TRANSPORT IN PLANTS AND ANIMALS: KCSE BIOLOGY NOTES, OBJECTIVES, SCHEMES OF WORK, QUESTIONS AND ANSWERS
Transport in Plants and Animals: Introduction
Transport in plants
Internal structure of roots and root hairs
The main functions of roots are;
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.
Pericyc1e 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 centre, with phloem in between the arms.
It has no pith. In monocotyledonous root, xylem alternates with phloem and there is a path in the centre.
Internal structure of a root hair cell
The main functions of the stem are;
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 schlerenchyma cells.
Is next to the epidermis and has thickened walls at the corners which strengthen the stem.
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.
Cells are closely connected to vascular bundles.
These cells are thickened by deposition of lignin and they provide support to plants.
This is the central region having parenchyma cells.
Absorption of Water and Mineral Salts Absorption of Water
Transpiration is the process by which plants lose water in the form of water vapour into the atmosphere.
Water is lost through stomata, cuticle and lenticels.
This accounts for 80-90% of the total transpiration in plants.
Stomata are found on the leaves.
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.
This is 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.
The mesophyll cells draw water from the xylem vessels by osmosis.
The xylem in the leaf is continuous with xylem in the stem and root.
Structure and function of Xylem
Movement of water is through the xylem.
Xylem tissue is made up of vessels and tracheids.
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
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 fibres and xylem parenchyma.
These are cells that are strengthened with lignin.
They form wood.
These are cells found between vessels.
They form the packing tissue.
Forces involved in Transportation of Water and Mineral Salts
As water vaporises 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 pulls 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.
This 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.
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:
The factors that affect transpiration are grouped into two. i.e. environmental and structural.
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 vapour is therefore lost from the leaf to the air.
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 carries away water vapour as fast as it diffuses out of the leaves.
This prevents the air around the leaves from becoming saturated with vapour.
On a windy day, the rate of transpiration is high.
When light intensity is high; more stomata open hence high rate of transpiration.
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.
Plants growing in arid or semi-arid areas have leaves covered with a thick waxy cuticle.
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 vapour accumulates in the pits.
Others have stomata on the lower leaf surface hence reducing the rate of water-loss.
Some plants have reversed stomatal 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 translocated include glucose, amino acids, and 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 is made up of;
TRANSPORT IN PLANTS.
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