por The plain diffusion It is the physical process by which substances move from a region where they are highly concentrated to areas where the concentration of that substance is lower. Simple diffusion is a spontaneous process that does not require energy, but is promoted by a concentration gradient.
Substances that are dissolved are called solutes, they move through a solution (or a gas) by random motion. Random motion is defined as motion that occurs by chance, where there is no regular order or system by which the motions of particles change in a solution or gas.
If the molecules are small enough, this simple diffusion can occur across cell membranes, between the individual phospholipids that make it up. Membranes can be permeable, waterproof, or semi-permeable.
A permeable membrane is one that allows any substance to pass through, an impermeable membrane does not allow any substance to pass through, and a semi-permeable membrane only allows certain substances to pass through.
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simple diffusion process
Simple diffusion is carried out by actions of the hydrogen bonds that are formed between the water molecules and the solutes dissolved in it. The water molecules move around randomly and eventually surround the individual solute molecules, maximizing the opportunity for hydrogen bonding.
Thus, if a mixture of a substance in water were made, initially the substance would be concentrated in one place, the initial one. At that time the concentration gradient is very high.
As time passes the molecules move and collide with each other; this movement promotes the formation of bonds between the solute particles and the solvent. However, these bonds break quickly because they are very weak and this is the property that allows them to distribute themselves throughout space until they reach equilibrium.
At that point, where the solute is evenly distributed throughout the solution, the strength of the concentration gradient is exhausted.
Examples of simple diffusion
Gas exchange in the respiratory system
In the respiratory system of pulmonary animals, oxygen (O2) and carbon dioxide (CO₂) are constantly exchanged. Animals absorb oxygen and expel carbon dioxide by simple diffusion.
In the lungs the concentration of oxygen is higher than in the blood, so there is a concentration gradient between these two regions, and this promotes the diffusion of oxygen from the lungs into the blood.
Similarly, since there are more carbon dioxide molecules in the blood than in the lungs, they will tend to move from the blood to the lungs.
Diffusion of tea in hot water
When a cup of tea is initially prepared there is only hot water without any solute. By placing the tea bag in the hot water, it begins to release the tea particles from the bag—where there is a higher concentration of tea—into the water, where the concentration of tea is lower.
The movement of the molecules is favored by the initial concentration gradient of the tea. Eventually, the diffusion of the tea causes the concentrations to equalize throughout the cup and the solution to become homogeneous (it all looks the same color). At this point there is no longer a concentration gradient.
Differences between simple diffusion and facilitated diffusion
– In simple diffusion, the particles cross the cell membrane freely, without the “help” of other structures. In contrast, in facilitated diffusion, molecules are transported through small protein channels that span the entire membrane.
– Simple diffusion is slower than facilitated diffusion.
– Simple diffusion is not specific to the solute, only that the particles to be transported have to be hydrophobic so that they can cross the cell membrane without problem. In contrast, in facilitated diffusion, specific solutes are transported.
– Simple diffusion cannot be inhibited, while facilitated diffusion can be inhibited by specific molecules that bind to transporter channels.
– Simple diffusion is always a passive process, that is, it does not require energy from ATP. In contrast, facilitated diffusion can be an active or passive mechanism, depending on whether or not it requires ATP energy.
– In simple diffusion, substances always move down their concentration gradient. Conversely, in facilitated diffusion substances cross the membrane against or in favor of their concentration gradient.
– Simple diffusion allows the passage of small, nonpolar (hydrophobic) molecules, while facilitated diffusion allows the passage of large, polar (hydrophilic) molecules.
Differences between simple diffusion and osmosis
– Simple diffusion refers to the movement of any substance as a function of its concentration gradient; Instead, osmosis refers exclusively to the movement of water based on its water potential.
– Simple diffusion occurs in solid liquids and gases. On the contrary, osmosis can only occur in a liquid medium.
– Simple diffusion does not require a semipermeable membrane, while osmosis is a process that occurs through a semipermeable membrane.
– Simple diffusion helps equalize the concentrations of dissolved solutes in the available space. Osmosis does not favor equal concentration of water on both sides of the semipermeable membrane.
– Simple diffusion can occur between different types of solvents. In contrast, osmosis can only occur between similar solvents and in biological systems. This solvent is generally water.
– In simple diffusion the particles are distributed in all directions, in osmosis the flow of molecules is basically in the same direction.
– The simple diffusion process cannot be stopped or reversed. Instead, osmosis can be stopped or reversed if additional pressure is applied to the side of the membrane where less water is available.
References
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