Difference Between Diffusion and Active Transport

Main Difference – Diffusion and Active Transport

Diffusion and active transport are two types of methods involved in the movement of molecules across the cell membrane. Cell membrane serves as a semi-permeable barrier to molecules which pass through it. On that account, only small nonpolar molecules are capable of freely moving across the cell membrane; movement of large and polar molecules across the cell membrane is restricted. It is active transport that facilitates the transportation of large and polar molecules. The main difference between diffusion and active transport is that diffusion is a passive transport method in which molecules move across the cell membrane through a concentration gradient whereas active transport requires cellular energy in order to transport molecules against the concentration gradient.

Key Areas Covered

1. What is Diffusion
      – Definition, Types, Process
2. What is Active Transport
      – Definition, Types, Process
3. What are the similarities between Diffusion and Active Transport
      – Common Features
4. What is the difference between Diffusion and Active Transport
      – Comparison of key differences

Key Terms: Active Transport, Antiporters, Carrier Proteins, Channel Proteins, Cotransporters, Diffusion, Facilitated Diffusion, Osmosis, Primary Active Transport, Secondary Active Transport, Simple Diffusion, Symporters

What is Diffusion

Diffusion is the passive movement of molecules along a concentration gradient of a higher concentration to a lower concentration. Three major diffusion methods can be identified: simple diffusion, facilitated diffusion, and osmosis.

Simple Diffusion

Simple diffusion is an unassisted type of diffusion in which a particle moves from a higher to a lower concentration. Once, the molecules are evenly distributed by simple diffusion, the molecules on the either sides of the cell membrane achieve an equilibrium where no net movement of molecules is observed. Small, nonpolar molecules like oxygen, carbon dioxide, and ethanol move across the cell membrane by simple diffusion.

Facilitated Diffusion

Facilitated diffusion is the transport of substances across a biological membrane through a concentration gradient by means of a carrier molecule. Large ions and polar molecules which are dissolved in water are transported by specific transmembrane proteins in the cell membrane. Polar ions diffuse through transmembrane channel proteins and large molecules diffuse through transmembrane carrier proteins. Aquaporins are the other type of transmembrane proteins, which transport water through the cell membrane quickly.

Figure 1: Facilitated Diffusion through Carrier Proteins

Osmosis

Osmosis refers the free diffusion of water molecules across the cell membrane through an osmotic pressure.

What is Active Transport

Active transport is the movement of particles across a cellular membrane from a lower to a higher concentration by the use of metabolic energy. Enzymes bound to the cellular membranes and metabolic energy in the form of ATP assist active transport. Primary active transport and secondary active transport are the two types of active transport. Molecules required by the cell are specifically recognized by transmembrane proteins in the cell membrane. These transmembrane proteins are powered by ATP. The sodium/potassium pump (Na+/K+ ATPase), which maintains the resting potential of the nerve cells, and the proton/potassium pump (H+/K+ ATPase), which maintains an acidic environment in the stomach, are examples of primary active transport. Secondary active transport is powered by an electrochemical gradient. The transmembrane proteins, which are involved in the secondary active transport are called cotransporters. Two types of cotransporters are found: antiporters and symporters. In symporters, ion and the specific solute are transported in the same direction, either into the cell or out of the cell. In antiporters, ion and the specific solute are transported in opposite directions. Active transport is shown in figure 2.

Figure 2: Active Transport

Similarities Between Diffusion and Active Transport

• Both diffusion and active transport allow the cell to maintain homeostasis inside the cell by transporting molecules across the cell membrane.

• Transportation of molecules occurs with the assistance of transmembrane proteins other than simple diffusion.

Difference Between Diffusion and Active Transport

Definition

Diffusion: Diffusion is the passive movement of molecules along a concentration gradient of higher concentration to a lower concentration.

Active Transport: Active transport is the movement of particles across a cellular membrane from a lower to a higher concentration by the use of metabolic energy.

Concentration Gradient

Diffusion: Diffusion occurs through a concentration gradient.

Active Transport: Active transport occurs against a concentration gradient.

Metabolic Energy

Diffusion: Diffusion is a passive process, which does not require metabolic energy in order to transport molecules across the cell membrane.

Active Transport: Active transport requires metabolic energy in the form of ATP for the transportation of molecules across the cell membrane.

Types of Particles

Diffusion: Water, oxygen, carbon dioxide, small monosaccharides, sex hormones and other small, hydrophobic molecules are transported through the cell membrane by diffusion.

Active Transport: Proteins, ions, complex sugars, and large cells are transported through the cell membrane by active transport.

Equilibrium

Diffusion: No net movement of molecules is observed after the establishment of equilibrium on either side of the membrane.

Active Transport: No equilibrium of molecules is established in active transport.

Functions

Diffusion: Diffusion maintains a dynamic equilibrium of water, gases, nutrients, and wastes in and out of the cell.

Active Transport: Active transport allows the transportation of molecules like nutrients and wastes against the concentration gradient.

Examples

Diffusion: Oxygen moving from the airways and diffusion of molecules from the blood to the cells through the interstitial fluid are examples of diffusion.

Active Transport: Plants taking up nutrients from the soil, endocytosis, exocytosis, sodium/potassium pump, and secretion of a substance into the blood stream are the examples of active transport.

Conclusion

Diffusion and active transport are two methods of transporting molecules across the cell membrane. Diffusion is a passive process, but active transport requires metabolic energy or an electrochemical gradient for the transportation of molecules across the membrane. Simple diffusion occurs directly through the cell membrane. But, other diffusion methods, as well as active transport specifically, occur via transmembrane proteins. The main difference between diffusion and active transport is energy their requirement for the transportation of molecules across the cell membrane.

Reference:

1.”Diffusion.” Biology-Online Dictionary. N.p., n.d. Web. Available here. 13 June 2017. 
2.”Passive transport and active transport across a cell membrane.” Khan Academy. N.p., n.d. Web. Available here. 13 June 2017. 

Image Courtesy:

1. “Figure 05 02 05” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “OSC Microbio 03 03 Transport” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia

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