Difference Between Active and Passive Transport

Active and passive transport are systems that are meant for transporting molecules through the cell membrane. A cell membrane is a multi-task component that gives structure to the cell while protecting the cytosolic content from the outer environment. The movement of molecules from in and out of the cell is guided by the phospholipid bilayer, sustaining a delicate homeostasis state of the cell. The phospholipid bilayer is semi-permeable in nature, permitting certain molecules to freely pass the membrane through a concentration channel and certain molecules to use distinct structures in order to travel the membrane and others to travel the membrane by consuming cellular energy. The key difference between active and passive transport is that active transport forces molecules against the concentration gradient with help of ATP energy whereas passive transport lets the molecules pass across the membrane through a concentration channel, requiring no cellular energy.

The main purpose of both transport systems is to transport molecules and ions across the cellular membrane. The external layer is made up of the phospholipid bilayers, which preserve the homeostasis condition of the cell and regulate the entry of the materials. Few particular proteins along with a semi-permeable membrane support the entrance of the molecules. In simple words, Active and passive transport are the two key biological processes that play a vital role in supplying nutrients, water, oxygen, and other vital molecules to cells and also by eliminating waste products. Both active and passive transport works for a similar cause, but with a different action.

What is Active Transport?

Active transport is the movement of molecules like water oxygen and other important molecules across the membrane against the concentration channel with the help of enzymes and usage of cellular energy. It is required for the gathering of molecules like amino acids, glucose, and ions inside the cell in high concentrations.

Active transports are of two types:

• Primary Active Transport: In the primary active transport, for transporting the molecules it uses chemical energy to push the molecule.

• Secondary Active Transport:  In the secondary active transport, proteins present in the cell membrane use the electromagnetic gradient to move across the membrane.

• Primary Active Transport

During primary active transport, the existence of molecules in the extracellular fluid that is necessary by the cell is recognized by the specific transmembrane proteins on the cell membrane, which acts as pumps for transferring the molecules. These transmembrane proteins are run by ATP. The primary active transport is of utmost obvious in the sodium/potassium pump (Na⁺/K⁺ ATPase), which regulates the resting potential of the cell.

The energy-free hydrolysis of ATP is used to force three sodium ions out of the cell and two potassium ions into the cell. Here, sodium ions are shifted from a lower concentration of 11 mM to a higher concentration of 146 mM. Potassium ions are transferred from a 146 mM concentration inside the cell to a 4 mM concentration of the extracellular fluid. The proton/potassium pump (H⁺/K⁺ ATPase) is present in the lining of the stomach, preserving an acidic environment inside the stomach. Omeprazole is a type of proton/potassium pump inhibitor that reduces acid reflux inside the stomach. Both oxidative phosphorylation and photophosphorylation of electron transport chains use the help of primary active transport to generate a reducing power as well.

• Secondary Active Transport

Secondary active transport is governed by an electrochemical gradient. Here, channels are made by pore-forming proteins (Pore are small holes). A simultaneous movement of another molecule against the concentration gradient can be seen with the secondary active transport. Therefore, the channel proteins which are involved in the secondary active transport can be recognized as co-transporters. There are two kinds of co-transporters: symporters and antiporters. 

Specific ions and the solute are shifted in opposite directions by antiporters. Calcium/Sodium exchanger, which permits the restoration of calcium ion concentration in the cardiomyocyte after the action potential, is the most common example for antiporters co-transporter. Ions are transferred through the concentration gradient while the solute is transferred against the concentration gradient by symporters. Here, both molecules are shifted in the same direction across the cell membrane. SGLT2 is a symporter co-transporter that transports glucose into the cell along with the sodium ions. The role of symporter and antiporter is shown in the image below.

Importance of Active Transport: 

In eukaryotic cells, sugar, lipids, and amino acids want to enter the cell by protein pumps, which require active transport. These items either cannot diffuse or diffuse too slowly for existence. Active transport is essential for the entry of large, insoluble molecules into the cell.

What is Passive Transport?

Passive transport is the transport of molecules across the membrane through a concentration gradient without the use of cellular energy by movement. It uses natural entropy to transport molecules from a higher concentration to a lower concentration until the concentration becomes balanced. Then, there will be no net transport of molecules at the equilibrium. 

Four Main Kinds of Passive Transport are Found: 

• Osmosis

• Simple diffusion

• Facilitated diffusion

• Filtration

• Osmosis

In the process of osmosis, the water, and other molecules or substances are transported through the selectively permeable cell membrane. There are many aspects that affect this transport. One of the main factors is the cell having less negative water potential and other factors are the solute potential of a molecule and the pressure potential of a cell membrane.

• Simple Diffusion

In the process of simple diffusion, the transportation of molecules or solute across a permeable membrane this process is known as simple diffusion. Mainly non-polar molecules use simple diffusion, to maintain the better flow of molecules the distance should be less.

• Facilitated Diffusion

Facilitated diffusion is the natural passive transportation of molecules or ions across the cell membrane through the specific-trans membrane of integral proteins. The molecules, which are big and insoluble, need a carrier molecule for their transportation through the plasma membrane. This process does not require any cellular energy or external energy.

• Filtration

The cardiovascular system (CVS) in the human body produces hydrostatic pressure, which helps water and other soluble biochemical molecules or substances to travel across the cell membrane. This process is named filtration because the cell membrane permits only substances that are soluble and could freely pass through the membrane’s pore.

Passive transport across the membrane is shown in the image below:

During facilitated diffusion, different transport proteins are used to monitor the movement of polar molecules and big ions. These carrying proteins are glycoproteins and are specific to a certain protein. The GLUT4 is a glucose transporter that helps to transport glucose from the bloodstream into the cell. 

It is typically found in fat and skeletal muscles. Three sorts of transport proteins are engaged in facilitated diffusion: channel proteins, carrier proteins, and aquaporins. Channel proteins make hydrophobic channels across the membrane, permitting the selected hydrophobic molecules to travel through the membrane. Certain channel proteins are opened at all times, and several are gated like ion channel proteins. Aquaporins permit water to cross the membrane swiftly. Carrier proteins alter their shape, transporting target molecules across the membrane.

Importance of Passive Transport

It maintains balance in the cell. Wastes like carbon dioxide, water, etc. are diffused out and excreted; nutrients and oxygen diffuse in to be used by the cell. Passive transport also allows the maintenance of a delicate homeostasis condition between the cytosol and extracellular fluid.

Difference Between Active and Passive Transport

Conclusion

Active and passive transport are the two systems of transporting molecules across the cell membrane. Active transport pumps molecules or substances against a concentration gradient using cellular energy. In primary active transport, ATP is used in form of the energy. In secondary active transport, the electrochemical gradient is used to transport molecules across the membrane. Nutrients are concentrated into the cell with the help of active transport. Passive diffusion also allows small, non-polar molecules or substances to travel across the membrane. It only happens through a concentration gradient. Therefore, no energy is utilized by the system. However, the key difference between active transport and passive transport is their mechanisms of transporting molecules or substances across the membrane.