MECHANISM FOR UPTAKE OF NUTRIENT IN PLANTS

M.Nadeem AshrafSoil & Environmental sciencesFaculty of agriculture, Rawalakot

RecallTransport Mechanism

Passive vs. ActivePlant Transport Tissues

Xylem Vessel elements Tracheids

Phloem Sieve tube member Companion cells

Within a plant, the apoplast is the free diffusional space outside the plasma membrane. It is interrupted by the Casparian strip in roots, air spaces between plant cells and the cuticula of the plant.

Structurally, the apoplast is formed by the continuum of cell walls of adjacent cells as well as the extracellular spaces, forming a tissue level compartment comparable to the symplast. The apoplastic route facilitates the transport of water and solutes across a tissue or organ.[1] This process is known as apoplastic transport.

The apoplastic and symplastic pathways

Apoplast

PATHWAYS FOR WATER UPTAKE

Plasmodesmata

Plasmodesmata Plasmodesma allow molecules to travel between plant cells through the symplastic pathway

symplast The symplast of a plant is

the inner side of the plasma membrane in which water (and low-molecular-weight solutes) can freely diffuse.

The plasmodesmata allow the direct flow of small molecules such as sugars, amino acids, and ions between cells. Larger molecules, including transcription factors and plant viruses, can also be transported through with the help of actin structures

MineralsH2O CO2

O2

CO2 O2

H2O Sugar

Light

A variety of physical processes Are involved in the different types of transport

Sugars are produced byphotosynthesis in the leaves.

5

Sugars are transported asphloem sap to roots and otherparts of the plant.

6

Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.

4

Transpiration, the loss of waterfrom leaves (mostly through

stomata), creates a force withinleaves that pulls xylem sap upward.

3

Water and minerals aretransported upward from

roots to shoots as xylem sap.

2

Roots absorb waterand dissolved minerals

from the soil.

1

Figure 36.2

Roots exchange gases with the air spaces of soil, taking in O2 and discharging CO2. In cellular respiration, O2 supports the breakdown of sugars.

7

Transport of Xylem Sap

Water Movement in Xylem through TACT Mechanism;

Four important forces combine to transport water solutions from the roots, through the xylem elements, and into the leaves. These TACT forces are:

transpirationadhesioncohesiontension

Transport of Xylem Sap

Transpiration involves the pulling of water up through the xylem of a plant utilizing the energy of evaporation and the tensile strength of water. The previous section describes transpiration more fully.

Adhesion is the attractive force between water molecules and other substances. Because both water and cellulose are polar molecules there is a strong attraction for water within the hollow capillaries of the xylem.

Transport of Xylem Sap

Cohesion is the attractive force between molecules of the same substance..

Tension can be thought of as a stress placed on an object by a pulling force. This pulling force is created by the surface tension which develops in the leaf's air spaces.

A combination of adhesion, cohesion, and surface tension allow water to climb the walls of small diameter tubes like xylem. This is called capillary action

Root Pressure: A Mechanism to "Push" Xylem Sap Up the Plant

At night, transpiration is almost nil.  However, the root cells continue to actively transport minerals into the xylem and the phloem).

This active transport lowers the water potential within the stele

Water passively flows into the roots, pushing the water up against gravity

Water that reaches the leaves is often forced out, causing a beading of water upon the leaf tips known as guttation

In most plants, however, root pressure is not the primary mechanism for transporting the xylem

Tall trees generate

The Translocation of Phloem

Phloem loading results in a high solute concentration at the source end. This creates hypnotic conditions in the phloem, causing

water to flow into the phloem Hydrostatic pressure builds in the sieve tube, but it is

greatest in the sourceAt the sink, osmosis occurs with the unloading of sugar -

water flows out of the phloem The buildup of pressure at the source and the reduction

of that pressure at the sink causes water to flow from source to sink, carrying the sugar along with it.

Water is recycled via transport in the xylem