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UNIT 4
PLANT PHYSIOLOGY
Chapter 11
Transport in Plants
Chapter 12
Mineral Nutrition
Chapter 13
Photosynthesis in Higher
Plants
Chapter 14
Respiration in Plants
Chapter 15
Plant Growth and
Development

The description of structure and variation of living organisms over a
period of time, ended up as two, apparently irreconcilable perspectives
on biology
...
One described at organismic
and above level of organisation while the second described at cellular
and molecular level of organisation
...
The second resulted in physiology and biochemistry
...
The processes of
mineral nutrition of plants, photosynthesis, transport, respiration and
ultimately plant growth and development are described in molecular
terms but in the context of cellular activities and even at organism
level
...


Melvin Calvin
(1911 – )

MELVIN CALVIN born in Minnesota in April, 1911 received his
Ph
...
in Chemistry from the University of Minnesota
...

Just after world war II, when the world was under shock
after the Hiroshima-Nagasaki bombings, and seeing the illeffects of radio-activity, Calvin and co-workers put radioactivity to beneficial use
...
A
...
Calvin proposed that
plants change light energy to chemical energy by transferring
an electron in an organised array of pigment molecules and
other substances
...

The principles of photosynthesis as established by Calvin
are, at present, being used in studies on renewable resource
for energy and materials and basic studies in solar energy
research
...
1 Means of
Transport
11
...
3 Long Distance
Transport of
Water
11
...
5 Uptake and
Transport of
Mineral
Nutrients
11
...
Plants need to move
molecules over very long distances, much more than animals do; they also
do not have a circulatory system in place
...
The
photosynthates or food synthesised by the leaves have also to be moved to
all parts including the root tips embedded deep inside the soil
...
To understand some of
the transport processes that take place in plants, one would have to recollect
one’s basic knowledge about the structure of the cell and the anatomy of
the plant body
...

When we talk of the movement of substances we need first to define
what kind of movement we are talking about, and also what substances
we are looking at
...
Over small distances substances move by diffusion
and by cytoplasmic streaming supplemented by active transport
...

An important aspect that needs to be considered is the direction of
transport
...
Organic and mineral
nutrients however, undergo multidirectional transport
...
From the storage organs
they are later re-exported
...
When any plant part undergoes senescence, nutrients may be
withdrawn from such regions and moved to the growing parts
...

Hence, in a flowering plant there is a complex traffic of compounds (but
probably very orderly) moving in different directions, each organ receiving
some substances and giving out some others
...
1 MEANS

OF

TRANSPORT

11
...
1 Diffusion
Movement by diffusion is passive, and may be from one part of the cell to
the other, or from cell to cell, or over short distances, say, from the intercellular spaces of the leaf to the outside
...
In diffusion, molecules move in a random fashion, the net result
being substances moving from regions of higher concentration to regions
of lower concentration
...
Diffusion is obvious in gases and liquids, but diffusion in
solids rather than of solids is more likely
...

Diffusion rates are affected by the gradient of concentration, the
permeability of the membrane separating them, temperature and pressure
...
1
...
The diffusion rate depends on the size of the substances; obviously
smaller substances diffuse faster
...
Substances soluble in lipids diffuse through the membrane
faster
...
Membrane
proteins provide sites at which such molecules cross the membrane
...

This process is called facilitated diffusion
...
Facilitated diffusion
cannot cause net transport of molecules from a low to a high concentration
– this would require input of energy
...
Facilitated

TRANSPORT IN PLANTS

177

Figure 11
...
It is
sensitive to inhibitors which react with
protein side chains
...

Some channels are always open; others
can be controlled
...

The porins are proteins that form huge Uniport A
pores in the outer membranes of the
plastids, mitochondria and some bacteria
allowing molecules up to the size of small
proteins to pass through
...
1 shows an extracellular
A
molecule bound to the transport protein; Antiport
the transport protein then rotates and
releases the molecule inside the cell, e
...
,
water channels – made up of eight
different types of aquaporins
...
1
...
1 Passive symports and
B
antiports
Some carrier or transport proteins allow
diffusion only if two types of molecules
move together
...
2)
...
2 Facilitated diffusion

178

BIOLOGY

molecule moves across a membrane independent of other molecules, the
process is called uniport
...
1
...
Active transport is carried out by membrane-proteins
...
Pumps are proteins that use energy to carry
substances across the cell membrane
...
Transport rate reaches a maximum when all the protein
transporters are being used or are saturated
...
These
proteins are sensitive to inhibitors that react with protein side chains
...
1
...
1 gives a comparison of the different transport mechanisms
...
But diffusion whether facilitated or not – take place
only along a gradient and do not use energy
...
1 Comparison of Different Transport Mechanisms
Property
Requires special membrane proteins

Simple
Diffusion

Facilitated
Transport

Active
Transport

No

Yes

Yes

Highly selective

No

Yes

Yes

Transport saturates

No

Yes

Yes

Uphill transport

No

No

Yes

Requires ATP energy

No

No

Yes

11
...
It provides the medium in
which most substances are dissolved
...
A watermelon has over 92 per cent water; most
herbaceous plants have only about 10 to 15 per cent of its fresh weight
as dry matter
...
A seed may appear dry but it still has water – otherwise it would
not be alive and respiring!
Terrestrial plants take up huge amount water daily but most of it is
lost to the air through evaporation from the leaves, i
...
, transpiration
...

Because of this high demand for water, it is not surprising that water is
often the limiting factor for plant growth and productivity in both
agricultural and natural environments
...
2
...
Water potential (Ψ
fundamental to understanding water movement
...

Water molecules possess kinetic energy
...
The greater
the concentration of water in a system, the greater is its kinetic energy or
‘water potential’
...
If two systems containing water are in contact, random
movement of water molecules will result in net movement of water
molecules from the system with higher energy to the one with lower energy
...
This process of movement
of substances down a gradient of free energy is called diffusion
...
By convention, the water potential
of pure water at standard temperatures, which is not under any pressure,
is taken to be zero
...
Hence, all solutions have a lower water potential than pure
water; the magnitude of this lowering due to dissolution of a solute is
called solute potential or Ψs
...
The more the
solute molecules, the lower (more negative) is the Ψs
...

If a pressure greater than atmospheric pressure is applied to pure
water or a solution, its water potential increases
...
Can you think of any system
in our body where pressure is built up? Pressure can build up in a plant
system when water enters a plant cell due to diffusion causing a pressure
built up against the cell wall, it makes the cell turgid (see section 11
...
2);

179

180

BIOLOGY

this increases the pressure potential
...
Pressure
potential is denoted as Ψp
...
The relationship between them is as follows:
Ψw = Ψs + Ψp

11
...
2 Osmosis
The plant cell is surrounded by a cell membrane and a cell wall
...
In plants the cells usually contain a large central
vacuole, whose contents, the vacuolar sap, contribute to the solute
potential of the cell
...

Osmosis is the term used to refer specifically to the diffusion of water
across a differentially- or semi-permeable membrane
...
The net direction and rate of
osmosis depends on both the pressure gradient and concentration gradient
...
At
equilibrium the two chambers should have the same water potential
...
If the tuber is placed in water, the cavity in the potato tuber
containing a concentrated solution of sugar collects water due to osmosis
...
3 in which the two chambers, A and B, containing
solutions are separated by a semi-permeable membrane
...
3

TRANSPORT IN PLANTS

Let us discuss another experiment where a
solution of sucrose in water taken in a funnel is
separated from pure water in a beaker through
a semi-permeable membrane (Figure 11
...
You
can get this kind of a membrane in an egg
...
The egg shell dissolves leaving the
membrane intact
...
This will continue till the equilibrium is
reached
...

This pressure required to prevent water from
diffusing is in fact, the osmotic pressure and this
is the function of the solute concentration; more
the solute concentration, greater will be the
pressure required to prevent water from diffusing
in
...
Osmotic pressure is the positive
pressure applied, while osmotic potential is
negative
...
2
...
If the external solution
balances the osmotic pressure of the cytoplasm,
it is said to be isotonic
...
Cells swell in
hypotonic solutions and shrink in hypertonic
ones
...
This occurs when

181

Pressure

Sucrose
solution
Membrane

water
(a)

(b)

Figure 11
...
A
thistle funnel is filled with
sucrose solution and kept
inverted in a beaker containing
water
...
5 Plant cell plasmolysis

the cell (or tissue) is placed in a solution that is hypertonic (has more solutes)
to the protoplasm
...
The water when drawn out of the cell through
diffusion into the extracellular (outside cell) fluid causes the protoplast to
shrink away from the walls
...
The movement
of water occurred across the membrane moving from an area of high water
potential (i
...
, the cell) to an area of lower water potential outside the cell
(Figure 11
...

What occupies the space between the cell wall and the shrunken
protoplast in the plasmolysed cell?
When the cell (or tissue) is placed in an isotonic solution, there is no
net flow of water towards the inside or outside
...

When water flows into the cell and out of the cell and are in equilibrium,
the cells are said to be flaccid
...
When the cells are
placed in a hypotonic solution (higher water potential or dilute solution
as compared to the cytoplasm), water diffuses into the cell causing the
cytoplasm to build up a pressure against the wall, that is called turgor
pressure
...
Because of the
rigidity of the cell wall, the cell does not rupture
...

What would be the Ψp of a flaccid cell? Which organisms other than
plants possess cell wall ?

11
...
4 Imbibition
Imbibition is a special type of diffusion when water is absorbed by solids
– colloids – causing them to enormously increase in volume
...

The pressure that is produced by the swelling of wood had been used by
prehistoric man to split rocks and boulders
...
Water potential gradient between
the absorbent and the liquid imbibed is essential for imbibition
...


11
...
On examining the cut end of the twig after a few
hours you had noted the region through which the coloured water moved
...
Now we
have to go further and try and understand the mechanism of movement
of water and other substances up a plant
...
Diffusion is a slow process
...
For example, the movement of a molecule
across a typical plant cell (about 50 µm) takes approximately 2
...
At this
rate, can you calculate how many years it would take for the movement
of molecules over a distance of 1 m within a plant by diffusion alone?
In large and complex organisms, often substances have to be moved
across very large distances
...
Special long distance transport systems
become necessary so as to move substances across long distances and at
a much faster rate
...
Mass flow is the movement of substances in
bulk or en masse from one point to another as a result of pressure
differences between the two points
...
This is unlike diffusion where different
substances move independently depending on their concentration
gradients
...
g
...
g
...


184

BIOLOGY

The bulk movement of substances through the conducting or vascular
tissues of plants is called translocation
...
Xylem is
associated with translocation of mainly water, mineral salts, some organic
nitrogen and hormones, from roots to the aerial parts of the plants
...


11
...
1 How do Plants Absorb Water?
We know that the roots absorb most of the water that goes into plants;
obviously that is why we apply water to the soil and not on the leaves
...
Root hairs are thin-walled slender extensions of root epidermal
cells that greatly increase the surface area for absorption
...

Once water is absorbed by the root hairs, it can move deeper into root
layers by two distinct pathways:

• apoplast pathway
• symplast pathway
The apoplast is the system of adjacent cell walls that is continuous
throughout the plant, except at the casparian strips of the endodermis
in the roots (Figure 11
...
The apoplastic movement of water occurs
exclusively through the intercellular spaces and the walls of the cells
...
6 Pathway of water movement in the root

TRANSPORT IN PLANTS

185

membrane
...
The apoplast
does not provide any barrier to water movement and water movement is
through mass flow
...

The symplastic system is the system of interconnected protoplasts
...
During symplastic movement, the water
travels through the cells – their cytoplasm; intercellular movement is
through the plasmodesmata
...
Movement is again
down a potential gradient
...
You may have observed cytoplasmic streaming
in cells of the Hydrilla leaf; the movement of chloroplast due to streaming
is easily visible
...
However, the inner boundary of the cortex, the endodermis,
is impervious to water because of a band of suberised matrix called the
casparian strip
...
The water then moves through the
symplast and again crosses a membrane to reach the cells of the xylem
...
This is the only
way water and other solutes can
Endodermis
Xylem
enter the vascular cylinder
...
In young
roots, water enters directly into the
xylem vessels and/or tracheids
...
The
path of water and mineral ions into
the root vascular system is
summarised in Figure 11
...

Phloem
Casparian
Some plants have additional
Apoplastic
strip
Cortex
structures associated with them
path
Pericycle
that help in water (and mineral)
absorption
...
7 Symplastic and apoplastic pathways of
water and ion absorption and movement in
symbiotic association of a fungus
roots
with a root system
...
The hyphae have a very large surface area that absorb mineral
ions and water from the soil from a much larger volume of soil that perhaps
a root cannot do
...
Some plants have an obligate association with the
mycorrhizae
...


11
...
2 Water Movement up a Plant
We looked at how plants absorb water from the soil, and move it into the
vascular tissues
...
Is the water movement active, or
is it still passive? Since the water has to be moved up a stem against
gravity, what provides the energy for this?
11
...
2
...
This positive pressure is called root
pressure, and can be responsible for pushing up water to small heights
in the stem
...
You will soon see drops of solution ooze out of the
cut stem; this comes out due to the positive root pressure
...
Effects of root pressure is also observable at night and early
morning when evaporation is low, and excess water collects in the form of
droplets around special openings of veins near the tip of grass blades,
and leaves of many herbaceous parts
...

Root pressure can, at best, only provide a modest push in the overall
process of water transport
...
The greatest contribution of root pressure
may be to re-establish the continuous chains of water molecules in the
xylem which often break under the enormous tensions created by
transpiration
...

11
...
2
...
How is this movement accomplished? A long
standing question is, whether water is ‘pushed’ or ‘pulled’ through the
plant
...
This is referred to as the cohesion-tension-transpiration pull
model of water transport
...
Less than 1 per cent of the water reaching
the leaves is used in photosynthesis and plant growth
...
This water loss is known as
transpiration
...
You could also study water loss from a leaf using cobalt chloride
paper, which turns colour on absorbing water
...
4 TRANSPIRATION
Transpiration is the evaporative loss of water by plants
...
Besides the loss of water vapour in
transpiration, exchange of oxygen and carbon dioxide in the leaf also
occurs through pores called stomata (sing
...
Normally stomata
are open in the day time and close during the night
...
The inner wall of each guard cell, towards the pore or stomatal
aperture, is thick and elastic
...
The opening of
the stoma is also aided due to the orientation of the microfibrils in the cell
walls of the guard cells
...
When the
guard cells lose turgor, due to water loss (or water stress) the elastic inner
walls regain their original shape, the guard cells become flaccid and the
stoma closes
...

Transpiration is affected by several
external factors: temperature, light,
humidity, wind speed
...
8 A stomatal aperture with guard cells

188

BIOLOGY

stomata open, per cent, water status of the plant, canopy structure etc
...

• Adhesion – attraction of water molecules to polar surfaces (such
as the surface of tracheary elements)
...

These properties give water high tensile strength, i
...
, an ability to
resist a pulling force, and high capillarity, i
...
, the ability to rise in thin
tubes
...

The process of photosynthesis requires water
...
But
what force does a plant use to move water molecules into the leaf
parenchyma cells where they are needed? As water evaporates through
the stomata, since the thin film of water over the cells is continuous, it
results in pulling of water, molecule by molecule, into the leaf from the
xylem
...
This creates a ‘pull’
(Figure 11
...

Measurements reveal that the forces generated by transpiration can
create pressures sufficient to lift a xylem sized column of water over 130
metres high
...
9 Water movement in the leaf
...
The gradient is transmitted into the photosynthetic cells and on
the water-filled xylem in the leaf vein
...
4
...

Photosynthesis is limited by available water which can be swiftly depleted
by transpiration
...

The evolution of the C4 photosynthetic system is probably one of the
strategies for maximising the availability of CO2 while minimising water
loss
...
However, a C4 plant loses only half as much water as a C3
plant for the same amount of CO2 fixed
...
5 UPTAKE

AND

TRANSPORT

OF

MINERAL NUTRIENTS

Plants obtain their carbon and most of their oxygen from CO2 in the
atmosphere
...


11
...
1 Uptake of Mineral Ions
Unlike water, all minerals cannot be passively absorbed by the roots
...
Therefore, most minerals must enter the root by
active absorption into the cytoplasm of epidermal cells
...
The active uptake of ions is partly responsible for the
water potential gradient in roots, and therefore for the uptake of water by
osmosis
...

Ions are absorbed from the soil by both passive and active transport
...
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
...
Note
that the root endodermis because of the layer of suberin has the ability to
actively transport ions in one direction only
...
5
...

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
...

Mineral ions are frequently remobilised, particularly from older,
senescing parts
...
Similarly, before leaf fall in decidous plants, minerals
are removed to other parts
...
Some elements that are
structural components like calcium are not remobilised
...
Similarly, small amounts
of P and S are carried as organic compounds
...

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
...
6 PHLOEM TRANSPORT: FLOW

FROM

SOURCE

TO

SINK

Food, primarily sucrose, is transported by the vascular tissue phloem
from a source to a sink
...
e
...
But, the source and sink may be
reversed depending on the season, or the plant’s needs
...
Since the source-sink
relationship is variable, the direction of movement in the phloem can
be upwards or downwards, i
...
, bi-directional
...
e
...
Hence, unlike one-way flow of water in transpiration, food
in phloem sap can be transported in any required direction so long
as there is a source of sugar and a sink able to use, store or remove
the sugar
...


TRANSPORT IN PLANTS

191

11
...
1 The Pressure Flow or Mass Flow Hypothesis
The accepted mechanism used for the translocation of sugars from source
to sink is called the pressure flow hypothesis
...
10)
...
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
...
Water in the adjacent xylem moves
into the phloem by osmosis
...
At the sink osmotic pressure
must be reduced
...
As sugars are removed, the
osmotic pressure decreases and water moves out of the phloem
...

Loading of the phloem sets up a water potential gradient that facilitates
the mass movement in the phloem
...
Cytoplasmic strands pass
through the holes in the sieve plates, so forming continuous filaments
...
Meanwhile, at the sink,
incoming sugars are actively transported out of the phloem and removed
Sugars leave sieve tubes;
water follows by osmosis

Tip of stem

Sugar solution flows
to regions of low
turgor pressure

Sugars enter sieve tubes;
water follows by osmosis
Phloem

=High
turgor
pressure

Sugars leave sieve tube
for metabolism and
storage; water follows
by osmosis

Root
Figure11
...
The loss of solute produces a high water
potential in the phloem, and water passes out, returning eventually to xylem
...
On the trunk of a tree a ring of bark
up to a depth of the phloem layer, can be carefully removed
...
This simple experiment
shows that phloem is the tissue responsible for translocation of food; and
that transport takes place in one direction, i
...
, towards the roots
...


SUMMARY
Plants obtain a variety of inorganic elements (ions) and salts from their
surroundings especially from water and soil
...
Transport across cell
membrane can be through diffusion, facilitated transport or active transport
...

Passive transport (diffusion, osmosis) and active transport are the two modes
of nutrient transport across cell membranes in living organisms
...

This diffusion of substances depends on their size, solubility in water or organic
solvents
...
In
active transport, energy in the form of ATP is utilised to pump molecules against
a concentration gradient across membranes
...
It is determined by solute potential
and pressure potential
...
If the surrounding solution of the cell is hypertonic, it gets plasmolysed
...

In higher plants, there is a vascular system, xylem and phloem, responsible
for translocation
...
They are therefore, transported by a mass flow system –
movement of substance in bulk from one point to another as a result of pressure
differences between the two points
...
e
...
Various ions, and water from soil can be
transported upto a small height in stems by root pressure
...
Transpiration is

TRANSPORT IN PLANTS

193

the loss of water in the form of vapours from the plant parts through stomata
...
Excess water is also removed through tips of leaves of plants by
guttation
...
The translocation in phloem is bi-directional; the source-sink
relationship is variable
...


EXERCISES
1
...
What are porins? What role do they play in diffusion?
3
...

4
...

5
...

Guttation and Transpiration
...
Briefly describe water potential
...
What happens when a pressure greater than the atmospheric pressure is applied
to pure water or a solution?
8
...

(b) Explain what will happen to a plant cell if it is kept in a solution having
higher water potential
...
How is the mycorrhizal association helpful in absorption of water and minerals
in plants?
10
...
Describe transpiration pull model of water transport in plants
...
Discuss the factors responsible for ascent of xylem sap in plants
...
What essential role does the root endodermis play during mineral absorption in
plants?
14
...

15
...

16

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