Transport of water from soil to the leaves

Up take of water also called absorption is a continuous stream through the plant.
Root hairs in the soil are surrounded by a film of water containing mineral salts/ soil solution.
The soil solution once inside the root hair vacuole is called cell sap and is a strong solution than the soil solution( has a lower osmotic potential and the cell membrane of the root hair is semi permeable.
The above conditions enable water to move from the soil, passes through the cell membrane in to the vacuole by osmosis.
Addition of water to the root hair all which is absorbed by osmosis makes it to attain higher osmotic potential as compared to the neighboring cells with stronger cell sap.
This enables water to move to and from the root hair to other cells of the cortex and through the cortex cells until it reaches the xylem which conducts water up the plant.
The water rises up the xylem by the following forces.

Capillarity
This is the ability of water to move up the fine tube. It is usually caused by the surface tension but because the capillary tube is narrow, the water rise is limited.

Cohesion – tension forces
This is a force of attraction between the molecules of the same substance.
Cohesion between water molecules allows water in a continuous column without breaking.
This occurs because as water is lost by transpiration from the leaves, the water potential at the top of xylem vessels falls below that at the bottom of the xylem in the root. Water is now pulled by this potential difference because of the cohesion of the water molecules.

Adhesion
This is the force of attraction between molecules of different substances (unlike)
Adhesion forces between walls of xylem and water molecules support a considerable weight of water within the xylem tissue and prevent the xylem vessels from collapsing.

Root pressure
This is regarded as the pressuring force of the water up the stem from the roots
The root pressure is an active process confirmed by the fact that:
It occurs only in living tissues/ plants.
It is affected by the same factors that affect respiration in living cells like oxygen supply, temperature, starch supply and the presence of respiratory poison like cyanides.
The root pressure theory has been suggested as a result of a common observation that water tends to exude from the cut stem indicating that some pressure in a root is actually pushing the water up. This pressure has been measured using a monometer attached to the stamp and it is this force which is normally called root pressure.
The root pressure depends on the type of plant species e.g. vines root pressure is up to 200 kpa has been demonstrated.
However, like capillary, not pressure is not sufficient on its own to push water to the leaves of the plant at the top of the tree and can slowly cause guttation in transpiring herbaceous plants.

Transpiration pull
This is the pulling force generated by the evaporation of water from the leaves.
This is caused when the cells of the spongy mesophyll layer in the leaf lose water by evaporation in to the air spaces causing their cell sap to become more concentrated and as a result they draw the water from the surrounding cells by osmosis.

These in cells in turn get water from the xylem in the veins and then water from the xylem moves to replace the lost water by evaporation. This evaporation sets up the passing action on water in the xylem called transpiration pull.

HOW IS THE ROOT HAIR ADAPTED TO ITS FUNCTION OF WATER ABSORPTION
i) The root hair is slender and flexible and can therefore flow between the soils particles.
ii) They are numerous which increase the surface area available for water absorption.
iii) They lack the cuticle which would restrict water absorption.
iv) They are long and narrow which increases surface area to volume ratio that increases the rate of water absorption.
v) The cytoplasm of the root hair contains numerous mitochondria where respiration occurs to release ATP needed for active transport of mineral salts from the soil solution to the cytoplasm of the root hairs.
vi) All the centre of the root hair is a vascular tissue which transports water and mineral salts to the rest of the plant.
vii) The cell sap of the root hair contains sugars, amino acids and salts, and so its concentrated than the soil solution and this low osmotic potential enables water to entre it by osmosis

Longitudinal section of the root hair

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Root hairs vacuoles contain a high concentration of solute than the surrounding water. Water is absorbed by root hairs by osmosis. This causes the root hair, vacuoles to become less concentrated than those of the adjacent cortex cell. Water is then passed into the cortex cell by osmosis. Water then enters the xylem tissue.
Water moves through the root cortex from cell to cell by 3 path ways:

i) Most of the water flows along the cell vacuole.
ii) Some water travels in the cytoplasm.
iii) Some water moves from vacuole to vacuole
The inner most region of cortex is made up of the endodermis strip which controls the movement of water from the cortex into the xylem.

EXPERIMENT TO SHOW THAT WATER TRAVELS UP THE PLANT THROUGH THE XYLEM
Apparatus

Small plant with flowers,
beaker,
water,
dye,
knife, and
microscope

Procedure
A small plant with light coloured flowers is placed in a beaker containing water with a dye.
It is allowed to stay in the water for 24 hrs
The stem and the roots are cut transversally and then observed under a microscope

Observation
The dye appears in the flower and along the veins of the leaves
It is observed that the xylem in the stem and roots are stained with the dye.

EXPERIMENT TO DEMONSTRATE ROOT PRESSURE
Apparatus

Potted plant with actively growing shoot, glass tubing, water, retort stand, rubber tubing.
Procedure
a) Cut the shoot of an actively growing potted plant leaving about 5 cm of stem above the ground/ soil
b) Firmly fix the glass tubing about 30cm long to the cut end of the stem using rubber tubing
c) Partly fill the glass tubing with coloured water and support it with a retort stand
d) Make the level of water in the glass tubing
e) Water the soil well and place the apparatus in a warm place for 3 hrs
f) Control experiment is a dry plant.

Diagram

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Observation
The level of coloured water rises in the glass tubing.
Conclusion
Water was absorbed by the plant and travelled upwards the stem due to root pressure

Importance of water to the plant

  • Raw material for photosynthesis
  • Solvent for mineral salts and oxygen that enable them to diffuse into the roots.
  • It is a constituent of the cytoplasm and all sap of the growing plants
  • Provides turgidity which provides support in non woody plants
  • Cools the leaves of the plants during transpiration


ABSORPTION OF MINERAL SALTS BY THE ROOT HAIRS
Mineral salts are moved in the plant in the xylem in solution with water. Roots absorb mineral salts in form of ions by diffusion and active transport. Active transport is the movement of the materials against the concentration gradient by the use of energy released from respiration.

TRANSPORT OF THE PRODUCTS OF PHOTOSYNTHESIS
The process by which the soluble products of photosynthesis are carried in plants is called translocation. Translocation is the movement of manufactured food from the side of photosynthesis. Throughout the plant, sugars and amino acids are transported in the phloem from the leaves to the growing parts of the plant or storage organs. Food substances may also move from the storage organs to the growing regions of the plants. In the phloem, food substances may move upwards/down wards.

EVIDENCE TO SHOW THAT FOOD MADE IN LEAVES IS TRANS LOCATED BY THE PHLOEM
1) The Ring Experiment:

Remove a ring of the bark from the stem at a point between the ground and the upper leaves. Leave another plant with the ring on.
The plants are left to stand for one week after which the observation is made.

Observation
The upper part of the stem of the ring plant swells immediately above the ring while the lower part of the stem remains un swollen.
The un ringed plant remains unchanged.
Conclusion
The phloem transports manufactured food.
Explanation
When a ring of a base is cut, the phloem tissue is removed along with it since it’s found within the bark. This cuts off the supply of manufactured food to the lower

parts of the plant as a result, the phloem in the upper part of the stem will transport the food to the part just above the ring. The food will then accumulate in this part hence it will swell.
When the ring is removed, the tree or plant also dries because the food supply to the root is cut off therefore the stored food in the roots gets exhausted then the roots die.

2) Feeding Aphids:
When the proboscis of the sucking aphid is cut, it is found to have penetrated into the phloem tube and when its contents of the proboscis are analyzed, it is found to contain products of photosynthesis (sucrose) which are transported to the bark through the phloem.

3) Radio Active Tracers:
If a plant is exposed to CO2 labeled with radioactive C-14, the C-14 becomes incorporated into the end products of photosynthesis which are subsequently detected in the stem. That these substances are confined to the phloem and can be shown by cutting sections of the stem, placing the sections in contact with photographic film and making auto radiographing it is found that the sites of radioactivity correspond precisely to the positions of the phloem.