8.3.6 Transport in Plants

Introduction

Plants need to transport water, minerals, and food to different parts of their body. Water and minerals are absorbed by the roots and transported to the leaves, while food (sugars) is produced in the leaves and transported to other parts of the plant. This transport occurs through specialized tissues called xylem and phloem. Understanding transport in plants helps us understand how plants survive, grow, and reproduce.

Definition

Transport in Plants: Transport in plants is the movement of water, minerals, and food (sugars) from one part of the plant to another through specialized tissues called xylem and phloem.

In simple words: Transport in plants is how water, minerals, and food move around inside a plant.

Core Concept

• Water and minerals – absorbed by roots, transported to leaves.
• Food (sugars) – produced in leaves, transported to other parts.
• Xylem – transports water and minerals.
• Phloem – transports sugars and organic nutrients.
• Transpiration – loss of water vapor from leaves.
• Root pressure – pressure that pushes water up the xylem.
• Cohesion-tension theory – explains water movement in xylem.
• Pressure-flow hypothesis – explains food movement in phloem.

Detailed Explanation

1. Transport of Water and Minerals (Xylem)

Definition: Xylem is the vascular tissue that transports water and minerals from roots to leaves.

• Transpiration pull: Loss of water vapor from leaves creates a negative pressure that pulls water up.
• Cohesion-tension theory: Water molecules are cohesive (stick together) and form a continuous column in the xylem. Transpiration creates tension that pulls this column upward.
• Root pressure: Pressure that helps push water up the xylem.
• Adhesion: Water molecules adhere to the walls of the xylem, helping to counteract gravity.

• Water is absorbed by root hairs from the soil.
• Water moves into the root cortex and then into the xylem.
• Water moves up the xylem due to transpiration pull, cohesion-tension, and root pressure.
• Water reaches the leaves and is used in photosynthesis or lost through transpiration.

2. Transport of Food (Phloem)

Definition: Phloem is the vascular tissue that transports sugars and organic nutrients from leaves to other parts of the plant.

• Composition: Phloem is made of sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
• Sieve tubes and companion cells: Living cells that form tubes for food transport.
• Mechanism: Food moves from source to sink due to:

• Sugars are produced in the leaves (source) by photosynthesis.
• Sugars are actively transported into the phloem.
• Water enters the phloem by osmosis, increasing pressure.
• Pressure pushes the sap towards the sink.
• Sugars are unloaded at the sink (roots, fruits, seeds) for use or storage.

3. Transpiration

Definition: Transpiration is the loss of water vapor from the aerial parts of the plant, mainly through stomata.

• Site: Mainly leaves, through stomata.
• Factors affecting transpiration:
• Temperature
• Humidity
• Wind
• Light
• Soil water
• Importance:
• Creates transpiration pull for water transport.
• Cools the plant.
• Helps in the absorption of minerals.

4. Root Pressure

Definition: Root pressure is the pressure that develops in the roots due to the active transport of minerals into the xylem, causing water to enter by osmosis.

• Cause: Active transport of minerals into the xylem.
• Effect: Water enters the xylem by osmosis, creating pressure.
• Importance: Helps push water up the xylem, especially at night when transpiration is low.

Summary Table of Transport in Plants

Summary Table of Transport Mechanisms

Intermediate Level

Transport of Water and Minerals in Detail

Transport of Food in Detail

Transpiration in Detail

Root Pressure in Detail

Summary Table of Transport in Plants

Important Points

• 1. Plants need to transport water, minerals, and food to different parts of their body.
• 2. Water and minerals are absorbed by the roots and transported to the leaves, while food (sugars) is produced in the leaves and transported to other parts of the plant.
• 3. This transport occurs through specialized tissues called xylem and phloem.
• 4. Transport of Water and Minerals (Xylem):
• - Xylem is the vascular tissue that transports water and minerals from roots to leaves.
• - Composition: Xylem is made of tracheids, vessels, xylem parenchyma, and xylem fibres.
• - Tracheids and vessels: Dead cells that form tubes for water transport.
• - Mechanism: Water moves from roots to leaves due to:
• - Transpiration pull: Loss of water vapor from leaves creates a negative pressure that pulls water up.
• - Cohesion-tension theory: Water molecules are cohesive (stick together) and form a continuous column in the xylem. Transpiration creates tension that pulls this column upward.
• - Root pressure: Pressure that helps push water up the xylem.
• - Adhesion: Water molecules adhere to the walls of the xylem, helping to counteract gravity.
• - Steps:
• 1. Water is absorbed by root hairs from the soil.
• 2. Water moves into the root cortex and then into the xylem.
• 3. Water moves up the xylem due to transpiration pull, cohesion-tension, and root pressure.
• 4. Water reaches the leaves and is used in photosynthesis or lost through transpiration.
• 5. Transport of Food (Phloem):
• - Phloem is the vascular tissue that transports sugars and organic nutrients from leaves to other parts of the plant.
• - Composition: Phloem is made of sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
• - Sieve tubes and companion cells: Living cells that form tubes for food transport.
• - Mechanism: Food moves from source to sink due to:
• - Pressure-flow hypothesis: Sugars are actively transported into the phloem at the source (leaves). This increases the solute concentration, causing water to enter by osmosis. The increased pressure pushes the sap towards the sink (roots, fruits, storage organs).
• - Source: Where sugars are produced (leaves).
• - Sink: Where sugars are used or stored (roots, fruits, seeds).
• - Steps:
• 1. Sugars are produced in the leaves (source) by photosynthesis.
• 2. Sugars are actively transported into the phloem.
• 3. Water enters the phloem by osmosis, increasing pressure.
• 4. Pressure pushes the sap towards the sink.
• 5. Sugars are unloaded at the sink (roots, fruits, seeds) for use or storage.
• 6. Transpiration:
• - Transpiration is the loss of water vapor from the aerial parts of the plant, mainly through stomata.
• - Site: Mainly leaves, through stomata.
• - Factors affecting transpiration: Temperature, humidity, wind, light, soil water.
• - Importance: Creates transpiration pull for water transport; cools the plant; helps in the absorption of minerals.
• 7. Root Pressure:
• - Root pressure is the pressure that develops in the roots due to the active transport of minerals into the xylem, causing water to enter by osmosis.
• - Cause: Active transport of minerals into the xylem.
• - Effect: Water enters the xylem by osmosis, creating pressure.
• - Importance: Helps push water up the xylem, especially at night when transpiration is low.

Remember

• - Xylem = transports water and minerals.
• - Phloem = transports sugars and organic nutrients.
• - Transpiration = loss of water vapor from leaves.
• - Root pressure = pressure that pushes water up.
• - Cohesion-tension theory = explains water movement in xylem.
• - Pressure-flow hypothesis = explains food movement in phloem.
• - Source = where sugars are produced (leaves).
• - Sink = where sugars are used or stored (roots, fruits, seeds).

Common Mistakes

• 1. Confusing xylem and phloem – Xylem = water and minerals; Phloem = sugars and organic nutrients.
• 2. Thinking xylem is living – Xylem is mostly dead.
• 3. Thinking phloem is dead – Phloem is mostly living.
• 4. Confusing transpiration and root pressure – Transpiration = loss of water vapor; Root pressure = pressure that pushes water up.
• 5. Thinking water moves down the xylem – Water moves up the xylem.
• 6. Thinking food moves only up the phloem – Food moves in both directions in the phloem.
• 7. Forgetting the role of cohesion-tension theory – Cohesion-tension theory explains water movement in xylem.
• 8. Forgetting the role of pressure-flow hypothesis – Pressure-flow hypothesis explains food movement in phloem.
• 9. Confusing source and sink – Source = where sugars are produced (leaves); Sink = where sugars are used or stored.
• 10. Thinking transpiration is not important – Transpiration is essential for water transport and cooling.
• 11. Confusing the factors affecting transpiration – Temperature, humidity, wind, light, soil water.
• 12. Thinking root pressure is the main mechanism of water transport – Root pressure is important at night; transpiration pull is the main mechanism during the day.

Quick Revision

• Plants need to transport water, minerals, and food to different parts of their body.
• Water and minerals are absorbed by the roots and transported to the leaves, while food (sugars) is produced in the leaves and transported to other parts of the plant.
• This transport occurs through specialized tissues called xylem and phloem.
• Transport of Water and Minerals (Xylem):
• Xylem is the vascular tissue that transports water and minerals from roots to leaves.
• Composition: Xylem is made of tracheids, vessels, xylem parenchyma, and xylem fibres.
• Tracheids and vessels: Dead cells that form tubes for water transport.
• Mechanism: Water moves from roots to leaves due to:
• Transpiration pull: Loss of water vapor from leaves creates a negative pressure that pulls water up.
• Cohesion-tension theory: Water molecules are cohesive (stick together) and form a continuous column in the xylem. Transpiration creates tension that pulls this column upward.
• Root pressure: Pressure that helps push water up the xylem.
• Adhesion: Water molecules adhere to the walls of the xylem, helping to counteract gravity.
• Steps:
• Transport of Food (Phloem):
• Phloem is the vascular tissue that transports sugars and organic nutrients from leaves to other parts of the plant.
• Composition: Phloem is made of sieve tubes, companion cells, phloem parenchyma, and phloem fibres.
• Sieve tubes and companion cells: Living cells that form tubes for food transport.
• Mechanism: Food moves from source to sink due to:
• Pressure-flow hypothesis: Sugars are actively transported into the phloem at the source (leaves). This increases the solute concentration, causing water to enter by osmosis. The increased pressure pushes the sap towards the sink (roots, fruits, storage organs).
• Source: Where sugars are produced (leaves).
• Sink: Where sugars are used or stored (roots, fruits, seeds).
• Steps:
• Transpiration:
• Transpiration is the loss of water vapor from the aerial parts of the plant, mainly through stomata.
• Site: Mainly leaves, through stomata.
• Factors affecting transpiration: Temperature, humidity, wind, light, soil water.
• Importance: Creates transpiration pull for water transport; cools the plant; helps in the absorption of minerals.
• Root Pressure:
• Root pressure is the pressure that develops in the roots due to the active transport of minerals into the xylem, causing water to enter by osmosis.
• Cause: Active transport of minerals into the xylem.
• Effect: Water enters the xylem by osmosis, creating pressure.
• Importance: Helps push water up the xylem, especially at night when transpiration is low.

60-Second Revision

• Xylem = transports water and minerals.
• Phloem = transports sugars and organic nutrients.
• Transpiration = loss of water vapor from leaves.
• Root pressure = pressure that pushes water up.
• Cohesion-tension theory = explains water movement in xylem.
• Pressure-flow hypothesis = explains food movement in phloem.
• Source = where sugars are produced (leaves).
• Sink = where sugars are used or stored (roots, fruits, seeds).
• Warning: Xylem = water and minerals; Phloem = sugars and organic nutrients. Xylem is mostly dead; Phloem is mostly living. Water moves up the xylem; food moves both directions in the phloem. Transpiration pull is the main mechanism for water transport. Root pressure is important at night. Cohesion-tension theory explains water movement. Pressure-flow hypothesis explains food movement. Source = leaves; Sink = roots, fruits, seeds.