how does transpritation help in upwards transport of substance
Answers
because the transportation of water in plant is from root to shoot.
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Uptake of Water
Water is absorbed in plants by young roots.
The water is absorbed in the dermal cells by osmosis.
The cell sap of the epidermal cells is much more concentrated than soil water.
Roots hairs greatly enhance water absorption.
Root hairs increase the root surface area and catchment space for water uptake.
Water uptake is fast as these epidermal cells do not have a cuticle.
Textbook Diagrams: transverse and longitudinal sections of a young root.
Water Transfer to Xylem
Water passes into the dermal cells by osmosis from the soil.
Water can move across the root ground tissue from cell to cell to the xylem by osmosis.
Water can also move towards the xylem through the intercellular spaces.
Water can pass through the cells walls towards the xylem.
Direct entry into the xylem tracheids and vessel members is by osmosis.
Upward Movement of Water
Root Pressure
The xylem sap is more concentrated than soil water.
Water passes from the soil into the xylem by osmosis.
The force of the osmotic flow of water drives the water from the roots up into the stem.
The rise of water is short – a metre at most.
The rise also varies with changes in the concentration of xylem sap.
The xylem sap concentration varies with mineral ion absorption.
Transpiration Pull
Transpiration is the loss of water from the surface of a plant by evaporation.
Transpiration mostly takes place from the leaves.
Water in soil, roots, stems and leaves is one continuous mass – the water in the leaves is in direct contact with water in the soil.
Water has great cohesion – water molecules are glued tightly to each other.
Due to its cohesion, a change in the movement of one water molecule affects all.
The loss of water from the leaf cells pulls water in from the xylem, this pulls water up the xylem, and this in turn pulls water from the ground tissue of the roots causing water to be pulled in from the soil.
Transpiration supplies the ‘pull’ or tension force that lifts the water up the plant.
The flow of water up the plant is called the ‘transpiration stream’.
The rate of water flow is directly related to the rate of transpiration.
The weight of the column of water is supported by water’s great adhesion to cell walls.
ATP is not used in water transport therefore it is a passive process.
Water flow is upward only in a plant.
The rate of transpiration from leaves is affected by the thickness of the cuticle.
The opening and closing of the stomata is a major factor in controlling water loss by transpiration.
Note: Dixon and Joly, two Irish professors, were the first to put forward the ‘cohesion-tension model’ of water transport in xylem.
Textbook Diagram: pathway of water flow into the plant and up to the leaves.
Mineral Nutrients
Plant mineral nutrients are in solution in soil water.
The root dermal cells absorb the mineral nutrients by diffusion and active transport.
The root dermal cells are rich in the ATP producing mitochondria.
The mineral nutrients are actively ‘pumped’ into the xylem.
The mineral nutrients are transported throughout the plant in the xylem.
Food Transport
Food is transported in the sieve tube elements of the phloem.
Food transport is an active process because ATP is needed.
Phloem sap is really a sugar solution.
In the growing season sugar made by in the leaves by photosynthesis is transported to the growing points and excess is often sent to the roots for storage.
At the start of the growing season the food is transported from the storage regions to the growing points.
Transport of food can be up and down the stem.
CO2 Transport
Carbon dioxide gas enters the leaves by diffusion through the stomata.
Carbon dioxide diffuses to the photosynthetic cells through the leaf’s air spaces.
Aerobic respiration produces carbon dioxide gas as a waste.
Carbon dioxide will diffuse into the intercellular air spaces from respiring plant tissue.
This carbon dioxide gas will diffuse through the air spaces to photosynthetic tissue.