Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the structure and function of xylem and phloem.
• Explain the cohesion-tension theory for water transport.
• Explain the pressure flow hypothesis for sugar transport.
• Compare and contrast xylem and phloem transport.

Lesson summary

The survival and growth of plants depend on efficient transport systems. Just like our own bodies need blood to deliver nutrients and remove waste, plants need systems to transport water, minerals, and sugars throughout their structures. This is especially important in a country like South Africa, where diverse climates and agricultural practices demand that plants are able to efficiently utilize available resources and withstand environmental stresses. Understanding these transport mechanisms is crucial for understanding plant physiology and how we can optimize crop production and manage our natural resources effectively.

Lesson notes

2.1 Vascular Tissue: Xylem and Phloem Plants possess specialized vascular tissues, xylem and phloem, that form a continuous network throughout the plant, facilitating the long-distance transport of water, minerals, and organic compounds.

Xylem: Structure: Xylem is primarily composed of dead cells, namely tracheids and vessel elements.

Tracheids: Elongated, spindle-shaped cells with tapered ends. Water moves between tracheids through pits (thin areas in the cell wall).

Vessel elements: Wider and shorter than tracheids. They are connected end-to-end, forming long continuous tubes called vessels. Perforations in the end walls of vessel elements allow for more efficient water flow compared to tracheids. Both tracheids and vessel elements are reinforced with lignin, a rigid polymer, providing structural support and preventing collapse under negative pressure.

Function: The primary function of xylem is to transport water and dissolved minerals (xylem sap) from the roots to the aerial parts of the plant (leaves, stems, flowers). Xylem also provides structural support to the plant.

Direction of Transport: Unidirectional - from roots to shoots.

Phloem: Structure: Phloem is composed of living cells, namely sieve tube elements and companion cells.

Sieve tube elements: Elongated cells connected end-to-end, forming sieve tubes. Unlike xylem, sieve tube elements are living, but they lack a nucleus and ribosomes at maturity.

Sieve plates: The end walls between sieve tube elements are perforated with sieve pores, allowing for the flow of phloem sap.

Companion cells: Located alongside sieve tube elements, providing metabolic support to the sieve tube elements. They contain a nucleus, ribosomes, and other organelles and are connected to sieve tube elements via plasmodesmata (cytoplasmic connections).

Function: The primary function of phloem is to transport organic compounds (primarily sucrose – sugars produced during photosynthesis) from source tissues (e.g., leaves) to sink tissues (e.g., roots, developing fruits, growing stems).

Direction of Transport: Bidirectional - can move sugars both up and down the plant. 2.2 Water Transport in Xylem: The Cohesion-Tension Theory The ascent of water in the xylem to great heights (e.g., in tall trees) is explained by the cohesion-tension theory. This theory relies on the following principles: Transpiration: The evaporation of water from the leaves through stomata. This process creates a negative pressure (tension) in the leaf cells.

Cohesion: Water molecules are attracted to each other through hydrogen bonds. This cohesion creates a continuous column of water throughout the xylem.

Adhesion: Water molecules are attracted to the walls of the xylem vessels through hydrogen bonds. This adhesion helps to counteract the force of gravity and prevents the water column from breaking.

Mechanism: Transpiration in the leaves creates a negative pressure (tension), which pulls water up the xylem vessels. The cohesion of water molecules ensures that the entire water column is pulled upwards. Adhesion of water molecules to the xylem walls helps to maintain the water column. The water is ultimately drawn from the roots, where water potential is higher. Think of it like sucking water up a straw. The tension created by suction pulls the water column.

Example: Consider a tall Eucalyptus tree in Mpumalanga. The sun's heat causes water to evaporate from the leaves through transpiration. This creates a pull that extends down the xylem vessels all the way to the roots, drawing water up from the soil to replace the water lost from the leaves. 2.3 Sugar Transport in Phloem: The Pressure Flow Hypothesis The translocation of organic compounds in the phloem from source to sink tissues is explained by the pressure flow hypothesis.

Source: A tissue where sugars are produced (e.g., leaves during photosynthesis) or released from storage (e.g., roots during the breakdown of starch).

Sink: A tissue where sugars are used or stored (e.g., roots, developing fruits, growing stems).

Mechanism: Loading at the Source: Sugars (primarily sucrose) are actively transported (requires energy) from the source cells into the sieve tube elements of the phloem. This increases the sugar concentration in the sieve tube elements.

Water Uptake: The high sugar concentration in the sieve tube elements causes water to move from the adjacent xylem into the sieve tube elements by osmosis. This increases the pressure (turgor pressure) in the sieve tube elements at the source.

Pressure Gradient: The increased pressure at the source creates a pressure gradient between the source and the sink.

Bulk Flow: Phloem sap (sugar-rich water) flows from the source to the sink along the pressure gradient. This bulk flow is driven by the difference in turgor pressure between the source and the sink.

Unloading at the Sink: At the sink, sugars are actively transported out of the sieve tube elements and into the sink cells.