CHAPTER 8(b) - TRANSPORT SYSTEM IN VASCULAR PLANTS

XYLEM AND PHLOEM 

-Xylem tissues transport water and dissolved minerals
-phloem tissue transport organic compound and dissolved  food substance
-transportation of water  substances by phloem is called translocation

UPTAKE OF WATER BY THE ROOT

-Water is absorbed into root hairs by osmosis as a result of water potential

-water potential exist because higher concentration of dissolved substance in the cell sap of the vacuoles

-the water potential of the soil solution is higher than the cell sap of the root hair

-water molecules will move to the root hair from the soil by osmosis

-via the fully permeable cellulose wall and semi permeable cell membrane

-At times , water is actively taken into the root and require energy

MOVEMENT OF WATER ACROSS THE CORTEX OF THE ROOT 

-From ROOT HAIR , water is passed through the cortical region of the root

-comprising mostly of parenchyma cells

-water movement across the root cortex may occur in three ways

1. APOPLAST PATHWAY

-water diffuse through the pores of cellulose cell wall

-pathway works only when there is a water potential gradient

-water potential gradient of the root cells is always negative than the soil solution

-water is also drawn is this pathway by effect of transpirational stream

-however, upon reaching the endodermis cells, water is forced to move from the apoplast pathway into the cytoplasm of the cells

-as movement of water is prevented by the waterproof suberin of the casparian strip

-in this way water does not leak easily

2. SYMPLAST PATHWAY

-water is able to move quickly across the root through pores called plasmodemata

-which link the cytoplasm of adjacent  cortical cells

-water moves down the water potential gradient

- water potential gradient is always maintained to the xylem through the cortex  from the root hairs

3. VACUOLAR PATHWAY

-water is transported by osmosis from the vacuole of one cell to the vacuole of another 

-through the cell wall

-in this pathway water is transported is a slower rate

-dependent on water potential gradient too

UPTAKE OF INORGANIC IONS BY ROOTS

-Inorganic ions are essential for the healthy growth of a plant

-typically , inorganic ions are taken up by roots by two ways

1. PASSIVE UPTAKE

-Inorganic ions are absorbed into root hairs from the soil by diffusion

-the concentration of the inorganic ions in the soil are higher than the cell sap of the root hairs

-no energy is required

2. ACTIVE UPTAKE

-Inorganic ions are actively transported into the root hair against the concentration gradient

-this is done with the help of channel protein, carrier protein and tonoplast found in root hair

-the ions that released from the complex into the cytoplasm of the root cells cannot move out

-will accumulate in the cytoplasm of the cell

-this causes concentration of ions in the root cells is higher than the soil ions 

-the carrier molecule that are released will return to carry new ions from outside cell membrane

-most of the inorganic ions are taken up using this method

-once absorbed by the root hairs ,the inorganic ions will move together with water as a solution across the root cortex either with that three ways

-like water, the movement of inorganic ions may be slowed down by the casparian strip when being transported through the endodermal cells

-inorganic ions will then actively transported  into the xylem from the endodermis

-this will be followed by an influx of water into the xylem which form the basic of root pressure

MOVEMENT OF WATER AND INORGANIC IONS UP THE XYLEM

-Water and inorganic ions move up the stem of a plant through the xylem

-three theories are used to explain the movement of xylem sap up the plant sap against gravity

-root pressure, cohsion tension and transpirational pull

1. ROOT PRESSURE THEORY

-the active transport of inorganic ions into the xylem from endodermal cells

-causes the water potential of the xylem sap to lower drastically

-draws the water  from surrounding root cells to enter the xylem by osmosis

-the hydrostatic pressure generated by this method helps to push water to stem

-but not strong enough to reach the top of tall trees

-root pressure is created by active process which requires energy

-this can be demonstrated when roots subjected to high temperature or poison

-wont show any water being forced out of the stem

-root pressure is believed to cause guttaiton 

-guttation is a process which water oozes out at the tips of the leaves

-normally occurs night or humid day

-guttation ensures water  and dissolved inorganic ions continue to move up a plant when transpiration rate are low

-root pressure alone are too weak to move water up to leaves

2. COHESION  TENSION  THEORY

-transportation of water up the stem was discovered to be pulled from the top of the plant

-this force is caused by the evaporation

-as we know , water will move from high water potential to low water potential

-evaporation occur because the water potential of the atmosphere is lower that in the leaves                 -this will create  a tension which pull water molecules through the xylem vessels

-As water molecules leave the xylem to replace those lost by evaporation, they pull other water molecules with them

-the strong cohesion or stickiness of water allow this to happen

-continuous column of water moving through the xylem vessels forms the transpiration stream

-when water is pulled out of the xylem vessel, a low pressure is created inside the xylem 

-called transpirational pull

-the pull generated this way is sufficient to move water up to the leaves