INTRODUCTION :
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 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 When any plant part undergoes senescence, nutrients may be withdrawn from such regions and moved to the growing parts.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 stimuli 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.
MEANS OF TRANSPORT :
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 rather than of solids is more likely. 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.
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 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 huge
pores in the outer membranes of the plastids, mitochondria and some bacteria allowing molecules up to the size of small proteins to pass through.
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
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 where a molecule moves across a membrane independent of other molecules, the
process is called uniport.
Active Transport :
Active transport uses energy to pump molecules against a concentration gradient. Active transport is carried out by 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.
Comparison of Different Transport Processes :
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.
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 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.
Water Potential :
To comprehend plant-water relations, an understanding of certain standard terms is necessary. 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. 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
potentialto 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 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 and the concentration 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. 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 . 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 Ψp.
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 semi-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 the same water potential. If the tuber is placed in water, the cavity in the potato tuber containing a concentrated solution of sugar collects water due to osmosis.
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 membra. 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.
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 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 .
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 Ψp.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.
Imbibition :
Imbibition is a special type of diffusion when water is absorbed by solids– colloids – causing them to enormously increase in volume. The classical 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.
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 across very large 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).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.
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 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
