Transport in Plants
How plants move water up and food around — a complete guide to xylem, phloem, and transpiration.
Chapter Overview
Plants need two separate transport systems to survive. One carries water and minerals from roots to leaves; the other carries sugars and amino acids from leaves to all parts of the plant. Understanding how these systems work — and what environmental factors affect them — is key to acing this topic.
Xylem
Transports water & minerals upward. Made of dead cells. One-way traffic only: roots → leaves.
Phloem
Transports sugars & amino acids. Made of living cells. Goes in multiple directions depending on plant needs.
Transpiration
Water loss from leaves by evaporation. Creates a "pull" that draws water up the xylem. Affected by temperature, wind, humidity.
1. Xylem & Phloem: The Two Transport Networks
The Big Picture
Imagine a plant as a factory with two delivery systems. Xylem is the water pipeline — it brings in raw materials (water and minerals) from the soil. Phloem is the product distributor — it carries finished goods (sugars and amino acids) made in the leaves to every part of the plant that needs them.
These two vessels run alongside each other through roots, stems, and leaves, bundled together in groups called vascular bundles.
Xylem
A transport vessel made of dead cells joined end-to-end without cross walls. Transports water and dissolved mineral ions from roots to leaves. Movement is always in one direction: upward from roots to leaves.
Phloem
A transport vessel made of living cells joined end-to-end with holes in the end walls (sieve plates). Transports dissolved food (mainly sucrose and amino acids) from leaves to storage organs and growing regions. Movement can go in multiple directions.
Why Xylem is Dead But Phloem is Alive
Xylem is dead: The cells die and lose their contents, leaving hollow tubes. This is actually perfect — there's nothing blocking the water flow, and the cell walls are thickened with a substance called lignin that strengthens the plant and prevents the tubes from collapsing.
Phloem is alive: Living cells are needed because transporting sugars requires active transport — which costs energy. Only living cells can do that. The sieve plates (holes between cells) let dissolved sugars flow through easily.
Key memory trick: Xylem = water (both start with X sound), Phloem = food (both have similar sounds). Xylem is "inside" (dead), Phloem is "outside" (living) — though this is just to help you remember the arrangement in a cross-section!
Practice Question 1
A student observes a cross-section of a plant stem. They see a ring of dead, hollow tubes strengthened with lignin in the center, and living cells with sieve plates near the edge. Which tube is which, and why is this arrangement useful?
2. Root Hair Cells: Where Water Enters the Plant
The Entry Point
Water doesn't just float into the plant. It enters through root hair cells — tiny, hair-like projections of epidermal cells in the root. These cells grow between soil particles and are where the plant's water journey begins.
Why Large Surface Area Matters
Why?
osmosis
number of contact points
active transport
Final Thoughts