CELL TO CELL TRANSPORT – FACILITATED DIFFUSION

The movement of materials on the other side of cell membranes is called cell transport. Cell transport includes two types, namely active transport and passive transport. Active transport requires energy to proceed whereas passive transport does not require energy to proceed. Passive transport is categorized into diffusion, facilitated diffusion, and osmosis.

Passive transport appears when substances move across the plasma membrane without the interference of energy from the cell. The energy is not needed for transport because the substances travel from an area of higher concentration to an area of lower concentration.

When water is mixed with other molecules, the mixture is called a solution. In a solution, the water is the solvent and the dissolved substance is the solute. A solution is considered by the solute. For example, a sugar solution means a solution of water as solvent and sugar as the solute. 

PASSIVE TRANSPORT

Passive transport is a type of transport of small molecules through membranes that do not need the energy to move substances across the cell membranes. Instead of using the energy of the cell, like active transport, the passive transport depends on the process to initiate the movement of substances across cell membranes.

 The main kinds of passive transport are simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

Simple Diffusion is the movement of material from a region of high concentration to a region with lower concentration. The rate of movement of the diffusing substance is proportional to the concentration gradient.  The difference in concentration between the two regions is termed as the concentration gradient and diffusion continues until the gradient has been removed. The exchange of gases during respiration is an example of simple diffusion.

Osmosis

Osmosis is the movement of molecules of water across a selectively permeable membrane. It is the net transport of water molecules through a partially permeable membrane from a high-water potential solution to a low-water potential substance. 

The osmosis solutions are classified into three types: the isotonic solution, hypotonic solution, and hypertonic solution.

Isotonic solution is the solution when the extracellular solute concentration is brought in parity with the concentration inside the cell. In the Isotonic solution, the water molecules move between the solutions, but the rates remain the same from both sides, thus the water movement is balanced between the inside of the cell as well as the outside of the cell.

A hypotonic solution is a solution when the solute concentration outside the cell is of a lesser amount than the concentration inside the cell. In hypotonic solutions, the water travels into the cell, down its concentration gradient from higher to lower water concentrations. This causes the cell to swell up. Cells without a cell wall, such as animal cells, could burst out in this solution. 

A hypertonic solution is a solution when the solute concentration is higher than the concentration within the cell. In a hypertonic solution, the water moves out causing the cell to shrink.

Facilitated Diffusion

Facilitated diffusion is the diffusion of solutes through transport proteins present in the plasma membrane. Even though facilitated diffusion includes transport proteins, it is still passive transport because the solute moves down the concentration gradient. Facilitated diffusion is also called carrier-mediated osmosis and it simply means the movement of molecules across the cell membrane through special transport proteins that are present in the plasma membranes. 

FACILITATED DIFFUSION

The transport of substances from a higher concentrated region to a lower concentrated region through a plasma membrane with the help of a transport molecule. The substances travel along the direction of their concentration gradient therefore the chemical energy is not directly required. 

The transport of glucose and amino acids, gas transport, and transport of ions are examples of facilitated diffusion. Facilitated diffusion is crucial because it adjusts the inputs and output of the cell. The plasma membrane is the cell organelle present in the cell structure that is accountable for the selection of movements of substances.

CHARACTERISTICS OF FACILITATED DIFFUSION

• The rate of transport is high in this type of diffusion.

• There are limited carriers that are fully active and hence, saturation might occur which leads to a reduction in transport through the membrane.

• Carriers are subjective for the substance that they choose to transport.

• Structures with a similar structure as substrate might occur as competitors with other substances.

• Facilitated diffusion is sensitive to temperature. The nature of the carriers can be changed at high temperatures. 

FACTORS AFFECTING THE RATE OF FACILITATED DIFFUSION

Concentration Gradient

Concentration gradient across the membrane is a necessary factor that regulates the process of diffusion. The diffusion always occurs from an area of high concentration to an area of lower concentration. The gradient produces potential energy which increases with the difference in concentration, which results in faster diffusion.

Temperature

The rate of carrier transport increases more speedily with temperature. The increase in temperature increases the rate of response between the carrier proteins and the ion in the molecules.

Saturation

The number of carrier proteins in the membrane is limited, hence, once all the proteins are bound, they are no longer able to bind more molecules. The rate of diffusion at this stage cannot be changed even with the change in the concentration gradient.

Selectivity

There is generally a mutual relationship between the rate of transportation and the selectivity of the process of transport. It is because selectivity is often reached by binding sites that separate the solutes available. These selective and intense interactions lead to reduce transport.

IMPORTANCE OF FACILITATED DIFFUSION

Every molecule cannot travel across the cell membranes. To cross the membrane, the molecules should be small and non-polar to cross the membrane. For example, glucose is a large molecule that cannot travel across the cell membrane. 

Charged ions like sodium, potassium, and calcium are prevented by the cell membrane. Amino acids and nucleic acids are polar and are too large to move across the cell membrane. Also, it is difficult at times to move the water molecules in large amounts to another side of the membrane.

For transfer of substances across the membrane, certain important membrane proteins or the transmembrane proteins are required which are channel proteins and carrier proteins.

TYPES OF FACILITATED DIFFUSION

The types of transfer proteins involved in facilitated diffusion are channel proteins and carrier proteins.

Channel Protein: A channel protein is a unique arrangement of amino acids which are implanted in the membrane of the cell, providing a hydrophilic gateway for water and small, polar ions. Each channel protein has a size and shape which contains the most specific molecules.

A basic channel protein is seen at the bottom, fixed within the membrane. The small green hexagons, called the ions, travel through the channel protein. They move from a region of high concentration to a region of a lower concentration.

Carrier Protein: Carrier proteins are proteins that carry substances from one part of a membrane to the other part. Carrier proteins are found mainly in the membrane of the cell. They may be found in the membranes of other organelles such as the mitochondria, chloroplasts, nucleolus, etc.

Carrier proteins have a binding place that will only bind the substance which they are required to carry. For example, the sodium-potassium pump has binding sites that will only bind to those ions.

Once the carrier protein has bound to a sufficient amount of its substance targeted, the protein changes its forms to carry the substance from one part of the membrane to the other part. 

CONCLUSION

In this article, we have learned the definition of the cell to cell transport that the movement of materials on the other side of cell membranes is called cell transport. We have also learned the two types of cell transport namely, active transport and passive transport. Passive transport is further segregated into three different types: diffusion, osmosis, and facilitated diffusion.

Facilitated diffusion as the transport of substances from a higher concentrated region to a lower concentrated region through a plasma membrane with the help of a transport molecule is explained in the article. The importance of facilitated diffusion and the various factors affecting diffusions like selectivity and temperature has also been discussed.