Transport in plants

 

INTRODUCTION

Transport is the movement of substances within an organism.

•           All living cells require oxygen and food for various metabolic processes.

•           These substances must be transported to the cells.

•           Metabolic processes in the cells produce excretory products which should be eliminated before they accumulate.

•           The excretory products should be transported to sites of excretion.

Organisms like amoeba are unicellular. They have a large surface area to volume ratio. The body is in contact with the environment. Diffusion is adequate to transport substances across the cell membrane and within the organism.

Large multi-cellular organisms have complex structure where cells are far from each other hence diffusion alone cannot meet the demand for supply and removal of substances. Therefore an elaborate transport system is necessary.

Transport in plants

Simple plants such as mosses and liverworts lack specialized transport system. Higher plants have specialized transport systems known as the vascular bundle. Xylem transports water and mineral salts . Phloem transports dissolved food substances like sugars.

 

Internal structure of roots and root hairs

•           The main functions of roots are:

Ø  Anchorage

Ø  Absorption.

Ø  Storage

Ø  Gaseous exchange.

 

•           The outermost layer in a root is the piliferous layer. This is a special epidermis of young roots whose cells give rise to root hairs. Root hairs are microscopic outgrowths of epidermal cells. They are found just behind the root tip. They are one cell thick for efficient absorption of substances. They are numerous and elongated providing a large surface area for absorption of water and mineral salts. Root hairs penetrate the soil and make close contact with it.

•           Below the piliferous layer is the cortex. This is made up of loosely packed, thin walled parenchyma cells. Water molecules pass through this tissue to reach the vascuiar bundles. In some young plant stems, cortex cells contain chloroplasts.

•           The endodermis (starch sheath) is a single layer of cells with starch grains. The endodermis has a casparian strip which has an impervious deposit controlling the entry of water and mineral salts into xylem vessels. Pericycle forms a layer next to the endodermis.

•           Next to the pericycle is the vascular tissue. In the Dicotyledonous root, xylem forms a star shape in the center; with phloem in between the arms. It has no pith. In monocotyledonous root, xylem alternates with phloem and there has pith in the center.

The Stem

 

•           The main functions of the stem are;

Ø  Support and exposure of leaves and flowers to the environment,

Ø  Conducting water and mineral salts

Ø  Conducting manufactured food from leaves to other parts of the plant.

•           In monocotyledonous stems, vascular bundles are scattered all over the stem, while in dicotyledonous stems vascular bundles are arranged in a ring.

•           Vascular bundles are continuous from root to stems and leaves.

•           The epidermis forms a single layer of cells enclosing other tissues. The outer walls of the cells have waxy cuticle to prevent excessive loss of water.

•           The cortex is a layer next to the epidermis. It has collenchyma, parenchyma and sclerenchyma cells.

Collenchyma: next to the epidermis and has thickened walls at the corners which strengthen the stem.

Parenchyma: cells are irregular in shape, thin walled and loosely arranged hence creating intercellular spaces filled with air. They are packing tissues and food storage areas.

Sclerenchyma: Cells are closely connected to vascular bundles. These cells are thickened by deposition of lignin and they provide support to plants.

 

Pith

•           Is the central region having parenchyma cells.

 

Absorption of Water and Mineral Salts

Absorption of Water

•           Root hair cell has solutes in the vacuole and hence a higher osmotic pressure than the surrounding soil water solution. Water moves into the root hair cells by osmosis along a concentration gradient. This makes the sap in the root hair cell to have a lower osmotic pressure than the surrounding cells. Therefore water moves from root hair cells into the surrounding cortex cells by osmosis. The process continues until the water gets into the xylem vessels.

Uptake of Mineral Salts

•           If the concentration of mineral salts in solution is greater than its concentration in root hair cell, the mineral salts enter the root hair cell by diffusion.

•           If the concentration of mineral salts in the root hair cells is greater than in the soil water, the mineral salts enter the root hairs by active transport. Most minerals are absorbed in this way. Mineral salts move from cell to cell by active transport until they reach the xylem vessel.

•           Once inside the xylem vessels, mineral salts are transported in solution as the water moves up due to root pressure, capillary attraction and cohesion and adhesion forces.

 

Transpiration

•           Transpiration is the process by which plants lose water in the form of water vapor into the atmosphere. Water is lost through stomata, cuticle and lenticels.

•           Stomata transpiration: Accounts for 80-90% of the total transpiration in plants. Stomata are found on the leaves.

•           Cuticular transpiration: The cuticle is found on the leaves, and a little water is lost through it. Plants with thick cuticles do not lose water through the cuticle.

•           Lenticular transpiration: Loss of water through lenticels. These are found on stems of woody plants.

Water lost through the stomata and cuticle by evaporation leads to evaporation of water from surfaces of mesophyll cells.

 

Structure and function of Xylem

•           Movement of water is through the xylem. Xylem tissue is made up of vessels and tracheids.

Xylem Vessels

•           Xylem vessels are formed from cells that are elongated along the vertical axis and arranged end to end. During development, the cross walls and organelles disappear and a continuous tube is formed.

The cells are dead and their walls are strengthened by deposition of lignin.The lignin has been deposited in various ways. This results in different types of thickening:

Ø  Annular.

Ø  Simple spiral.

Ø  Double spiral.

Ø  Reticulate.

•           The bordered pits are areas without lignin on xylem vessels and allow passage of water in and out of the lumen to neighboring cells.

Tracheids

•           Tracheids have cross-walls that are perforated. Their walls are deposited with lignin.

•           Unlike the xylem vessels, their end walls are tapering or chisel-shaped. Their lumen is narrower.

•           Besides transport of water, xylem has another function of strengthening the plant which is provided by xylem fibers and xylem parenchyma.

Xylem fibers:

•           Cells that are strengthened with lignin. They form wood.

Xylem parenchyma:

•           These are cells found between vessels. They form the packing tissue.

 

 

Forces involved in Transportation of Water and Mineral Salts

Transpiration pull

As water vaporizes from spongy mesophyll cells into sub-stomatal air spaces, the cell sap of mesophyll cells develop a higher osmotic pressure than adjacent cells. Water is then drawn into mesophyll cells by osmosis from adjacent cells and finally from xylem vessels. A force is created in the leaves which pull water from xylem vessels in the stem and root. This force is called transpiration pull.

 

Cohesion and Adhesion

•           The attraction between water molecules is called cohesion.

•           The attraction between water molecules and the walls of xylem vessels is called adhesion.

•           The forces of cohesion and adhesion maintain a continuous flow of water in the xylem from the root to the leaves.

 

Capillarity

•           Is the ability of water to rise in fine capillary tubes due to surface tension.

•           Xylem vessels are narrow, so water moves through them by capillarity.

 

Root Pressure

•           If the stem of a plant is cut above the ground level, it is observed that cell sap continues to come out of the cut surface.

•           This shows that there is a force in the roots that pushes water up to the stem.

•           This force is known as root pressure.

 

Importance of Transpiration

•           Transpiration leads to excessive loss of water if unchecked.

Some beneficial effects are:

•           Replacement of water lost during the process.

•           Movement of water up the plant is by continuous absorption of water from the soil.

•           Mineral salts are transported up the plant.

•           Transpiration ensures cooling of the plant in hot weather.

•           Excessive loss of water leads to wilting' and eventually death if water is not available in the soil.

 

Factors Affecting Transpiration

The factors that affect transpiration are grouped into: Environmental and structural.

Environmental factors

Temperature

•           High temperature increases the internal temperature of the leaf which in turn increases kinetic energy of water molecules which increases evaporation. High temperatures dry the air around the leaf surface maintaining a high concentration gradient.

More water vapor is therefore lost from the leaf to the air.

Humidity

•           The higher the humidity of the air around the leaf, the lower the rate of transpiration. The humidity difference between the inside of the leaf and the outside is called the saturation deficit.

•           In dry atmosphere, the saturation deficit is high. At such times, transpiration rate is high.

 

Wind

•           Wind carries away water vapor as fast as it diffuses out of the leaves. This prevents the air around the leaves from becoming saturated with vapor.

•           On a windy day, the rate of transpiration is high.

 

Light Intensity

•           When light intensity is high; more stomata open hence high rate of transpiration.

 

Atmospheric Pressure

•           The lower the atmospheric pressure the higher the kinetic energy of water molecules hence more evaporation. Most of the plants at higher altitudes where atmospheric pressure is very low have adaptations to prevent excessive water-loss.

Availability of Water

•           The more water there is in the soil, the more is absorbed by the plant and hence a lot of water is lost by transpiration.

 

Structural Factors

Cuticle

•           Plants growing in arid or semi-arid areas have leaves covered with a thick waxy cuticle.

Stomata

•           The more the stomata, the higher the rate of transpiration. Xerophytes have few stomata which reduce water-loss. Some have sunken stomata which reduces the rate of transpiration as the water vapor accumulates in the pits.

•           Others have stomata on the lower leaf surface hence reducing the rate of water-loss. Some plants have reversed stomata rhythm whereby stomata close during the day and open at night.

•           This helps to reduce water-loss.

Leaf size and shape

•           Plants in wet areas have large surface area for transpiration.

•           Xerophytes have small narrow leaves to reduce water-loss.

•           The photometer can be used to determine transpiration in different environmental conditions.

 

Translocation of organic compounds

•           Translocation of soluble organic products of photosynthesis within a plant is called translocation. It occurs in phloem in sieve tubes. Substances trans located include glucose, amino acids, vitamins.

•           These are translocated to the growing regions like stem, root apex, storage organs e.g. corms, bulbs and secretory organs such as nectar glands.

 

Phloem

Phloem is made up of: sieve tubes, companion cells, parenchyma, a packing tissue and sclerenchyma, a strengthening tissue

 

Sieve Tubes

•           These are elongated cells arranged end to end along the vertical axis.

•           The cross walls are perforated by many pores to make a sieve plate.

•           Most organelles disappear and those that remain are pushed to the sides of the sieve tube.

•           Cytoplasmic strands pass through the pores in the plate into adjacent cells.

•           Food substances are trans-located through cytoplasmic strands.

Companion Cells

•           Companion cells are small cells with large nuclei and many mitochondria.

•           They are found alongside each sieve element.

•           The companion cell is connected to the tube through plasmodesmata.

•           The mitochondria generate energy required for translocation.

Phloem Parenchyma

•           These are parenchyma cells between sieve elements.

•           They act as packing tissue.