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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

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