UNrrS: TRANSPORT IN PTANTS

Learning Objectives:g ldentify the positions and explain the functions of xylem and phloemg Relate the structure andfunctions ofroot hairs to their surface areag Explain the movement of waterA Outline the pathway by which is transported in a plantg Define and explain transpirationg Describe and explain the effects offactors on transpirotion rateg Describe and explain how wilting occursg Define translocation

TRANSPORT SYSTEM IN PLANTS

. The transport system in plants is called the vascular system.

. lt consists of xylem tissue and phloem tissue./ The xylem tissue transports water and minerals from the roots to the leaves.,/ The phloem tissue transports food made in the leaves to all parts of a plant.

' ln flowering plants, the arrangement of xylem tissue and phloem tissue in the root, stem and leafare different.

The stem

. The stem of a plant provides the following important functions:

Function Explanation

ConductionIt conducts water and dissolved mineral salts from the roots to theleaves as well as sugars from the leaves to the different parts of theplant, especially to the storage organs and the growing regions.

Support

a It acts like a strong pillar to support the branches, which spread outin all directions so as to expose a large surface area of leaves andflowers.

StorageThe underground modified stems serve as storage organs for foodstorage e.g. potato. The fleshy stems of the cactus stores a largequantity of water.

Manufacture offood

a The young green shoots can synthesise food in the presence ofsunlight, using the chloroplasts contained in it.

Bearing leavesand flowers

The stem and branches bear numerous leaves and flowers. Theyspread them out in all directions so that the leaves obtain sufficientsunlight to manufacture food and the flowers are able to attractinsects for pollination and reproduction.

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Uosculor tissues in stem

. The xylem tissue and phloem tissue are arranged in units called vascular bundles.

. The arrangement of xylem and phloem in a stem:

pith

phloem

cambium

xylem

cortex

vascular bundle

epidermis

o

Part Description

EpidermisOutermost layer which consists of a layer of cellsCovered by a layer of cuticle which prevents the evaporation of waterfrom the stem

Cortexa

The middle layer that consists of a few layers of thin walled, large androunded cells with intercellular spaces.Functions mainly as a storage of food substances

Pithlnside of the ring of vascular bundlesMade up if thin walled cells which serve as storage tissue

Vascularbundle

a

It is composed of outer phloem and inner xylem, separated by thecambium.The bundles are arranged in a ring with the phloem and xylem along thesame radius.

1. Phloem tissue

- Consists of sieve tubes and companion cells,

- Companion cells have mitochondria to provide energy needed tomove food from the mesophyll cells into the sieve tubes by activetransport.

- Sieve plates contain holes that allows food move through the sievetubes rapidly.

sieveplate

SICVC

tube

companioncell

Conducts manufactured food from the leaves to all parts of theplant (translocation)

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2. Xylem tissue

- Consists of xylem vessels mainly which are long hollow tube madeup of dead cells

- Has lignified walls that provide mechanical strength to the plant

- Lignin is deposited within the cellulose cell walls in the forms ofrings, spiral networks or pit.

lumen lumen lumen

[']z[l -'\-'It-{lr=ll

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cell wall

pit

lignin in the lignin in the lignified cellform of ring form of spiral wall

Conducts water and minerals salts form the roots to the leaves

3. CambiumLying in between the phloem and xylemConstantly adds new cells to xylem and phloem that thicken thestem

The roots

. The roots of a plant provides the following important functions:

1. To hold plants firmly in the soil

2. To absorb water and dissolved mineral salts from the soil and distribute them to variousparts of the plant

3. To serve as food storage organs e.g. carrot, beetroot

4. For climbing and creeping purposes e.g. vines, money plant

5. For breathing purposes e.g. mangrove

6. For absorbing water from the air e.g. adventitious roots of orchids and banyan tree

Uosculor tissues in roots

. The arrangement of xylem and phloem in roots

piliferous layer

root hair

phloem

xylem

vasculartissue

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cortex

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Part Description

Piliferous layer

Outermost layer

Consists of a single layer of box like cells

Outer walls of these cells extend to form unicellular root hairs whichincrease the surface area tor absorption of water

Cuticle is absent in the piliferous layer

Cortex

Consists of many layers of thin walled, large and rounded cells withintercellular spaces

Functions mainly as a storage of food substances e.g. starchgra ins

a

Vascular tissueThe phloem and xylem are not bundled together but lie on differentradii, alternating with each other

ABSORPTION OF SUBSTANCES

Water

Absorption of water takes place in the root hairs which are tubular outgrowth of root epidermalcells. An epidermal cell with root hair is called a root hair cell.

Soil water contains dissolved mineral salts, so it normally has a higher water potential than thecell sap of the root hair cells. This allows water to enter the root hair cells by osmosis.

Qnce water enters the root hair cell, its cell sap has a higher water potential. As a result, waterpasses from the root hair cells into the adjacent cells of the cortex. These cortical cells now

contain cell sap which has a higher water potential than the next inner cortical cells and so inthis way, water moves from one cell to another.

direction ofwater movement

to stem

xylem vessels

water from soilentering root hair

soil particles

root hair cell cortical cells

The result is a flow of water across the plant by osmosis, until water finally enters the xylemvessels.

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Mineral salts and ions

. Mineral salts and ions are absorbed in aqueous form from the soil.

. The concentration of mineral salts and ions in the cell sap of the root cells may be considerablyhigher than that of In the soil solution. Hence, the movement of these substances against theconcentration gradient into the roots involves the process of active transport. This processrequires energy from respiration.

. A root hair cell has the following adaptations to absorb mineral salts and ions more efficiently:

Adaptation Explanation

Long and narrowThis increases the surface area to volume ratio that enhancesthe rate of absorption of water and mineral salts.

Higher concentrationof cell sap

The cell sap is more concentrated than the soil solution asit contains sugars, amino acids and salts. Water will thusenter the roots by osmosis.

Able to carry outactive transport

It is able to carry out cellular respiration to provide energyfor active transport.

Evidence to show the movement of woter in the xylem

. Place the shoots of a balsam plant in a solution of blue dye.

. Leave the shoots in the light for a period of time.

' The blue dye will appear in the veins of the leaves. When the stems are cut transversely, the dyecan be seen in the vascular bundles.

' The dye, together with water, travels up the stem in the vascular bundles. Upon closer inspectionwith a microscope, it is confirmed the dye can be seen in the xylem vessels.

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1. (a) What is the name for the transport system in plants?

(b) This system consists of xylem tissue and phloem tissue.(i) State two functions of xylem tissue.

(ii) State one function of phloem tissue

OREVISION

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2 The diagrams below shows sections of the root, stem and leaf of a dicotyledon plant. Label thestructures in the box provided for each diagram.

(a) Transverse section of a root

(b) Transverse section of a stem

(c) Transverse section of a leaf

3. (a) What is a vascular bundle in the dicotyledonous stem?

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(b) Explain how the stem is thickened when a plant grows.

(c) How is the loss of water from the stem prevented in the dicotyledonous stem?(-

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4. The diagrams below show different types of xylem vessels

lumen

XYZ(a) Xylem vessels X, Y and Z contain substance P. lt is an adapted feature for the vessels to

function efficiently.(i) Name substance P,

(ii) What is the role of substance P in these vessels?

(b) State the other adaptation of these vessels for their function.

5. The diagram below shows a section of phloem

(a) Name labelled structures P, Q and R.

P

(b) Describe the characteristics of each structure shown in the diagram.(i) Structure P

(ii) Structure Q

P

u

R

a

R

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(iii) Structure R

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(c) Why is structure R important to the function of structure Q?

(d) State two adaptations of phloem for its function

MOVEMENT OF WATER IN XYLEM VESSEL

. The movement of water in the xylem vessels is aided by three forces

1. Transpiration pull

2. Root pressure

3. Capillary action

Transpiration

. Transpiration is the loss of water vapour from the stomata of the plant.

. lt is the main force that draws water from the soil and through the plant.

. When water evaporates from the leaves, it causes a suction force which pulls water up thevessels in the vascular bundles.

. This stream of water moving up the xylem vessels against gravity is known as the transpirationstream.

. The cell sap in each cell is exerting a turgor pressure outwards on the cell wall. This pressure

forces some water out of the cell wall and into the air space between the cells.

How tronspirotion tokes ploce

. When water moves out from the mesophyll cells, it forms a thin film of moisture over theirsurfaces.

. lt will then evaporate from this thin film of moisture into the intercellular spaces.

. When the concentration of water vapour in the intercellular spaces becomes higher than that inthe atmosphere, it diffuses out of the spaces through stomata. The loss of water vapour from theleaves known as transpiration.

movementof water

xylem vesselsthin film ofmoisture

evaporation fromcell walls to theintercellular spaces

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i / guard cell

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The cells that are losing water in this way replace it by drawing water from the nearest xylemvessels.

As a result, water is pulled up through the xylem vessels and into the stem from the roots causedby transpiration. lt is known as transpiration pull.

lmportance of transpiration

Tronsport ol woter und minerol solts

. The transpiration stream moves water up through the plant to the leaves for photosynthesis.

. Most of the water evaporates from the leaves, only a tiny fraction is retained for photosynthesisand to maintain turgor in the cells.

. One advantage of this excessive evaporation of water is that the rapid flow of water is needed toobtain sufficient mineralsalts, which are in very dilute concentration in the soil.

Gooling of leof

. A leaf exposed to direct sunlight will absorb heat and its temperature may rise to a level thatcould kill the leaf.

. Water evaporation from the leaf absorbs latent heat and cools the leaf down.

WILTING OF PLANT

When a leaf mesophyll cell loses water, its turgor pressure will fall.

This fall in pressure allows water in the cell wall to enter the vacuole so as to restore the turgorpressure.

During times of water shortage, this particular cell may be able to get water by osmosis fromneighbouring cells more readily than it can get from the xylem vessels.

ln this case, all the mesophyll cells will be losing water faster than they can absorb it from thevessels. This happens when the rate of transpiration exceeds the rate of absorption of water bythe roots under strong sunlight.

As a result, the plant cells become flaccid and the plant wilts.

The advantage and disadvantage of wilting in a plant:

Advantage Disadvantage

Wilting causes the plant leaves to fold up.

This action reduces the surface area that is

exposed to sunlight.

The guard cells become flaccid and close thestomata to reduce the rate of transpiration.

a

Wilting reduces the amount of carbondioxide entering the leaves.

As a result, carbon dioxide becomes a

limiting factor that reduces the rate ofphotosynthesis in the plant.

Contro! of transpiration

. During photosynthesis, a plant has to take in carbon dioxide from the atmosphere.

. The pathway that allows carbon dioxide to enter will also let water to leave simultaneously.

. Plants have to maintain a careful balance between the optimum intake of carbon dioxide and

a damaging loss of water. They achieve this balance through stomata, leaf fall, leaf shape and

cuticle.

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Control mechanism Elaboration

Stomata

When loss of water greatly exceeds uptake of water, the plantwilts. As a result, the leaf cells, including the guard cells, becomeflaccid and the stomata close, preventing further evaporation.

The movements of the stomata depend on light intensity so theyare normally open during the day and closed at night.

Hence in darkness when photosynthesis is not in progress andcarbon dioxide is not needed, less water vapour is lost.

Leaf fall

Water supply from cold or frozen soil during wintertime is verylimited.

Hence deciduous trees shed their leaves in winter so as to reduce

water loss via transpiration.

Leaf shape andcuticle

Cacti have small pine needle like leaves so as to reduce water loss

in the hot desert climate.

Plants, living in areas where it is hot and dry, also have waxycuticles and sunken stomata that lie below the epidermal level so

as to reduce water loss.

Evergreen plants have one or more of these characteristics to help

them retain their leaves during winter.

However, a small amount of water may also escape from the leaves

through the epidermal cells known as cuticle transpiration.

a

a

a

Factor Elaboration

Light intensity

Light itself does not affect evaporation but in daylight the stomataare open. Hence, water vapour in the leaves will diffuse out intothe atmosphere.

At night when the stomata close, transpiration is greatly reduced.

Essentially transpiration speeds up when light intensity increases

because stomata respond to changes in light intensity. Sunlightwill also warm the leaves up and increase transpiration.

Humidity

At high humidity levels, the air around the plant only allowslimited amount of water vapour to escape from the leaf. Hence

transpiration slows down.

ln dry air, there is very little watervapour in the airso diffusion ofwater vapour from the leaf into the atmosphere will be rapid.

Presence of wind

ln still air, the region around a transpiring leaf will be saturatedwith water vapour so no more can escape from the leaf and

transpiration will slow down.

ln moving air, water vapour around the leaf will be blown away asquickly as it diffuses out, increasing the rate of transpiration.

Temperature

a Evaporation will take place more rapidly when the surroundingtemperature is high as water from the leaves will evaporate morereadily at higher temperatures.

Warm air can also hold more water vapour than cold air.

Factors affecting the rate of transpiration

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Factor Key steps

Light intensity

Experimental set-up

plant

oil

balance water

Objective

To investigate the effect of light intensity on the rate of transpiration

Manipulation of factor being investigated

Place one apparatus in a sunny area and another one in a dark room

Factors that kept constant(a) Humidity in air(b) Wind condition(c) Temperature

How to check the effect on the rate of transpiration

1. Record the initial weight of the apparatus

2. Record the final weight of the apparatus

3. Rate of transpiration = Loss in weighVTime taken

EXPERIMENT TO INUESTIGATE FACTORS AFFECTING THE RATE OF TRANSPIRATION

Using a Balance

* Note; You should be able to describe the similar experiment to investigate the other factors.

Using a potometer

. Potometer is an instrument used to measure the uptake of water by a plant such as a leafyshoot.

. When water is taken up by the plant, a small air bubble moves along a horizontal capillarytube. ln this way, the rate of movement of the air bubble corresponds to the rate of wateruptake by the leafy shoot.

. The assumption using a potometer is that water uptake is equal to water loss by transpirationThis is because some of the water may be used by plants for the process of photosynthesis.

. The end of the leafy shoot should be cut under water to prevent air trapped in the xylem.

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leafy shoot(transpirationtakes place) water reservoir

(refill water in thecapillary tube)

water taken up bythe stem to replacewater loss through

transpirationtap

-

-Il urr lrr rrlrr rrlr rrr lrr rrl-

capillary tubeair bubble

The rate of transpiration can be calculated by the following method

Rateoftranspiration = /cm + fsecond

Where / : Length of air bubble travelled from X to Y

t = Time take for air bubble to move from X to Y

graduatedmeasurement

ROOT PRESSURE

. When water enters root hair cells and the cortical cells, the cells become turgid. This turgiditygenerates a pushing force called root pressure.

. This pressure can push the water and mineral ions towards the xylem vessels in the roots and

upwards the stem.. However, root pressure seems to occur mainly in young herbaceous (non woody) plants or in

woody plants early in the growing season.

. Root pressure contributes to water movements in the stem, but the observed rates of flow are too

fast to be explained by root pressure alone.

. Killed root systems do not exhibit any root pressure, so it is clearly dependent on intact cytoplasmand living processes in the roots. Reducing the osmotic potential of soil water also reduces rootpressure.

Experiment to show the root pressure

. Connect a piece of glass tubing by means of a rubber stopper to a freshly cut stem of a potted plant.

. Place a little coloured water in the tube and mark its level.

. lf the roots are watered well, the coloured water will rise a few centimeters up the tube.

Experimental set-up Explanation

glass tube

coloured waterfinal level

initial level

rubber tubingcut

The cell sap in the root hairs is moreconcentrated than the soil water, so

water enters the root by osmosis.The water passes from cell to cell byosmosis. lt is finally forced in to thexylem vessels in the centre of the rootand up the stem. t-

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CAPILLARY ACTION

' ln long vertical columns of water containing in very thin tubes, the attractive forces between thewater molecules are greater than the forces trying to separate them.

. Hence the transpiration stream is actually pulling up thin threads of water which are resistingthe tendency to break.

Transport of food

. The xylem sap is always a very dilute solution but the phloem sap may contain up lo 257" ofdissolved solids, the bulk of which consists of sucrose and amino acids.

. The movement of water and salts in the xylem is always upwards from soil to leaf. However, inthe phloem, the solution may be traveling in all directions, Llp or down the stem.

. During photosynthesis, the manufactured carbohydrates are converted into sucrose and aretransported out of the leaf to the stem.

' From here the sucrose may pass upwards to growing buds and fruits or downward! to the rootsand storage organs.

. This transport of food and substances such as amino acids in plants is known as translocation.

Experiment to show the evidence of translocation in the phloem

' lf a leaf is exposed to radioactive carbon dioxide, it will appear in the sucrose in the phloem

radioactive productstravel up stem

'oco,

phloem killedby heat

phby

loem killedheat

"co.

radioactive productstravel down stem

a

Furthermore, if the phloem of the stem below the leaf is killed by a jet of steam or removed bycutting away a ring or bark, the substances containing radioactive carbon are found to move upthe stem only.

When the phloem above the leaf above the leaf is killed or removed, the radioactive substancesare found to move down the stem only.

lf the oxygen supply to the phloem is cut off, translocation of sugars ceases.

Hence it can be concluded that:

1. Sugars travel in the phloem.

2. Sugars can travel in upward and downward directions in a plant.

3. Translocation can only take place in living tissues.

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OREUISIO}I 2

1-. The diagram below shows sections of a root.

xylem vessel

cell Acell B cell C

(a) Describe two adaptations of cell A for its function.

(b) Describe how the movement of water(i) from soil to cellA

(ii) from cell A to xylem vessel

The diagram below shows how water moves against gravity in the xylem vessel from the roots to themesophyllcells.

2.

mesophyll cellsin a leaf

xylem vesselin the stem

direction of watermovement

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There are three ways to move water against gravity from the roots to the mesophyll cells. They arecapillary action, root pressure and transpiration pull.

(a) Briefly describe the following.(i) Capillary action

(ii) Root pressure

(b) (i) Through what process is transpiration pull generated in a plant?

(ii) Describe how this transpiration pull can be generated by this process named in (b)(i) tomove water up the plant.

(c) Name four factors that can affect the force of transpiration pull

3. (a) Define transpiration

(b) The diagram below shows the internal structures of a leaf

A

B

\E

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By identify all the labelled regions in the diagram, describe how transpiration takes place in theleaf.

(c) Why is transpiration important to plants? Give three reasons

4. (a) Which process affects the rate of transpiration?

(b) State the four external factors that affect the rate of transpiration

5. (a) Explain why a plant wilts in a sunny day,

(b) (i) State one advantage of wilting in plants.

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(ii) State one disadvantage of wilting in plants.

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