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 and movement through the xylem.
• Explain the cohesion-tension theory for water transport.
• Describe sugar translocation using the pressure flow hypothesis.
• Discuss factors affecting transpiration.

Lesson summary

In South Africa, agriculture is a cornerstone of our economy and food security. Understanding how plants transport water, nutrients, and sugars is crucial for improving crop yields, managing water resources effectively, and developing sustainable farming practices. This week, we delve into the intricate transport systems that sustain plant life, building upon our understanding of plant cell structure and function. From the sprawling vineyards of the Western Cape to the maize fields of the Free State, the principles we learn this week directly impact the success of South African agriculture.

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 all other parts of the plant. Xylem is composed of dead cells, primarily tracheids and vessel elements. These cells are hollow and interconnected, forming a continuous pipeline for water movement.

Tracheids: Elongated cells with tapered ends. Water moves between tracheids through pits, which are thin, porous regions in the cell walls. Tracheids provide structural support to the plant.

Vessel elements: Wider and shorter than tracheids. They are arranged end-to-end to form long vessels. Perforations in the end walls of vessel elements allow for more efficient water flow.

Water Uptake by Roots: Water enters the plant through root hairs, which are extensions of epidermal cells. Root hairs increase the surface area for water absorption. Water moves from the soil into the root hairs by osmosis, down a water potential gradient. The water potential is lower in the root cells than in the soil solution.

Pathways of Water Movement to the Xylem: Water can move from the root hairs to the xylem via two pathways: Apoplast pathway: Water moves through the cell walls and intercellular spaces. This pathway is relatively unrestricted, but water cannot enter the xylem directly via the apoplast pathway because of the Casparian strip.

Symplast pathway: Water moves through the cytoplasm of cells, passing from cell to cell via plasmodesmata (channels that connect the cytoplasm of adjacent cells). The Casparian strip, a band of suberin (a waxy substance) in the cell walls of the endodermis, blocks the apoplast pathway and forces water to enter the symplast pathway. This ensures that the plant controls which minerals enter the xylem, preventing the uptake of potentially harmful substances. Once inside the endodermal cells' cytoplasm, water and selected minerals are released into the xylem. 2.2 Transpiration and the Cohesion-Tension Theory Transpiration is the loss of water vapor from the leaves and other aerial parts of the plant through stomata (small pores on the leaf surface). Transpiration creates a negative pressure or tension in the xylem, which pulls water up from the roots.

Cohesion-Tension Theory: This theory explains how water moves up the xylem against gravity. It relies on three key properties of water: Cohesion: Water molecules are attracted to each other through hydrogen bonds. This creates a continuous column of water within the xylem.

Adhesion: Water molecules are attracted to the walls of the xylem vessels. This helps to counter the force of gravity.

Tension: Transpiration creates a negative pressure (tension) in the leaves. This tension pulls the water column up the xylem from the roots. Imagine sucking water through a long straw. The tension you create in your mouth pulls the water up the straw. Similarly, transpiration creates tension in the leaves, pulling water up the xylem.

Factors Affecting Transpiration Rate: Temperature: Higher temperatures increase transpiration rate.

Humidity: Higher humidity decreases transpiration rate.

Wind speed: Higher wind speed increases transpiration rate.

Light intensity: Higher light intensity increases transpiration rate (because it stimulates stomatal opening).

Water availability: Decreased water availability decreases transpiration rate. 2.3 Phloem: Sugar Transport (Translocation) Phloem is the vascular tissue responsible for transporting sugars (produced during photosynthesis) from source to sink.

Source: A source is a part of the plant that produces sugars (e.g., leaves).

Sink: A sink is a part of the plant that uses or stores sugars (e.g., roots, developing fruits, growing stems). Phloem is composed of living cells called sieve tube elements and companion cells.

Sieve tube elements: Elongated cells that are connected end-to-end to form long sieve tubes. Sieve tube elements lack a nucleus and other organelles, but they contain cytoplasm and are connected by sieve plates (perforated end walls).

Companion cells: Located adjacent to sieve tube elements and provide them with metabolic support. They are connected to the sieve tube elements by plasmodesmata.

Pressure Flow Hypothesis: This theory explains how sugars are translocated in the phloem.

Loading: Sugars are actively transported from the source cells into the sieve tube elements. This increases the sugar concentration in the sieve tube elements, lowering the water potential.

Water Uptake: Water enters the sieve tube elements from the adjacent xylem by osmosis, due to the lower water potential. This increases the pressure in the sieve tube elements.

Bulk Flow: The increased pressure pushes the sugar-rich sap down the sieve tube towards the sink.

Unloading: Sugars are actively or passively transported from the sieve tube elements into the sink cells. This increases the water potential in the sieve tube elements.