Lesson 16 - Transpiration

Lesson Objectives

Content Objective: SL - Students will understand the mechanisms of water transport from roots to leaves, focusing on the role of transpiration, capillary action, and cohesion in moving water through xylem vessels. They will explore the adaptations of xylem for efficient water transport and compare the tissue distribution in transverse sections of dicotyledonous stems and roots.

HL - Students will analyze the generation of root pressure in xylem vessels and the role of active transport of mineral ions in facilitating water movement when transpiration is inadequate. They will also examine the structural adaptations of phloem sieve tubes and companion cells that optimize the translocation of sap within plants.

Language Objective:  SL - Students will explain the processes and adaptations related to water transport in plants using specific scientific terms. They will demonstrate their understanding by drawing, labeling, and annotating diagrams of transverse sections of dicotyledonous stems and roots, describing each tissue's function and location.

HL - Students will describe the physiological processes of root pressure and sap translocation using precise botanical terms. They will articulate the significance of specific adaptations in phloem sieve tubes and companion cells, and explain their functions in facilitating sap flow through oral or written presentations, including detailed diagrams.

Syllabus Details: 

B3.2.7—Transport of water from roots to leaves during transpiration - "Students should understand that loss of water by transpiration from cell walls in leaf cells causes water to be drawn out of xylem vessels and through walls by capillary action, generating tension (negative pressure potentials). It is this tension that draws water up in the xylem. Cohesion ensures a continuous column of water."

B3.2.8—Adaptations of xylem vessels for transport of water - "Include the lack of cell contents and incomplete or absent end walls for unimpeded flow, lignified walls to withstand tensions, and pits for entry and exit of water."

B3.2.9—Distribution of tissues in a transverse section of the stem of a dicotyledonous plant - "Students should be able to draw plan diagrams from micrographs to identify the relative positions of vascular bundles, xylem, phloem, cortex and epidermis. Students should annotate the diagram with the main functions of these structures."

B3.2.10—Distribution of tissues in a transverse section of a dicotyledonous root - "Students should be able to construct diagrams from microscope images to identify vascular bundles, xylem and phloem, cortex and epidermis."

B3.2.17(HL)—Generation of root pressure in xylem vessels by active transport of mineral ions - "Root pressure is positive pressure potential, generated to cause water movement in roots and stems when transport in xylem due to transpiration is insufficient, for example when high humidity prevents transpiration or in spring, before leaves on deciduous plants have opened."

B3.2.18 (HL)—Adaptations of phloem sieve tubes and companion cells for translocation of sap - "Include sieve plates, reduced cytoplasm and organelles, no nucleus for sieve tube elements and presence of many mitochondria for companion cells and plasmodesmata between them. Students should appreciate how these adaptations ease the flow of sap and enhance loading of carbon compounds into phloem sieve tubes at sources and unloading of them at sinks."

Activites

Activity 1  - Notes on Transpiration

Activity 2 - Assignment: Microscopic Analysis of Dicotyledonous Plant Tissues

Instructions:

Part 1: Observation of Stem Tissues

• Place the prepared slide of a dicotyledonous plant stem under the microscope.

• Observe at low magnification first to locate the vascular bundles, then switch to a higher magnification to view detailed cellular structures.

• Identify the following tissues:

  • Epidermis: Note its position and protective features.

  • Cortex: Observe the type of cells and their storage capabilities.

  • Vascular Bundles: Focus on the arrangement in a ring, differentiating between xylem and phloem.

• Draw a labeled diagram of the stem section. Indicate the location and function of each tissue.

Part 2: Observation of Leaf Tissues

Place the prepared slide of a dicotyledonous plant leaf under the microscope.

Low Magnification Observation:

1. • Begin by viewing the leaf at low magnification to identify the general layout of tissues.

   • Locate the larger structures such as veins and the overall distribution of the spongy and palisade mesophyll.

High Magnification Observation:

1. • Switch to a higher magnification to observe detailed cellular structures.

   • Focus on specific adaptations for gas exchange.

Identify the Following Tissues and Adaptations:

1. • Waxy Cuticle: Observe the shiny, thick layer on the top surface of the leaf. Note how this layer helps reduce water loss.

   • Epidermis: Identify the upper and lower epidermis. Observe their compact structure and lack of chloroplasts, which helps in reducing water loss while protecting the leaf.

   • Palisade Mesophyll: Locate these column-shaped cells just below the upper epidermis. Note their high concentration of chloroplasts for maximum light absorption.

   • Spongy Mesophyll: Identify this layer beneath the palisade mesophyll. Observe the loose arrangement of cells with air spaces. Discuss how these air spaces facilitate the exchange of gases (CO2 and O2) between the leaf and the environment.

   • Stomata and Guard Cells: Find these structures mainly on the lower epidermis. Examine how guard cells control the opening and closing of stomata, thus regulating gas exchange and transpiration.

   • Veins (Vascular Bundles): Focus on the arrangement of veins within the leaf. Differentiate between the xylem (carries water and minerals) and phloem (transports sugars), noting their crucial role in the leaf’s function and overall plant health.

Draw a Labeled Diagram:

1. • Create a detailed, labelled diagram of the leaf section.

   • Indicate the location and function of each tissue and adaptation you observed.

Part 3: Comparative Analysis

• Discuss the differences and similarities in the tissue distribution between the stem and root sections, put this in a table.

Activity 3  - Transpiration Lab Analysis

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Activity 4  - Translocation (HL) Notes

Home Learning

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 B.  Expand Your Knowledge