UNIT 4

The description of structure and variation of living organisms over a

period of time, ended up as two, apparently irreconcilable perspectives

on biology. The two perspectives essentially rested on two levels of

organisation of life forms and phenomena. One described at organismic

and above level of organisation while the second described at cellular

and molecular level of organisation. The first resulted in ecology and

related disciplines. The second resulted in physiology and biochemistry.

Description of physiological processes, in flowering plants as an

example, is what is given in the chapters in this unit. 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. Wherever appropriate, the relation of the physiological processes

to environment is also discussed.

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

2020-21

MELVIN CALVIN born in Minnesota in April, 1911, received his

Ph.D. in Chemistry from the University of Minnesota. He served

as Professor of Chemistry at the University of California,

Berkeley.

Just after world war II, when the world was under shock

after the Hiroshima-Nagasaki bombings, and seeing the ill-

effects of radio-activity, Calvin and co-workers put radio-

activity to beneficial use. He along with J.A. Bassham studied

reactions in green plants forming sugar and other substances

from raw materials like carbon dioxide, water and minerals

by labelling the carbon dioxide with C

. 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 mapping of the pathway of carbon

assimilation in photosynthesis earned him Nobel Prize in 1961.

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.

Melvin Calvin

2020-21

Have you ever wondered how water reaches the top of tall trees, or for that

matter how and why substances move from one cell to the other, whether

all substances move in a similar way, in the same direction and whether

metabolic energy is required for moving substances. Plants need to move

molecules over very long distances, much more than animals do; they also

do not have a circulatory system in place. 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 synthesised 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 and the anatomy of

the plant body. We also need to revisit our understanding of diffusion,

besides gaining some knowledge about chemical potential and ions.

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. In a flowering plant the substances that would need to

be transported are water, mineral nutrients, organic nutrients and plant

growth regulators. Over small distances substances move by diffusion

and by cytoplasmic streaming supplemented by active transport.

Transport over longer distances proceeds through the vascular system

(the xylem and the phloem) and is called translocation.

An important aspect that needs to be considered is the direction of

transport. In rooted plants, transport in xylem (of water and minerals) is

essentially unidirectional, from roots to the stems. Organic and mineral

nutrients however, undergo multidirectional transport. Organic

RANSPORT IN

LANTS

HAPTER

11.1 Means of

Transport

11.2 Plant-Water

Relations

11.3 Long Distance

Transport of

Water

11.4 Transpiration

11.5 Uptake and

Transport of

Mineral

Nutrients

11.6 Phloem

Transport: Flow

from Source to

Sink

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176 BIOLOGY

compounds synthesised in the photosynthetic leaves are exported to all

other parts of the plant including storage organs. From the storage organs

they are later re-exported. The mineral nutrients are taken up by the

roots and transported upwards into the stem, leaves and the growing

regions. 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.

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.

11.1 MEANS OF TRANSPORT

11.1.1 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 inter-

cellular 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 is a slow process and is not dependent on a ‘living

system’. Diffusion is obvious in gases and liquids, but diffusion in solids is

more likely rather than of solids. Diffusion is very important to plants since

it is the only means for gaseous movement within the plant body.

Diffusion rates are affected by the gradient of concentration, the

permeability of the membrane separating them, temperature and pressure.

11.1.2 Facilitated Diffusion

As pointed out earlier, a gradient must already be present for diffusion to

occur. The diffusion rate depends on the size of the substances; obviously

smaller substances diffuse faster. The diffusion of any substance across a

membrane also depends on its solubility in lipids, the major constituent of

the membrane. 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. Membrane

proteins provide sites at which such molecules cross the membrane. 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.

In facilitated diffusion special proteins help move substances across

membranes without expenditure of ATP energy. 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). Facilitated

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diffusion is very specific: it allows cell to

select substances for uptake. It is

sensitive to inhibitors which react with

protein side chains.

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.

Figure 11.1 shows an extracellular

molecule bound to the transport protein;

the transport protein then rotates and

releases the molecule inside the cell, e.g.,

water channels – made up of eight

different types of aquaporins.

11.1.2.1 Passive symports and

antiports

Some carrier or transport proteins allow

diffusion only if two types of molecules

move together. In a symport, both

molecules cross the membrane in the same

direction; in an antiport, they move in

opposite directions (Figure 11.2). When a

Figure 11.1 Facilitated diffusion

Uniport

Carrier protein

Membrane

Antiport

Symport

Figure 11.2 Facilitated diffusion

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molecule moves across a membrane independent of other molecules, the

process is called uniport.

11.1.3 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). Transport rate reaches a maximum

when all the protein transporters are being used or are saturated. Like

enzymes the carrier protein is very specific in what it carries across the

membrane. These proteins are sensitive to inhibitors that react with protein

side chains.

11.1.4 Comparison of Different Transport Processes

Table 11.1 gives a comparison of the different transport mechanisms.

Proteins in the membrane are responsible for facilitated diffusion and

active transport and hence show common characterstics of being highly

selective; they are liable to saturate, respond to inhibitors and are under

hormonal regulation. But diffusion whether facilitated or not – take place

only along a gradient and do not use energy.

TABLE 11.1 Comparison of Different Transport Mechanisms

Property Simple Facilitated Active

Diffusion Transport Transport

Requires special membrane proteins No Yes Yes

Highly selective No Yes Yes

Transport saturates No Yes Yes

Uphill transport No No Yes

Requires ATP energy No No Yes

11.2 PLANT-WATER RELATIONS

Water is essential for all physiological activities of the plant and plays a

very important role in all living organisms. It provides the medium in

which most substances are dissolved. The protoplasm of the cells is

nothing but water in which different molecules are dissolved and (several

particles) suspended. 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. Of course, distribution of water within a plant varies –

woody parts have relatively very little water, while soft parts mostly contain

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water. 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.e., transpiration. A

mature corn plant absorbs almost three litres of water in a day, while a

mustard plant absorbs water equal to its own weight in about 5 hours.

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.

11.2.1 Water Potential

To comprehend plant-water relations, an understanding of certain

standard terms is necessary. Water potential (

ΨΨ

) is a concept

fundamental to understanding water movement. Solute potential

(

ΨΨ

) and pressure potential (

ΨΨ

) are the two main components that

determine water potential.

Water molecules possess kinetic energy. In liquid and gaseous form

they are in random motion that is both rapid and constant. The greater

the concentration of water in a system, the greater is its kinetic energy or

‘water potential’. Hence, it is obvious that pure water will have the greatest

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.

Thus water will move from the system containing water at higher water

potential

to the one having low 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

ΨΨ

Ψ and is expressed in

pressure units such as pascals (Pa). By convention, the water potential

of pure water at standard temperatures, which is not under any pressure,

is taken to be zero.

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

ΨΨ

is always negative. The more

the solute molecules, the lower (more negative) is the Ψ

For a solution at

atmospheric pressure (water potential) Ψ

= (solute potential)

If a pressure greater than atmospheric pressure is applied to pure

water or a solution, its water potential increases. It is equivalent to

pumping water from one place to another. 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 11.2.2);

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this increases the pressure potential. Pressure potential is usually

positive, though in plants negative potential or tension in the water column

in the xylem plays a major role in water transport up a stem. Pressure

potential is denoted as

ΨΨ

Water potential of a cell is affected by both solute and pressure

potential. The relationship between them is as follows:

ΨΨ

11.2.2 Osmosis

The plant cell is surrounded by a cell membrane and a cell wall. The cell

wall is freely permeable to water and substances in solution hence is not

a barrier to movement. In plants the cells usually contain a large central

vacuole, whose contents, the vacuolar sap, contribute to the solute

potential of the cell. In plant cells, the cell membrane and the membrane

of the vacuole, the tonoplast together are important determinants of

movement of molecules in or out of the cell.

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. At equilibrium

the two chambers should have nearly the same water potential.

You may have made a potato osmometer in your earlier classes in

school. If the potato tuber is placed in water, the water enters the cavity in

the potato tuber containing a concentrated solution of sugar due to osmosis.

Study Figure 11.3 in which the two chambers, A and B, containing

solutions are separated by a semi-permeable membrane.

(a) Solution of which chamber has a lower water potential?

(b) Solution of which chamber has a lower solute potential?

(d) Which solution has a higher solute

potential?

(e) At equilibrium which chamber will

have lower water potential?

(f) If one chamber has a

Ψ Ψ

Ψ of – 2000

kPa, and the other – 1000 kPa, which

is the chamber that has the higher

ΨΨ

Ψ?

(g) What will be the direction of the

movement of water when two

solutions with Ψ

= 0.2 MPa and

= 0.1 MPa are separated by a

selectively permeable membrane?

Figure 11.3

Solute

molecule

Water

Semi-permeable

Selectively permeable

membrane

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Let us discuss another experiment where a

solution of sucrose in water taken in a funnel is

separated from pure water in a beaker by a

selectively permeable membrane (Figure 11.4).

You can get this kind of a membrane in an egg.

Remove the yolk and albumin through a small

hole at one end of the egg, and place the shell

in dilute solution of hydrochloric acid for a few

hours. The egg shell dissolves leaving the

membrane intact. Water will move into the funnel,

resulting in rise in the level of the solution in the

funnel. This will continue till the equilibrium is

reached. In case sucrose does diffuse out

through the membrane, will this equilibrium be

ever reached?

External pressure can be applied from the

upper part of the funnel such that no water

diffuses into the funnel through the membrane.

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. Numerically osmotic pressure is equivalent

to the osmotic potential, but the sign is

opposite.Osmotic pressure is the positive

pressure applied, while osmotic potential is

negative.

11.2.3 Plasmolysis

The behaviour 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.

Plasmolysis occurs when water moves out of

the cell and the cell membrane of a plant cell

shrinks away from its cell wall. This occurs when

Figure 11.4 A demonstration of osmosis. A

thistle funnel is filled with

sucrose solution and kept

inverted in a beaker containing

water. (a) Water will diffuse

across the membrane (as

shown by arrows) to raise the

level of the solution in the

funnel (b) Pressure can be

applied as shown to stop the

water movement into the

funnel

Sucrose

solution

Membrane

water

(a)

(b)

Pressure

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the cell (or tissue) is placed in a solution that is hypertonic (has more solutes)

to the protoplasm. Water moves out; it is first lost from the cytoplasm and

then from the vacuole. 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

(Figure 11.5).

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. If the external solution

balances the osmotic pressure of the cytoplasm it is said to be isotonic.

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

ΨΨ

. Because of the

rigidity of the cell wall, the cell does not rupture. This turgor pressure is

ultimately responsible for enlargement and extension growth of cells.

What would be the

ΨΨ

of a flaccid cell? Which organisms other than

plants possess cell wall ?

11.2.4 Imbibition

Imbibition is a special type of diffusion when water is absorbed by

solids – colloids – causing them to increase in volume. The classical

Figure 11.5 Plant cell plasmolysis

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examples of imbibition are absorption of water by seeds and dry wood.

The pressure that is produced by the swelling of wood had been used by

prehistoric man to split rocks and boulders. If it were not for the pressure

due to imbibition, seedlings would not have been able to emerge out of

the soil into the open; they probably would not have been able to establish!

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. Water potential gradient between

the absorbent and the liquid imbibed is essential for imbibition. In addition,

for any substance to imbibe any liquid, affinity between the adsorbant and

the liquid is also a pre-requisite.

11.3 LONG DISTANCE TRANSPORT OF WATER

At some earlier stage you might have carried out an experiment where

you had placed a twig bearing white flowers in coloured water and had

watched it turn colour. On examining the cut end of the twig after a few

hours you had noted the region through which the coloured water moved.

That experiment very easily demonstrates that the path of water movement

is through the vascular bundles, more specifically, the xylem. Now we

have to go further and try and understand the mechanism of movement

of water and other substances up a plant.

Long distance transport of substances within a plant cannot be by

diffusion alone. Diffusion is a slow process. It can account for only short

distance movement of molecules. For example, the movement of a molecule

across a typical plant cell (about 50 µm) takes approximately 2.5 s. 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

to long distances. Sometimes the sites of production or absorption and

sites of storage are too far from each other; diffusion or active transport

would not suffice. 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. 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. It is a characteristic of mass flow that substances,

whether in solution or in suspension, are swept along at the same pace,

as in a flowing river. This is unlike diffusion where different substances

move independently depending on their concentration gradients. Bulk

flow can be achieved either through a positive hydrostatic pressure

gradient (e.g., a garden hose) or a negative hydrostatic pressure gradient

(e.g., suction through a straw).

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The bulk movement of substances through the conducting or vascular

tissues of plants is called translocation.

Do you remember studying cross sections of roots, stems and leaves

of higher plants and studying the vascular system? The higher plants

have highly specialised vascular tissues – xylem and phloem. Xylem is

associated with translocation of mainly water, mineral salts, some organic

nitrogen and hormones, from roots to the aerial parts of the plants. The

phloem translocates a variety of organic and inorganic solutes, mainly

from the leaves to other parts of the plants.

11.3.1 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.

The responsibility of absorption of water and minerals is more specifically

the function of the root hairs that are present in millions at the tips of the

roots. 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:

• 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 11.6). The apoplastic movement of water occurs

exclusively through the intercellular spaces and the walls of the cells.

Movement through the apoplast does not involve crossing the cell

Figure 11.6 Pathway of water movement in the root

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membrane. This movement is dependent on the gradient. The apoplast

does not provide any barrier to water movement and water movement is

through mass flow. As water evaporates into the intercellular spaces or

the atmosphere, tension develop in the continuous stream of water in the

apoplast, hence mass flow of water occurs due to the adhesive and cohesive

properties of water.

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. Movement is again

down a potential gradient. Symplastic movement may be aided by

cytoplasmic streaming. You may have observed cytoplasmic streaming

in cells of the Hydrilla leaf; the movement of chloroplast due to streaming

is easily visible.

Most of the water flow in the roots occurs via the apoplast since the

cortical cells are loosely packed, and hence offer no resistance to water

movement. However, the inner boundary of the cortex, the endodermis,

is impervious to water because of a band of suberised matrix called the

casparian strip. Water molecules are unable to penetrate the layer, so

they are directed to wall regions that are not suberised, into the cells

proper through the membranes. The water then moves through the

symplast and again crosses a membrane to reach the cells of the xylem.

The movement of water through the root layers is ultimately symplastic

in the endodermis. This is the only

way water and other solutes can

enter the vascular cylinder.

Once inside the xylem, water is

again free to move between cells as

well as through them. In young

roots, water enters directly into the

xylem vessels and/or tracheids.

These are non-living conduits and

so are parts of the apoplast. The

path of water and mineral ions into

the root vascular system is

summarised in Figure 11.7.

Some plants have additional

structures associated with them

that help in water (and mineral)

absorption. A mycorrhiza is a

symbiotic association of a fungus

with a root system. The fungal

Pericycle

Phloem

Casparian

strip

Apoplastic

path

Symplastic

path

Endodermis Xylem

Cortex

Figure 11.7 Symplastic and apoplastic pathways of

water and ion absorption and movement in

roots

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filaments form a network around the young root or they penetrate the

root cells. 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. The fungus provides minerals and water to the roots, in

turn the roots provide sugars and N-containing compounds to the

mycorrhizae. Some plants have an obligate association with the

mycorrhizae. For example, Pinus seeds cannot germinate and establish

without the presence of mycorrhizae.

11.3.2 Water Movement up a Plant

We looked at how plants absorb water from the soil, and move it into the

vascular tissues. We now have to try and understand how this water is

transported to various parts of the plant. 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?

11.3.2.1 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.

How can we see that root pressure exists? Choose a small

soft-stemmed plant and on a day, when there is plenty of atmospheric

moisture, cut the stem horizontally near the base with a sharp blade,

early in the morning. You will soon see drops of solution ooze out of the

cut stem; this comes out due to the positive root pressure. If you fix a

rubber tube to the cut stem as a sleeve you can actually collect and

measure the rate of exudation, and also determine the composition of the

exudates. 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.

Root pressure can, at best, only provide a modest push in the overall

process of water transport. They obviously do not play a major role in

water movement up tall trees. 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. Root pressure does not account for the majority of water

transport; most plants meet their need by transpiratory pull.

11.3.2.2 Transpiration pull

Despite the absence of a heart or a circulatory system in plants, the

upward flow of water through the xylem in plants can achieve fairly high

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rates, up to 15 metres per hour. How is this movement accomplished? A

long standing question is, 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. But, what generates this transpirational pull?

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.

You have studied transpiration in an earlier class by enclosing a healthy

plant in polythene bag and observing the droplets of water formed inside

the bag. You could also study water loss from a leaf using cobalt chloride

paper, which turns colour on absorbing water.

11.4 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 turgidity increases within the two guard cells flanking each stomatal

aperture or pore, the thin outer walls bulge out and force the inner walls

into a crescent shape. The opening of the stoma is also aided due to the

orientation of the microfibrils in the cell walls of the guard cells. Cellulose

microfibrils are oriented radially rather than longitudinally making it easier

for the stoma to open. 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. Plant factors that

affect transpiration include number and

distribution of stomata, per cent of open

stomata, water status of the plant, canopy

structure etc.

Figure11.8 A stomatal aperture with guard cells

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The transpiration driven ascent of xylem sap depends mainly on the

following physical properties of water:

• Cohesion – mutual attraction between water molecules.

• Adhesion – attraction of water molecules to polar surfaces (such

as the surface of tracheary elements).

• 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 tracheary

elements – the tracheids 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. But

what force does a plant use to move water molecules into the leaf

parenchyma cells where they are needed? As water evaporates through

the stomata, since the thin film of water over the cells is continuous, it

esults in pulling of water, molecule by molecule, into the leaf from the

xylem. Also, because of lower concentration of water vapour in the

atmosphere as compared to the substomatal cavity and intercellular

spaces, water diffuses into the surrounding air. This creates a ‘pull’

(Figure 11.9).

Measurements reveal that the forces generated by transpiration can

create pressures sufficient to lift a xylem sized column of water over 130

metres high.

Xylem

Phloem

Diffusion into

surrounding air

Stoma

Guard Cell

Palisade

Figure11.9 Water movement in the leaf. Evaporation from the leaf sets up

a pressure gradient between the outside air and the air spaces of the

leaf. The gradient is transmitted into the photosynthetic cells and on

the water-filled xylem in the leaf vein.

Stomatal

pore

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11.4.1 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 photosynthesising plant has an insatiable need for water.

Photosynthesis is limited by available water which can be swiftly depleted

by transpiration. The humidity of rainforests is largely due to this vast

cycling of water from root to leaf to atmosphere and back to the soil.

The evolution of the C

photosynthetic system is probably one of the

strategies for maximising the availability of CO

while minimising water

loss. C

plants are twice as efficient as C

plants in terms of fixing carbon

dioxide (making sugar). However, a C

plant loses only half as much water

as a C

plant for the same amount of CO

fixed.

11.5 UPTAKE AND TRANSPORT OF MINERAL NUTRIENTS

Plants obtain their carbon and most of their oxygen from CO

in the

atmosphere. However, their remaining nutritional requirements are

obtained from water and minerals in the soil.

11.5.1 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. The active uptake of ions is partly responsible for the

water potential gradient in roots, and therefore for the uptake of water by

osmosis. Some ions also move into the epidermal cells passively.

Ions are absorbed from the soil by both passive and active transport.

Specific proteins in the membranes of root hair cells actively pump ions

from the soil into the cytoplasms of the epidermal cells. Like all cells, the

endodermal cells have many transport proteins embedded in their plasma

membrane; they let some solutes cross the membrane, but not others.

Transport proteins of endodermal cells are control points, where a plant

adjusts the quantity and types of solutes that reach the xylem. Note

that the root endodermis because of the layer of suberin has the ability to

actively transport ions in one direction only.

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11.5.2 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.

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 remobilised.

An analysis of the xylem exudates shows that though some of the

nitrogen travels as inorganic ions, much of it is carried in the organic

form as amino acids and related compounds. Similarly, small amounts

of P and S are carried as organic compounds. In addition, small amount

of exchange of materials does take place between xylem and phloem.

Hence, it is not that we can clearly make a distinction and say categorically

that xylem transports only inorganic nutrients while phloem transports

only organic materials, as was traditionally believed.

11.6 PHLOEM TRANSPORT: FLOW FROM SOURCE TO SINK

Food, primarily sucrose, is transported by the vascular tissue phloem

from a source to a sink. Usually the source is understood to be that

part of the plant which synthesises the food, i.e., the leaf, and sink, the

part that needs or stores the food. But, the source and sink may be

reversed depending on the season, or the plant’s needs. Sugar stored

in roots may be mobilised to become a source of food in the early spring

when the buds of trees, act as sink; they need energy for growth and

development of the photosynthetic apparatus. 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. Hence, unlike one-way flow of water in transpiration, food

in phloem sap can be transported in any required direction so long

as there is a source of sugar and a sink able to use, store or remove

the sugar.

Phloem sap is mainly water and sucrose, but other sugars, hormones

and amino acids are also transported or translocated through phloem.

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11.6.1 The Pressure Flow or Mass Flow Hypothesis

The accepted mechanism used for the translocation of sugars from source

to sink is called the pressure flow hypothesis. (see Figure 11.10). As

glucose is prepared at the source (by photosynthesis) it is converted to

sucrose (a dissacharide). The sugar is then moved in the form of sucrose

into the companion cells and then into the living phloem sieve tube cells

by active transport. This process of loading at the source produces a

hypertonic condition in the phloem. Water in the adjacent xylem moves

into the phloem by osmosis. As osmotic pressure builds up the phloem

sap will move to areas of lower pressure. At the sink osmotic pressure

must be reduced. Again active transport is necessary to move the sucrose

out of the phloem sap and into the cells which will use the sugar –

converting it into energy, starch, or cellulose. As sugars are removed, the

osmotic pressure decreases and water moves out of the phloem.

To summarise, the movement of sugars in the phloem begins at the

source, where sugars are loaded (actively transported) into a sieve tube.

Loading of the phloem sets up a water potential gradient that facilitates

the mass movement in the phloem.

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

Sugars leave sieve tube

for metabolism and

storage; water follows

by osmosis

=High

Phloem

turgor

pressure

Root

Sugars enter sieve tubes;

water follows by osmosis

Sugar solution flows

to regions of low

turgor pressure

Tip of stem

Sugars leave sieve tubes;

water follows by osmosis

Figure11.10 Diagrammatic presentation of mechanism of translocation

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as complex carbohydrates. The loss of solute produces a high water

potential in the phloem, and water passes out, returning eventually to xylem.

A simple experiment, called girdling, was used to identify the tissues

through which food is transported. On the trunk of a tree a ring of bark

up to a depth of the phloem layer, can be carefully removed. In the absence

of downward movement of food the portion of the bark above the ring on

the stem becomes swollen after a few weeks. This simple experiment

shows that phloem is the tissue responsible for translocation of food; and

that transport takes place in one direction, i.e., towards the roots. This

experiment can be performed by you easily.

SUMMARY

Plants obtain a variety of inorganic elements (ions) and salts from their

surroundings especially from water and soil. The movement of these nutrients

from environment into the plant as well as from one plant cell to another plant cell

essentially involves movement across a cell membrane. Transport across cell

membrane can be through diffusion, facilitated transport or active transport. Water

and minerals absorbed by roots are transported by xylem and the organic material

synthesised in the leaves is transported to other parts of plant through phloem.

Passive transport (diffusion, osmosis) and active transport are the two modes

of nutrient transport across cell membranes in living organisms. In passive

transport, nutrients move across the membrane by diffusion, without any use of

energy as it is always down the concentration gradient and hence entropy driven.

This diffusion of substances depends on their size, solubility in water or organic

solvents. Osmosis is the special type of diffusion of water across a selectively

permeable membrane which depends on pressure gradient and concentration

gradient. In active transport, energy in the form of ATP is utilised to pump

molecules against a concentration gradient across membranes. Water potential is

the potential energy of water molecules which helps in the movement of water. It is

determined by solute potential and pressure potential. The osmotic behaviour of

cells depends on the surrounding solution. If the surrounding solution of the cell

is hypertonic, it gets plasmolysed. The absorption of water by seeds and drywood

takes place by a special type of diffusion called imbibition.

In higher plants, there is a vascular system comprising of xylem and phloem,

responsible for translocation. Water minerals and food cannot be moved within

the body of a plant by diffusion alone. They are therefore, transported by a mass

flow system – movement of substance in bulk from one point to another as a

result of pressure differences between the two points.

Water absorbed by root hairs moves into the root tissue by two distinct

pathways, i.e., apoplast and symplast. Various ions, and water from soil can be

transported upto a small height in stems by r

oot pressure. Transpiration pull

model is the most acceptable to explain the transport of water. Transpiration is

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the loss of water in the form of vapours from the plant parts through stomata.

Temperature, light, humidity, wind speed and number of stomata affect the rate

of transpiration. Excess water is also removed through tips of leaves of plants by

guttation.

Phloem is responsible for transport of food (primarily) sucrose from the source

to the sink. The translocation in phloem is bi-directional; the source-sink

relationship is variable. The translocation in phloem is explained by the pressure-

flow hypothesis.

EXERCISES

1. What are the factors affecting the rate of diffusion?

2. What are porins? What role do they play in diffusion?

3. Describe the role played by protein pumps during active transport in plants.

4. Explain why pure water has the maximum water potential.

5. Differentiate between the following:

(a) Diffusion and Osmosis

(b) Transpiration and Evaporation

(d) Imbibition and Diffusion

(e) Apoplast and Symplast pathways of movement of water in plants.

(f) Guttation and Transpiration.

6. Briefly describe water potential. What are the factors affecting it?

7. What happens when a pressure greater than the atmospheric pressure is applied

to pure water or a solution?

8. (a) With the help of well-labelled diagrams, describe the process of plasmolysis

in plants, giving appropriate examples.

(b) Explain what will happen to a plant cell if it is kept in a solution having

higher water potential.

9. How is the mycorrhizal association helpful in absorption of water and minerals

in plants?

10. What role does root pressure play in water movement in plants?

11. Describe transpiration pull model of water transport in plants. What are the

factors influencing transpiration? How is it useful to plants?

12. Discuss the factors responsible for ascent of xylem sap in plants.

13. What essential role does the root endodermis play during mineral absorption in

plants?

14. Explain why xylem transport is unidirectional and phloem transport

bi-directional.

15. Explain pressure flow hypothesis of translocation of sugars in plants.

16. What causes the opening and closing of guard cells of stomata during

transpiration?

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