Download Chapter 9: Transport in Flowering Plants

Seng Kang Secondary School
Secondary 3 Biology
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Chapter 9: Transport in Flowering Plants
Learning outcomes
1) Identify the positions and explain the functions of xylem vessels, phloem (sieve tube
elements and companion cells) in sections of a herbaceous dicotyledonous leaf and
stem, under microscope
2) Relate the structure and functions of root hairs to their surface area, and to water and
ion uptake
3) Explain the movement of water between plant cells, and between them and the
environment in term of water potential
4) Outline the pathway by which water is transported from the roots to the leaves through
the xylem vessels
5) Define the term transpiration and explain that transpiration is a consequence of
gaseous exchange in plant
6) Describe
i.
the effects of variation of air movement, temperature, humidity and light intensity
on transpiration rate
ii.
how wilting occurs
7) Define the term translocation as the transport of food in the phloem tissue and
illustrate the process through translocation studies
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9.1 Transport Structures – Xylem and Pholem
Vascular Tissues

Vascular tissues are made up of xylem, phloem and cambium. Xylem and phloem are
involved in the transport system of plants.
Xylem

Structure
o Mainly made up of xylem vessels
 Made up of dead, tube like cells joined end-to-end
 Inner walls strengthen by deposits of lignin

Adaptations
o An empty lumen without protoplasm. The vessel is a continuous tube without
cross wall
 Reduces resistance to water flow
o Lignified walls (contains deposits of lignin – a rigid impermeable substance)
 Strengthens the xylem vessel so as it does not collapse easily, providing
mechanical support to the plant

Different terms for xylem on the pattern of ligification
o Annular xylem: Thickenings in the form of ring.
o Spiral xylem:
Lignin is deposited in the form of a continuous spiral
o Pitted xylem:
The entire wall is uniformly thickened leaving small areas
called pits
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Phloem

Transports manufactured sugars and amino acids from the leaves to the other parts of the
plant

Structure
o Sieve tubes
 Consist of a single column of elongated, thin-walled living cells called
sieve tube elements
 The end walls are called sieve plates. Each plate is perforated with minute
holes
 Mature sieve tube element has only thin layer of cytoplasm lining at the
thin wall of the cell
 Sieve tube element lost most of its organelles, including its vacuole and
nucleus
o Companion cells
 Next to the sieve tube elements
 Do not lose their nuclei or cytoplasmic organelles

Adaptions
o Companion cells have thin walls and numerous mitochondria
 Supports sieve tube elements metabolically
 Move sugars and amino acids into sieve tube elements by active transport
(phloem loading)
o Pores in sieve plates contain thin strands of cytoplasm
 Allow rapid flow of manufactured food substances through the sieve tubes
 Process of transport called translocation
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Cambium
 The cambium consists of a group of cells which can divide and differentiate to form new
xylem and phloem when the plant grows.
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Structure of dicotyledonous root
Structure of dicotyledonous stem
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9.2 Entry of water and mineral salts into plants
Uptake of water and ions into the plants involved the xylem.
Entry of water into the plant

Absorption of water and mineral salts takes place through the root hairs
o Mineral salts are found in the water the coats soil particle
o Root hairs grow among the soil particles in contact with the soil

The sap in the root hair cell has a higher concentration of sugars, amino acids and various
mineral salts.
o Sap has low water potential compared to the surrounding soil
o Hence, water enters the root hair cell by osmosis through a selectively permeable cell
membrane
o With the entry of water, the sap of the cell A has higher water potential than its
adjacent cell, cell B. Under the same process, water will move from cell B to cell C of
the root cortex.
o This process continues until water enters the xylem vessels and moves up the plant.
C B A
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Absorption of dissolved mineral salts by diffusion and active transport
o Partially permeable cell surface membrane in the root hair cell
o Prevents sugar and starch to pass out into the soil
o Allows dissolved mineral salts in the soil solution to pass through by diffusion into
the root hair
o From there, mineral salts diffuse inwards to cortex, then to xylem vessels which will carry
them upwards
o
If the concentration of mineral salts in the root
hair cells is greater than that in the soil solution 
intake of mineral salts by diffusion is not possible.
o In this case, the root hair needs to absorb
mineral salts against a concentration gradient
o This process is known as active transport.
o Active transport requires energy and occurs
only in living cells because only living cells respire.
 Tissue respiration is a process whereby
energy is set free.
 Part of this energy can be used in active
transport.
Adaptation of root hair cell to absorption
 Outgrowth of root hair is long and narrow:
 Increase in surface area to volume ratio
o Enhances the rate of absorption of water and mineral salts per unit time
 The cell sap of the root hair cell :
 Contains sugar, amino acids and salts
 More concentrated than the surrounding soil solution
o Osmosis can then occur
o Water from the surrounding water solution enters
 The root hair is living and is able to provide energy for active transport of mineral ions
against a concentration gradient by carrying out cellular respiration.
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9.3 Translocation
Translocation is the transport of manufactured foods (sugar and amino acids) in plants.
 In the leaves, sucrose produced by photosynthesis
 Sucrose actively pumped into the phloem vessel by the companion cells
 Water potential in the leaf phloem decreases
 Water diffuses from the neighboring xylem vessels by osmosis
 This increase the hydrostatic pressure in the phloem
 Water and dissolved solutes are forced downwards to relieve the pressure
 Lead to mass flow, the flow of water together with its dissolved solutes due to a force
 In the roots, solutes are removed from the phloem by active transport into the root cells
 Increase in water potential in phloem
 At the same time, ions are being pumped into the xylem from the soil by active
transport.
 Water potential in the xylem is lower as compare to the phloem.
 Water moves in by osmosis from the phloem to the xylem.
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Translocation studies

Aphid feeding
o Aphids pierce into plant leaves or stems with their stylets to feed on sap within
phloem sieve tubes
o Procedure
1. Anaesthesise the aphids with carbon dioxide while its feeding
2. Remove the body of aphid, leaving stylet in plant tissue
3. Analysis liquid from cut end of stylet

Ringing experiment
o Refer to worksheet

Carbon-14 radioactive tracing
o Carbon-14 is a radioactive isotope of carbon that can be used by plants for
photosynthesis. It can be detected with an X ray flim
o Procedure:
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9.4 Vertical transport of water in the stem

Differences in water potentials of a plant and its surroundings determines the
movement of water into and within the plant

Three theories are proposed to account for the vertical transport in plants.
1) Root pressure:



Cells surrounding xylem vessels in the root actively pump ions into xylem vessels
Water potential decreases in the xylem vessel, creating a water potential gradient
Water moves by osmosis from these cells into xylem vessels. As water continues to
enter the xylem, the increasing pressure pushes the water column upwards
Is root pressure effective in water transport in plants?
No. It is unable to bring water all the way to tops of tall trees
2) Capillary action:

Water tends to move up inside fine capillary tubes by capillary action. This
contributes upward movement of water in small plants.

Capillary action is due to:
o Force of attraction between water molecules ( cohesion )
o Force of attraction between water molecule and inner wall of capillary
tube ( adhesion )
Is capillary action effective in water transport in plants?
No. Maximum height reached by capillary action is 3 metre.
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3) Transpiration Pull






Main force that pulls water up the plant
Transpiration : The loss of water vapor from the aerial parts of a plant,
especially through the stomata, as a consequence of gaseous exchange
Transpiration lowers the water potential of the mesophyll cells, which causes
water to move from xylem vessels into the mesophyll cells
As water leaves the xylem vessels, a tension (pulling force) is created
Water is able to move up due to
o Adhesive and;
o Cohesive property of water
Continuous column of water moving up the xylem due to transpiration pull is
called the transpiration stream .
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Importance of transpiration
Although plants risk death if excessive water is lost through transpiration, they have the
following advantages

Cooling of leaves
o As water evaporates from the leaves, it will remove the latent heat (heat
released or absorbed during a change in state).

Supply sufficient water and mineral salts for photosynthesis
o Suction force to pull water and mineral salts from roots to the leaves

Brings water to various part of the plant to maintain turgidity
Moving water against gravity
 Transpiration Pull (the driving force)
 Upon evaporation, the water deficit
generates pressure strong enough to
pull more water up the tree.
 Cohesion (in xylem)
 Water molecules form an unbroken
water column that extends from the
leaf vein to the roots.
 When one water molecule exits the
xylem to replace water lost through
transpiration, it exerts a pull on
adjacent water molecules.
 Water uptake (from soil)
 Lower water potential in root cells
draw water from soil.
 The absorptive surface is increased
with more root hairs.
 Water moves through endodermis by
osmosis.
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9.5 Factors that affect the rate of transpiration

Since transpiration involves evaporation, factors that influences evaporation of water
affects rate of transpiration

Air humidity
o Air spaces in leaf usually saturated with water vapor  water potential
gradient exist between leaf and surrounding air
o In drier conditions, water potential gradient becomes steeper  rate of
transpiration is higher
o As humidity increases , water potential gradient decreases  transpiration rate
decreases

Wind
o Water vapor that diffuses out of leaf tends to accumulate in a layer around leaf
 increases humidity around leaf  reducing transpiration.
o Wind blows away layer of water vapor  decreasing humidity around the leaf
 increasing rate of transpiration
o Rate of transpiration increases with wind speed up to a certain extent
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
Temperature
o At higher temperature, rate of evaporation increases  transpiration rate
increases with temperature until it reaches maximum

Light
o Light stimulates the opening of stomata
o In light, stomata open  transpiration rate increases
o In darkness, stomata close  transpiration rate decreases
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9.6 Wilting

Wilting refers to the loss of turgidity in non-woody plants. It occurs when the rate of
water loss from the leaves exceed the rate of water replacement.

The cells become flaccid and the plant wilts

Leaves are usually the first to wilt
o

Decrease exposed surface area Decrease in transpiration
Stomata close which lead to a decrease in the amount of carbon dioxide entering the
leaves
o Decrease in photosynthesis
Wilting leaf
folds up,
reducing
exposed
surface area
Leaf cells
lose their
turgor and
become
flaccid
transpiration
reduced
Flacid guard
cells cause
stomata to
close
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Summary
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