Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Explain the structure and function of xylem and phloem.
• Describe water uptake by roots via osmosis.
• Explain the transpiration stream (cohesion, adhesion, pull).
• Describe translocation in the phloem.
• Discuss factors affecting transpiration rate.

Lesson summary

This week, we delve into the fascinating world of plant transport systems. Understanding how plants move water, nutrients, and sugars is crucial. Think about the delicious fruit you eat – how did the plant deliver the sugars to make it sweet? Think about the trees providing shade in our hot South African summers – how do they get water all the way to their leaves? This is all thanks to sophisticated transport mechanisms. These mechanisms are essential not only for plant survival but also for the ecosystems they support and the food security we rely on. South Africa's diverse flora, from the fynbos to the savanna, depends entirely on efficient transport systems.

Lesson notes

2.1 Xylem: The Water Highway Xylem is the vascular tissue responsible for transporting water and dissolved minerals from the roots to the rest of the plant. Imagine it as the plant's plumbing system. Xylem tissue is composed of several cell types, primarily: Tracheids: These are elongated, dead cells with tapered ends and thickened, lignified cell walls. The lignin provides structural support and prevents the cells from collapsing under pressure. Tracheids have pits (small pores) in their walls that allow water to move laterally between adjacent tracheids.

Vessel Elements: These are shorter and wider than tracheids, also dead at maturity, and arranged end-to-end to form long, continuous tubes called vessels. The end walls of vessel elements are either perforated (with many holes) or completely absent, creating a less resistant pathway for water flow than tracheids. Vessels are more efficient at water transport than tracheids but are also more prone to cavitation (air bubble formation).

Structure-Function Relationship of Xylem: Dead Cells: Being dead at maturity allows xylem cells to form hollow tubes with no cytoplasm, minimizing resistance to water flow.

Lignified Walls: Lignin provides strength and rigidity, preventing the xylem vessels from collapsing under negative pressure created by transpiration.

Pits: Allow for lateral movement of water between adjacent xylem vessels, providing alternative pathways if one vessel is blocked.

Vessels (in flowering plants): The continuous tubes formed by vessel elements provide a less resistant pathway for rapid water transport. 2.2 Water Uptake by Roots: Water moves from the soil into the roots through osmosis. The soil water typically has a higher water potential (lower solute concentration) than the cytoplasm of root hair cells.

Therefore, water moves down the water potential gradient into the root hair cells.

Root Hair Cells: These are specialized epidermal cells with long, thin extensions that greatly increase the surface area for water absorption.

Osmosis: The movement of water from a region of high water potential to a region of low water potential across a semi-permeable membrane.

Apoplast Pathway: Water moves through the cell walls and intercellular spaces without entering the cytoplasm. This pathway is relatively fast but is blocked by the Casparian strip in the endodermis.

Symplast Pathway: Water enters the cytoplasm of the root hair cells and moves from cell to cell through plasmodesmata (cytoplasmic connections). This pathway is slower but allows the plant to regulate the uptake of water and minerals.

Casparian Strip: A band of suberin (a waxy, waterproof substance) in the endodermal cell walls that forces water and minerals to enter the symplast pathway before reaching the xylem. This allows the plant to control which minerals enter the xylem.

Root Pressure: The positive pressure in the xylem of roots caused by the accumulation of water. Root pressure can force water up the xylem to some extent, especially in small plants. It is most noticeable at night when transpiration rates are low. Guttation (the exudation of water droplets from leaf tips) is a result of root pressure.

However, root pressure is not the primary mechanism for water transport in tall plants.

Example: Imagine a farmer in KwaZulu-Natal who over-fertilizes his maize field. The high concentration of salts in the soil decreases the water potential of the soil water. This makes it difficult for the maize roots to absorb water, leading to wilting even though the soil is wet. 2.3 Transpiration Stream: The Engine of Water Transport The transpiration stream is the continuous flow of water from the roots to the leaves, where it evaporates through the stomata. This process is driven by transpiration, the evaporation of water from the leaves.

Transpiration: The loss of water vapor from the leaves and other aerial parts of the plant. Most transpiration occurs through the stomata (small pores on the leaf surface).

Cohesion: The attraction between water molecules due to hydrogen bonding. This allows water molecules to stick together and form a continuous column in the xylem.

Adhesion: The attraction between water molecules and the hydrophilic walls of the xylem vessels. This helps to prevent the water column from breaking.

Transpiration Pull: The negative pressure created by transpiration in the leaves pulls water up the xylem from the roots. This is the primary force driving water transport in plants, especially tall trees.

Stomata: Pores on the leaf surface that allow for gas exchange (CO2 uptake for photosynthesis and O2 release) and transpiration. The opening and closing of stomata are regulated by guard cells, which respond to environmental factors such as light intensity, CO2 concentration, and water availability.

Explanation of the Transpiration Stream: Water evaporates from the mesophyll cells in the leaves into the air spaces surrounding them.