Chapter-11TRANSPORT IN PLANTS

Presented by B.Prabhakar

PrincipalJNV. Panchmahal (Guj)

Transport in Plants

• Explain the need for transport systems in multicellular plants in terms of size and surface-area-to-volume ratio.

• Describe the distribution of xylem and phloem tissues in roots, stems and leaves of dicotyledonous plants.

Transport systems and surface area to volume ratio

• Single celled organisms have large surface area: vol ratio and can obtain all their requirements (oxygen, CO2, water and minerals) by diffusion. The sugar they make is available throughout the cell.

• Large plants need to transport water and minerals up from the roots. Sugars need to be moved from the leaves where they are made to other areas of the plant. In order to do this they need transport systems

Tissues that carry out movement of water, minerals and sugars

• A Tissue is group of similar cells that carry out a particular function

• Xylem tissue is made of Xylem vessels, fibres and parenchyma cells. Xylem transports water and minerals.

• Phloem tissue is made of Sieve tubes and companion cells. Phloem transports sugars.

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Distribution of vascular tissue in plant roots

• Xylem forms a cross shape in the centre with areas of phloem between the arms

• The vascular (transport) tissue is surrounded by a layer of endodermis

• The Pericycle is a layer of meristem (dividing) cells inside the endodermis

Distribution of vascular bundles in stem

• In the stem the vascular bundles of Xylem, Cambium and Phloem are arranged around the edge of the stem

• Xylem is always on the inside of the bundle,

• Cambium is a layer of meristem cells in the middle of the bundle

• Phloem is always on the outside of the bundle

Distribution of vascular tissue in leaves

• In leaves the vascular tissue is seen as the midrib and veins in the leaf

• The xylem is always closer to the top surface of the leaf

• The phloem is below the xylem

Describe the structure and function of xylem vessels, sieve tube elements and companion cells.

Xylem tissue

Phloem (longitudinal section)

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Week 10

Explain, in terms of water potential, the movement of water between plant cells, and between plant

cells and their environment.

• Pure water has a water potential of zero• Any solute dissolved in water lowers the

water potential and makes it more negative

• Water always moves from an area of higher water potential to an area of lower water potential

Movement of water into plant cells• This cell has a water potential of

-500kPa• It is bathed in pure water with a water

potential of 0kPa• So water enters the cell down the

water potential gradient (from higher to lower) by osmosis

• Eventually the pressure exerted by the water entering the cell equals the pressure exerted by the cell wall on the contents, water stops going into the cell

• The cell becomes turgid

A pair of adjacent cells, A and B, have water potentials of -1000kPa and 1200kPa respectively. Which cell will gain water from the other

Movement of water out of plant cells• If a plant cell is put into a

solution with a much lower water potential than the cell then water will leave the cell by osmosis.

• The vacuole shrinks and then the cytoplasm shrinks and becomes smaller in volume

• Eventually the cell membrane pulls away from the cell wall

• The cell becomes plasmolysed.

What is in the space between the cell wall and the plasma membrane of the plasmolysed cell?

© Pearson Education Ltd 2008This document may have been altered from the original

Movement of water between plant cells

• If adjacent cells have different water potentials then water will move between them, by osmosis, down the water potential gradient

http://images.yourdictionary.com/images/main/A4reosmo.jpg

http://biopleasure.files.wordpress.com/2011/03/scheme_facilitated_diffusion_in_cell_membrane-de.png

Complete the question on water potential

Movement of water through plants• A, the Apoplast pathway. Water and

dissolved ions move through the cell walls between the cellulose molecules

• B, the Symplast pathway. Water goes into the cell through the plasma membrane into the cytoplasm. It moves from cell to cell through the plasmodesmata

• C, the Vacuolar pathway. Water goes into the cell through the plasma membrane into the cytoplasm and then into the vacuole.

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A4493

• Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata.

• Explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream.

© Pearson Education Ltd 2008This document may have been altered from the original

• Describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to: the Casparian strip, apoplast pathway symplast pathway, xylem and stomata.

Water movement into the plant• Water goes into the root cells and

moves across the cortex by osmosis, water can move by any of the pathways

• At the Casparian strip in the Endodermis water is forced out of the Apoplast pathway and into symplast or vacuolar pathway

• Water and ions pass through proteins in the plasma membrane into the cytoplasm

• Nitrate ions are actively pumped from the endodermis cells into the xylem

• This lowers the water potential in the xylem so water follows by osmosis

Movement up the stem and out of the leaves

• Pumping ions into the xylem forces water to follow by osmosis

• Water can rise up stems about 3 metres by this process

• Water evaporates from leaves through the stomata, water moves through the leaf by osmosis down the water potential gradient

• Water leaves the xylem creating tension in the xylem, this is why the xylem needs to be strengthened with lignin, to prevent collapse.

• Cohesion between water molecules means that the whole column of water is pushed upwards from below and pulled upwards from above

Loss of water from leaf by Transpiration• Osmosis moves water

from xylem to palisade and spongey mesophyll

• Water evaporates from the mesophyll cells into the intercellular spaces

• Water diffuses from intercellular spaces out through stomata

Factors that increase transpiration rates

• Number of leaves• Number of

stomata • Cuticle• Light• Temperature

• Relative humidity• Air movement/ wind• Water available

• More leaves = larger surface area• More stomata = more spaces for

evaporation • More cuticle= slower evaporation• Stomata open in sunlight• Higher temp = faster evaporation,

faster diffusion through stomata• Lower gradient slows water loss• Maintains water potential gradient• Little water in plant closes stomata

and reduces water loss

Animation link

• http://www.kscience.co.uk/animations/transpiration.swf

Wilting: Loss of turgidity of the cells results in leaves andSometimes the stems, becoming limp, causing them to droop.Then the plant is called wilting.

Guttation: Loss of liquid water through tiny pores, called hydathodes on margin of leaf.

Wilting Plant Guttation

• The transpiration driven ascent of xylem sap depends mainly on the• following physical properties of water:• • Cohesion – mutual attraction between water molecules.• • Adhesion – attraction of water molecules to polar surfaces (such• as the surface of tracheary elements).• • Surface Tension – water molecules are attracted to each other in• the liquid phase more than to water in the gas phase.• These properties give water high tensile strength, i.e., an ability to• resist a pulling force, and high capillarity, i.e., the ability to rise in

thin tubes. In plants capillarity is aided by the small diameter of the tracheary elements – the tracheids and vessel elements.

Transpiration and Photosynthesis

• Transpiration has more than one purpose; • It creates transpiration pull for absorption and

transport of plants• • supplies water for photosynthesis• • transports minerals from the soil to all parts of

the plant• • cools leaf surfaces, sometimes 10 to 15

degrees, by evaporative cooling• • maintains the shape and structure of the plants

by keeping cells turgid

• Photosynthesis is limited by available water which can be swiftly depleted by transpiration. The humidity of rainforests is largely due to this vast cycling of water from root to leaf to atmosphere and back to the soil.

• The evolution of the C4 photosynthetic system is probably one of the strategies for maximising the availability of CO2 while minimising water loss.

• C4 plants are twice as efficient as C3 plants in terms of fixing carbon (making sugar). However, a C4 plant loses only half as much water as a C3 plant for the same amount of CO2 fixed.

• Unlike water, all minerals cannot be passively absorbed by the roots.• Two factors account for this:• (i) minerals are present in the soil as charged• particles (ions) which cannot move across cell membranes and• (ii) the concentration of minerals in the soil is usually lower than the

concentration of minerals in the root. • Therefore, most minerals must enter the root by active absorption into the

cytoplasm of epidermal cells. This needs energy in the form of ATP. The active uptake of ions is partly responsible for the

• water potential gradient in roots, and therefore for the uptake of water by• osmosis. Some ions also move into the epidermal cells passively.• Ions are absorbed from the soil by both passive and active transport.• Specific proteins in the membranes of root hair cells actively pump ions• from the soil into the cytoplasms of the epidermal cells. Like all cells, the• endodermal cells have many transport proteins embedded in their plasma• membrane; they let some solutes cross the membrane, but not others.• Transport proteins of endodermal cells are control points, where a plant• adjusts the quantity and types of solutes that reach the xylem.

• After the ions have reached xylem through active or passive uptake, or a• combination of the two, their further transport up the stem to all parts of• the plant is through the transpiration stream.• The chief sinks for the mineral elements are the growing regions of the• plant, such as the apical and lateral meristems, young leaves, developing• flowers, fruits and seeds, and the storage organs. Unloading of mineral• ions occurs at the fine vein endings through diffusion and active uptake• by these cells.• Mineral ions are frequently remobilised, particularly from older,• senescing parts. Older dying leaves export much of their mineral content• to younger leaves. Similarly, before leaf fall in decidous plants, minerals• are removed to other parts. Elements most readily mobilised are• phosphorus, sulphur, nitrogen and potassium. Some elements that are• structural components like calcium are not remobilised.

• An analysis of the xylem exudates shows that though some of the• nitrogen travels as inorganic ions, much of it is carried in the organic• form as amino acids and related compounds. Similarly, small

amounts• of P and S are carried as organic compounds. In addition, small

amount• of exchange of materials does take place between xylem and

phloem.• Hence, it is not that we can clearly make a distinction and say

categorically• that xylem transports only inorganic nutrients while phloem

transports• only organic materials, as was traditionally believed.

Pressure flow hypothesis• The accepted mechanism used for the translocation of sugars from source• to sink is called the pressure flow hypothesis. • As glucose is prepared at the source (by photosynthesis) it is converted to• sucrose (a dissacharide). The sugar is then moved in the form of sucrose• into the companion cells and then into the living phloem sieve tube cells• by active transport. • This process of loading at the source produces a hypertonic condition in the

phloem. Water in the adjacent xylem moves into the phloem by osmosis. • As osmotic pressure builds up the phloem sap will move to areas of lower

pressure.• At the sink osmotic pressure must be reduced. Again active transport is

necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar – converting it into energy, starch, or cellulose. As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.