C4 PHOTOSYNTHESIS

 

PEP + HCO3- ---> OAA ---> malate

 

         IN THE CHLOROPLAST IN

         MESOPHYLL CELLS

________________________________

 

         -CO2

malate  ---> PCR CYCLE

 

          ---> PYRUVATE

 

         IN THE CHLOROPLASTS IN

         BUNDLE SHEATH CELLS

________________________________

 

PYRUVATE   ---> PEP ---> OAA

 

         IN THE CHLOROPLASTS IN

         MESOPHYLL CELLS

 

________________________________

 

 

 

 

C4 PLANTS DIFFER ANATOMICALLY FROM A C3 LEAF IN THAT THEY:

 

   1.   HAVE THINNER LEAVES

 

   2.   HAVE VASCULAR BUNDLES THAT

      ARE CLOSER TOGETHER

 

   3.   HAVE SMALLER AIR SPACES

 

   4.   HAVE KRANZ ANATOMY

 

      a.   EACH VASCULAR BUNDLE IS

         SURROUNDED BY SHEATH OF

         THICK WALLED CELLS                   CONTAINING MANY                   CHLOROPLASTS.

 

      b.   KRANZ = WREATH

 

         NAMED BY HABERLANDT

 

      c.   INSURES CLOSE LOCATION            OF MESOPHYLL CELLS AND

         BUNDLE SHEATH CELLS

 

ECOLOGICAL SIGNIFICANCE OF C4 PHOTOSYNTHESIS

 

C4 PHOTOSYNTHESIS IS NOT INHIBITED BY OXYGEN

 

C4 PLANTS HAVE VERY LOW RATES OF PHOTORESPIRATION

 

C4 PLANTS PERFORM BETTER THAN C3 PLANTS AT HIGH TEMPERATURES AND WORSE THAN C3 PLANTS AT LOW TEMPERATURES.  WHY?

 

   a.   HIGH TEMPERATURES FAVOR THE

      OXYGENATION OF RuBP SO THAT

      THERE IS LESS PGA                  PRODUCTION

 

   b.   LOW TEMPERATURES INHIBIT

      PEP CARBOXYLASE ACTIVITY

 

C4 PLANTS HAVE LOWER TRANSPIRATION RATIOS THAN C4 PLANTS

 

   TR =   moles of H2O TRANSPIRED

         moles of CO2 ASSIMILATED

 

   C4 PLANTS CAN FIX MORE CO2    WHEN ITS STOMATES ARE CLOSED    DUE TO HAVING A LOWER CO2    COMPENSATION POINT.

 

   THE CO2 COMPENSATION POINT IS    WHERE THE RATE OF CO2 UPTAKE =    CO2 EVOLUTION

 

      C3 PLANTS HAVE A CO2                COMPENSATION POINT OF             BETWEEN 20-100 ul CO2/l

 

      C4 PLANTS HAVE A CO2

      COMPENSATION POINT OF

      BETWEEN 0-5 ul CO2/l

 

CRASSULACEAN ACID METABOLISM (CAM)

 

ORIGINIALLY STUDIED IN THE FAMILY, CRASSULACEAE

 

 

SPECIALLY ADAPTED TO SURVIVAL UNDER EXTREMELY DRY CONDITIONS

 

*ARE SUCCULENT

 

   a.   HAVE THICKY FLESHY LEAVES

 

   b.   HAVE CELLS THAT CONTAIN

      LARGE, FLUID FILLED              VACUOLES

 

HAVE AN INVERTED STOMATAL CYCLE

 

   a.   STOMATES OPEN AT NIGHT AND

      CLOSE DURING THE DAY SO             THAT CO2 UPTAKE OCCURS AT

      NIGHT

 

ARE CHARACTERIZED BY ACCUMULATING MALATE AT NIGHT AND SUBSEQUENTLY DEPLETING IT DURING THE DAY

 

 

 

ECOLOGICAL SIGNIFICANCE OF CAM

 

AN ADAPTATION TO EXTREMELY DRY CONDITIONS

 

A LOW RATE OF CARBON ASSIMILATION WILL RESULT IN SLOWER GROWTH BUT GROWTH CAN STILL OCCUR WHEN C3 AND C4 PLANTS WOULD HAVE NO GROWTH

 

SOME PLANTS ARE OBLIGATE CAM PLANTS WHILE OTHERS ARE FACULTATIVE CAM PLANTS

 

EXPORT AND STORAGE OF PHOTOSYNTHETIC PRODUCT

 

PRODUCTS OF THE PCR CYCLE ARE EITHER CONVERTED TO STARCH IN THE CHLOROPLAST OR EXPORTED TO THE CYTOPLASM TO BE CONVERTED TO SUCROSE

 

 

 

THE PURPOSE OF PHOTOSYNTHESIS IS TO PROVIDE ENERGY AND CARBON TO SUPPORT MAINTENANCE AND GROWTH OF THE PLANT.

 

   DURING THE DAY,    PHOTOASSIMILATE IS EXPORTED TO    OTHER TISSUES, AS SUCROSE.

 

   SOME PHOTOASSIMILATE IS    RETAINED AS STARCH IN    CHLOROPLASTS

 

   SOME PHOTOASSIIMILATE IS    RETAINED AS SUCROSE IN THE    VACUOLE

 

STARCH SYNTHESIS IN THE CHLOROPLAST

 

   STARCH EXISTS IN TWO FORMS

 

   a.   AMYLOSE

 

   b.   AMYLOPECTIN

 

 

FRUCTOSE-6-P ----> GLUCOSE-6-P

             <----

 

GLUCOSE-6-P ----> GLUCOSE-1-P

            <----

 

GLUCOSE-1-P + ATP -->ATP-GLUCOSE

                   <-- +PPi

         PPi + H2O ---> 2Pi

 

ADP-GLUCOSE + (1-->4)-GLUCAN -->

                               <--

ADP +  -(1-->4)-GLUCOSYL-GLUCAN

 

SUCROSE SYNTHESIS

 

SUCROSE = GLUCOSE + FRUCTOSE

 

SUCROSE SYNTHESIS OCCURS IN THE CYTOPLASM OF PHOTOSYNTHETIC CELLS

 

 

 

 

UDP-GLUCOSE + FRUCTOSE-6-P --->

                           <---

UDP + SUCROSE-6-P

 

 

SUCROSE-6-P + H2O ---> SUCROSE +

                           Pi 

 

              -OR-

 

UDP-GLUCOSE + FRUCTOSE --->UDP +      

                           SUCROSE

 

SUCROSE OR STARCH

 

SOURCE-SINK RELATIONSHIPS

 

SOURCES:

 

   1.   PHOTOSYNTHETICALLY ACTIVE

      LEAF

 

SINKS:

 

   1.   STORAGE ORGAN

 

   2.   FLOWERS

 

   3.   DEVELOPING FRUITS/SEEDS