Diffusion in Plants: Definition, Types & Examples Of Diffusion | Plant Physiology

In this article, we will discuss about:- 1. Diffusion in Plants Definition 2.Types of Diffusion 3.Factors that Affect Diffusion and 4.significance of Diffusion in Plants

1. Diffusion in Plants Definition

Diffusion is a very important process for photosynthesis where carbon dioxide from the stomata diffuses into the leaves and finally into the cells. Also, during transpiration, the water and oxygen diffuse from the leaves into the environment.

It includes the movement of particles of a medium from the region of its higher concentration to the region of its lower concentration without the expenditure of energy. This process is slow and occurs mostly in gases and liquids. The rate of diffusion is affected by various factors like temperature and pressure, concentration gradient, separating membrane’s permeability etc.

2.Types of Diffusion

Diffusion can either be simple diffusion and be facilitated by another molecule

i) Simple Diffusion

Simple diffusion is merely the movement of molecules along their concentration gradient without the direct involvement of any other molecules. It can involve either the spreading of a material through a medium or the transport of a particle across a membrane. All the examples given above were instances of simple diffusion.

The image is a simple representation of the diffusion of one particle in another medium.

Simple diffusion is relevant in chemical reactions, in many physical phenomena, and can even influence global weather patterns and geological events. In most biological systems, diffusion occurs across a semi-permeable membrane made of a lipid bilayer. The membrane has pores and openings to allow the passage of specific molecules.

ii) Facilitated Diffusion

On the other hand, facilitated diffusion, as the term indicates, requires the presence of another molecule (the facilitator) in order for diffusion to occur. Facilitated diffusion is necessary for the movement of large or polar molecules across the hydrophobic lipid bilayer. Facilitated diffusion is necessary for the biochemical processes of every cell since there is communication between various subcellular organelles. As an example, while gases and small molecules like methane or water can diffuse freely across a plasma membrane, larger charged molecules like carbohydrates or nucleic acids need the help of transmembrane proteins forming pores or channels.

The image shows the movement of an insoluble molecule from the extracellular space towards the cytoplasm.

Since they are relatively large openings in the plasma membrane, these integral membrane proteins also have high specificity. For instance, the channel protein that transports potassium ions has a much higher affinity for that ion than a very similar sodium ion, with nearly the same size and charge.

3.Factors that Affect Diffusion

Diffusion is affected by temperature, area of interaction, steepness of the concentration gradient and particle size. Each of these factors, independently and collectively can alter the rate and extent of diffusion.

i) Temperature

In any system, molecules are moving with a certain amount of kinetic energy. This is usually not directed in any particular manner, and can appear random. When these molecules collide with one another, there is a change in the direction of movement as well as changes to momentum and velocity. For example, if a block of dry ice (carbon dioxide in solid form) is placed inside a box, carbon dioxide molecules in the center of the block mostly collide with each other and get retained within the solid mass. However, for molecules in the periphery, rapidly moving molecules in the air also influence their movement, allowing them to diffuse into the air. This creates a concentration gradient, with concentration of carbon dioxide gradually decreasing with distance from the lump of dry ice.

With increase in temperature, the kinetic energy of all particles in the system increases. This increases the rate at which solute and solvent molecules move, and increases collisions. This means that the dry ice (or even regular ice) will evaporate faster on a warmer day, simply because each molecule is moving with greater energy and is more likely to quickly escape the confines of a solid state.

ii) Area of interaction

To extend the example given above, if the block of dry ice is broken into multiple pieces, the area that interacts with the atmosphere immediately increases. The number of molecules that only collide with other carbon dioxide particles within dry ice decreases. Therefore, the rate of diffusion of the gas into air also increases.

This property can be observed even better if the gas has an odor or color. For instance, when iodine is sublimated over a hot stove, purple fumes begin to appear and mix with air. If sublimation is carried out in a narrow crucible, the fumes diffuse slowly out towards the mouth of the container and then rapidly disappear. While they are confined to the smaller surface area within the crucible, the rate of diffusion remains low.

This is also seen when two liquid reactants are mixed with one another. Stirring increases the area of interaction between the two chemicals and allows these molecules to diffuse towards each other more quickly. The reaction proceeds towards completion at a faster rate. On a similar note, any solute that is broken into small pieces and stirred into the solvent dissolves rapidly – another indicator of molecules diffusing better when the area of interaction increases.

iii) Particle Size

At any given temperature, the diffusion of a smaller particle will be more rapid than that of a larger-sized molecule. This is related to both the mass of the molecule and its surface area. A heavier molecule with a larger surface area will diffuse slowly, while smaller, lighter particles will diffuse more quickly. For example, while oxygen gas will diffuse slightly more quickly than carbon dioxide, both of them will move more quickly than iodine gas.

4. Significance of Diffusion in Plants

The process of diffusion is important for the plants in the following ways:

• The exchange of gases through stomata takes place by the process of diffusion.
• Transpiration occurs by the principle of diffusion.
• The ions are absorbed by simple diffusion.
• The food material is translocated by this process.
• This process keeps the walls of the internal tissues of the plant moist.
• It is responsible for spreading the ions and molecules throughout the protoplast.
• Aroma of flowers is due to the diffusion of aromatic compounds to attract insects.