Chapter 9 Transport in Plants

How is water transported against gravity from the roots, up the xylem and to the leaves?

I wonder where trees get water from?

Well, obviously from the ground.

What are the processes involved?

How does water move through the transport system of a plant IFit does not have a heart to act as a pump?

PAUSE toPONDER

•How is water lifted against gravity from the ground to the leaves through this transport system?

• Are the products of photosynthesis also carried in a set of vessels from the leaves to the roots?

4

Diagram of Xylem Vessel

Transport Structures of Flowering Plants

1) Xylem

• Consists of mainly xylem vessels that are made up of dead cells.

• Inner walls of xylem vessels are strengthened by lignin.

• Lignin deposited in the form of rings or spirals.

1) Xylem (cont)

• Function:

– Conduct water and mineral salts from roots to stems and leaves.

– Provide mechanical support for plant.

Transport Structures of Flowering Plants

Adaptations of the xylem vessel for its function

1) Empty lumen, no protoplasm or cross-walls.

– Reduces resistance to water flowing through

2) Walls thickened with lignin.– Lignin is a hard and rigid substance.– Prevents collapse of the vessel

(Mechanical support)

2) Phloem• Consists mainly of sieve tubes and companion

cells.

• Sieve tube consists of columns of sieve tube cells, that are elongated and thin-walled.

• Companion cells provide the nutrients and help the sieve tube cells transport manufactured food. (Sucrose)

Transport Structures of Flowering Plants

Diagram of Phloem

2) Phloem (cont)

• Function:– Conducts manufactured food (sucrose and

amino acids) from the leaves to the other parts of the plant.

Transport Structures of Flowering Plants

Monocotyledon VS Dicotyledon

1 Vascular bundles

14

How Are the Vascular Tissues Organised in Stems?

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

xylemcambium

phloem vascular bundle

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.

2 Cambium

How Are the Vascular Tissues Organised in Stems?

Pith

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

3

Cortex4

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.

How Are the Vascular Tissues Organised in Stems?

Epidermis

The stem is covered by a layer of cells called the epidermis.

•The epidermal cells are protected by a waxy, waterproof cuticle

•Which greatly reduces evaporation of water from the stem.

5

2 Pith

1 Vascular Bundles

3

4 Cortex

5 Epidermis

Xylem

Cambium

Phloem

Vascular bundle

17

How Are the Vascular Tissues Organised in Stems?

How Are the Vascular Tissues Organised in Roots?

xylem

phloem

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

1 Vascular bundles

2 Cortex

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

How Are the Vascular Tissues Organised in Roots?

Piliferous layer

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

Root hair

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.

4

3

1 Xylem and phloem alternate with each other.

2 Cortex

Endodermis3 Piliferous layer

4 Root hair

20

How Are the Vascular Tissues Organised in Roots?

How Are the Vascular Tissues Organised in Leaves?

Distribution of vascular bundles in leaves.

xylemphloem

xylem

phloem

Diagram showing a section through a leaf.

How you ever noticed such dots on tree trunks? WH AT IS

IT?

Formation of Lenticles• In woody stems the stomata are blocked by the

presence of cork cells• The epidermis of woody stems breaks up to form tiny

pores called lenticles which allow gaseous exchange.

Test yourself! (pg.179)

• Qn 1

The Journey so far…

• Xylem– Structure: Arrangement of lignin– 2 functions– 2 adaptations

• Phloem– Structure: Made of ? & ?– Function– Adaptation

• Arrangements of X & P (VB) in roots, stems, leaves

Lesson Objectives

• Movement of substances and the processes involved in the roots and leaves

29

cytoplasm

vacuole

nucleus

cell wall

cell surface membrane of root hair cell

film of liquid (dilute solution of mineral salts)

soil particles

Entry of Water into a Plant

Entry of Water into a Plant

30

ABC

xylem

phloem cortex

root hair

piliferous layer

A section of root showing the path of water through it

31

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.

1

Entry of Water into a Plant

1

32

cytoplasm

vacuole

nucleus

cell wall

cell surface membrane of root hair cell

film of liquid (dilute solution of mineral salts)

soil particles

1

Entry of Water into a Plant

2

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

2

Entry of Water into a Plant

33

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.

3

ABC

xylemphloem cortex

root hairpiliferous layer

water entering the root hair

A section of root showing the path of water through it

3

Entry of Water into a Plant

34

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

ABC

xylem

phloemcortex

root hair

piliferous layer

water entering the root hair

A section of root showing the path of water through it

34

Entry of Water into a Plant

35

Similarly, water passes from cell B into the next cell (cell C) of the cortex.

•This continues until the water enters the xylem vessels and moves up the plant.

5

ABC

xylem

phloemcortex

root hairpiliferous layer

water entering the root hair

A section of root showing the path of water through it

34

5

How do nitrate ions get into plants?Are they directly absorbed from the air?

No. Even though the air has 79% of nitrogen, it is highly unreactive.PAUSE to

PONDER•How are ions transported around in plants?

How do root hairs absorb ions and mineral salts?

1) Active Transport• When the concentration of ions in the salt solution

is lower than that in the root hair cell sap

• Root hairs absorb the salts against a concentration gradient

• Energy for this process comes from cellular respiration in the root hair cell.

2) Diffusion• When concentration of certain ions in the soil

solution are higher than that in the root hair cell.

How do root hairs absorb ions and mineral salts?

How the root hair cell is adapted to its function of absorption

1) Root hair is long and narrow• Increases surface area to volume ratio• Which in turn increases the rate of absorption of

minerals and water.

2) Cell surface membrane prevents cell sap from leaking out

• Cell sap contains sugars, amino acids and salts• Has lower water potential than soil solution• Results in water entering the root hair by osmosis

3) Living root hair cell• Able to provide energy for active transport of ions

(mineral salts) into the cell.• Energy produced by the mitochondria during

cellular respiration.

How the root hair cell is adapted to its function of absorption

Movement of Water inside

a Leaf

41

Cuticle

Film of water

Xylem

Phloem

Intercellular air space

Upper epidermis

Palisade mesophyll

Spongy mesophyll

Lower epidermis

Guard cell

Stomatal pore

Movement of Water inside

a Leaf

42

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

Stomatal pore

Cell sap

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

1

43

cuticlechloroplasts

nucleus

film of water

xylem of vein

phloem

intercellular air space

sub-stomatal air space

upper epidermis

palisade mesophyll

spongy mesophyll

lower epidermis

1

guard cell

Water evaporates from this thin film of moisture

• moves into the intercellular air spaces.

• Water vapour accumulates in the large air spaces near the stomata (sub-stomatal air spaces).

2

2

stomatal pore

Movement of Water inside

a Leaf

Movement of Water inside

a Leaf

44

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

guard cell

cell sap

2

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

3

3stomatal pore

45

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

guard cell

2

stomatal pore

Movement of Water inside a

Leaf

3

4cell sap

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.

4

46

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

guard cell

2

stomatal pore

Movement of Water inside

a Leaf

3

4cell sap

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

5

Transpiration

Definition:

It is the loss of water vapour through the stomata of the leaves.

Importance of Transpiration

1) Transpiration pull • helps to draw water and mineral salts from the

roots to the leaves

2) Evaporation of water removes latent heat from the plant.

• prevent the plant from overheating

3) Water transported to the leaves can:• be used for photosynthesis• keep cells turgid• replace water lost by the cells.

Factors affecting the rate of Transpiration

1) Humidity of the air • Humidity refers to the amount of water vapor in

the air.• Increasing humidity decreases the concentration

gradient between air and leaf.• Therefore, deceasing the rate of transpiration

2) Wind or Air movement• The stronger the wind, the faster the rate of

transpiration.

3) Temperature of air• Increasing temperatures increases the rate of

evaporation.• Thus increasing the rate of transpiration

4) Light • Light affects the stomata.• With increasing light intensity, stomata open and

become wider.• Therefore, this increases the rate of transpiration

Factors affecting the rate of Transpiration

Wilting

Wilting• Occurs when excess transpiration occurs• In strong sunlight, rate of transpiration exceed

rate of water absorption by the roots• cells lose more water than they absorb water,

resulting in plasmolysis water leaves the cells, causing the cell membrane to shrink from the cellulose cell wall

• cells lose their turgor pressure and become flaccid

• This results in the plant wilting.

Advantages of Wilting

• The folding up of the leaves– Surface area exposed to sunlight is reduced

• Leads to the guard cells becoming flaccid and close up– Therefore reducing the rate of transpiration

Disadvantages of Wilting• Water is the limiting factor for photosynthesis

• Stomata is closed, – Amount of CO2 entering the leaf is also reduced.

– CO2 becomes a limiting factor,

• Therefore decreasing the rate of photosynthesis.

Experiments

Investigation 9.1

• What is transverse?

• Which tissue has been stained red?

• What conclusion can you draw from your investigation?

Investigation 9.2

• Make drawings of your observations

• What does this experiment tell you about the phloem?

• Suggest an explanation of your observations.

Investigation 9.2

• The phloem tissues have been removed.

• Manufactured food substances (e.g sugar and amino acid) accumulate above the cut region and cause swelling in twigs A and C. However in twig B, manufactured food can pass through the phloem without any barrier.

• This suggests that food is made in the leaves and are transported through the phloem.

twig A twig B twig C