krit.club logo

Transport in Plants - Xylem and phloem

Grade 12IGCSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

Xylem Structure and Function: Xylem is responsible for the transport of water and mineral ions from the roots to the leaves. It consists of dead cells called vessel elements and tracheids. The cell walls are thickened with lignin, providing structural support to withstand the negative pressure created during the transpiration stream.

•

The Transpiration Stream: Water moves upwards through the xylem due to the cohesion-tension theory. H2OH_2O molecules are polar and form hydrogen bonds (cohesion). Adhesion occurs between H2OH_2O and the xylem walls. As water evaporates from the mesophyll cells into air spaces and out through stomata, it creates a tension (negative pressure) that pulls the water column upwards.

•

Phloem Structure and Function: Phloem transports organic solutes, primarily sucrose and amino acids, via a process called translocation. It consists of living sieve tube elements (which have no nucleus or ribosomes to allow free flow) and companion cells, which provide metabolic support and ATPATP for active loading.

•

Translocation (Source to Sink): Solutes move from 'sources' (e.g., photosynthesizing leaves) to 'sinks' (e.g., growing roots, fruits, or storage organs). This movement is explained by the pressure-flow hypothesis, where high hydrostatic pressure at the source and low pressure at the sink drive the flow of sap.

•

Water Potential (Ψ\Psi): Water moves from an area of higher water potential to an area of lower (more negative) water potential. In plants, this is influenced by solute potential (Ψs\Psi_s) and pressure potential (Ψp\Psi_p).

•

Apoplast and Symplast Pathways: Water moves through the root cortex via the apoplast pathway (through cell walls) and the symplast pathway (through the cytoplasm via plasmodesmata). The Casparian strip in the endodermis blocks the apoplast pathway, forcing water into the symplast to regulate mineral uptake.

📐Formulae

Ψ=Ψs+Ψp\Psi = \Psi_s + \Psi_p

Rate of Transpiration=Volume of H2O lostTime\text{Rate of Transpiration} = \frac{\text{Volume of } H_2O \text{ lost}}{\text{Time}}

Velocity of flow=DistanceTime\text{Velocity of flow} = \frac{\text{Distance}}{\text{Time}}

💡Examples

Problem 1:

A plant cell has a solute potential (Ψs\Psi_s) of −0.7-0.7 MPa and is placed in a solution with a water potential (Ψ\Psi) of −0.3-0.3 MPa. At equilibrium, the cell becomes turgid and reaches a pressure potential (Ψp\Psi_p) of 0.40.4 MPa. Calculate the final water potential (Ψ\Psi) of the cell and determine the direction of water movement.

Solution:

Using the formula Ψ=Ψs+Ψp\Psi = \Psi_s + \Psi_p: Ψcell=−0.7 MPa+0.4 MPa=−0.3 MPa\Psi_{cell} = -0.7\text{ MPa} + 0.4\text{ MPa} = -0.3\text{ MPa}. Since the external solution Ψ=−0.3 MPa\Psi = -0.3\text{ MPa} and the cell Ψ=−0.3 MPa\Psi = -0.3\text{ MPa}, the system is at equilibrium.

Explanation:

Water moves from higher water potential to lower water potential. Initially, the external solution (−0.3-0.3 MPa) is higher than the cell's initial Ψ\Psi (assuming Ψp=0\Psi_p=0 initially). Water enters the cell until the internal pressure potential increases the cell's total Ψ\Psi to match the environment.

Problem 2:

Describe the mechanism of active loading in the phloem using the concept of electrochemical gradients.

Solution:

H+H^+ ions are actively pumped out of the companion cells into the leaf mesophyll cell walls using ATPATP. This creates a high concentration of H+H^+ outside. The H+H^+ ions then diffuse back into the companion cells through a co-transporter protein, carrying sucrose molecules against their concentration gradient.

Explanation:

This process uses the H+H^+ gradient to drive the transport of sucrose into the sieve tube elements, which subsequently lowers the water potential (Ψ\Psi), causing H2OH_2O to enter from the xylem and create high hydrostatic pressure.

Xylem and phloem - Revision Notes & Key Diagrams | IGCSE Grade 12 Biology