BIOLOGY O LEVEL(FORM TWO) NOTES – TRANSPORTATION OF LIVING MATERIALS -2

TRANSPORTATION OF LIVING MATERIALS -2

TRANSPORT IN PLANTS

Introduction

The transport system in plants is less complex than that of animals. Materials are transported by vascular bundles composed of xylem and phloem tissues. Xylem tissue transports water and mineral salts from the soil to all parts of the plant. Phloem tissue transports manufactured food from the sites of photosynthesis to all parts of the plant. Between the xylem and phloem lies the cambium, which divides to form new xylem and phloem tissues.

Vascular bundle in a stem showing the position of cambium

Components of the Vascular System

Xylem

Xylem tissue consists of xylem vessels and tracheids. Mature xylem vessels and tracheids are composed of hollow, dead cells. Their walls are made of cellulose and lignin. Lignin strengthens the cell walls and makes them rigid, giving xylem the additional function of supporting the plant.

Xylem vessels

Movement of substances in the xylem is always upward and occurs by conduction. A xylem vessel is made of hollow cells without end walls, joined end to end to form a pipe-like structure. Xylem vessels begin in the roots, extend through the stem, and branch into every leaf of the plant.

Xylem vessels lack cytoplasm and nuclei, enabling them to transport a larger volume of water and mineral salts efficiently.

Tracheid Elements

Tracheids are elongated cells with pointed (tapering) ends. They are also arranged end to end to allow continuous water flow. Their end walls have perforations called pits, unlike xylem vessels where end walls are absent. This makes tracheids less efficient in conducting water compared to xylem vessels.

Phloem

Phloem tissue is composed of sieve-tube elements and companion cells.

Phloem tissue

Like xylem vessels, sieve-tube elements are made of cells joined end to end. However, their end walls are not completely broken down; they have perforations or pores forming sieve plates. These cells contain cytoplasm but lack a nucleus. Fibres run through the pores, connecting adjacent sieve-tube cells.

Each sieve-tube element is associated with a companion cell. They are separated by a thin wall made of parenchyma cells with pores called plasmodesmata, which allow exchange of materials between them.

Companion cells have a high concentration of mitochondria and provide sieve-tube elements with energy.

Movement of substances in the phloem occurs by translocation, which can proceed in any direction depending on the plant’s needs.

Distribution of Vascular Bundles in Plants

The arrangement of vascular bundles differs in the roots, stems, and leaves of monocots and dicots. This arrangement also varies between roots and stems within these two categories of plants.

Monocotyledonous Root

The vascular bundles in monocot roots are arranged as shown below:

Dicotyledonous Root

In dicot roots, the xylem is centrally positioned and star-shaped. The phloem is located between the arms of the xylem, as shown below.

Monocotyledonous Stem

In monocot stems, vascular bundles are arranged randomly throughout the stem, as illustrated below.

Dicotyledonous Stem

In dicot stems, vascular bundles are arranged in a ring around the central pith, as shown below.

Absorption and Movement of Water and Mineral Salts

Plants absorb water and mineral salts from the soil through root hairs.

Structure and Functions of Root Hairs

Root hairs are extensions of the epidermal cells of the root. The figure below shows the structure of a root hair.

Structure of root hair

Root hairs are long and slender, providing a large surface area for absorption of water and mineral salts from the soil. The large number of root hairs further increases the total surface area of the roots, enhancing absorption efficiency.

The cell sap inside root hair cells is usually hypertonic compared to the surrounding soil solution, causing water to enter the cells by osmosis.

Root hair cells have a higher concentration of minerals than the surrounding soil, so mineral salts are absorbed by active transport.

The thinness of root hairs ensures a short distance for absorption of water and mineral salts, facilitating efficient uptake.

Movement of Water and Dissolved Mineral Salts

When water is absorbed by root hairs, it dilutes the contents of the cell sap vacuole. Consequently, the adjacent cortex cells have less water than the root hair cells. Water moves from root hair cells to cortex cells by osmosis. It continues moving similarly into the endodermis cells, then into the pericycle, and finally into the xylem vessels.

Movement of water from root hair to xylem

Once in the xylem, water and dissolved mineral salts move upward through the xylem vessels by transpirational pull, capillarity, and root pressure.

Transpirational Pull

Transpiration occurs when water evaporates from the plant through stomata in the leaves. As water is lost, mesophyll cells draw water from the xylem in the leaf, which in turn draws water from the xylem in the stem. This creates a tension called transpirational pull, which draws water from the roots upward.

This results in a continuous column of water from the roots, through the xylem, to the leaves. This column is called the transpirational stream.

Transpirational stream

Capillarity

Capillarity is the action that causes water to rise in narrow tubes. Xylem vessels have narrow lumens, allowing water to rise through them by capillarity.

Capillarity is made possible by cohesion and adhesion forces. Cohesion is the attraction between like molecules, causing water molecules to stick together. Adhesion is the attraction between different molecules, causing water molecules to adhere to the walls of xylem vessels.

Root Pressure

Root pressure pushes water and dissolved mineral salts upward from the root. This occurs because endodermis cells actively push minerals into the xylem, increasing osmotic pressure in the xylem and creating a force that moves water and minerals upward. When a plant is cut, fluid oozes from the remaining stem (stump), which is evidence of root pressure.

TRANSPIRATION

Transpiration is the process by which plants lose water through stomata in the leaves.

Water flows from the roots to the leaves through xylem vessels. It enters the spongy mesophyll cells by osmosis. The spongy mesophyll contains substomatal air spaces where water evaporates as water vapor. This causes the concentration of water vapor in the substomatal air spaces to be higher than in the surrounding air, leading to diffusion of water vapor into the atmosphere through the stomata.

Movement of water through leaves

Note: Another process called guttation also occurs in plants. It is the loss of water as droplets through special glands located where veins contact the leaf margin. Guttation differs from transpiration, which is the loss of water vapor mainly through stomata. Guttation mostly occurs at night or in plants growing in wet areas.

Types of Transpiration

There are three types of transpiration:

1. Stomatal transpiration: Occurs through stomata on the leaves and accounts for approximately 90% of water loss in plants.
2. Cuticular transpiration: Occurs through the cuticle, a waxy layer covering the leaf surface. A thick cuticle reduces water loss.
3. Lenticular transpiration: Occurs through lenticels, which are pores found on the bark of stems or roots in woody plants.

Factors Affecting the Rate of Transpiration

The rate of transpiration is influenced by both plant features and environmental factors.

Plant Features

• Leaf size: Larger leaves have more stomata and lose more water than smaller leaves.
• Root system extent: Plants with extensive root systems absorb more water and can lose more water than those with fewer roots.
• Leaf cuticle thickness: A thick cuticle resists water loss, while a thin cuticle allows easier water loss.
• Number of stomata: More stomata increase the rate of transpiration.
• Position of stomata: Stomata on the upper leaf surface lose water more easily than those on the lower surface.
• Size of substomatal air spaces: Larger air spaces allow faster transpiration by holding more water vapor; smaller spaces slow it down.
• Sunken stomata: Located in pits, sunken stomata reduce exposure to moving air and slow transpiration.
• Epidermal hairs: Trap water on leaf surfaces, reducing water loss.

Environmental Factors

• Temperature: Higher temperatures increase transpiration by causing stomata to open and water to evaporate faster; lower temperatures reduce transpiration.
• Relative humidity: Higher humidity reduces transpiration because water evaporates less readily into moist air.
• Wind and air movement: Wind removes saturated air around leaves, increasing transpiration rate.
• Availability of soil moisture: Dry soil reduces water absorption, causing plants to lose leaves and reduce transpiration.
• Light intensity: Increased sunlight raises photosynthesis and internal temperature, opening stomata and increasing transpiration.
• Atmospheric pressure: Low atmospheric pressure (e.g., at high altitudes) increases water loss; high pressure reduces transpiration.

Significance of Transpiration

1. Maintains transpirational pull, essential for continuous water movement from roots to leaves.
2. Enables the plant to lose excess water, preventing waterlogging.
3. Helps cool the plant by evaporative cooling.
4. Facilitates absorption and distribution of water and mineral salts throughout the plant.

Summary

1. The vascular system in plants consists of xylem and phloem tissues.
2. Xylem transports water and mineral salts from roots to all parts of the plant.
3. Phloem transports manufactured food from photosynthesis sites to all parts of the plant.
4. Vascular bundle distribution differs between roots and stems, and between dicotyledonous and monocotyledonous plants.
5. Root hairs are extensions of epidermal root cells that absorb water and mineral salts from the soil.
6. Water is absorbed from the soil by osmosis.
7. Mineral salts are absorbed by active transport.
8. Water and dissolved minerals move up the xylem by transpirational pull, capillarity, and root pressure.
9. Transpiration is the process by which plants lose excess water through their leaves. It is important because it:

• Maintains the transpirational stream.
• Enables loss of excess water.
• Facilitates absorption and distribution of water and mineral salts.
• Helps cool the plant.

10. Transpiration is affected by plant features such as leaf size, root system size, cuticle thickness, air space size, stomata number and position, presence of sunken stomata, and epidermal hairs.
11. Environmental factors affecting transpiration include moisture in the air, temperature, air movement, soil moisture availability, light intensity, and atmospheric pressure.