Facilitated Diffusion vs Active Diffusion: Detailed Comparative Analysis

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In this post you will find the key differences between facilitated diffusion vs active diffusion and simple diffusion.

Facilitated diffusion is the movement of substances form one medium to another medium along the concentration gradient while diffusion allows the substances in the same medium from region of higher to lower concentration while active diffusion involves expenditure of energy and is against the concentration gradient.

Facilitated Diffusion Vs Active Diffusion

Facilitated DiffusionActive Diffusion

It is the movement along concentration gradient through certain locations in the cell membranes without the cell expelling any energy. Naturally, smaller particles will show higher rate of facilitated diffusion as compared to the larger particles.

The passage of substances in the cell membrane is take place by protein transporters.

The transporters have specific configuration suitable to particular particles. Facilitated diffusion can show saturation effect like the active transport.

Protein transporters often form channels and aquaporins. Ion channels are specific for different ions, e.g., K+, Cl–, NO3–, PO43-, Mg2+.

Aquaporins or water channels are the specialized channels that  allows the passage of water through them. Each water channel is surrounded by eight proteins (also called aquaporins, though the term is also used for water channel).

Porins are proteins lined pores found in the outer membrane of mitochondria, plastids and several bacteria which allow passage of small sized solutes.

It is the mode of transport which involves the expenditure of energy, special carrier proteins and is often against concentration gradient (uphill transport). Pumps are the carrier proteins engaged in active transport because it occurs against a concentration gradient. It is believed that energy is used in activating the carrier proteins. The activated carrier picks up solute from the membrane surface and forms solute carrier complex.The complex moves in the membrane to bring the solute bearing part towards the other surface. Now the complex breaks and solute is released. The free carrier protein again changes its position to pick up solute again.
facilitated diffusion vs active diffusion
Facilitated diffusion in cell membrane, showing ion channels and carrier proteins from Wikipedia

Means of transport across the cell : Facilitated diffusion Vs Active Diffusion

Materials are transported across the cells in three ways. Both the types of diffusion constitute passive transport as they do not use energy and are always along the gradient.

Types of Facilitated Diffusion

  • Movement of particles of solute across a membrane independent of particles of other solutes is called Uniport.
  • Symport is the movement of the particles across the membrane in the same direction.
  • Movement of one solute in one direction is accompanied by transport of another solute in the opposite direction is called Antiport.

Active Transport

It is the mode of transport which involves the expenditure of energy, special carrier proteins and is often against concentration gradient (uphill transport).

Scheme sodium potassium pump en.svg
The action of sodium potassium pump is an example of Active transport from Wikipedia

Diffusion

It is the movement of molecules across the cell membrane along the concentration gradient without expending any energy. It is a passive movement.

 It is a slow process and occurs in mostly lipid soluble substances because the matrix of cell membranes if formed of lipids. Diffusion is commonly observed in gases and liquids. It may also occur in solids. Diffusion is important as it is the only means of transport of gases inside the plants. Concentration gradients, temperature, pressure, and membrane permeability all influence the rate of diffusion.

Diffusion Pressure

The pressure applied by the particles across the cell membrane to diffuse from higher to lower concentration of area. The concentration of particles in the diffusing substance determines the diffusion pressure.

Independent Diffusion

Two or more types of diffusing particles, such as oxygen and carbon dioxide, may be present in a system. Diffusion pressure is exerted by each diffusing substance. It’s known as partial pressure. Different molecules will move across the membrane according to their partial pressure. Independent diffusion allows different molecules to pass across the membrane depending upon the concentration or partial pressure.

Factors affecting Diffusion

The rate of diffusion is influenced by a number of things. Temperature, diffusing substance density, medium density, diffusion pressure, and diffusion distance are the factors to take into account.

  • Temperature

The rate of diffusion increases as the temperature rises. It’s because the diffusing particles kinetic energy has increased.

  • Density of substance

Density of a substance plays a crucial role in diffusion. Light weight substances passes across membrane faster while heavy weight substances take time and diffuse slowly. Molecules of liquid diffuse at very high rate compared to solids. The lighter gases will have a faster rate of diffusion than the heavier ones. For example, hydrogen diffuses faster  as compared to oxygen.

  • Density of the Medium

If the medium has a lower density, diffusion occurs more quickly. As the density of the medium increases, diffusion decreases.

  • Concentration or Diffusion Pressure

Diffusion is proportional to the concentration or diffusion pressure of the material. If the difference in concentration or diffusion pressure is greater, it is faster; if the difference in diffusion pressure is modest, it is slower. Diffusion will continue till the diffusing substance gets distributed evenly in the area. At the time of equilibrium, diffusion pressure will be uniform throughout.

  • Distance

If the distance over which diffusion is to occur is modest, net diffusion will be high. It will be low if the distance is great.

Importance of Diffusion

  • Intracellular Transport : Ions and other substances rapidly spread throughout the cytoplasm due to diffusion.
  • Osmosis: Only the solvent particles are allowed to diffuse in this sort of diffusion.
  • Transpiration: It is loss of water in the vapour form from the aerial parts of the plants. Transpiration occurs through diffusion.
  • Exchange of gases: During daytime, oxygen diffuses from photosynthesizing organs to the atmosphere while carbon dioxide of atmosphere diffuses into them.
  • Wetting of cell walls: Diffusion of water from xylem channels keeps the walls of plant cells moist.
  • Distribution: Diffusion brings about distribution of water and solutes inside the plants.
  • Aroma: Many plants produce aromatic substances for attracting particular type of animals for performing pollination.

Simple Diffusion

It is a type of passive diffusion which means it requires no ATP. It is along the electrochemical gradient i.e., from higher concentration of a substance to a lower concentration of the substance. There is no need for carrier. Osmosis is the subdivision of simple diffusion.

Only water or solvent molecules move according to the gradient of their chemical potential in osmosis.

0307 Osmosis 1
The process of osmosis over a semi-permeable membrane. The blue dots represent particles driving the osmotic gradient from Wikipedia

Chemical potential is the free energy of one mole of  a substance in a system under constant temperature and pressure. The chemical potential of pure water is considered as zero at normal temperature and pressure. In a system its chemical potential is called water potential. It is always less than zero,i.e., with a negative value.

Importance of Osmosis

  • Soil Water – Roots absorb water through the process of osmosis.
  • Cell Absorption – Cell absorb or lose water on the basis of their osmotic relations.
  • Cell organelles – For their optimum working, cell organelles must be fully distended. This they do so by having a higher osmotic concentration than the surrounding cytosol.
  • Cell Turgidity – Cells maintain their turgidity through having a proper osmotic concentration of solutes and osmotic absorption of water.
  • Cell Growth – Cells enlarge only in response to entry of water into them.
  • Soft Organs – Osmosis helps soft organs like juvenile stems, leaves, flowers, and squishy fruits to keep their shape and expanded form.
  • Growth of Seedling – The young radicle and plumule are able to come out of the seed and grow to form the seedling by maintaining a high osmotic concentration.
  • Root Growth – Young roots are able to penetrate the soil because of their rigidity.
  • Autochory – Many fruits burst on maturity due to osmotic changes in their interior.
  • Plant Movements – Osmotic alterations cause cyclic movements in Indian telegraph plant leaflets and seismonastic motions in Mimosa pudica.
  • Stomatal Movements – The osmotic changes in the  guard cells of stomata in respect to other epidermal cells cause stomata to open and close.
  • Desiccation – Plants protect themselves from desiccation or excessive loss of water by maintaining a high osmotic concentration of solutes.

Frequently Asked Questions

Q1. Give an example of Independent diffusion.

Example: It is seen during day time in the diffusion of water, carbon dioxide and oxygen between air and leaf interior through the stomata.

Q2. What type of diffusion is found in osmosis?

In osmosis, there is diffusion of water from the area of its higher chemical potential to the area of its lower chemical potential through a semipermeable membrane.

Q3. What is DPD ( Diffusion Pressure Deficit)?

The presence of solutes in a system reduces the diffusion pressure of water in that system over its pure condition.

Conclusion

To wrap this post, we conclude that facilitated and simple diffusion works along the concentration gradient while active diffusion or transport works against the concentration gradient along with the expenditure of energy. Osmosis is an example of simple diffusion with the use of water molecules.

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