SELECTIVE ACCUMULATION OF IONS

 

ACCUMULATION - THE ACQUISITION OF IONS AGAINST A CONCENTRATION GRADIENT

 

ACCUMULATION RATIO =

 

 CONC. INSIDE OF CELL (Ci)

 CONC. OUTSIDE OF CELL (CO)

 

EX. K+

 

CO = 0.14   Ci = 160   

 

   Ci/CO = 160/.14 = 1142

 

A LARGE RATIO USUALLY INDICATES THAT ACTIVE TRANSPORT IS OCCURRING AND THE RESULT IS AN ACCUMULATION OF POTASSIUM IONS

 

A RATIO OF LESS THAN 1 INDICATES

THAT IONS ARE BEING ACTIVELY EXCLUDED OR ITíS BEING EXPELLED FASTER THAN IT CAN BE TAKEN UP

ACCUMULATION FOR UNCHARGED PARTICLES IS STRAIGHTFORWARD.  IT DEPENDS ON THE CONCENTRATION GRADIENT (CHEMICAL GRADIENT)

 

ELECTROCHEMICAL GRADIENTS AND ION MOVEMENT

 

IN IONS, THE CHARGED SOLUTES DEPEND ON A CHEMICAL GRADIENT AND AN ELECTRICAL POTENTIAL GRADIENT

 

A VOLTAGE OR POTENTIAL DIFFERENCE DEVELOPS ACROSS A MEMBRANE DUE TO DIFFERENCES IN CHARGE DISTRIBUTION

 

 

 

 

 

 

 

 

 

ACTIVE TRANSPORT AND ELECTROGENIC PUMPS

 

ATP-ase PROTON PUMPS - LARGE MULTIPROTEIN COMPLEXES

 

FOUND IN THE MEMBRANES OF CHLOROPLASTS AND MITOCHONDRIA

 

USE ENERGY FROM THE HYDROLYSIS OF ATP TO FORM A GRADIENT ACROSS A MEMBRANE TO FORM ATP

 

THREE TYPES OF COTRANSPORT ARE KNOWN TO OCCUR AS FORMS OF ACTIVE TRANSPORT MEDIATED BY CARRIER PROTEINS:

 

1.   WHEN TWO IONS ARE MOVING IN    OPPOSITE DIRECTIONS ACROSS A    MEMBRANE, IN RESPONSE TO ONE    ANOTHER, IT IS KNOWN AS AN    ANTIPORT

 

2.   WHEN TWO IONS ARE MOVING IN    THE SAME DIRECTION, AT THE    SAME TIME, ACROSS A    MEMBRANE IT IS A SYNPORT

 

   a.   PROTON UPTAKE IS COUPLED

      WITH AN UPTAKE OF ANIONS

      AGAINST THEIR ELECTRO-            CHEMICAL GRADIENT

 

   b.   PROTON UPTAKE IS COUPLED

      WITH AN UPTAKE OF UNCHARGED

      SOLUTES (ie, sugars)

 

ION UPTAKE BY ROOTS

 

   DIFFUSION AND APPARENT FREE

   SPACE - IONS ABSORBED WITHOUT

   ENTERING A CELL

 

   EXPERIMENT -

 

      1.   ROOTS GROWN IN NUTRIENT

         DEFICIENT CONDITIONS

 

      2.   ROOTS EXCISED AND PUT IN

         BAGS AND DIPPED INTO

         SOLUTIONS CONTAINING

         THE ION IN QUESTION

         (CaCl2)

 

      3.   UPTAKE MONITORED OVER

         TIME

 

      4.   ROOTS PUT IN WATER

 

      5.   ROOTS PUT IN MgSO4

 

   RESULTS

 

 

 

 

 

 

 

 

 

 

   1.   RAPID UPTAKE OF CALCIUM

      FOLLOWED BY A SLOWER UPTAKE

 

   2.   WHEN PUT IN DISTILLED WATER

      MUCH OF THE CALCIUM IS LOST

 

   3.   WHEN Mg2+ IS ADDED EVEN MORE

      CALCIUM COMES OUT

 

WHATíS HAPPENING?

 

   1. Ca2+ IS RAPIDLY ENTERING

      THE FREE SPACE IN THE CELL

      WALL AND BETWEEN CELLS AS

      WELL AS ENTERING THE CELL

 

   2.   WHEN THE ROOTS ARE PUT IN

      WATER, SOME OF THE Ca2+ IN          THE FREE SPACE WASHES OUT

 

   3.   WHEN THE ROOT IS PUT IN

      MgSO4, SOME OF THE Mg2+          IS EXCHANGED FOR Ca2+

 

THIS EXPERIMENT MEASURES APPARENT FREE SPACE (AFS).

 

APPARENT FREE SPACE = CELL WALL AND INTERCELLULAR SPACES.

 

VARIES FROM PLANT TO PLANT (10-25%)

 

THIS IS THE VOLUME OF A ROOT THAT CAN BE ENTERED WITHOUT CROSSING A MEMBRANE

 

FACTORS INFLUENCING AFS ARE:

 

   1.   SPECIES OF PLANT

   2.   CONDITIONS UNDER WHICH             ROOTS ARE GROWN

   3.   CORRECTION FOR SURFACE FILM

      (AMOUNT OF SURFACE

      ATTRACTION)

 

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