Passive Transport – Definition, Types, Examples

Passive Transport Definition

• Passive transport, also known as passive diffusion, is the passage of an ion or molecule through a cell wall along a concentration gradient, or from a region of high concentration to a region of low concentration.
• Comparable to transferring from the metro train to the platform or leaving a packed area. Passive transport essentially allows an ion or molecule “space to breathe.”
• This concept is most easily recalled when contrasted with its opponent, active transportation. Active transport, like physical activity, requires energy. In contrast, passive transport requires no energy at all.
• Passive transport, also known as passive diffusion, is the passage of an ion or molecule through a cell wall along a concentration gradient, or from a region of high concentration to a region of low concentration.
• Comparable to transferring from the metro train to the platform or leaving a packed area. Passive transport essentially allows an ion or molecule “space to breathe.”
• This concept is most easily recalled when contrasted with its opponent, active transportation. Active transport, like physical activity, requires energy. In contrast, passive transport requires no energy at all.

What is Diffusion?

• Diffusion is a passive transport method. The concentration of a single material tends to migrate from areas of high concentration to areas of low concentration until the concentration is uniform throughout the space.
• You are familiar with substances diffusing through the air. Consider, for instance, a person opening a bottle of perfume in a crowded room.
• The concentration of the fragrance is strongest in the bottle and lowest at the room’s perimeter. The perfume vapour will diffuse, or travel out from the bottle, and as it spreads, an increasing number of individuals will smell it.
• Through diffusion, substances travel within the cytoplasm of the cell, and by diffusion, certain substances migrate past the plasma membrane.
• The process of diffusion consumes no energy. Rather, the varying concentrations of materials in various regions represent a sort of potential energy, and diffusion is the dissipation of this potential energy as materials travel down their concentration gradients, from high to low.
• Each substance in a medium, such as extracellular fluid, has its own concentration gradient that is independent of the gradients of other substances. Moreover, each component will disperse in accordance with the gradient.

Factors affects the rate of Diffusion

• Extent of the concentration gradient: The larger the concentration gradient, the faster the diffusion. The closer a material’s distribution approaches equilibrium, the slower its rate of diffusion.
• Mass of the diffusing molecules: Molecules that are more massive diffuse more slowly because it is more difficult for them to migrate between the molecules of the substance they are moving through.
• Temperature:  Greater temperatures enhance the energy and, consequently, the movement of molecules, so accelerating the rate of diffusion.
• Density of the solvent: When the density of the solvent increases, the diffusion rate drops. The molecules move more slowly because it is more difficult for them to pass through the denser material.

Characteristics of Passive Transport

1. No energy expenditure: Passive transport does not require the cell to expend energy in order to move molecules or ions across the cell membrane.
2. Movement down the concentration gradient: Passive transport moves molecules or ions from areas of high concentration to areas of low concentration, which is also known as moving down the concentration gradient.
3. No transport proteins required for simple diffusion: In simple diffusion, molecules move directly across the cell membrane, without the need for transport proteins.
4. Transport proteins required for facilitated diffusion: Facilitated diffusion requires transport proteins to move molecules or ions across the cell membrane.
5. Saturation: Facilitated diffusion can become saturated when all the transport proteins are occupied by molecules or ions.
6. No specificity: Passive transport does not exhibit specificity, meaning that any molecule or ion that can pass through the cell membrane can be transported.
7. Rate dependent on temperature and concentration gradient: The rate of passive transport is dependent on temperature and the concentration gradient of the molecules or ions being transported.

Overall, passive transport allows for the movement of molecules or ions across the cell membrane without requiring the cell to expend energy.

Types Of Passive Transport

There are four types of passive transport:

1. Simple Diffusion: Diffusion is the movement of chemicals from a higher concentration region to a lower concentration region. The difference in concentration between two locations is referred to as a concentration gradient, and diffusion continues until this gradient is neutralised. Liquids and gases undergo diffusion because their particles travel randomly from one location to another. It is a necessary process for several life processes in living organisms. Substances enter and exit cells through simple diffusion.
2. Facilitated Diffusion: Facilitated diffusion is the passive movement of ions or molecules across the cell membrane by means of certain transmembrane integral proteins. Large and insoluble molecules necessitate a carrier substance for their transport across the plasma membrane. This procedure requires no external or cellular energy. Glucose transporter, ion channels, and aquaporins are assisted diffusion examples. The cell membrane is only permeable to a restricted number of tiny, non-polar molecules. Hence, transmembrane protein-mediated diffusion facilitation is essential.
3. Filtration: The process of separating particles from liquids and gases called filtration. Filtration is illustrated by the selective absorption of nutrients by the body. This mechanism occurs along the concentration gradient without requiring any energy. An example of a biological filter is the kidney. The glomerulus filters the blood and reabsorbs the essential chemicals. In the process of filtration, the cell membrane allows only soluble and easily permeable substances to pass through its pores.
4. Osmosis: In order to equalise the concentration of other substances, water and other molecules flow across a selectively permeable membrane during osmosis. Temperature and concentration gradient influence osmosis. The higher the concentration gradient, the larger the osmosis rate. In addition, the rate of osmosis increases as the temperature rises. There is a conflict theory regarding the osmosis process. Few biologists argue that osmosis is an active transport rather than a passive one.

Simple Diffusion

• Until equilibrium is established, chemicals or particles will diffuse from high-concentration to low-concentration regions along their concentration gradient. The random thermal motion of the particles is responsible for this motion, and no more energy is needed to sustain it.
• Diffusion is the process by which molecules pass unimpeded through a selectively permeable barrier, such as a cell membrane. Diffusion rates are affected by variables such as molecule size, concentration gradient, temperature, and membrane permeability.
• The exchange of gases in the lungs, the transport of nutrients into cells, and the combination of chemicals in a solution are all examples of simple diffusion occurring in diverse biological and physical systems. It’s vital for regulating the movement of chemicals and ions between cells and between organs.

Factors affecting on Simple Diffusion

1. Concentration gradient: The larger the difference in concentration between two regions, the faster the rate of diffusion.
2. Temperature: As the temperature increases, the rate of diffusion also increases. This is because molecules have more kinetic energy, and therefore move faster.
3. Molecular weight: Smaller molecules will diffuse faster than larger molecules.
4. Surface area: A larger surface area available for diffusion will increase the rate of diffusion.
5. Distance: The greater the distance between two regions, the slower the rate of diffusion.
6. Solubility: Molecules that are more soluble in a given medium will diffuse more easily through that medium.
7. Presence of barriers: Barriers such as membranes or other physical barriers can slow down the rate of diffusion.
8. Pressure: An increase in pressure can increase the rate of diffusion by forcing more molecules to move through a given area

Facilitated diffusion

• Facilitated diffusion (also known as facilitated transport or passive-mediated transport) is the spontaneous passive transfer of molecules or ions across a biological membrane by certain transmembrane integral proteins.
• Being passive, assisted transport does not directly require chemical energy from ATP hydrolysis in the transport phase itself; instead, molecules and ions travel along their concentration gradient, indicating the diffusive nature of the transport.
• In numerous ways, facilitated diffusion differs from simple diffusion.
  • The transport is dependent on the molecular binding of the payload to the membrane-embedded channel or carrier protein.
  • Unlike free diffusion, which is linear in the concentration difference between the two phases, the rate of assisted diffusion is saturable with regard to the concentration difference between the two phases.
  • Due to the presence of an active binding event, the temperature dependency of assisted transport is significantly different from that of free diffusion, where the dependence on temperature is minimal.

• Due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the lipid bilayer, polar molecules and large ions dissolved in water cannot pass freely across the plasma membrane.
• Small, non-polar molecules, such as oxygen and carbon dioxide, can easily diffuse through the membrane. Hence, tiny polar molecules are transported via transmembrane channels formed by proteins.
• These channels are gated, which means they can open and close, hence deregulating the passage of ions or tiny polar molecules across membranes, sometimes against the osmotic gradient. Larger molecules are transported by transmembrane carrier proteins, such as permeases, whose conformation changes as the molecules are moved across the membrane (e.g. glucose or amino acids).
• In water, nonpolar compounds, such as retinol or lipids, are insoluble. Water-soluble carriers move them through watery compartments of cells or extracellular space (e.g. retinol binding protein). The metabolites are not changed since enhanced diffusion requires no energy.
• Only permease is capable of changing its form to transport metabolites. The process of transporting a changed metabolite across a cell membrane is known as group translocation transport.
• Glucose, sodium ions, and chloride ions are examples of chemicals and ions that must easily pass the plasma membrane but are virtually impermeable to the lipid bilayer of the membrane.
• Thus, their transport must be “facilitated” by transmembrane proteins that provide an alternate route or bypass mechanism. Proteins such as glucose transporters, organic cation transport proteins, urea transporter, monocarboxylate transporter 8 and monocarboxylate transporter 10 mediate this process.

Filtration

• Filtering is a physical separation procedure that separates solid matter and fluid from a mixture by passing only the fluid through a filter media with a complicated structure.
• Solid particles that are too large to pass through the filter medium are referred to as oversize, while the fluid that does pass through is known as the filtrate.
• Oversize particles can create a filter cake on top of the filter and obstruct the filter lattice, preventing the fluid phase from passing through the filter. This phenomenon is known as blinding. The size of the largest particles that can pass through a filter is known as the filter’s effective pore size.
• The separation of solids and liquids is poor; some liquid will contaminate the solids, and the filtrate will contain minute particles (depending on the pore size, filter thickness and biological activity).
• There are biological, geological, and industrial forms of filtration. Filtration happens in both natural and manmade systems.
• In addition to separating particulates from a fluid stream, biological and physical filtration devices also remove chemical species and living organisms by entrainment, phagocytosis, adsorption, and absorption. Slow sand filters and trickling filters are examples.
• It is often used as a broad word for microphagy, in which organisms filter minute food particles from their surroundings through a variety of mechanisms. Filter-feeding organisms range in size from the microscopic Vorticella to the basking shark, one of the largest fishes, and the baleen whales.
• Filtration is used to separate particles and fluid in a suspension, with the fluid being a liquid, gas, or supercritical fluid. One or both of the components may be separated, depending on the application.
• As a physical process, filtration permits the separation of substances with distinct chemical compositions. A solvent is selected that dissolves one component, but not the other. By dissolving the mixture in the selected solvent, one component will enter the solution and pass through the filter, while the remaining component will be retained.
• In chemical engineering, filtration is commonly used. It can be integrated with other unit operations to process the feed stream, such as in the biofilter, a filter and biological digesting device.
• In sieving, separation happens at a single perforated layer, whereas in filtration, separation occurs at many perforated layers (a sieve). In sieving, particles too large to pass through the sieve’s holes are kept (see particle size distribution). In filtration, a multilayer lattice holds particles incapable of following the filter’s tortuous pathways. Oversize particles may form a cake layer on the filter surface and may also obstruct the filter lattice, preventing the fluid phase from passing through the filter (blinding). Commercially, the word filter is applied to membranes whose separation lattice is so thin that the surface becomes the primary zone of particle separation, despite the fact that these products might also be referred to as sieves.
• Filtration is distinct from adsorption, which relies on surface charge for separation. Commercially, some adsorption devices incorporating activated charcoal and ion-exchange resin are referred to as filters, despite the fact that filtration is not their primary mechanical function.
• Because there is no filter medium, filtration varies from magnetic removal of magnetic impurities from fluids (usually lubricating oil, coolants, and fuel oils). There are commercial devices referred to as “magnetic filters,” although their name does not reflect their operation.
• In biological filters, particles of excessive size are captured and ingested, and the resulting metabolites may be discharged. In animals (including humans), for instance, renal filtration removes waste from the blood, and in water treatment and sewage treatment, undesired elements are removed by adsorption into a biological film developed on or within the filter media, as in slow sand filtration.

Osmosis

• Osmosis is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of higher solute concentration) in the direction that tends to equalise the solute concentrations on both sides.
• It can also be used to describe a physical process in which any solvent travels through a selectively permeable barrier (permeable to the solvent, but not to the solute) that separates two solutions of different concentrations. It is possible to make osmosis function.
• Osmotic pressure is defined as the external pressure necessary to prevent any net passage of solvent across a membrane. Osmotic pressure is a colligative property, which means that it depends on the solute’s molar concentration but not its identity.
• Biological systems require osmosis because biological membranes are semipermeable. In general, these membranes are impermeable to big and polar molecules, such as ions, proteins, and polysaccharides, but they are permeable to non-polar or hydrophobic molecules, such as lipids, and tiny molecules, such as oxygen, carbon dioxide, nitrogen, and nitric oxide.
• Solubility, charge, or chemistry, as well as solute size, determine permeability. Aquaporins enable the diffusion of water molecules through the phospholipid bilayer of the plasma membrane, tonoplast membrane (vacuole), and organelle membranes (small transmembrane proteins similar to those responsible for facilitated diffusion and ion channels).
• Osmosis is the fundamental mechanism for transporting water into and out of cells. The majority of a cell’s turgor pressure is maintained through osmosis across the cell membrane between the cell’s interior and its comparatively hypotonic surroundings.

The diffusion of water molecules through a selectively permeable membrane is osmosis. The net migration of water molecules from a solution with a high water potential through a partially permeable barrier into a region with a low water potential. A cell with a reduced negative water potential will absorb water, but this also depends on other elements such as solute potential (the pressure of solute molecules within the cell) and pressure potential (external pressure e.g. cell wall). There are three different kinds of Osmosis solutions: isotonic, hypotonic, and hypertonic. Isotonic solution occurs when the extracellular solute concentration is equal to the intracellular solute concentration. In an isotonic solution, water molecules continue to travel between the solutions, but at the same rate in both directions. As a result, water movement is balanced between the inside and outside of the cell. A hypotonic solution is one in which the concentration of solutes outside the cell is lower than the concentration inside. In hypotonic solutions, water enters the cell via the concentration gradient (from higher to lower water concentrations). This can lead to cell swelling. In this fluid, cells without a cell wall, such as animal cells, could explode. A hypertonic solution is one in which the solute concentration is greater than the concentration within the cell. In hypertonic solution, water will migrate out of the cell, causing it to shrink.

Mechanism

• The mechanism responsible for driving osmosis is typically represented in biology and chemistry textbooks as either the dilution of water by solute (resulting in a lower concentration of water on the higher solute concentration side of the membrane and thus a diffusion of water along a concentration gradient) or as a solute’s attraction to water (resulting in less free water on the higher solute concentration side of the membrane and therefore net movement of water towards the solute). Both of these claims have been rejected with absolute certainty.
• The inapplicability of the diffusion model of osmosis is caused by the fact that osmosis can move water across a membrane towards a higher concentration of water.
• The “bound water” concept is challenged by the fact that osmosis is independent of the solute molecules’ colligative property or hydrophilicity.
• It is difficult to define osmosis without a mechanical or thermodynamic explanation, but fundamentally, there is an interaction between the solute and water that opposes the pressure that otherwise free solute molecules would exert. Notable is the ability to transfer ambient heat into mechanical energy (water rising).
• Many thermodynamic explanations are provided for the idea of chemical potential and how the function of water in a solution differs from that of pure water as a result of the increased pressure and the presence of a counteracting solute such that the chemical potential remains intact.
• The virial theorem demonstrates that the attraction between the molecules (water and solute) lowers the pressure; consequently, the pressure exerted by water molecules on each other in solution is less than in pure water, allowing pure water to “force” the solution until the pressure reaches equilibrium.

Passive Transport using Membrane Proteins

• Occasionally, molecules are unable to traverse the cell membrane on their own. These chemicals require transport proteins to promote their movement across the membrane, a process referred to as facilitated diffusion. Attached to the cell membrane, these specialised proteins are referred to as channel proteins or carrier proteins (Figure). In reality, they traverse the cell membrane from the inside to the exterior of the cell.
• Channel proteins create an open channel or passageway for molecules to cross the cell membrane. Many channel proteins permit ion diffusion. Ions are atomic charges. It is difficult to cross the cell membrane without aid due to the charge. Specificity of channel proteins for the molecules they transport. For instance, sodium ions cross the membrane via a sodium-specific channel protein.
• Carrier proteins bind the chemicals and transport them across the cell membrane. These proteins bind a molecule on one side of the membrane, transform as they transport it across the membrane, and deposit it on the opposite side. Although though proteins are involved in each of these transport mechanisms, neither requires energy. Hence, these remain forms of passive mobility.

Significance of Passive Transport

• Transmission across membranes is crucial to cellular survival. Throughout their existence, cells must constantly communicate and trade with one another. Biological molecules and waste products essential to regular function may be taken in and excreted, respectively, during transport.
• In a process known as passive diffusion, all solute particles are passed back and forth between mother and foetus.
• Certain chemicals, including sodium thiopental, are able to diffuse across the blood-brain barrier.
• Small molecular weight compounds can be transported passively across membranes by diffusion and osmosis.
• The molecules of digested food (amino acids, glucose) go from the intestine to the bloodstream by following a concentration gradient. Through diffusion, metabolic byproducts like carbon dioxide and urea move from cells to the circulatory system.
• A large amount of oxygen (from the air sac) diffuses down to a smaller amount (in the blood). Changes in the concentration of carbon dioxide occur from high (in the blood) to low (in the air sac).
• Renal filtration is the process by which animals (including humans) cleanse their blood of impurities.
• Osmosis plays a crucial role in biology since it aids in the transport of nutrients and the elimination of metabolic waste. Since the membranes of cells are only partially permeable, osmosis is responsible for the diffusion of liquid solvents across their surfaces.

Examples of Passive Transport

1. BAC Goin’ Up

• After ethanol – the “alcohol” component in beer, wine, and spirits – enters the body, it enters the bloodstream at a rapid rate. This is the reason why you can have a BAC without feeling intoxicated, and why some people become extremely drunk within minutes after having a shot.
• This occurs because ethanol molecules effortlessly execute simple diffusion, a form of passive transport. Its ultramicroscopic size enables them to traverse cell and tissue membranes without assistance and to exert their effect on the body without spending energy.

2. Neurotransmission Impossible

• well, not exactly. It is easy to overlook the fact that neurons, or brain cells, rely on passive transport to communicate, in part because we make them appear so sophisticated.
• The intricate network of synapses (brain activity) in our heads is dependent on two ions, sodium (Na+) and potassium (K+), which operate along a gradient. A neuron in resting potential (not firing) has an inside concentration of K+ ions and an outside concentration of Na+ ions. As a neuron fires (active potential), protein “pumps” on its outer membrane let Na+ ions to enter the cell and K+ ions to leave.
• As can be observed, Na+ and K+ ions travel from a region of high concentration to a region of lower concentration, much as the ethanol molecules in Example 1 do. Yet, they require assistance. Because they require some assistance, they engage in assisted diffusion as opposed to simple diffusion.

3. (Not) a Pile of Waste

• Our intestines do much more than move waste through the body. In fact, you could say that their primary responsibility is to extract minerals from our meals. Although vitamins and minerals are often significantly larger than ethanol and ions, our bodies remove them via a passive transport mechanism.
• Specifically, filtration occurs when a membrane is used to separate solids from liquids and liquids from gases. By flowing through the intestinal wall and into the bloodstream, nutrients (liquid) are distinguished from waste (solid).

4. Fresh Veggies

• When a raisin is soaked in water, it transforms into a grape. Soaking raisins constitutes an additional instance of passive transport – in this case, osmosis.
• It seeks equilibrium as opposed to mere movement along a concentration gradient, distinguishing it from other forms of passive transport. Water goes through the raisin’s membrane not just to dilute its inside, but also to “equalise” the grape with its surroundings. This process can also occur with other fruits and vegetables, so long as they have been dehydrated in some way.

FAQ

References

1. Feher, J. (2017). Passive Transport and Facilitated Diffusion. Quantitative Human Physiology, 161–169. doi:10.1016/b978-0-12-800883-6.00015-x
2. Chen I, Lui F. Physiology, Active Transport. [Updated 2022 Sep 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK547718/
3. https://flexbooks.ck12.org/cbook/ck-12-biology-flexbook-2.0/section/2.13/primary/lesson/passive-transport-bio/
4. https://ib.bioninja.com.au/standard-level/topic-1-cell-biology/14-membrane-transport/types-of-transport.html
5. https://www.aakash.ac.in/important-concepts/biology/passive-transport
6. https://humanbiology.pressbooks.tru.ca/chapter/4-7-passive-transport/
7. https://study.com/academy/lesson/active-and-passive-transport-across-the-cell-membrane.html
8. https://conductscience.com/passive-transport/
9. https://www.diffen.com/difference/Active_Transport_vs_Passive_Transport
10. https://openoregon.pressbooks.pub/mhccmajorsbio/chapter/passive-transport-diffusion/
11. https://www.futurelearn.com/info/courses/introduction-to-human-cells-the-basis-of-life/0/steps/330524
12. https://www.biologyonline.com/dictionary/passive-transport
13. https://www.khanacademy.org/test-prep/mcat/cells/transport-across-a-cell-membrane/a/passive-transport-and-active-transport-across-a-cell-membrane-article
14. https://biologydictionary.net/passive-transport/
15. https://opentextbc.ca/biology/chapter/3-5-passive-transport/
16. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/passive-transport