Difference Between Active and Passive Diffusion

Main Difference – Active vs Passive diffusion

The membrane of the cell serves as a semi-permeable barrier, controlling the movement of molecules across it in order to maintain a constant cytosolic environment. The phospholipid bilayer allows some molecules to freely pass the cell membrane through its concentration gradient and some other molecules to use special structures in order to pass the membrane. These structures are transmembrane proteins. The rest of the molecules would pass the cell membrane by utilizing cellular energy. Active and passive diffusion are two methods that are involved in the transport of molecules across the cell membrane. The main difference between active and passive diffusion is that active diffusion pumps molecules against the concentration gradient by using ATP energy whereas passive diffusion allows the molecules to pass the membrane through a concentration gradient. Hence, passive diffusion does not utilize cellular energy for the transport of molecules.

Key Areas Covered

1. What is Active Diffusion
     – Definition, Types of molecules, Transport Mechanism
2. What is Passive Diffusion
     – Definition, Types of Molecules, Transport Mechanism
3. What are the Similarities Between Active and Passive Diffusion
     – Outline of Common Features
4. What is the Difference Between Active and Passive Diffusion
     – Comparison of Key Differences

Key Terms: ATP, Cell Membrane, Electrochemical Gradient, Facilitated Diffusion, Osmosis, Primary Active Diffusion, Secondary Active Diffusion, Simple Diffusion

What is Active Diffusion

Active diffusion refers to the movement of molecules or ions from an area of lower concentration to a higher concentration with the assistant of carrier proteins in the cell membrane, utilizing cellular energy. Cells accumulate glucose, amino acids, and ions by means of active diffusion. Primary active diffusion and secondary active diffusion are the two types of active diffusion mechanisms used by cells.

Primary Active Diffusion

Primary active diffusion refers to the transport of molecules against the concentration gradient by utilizing cellular energy in the form of ATP. Therefore, primary active transport utilizes carrier protein molecules powered by ATP. The primary active transport is most obvious in the sodium/potassium pump (Na+/K+ ATPase), which maintains the resting potential of the cell. The energy released by the hydrolysis of ATP is used to pump three sodium ions out of the cell and two potassium ions into the cell. Here, sodium ions are transported from a lower concentration of 10 mM to a higher concentration of 145 mM. Potassium ions are transported from a 140 mM concentration inside the cell to a 5 mM concentration of the extracellular fluid. The action of sodium/potassium pump is shown in figure 1.

Figure 1: Sodium-Potassium Pump

The proton/potassium pump (H+/K+ ATPase) is found in the lining of the stomach, maintaining an acidic environment inside the stomach. Omeprazole is a proton/potassium pump inhibitor, reducing the acid reflux inside the stomach. Both oxidative phosphorylation and photophosphorylation of electron transport chain use primary active transport to create a reducing power as well.

Secondary Active Diffusion

Secondary active diffusion refers to the transport of molecules against the concentration gradient by the energy released from an electrochemical gradient. Here, the transmembrane proteins are made by channel proteins (pore-forming proteins). A simultaneous movement of another substance against the concentration gradient is observed with the secondary active transport. Hence, the channel proteins involved in the secondary active diffusion can be identified as cotransporters. The two types of cotransporters are antiporters and symporters. The action of the cotransporters is shown in figure 2.

Figure 2: Cotransporters

Particular ion and the solute are transported in the opposite directions by antiporters. Sodium/calcium exchanger, which allows the restoration of calcium ion concentration in the cardiomyocyte after the action potential, is the most common example of antiporters. Ions are transported through the concentration gradient while the solute is transported against the concentration gradient by symporters. Here, both molecules are transported in the same direction across the cell membrane. SGLT2 is a symporter that transports glucose into the cell along with the sodium ions.

What is Passive Diffusion

Passive diffusion refers to the movement of ions or molecules across the cell membrane through a concentration gradient without utilizing the cellular energy. Therefore, passive diffusion uses the natural entropy of the molecules to pass through the cell membrane. The movement of molecules occurs until their concentration becomes equal on both sides. The four main types of passive diffusion are osmosis, simple diffusion, facilitated diffusion, and filtration.

Simple Diffusion

The simple movement of molecules across a permeable membrane is called simple diffusion. Small, non-polar molecules use simple diffusion. The diffusion distance should be less in order to maintain a better flow. Simple diffusion is shown in figure 3.

Figure 3: Simple Diffusion

Facilitated Diffusion

Polar molecules and large molecules pass through the cell membrane by facilitated diffusion. The three types of transport proteins involved in the facilitated diffusion are channel proteins, aquaporins, and carrier proteins. Channel proteins make hydrophobic tunnels across the membrane, allowing the selected hydrophobic molecules to pass through the membrane. Some channel proteins are opened at all times, and some are gated like ion channel proteins. Aquaporins allow water to cross the membrane quickly. Carrier proteins change their shape, transporting target molecules across the membrane. Facilitated diffusion is shown in figure 4.

Figure 4: Facilitated Diffusion

Filtration

Filtration is the movement of solutes along with water due to the hydrostatic pressure generated by the cardiovascular system. It occurs in Bowman’s capsule in the kidney. Filtration is shown in figure 5.

Figure 5: Filtration

Osmosis

Osmosis is the movement of water across a selectively permeable membrane. It occurs from a high water potential to a low water potential. The effect of the osmotic pressure on red blood cells is shown in figure 6. Red blood cells in a hypertonic solution may lose water from cells. Hypertonic solutions contain a higher concentration of solutes than the cytoplasm of red blood cells. Isotonic solutions contain a similar concentration of solutes as in the cytoplasm. So, the net movement of water in and out of the cell is zero. Hypotonic solutions contain low solute concentrations than the cytoplasm. Red blood cells receive water from hypotonic solutions.

Figure 6: Osmotic Pressure on Red Blood Cells

The lipid soluble molecules passively pass through the phospholipid bilayer. Water soluble molecules pass through the cell membrane by means of transmembrane proteins.

Similarities Between Active and Passive Diffusion

• Both active and passive diffusion are involved in the transport of molecules through the cell membrane.
• Both active and passive diffusion use transmembrane proteins to transport molecules.

Difference Between Active and Passive Diffusion

Definition

Active Diffusion: Active diffusion is the movement of molecules or ions from an area of lower concentration to a higher concentration with the assistant of carrier proteins in the cell membrane, utilizing cellular energy.

Passive Diffusion: Passive diffusion is the movement of ions or molecules across the cell membrane through a concentration gradient without utilizing the cellular energy.

Cellular Energy Usage

Active Diffusion: Active diffusion utilizes cellular energy to transport molecules across the cell membrane.

Passive Diffusion: Passive diffusion does not utilize cellular energy.

Type of Transport

Active Diffusion: Primary active diffusion and secondary active diffusion are the two types of active diffusion.

Passive Diffusion: Simple diffusion, facilitated diffusion, filtration, and osmosis are the four types of passive diffusion.

Transporting Molecules

Active Diffusion: Ions, large proteins, complex sugars as well as cells are transported by active diffusion.

Passive Diffusion: Water soluble molecules like small monosaccharides, lipids, sex hormones, carbon dioxide, oxygen, and water are transported by passive diffusion.

Role

Active Diffusion: Active diffusion allows molecules to pass the cell membrane, disrupting the equilibrium established by the diffusion.

Passive Diffusion: A dynamic equilibrium of water, nutrients, gasses, and wastes is maintained by passive diffusion between cytosol and extracellular environment.

Importance

Active Diffusion: Active transport is required for the entrance of large, insoluble molecules into the cell.

Passive Diffusion: Passive diffusion allows the maintenance of a delicate homeostasis between the cytosol and extracellular fluid.

Conclusion

Active diffusion and passive diffusion are the two types of membrane transport mechanisms used by cells. Both processes occur through the cell membrane. Cell membrane serves as a selectively permeable barrier, only allowing small, uncharged molecules to pass through the cell membrane freely. Large molecules, as well as charged ions, are passed through the cell membrane through active diffusion. Small, uncharged molecules pass through the passive diffusion. Since active diffusion occurs against the concentration gradient, it uses cellular energy in the form of ATP or electrochemical gradient. But, passive diffusion occurs through a concentration gradient and does not require cellular energy for the transport of molecules. The main difference between active and passive diffusion is the type of molecules pass and the utilization of cellular energy by each process. 

Reference:

Image Courtesy:

1. “Blausen 0818 Sodium-PotassiumPump” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
2. “Cotransporters” By Wikimedia user: Lupask – Wikimedia Commons (CC BY-SA 3.0) via Commons Wikimedia    3. “Scheme simple diffusion in cell membrane-en” By LadyofHats Mariana Ruiz Villarreal – Own work (Public Domain) via Commons Wikimedia
4. “Blausen 0394 Facilitated Diffusion” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
5. “Filtration diagram” By LadyofHats Mariana Ruiz (Public Domain) via Commons Wikimedia
6. “Osmotic pressure on blood cells diagram” By LadyofHats (Public Domain) via Commons Wikimedia