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Transport in Plants
ICSE Class 8 Biology • Chapter 1 (Detailed Master Notes)
Chapter Overview
How does a giant 300-foot Redwood tree get water from deep underground all the way up to its topmost leaves? Animals have a pumping heart and a network of blood vessels to transport nutrients. Plants don't have hearts, but they have a brilliant system of internal plumbing called the Vascular System.
1.1 The Need for Transport
A plant is a living organism. Its leaves act as chemical kitchens, using sunlight, water, and carbon dioxide to cook food (glucose) during photosynthesis. But leaves cannot absorb water directly from the air—they need the roots to do it. Similarly, the roots cannot make food because they live in darkness—they rely on the leaves to send food down.
Therefore, a two-way transport highway is strictly required: water going UP, and food going DOWN.
1.2 The Vascular System (Xylem and Phloem)
Higher plants possess complex conducting tissues. Together, they form the Vascular Bundle.
| Feature | Xylem ($UPWARD$) | Phloem ($BIDIRECTIONAL$) |
|---|---|---|
| Primary Function | Transports Water and dissolved Minerals strictly from roots to leaves. | Transports manufactured Food (glucose) from leaves to all other parts of the plant. |
| Direction of Flow | Unidirectional (Only Upwards). | Bidirectional (Upwards to growing buds, Downwards to roots). |
| Cell Type | Mostly dead cells (act as hollow pipes). | Living cells (sieve tubes and companion cells). |
| Location | Usually located deeper inside the stem. | Usually located towards the outer side of the stem (just under the bark). |
AI Image Prompt: A microscopic cross-section view of a plant stem. Highlight a distinct circular 'Vascular Bundle'. The inner part of the circle (Xylem) should be colored bright blue, with tiny arrows pointing straight UP. The outer part of the circle (Phloem) should be colored rich green, with tiny arrows pointing both UP and DOWN.
1.3 Absorption of Water by Roots
Roots are uniquely designed for absorption. They are heavily branched, providing a massive surface area. The actual heroes of absorption are microscopic, hair-like extensions called Root Hairs.
Water enters the roots strictly through a physical process called Osmosis.
Osmosis: The strict movement of water molecules from a region of their higher concentration (dilute solution) to a region of their lower concentration (concentrated solution) through a semi-permeable membrane.
The soil outside has plenty of water (dilute). The sap inside the root hair cell is highly concentrated with dissolved salts. The cell membrane acts as a semi-permeable barrier. Because of the concentration difference, water rushes into the root hair cell via osmosis.
1.4 Ascent of Sap
Once water enters the root, how does it physically climb against the immense force of gravity?
- Root Pressure: The continuous incoming flow of water via osmosis creates a physical pushing pressure at the base of the roots. This gently pushes the water column upwards.
- Transpiration Pull (The Main Force): Transpiration is the constant loss of water vapor from the leaves (stomata). As water evaporates, it literally "pulls" the water molecules below it to fill the gap (like drinking through a straw). Because water molecules stick strongly to each other (Cohesion), the entire water column is yanked upward from the roots all the way to the top leaves.
1.5 Translocation of Food
The organized transport of manufactured food (mostly simple sugars) from the green leaves to other plant parts is called Translocation. This happens specifically inside the living Phloem tissue. Unlike water transport, translocating thick, sugary food requires the plant to actively spend chemical energy ($ATP$).
Q1. Why are most of the cells composing mature Xylem tissue entirely dead?
Answer: Dead cells lose their cytoplasm and internal organelles, effectively turning into empty hollow pipes. This hollow structure is functionally perfect because it provides zero resistance, allowing rapid and continuous flow of large volumes of water straight up the stem.
Q2. What would happen to a plant if you precisely peel off a complete ring of bark from its stem?
Answer: The Phloem tissue is located just directly underneath the extreme outer bark. If a complete ring of bark is meticulously removed, the Phloem is severed. Downward translocation of food stops immediately. The roots will eventually starve to death, subsequently resulting in the death of the entire plant.