Transportation in Plants Chapter Notes | Biology Class 8 ICSE PDF Download

Introduction

Plants need food, water, and minerals to grow and stay healthy. But how do these things move inside a plant? This chapter, "Transport in Plants," teaches us how plants carry water, minerals, and food to different parts. Plants have special tissues called xylem and phloem that act like pipes to move these things. Xylem carries water and minerals from the roots to the leaves, while phloem moves food from the leaves to other parts of the plant. We will also learn how roots absorb water and how plants use different ways to transport these things.

Transport in Plants

• Plants make their own food using a process called photosynthesis.
• In this process, plants use carbon dioxide and water to make food.
• Plants get carbon dioxide from the air through tiny holes in their leaves.
• They get water from the soil through their roots using a process called absorption. 
• The food made by leaves is sent to all parts of the plant, like the stem and roots.
• Roots also take in minerals from the soil, which are important for the plant to grow.
• Special tissues called xylem and phloem help move water, minerals, and food inside the plant.
• Transportation in plants happens through two special tissues: xylem and phloem.
• Xylem and phloem are also called vascular bundles.
• Xylem carries water and minerals from the roots to other parts of the plant.
• Phloem carries food made by the leaves to all parts of the plant.

Transport of Water and Minerals

Roots of the plant take in water and minerals from the soil. This process of taking in water and minerals is called absorption. The water and minerals are then moved to other parts of the plant through the xylem.

Xylem

Xylem cells have thick walls and create a tube-like structure that transports water and minerals absorbed by the roots from the soil to various parts of the plant. 
The xylem tissue consists of four types of cells: xylem tracheids, xylem vessels, xylem fibres, and xylem parenchyma.

• Tracheids are elongated with varying types of wall thickenings, aiding in the upward movement of water and providing structural support. 
• Vessels, positioned above one another, form long, tube-like channels that are open at both ends, with different thickening patterns in the xylem vessels to facilitate the lateral and vertical transport of water and mineral salts. 
• Fibres are sclerenchymatous, long, and narrow, offering only mechanical support.
• Parenchyma consists of living cells that assist in transporting water and minerals, and also play a role in storing food. Xylem primarily functions in moving water and dissolved minerals upward from the roots to various parts of the plant. This process is aided by xylem vessels and tracheids.

Phloem

Phloem is a type of complex permanent tissue that plays a crucial role in transporting organic food, produced in the leaves, to different parts of the plant. It is a component of the vascular bundle and, along with xylem, runs throughout the plant's structure—including the roots, stem, branches, and leaves.
Phloem consists of both living and dead cells, which are categorized into four main types: sieve tubes, companion cells, phloem parenchyma, and phloem fibres.

• Sieve tubes are the primary conducting elements of the phloem. These are long, cylindrical cells aligned vertically and connected end-to-end. Their end walls, known as sieve plates, have small pores that allow the passage of food from one cell to another.
• Companion cells lie next to sieve tubes and are elongated, thin-walled parenchyma cells. They assist the sieve tubes in transporting food.
• Phloem parenchyma consists of unspecialized, thin-walled parenchymatous cells responsible for storing food.
• Phloem fibres are non-living sclerenchyma cells that are elongated and contribute to mechanical support.

Functions of phloem

Sieve tubes transport organic food from the leaves to other parts of the plant, including roots and storage organs. Phloem fibres offer mechanical strength, while phloem parenchyma aids in food storage.

Transport of Food

• During photosynthesis, plants make food in the form of glucose, which is later turned into starch.
• This food is made in the leaves, which act like the kitchen of the plant.
• The food is then moved to all parts of the plant through phloem cells.
• Phloem cells are also called sieve tubes because they help move food.

Water Absorption by the Roots

• Plants take in water by a process called absorption, either by soaking it up or through tiny holes in the leaves.
• In both cases, water is taken in by cotton dipped in water or through the roots.
• The ability of tiny spaces between soil particles to hold water helps the roots take it in.
• But the way water moves through the plant is quite different.
• Let us learn about the roots of plants.

Root hairs

Magnified illustration of a root-hair, which is an extension of an epidermal cell of the root. As a plant cell, it has two main coverings—an outer cell wall and an inner cell membrane. 

• The cell wall is freely permeable, meaning it allows all substances to pass through. In contrast, the inner cell membrane is semi-permeable; it permits the passage of water molecules but restricts larger molecules.
• Inside the cell membrane lies the cytoplasm and other cellular contents. This setup aids in the absorption of water and dissolved minerals from the soil. 
• The cell sap within the root-hair cell has a higher concentration of solutes compared to the surrounding soil water, making it more concentrated.

Speciality of Root-Hairs

Root-hairs are special because they help the plant take in water from the soil in three ways:

• The surface of root-hairs is very large, which helps them take in more water.
• Root-hairs have a higher amount of water inside than the soil around them.
• The cell wall of root-hairs is fully open, which means it allows all types of things to pass through easily.
• But the inner cell membrane is only partly open (semi-permeable), which allows only some things to pass through.
• The semi-permeable membrane separates water on one side from a concentrated sugar solution on the other. 
• The sugar solution contains a high number of sugar molecules, while the water side has none, meaning there are more water molecules on the left side of the membrane compared to the right. 
• Over time, you’ll observe that the concentrated sugar solution becomes diluted as water molecules move from the water side into the sugar solution. 
• This movement of water molecules across a semi-permeable membrane to dilute a concentrated solution is known as osmosis.

Movement of Water in Plants

• Diffusion: It is the movement of gas, liquid, or solid from a place of higher amount to a place of lower amount.
• Osmosis: It is the movement of water through a semi-permeable membrane from a place of higher amount to a place of lower amount.
• Active Transport: It is the movement of minerals that needs energy.
• The water taken in by the root-hairs moves into the root by osmosis.
• It then moves deeper into the central part of the root, called the xylem.
• The xylem carries the water and minerals upward into the stem and leaves.
• This upward movement of water and minerals is called ascent of sap.

Root Pressure

• Root pressure is the force created in the root that helps push the water upward through the xylem vessels.
• This pressure helps the water move up to a certain height in the plant.

Transpiration

• Transpiration is when plants lose water through tiny holes in their leaves called stomata.
• These holes are mostly on the leaves, but they can also be on the stem.
• Only a small amount of water taken by the plant is used, and the rest goes out as water vapor through transpiration.
• Transpiration happens when the air around the plant takes in this water vapor by a process called evaporation.
• Transpiration helps pull water up from the roots to the leaves through special tubes in the stem called xylem.
• This pulling of water is called the transpiration pull.
• Water moves from the roots to the stem and then to the leaves through xylem tubes.
• The force that helps move water upward in the xylem is called the transpiration pull.
• Transpiration helps the plant in three main ways:
  • It helps the plant cool down by evaporating water.
  • It helps move water and food through the plant.
  • It helps the plant get important minerals from the soil.

Factors Affecting the Rate of Transpiration

Some things can change how fast transpiration happens in plants:

• Sunlight: More sunlight makes transpiration faster because the tiny holes (stomata) on leaves open up to take in sunlight for making food. At night, when there is no sunlight, these holes close, and transpiration becomes very slow.
• Temperature: Hot summer days make transpiration faster than cold winter days because heat makes water evaporate quickly.
• Wind: Wind makes transpiration faster because blowing air takes away the water vapor around the leaves quickly.
• Humidity: Humid air slows down transpiration because the air already has a lot of water, so it can’t take more water vapor from the leaves. In dry air, transpiration happens faster because the air can hold more water vapor.

Importance of Transpiration in Plants

Transpiration is very important for plants in two main ways:

• Cooling Effect: During transpiration, water evaporates from the plant's surface. The energy required for this evaporation comes from the plant itself (latent heat), helping it to cool down, especially in hot weather.
• Transpiration maintains the concentration of sap inside the plant body: Roots keep absorbing water from the soil. If this water isn't released through transpiration, the sap would get diluted, which could hinder further water absorption along with essential minerals.

Use of water in the plant

Water absorbed by the roots serves three key purposes for the plant: transportation, food production, and temperature regulation.

• Transportation: Transpiration helps move water and minerals from the roots to the leaves through the xylem tubes. It also helps move food made in the leaves to other parts of the plant through special tubes called phloem.
• Food production: Water plays a vital role in photosynthesis. It combines with carbon dioxide from the air in the presence of sunlight and chlorophyll to produce food for the plant.
• Cooling: When water absorbed by the plant is released as water vapour from the leaves, it creates a cooling effect. You might have noticed that standing beneath a tree on a hot summer day feels cooler. This effect is not just from the shade, but also from water evaporating from the leaf surfaces. The evaporation process cools the surrounding air, making it more comfortable. Since cool air is denser, it settles down, enhancing the pleasant environment around the tree.
  

Important Minerals

• Plants need certain minerals to grow well and stay healthy.
• These minerals are divided into two types: macro-nutrients and micro-nutrients.
• Macro-nutrients are needed in larger amounts, and micro-nutrients are needed in very small amounts.
• Both types of minerals are very important for the plant’s growth.
• If the plant does not get enough of these minerals, it can get sick and show deficiency symptoms.
• Here is a table showing the role of these minerals and what happens if the plant doesn’t get them: