Transport in Plants and Animals
Transport in plants
Internal structure of roots and root hairs
The main functions of roots are;
Internal structure of a root hair cell
The main functions of the stem are;
Absorption of Water and Mineral Salts Absorption of Water
Structure and function of Xylem
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.
EXCRETION AND HOMEOSTASIS
Gaseous Exchange In Animals
GASEOUS EXCHANGE IN PLANTS AND ANIMALS
GROWTH & DEVELOPMENT
INTRODUCTION TO BIOLOGY
MEASUREMENT OF GROWTH
NUTRITION IN ANIMALS
NUTRITION IN PLANTS AND ANIMALS
REPRODUCTION IN PLANTS AND ANIMALS
Transport In Animals
TRANSPORT IN PLANTS AND ANIMALS