Transport in plants


Plants  lack both interstitial  fluid as well as a regular circulation  system. Even then they have to move ( transport) various  types of substances  not only to short distance  but also to every long distances. Substances  moves over short distances through diffusion  and active transport  supplemented by Cytoplasmic streaming. Long distance transport  occurs through vascular  systems, xylem and phloem. This transport of substances  over longer distances through the vascular  tissue  is called translocation. It occurs through mass flow. Translocation  operates either  due to positive hydrostatic  pressure gradient  as in phloem  or a negative  hydrostatic  pressure gradient as in xylem. The direction of translocation is essentially  unidirectional  in case of water ( from root to stem, leaves, flowers and fruits). It is multidirectional in case  of minerals and organic solutes. Organic compounds are synthesized in leaves. They are exported to all other parts including  every living cell, growing  points, fruits, and storage organs. Senescent  organs and leaves  pass out most of their nutrients, especially  the mineral ones, before falling  down from the plant. Plant hormones and other chemical stimuli are transported in every small amounts. Some of them are transported  in polarised  or unidirectional  manner while others diffuse to all parts. 

Means of Transport 

Passage of materials into and out of the cells is carried out by a number of methods _ diffusion, facilitated diffusion,  active transport. 

(a) Diffusion : Movement  by diffusion is passive and slow. It occurs along the concentration gradient. 

(b) Facilitated  Diffusion : Particles which are lipid soluble can easily pass directly  through the cell membrane  as it is mainly  made of it. The hydrophilic  solutes, find it difficult to pass through  the membrane. Their movement  has to be facilitated. For this the membranes  possess aquaporins and ion channels. 

Elight different  types of aquaporins or water channels have been recorded. Aquaporins are membrane proteins for passive transport of water soluble substance. They do not set up a concentration gradient.  No energy is utilized. The diffusion of hydrophilic substances along the concentration  gradient through fixed membranes transport  protein without involving energy expenditure,  is called facilitated diffusion. This diffusion  is very  specific as it  allows  cell to select substances for uptake. It is sensitive to inhibitors  ( which react with protein side chains) as well as  show  saturation effect. Two major types of transport   proteins are known viz., carrier proteins. Carrier proteins bind the particular solute to be transported and deliver the same to the other side of the membrane. Channel proteins are  usually gated, i.e., they may be open or closed. When the ' gate ' is open, the solute of an appropriate size may diffuse.  

  Ion channels allow passage of their own specific ions. They  are  often gated _ voltage gated, mechanical  gated,  ligand gated. The gates  open under specific conditions, e.g., K+ channels  in nerve  conduction. Certain pores called porins  are present in the outer membrane of plastids, mitochondria and some bacteria. They are large protein pores which allow small sized proteins to pass through. Normally passage is allowed for only small sized particles. Passage of some important solutes is connected with the occurrence of transport  or carrier proteins.

Active Transport:

Pumps are proteins that use energy to carry substances  across the cell membrane. These pumps can transport  substances from a low concentration  to a high concentration  ( ' uphill' transport. Transport rate reaches a maximum  when all the protein transporters are being used or are saturated . Like enzymes  the carrier protein is very specific  in what it carriers  across the membrane. These proteins are sensitive to inhibitors that react with protein side chains. Active transport is faster than passive transport. Some carrier proteins allow Transport only if two types of molecules move together. This is called cotransport. It is of two types. In symport method of cotransport, both molecules cross the membrane in the same direction at the same time. In antiport method of cotransport, both molecules move in opposite direction. When a molecule moves across a membrane independent of other molecule,  the process is called uniport. 

Comparison of Different Transport Processes:

Proteins in the membrane are responsible  for facilitated  diffusion  and active transport  and hence show common characteristics of being highly selective; they are liable to saturate, respond to inhibors  and are under  hormonal Regulation.  

Movement  of substances: 

Substances   move through  the plant  by means of  diffusion, facilitated , active transport and mass or bulk flow. Gases move into and out of the plant entirely through diffusion. Movement  of water into and out of cells occurs by a special type of diffusion called osmosis. Ions move into cells through diffusion, facilitated diffusion  and active transport. These methods cannot provide a mechanism  for translocation or long distance Transport . For example movement  of a molecule through diffusion across a cell of 500 im size will require 2.5 seconds. 1 metre transport through diffuion  will approximately  take 32 years. To overcome this deficiency, plants have developed  a mass or  Bulk flow  system which operates through development  of pressure  difference between  the source and sink. In mass  or bulk  flow all the substances  dissolved  or suspended  in solution travel at the same pace, just as slit suspended in the flowing river. It is quite different where substances  move independently  according  to their concentration  gradients. 

Mineral uptake by  Roots 

Minerals exist in the soil as ions which cannot directly  cross the cell membranes. The concentration  of ions is some 100 times more in root interior than in the soil. All minerals  cannot be passively  absorbed.  The movement of ions from soil to interior of root is a against  concentration  gradient and requires an active transport. Specific  ion pumps  occurs in the membrane  of root hairs. They pump mineral ions from soil to cytoplasm  of epidermal  cells of root hairs. Energy is provided by ATP. Respiratory  passes into the root without ATP, must be through a passive technique. For active transport, AT_ Pases are present  over the plasma membranes of root epidermal  cells. They  establish  an electro _ chemical proton gradient for supplying energy  for movement  of ions. The ions are again checked and transported inwardly by transport  proteins  present over the endodermal cells. Endodermis allows the passage of ions inwardly but not outwardly. Because of the layer of suberin  it also controls the quantity  and type of ions to be passed into xylem. Inward flow of ions from epiblema to xylem is along the  concentration  gradient.  The collection  of ions in the xylem is responsible  for water potential  gradient  in the root  that helps in osmotic entry of water as  well as its passage to xylem. In the xylem, minerals are carried up along with the  flow of xylem solution. In leaves the cells absorb the minerals selectively  through  membrane  pumps.

Translocation  of Mineral Ions in the Plant 

Though it is generally  considered that xylem transports  inorganic nutrients while phloem transports organic nutrients,  the same is not exactly true.In xylem sap, nitrogen travels as inorganic ions, as well as organic form of amino acids  and  related  compounds.  Small amounts of P and S are passed in Xylem as organic compounds.  There is also exchange of materials between xylem and phloem. Mineral elements  pass up xylem in both inorganic and organic form. They reach meristems and individual  cells for storage.  Minerals are unloaded  at fine vein endings through  diffusion. They are picked up by cells through active  uptake. 

There is remobilization  of minerals from older senescing  parts. Nickel has a prominent role in this activity. The senescing  leaves send out many minerals  like nitrogen, sulphur, phosphorous  and potassium.  Elements  incorporated  in structural  components are, however,  not remobilised,  e.g., calcium . The remobilised minerals become available  to young growing  leaves and other sinks.


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