
  1. As water is pulled out of the xylem in the leaves, a negative pressure (suction) is created.
  1. Water is cohesive (sticks to itself) and so the water is pulled up from the roots in bulk (mass flow)
  1. As the water is pulled up out of the roots by the xylem, it must be re-supplied by the soil. Water is drawn into the roots by Osmosis and by adhesion to the root surfaces

*Note that the process starts with the leaves, NOT the roots.

    1. Guttation The second way water is moved through plants. =Water pushed up and out of the pant due to root pressure. Very little water is moved this way. It is often the cause of "dew drops" on leaf ends when the air is dry. 
  1. Control of water movement
  1. Stomates control the loss of water from the leaves.
  1. Their primary function, however, is to control CO2 and O2 exchange in the leaf. Therefore, transpiration is simply a consequence of that.
  2. Transpiration can cause the loss of 200 -1000 pds of water during the production of 1 pd of dry matter. 
  3. Nutrient transport from the roots does Not require rapid water movement from transpiration.
  1. Control of Stomates
    1. Guard cells are found on both sides of the pore opening.
    2.  K is pumped into the cells using sun energy. This causes water to be sucked into the guard cells. (Osmosis)
    3. This causes the pores to open
  2. Morphological Adaptations on leaves for water conservation
  1. Thickened Cuticle
  2. Fewer Stomates
  3. Stomata sunken into epidermis (reduces wind)
  1. Physiological adaptations to conserve water
  1. Alternative photosynthetic system. CAM. In cacti.
  1. Soils and Mineral Nutrients.
    1. Soil functions for the plants.
      1. Supply water 
      2. Supply minerals 
      3. Anchor Roots. 
    2. Soil water 
      bulletThe amount of water the soil can supply depends on... 
      1. texture (sand, silt and clay) 
      2. Organic Matter content. 
      3. Soil Structure 
    3. 16 Essential Elements (minerals) for Plants = those absolutely needed for plant survival 
      1. From Air:   Hydrogen, Carbon, Oxygen 
      2. From Soil:  
        bulletMacronutrients (= Needed in large quantities.)  Nitrogen, Phosphorous, Potassium, Calcium, Magnesium, Sulfur - 
        bullet Micronutrients (=  Needed in small quantities.)  Boron, Cobalt, Copper, Iron, Manganese, Molybdenum, Zinc

           

Stop Here!

Review the questions for this section. 

See you Tuesday!