The Architecture of a Vascular Plant
Vascular plants have specialized tissues for transport, allowing them to grow much larger than non-vascular plants like mosses. They have three main organs.
Plant Organs
1.Roots: Anchor the plant and absorb water and mineral nutrients from the soil.
2.Stems: Support the leaves and reproductive structures, and transport substances between the roots and leaves.
3.Leaves: The primary sites of photosynthesis. Their flat shape maximizes surface area for sunlight absorption.
Transport Tissues: Xylem and Phloem
Xylem: This tissue transports water and dissolved minerals from the roots to the rest of the plant. The cells that make up xylem (tracheids and vessel elements) are dead at maturity, forming hollow tubes. The movement of water in xylem is a one-way street: up.
Phloem: This tissue transports sugars (produced during photosynthesis) from the leaves (the 'source') to other parts of the plant where they are needed for growth or storage (the 'sink'), such as roots, fruits, or flowers. The cells (sieve-tube elements) are living.
Water Transport in Xylem: Cohesion-Tension Theory
The movement of water up a tall tree defies gravity and is explained by this theory:
1.Transpiration: The process starts at the leaves. Water evaporates from the surface of leaf cells and exits the leaf through small pores called stomata. This creates a negative pressure potential, or tension.
2.Cohesion: Water molecules are highly cohesive (they stick to each other) due to hydrogen bonding.
3.Adhesion: Water molecules also adhere (stick) to the cellulose walls of the xylem.
4.Pulling Action: As one water molecule is 'pulled' out of the leaf by transpiration, it pulls the entire column of water up behind it, all the way from the roots, like drinking through a straw.
Sugar Transport in Phloem: Pressure-Flow Hypothesis
The movement of sugar (translocation) is an active process:
1.Loading at the Source: Sugar is actively transported from leaf cells into the phloem. This high concentration of sugar draws water from the nearby xylem into the phloem via osmosis.
2.Bulk Flow: This influx of water creates high hydrostatic pressure at the source end of the phloem. This pressure pushes the sugar-rich sap along the phloem tube towards areas of lower pressure.
3.Unloading at the Sink: At the sink (e.g., a root or fruit), sugar is actively transported out of the phloem for use or storage. Water then follows by osmosis, moving back into the xylem and reducing the pressure at the sink.