Simple and Facilitated Diffusion

Cell to cell transport is movement of substances out of or into the cell through cell membrane. It’s also called cell transport. It is the basic process of the cell of how things move into or out of the cell through the cell membrane.  

Substances can sometimes just go across the pf plasma membrane phospholipid bilayer. Sometimes, another transmembrane protein or a channel protein is used to aid the molecules to go through the membrane of the cell. Based on this phenomenon, molecules can flow through a phospholipid plasma membrane in two ways: Passive transport and Active transport.

• Passive transport is automatic and does not require energy or ATP to proceed. Osmosis and Diffusion are examples of this sort of transport
• Active transport requires energy to pull in or remove specific molecules and ions. Proteins called pumps are involved in active transport

Osmosis is the movement of particles from a dilute solution into a concentrated solution across a semipermeable membrane until equilibrium is reached.

Diffusion is defined as “the movement of molecules down a concentration gradient i.e., from an area of higher concentration to a region of lower concentration.”

• Diffusion is the first method, which requires no energy input from the cell. It is a natural and fundamental process that occurs in all beings that are living, breathing. Diffusion is a process that aids in the motion of molecules out of and into cells where molecules travel from a high-concentration area to a low-concentration area until the concentration is uniform throughout. We can see diffusion in liquids and gases because molecules move randomly. The molecules collide with each other and change their direction of flow. Diffusion is essential for the movement of molecules within cells during metabolic activities

Examples of diffusion

1. A spray or perfume or room freshener gets diffused into the air and we will be able to sense the odour.
2. Salt gets dissolved evenly and makes the water salty without having to stir it.
3. In plants, Water from the soil diffuses into plants via their root hair cells.

There are two types of diffusion: simple diffusion and facilitated diffusion.

Facilitated diffusion 

Facilitated diffusion is a type of passive transport through the cell membrane of molecules by a protein molecule to a region of lower concentration from an area of higher concentration. The involvement of transport proteins is required for facilitated diffusion. No energy or ATP is required.

Some molecules, like oxygen and carbon dioxide, can pass through the plasma membrane without assistance, but others require assistance to pass through its hydrophobic core (water resistant membrane). The compounds that are transferred via facilitated diffusion would not otherwise be able to pass the membrane easily or quickly. 

Simple diffusion

“Simple diffusion is the movement of molecules across a semipermeable membrane from a higher concentration to a lower concentration”. It is also called as passive diffusion as it is a type of passive transport. For example, movement of oxygen, CO2 and water molecules into the cytoplasm through simple diffusion.

Facilitated diffusion via proteins

Transport mediators are the membrane components that are responsible for facilitated diffusion. 

Hydrophilic (water loving) as well as some highly polar m molecules cannot pass through the membrane because it is hydrophobic.

The movement across the membrane can be accomplished in one of two ways: one using carrier proteins and the other using channel proteins.

Facilitated diffusion via carrier proteins

In case of transporters or carrier proteins which are embedded in the cellular membrane are used.

Some substances in the extracellular matrix have a high affinity for these proteins by binding to them. The carrier proteins bind to the molecules, causing conformational changes in the molecules that allow them to pass through the membrane and into the cytosol. 

This facilitated diffusion process is used by larger molecules, such as enzymes. 

Facilitated diffusion via channel proteins

In the case of channel proteins, the transmembrane proteins present in the membrane, act as a channel (pore) in the membrane, allowing molecules to pass through. These channels run through the plasma membrane, linking the cytosol to the outside environment or crossing the biological membranes of different cellular organelles. Molecules transfer similar-charged ions across transmembrane channels produced by protein complexes. The species that pass-through channels, also called the transmembrane proteins, are almost always ions, as they span across membranes.

Aquaporins are a type of channel protein that transports water across the plasma membrane at a very high rate.