Cell Transport and Its Types

The movement of a substance across the cell membrane is known as cell transport. The substance can move either in or out of the cells. There are various molecules that pass through the plasma membrane or the cell membrane. The plasma membrane is highly selective in nature. 

Introduction to types of transport

Types of transport across cell membranes are listed below.

1. Active Transport

2. Passive Transport

3. Facilitators   

Active Transport: Active transport requires energy in the form of ATP, solute from lower concentration to higher concentration transport through cell membrane.  

Passive Transport: Passive transport does not require any energy and it transmits solute from high concentration to lower concentration through the transport through cell membrane.  

Facilitators: The facilitators will allow the diffusion process to take place through the membrane made up of glycoprotein.  

Types of Active Transports

The active transports are classified into four types based on their action mechanism. They are listed below. 

1. Antiport Pumps

2. Symport Pumps

3. Endocytosis

4. Exocytosis 

Antiport Pumps: The transmembrane is made up of co-transporter protein. This will pump a substance in one direction and transport the substance to another direction. The ATP molecules are enough to perform this process.  An example of an antiport pump is the sodium-potassium pump. 

Symport Pumps: The molecules of two different substances can move in the same direction related to each other through the protein transmembrane. Here, the movement of molecules or substances occurs from higher concentration to lower concentration. An example of symport pumps transport is a sodium-glucose transport protein

Endocytosis: The larger molecules or large substances of extracellular fluid will enter into the cell through the process of endocytosis. The cell utilizes its protein membrane to fold the membrane into the pockets. The pocket formed around the larger molecules enters the cell. These membrane packets, which carry materials inside the cells are termed vesicles. 

Exocytosis: The process of exocytosis is the opposite of endocytosis. The vesicle present inside the cell moves outside of the cell membrane is known as exocytosis. This is commonly occurring, when the cell needs to export a molecule, enzymes and hormones  In eukaryotic cells, the protein products are made up of endoplasmic reticulum. The Golgi apparatus helps in packaging and removing these substances.

Transport Mechanism Across Cell Membrane 

The chemical structure of the cell membrane is flexible, it is because of the rapid growth of cells and cell division. The cell membrane is also known as a formidable barrier. This allows and blocks the dissolved substances or solutes to pass through the membrane. The Lipid soluble molecules and some other molecules can fill the membrane, but the bilayer lipid effectively repels the entry of larger water-soluble molecules. In order to make the cell live, the electrically charged ions must be imported or exported from the cell. 

Transport systems

The transport systems are carried out by different intrinsic proteins to perform the transportation of vital substances in cells. The types of intrinsic proteins are open channels, facilitators, and pumps. The open channel allows the ion to directly diffuse into the cell. Facilitators will allow the little chemical transformation. It helps to diffuse solutes to pass the lipid screen. The pump will force solutes to pass through the membrane if they are not concentrated enough to diffuse into the cell membrane spontaneously. The large particles, which are pumped or diffused can occur only by opening or closing the membrane. 

Diffusion in the cell membrane

The major principle behind the movement of solutes transport across the cell membranes is based on the diffusion process. According to the diffusion process, dissolved substances transported across membranes through a concentration gradient. This does not require external energy to move from a higher concentration to a lower concentration. This diffusion continues and starts decreasing gradually till it attains the equilibrium state. The random diffusion occurs from both places at an equal ratio during the equilibrium state.

A solute at a high concentration has high free energy. These are capable to do more work than the solute at low concentration. While performing the diffusion process, the solutes lose their free energy. So, the solutes are unable to return to the high concentration, after attaining the lower concentration or equilibrium state. But, it is possible to perform the transport of ions across the cell membrane to higher concentrations through an ion pump.  

Role of pumps in transport

For many substances, the concentration present inside the cells is different from the concentration present outside the cell. This can create the concentration gradient and solutes start diffusing from higher concertation to lower concentration of cell through the lipid bilayer, membrane channel, and diffusion facilitators. The changes in protein help to take place to facilitate diffusion. For the healthy cell function, some solutes in each side of the membrane must remain at different concentrations. If the cells undergo diffusion and approach equilibrium, they must be pumped back to their gradient concentration using active transport. The membrane proteins, which serve as pumps will provide the energy for transport across plasma membrane for cell metabolism or diffusion of other solutes.

Conclusion

Cell transport is a very important phenomenon for the working of cells. Different types of substances are transported across cells via different means. In cells, diffusion is the simplest means of transport. Diffusion is the movement of substances from an area of high concentration to an area of low concentration. It is one of the slowest means of transport across cells. Diffusion does not help in the transport of substances across a concentration gradient. For that purpose, we need a pump. Sodium-potassium pumps are the means by which active transport takes place. The substances are transported against the concentration gradient at the expense of energy.