Simple Diffusion

A substance tends to move from a region of high concentration to a region of lower concentration via diffusion until equilibrium is reached. This movement of molecules is done until the concentrations of both regions become equal, and does not require any input of energy. The concentration gradient itself is a form of stored energy, which is utilized as concentrations tend to equalize.

The directional movement of diffusion along a gradient is passive and continues until the molecules are dispersed evenly. Non-polar and small molecules can diffuse freely across cell membranes. The rate of diffusion can be affected by several factors, such as:

The size of molecules - larger particles have more resistance when traveling through a fluid medium.

Temperature

The steeper the concentration gradient, the higher the rate of diffusion.

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Simple Diffusion Definition: The movement of molecules from an area of higher concentration to an area of lower concentration.

The process of diffusion involves the passing of solutes through a concentration gradient in a solution across a semipermeable membrane, without the need for membrane proteins. This process is driven by the hydrogen bonds formed between the solutes and water molecules, resulting in water molecules surrounding the solute molecules in order to maximize hydrogen bonding.

Hydrogen bonds are temporary and stirring the solution constantly helps to evenly distribute solutes. If molecules are small enough, simple diffusion can occur across cell membranes made of phospholipids. Water then passes through the cell membrane along its concentration gradient, which is a type of osmosis.

Mechanism of Simple Diffusion

ATP in biological systems does not directly drive simple diffusion. Instead, it is the kinetic energy and concentration gradient that provides the energy for the process. This type of diffusion is characterized by molecules that are in a random motion due to collisions with one another, known as pedesis.

Molecules tend to be more compact and motion is reduced when an area is concentrated. When more space is available, molecules move towards the region with more space. For this to happen, there must be a concentration gradient, or a difference in concentration between two regions. Molecular movement will continue between both areas until a concentration gradient is established.

Simple Diffusion Example

Simple diffusion can be better understood with the following example:

Bacteria are simple entities that can only intake nutrients via diffusion across the cell membrane. Most nutrients are transported via facilitated diffusion, while water, oxygen, and small nutrients are transported via simple diffusion. As bacteria lack special organelles to transport or store substances, they rely on simple diffusion to ensure matter is present in the cell for reactions to regulate life processes.

Facilitated Diffusion

While some molecules, such as oxygen and carbon dioxide, can diffuse across the plasma membrane, other molecules require assistance to cross the hydrophobic core.

Molecules that are polar or charged are unable to cross the phospholipid part of the membrane without help. This is where facilitated diffusion comes in, as the membrane proteins such as carrier proteins and channels provide assistance. These proteins create a path for the molecules to cross, allowing them to move down the concentration gradient and into the cells. Thus, facilitated transport proteins are essential in allowing molecules to pass through the hydrophobic core of the membrane.

Osmosis

The spontaneous diffusion of solvent molecules through a selectively permeable membrane from an area of high water potential to an area of lower water potential, in order to equalize concentrations of solutes on both sides of the membrane, can be described as a physical phenomenon.

Osmosis is a primary method by which water is transported in and out of cells. It is also responsible for maintaining turgor pressure across the cell membrane between the cell interior and its surrounding hypotonic environment.

The osmotic pressure is the external pressure needed to be applied so that there is no net movement of solvent across the membrane. This pressure is a colligative property, meaning it depends on the molar concentration of the solution.

Active Transport

Active transport is a type of cellular transportation that facilitates the movement of molecules across the membranes of cells from an area of lower concentration to one of higher concentration, which goes against the concentration gradient. This process requires cellular energy to take place and is commonly seen in root hair cells and the walls of the small intestine (villi).

Active transport is of two types:

Primary Active Transport – Utilizes Adenosine Triphosphate

Secondary Active Transport – Utilizes an electrochemical gradient

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