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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
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A
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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
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1 Means of
Transport
11
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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
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Over small distances substances move by diffusion
and by cytoplasmic streaming supplemented by active transport
...
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
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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
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The net direction and rate of
osmosis depends on both the pressure gradient and concentration gradient
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At
equilibrium the two chambers should have the same water potential
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If the tuber is placed in water, the cavity in the potato tuber
containing a concentrated solution of sugar collects water due to osmosis
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3 in which the two chambers, A and B, containing
solutions are separated by a semi-permeable membrane
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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
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The egg shell dissolves leaving the
membrane intact
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This will continue till the equilibrium is
reached
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The symplastic system is the system of interconnected protoplasts
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During symplastic movement, the water
travels through the cells – their cytoplasm; intercellular movement is
through the plasmodesmata
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Movement is again
down a potential gradient
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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
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The water then moves through the
symplast and again crosses a membrane to reach the cells of the xylem
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This is the only
way water and other solutes can
Endodermis
Xylem
enter the vascular cylinder
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In young
roots, water enters directly into the
xylem vessels and/or tracheids
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The
path of water and mineral ions into
the root vascular system is
summarised in Figure 11
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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
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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
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11
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2 Water Movement up a Plant
We looked at how plants absorb water from the soil, and move it into the
vascular tissues
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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
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2
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This positive pressure is called root
pressure, and can be responsible for pushing up water to small heights
in the stem
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You will soon see drops of solution ooze out of the
cut stem; this comes out due to the positive root pressure
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Effects of root pressure is also observable at night and early
morning when evaporation is low, and excess water collects

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