CELLULAR RESPIRATION:RETRIEVING THE ENERGY IN PHOTOASSIMILATES

 

SUCROSE:

 

   HIGHLY MOBILE

 

   SOURCE OF CARBON SKELETONS

 

  

CELLULAR RESPIRATION: AN OVERVIEW

 

   DEFINITION- A SERIES OF PATH-

   WAYS BY WHICH CARBOHYDRATES    AND OTHER MOLECULES ARE

   OXIDIZED FOR THE PURPOSE OF

   RETRIEVING THE ENERGY STORED

   IN PHOTOSYNTHESIS AND TO

   OBTAIN CARBON SKELETONS USED

   IN THE GROWTH AND MAINTENANCE

   OF A CELL

 

   C6H12O6 + 6O2 + 6H2O <--->

 

         6CO2 + 12H2O

 

 

HEXOSE IS OXIDIZED TO CO2, WITH

WATER AS A PRODUCT

 

RESPIRATION OCCURS IN ALL LIVING CELLS OF THE PLANT

 

RESPIRATION OCCURS IN THREE

DIFFERENT STAGES:

 

   1.   GLYCOLYSIS

   2.   CITRIC ACID CYCLE

   3.   ELECTRON TRANSPORT CHAIN

 

THE FINAL STEP IN RESPIRATION IS THE TRANSFER OF ELECTRONS TO AN

OXYGEN, PRODUCING WATER

 

THE SUGAR BROKEN DOWN CAN BE ONE OF MANY:

 

   1.   SUCROSE (GLUCOSE +                   FRUCTOSE)

   2.   FRUCTANS (FRUCTOSE)

   3.   STARCH (GLUCOSE)

   LIPIDS, ORGANIC ACIDS AND

   PROTEINS CAN ALSO BE RESPIRED

 

   WHAT IS BEING RESPIRED IS

   DEPENDENT ON THE:

 

   1.   SPECIES OR ORGAN

   2.   STAGE OF DEVELOPMENT

   3.   PHYSIOLOGICAL STATE

 

   WHAT IS BEING RESPIRED CAN BE

   DETERMINED BY MEASURING O2

   BEING CONSUMED AND CO2 BEING

   GIVEN OFF TO DETERMINE A

   RESPIRATORY QUOTIENT (RQ)

 

   RQ =   MOLES CO2 EVOLVED

         MOLES O2 CONSUMED

 

   WHEN CARBOHYDRATE IS BEING

   RESPIRED THE RQ IS 6CO2/6O2= 1

 

   WHEN LIPIDS AND PROTEINS ARE

   BEING RESPIRED THE RQ IS 0.7

 

   WHEN ORGANIC ACIDS ARE BEING

   RESPIRED THE RQ IS 1.3

 

THINGS THAT CAN AFFECT THE RQ VALUE OBTAINED ARE:

 

   1.   A MIXTURE OF COMPONENTS

      BEING RESPIRED

 

   2.   IF FERMENTATION IS OCCURING

      THERE IS LITTLE CONSUMPTION

      OF OXYGEN AND THE RQ WILL

      BE ABNORMALLY HIGH

 

   3.   TRAPPED O2 AND CO2

  

   4.   PLANTS UNDER STARVATION

      CONDITIONS

 

BREAKDOWN OF SUCROSE AND STARCH

 

STARCH STORED AS:

 

   1.   AMYLOSE

 

      a.   ONE THIRD OF STARCH              IN ALL HIGHER PLANTS

      b.   STRAIGHT CHAINS OF

         GLUCOSE MOLECULES HELD

         TOGETHER BY (1-4)-LINKED

           D-GLUCOSE UNITS

 

   2.   AMYLOPECTIN

 

      a.   HIGHLY BRANCHED CHAINS

         OF GLUCOSE WHERE SHORT

         (1->4)-LINKED  D-GLUCOSE

         UNITS ARE CONNECTED BY

         (1->6)LINKS

 

   BOTH NORMALLY STORED IN

   PLASTIDS AS WATER SOLUBLE

   GRAINS

 

STARCH BREAKDOWN REQUIRE   SEVERAL HYDROLYTIC ENZYMES:

 

   1.   -AMYLASE CLEAVES  -(1->4)

      GLUCOSYL BONDS IN AMYLOSE

      AND AMYLOPECTIN TO FORM:

 

      a.   MALTOSE-     DISACCHARIDES

     

 

 

      b.   LIMIT DEXTRINS- SMALL             NUMBER OF GLUCOSE                   RESIDUES (4 TO 10)

 

      -AMYLASE CAN USE STARCH

   GRAINS AS A SUBSTRATE

 

  

   2.   B-AMYLASE - DEGRADES               AMYLOSE BY SELECTIVELY           HYDROLYZING EVERY SECOND        BOND TO FORM MALTOSE

 

   3.   LIMIT DEXTRINASE - A DE-

      BRANCHING ENZYME

 

      a.   CLEAVES THE (1->6)                  BRANCHING BOND

 

   4.   -GLUCOSIDASE - FINAL

      ENZYME IN HYDROLYSIS

 

      a.   HYDROLYZES MALTOSE TO

         TWO MOLECULES OF GLUCOSE

ENZYMES 1 THRU 4 MEDIATED THE HYDROLYTIC BREAKDOWN OF STARCH. WATER IS ADDED

 

WHEN THE INORGANIC PHOSPHATE (Pi)LEVEL IS HIGH STARCH BREAKS DOWN TO PHOSPHORYLATED SUGARS

 

5.   STARCH PHOSPHORYLASE

 

   STARCH + nPi <-->(GLUCOSE-1-

                     PHOSPHATE)n

 

   a.   CANNOT DEGRADE STARCH           GRAINS

 

   b.   MUST WORK IN CONJUNCTION

      WITH   -AMYLASE

 

      1.   -AMYLASE EXPOSES THE

         INTERIOR OF A STARCH              GRAIN

 

   c.   ACCOUNTS FOR LESS THAN THAN

      HALF OF STARCH DEGRADATION

 

STARCH DEGRADATION OCCURS IN THE PLASTIDS

 

THE FIRST STEPS OF RESPIRATION OCCUR IN THE CYTOPLASM

 

STARCH BREAKDOWN PRODUCTS (GLUCOSE) MUST BE TRANSPORTED FROM INSIDE THE PLASTID TO THE CYTOPLASM

 

SUCROSE BREAKDOWN

 

SUCROSE BROKEN DOWN BY TWO ENZYMES:

 

   1.   SUCROSE SYNTHASE

 

   2.   INVERTASE

 

      a.   ALKALINE (pH OPTIMUM

         NEAR 7.5)

 

      b.   ACID (pH OPTIMUM NEAR

         5)