Introduction

Like animals, plants also have transportation systems to circulate useful substances and excrete waste materials. The movement of water, nutrients, and minerals across the plant body is called transportation in plants. Based on distance, transportation in plants takes place in two ways – Short distance (cell to cell) and long distance. Short-distance transportation can be active (requires ATP) or passive (diffusion), depending on the requirement of energy. Long-distance transportation (translocation) takes place with the help of vascular tissues (xylem and phloem).

Diffusion

• Diffusion is passive transport in which the movement of molecules takes place from a higher concentration to a lower concentration (along the potential gradient) until equilibrium is achieved

• The process of diffusion occurs everywhere; it is independent of the living system

For example, if we burn incense sticks in a room, its aroma spreads throughout the room after some time because of the diffusion of particles of essence stick (higher concentration) into the air (lower concentration) of the room.

• In diffusion, particles move randomly; they do not follow a particular path

• The process of diffusion takes a very long time to complete

• Diffusion is a thermodynamically favourable process that does not require an external energy source

• The energy for diffusion is derived from the energy stored in the concentration gradient

• The rate of diffusion is highest in gases and lowest in solids because particles move in random motion during diffusion. In gases, a higher degree of randomness is found compared to liquid and solids

Diffusion rate in different states : Gas > Liquid > Solid

Importance

• Most important physiological functions of plant-like transpiration, absorption of water and minerals from the soil, and respiration work on the principle of diffusion

• This is the only way through which plants exchange gases

Factors affecting the process of diffusion

Temperature: The rate of diffusion increases with increasing temperature.

• Due to the increase in kinetic energy of the particles with increasing thermal energy, particles are more likely to move.

Rate of diffusion ∝ Temperature

Concentration of medium: Molecules move slowly in a concentrated medium because a concentrated medium provides more restriction to diffusing molecules. 

Rate of diffusion ∝ 1 / Concentration of medium

Density: The rate of diffusion decreases with an increase in the density of diffusing particles.

• Particles with higher mass (higher density) require more time to diffuse.

Rate of diffusion ∝ 1 / Density

Diffusion is of two types – simple diffusion and facilitated diffusion 

• In simple diffusion, molecules do not require any special type of protein molecules for movement.

• In facilitated diffusion, a special type of protein molecule is required to transport the molecules from a region of higher concentration to lower concentration assisted by a carrier.

Osmosis and osmotic pressure

Osmosis

The plant cell wall is permeable; hence, it allows both water molecules and solutes to pass through it, but the cell membrane and tonoplast are selectively permeable. It allows only water molecules to pass through and restricts the movement of solutes through it. The cell membrane and the vacuole membrane are significant determining factors for the flow of substances throughout the plant cells.

• The term osmosis is used to describe the diffusion of water through a membrane that is semi-permeable or selectively permeable

• If two solutions with varying solute concentrations are divided by the membrane, water has a fundamental tendency to flow across the solution with a higher solute concentration. So, we can say that water has a tendency to maintain equilibrium. This type of movement of water is called osmosis

Osmosis is defined as the migration of water over a semi-permeable membrane from a region of its higher concentration to a region of its lower concentration.

Two solutions, one with lower solute concentration and the other with higher solute concentration, are separated by a semi-permeable membrane in a U-shaped glass tube, as shown in the above figure. The water will move from the solution with a lower solute concentration to a higher solution until the equilibrium is achieved.

Osmotic pressure

The pressure required to prevent the passage of solvent due to osmosis is known as osmotic pressure.

Osmotic pressure is the maximum amount of pressure generated by the solution, which is separated from its pure solvent by a semipermeable membrane. 

Closed system is required to develop the osmotic pressure.

Explanation of osmotic pressure with the help of thistle funnel experiment

The thistle funnel is filled with a sucrose solution placed in a beaker containing pure water separated by a semipermeable membrane. The pure water will move from its higher concentration to its lower concentration from beaker to thistle funnel. The movement of water inside the funnel can be prevented by applying the pressure. The pressure applied to prevent the movement of water inside the thistle funnel is equivalent to osmotic pressure. 

Transpiration stream

The transpiration stream in plants is the continuous flow of water and solutes taken in by the roots and transferred to the leaves via the xylem. It evaporates into the air/apoplast interface of the substomatal cavity. Capillary action and  in some plants, root pressure, drive it.

Needs of transportation

• To maintain the normal function of the plant

• To transport essential nutrients, growth hormones, water, and minerals throughout the plant body

• To carry out photosynthetic activity

• To remove the waste products out of the plant body

• To perform metabolic activities which are vital for the survival of plants

• To exchange the gases

Conclusion

 Special membrane proteins in the membranes are responsible for facilitated diffusion and active diffusion as well, and therefore, they share common characteristics of being selective, being liable to saturation, and also to make a response against various inhibitors coming in contact with them. All these transport processes are under hormonal regulation. Whether the diffusion is facilitated or not, it always takes place only along a concentration gradient and does not use energy for the same to occur in plants.  Here is the comparison of different transport mechanisms.