Plant Physiology
Retno Mastuti
Department of Biology
Faculty of Mathematics and Natural Sciences
University of Brawijaya
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CONTENTS
Water and Solutes in Plants
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Plant Cells - Water Relations
Objective of General Instruction........................
Objective of Specific Instruction........................
The water in the plant body......................
Polar and hydrogen bonding properties of
water..................................................................
Properties of water in plant physiology...............
A. The thermal properties of water…………
B. Cohesion, adhesion and surface tension
supporting water capillarity………………
C. Water in biochemical reactions………….
Cell water potential…………………………………
Water transport .................................................
A. Diffusion ……………………………………
B. Bulk flow / mass flow……………………..
C. Osmosis…………………………………….
Summary…………………………………………….
Quis………………………………………………….
Refferences………………………………………….
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Chapter 1
Plant Cell – Water Relations
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Objective of General Instruction
Students are expected to know and be able to explain the
properties and functions of water for the plant.
Students are able to understand and explain the concept of
the movement of water that affects various physiological
processes in plants.
Objective of Specific Instruction
Students are expected to understand and be able to explain
the nature and function of water as an essential component in the
life which is necessary to understand all of the physiological
processes in plants.
Students are able to understand and explain the mass flow,
diffusion and osmosis. Understanding of the movement of water
will facilitate the understanding of the process of mineral
nutrients absorption from the soil, transpiration and
translocation.
The water in the plant body
Water is essential component for all living things including
plants. Why do plants need water? Water maintains turgor
pressure of plant cells. Water facilitates the transport of nutrients
absorbed by the roots from the soil to be distributed to all parts
of the plant. Water is also a major component in the process of
photosynthesis to produce carbohydrates that determine the
productivity of agricultural crops. On the other hand, the water
is also involved in the regulation of temperature in the plants
body through the process of transpiration. In this condition
water contributes in cooling system of plant body.
Some plants have a water content reaches about 90-95 % (Taiz
and Zeiger, 2013). When the water content is very low, for
example in seeds caused them to enter a dormancy phase in
which the metabolic processes occur very slow or even stopped.
High water content in plant cells shows that water has very
important role in the biochemical reactions that occur in the
plant body. Plant physiology is the study of plant processes and
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functions at the level of cell, tissue, organ or individual plants.
Therefore, in studying the physiology of the plant and for
understanding plant function it should begin with an
understanding of the properties of water that support metabolic
processes in plant cells.
Polar and hydrogen bonding properties of water
Water, H2O, is a molecule composed of one oxygen atom with
two hydrogen atoms. Position of the H atoms in water molecules
forms an angle of 104.45  ºC with O atoms to form strong
polar covalent bonds (Figure 1). Oxygen has 8 protons, while
hydrogen only has 1 proton. Electron in water molecule is not
evenly distributed between the negatively charged (-) oxygen
and the positively charged (+) hydrogen atom. This means that
oxygen has higher electron affinity (more electronegative) to
attract electrons than hydrogen. As a result, water is a polar
molecule. Polarity of the water molecules creates all the unique
properties of water such as surface tension, freezing point, and
solubility.
Figure 1. Polar covalent bond to water molecules (Lodish et al.,
2007)
In addition covalent bonds, liquid water also has hydrogen
bonds. Hydrogen bonding occurs when an atom of hydrogen is
attracted by rather strong forces to two atoms instead of only
one (Pauling, 1948).
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A hydrogen bond in water molecules is the attractive forces
between polar molecules in which the hydrogen (H) bound to
the high electronegative atom, oxygen (O) (Figure 2). Hydrogen
bonds can occur between molecules (intermolecular) or in a
different part of the molecule (intramolecular). Hydrogen bonds
between water moleculs result cohesion while hydrogen bonds
between water molecules and other molecules such as cellulose
result adhesion. Intermolecular hydrogen bonds lead to the high
boiling point of water (100 ° C).
Figure 2.
Model of the hydrogen bonds between water
molecules (Hayley Biology, 2013)
Properties of water in plant physiology
As mentioned in the former that many biochemical processes in
plants involve water which is supported by its specific
properties. The shape of water molecules, its polarity, and
hydrogen/covalent bonding make them as unique molecules.
Some properties of water are relatively high cohesive and
adhesive forces, high latent heat of vaporization, high heat
capacity and incompressible.
A. The thermal properties of water
Water is liquid at physiological temperature (i.e between 0-100
ºC). It means that water involves in physiological processes only
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at liquid phase. In other words, in any place life can only occur
between a temperature of 0 and 100 ºC. Temperatures below 0
°C will inhibit a significant chemical metabolism, while
temperatures above 100 ºC tend to damage / break the chemical
bonds.
Water has a high heat of vaporization. Large quantities of
energy (about 44 kJ mol-1) is required to change water from
liquid phase to gas phase at constant temperature. Those high
energy is required to break hydrogen bonds between water
molecules. When large quantities of heat from radiant of the sun
captured by the leaf surfaces are used to form gas phase
consequently the plant temperature will decrease. These
properties are responsible for the use of water as an evaporative
cooling system. This may explain why the leaves transpiration
has an important role in plants temperature regulation. In is
noted that the water in liquid phase seemed to 'postpone'
evaporation.
Water has a high specific heat (heat capacity). Heat capacity is
the capacity of water to raise the temperature of a substance.
Heat capacity shows the capability of substance to absorb heat
energy. While specific heat is the specific amount of heat in
calories needed to raise the temperature of 1 gram of water by 1
degree celsius. The raise of temperature is required to break the
hudrogen bonds between water molecules. Water has a quite
large specific heat. This means that liquid water can absorb a
relatively large amount of heat before boiling and evaporating.
A lot of energy (4.184 J g-1 C-1, or the unit non - SI is the
calories that 1 cal = 4.184 J) is required to raise the temperature
of water to break the hydrogen bonds.
Both of high specific heat capacity and high heat of vaporization
of water result of the strong hydrogen bonds between the
molecules. Thus, water is slow to heat up and cool down, or in
other words the water 'slow' changes in temperature. This trait is
important in the role of water as a thermal buffer. So it is not
surprising that the desert plant is a succulent that can tolerate
against temperature fluctuations.
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B. Cohesion, adhesion and surface tension supporting water
capillarity
Large amounts of energy are needed to break through the
surface of the water, because the water molecules at the surface
are more strongly attracted by other water molecules in the
liquid (cohesion) more than attracted by the water molecules in
the air. Cohesion cause the water has high tensile strength
known as the maximum force per unit area. Water has a very
high surface tension (Figure 3) – the energy required to
increase the surface area. Due to the high tension of water the
drops of water are spheric (molecules tend to stick together)
(Figure 4) and the existence of meniscus shape at leaf surface
during transpiration processes.
When water is transported through the xylem there is an
attraction between water molecules and solid phase of cell walls
known as adhesion. The surface tension together with cohesion
and adhesion can move up the water through the xylem vessels.
Capillary action refers to the tendency of water to move up a
narrow tube against the force of gravity (Figure 3, 4). This trait
is very important for growth of all vascular plants, such as trees.
Figure 3. Surface tension and capillary action of water
(Hyperphysic, 2013)
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Figure 4. The surface tension of water that causes the water to
form drop (Anonym, 2013)
If the water is loaded into the tube and at both ends put piston
then at the certain time the piston will not be able to push
together. This shows that the water is a good hydraulic system
because when pressed does not compress and generate positive
pressure (hydrostatic pressure). This pressure provides the
driving force for cell growth and movement in plants. Pressure
is measured in Pascals (or actually megapascals, MPa). One
MPa is roughly equal to ten atmospheres or 10 bar. Conversely,
when the water resists the pull it gives negative hydrostatic
pressure.
C. Water in biochemical reactions
Water is the universal solvent. Water is a good polar solvent and
is often referred as the universal solvent. Water dissolves more
different kinds of molecules than other solvents. Substances that
dissolve in water, e.g, salts, sugars, acids, alkalis, and some
gases - especially oxygen, carbon dioxide (carbonation) are
known as hydrophilic substances (like water), while those who
do not mix with water (eg, fats and oils ), known as
hydrophobic substance (not like water). Most of the major
components in cells (proteins, DNA and polysaccharides) are
also in the aquatic environment.
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Water transparent to light. Water is transparent in the visible
electromagnetic spectrum. This is important because the
chloroplasts in the cell obviously surrounded by transparent
water so that photosynthesis process can occur. From an
ecological perspective, light penetration in the depth of water
determine the distribution of aquatic plants. Aquatic plants can
live in water because sunlight can reach them.
Water is inert / chemically inert. This means that the water does
not react unless the water reacts enzymatically.
Water dissociates into protons and hydroxide ions. This
character is important to determine the system pH. Biological
systems are very sensitive to pH. Most of the biologically active
system requires buffer. Water can ionize to form a hydrogen ion
(proton / H+) and hydroxide ions (OH-). Acidic compounds act
as a proton donor, potentially increasing [H+]. For example HCl
 H+ + Cl-.
In contrast, an alkaline compound which acts as a proton
acceptor, potentially increasing [OH-]. This compound has the
potential to reduce the concentration of protons.
Contoh : NaOH  Na+ + OH- (if accept proton will form wate)
NH3 (ammonia ) + H  NH4 + (ion ammonium)
The pH scale ranges from 0 to 14. Water is neutral with a pH of
7. Compounds having acidic pH range of 0 to 7, while
approaching the basic compound having a pH range of 7-14.
Enzyme reactivity is very sensitive to pH and buffer solutions
needed to function.
Water affects the shape, stability and properties of biological
molecules. For example, many ions (such as sodium) and
molecules (such as DNA and wall components) are usually
hydrated. This means that water is hydrogen bonded to them and
in some cases (e.g, sodium) form a hydration shell around it.
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Based on some properties above the function of water is as
follows: the main component of the cell, solvent for absorption
and transport of minerals, a good medium for chemical
reactions, reactants in some biochemical reactions
(photosynthesis), support structure through turgor pressures,
gamete transfer medium for plants and spread of propagules
(coconut). The water also serves in the 'movement' of plants due
to the movement of water in and out at certain parts (the opening
of the stomata, flowers blooming and the diurnal movement),
elongation and cell growth and thermal buffer.
Cell water potential
Free energy of water is an inevitable topic when we discuss
about water potential. Thermodinamycally, free energy is
defined as potential for doing works. Water potential is usefull
to know the energy status of water to do work and the direction
of water movement. Firstly, we should know that the chemical
potential of water is quantitative measurement of water free
energy. Water potential is chemical water potential of water
divided by the partial volume of water. In other words, water
potential is a measure of water free energy per unit volume (J m3
in MPa) causing water movement in plants. With its potential
the water move from high to low water potential (Figure 5).
Figure 5. The important concept of water potential
Water potential is a measure of water energy compare to free
energy of pure water. The water potential of pure water is zero.
There are four major components affecting plant water potential
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(), namely solute/osmotic (s), pressure (p), matrix (m)
and gravity (g) as the equation below:
 = + + +
w

s
p
m
g
= solute or osmotic potential is a measure of solute
potential in solution which reduce free water energy;
Solutes decrease of water free energy then the free energy
of water potential in plants usually less than that of pure
water. Therefore, solut potential values are always
negative. This is important for measuring water
movement into and out of cells such as guard cells
function and nutrient uptake.
 = pressure potential is positive (due to turgor pressure) or
negative (during transpiration) hydrostatic pressure in the
system. Pressure potential is usually positive in living
cells. While in death cells like xylem pressure potential is
negative because tension which develop water column is
more dominant.
 = matrix potential is reduction of water energy due to the
attraction of water molecules to surfaces such as cell
walls, soil particles surfaces or oher particles in the
system/water; it always negative. This potential based on
tendency of water to adhere to the surfaces. This potential
is usually ignored because the effect of the surface
isnteraction is too small to change the water potential in
the system. In saturated soil, water free to flow,and Ψm is
not a factor and value is 0. Conversely, in unsaturated soil
matrix potenial results from capillary water and adhesion
force. The more negative matrix potential the more
difficult to remove water from the surface. Consequently,
plants have difficulties to extract water from the soil.
 = gravity potential is a measure of water energy which
depends on the height of water. This potential component
is frequently ommited related to water transport at the cell
level.
s
p
m
g
Therefore, the preview equation can be simplified as follows:
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 = +
w
s
p
Water potential can change by changes in pressure potential or
solute potential. Water potential gradient between soil and root
hair cells is important to move the water into the cells.
Water transport
To fulfill the water needs of plants the water must flow from soil
at the out side into plant cells and finally released out to
atmosphere through transpiration processes involving stomata.
In plant body water movement occurs in two ways due to the
differences of concentration and pressure. Water movement in
plants is driven by three processes: diffusion, mass flow and
osmosis.
A. Diffusion
Diffusion is the random movement of individual molecules
caused by the difference in concentration. Diffusion of
molecules will move from high concentration to the low
concentration. Dye dripped into the water in the area has a high
concentration of the first droplet (Figure 6). Gradually, the dye
will spread moving toward areas that do not contain dye. The
movement will stop when the dye concentration is the same in
all areas so that the water is initially colorless changes into
corresponding colored dye dripped.
Figure 6. Diffusion process (University of California, 2013)
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Fick's law states the relationship between the rate of diffusion
of the gradient concentration (C1-C2) and the resistance /
resistance (r). The rate of diffusion is also called flux density
(with units J mol m-2s-1) and is expressed by the following
formula:
Jv =
(C1 - C2)
r
Based on the formula, there are three things that need to be
observed, namely:
1. Diffusion rate is directly proportional to the concentration
gradient. The greater the difference in concentration
between two areas the greater the rate of diffusion. When
the gradient reaches 0 indicates that diffusion does not
occure anymore.
2. The rate of diffusion is inversely proportional to the
resistance. The bigger the barriers the lower the diffusion
rate. Therefore the barrier is anything that reduces the rate of
diffusion. Membrane is inhibiting the movement of the
charged ions and compounds both inside and outside the
cell.
3. Diffusion rate is also inversely proportional to the distance
of displacement or movement as well as the function of the
barriers. In other word, the longer the distance the slower the
water diffusion.
Molecular speed. The rate of diffusion is also determined by
speed of moving molecules. It will be a) directly proportional
to temperature and b) inversely proportional to the molecular
weight (heavy particles move slower than light particles).
Temperature. Temperature will increase molecular speed. So,
increased movement consequently will increase rate of diffusion
(Gambar 7).
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Gambar 7. The effect of temperature on the rate of diffusion
(Chaplin, 2013)
Pressure. Like temperature, the higher the pressure the higher
the molecular speed which consequently increase the rate of
diffusion.
Effect of solute (dissolved substance) on the chemical potential
of solvent. Particles of solut will decrease free energy of solvent.
In this case, the numbers of particles have more influent than
charge of particles.
B. Bulk flow / Mass flow
Mass flow is the movement of the molecular mass (water and
solute) due to the pressure difference from the high pressure
area toward a low pressure area. Bulk flow of water molecules
facilitates the farthest plant parts from soil for obtaining water.
C. Osmosis
Osmosis is diffusion of solvent especially water through
semipermeable membran from high to low concentration.
Movement of water by osmosis is passive beacuse the water
moves spontaneously. It does not require energy because this
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movement is combination between diffusion (from high
concentration to low concentration) and mass flow (from high
pressure to low pressure) (Fig. 8).
Figure 8. Osmosis process
Recent studies show that water movement into the plants cells
not only involves phospholipid bilayers through diffusion but
also involves aquaporines membrane integral proteins which
develop selective pore through bulk flow.
Figure 9. Aquaporine facilitating bulk flow of water (Taiz and
Zeiger, 2013)
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Summary
Chemical and physical properties of water support
continuation of the physiological processes in plant. Therefore
understanding the properties of water are very important for
studying other physiological processes in plants.
The movement of water is due to the difference in pressure
and or difference in concentration. Direction of movement is
from high concentration / high pressure to low concentration /
pressure. Transfer speed is affected by the pressure, the speed of
the molecules, and solute temperature in the system.
Quis
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Why is water so important to living things including plants?
How the physical form of water at physiological
temperature? Explain it.
What are the advantages of high evaporation in the heat
properties of water for the plant?
What is the relationship between the surface tension of
water itnggi owned by physiological processes in plants?
What are the advantages of water as the universal solvent?
Describe the nature of the transparency of water to light for
plant survival.
Why the water dissociation process is very important for the
process plant physiology?
Explain what is mean by the polar nature of water.
Explain the functions of water for the survival of plants
based on the properties of water.
What are two ways of water movement in plants. What is
the difference between both of them?
Explain how Fick's law describes the rate of diffusion of the
gradient concentration.
What factors affect the rate of diffusion? Explain it.
Osmometer which has selective membrane containing
solution is placed into the beaker containing pure water.
Explain how the process of water diffusion. If the pressure
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is applied to the osmometer then how the possibility of its
water potential?
14. What is osmosis?
Refferences
Anonym. 2013. Nature wallpapers and desktop background.
http://wallpaperdreams.com/background/114/water-dropletcluster-on-leaf.html
Hayley
Biology.
2013.
Chemistry
of
Life.
http://hayleybiology.weebly.com/chemistry-of-life.html)
Hyperphysic. 2013.
Surface tension and bubles.
http://hyperphysics.phy-astr.gsu.edu/hbase/surten2.html
Harvey Lodish , Arnold Berk , Chris A. Kaiser , Monty
Krieger , Matthew P. Scott , Anthony Bretscher , Hidde
Ploegh , Paul Matsudaira, 2008. Molecular Cell Biology.
Ed. 6th. W.H. Freeman Publ. Los Angeles.
M. Chaplin. 2013. Water Structure and Science.
http://www1.lsbu.ac.uk/water/explan5.html
Pauling, L. 1948. The Nature of the Chemical Bond, 2nd ed.
Cornell University Press, New York.
Climate Science Investigation (CSI), 2013. Temperature
Overtime. http://www.ces.fau.edu/nasa/module-3/.
University of California. 2013. The Science of Solar.
http://solarwiki.ucdavis.edu/The_Science_of_Solar/Solar_B
asics/C._Semiconductors_and_Solar_Interactions/II._Condu
ction_in_Semiconductors/5._Diffusion_Current
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