TRANSPORT IN PLANTS
Transport in plants notes class 11
INTRODUCTION
·
Water taken up by the roots has to reach all
parts of the plant, up to the very tip of the growing stem.
·
The photosynthates or food synthesized by the
leaves have also to be moved to all parts including the root tips embedded deep
inside the soil.
·
Movement across short distances, say within the
cell, across the membranes and from cell to cell within the tissue has also to
take place.
·
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 be
anatomy of the plant body.
·
When any plant part undergoes senescence,
nutrients may be withdrawn from such regions and moved to the growing parts.
·
Hormones or plant growth regulators and other
chemical signals are also transported, though in very small amounts, sometimes
in a strictly polarised or unidirectional manner from where they are synthesised
to other parts.
Transportation in plants notes
Transport in plants notes class 11
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.
·
No energy expenditure takes place. 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 rates are affected by the gradient of
concentration, the permeability of the membrane separating them, temperature
and pressure.
Facilitated
Diffusion
· Substances soluble in lipids diffuse through
the membrane faster. Substances that have a hydrophilic moiety, find it
difficult to pass through the membrane; their movement has to be facilitated.
·
They do not set up a concentration gradient: a
concentration gradient must already be present for molecules to diffuse even if
facilitated by the proteins. 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. Transport rate reaches a maximum when all of the protein transporters
are being used (saturation).
· The proteins form channels in the membrane for
molecules to pass through. Some channels are always open; others can be
controlled. Some are large, allowing a variety of molecules to cross.
· The porins are proteins that form large pores
in the outer membranes of the plastids, mitochondria and some bacteria allowing
molecules up to the size of small proteins to pass through.
· The transport protein then rotates and releases
the molecule inside the cell, e.g., water channels – made up of eight different
types of aquaporins.
Passive
symports and antiports
·
In a symport,
both molecules cross the membrane in the same direction; in an antiport, they move in opposite
directions.
· When a molecule moves across a membrane
independent of other molecules, the process is called uniport.
Transportation in plants notes
Transport in plants notes class 11
Active
Transport
·
Active transport uses energy to transport and
pump molecules against a concentration gradient. Active transport is carried
out by specific membrane-proteins.
·
Hence different proteins in the membrane play a
major role in both active as well as passive transport.
·
Pumps are proteins that use energy to carry
substances across the cell membrane. These pumps can transport substances from
a low concentration to a high concentration (‘uphill’ transport).
·
These proteins are sensitive to inhibitors that
react with protein side chains.
· Terrestrial plants take up huge amount water
daily but most of it is lost to the air through evaporation from the leaves,
i.e., transpiration.
COMPARISON OF DIFFERENT TRANSPORT PROCESSES
RELATED POSTS:
·
PLANT GROWTH AND DEVELOPEMENT
Water Potential
· Water potential (Ψw ) is a concept fundamental
to understanding water movement. Solute
potential (Ψs ) and pressure potential (Ψp ) are the two main components
that determine water potential.
·
This process of movement of substances down a
gradient of free energy is called diffusion. Water potential is denoted by the
Greek symbol Psi or Ψ and is expressed in pressure units such as pascals (Pa).
·
If some solute is dissolved in pure water, the
solution has fewer free water molecules and the concentration (free energy) of
water decreases, reducing its water potential.
· 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 . Ψs is always negative.
· The more the solute molecules, the lower (more
negative) is the Ψs .For a solution at atmospheric pressure (water potential)
Ψw = (solute potential) Ψs .
· When water enters a plant cell due to diffusion
causing a pressure built up against the cell wall, it makes the cell turgid this
increases the pressure potential.
· Pressure potential is denoted as Ψp . Water
potential of a cell is affected by both solute and pressure potential. The
relationship between them is as follows:
Ψw = Ψs + Ψp
Osmosis
·
Osmosis is the term used to refer specifically
to the diffusion of water across a differentially- or selectively permeable
membrane.
·
Osmosis occurs spontaneously in response to a
driving force. The net direction and rate of osmosis depends on both the
pressure gradient and concentration gradient.
·
Water will move from its region of higher
chemical potential (or concentration) to its region of lower chemical potential
until equilibrium is reached.
Transport in plants Class 11 notes for NEET PDF
Plasmolysis
·
The behavior of the plant cells (or tissues)
with regard to water movement depends on the surrounding solution. If the
external solution balances the osmotic pressure of the cytoplasm, it is said to
be isotonic.
·
If the
external solution is more dilute than the cytoplasm, it is hypotonic and if the external solution is more concentrated, it is hypertonic. Cells swell in hypotonic
solutions and shrink in hypertonic ones.
·
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 cell is said to be plasmolysed.
·
The movement of water occurred across the
membrane moving from an area of high water potential (i.e., the cell) to an
area of lower water potential outside the 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.
·
The process of plasmolysis is usually
reversible. 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.
·
The pressure exerted by the protoplasts due to
entry of water against the rigid walls is called pressure potential Ψ
Imbibition
·
Imbibition is a special type of diffusion when
water is absorbed by solids-colloids-causing them to increase in volume.
·
Imbibition is also diffusion since water
movement is along a concentration gradient; the seeds and other such materials
have almost no water hence they absorb water easily
·
Examples of imbibition are absorption of water
by seeds and dry wood.
LONG
DISTANCE TRANSPORT OF WATER
·
Special long distance transport systems become necessary
so as to move substances across long distances and at a much faster rate. Water
and minerals, and food are generally moved by a mass or bulk flow system.
·
The bulk movement of substances through the
conducting or vascular tissues of plants is called translocation.
How
do Plants Absorb Water?
·
Root hairs are thin-walled slender extensions
of root epidermal cells that greatly increase the surface area for absorption.
·
Water is absorbed along with mineral solutes,
by the root hairs, purely by diffusion. Once water is absorbed by the root
hairs, it can move deeper into root layers by two distinct pathways:
(i)
apoplast
pathway
(ii)
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.
·
This movement is dependent on the gradient. 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. Neighbouring cells are
connected through cytoplasmic strands that extend through plasmodesmata.
·
During symplastic movement, the water travels
through the cells – their cytoplasm; intercellular movement is through the
plasmodesmata. Water has to enter the cells through the cell membrane, hence
the movement is relatively slower.
·
However, the inner boundary of the cortex, the
endodermis, is impervious to water because of a band of suberised matrix called
the casparian strip.
·
A mycorrhiza is a symbiotic association of a fungus 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. For example, Pinus seeds cannot germinate and establish
without the presence of mycorrhizae.
ALSO CHECK:
Transport in plants notes class 11
Root
Pressure
·
As various ions from the soil are actively
transported into the vascular tissues of the roots, water follows (its
potential gradient) and increases the pressure inside the xylem. This positive
pressure is called root pressure, and can be responsible for pushing up water
to small heights in the stem.
·
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. Such water loss in its liquid
phase is known as guttation.
Transpiration
pull
·
Whether water is ‘pushed’ or ‘pulled’ through the
plant. Most researchers agree that water is mainly ‘pulled’ through the plant,
and that the driving force for this process is transpiration from the leaves.
This is referred to as the cohesion-tension-transpiration pull model of water
transport.
·
Water is transient in plants. Less than 1 per
cent of the water reaching the leaves is used in photosynthesis and plant
growth. Most of it is lost through the stomata in the leaves. This water loss
is known as transpiration.
Transport in plants Class 11 notes for NEET PDF
TRANSPIRATION
·
Transpiration is the evaporative loss of water
by plants. It occurs mainly through stomata (sing: stoma).
·
Besides the loss of water vapour in
transpiration, exchange of oxygen and carbon dioxide in the leaf also occurs
through these stomata.
·
Normally stomata are open in the day time and
close during the night. The immediate cause of the opening or closing of
stomata is a change in the turgidity of the guard cells.
·
The inner wall of each guard cell, towards the
pore or stomatal aperture, is thick and elastic.
·
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.
·
Usually the lower surface of a dorsiventral
(often dicotyledonous) leaf has a greater number of stomata while in an
isobilateral (often monocotyledonous) leaf they are about equal on both
surfaces. Transpiration is affected by several external factors: temperature,
light, humidity, wind speed.
·
The transpiration driven ascent of xylem sap
depends mainly on the following physical properties of water:
(i)
Cohesion –
mutual attraction between water molecules.
(ii) Adhesion –
attraction of water molecules to polar surfaces (such as the surface of
tracheary elements).
(iii) Surface
Tension – water molecules are attracted to each other in the
liquid phase more than to water in the gas phase.
·
These properties give water high tensile
strength, i.e., an ability to resist a pulling force, and high capillarity,
i.e., the ability to rise in thin tubes.
·
In plants capillarity is aided by the small
diameter of the treachery elements-the tracheid’s and vessel elements. The
process of photosynthesis requires water. The system of xylem vessels from the
root to the leaf vein can supply the needed water.
·
Because of lower concentration of water vapor
in the atmosphere as compared to the sub stomatal cavity and intercellular
spaces, water diffuses into the surrounding air. This creates a ‘pull’.
·
Because of lower concentration of water vapor
in the atmosphere as compared to the sub stomatal cavity and intercellular
spaces, water diffuses into the surrounding air. This creates a ‘pull’.
·
Transpiration and Photosynthesis – a Compromise
Transpiration has more than one purpose; it
·
Creates transpiration pull for absorption and
transport of plants. supplies water for photosynthesis transports minerals from
the soil to all parts of the plant .cools leaf surfaces, sometimes 10 to 15
degrees, by evaporative cooling
·
Maintains the shape and structure of the plants
by keeping cells turgid An actively photosynthesizing plant has an insatiable
need for water.
·
The evolution of the C4 photosynthetic system
is probably one of the strategies for maximizing the availability of CO2 while minimizing
water loss. C4 plants are twice as efficient as C3 plants in terms of fixing
carbon dioxide (making sugar).
·
However, a C4 plant loses only half as much
water as a C3 plant for the same amount of CO2 fixed.
Transport in Plants Class 11 NCERT notes
UPTAKE
AND TRANSPORT OF MINERAL NUTRIENTS
Uptake
of Mineral Ions
·
Unlike water, all minerals cannot be passively
absorbed by the roots. Two factors account for this:
(i)
Minerals are present in the soil as charged
particles (ions) which cannot move across cell membranes and
(ii)
The concentration of minerals in the soil is
usually lower than the concentration of minerals in the root. Therefore, most
minerals must enter the root by active absorption into the cytoplasm of
epidermal cells. This needs energy in the form of ATP
·
Translocation of Mineral Ions After the ions
have reached xylem through active or passive uptake, or a combination of the
two, their further transport up the stem to all parts of the plant is through
the transpiration stream.
Translocation
of Mineral Ions
·
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.
·
Unloading of mineral ions occurs at the fine
vein endings through diffusion and active uptake by these cells.
·
Mineral ions are frequently remobilised,
particularly from older, senescing parts. Older dying leaves export much of
their mineral content to younger leaves.
·
Similarly, before leaf fall in decidous plants,
minerals are removed to other parts. Elements most readily mobilised are
phosphorus, sulphur, nitrogen and potassium. Some elements that are structural
components like calcium are not remobilized.
Transport in plants Class 11 Handwritten Notes
PHLOEM
TRANSPORT: FLOW FROM SOURCE TO SINK
·
Since the source-sink relationship is variable,
the direction of movement in the phloem can be upwards or downwards, i.e.,
bi-directional. This contrasts with that of the xylem where the movement is always
unidirectional, i.e., upwards.
·
Phloem sap is mainly water and sucrose, but
other sugars, hormones and amino acids are also transported or translocated
through phloem.
Transport in plants notes class 11
The
Pressure Flow or Mass Flow Hypothesis
·
Phloem tissue is composed of sieve tube cells,
which form long columns with holes in their end walls called sieve plates.
·
Cytoplasmic strands pass through the holes in
the sieve plates, so forming continuous filaments. As hydrostatic pressure in
the sieve tube of phloem increases, pressure flow begins, and the sap moves
through the phloem. Meanwhile, at the sink, incoming sugars are actively
transported out of the phloem and removed.
·
A simple experiment, called girdling, was used
to identify the tissues through which food is transported.