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TRANSPORT TRANSPORT IN PLANTSIN PLANTS

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Lesson 1Learning Objectives1.Draw a labelled transverse section drawing of dicot roots,

stems and leaves to show positions of xylem and phloem tissue.

2.State the functions of xylem and phloem and how they are adapted for their functions.

Outline Vascular bundle Xylem Phloem

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Transport system in plants

Xylem Phloem

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Vascular Bundle

The vascular bundles extend continuously from root to the stem and leaves.

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Vascular bundle in root

Cross section of a dicot root

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phloemxylem

1 In a dicotyledonous root, the xylem and phloem are not bundled together. Instead, they alternate with each other.

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1 xylem and phloem alternate with each other.

2 The cortex of the root is also a storage tissue. The innermost layer of root cortex is called the endodermis.

endodermis

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1 xylem and phloem alternate with each other.

2 cortex

3 The epidermis of the root is the outermost layer of cells. It bears root hairs. It is also called the piliferous layer.

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1 xylem and phloem alternate with each other.

2 cortex

3 piliferous layer

4 Each root hair is a tubular outgrowth of an epidermal cell. This outgrowth increases the surface area to volume ratio of the root hair cell. The absorption of water and mineral salts is increased through this adaptation.

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Vascular bundle in root

1 xylem and phloem alternate with each other.

2 cortex

endodermis3 piliferous layer

4 root hair

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Vascular bundle in stem

Cross section of a dicot stem

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1 In a dicotyledonous stem, the xylem and phloem are grouped together to form vascular bundles.

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2 The vascular bundles are arranged in a ring around a central region called the pith.

1 vascular bundles

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2 pith

1 vascular bundles

3 The phloem lies outside the xylem with a tissue called the cambium between them. Cambium cells can divide and differentiate to form new xylem and phloem tissues, giving rise to a thickening of the stem.

xylemcambium

phloemvascular bundle

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2 pith

1 vascular bundles

3 cambium

4 The region between the vascular bundles and the epidermis is the cortex. Both the cortex and the pith serve to store up food substances, such as starch.

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2 pith

1 vascular bundles

3 cambium

4 cortex

5 The stem is covered by a layer of cells called the epidermis. The epidermal cells are protected by a waxy, waterproof cuticle that greatly reduces evaporation of water from the stem.

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Vascular bundle in stem

2 pith

1 vascular bundles

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4 cortex

5 epidermis

xylem

cambium

phloem

vascular bundle

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Test yourself: Root? Stem? Leaf?

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Vascular bundle

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How to identify xylem vessels from phloem?

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Xylem

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Different pattern of lignification

annularspiral pitted

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PhloemPhloem consists mainly of sieve tubes and companion cells.

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Phloem – Sieve tubes

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Phloem – Companion cell

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Exercise

1.Draw and label a vascular bundle in root and stem. Show the positions of xylem and phloem clearly.

2.Draw a table of comparison between xylem and phloem.

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Lesson 2

Learning Objective1.Define translocation

Outline Translocation

– What is translocation?– Pressure-flow hypothesis– Translocation studies

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Translocation

The process by which solutes or manufactured food (e.g. sugars – sucrose

and amino acids) are transported throughout the plant by phloem.

It occurs by diffusion and active transport.

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Translocation

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• The best supported theory to explain the movement of food in plant through phloem is called pressure-flow hypothesispressure-flow hypothesis..

• The glucose molecules made during photosynthesis in the leaves (source) are converted to sucrose and carried out of the leaf to the stem. From here, sucrose may travel upwards (growing buds and fruits) or downwards (roots and storage organs).

Pressure-flow hypothesis

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• Water containing food molecules flows under pressure through the phloem. The pressure is created by the difference of water concentration in phloem and in xylem.

• At the leaf (source), sugars are pumped by active transport into the companion cell and sieve elements of phloem.

• As sugar accumulates in phloem, water enters by osmosis.

• Turgor pressure builds up in sieve tubes. As the fluid is pushed up or down the phloem, sugars are actively removed from phloem by stem or root (sink).

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Translocation studies

1. Use of aphids2. Ringing experiment3. Use of isotopes

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1. Use of Aphids• Aphids are insects which feed on plant juices.• They penetrate phloem tissue with their stylets.• Aphids are anaesthesised with carbon dioxide while

it is feeding. Body is cut off.• The stylet remains in place.• Liquid exudes from stylet.• Analysis shows liquid contains sucrose and amino

acids.

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2. Ringing Experiment

• A ring of bark is scraped away including the phloem.

• After a while, sucrose accumulates above the ring forming a bulge.

• Eventually, the bark below the ring dies, while the bark above the ring swells and remains healthy.

• This suggests sucrose moves down the stem in phloem.

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3. Use of isotopes

• Provide a leaf using carbon-14 radioactive isotope.

• Stem is cut off.• Exposed it onto an x-ray photographic

film.• Radioactive substances are present in

phloem.

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Lesson 3Learning Objectives1.Relate the structure and functions of root hairs to their surface area and to water

and ion uptake.2.Explain the movement of water from the soil to the root hairs.3.Explain the mechanism of root pressure.4.Explain the mechanism of capillary action.5.State that root pressure and capillary action are ways to draw water up the stem6.Define transpiration7.Explain the mechanism of transpiration producing a pull from above.

Outline Transport of water

– Soil to root hairs (osmosis)

– Up the stem (root pressure, capillary action, transpiration pull)

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Transport of water

• From soil into root hairs– Osmosis

• Up the stem– Root pressure– Capillary action– Transpiration pull

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Transport of water into the root hairs

cytoplasm

vacuole

nucleus

cell wall

cell surface membrane of root hair cell

film of liquid (dilute solution of mineral salts)

soil particles

Each root hair is a fine tubular outgrowth of an epidermal cell. It grows between the soil particles, coming into close contact with the water surrounding them.

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1The thin film of liquid surrounding each soil particle is a dilute solution of mineral salts.

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The sap in the root hair cell is a relatively concentrated solution of sugars and various salts. Thus, the sap has a lower water potential than the soil solution. These two solutions are separated by the partially permeable cell surface membrane of the root hair cell. Water enters the root hair by osmosis.

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The entry of water dilutes the sap. The sap of the root hair cell now has a higher water potential than that of the next cell (cell B). Hence, water passes by osmosis from the root hair cell into the inner cell.

4 Similarly, water passes from cell B into the next cell (cell C) of the cortex. This process continues until the water enters the xylem vessels and moves up the plant.

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ABC

xylemphloem

cortex

root hair

piliferous layer

water entering the root hair

a section of root showing the path of water through it

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As water enters the cells, it can take one of two routes depending upon whether or not it has entered the cytoplasm of the cell.

These routes are called the– Apoplastic

• Water is transported between cells up to the endodermis.

– Symplastic• Water is transported through the cell

membrane and cytoplasm. • It takes longer to transport and requires ATP.• Water can pass through the endodermis.

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Transport of mineral salts into root hairs

• Diffusion– When the concentration of mineral salts in

the soil solution is higher than that in the root hair cell.

• Active transport– When the concentration of ions in the soil

solution is lower than that in the root hair cell.

– The energy comes from respiration in the root hair cells.

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Transport of water up the stem

• Root pressure• Capillary action• Transpiration pull

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Root Pressure

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• By using active transport, ions in the living cells around the xylem vessels in the root are pumped into the vessels.

• Water potential in the xylem vessels is lowered.• Water passes from the living cells into the xylem

vessels by osmosis. • Accumulation of water in the xylem vessels create

a pressure called root pressure.• Root pressure pushes the water up the stem.

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Capillary Action

Water also moves up inside fine capillary tube by capillary action

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• Water molecules attract other water molecules by the force of cohesion.

• Water sticks to the inner surface of the xylem vessels by adhesion.

• Both cause water to move up xylem stem without breaking.

• This mechanism does not account for the greater distance that water can travel in trees.

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Transpiration PullTranspiration is the loss of water vapour from stomata

of the leaves of the plant.

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• When water vapour is lost from the leaves, there is a decrease in water concentration in the leaves.

• This creates a suction force that pull water up from the stems.

• This is called transpirational pull.• Transpiration stream is the stream of water

going up the stem.

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Lesson 4Learning Objectives1.Explain the effect of temperature, humidity, wind strength,

and light intensity on rate of transpiration.2.Describe wilting.3.State the appearance of a wilted plant.4.State how some plants are adapted to dry hot condition.

Outline Transpiration Factors affecting transpiration Wilting Adaptation of plants in dry hot condition

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Transpiration

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

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cuticle chloroplasts

nucleus

film of water

xylem of vein

phloem

intercellular air space

sub-stomatal air space

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

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arrows show path of water vapour and water

guard cellstomatal pore

cell sap

Water continuously moves out of the mesophyll cells to form a thin film of moisture over their surfaces.

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cuticle chloroplasts

nucleus

film of water

xylem of vein

phloem

intercellular air space

sub-stomatal air space

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

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arrows show path of water vapour and water

guard cell

cell sap

Water evaporates from this thin film of moisture and moves into the intercellular air spaces. Water vapour accumulates in the large air spaces near the stomata (sub-stomatal air spaces).

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stomatal pore

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cuticle chloroplasts

nucleus

film of water

xylem of vein

phloem

intercellular air space

sub-stomatal air space

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

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arrows show path of water vapour and water

guard cell

cell sap

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Water vapour then diffuses throughout the stomata to the drier air outside the leaf. This is transpiration.

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3stomatal pore

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cuticle chloroplasts

nucleus

film of water

xylem of vein

phloem

intercellular air space

sub-stomatal air space

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

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arrows show path of water vapour and water

guard cell

cell sap

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As water evaporates from the mesophyll cells, the water potential of the cell sap decreases. The mesophyll cells begin to absorb water by osmosis from the cells deeper inside the leaf. These cells, in turn, remove water from the vein, that is, from the xylem vessels.

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stomatal pore

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cuticle chloroplasts

nucleus

film of water

xylem of vein

phloem

intercellular air space

sub-stomatal air space

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

1

arrows show path of water vapour and water

guard cell

cell sap

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This results in a suction force which pulls the whole column of water up the xylem vessel.

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stomatal pore

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Importance of transpiration

1.Facilitates in transporting water and mineral salts upwards from roots to leaves

2.Cools the plants; prevents scorching.

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Experiment to investigate transpiration

Potometer

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Group Discussion

Discuss what could be the factors that affect transpiration rate.

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Factors affecting transpiration rate

• Temperature• Air humidity• Light intensity• Wind• Carbon dioxide concentration

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Temperature

30

T

degrees

Stomata closed

• As temperature increases, the rate of transpiration also increases up to a certain point.

• Beyond 30ºC, the stomata close to prevent more water loss.

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Air humidity

• As the air humidity increases, the rate of transpiration decreases.

• When the atmospheric air is fully saturated with water vapours, there is no possibility that more water vapours move into it.

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Humidity

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Light intensity

• As light intensity increases, the rate of transpiration increases up until a certain point.

• When there is too much light, the heat given out from the light can be very hot hence the stomata will close.

Stomata closedT

Light intensity

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Wind• As the wind gets

stronger, the rate of transpiration is faster.

• The wind blows away the water vapour on the surface of the leaf. This increases the water vapour concentration gradient between inside and outside the leaf. More water vapour will be lost.

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Wind

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Carbon dioxide concentration• As the CO2

concentration in the leaf increases, the rate of transpiration decreases.

• An increase in CO2 concentration in leaf causes the stomata to close. When stomata close, water vapour is not lost to the atmosphere.

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CO2 concentration in

leaf

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What happen when there is excessive transpiration?

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Adaptation of some plants in hot dry condition

• To reduce water loss– A thick, waxy, waterproof cuticle– A reduced number of stoma– Stomata sunken into pits to trap water vapour– Hairs on the surface to trap water vapour– Curled up leaves to trap water vapour– A reduced surface area e.g. spines instead of leaves)

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• To increase water uptake– A large, deep root system to reach water deep

in the ground.• To store water

– Succulent tissues to store large amounts of water.

Adaptation of some plants in hot dry condition