Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Differentiate between xylem and phloem roles.
• Explain water and mineral uptake by roots.
• Describe the cohesion-tension theory for water transport.
• Explain the pressure-flow hypothesis for sugar transport.
• Describe plant adaptations to dry conditions in South Africa.

Lesson summary

The efficient transport of water, minerals, and food is crucial for the survival and growth of plants. Understanding how plants achieve this is not just academic; it's deeply relevant to South Africa's agricultural sector, which is vital for food security and economic stability. From the maize fields of the Free State to the vineyards of the Western Cape, the health and productivity of our crops depend on the effective functioning of plant transport systems. Problems with water uptake and nutrient distribution can lead to stunted growth, reduced yields, and ultimately, economic losses for farmers and food insecurity for communities.

Lesson notes

2.1 Xylem: Water and Mineral Transport Xylem is the vascular tissue responsible for transporting water and dissolved minerals from the roots to the rest of the plant, primarily the leaves for photosynthesis. Xylem tissue is composed of dead cells, specifically tracheids and vessel elements, which form continuous, hollow tubes.

Tracheids: These are elongated cells with tapered ends and pits in their cell walls. Water moves from one tracheid to another through these pits.

Vessel Elements: These are wider and shorter than tracheids. They are connected end-to-end, forming long vessels. The end walls of vessel elements may have perforations (holes), or they may be completely absent, allowing for efficient water flow.

Water and Mineral Uptake by Roots: Root Hairs: These are extensions of epidermal cells in the root, dramatically increasing the surface area available for water and mineral absorption. Think of them as tiny straws drawing water from the soil.

Pathways of Water Movement: Water moves from the soil into the root via two main pathways: Apoplast Pathway: Water moves through the cell walls and intercellular spaces. This pathway is relatively fast but is blocked by the Casparian strip in the endodermis.

Symplast Pathway: Water moves through the cytoplasm of cells, passing from one cell to the next via plasmodesmata (small channels connecting the cytoplasm of adjacent cells). This pathway is slower but allows for selective uptake of minerals.

Endodermis and Casparian Strip: The endodermis is a layer of cells surrounding the vascular cylinder (stele) in the root. The Casparian strip, a band of suberin (a waxy substance) embedded in the cell walls of endodermal cells, is impermeable to water and ions. This forces water and minerals to enter the symplast pathway, allowing the plant to control which minerals are absorbed. It prevents backflow of water and dissolved minerals out of the xylem.

Osmosis and Water Potential: Water moves from an area of high water potential (e.g., moist soil) to an area of low water potential (e.g., xylem). Osmosis is the movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration. Water potential is influenced by factors like solute concentration and pressure.

Active Transport of Minerals: Mineral ions are often present in the soil in low concentrations. Plants use active transport (requiring energy in the form of ATP) to move these ions into the root cells against their concentration gradient. Specific transport proteins in the cell membranes are involved in this process. Cohesion-Tension Theory of Water Transport in Xylem: This theory explains how water moves up tall trees against gravity.

Transpiration: The evaporation of water from the leaves through stomata creates a negative water potential (tension) in the leaves. Imagine this as the "engine" driving water transport.

Cohesion: Water molecules are attracted to each other due to hydrogen bonds (cohesion). This creates a continuous column of water in the xylem, from the roots to the leaves.

Adhesion: Water molecules are also attracted to the walls of the xylem vessels (adhesion). This helps to counteract the force of gravity.

Tension: The tension created by transpiration pulls the water column up the xylem. The strong cohesive forces between water molecules prevent the column from breaking. The water moves from the roots where water potential is higher, to the leaves, where water potential is lower (more negative) due to transpiration.

Factors Affecting Transpiration Rate: Temperature: Higher temperatures increase the rate of evaporation, leading to increased transpiration.

Humidity: High humidity decreases the rate of evaporation, leading to decreased transpiration.

Wind: Wind removes humid air surrounding the leaves, increasing the rate of evaporation and transpiration.

Light Intensity: Light stimulates the opening of stomata, increasing transpiration. Stomata open to allow CO2 in for photosynthesis.

Water Availability: If the soil is dry, plants will close their stomata to conserve water, decreasing transpiration. 2.2 Phloem: Sugar Transport Phloem is the vascular tissue responsible for transporting sugars (produced during photosynthesis) from source tissues (e.g., leaves) to sink tissues (e.g., roots, fruits, developing leaves).

Phloem consists of living cells: sieve tube elements and companion cells.

Sieve Tube Elements: These are elongated cells connected end-to-end, forming long sieve tubes. They lack nuclei and ribosomes but contain cytoplasm.

Companion Cells: These are adjacent to sieve tube elements and provide them with metabolic support. They contain nuclei, ribosomes, and other organelles. They are connected to the sieve tube elements by plasmodesmata. Pressure-Flow Hypothesis of Translocation in Phloem: This hypothesis explains how sugars are transported in the phloem.