06 Plant Water Relation - part 11 - Transport of food

06 Plant Water Relation - part 11 - Transport of food


Transport of food : 
  • All the plant parts require continous supply of food for nutrition and developement. 
  • In higher plants, there is a great differentiation and division of labour. 
  • Chloroplasts are confined to green cells of leaves where food is synthesized. 
  • The non-green parts like root and stem must received food from leaves. 
  • The part where food is synthesized is called source and while part where it is utilized, is called sink. 
  • Food has to travel from source to sink. 
  • This movement of food from one part to the other part, is called translocation of food. 
Path of translocation : 
  • Food is to be translocated to longer distances in higher plants. Hence plants must have adequate channels for the transport of food. 
  • Sieve tubes and vessels are structurally ideally suited for longitudinal (vertical) translocation. 
  1. The ringing experiment
  2. structure and distribution of phloem
  3. chemical analysis of phloem sap and 
  4. use of isotope 14C, clearly point out that the phloem tissue is primarily responsible for flow of food in longitudinal downward direction. 
  • The horizontal (lateral) translocation occurs from phloem to pith or phloem to cortex via medullary rays in the stem. 
  • Food is always translocated in the form of sucrose (soluble form) and always along the concentration gradient from source to sink. 
  • The transport of food occurs in  - 
  1. Vertical translocation and 
  2. Lateral translocation
1. Vertical translocation : 
  • In vertical (longitudinal) transport, food is translocated in downward direction from leaves (source) to stem and root (sink). 
  • It also occurs in upward direction during germination of seed, bulbils, corm, etc. 
  • Upward translocation also occurs from leaves to growing point of stem, to developing flowers and fruits situated near the ends of the branches of stem.

2. Lateral translocation :
  • It occurs in the root and stem. 
  • When food is translocated from phloem to pith, it is called radial translocation and from phloem to cortex, it is called tangential translocation. 
  • The transport of food through phloem is bidirectional. 
  • Phloem sap contains mainly water and food in the form of sucrose. But sugars, amino acids and hormones are also transported through phloem. 
Mechanism of sugar transport through phloem : 
  • Several mechanisms/ theories like diffusion, activated diffusion, protoplasmic streaming, electro-osmosis, pressure-flow, etc. are put forth. 
  • The most convincing theory is Munch's pressure flow theory or mass flow hypothesis.
  • Ernst Munch proposed that photosynthetic cell synthesizes glucose. Hence, its osmotic concentration increases. 
  • Due to endo-osmosis water from surrounding cells and xylem, is absorbed. 
  • The cell becomes turgid. 
  • Due to increase in turgor pressure, sugar from photosynthetic cell is forced ultimately into the sieve tube of the vein. This is called loading of Vein. 
  • At the sink end, root cell utilizes sugar and also polymerizes excess sugar into the starch. Its osmotic concentration is lowered. 
  • Exo-osmosis occurs. 
  • Water in the root cell is lost to surrounding cells, thereby decreasing the turgidity of cell. 
  • Turgor pressure is lowered. Hence, a turgor pressure gradient is developed from sieve tube in the leaf to the root cell. 
  • Consequently, food is translocated along the concentration gradient, passively. This is Vein unloading. 
  • At the sink end sugar is used and excess water exudes into the xylem. 
  • Main objection to this theory is that this mechanism does not explain bidirectional transport of food. 
  • More over, according to Munch, pressure flow is purely a physical process.

Comments

Post a Comment

Popular posts from this blog

2. Systematics of Living Organisms part 03 - Units of Classification

  2. Systematics of Living Organisms part 03 - Units of Classification Units of Classification : Species : Species is the  principal natural taxonomic unit , ranking below a genus and denoted by  latin binomial  (considered as the basic) unit of classification.  It is a  group of organisms  that can interbreed under natural condition to produce fertile offspring.  It was thought to be an  indivisible, stable and static unit.   However in the modern taxonomy, subdivision of species such as  sub-species, varities and populations  are seen and given more importance. Genus :  Genus is a taxonomic rank or category  larger than species  used in the biological classification of living and fossil organisms.  Genus is a  group of species   bearing close resemblance to one another in their morphological characters but they do not interbreed.  e.g.  Tiger, Leopard, Lion  all three belong to sam...
Read more

2. Systematics of Living Organisms - part 04 - Nomenclature

  2. Systematics of Living Organisms - part 04 - Nomenclature Nomenclature : The art of naming the objects is in fact, a science called  nomenclature.  All living organisms are known by a particular name. 1.Vernacular / Local names / Common names:  Widely distributed organisms have a large number of common names.  Pansy (Viola  tricolor  L.)  grown in most European and American gardens has about 50 common English names.  In a multilingual country like India, almost all useful plants have local names which differ from language to language and even from dialect to dialect.  As in Ayurveda,  mango (Mangifera  indica  L.)  is known by over 50 different names, all in the Sanskrit language. Hence the common names obviously have limited usage and for universal applications, a unique name for a particular individual is very much essential. 2. Scientific Names :  To overcome the difficulties raised by common names, scient...
Read more

2. Systematics of Living Organisms - part 05 -Salient features of Five Kingdoms

Image
  2. Systematics of Living Organisms - part 05 -Salient features of Five Kingdoms  Salient features of Five Kingdoms : 1. Kingdom Monera : It contains unicellular organisms with prokaryotic cellular organization.  Monera includes  unicellular prokaryotic organisms. These are omnipresent.  They are found in all types of environment which are not generally inhabited by other living beings.  Few are  photoautotrophs or chemoautotrophs ; but  majority are heterotrophic  in nature.  These organisms do  not have well defined nucleus . DNA exists as a simple double stranded circular single chromosome called as  nucleoid. Smaller circular molecules of DNA as extrachromosomal genetic elements called  plasmids  are often present.  Cell wall is made up of  peptidoglycan  (also called  murein ) which is a polymer of sugars and amino acids. Membrane bound organelles e.g.  mitochondria, chloroplast, endopla...
Read more