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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
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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
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It is
sensitive to inhibitors which react with
protein side chains
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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
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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

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BIOLOGY

molecule moves across a membrane independent of other molecules, the
process is called uniport
...
1
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Active transport is carried out by membrane-proteins
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Pumps are proteins that use energy to carry
substances across the cell membrane
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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
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But diffusion whether facilitated or not – take place
only along a gradient and do not use energy
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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
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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
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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’
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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
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
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Less than 1 per cent of the water reaching
the leaves is used in photosynthesis and plant growth
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This water loss is known as
transpiration
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You could also study water loss from a leaf using cobalt chloride
paper, which turns colour on absorbing water
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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
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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
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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

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

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