There is a constant motion of molecules and ions dissolved in water, moving about randomly. This random motion causes these substances to move neatly from regions where their concentration is high to regions where their concentration is lower, a process called diffusion.Net diffusion-driven movement will continue until the concentrations are the same in all regions. By filling a jar to the brim with ink, capping it, placing it at the bottom of a water bucket, and then carefully removing the cap, you can demonstrate diffusion. The ink molecules will spread out of the jar until the bucket and jar have a uniform concentration. This uniformity in molecular concentration is a kind of equilibrium.

Diffusion is the net movement of substances through random spontaneous movement to regions of lower concentration. It tends to uniformly distribute substances. Membrane transport proteins allow diffusion through the plasma membrane only for certain molecules and ions.

Diffusion occurs because of the free energy of molecules; that is, they are always in motion. The molecules in a solid move very slowly; those in a liquid move faster; and those in a gas move even faster, as ice absorbs heat energy, melts, and then evaporates. Imagine a green sugar cube in a glass of water at the bottom (green so we can see it). The sugar molecules collide with each other or the water molecules as the sugar dissolves, and the green colours appear to rise in the glass. These collisions spread the sugar molecules until they are evenly dispersed between the water molecules (this would take a long time) and eventually the water becomes completely green. The molecules are still moving, but as some are moving up, others are moving down, and so on. A balance (or a stable state) is thus reached.

Diffusion is a very slow process, but can be an effective mechanism for transportation over microscopic distances. The oxygen and carbon dioxide gasses move in the body by diffusion. For example, in the lungs, there is a high oxygen concentration in the alveoli (air sacs) and a low oxygen concentration in the blood of the surrounding pulmonary capillaries. The opposite is true for carbon dioxide: low air concentration in the alveoli and high blood concentration in the capillaries of the lungs.

These gasses are spreading in opposite directions, each moving from where there is more to less. Oxygen diffuses to circulate throughout the body from the air into the blood. Carbon dioxide spreads to the exhaled air from the blood.

Facilitated Transport

Many cell - needed molecules, including glucose and other sources of energy, are polar and cannot pass through the phospholipid bilayer's nonpolar interior. These molecules enter the cell in the plasma membrane through specific channels. The inside of the channel is polar and thus “friendly” to the polar molecules, facilitating their transport across the membrane. Each type of the biomolecule that is transported across the plasma membrane has its own type of transporter. Each channel is said to be selective for that type of the molecule, and thus to be selectively permeable, as only molecules admitted by the channels it possesses can enter it. The plasma membrane of a cell has many types of channels, each selective for a different type of molecule. The word ‘facilitate’ means to help or assist. In facilitated diffusion, molecules move through a membrane from an area of greater concentration to an area of lesser concentration, but they need some help to do this.

Facilitated diffusion is the transportation by specific carriers of the molecules and ions across a membrane in the direction of the lower concentration of those molecules or ions.

Diffusion of Ions through Channels

One of the simplest ways for a substance to diffuse across a cell membrane is through a channel, as ions do. Ions are solutes (substances dissolved in water) with an unequal number of protons and electrons. Those with large amounts of protons are charged positively and are called cations. Ions with more electrons are charged negatively and referred to as anions. Since they are charged, the ions interact well with polar molecules such as water but are repelled by a phospholipid bilayer's non - polar interior. Therefore, ions cannot move between the cytoplasm of a cell and the extracellular without the assistance of membrane transport proteins. Ion channels have an interior that is hydrated and spans the membrane. Ions can diffuse in either direction through the channel without coming in contact with the hydrophobic tails of the membrane phospholipids, and the transported ions do not bind or interact with the channel proteins. The ions ' net movement direction is determined by two conditions: their relative concentrations on either side of the membrane, and the voltage across the membrane. Each of the channel types is specific to a specific ion, such as calcium (Ca++) or chloride (Cl–), or in some cases to a few ion types. Ionic channels play an essential role in the nervous system signalling.

Carriers are membrane protein classes, transport ions, and other membranes - wide solutes such as sugars and amino acids. Carriers are like channels, they are specific to a specific type of solution and can transport substances in either direction across the membrane. The solution on the cytoplasmic side of the membrane is more likely to bind to the carrier and release on the extracellular side if the cytoplasm concentration is higher. If there is a higher concentration of the extracellular fluid, the net movement will be from outside to inside. Thus, net movement always takes place from high to low concentration areas, just as it does in simple transport, but carriers facilitate the process. This transport mechanism is therefore called facilitated transport.

Facilitated transport in Red Blood Cells

The membranes of vertebrate red blood cells (RBCs) can find various examples of facilitated transport by carrier proteins. For example, one RBC carrier protein carries in each direction a different molecule: Cl–in one direction and HCO3–in the opposite direction. In transporting carbon dioxide in the blood, this carrier is important.

Another important facilitated transport carrier in red blood cells is the glucose transporter. RBC keep their internal concentration of glucose low through a chemical trick: they immediately add a phosphate group to any entering glucose molecule, converting it to a highly charged glucose phosphate that cannot pass back across the membrane. This maintains a steep concentration gradient for glucose, favouring its entry into the cell. Instead, the transmembrane protein appears to bind the glucose and then flip its shape, dragging the glucose through the bilayer and releasing it on the inside of the plasma membrane. Once it releases the glucose, the glucose transporter reverts to its original shape. Figure 1 describes the difference between simple transport and facilitated transport.

Figure 1: Difference between simple transport and facilitated transport

Transport through Selective Channels Saturates

The characteristic of selective channel transport is that its rate is saturable. In other words, if a substance's concentration gradient is gradually increased, it will also increase its transport rate to a certain point and then level off. Further gradient increases will not result in any additional rate increases. The scientific explanation for this kind of observation is that the membrane contains a limited number of carriers.

Facilitated transport has three main features:

• It is specific. Any given carrier transports only certain molecules or ions.

• It is passive. The direction of net movement is determined by the relative concentrations within and outside the cell of the transported substance.

• It saturates. If all relevant protein carriers are in use, increases in the concentration gradient do not increase the transport rate

Sl. No.	Simple Diffusion	Facilitated Diffusion
1	Simple diffusion does not require a carrier molecule's assistance.	Facilitated diffusion can only take place with the help of a carrier molecule.
2	Simple diffusion speeds are relatively slow.	The speed of diffusion facilitated is relatively fast.
3	Simple diffusion is not specific to the solution.	Facilitated diffusion is always a specific solution.
4	A molecule inhibitor cannot inhibit the simple process of diffusion.	Specific inhibitors that bind to the carrier molecules can inhibit the facilitated diffusion.
5	Simple diffusion is a passive process.	Facilitated diffusion can be an active or passive process.
6	Simple diffusion does not require ATP's energy.	Facilitated diffusion may require or may not require ATP energy.
7	The molecules can only pass in the direction of the concentration gradient in simple diffusion.	The molecules can pass in the direction and opposite of the concentration gradient in facilitated diffusion.
8	Simple diffusion allows the passage through the plasma membrane of only small and non - polar molecules.	Facilitated diffusion allows large and polar molecules to pass through the plasma membrane.