Means of Transport in Plants: Meaning, Different Pathways

Like all other living organisms on this earth, plants also have a well-developed transport system that ensures the plants’ smooth conduction of water and nutrients. All Plants are different in height, from shrubs and herbs to tall trees.

The transport system present in the plants also ensures the long-distance transport of the plant. Normally there are two means of transportation in plants one takes place from cell to cell, and the second transports water and minerals through vascular tissues. To learn more about transportation in plants, let’s go through this article.

Means of Transportation in Plants

The transport of materials can take place by several methods like diffusion, facilitated diffusion, and active transport.

Diffusion

a) Diffusion is the transportation of particles like molecules, atoms, ions, etc., of gases, liquids, and solids from the region of a higher concentration, higher energy, or higher diffusion pressure to the region of their lower concentration, lesser energy, or lower diffusion pressure.
b) No expenditure of energy takes place.
c) Diffusion is due to the random kinetic motion of particles.
d) Different substances present in the same medium perform independent diffusion as per their partial pressure or individual diffusion pressure.
e) Diffusion pressure is the pressure expended by the tendency of particles of a substance to diffuse from the area of its higher concentration to the region of its lower concentration. This pressure is directly proportional to the concentration or the number of diffusing particles.
f) Diffusion will be more rapid when the density of the diffusing substance is low, e.g., gases as compared to liquids and liquids as compared to solids. It also depends upon temperature, the density of medium, and diffusion pressure gradient.
g) Diffusion will continue till it reaches a state of equilibrium.
h) Osmosis is a type of diffusion. Transpiration, exchange of gases, and a part of solute absorption all take place due to diffusion.

Independent Diffusion

a) A system may have two or more types of diffusing particles.
b) Each diffusing particle exerts its own diffusion pressure called partial pressure. Particles of different substances diffuse according to their own partial pressure.
c) The tendency of different substances to diffuse according to their own partial pressures or concentrations is known as independent diffusion.

Factors Affecting Diffusion

There are several factors that affect the rate of diffusion are as follows:

1. Temperature: Higher temperature increases the kinetic energy, so the rate of diffusion increases and lower temperature decreases the kinetic energy, so the rate of diffusion becomes lower.
2. Solvent density: The rate of diffusing substances is inversely proportional to the density of diffusing substances that means as the density of a solvent increases, the rate of diffusion decreases.
3. Medium in which diffusion occurs: Gas particles diffuse more rapidly through a vacuum as compared to air.
4. Mass of the diffusing molecules: Heavier molecules diffuse more slowly as they move slowly vice versa; lighter molecules diffuse faster as they move faster.

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Facilitated Diffusion

a) A gradient is required to be present for diffusion to occur.
b) The rate of diffusion depends on the size of the substances; smaller substances diffuse faster as compared to larger ones.
c) The rate of diffusion across a membrane also depends on its solubility in liquids.
d) Substances soluble in lipids easily pass through the membrane, but the hydrophilic solutes are difficult to pass through the membrane; therefore, their movement has to be facilitated.
e) In this, the membrane proteins provide sites at which such molecules cross the membrane.
f) A concentration gradient must be present for molecules to diffuse even if facilitated by the proteins. This process is called facilitated diffusion.

g) In facilitated diffusion, special proteins such as permeases help to move substances across membranes without the expenditure of ATP energy. It is also a passive process.
h) Facilitated diffusion cannot cause net transport of molecules, and the transport rate reaches the maximum when all of the protein transporters are being used.
i) The facilitated diffusion is very specific. It only allows the cell to select substances for uptake.
j) The proteins form water channels or aquaporins in the membrane for molecules to pass through; some channels are always open; others can be controlled. Some channels are large, allowing a variety of molecules to cross.
k) The porins are a type of protein that form huge pores in the outer membranes of the plastids, mitochondria, etc.
l) The water channels are composed of eight different types of aquaporins.
m) Carrier and channel proteins are two special types of transport proteins.
n) Carrier proteins can transport the ion or molecule across the membrane by changing shape after the binding of the transport substances both down and against the concentration gradient.
o) Channel proteins allow the diffusion of substances of appropriate sizes.

Importance of Facilitated Diffusion

1. It plays an important role in maintaining equilibrium between the exterior and interior environments.
2. It ensures the selectivity of different biological membranes.
3. The important cellular mechanisms like the transport of oxygen, nutrients, and ions that are essential to maintain optimal homeostasis in the cell are carried through facilitated diffusion.
4. It helps in the transport of water-soluble molecules like glucose, sodium ions, chloride ions by transport proteins.

The facilitated diffusion can take place by the following process:
1. Symport: It is the transport of both molecules in the same direction at the same time.
2. Antiport: It is the movement of both molecules in the opposite direction.
3. Uniport: It is the movement of a single molecule across a membrane in any direction.

Active Transport

The transport of molecules against a concentration gradient (from lower concentrated region to higher concentrated region) with the expenditure of energy is called active transport. Pumps are the movable carrier proteins in active transport. To drive the pump, the energy of ATP is used. This transport can be of two types: Primary and secondary active transport.

a) Primary active transport: This transport uses chemical energy in the form of ATP. This energy is directly incorporated into the movement of sodium ions across a biological membrane. Na⁺, K⁺. Mg²⁺ and Ca²⁺ are the substances that move in this type of active transport. One example is the active transport between the sodium-potassium pump. It is an active transport system that takes place in a biological membrane where three Na⁺ ions are taken out while two K⁺ ions are taken into the cell against their respective concentration gradients.

b) Secondary active transport: This transport uses potential energy from an electrochemical potential difference. It does not take place in the direct coupling of ATP. The transport is powered by the energy from electrochemical potential difference as the ions are pumped into and out of the cell. In this active transport, only one ion is allowed to move down its electrochemical gradient while the other moves against the concentration gradient. It is also known as coupled transport or cotransport. It can be of symport or antiport depending on the direction of movement of the two substances.

Transportation in Plants: Overview

Plants transport various substances like gases, minerals, water, hormones, organic solutes to short distances from one cell to another. It also transports substances to long distances as water from roots to tips of the stem. In plants, long-distance transport takes place mainly through vascular tissue, i.e., xylem and phloem called translocation. The direction of translocation can be unidirectional (in the case of water) and multidirectional (in the case of minerals and organic solutes). Xylem is involved with the translocation of water and mineral salts from roots to the aerial parts of the plant. Phloem is involved with the translocation of organic and inorganic solutes from leaves to other parts of the plants.

Comparison of Different Types of Transport

Plant Water Relation

Water Potential: The energy of water is called water potential. It is also defined as the chemical potential of water. The water potential of pure water at standard temperatures is zero (not under any pressure). If a solute is dissolved in pure water, the concentration of water decreases, reducing its water potential. The magnitude of this lowering due to the dissolution of a solute is called solute potential Ѱs. Ѱs is always negative. At atmospheric pressure, Ѱw=Ψs. Pressure can build up in a plant system when water enters a plant cell due to diffusion, causing pressure to build up against the cell wall, making the cell turgid. This increase in pressure is called pressure potential or hydrostatic pressure Ѱp. Ѱp is usually positive. Ѱw= Ψs +Ψp

Osmosis: The spontaneous diffusion of water across a differentially permeable or semipermeable membrane is called osmosis. It mainly depends on two factors: Concentration of dissolved solutes in a solution and pressure gradient. The pressure required to prevent water from diffusing is the osmotic pressure.

Osmotic pressure (π) = -Osmotic potential (Ѱs)

Plasmolysis: Plasmolysis is a condition that occurs when water tends to move out of the cells (area of high-water potential) across the membrane to the outside (area of lower water potential), and the cell shrinks away from its cell wall when placed in a hypertonic solution. Plasmolysis is reversible.

Turgor Pressure: When the cells are placed in a hypotonic solution, water diffuses into the cell. As a result, the cytoplasm builds up pressure against the wall. It is called turgor pressure.

Imbibition
Imbibition is the process of absorption of water by the solid particles of a substance without forming a solution.

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Long Distance Transport of Water (Xylem, Phloem)

a) The long-distance transport of water in plants occurs by mass or bulk flow system.
b) Bulk flow is achieved either through a +ve hydrostatic pressure gradient or a -ve hydrostatic pressure gradient.
c) Bulk movement of substances in long-distance through the conducting tissues (xylem & phloem) is called translocation.
d) Xylem is involved with the translocation of water and mineral salts from roots to the aerial parts of the plant.
e) Phloem is involved with the translocation of organic and inorganic solutes from leaves to other parts of the plants.

Different Pathways of Water Absorption in Plants

Water, along with the minerals, is absorbed by root hairs. There are mainly two pathways that take place for the movement of water to the different parts of the plant. These pathways are described below:

Apoplast pathway: In the apoplast pathway, water passes from root hairs to xylem through the cell wall only without crossing any membranes. The apoplast involves a continuous system of cell walls and intercellular spaces in plant tissues, i.e., the nonliving part of the cell. Water movement takes place through mass flow as the apoplast does not provide any barrier to water movement. Water movement is dependent on the gradient. The endodermis is covered by Casparian strips that restrict the movement of water.

Symplast pathway: In the symplast pathway, water movement takes place from cell to cell through protoplasm with the help of plasmodesmata. In this pathway, direct movement occurs from cytoplasm to cytoplasm as water does not enter cell vacuoles. This pathway is comparatively slower than the apoplastic pathway.

Fig: Pathways of Water Movement

Summary

Plants transport various substances like gases, minerals, water, hormones, organic solutes to short distances from one cell to another and long-distance as water from roots to tips of the stem. In plants, long-distance transport takes place mainly through vascular tissue, i.e., xylem and phloem called translocation. The transport of materials can take place by several methods like diffusion, facilitated diffusion (passive transport), and active transport.

Osmosis is the spontaneous diffusion of water across a differentially permeable or semipermeable membrane. Water, along with the minerals, is absorbed by root hairs. There are mainly two pathways, i.e., apoplast and symplast pathways, that take place for the movement of water to the different parts of the plant.

FAQs

Q.1. What is osmosis?
Ans: Osmosis is the spontaneous diffusion of water across a differentially permeable or semipermeable membrane.

Q.2. What are modes of transport in plants?
Ans: The different modes of transport in plants are diffusion, facilitated diffusion, osmosis, and active transport.

Q.3. What are porins?
Ans: The porins are a type of protein that form huge pores in the outer membranes of the plastids, mitochondria, etc.

Q.4. What is the difference between the apoplast and symplast pathways?
Ans: The difference between apoplast and symplast pathways are:
The apoplastic pathway contains nonliving parts of the plant body, such as cell walls and intercellular spaces, whereas the symplastic pathway contains living parts of the plant body such as protoplasm.

Q.5. What is facilitated diffusion?
Ans: The transport of substances across a biological membrane from an area of higher concentration to an area of lower concentration with the help of a transport molecule is called facilitated diffusion.

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