Transport in Plants

Translocation is the transportation of manufactured food substances such as sugars and amino acids in the phloem tissue of plants.

Apids

Apids feed on plants by their proboscis by penetrating the leaf of stem.

Cut off the body of aphids, leaving only proboscis.

Liquid coming out from proboscis contains sucrose and phloem.

Probosics is inserted into phloem.

This shows that translocation occurs in phoem.

Isotopes

Provide leaf with carbon dioxide, containing radioactive 14C , which is detectable by an X-ray photographic flim.

Sugars formed will contain 14C.

Exposed section of stem onto photographic flim.

It is found that radioactive substances are present in phloem.

This shows that translocation only occurs in phloem.

'Ringing'

To remove a complete ring of bark including phloem and cabium from main stream. Place roots in water.

After a few days, ring will swell just above the cut.

This is due to food stuck there as food substances cannot be transported.

This also shows that translocation occurs in phloem.

Defination

Transpiration is the loss of water vapour from the aerial parts of a plant especially through the stomata of the leaves.

Importance

Transpiration pulls water & mineral salts from roots to stems and leaves.

Evaporation of water helps to cool the plant and preventing it from overheating as it also removes latent heat of vapourisation.

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.

Factors affecting

Temperature

The higher the temperature, the higher the rate of evaporation, which in also increases the rate of transpiration.

Humidity

The greater the humidity, the lower the rate of transpiration.

This is due to water entering the cells as water concentration is higher outside the leaf.

Wind

The stronger the wind, the higher the rate of transpiration.

As wind blows away the water vapour outside the leaf, water will diffuse out.

Light

The greater the light intensity, the greater the rate of transpiration.

As stomata opens wider in sunlight, this increases the rate of transpiration as water comes out.

Ways to control transpiration

Root pressure

Living cells in roots use active transport to pump ions into xylem to lower water potential in xylem.

Water therefore passes from living cells into xylem vessels by osmosis and flows upwards.

Water continue t o flow upwards and can even force water out of leaves or small plants.

Capillary action

Water tends to move up inside very narrow tubes due to the interaction between water molecules and surface of the tube.

Since xylem vessels in plants are very narrow tubes, capillary action helps in moving water up the vessels.

Transpiration pull

1.In the leaves, water constantly moves out of mesophyll cells to form thin flim of moistureover the surface of the mesophyll cells.

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.

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.

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.

Stems

Epidermis

cover with a layer of waxy , waterproof cuticle.

To reduce evaporation of water from stems.

Cortex

To store food substance

Pith

Central region to store food substance

Vascular bundle

Xylem

To conduct water and dissolved mineral salts from roots to stem and leaves.

To provide mechanical support.

Cabium

To divide and differentiate to form new xylem or phloem tissues.

To thicken stems

Phloem

conduct manufactured food from green parts of plants to other parts of plant.

Roots

root hair

Tubular outgrowth of an epidermal cell

Cortex

Storage of food substances.

Piliferous layer

Epidermis of root that bears root hairs.

xylem

To conduct water and dissolved mineral salts from roots to stem and leaves.

To provide mechanical support.

phloem

conduct manufactured food from green parts of plants to other parts of plant.

Xylem

Its empty lumen without crosswalls(protoplasm) helps to reduce resistance to water flowing through xylem.

Walls thicken with hard and rigid lignin prevents vessel from collapsing and provide mechanical support.

Phloem

Companion cells have many mitochondria to provide energy needed to load sugars from palisade mesophyll cells into sieve tubes by active transport.

Holes in sieve plates allows rapid flow of manufactured food substances through sieve tubes.

Root hair cell

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.

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.

Presence of mitochondria provides energy for the active transport of mineral salts and ions into cells.(This energy comes from cellular respiration in mitochondria.)

Advantages

-Leaf folds up to reduce surface area exposed to sunlight which causes guard cells to become flaccid.

-This causes stomata to close.

-Rate of transpiration is reduced, hence plants will not die from transpiration.

Disadvantage

Water is limited

As stomata are closed,carbon dioxide cannot enter.

As leaf folds up, surface area to capture sunlight decreased.

Rate of photosynthesis is greatly reduced by above factors.

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.

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.

3.Water will cross the partially permeable cell surface membrane of the root hair cell and into the root hair cell via osmosis.

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.

5.This process carries on until water enters the xylem vessels via osmosis as well.