Transport in Plants
Movement of water into the plant
5.This process carries on until water enters the xylem vessels via osmosis as well.
4.Entry of water dilutes the sap and causes the water potential of root hair cell to be higher than that of an inner neighbouring cell. This causes water to move from the root hair cell and into the neighbouring cell by osmosis.
3.Water will cross the partially permeable cell surface membrane of the root hair cell and into the root hair cell via osmosis.
2.As the sap in the root hair cell is relatively concentrated with sugars , mineral salts and ions, it has a lower water potential than the soil solution.
1.Each root hair is a fine tubular growth of an epidermal cell which grows between soil particles , coming in close contact with the thin film of dilute liquid surrounding each soil particles.
Wilting
Disadvantage
Rate of photosynthesis is greatly reduced by above factors.
As leaf folds up, surface area to capture sunlight decreased.
As stomata are closed,carbon dioxide cannot enter.
Water is limited
Advantages
-Rate of transpiration is reduced, hence plants will not die from transpiration.
-This causes stomata to close.
-Leaf folds up to reduce surface area exposed to sunlight which causes guard cells to become flaccid.
Adaptations
Root hair cell
Presence of mitochondria provides energy for the active transport of mineral salts and ions into cells.(This energy comes from cellular respiration in mitochondria.)
Presence of cell surface membrane prevents cell sap(sugars,amino acids and salts) from leaking out. It has a lower water potential than the soil solution which results in water entering the root hair by osmosis.
Its long and narrow structure increases the surface area to volume ratio which in turn increases the rate of absorption of water and mineral salts by root hair cell.
Holes in sieve plates allows rapid flow of manufactured food substances through sieve tubes.
Companion cells have many mitochondria to provide energy needed to load sugars from palisade mesophyll cells into sieve tubes by active transport.
Walls thicken with hard and rigid lignin prevents vessel from collapsing and provide mechanical support.
Its empty lumen without crosswalls(protoplasm) helps to reduce resistance to water flowing through xylem.
Parts & Function
Roots
phloem
xylem
Piliferous layer
Epidermis of root that bears root hairs.
Storage of food substances.
root hair
Tubular outgrowth of an epidermal cell
Stems
Vascular bundle
Phloem
conduct manufactured food from green parts of plants to other parts of plant.
Cabium
To thicken stems
To divide and differentiate to form new xylem or phloem tissues.
Xylem
To provide mechanical support.
To conduct water and dissolved mineral salts from roots to stem and leaves.
Pith
Central region to store food substance
Cortex
To store food substance
Epidermis
To reduce evaporation of water from stems.
cover with a layer of waxy , waterproof cuticle.
Movements of water up the plant
Transpiration pull
4.Eventually, water is absorbed fromylem vessels. This results in a suction force to pull up the whole column of water up xylem, known as transpiration pull.
3.As water evaporates from mesophyll cells, the water potential inside the mesophyll cells becomes lower. Mesophyll cells absorb water by osmosis from cells deeper inside.
2.Water evaporates from thin film of moisture and moves into intercellular air spaces. Water accumulates in large air spaces near stomata and diffuses out of the stomata via transpiration.
1.In the leaves, water constantly moves out of mesophyll cells to form thin flim of moistureover the surface of the mesophyll cells.
Capillary action
Since xylem vessels in plants are very narrow tubes, capillary action helps in moving water up the vessels.
Water tends to move up inside very narrow tubes due to the interaction between water molecules and surface of the tube.
Root pressure
Water continue t o flow upwards and can even force water out of leaves or small plants.
Water therefore passes from living cells into xylem vessels by osmosis and flows upwards.
Living cells in roots use active transport to pump ions into xylem to lower water potential in xylem.
Transpiration
Ways to control transpiration
Factors affecting
Light
As stomata opens wider in sunlight, this increases the rate of transpiration as water comes out.
The greater the light intensity, the greater the rate of transpiration.
Wind
As wind blows away the water vapour outside the leaf, water will diffuse out.
The stronger the wind, the higher the rate of transpiration.
Humidity
This is due to water entering the cells as water concentration is higher outside the leaf.
The greater the humidity, the lower the rate of transpiration.
Temperature
The higher the temperature, the higher the rate of evaporation, which in also increases the rate of transpiration.
Importance
Water transported to leaves are used in photosynthesis, keep cells turgid so that the leaves are spread out widely to trap sunlight for photosynthesis, which is also used to replace water lost by the cells.
Evaporation of water helps to cool the plant and preventing it from overheating as it also removes latent heat of vapourisation.
Transpiration pulls water & mineral salts from roots to stems and leaves.
Defination
Transpiration is the loss of water vapour from the aerial parts of a plant especially through the stomata of the leaves.
Translocation
Translocation is the transportation of manufactured food substances such as sugars and amino acids in the phloem tissue of plants.
'Ringing'
This also shows that translocation occurs in phloem.
This is due to food stuck there as food substances cannot be transported.
After a few days, ring will swell just above the cut.
To remove a complete ring of bark including phloem and cabium from main stream. Place roots in water.
Isotopes
This shows that translocation only occurs in phloem.
It is found that radioactive substances are present in phloem.
Exposed section of stem onto photographic flim.
Sugars formed will contain 14C.
Provide leaf with carbon dioxide, containing radioactive 14C , which is detectable by an X-ray photographic flim.
Apids
This shows that translocation occurs in phoem.
Probosics is inserted into phloem.
Liquid coming out from proboscis contains sucrose and phloem.
Cut off the body of aphids, leaving only proboscis.
Apids feed on plants by their proboscis by penetrating the leaf of stem.
