Passive transport is the process of transporting molecules from one side of the membrane to the other without any energy requirements.
The transport of materials across the cell membrane is necessary to uphold cellular homeostasis. It’s required to maintain the pH, volume, and accumulation of nutrients for protein synthesis and cell metabolism for organisms to thrive.
The cell membrane is composed of lipids, proteins, carbohydrates, and sterols that interact with each other to facilitate transmembrane transport. The phospholipid bilayer of the membrane has hydrophilic lipid heads facing outside and the hydrophobic tails facing towards each other on the inner leaflet of the membrane. This orientation supports the amphipathic nature of the biological membrane.
The hydrophobicity of the membrane makes it challenging to transport ions, solutes, and other hydrophilic molecules across the membrane, making it selectively permeable. Membrane permeability which is affected by several factors is the physiological property that allows the selective passage of hydrophilic solutes across the hydrophobic barrier. Transport proteins like carriers and channel proteins facilitate the transport of such hydrophilic molecules.
This article provides an in-depth discussion about passive transport, its types, and the molecules or ions it supports to cross the membrane.
Transport Across the Cell Membrane
Humans, animals, plants, and other organisms utilize different means of transporting materials from one place to another. This unique transportation network circulates food, minerals, hormones, oxygen, carbon dioxide, waste products, etc.
The movement of molecules across the membrane is categorized into two classes, depending on the energy required to execute the process. It includes active transport and passive transport.
1. Active Transport
It’s the transport of molecules across the membrane against the concentration gradient from low concentration to high concentration. It involves an expenditure of energy in the form of ATP.
The two types of active transport include:
- Primary (direct) active transport: It’s the transport of a single molecule across the membrane against its electrochemical gradient by using ATP as energy. An example is the plasma membrane sodium-potassium pump (Na+ – K+ -ATPase).
Image: An illustration of sodium-potassium transport against the electrochemical gradient using ATP as energy.
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Secondary (indirect) active transport: It involves coupling the transport of one molecule with another. The energy-dependent transfer of an ion (Na+, K+, or H+) generates an electrochemical gradient of the ion across the membrane. This ion gradient is coupled to the movement of solutes on the same side (symport) or opposite side (antiport) of the membrane.
2. Passive Transport
It’s the movement of substances across the membrane along the concentration gradient from higher concentration to lower concentration. Thus, it doesn’t require energy.
Passive Transport and Its Types
Passive transport is involved in transferring small molecules of low molecular weight and gases across the membrane. But, instead of utilizing cellular energy, the process relies on the second law of thermodynamics to drive the movement of substance.
Fick’s law of diffusion predicts the rate of diffusion through passive transport. It states that the molar flux due to diffusion is proportional to the concentration gradient.
The two types of passive transport include diffusion and facilitated diffusion.
1. Simple Diffusion
It’s the movement of materials from an area of high concentration to that of low concentration until the concentration is equal on both sides (gradient neutralization). Diffusion requires no energy expenses; instead, the concentration gradient (in the form of potential energy) is created and utilized during the transport of molecules.
In simple diffusion, molecules or solute particles move in random Brownian motion. And, their flux across the membrane can be calculated using an equation proposed by Torrell in 1953.
Flux = Mobility x Concentration x Driving Force
Here, flux is the number of moles of solute crossing one square centimeter of membrane per second. The concentration measures the amount of material available to participate in the diffusion process, while solute mobility is the ease of transport of molecules.
The difference in the concentration gradient on both sides of the membrane acts as a driving force for molecular transport. When the solute equilibrium is achieved on both sides of the membrane, the flux across the membrane becomes zero.
Simple diffusion occurs only in liquid and gases because of random movements of their particles from one place to another. Examples of molecules transported by simple diffusion include oxygen, carbon dioxide, and ethanol.
Another example of simple diffusion is the transport of water, nutrients, and other gases in prokaryotes like bacteria. In addition, the excretion of waste material is also through simple diffusion in these organisms.
Image: An illustration of simple diffusion through lipid bilayer membrane.
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Factors That Affect Diffusion Process:
- The extent of the concentration gradient: The greater the difference in the concentration of a particular molecule on both sides of the membrane, the higher the diffusion rate will be. But, as the concentration on both sides starts reaching equilibrium, the diffusion rate becomes lower.
- Mass of the molecules: The diffusion of materials of higher molecular weight will be slower than materials of lower molecular weight.
- Temperature: The rate of diffusion increases by increasing the system’s temperature, while the rate decreases if the temperature decreases. So, one can say the rate of diffusion is directly proportional to the temperature of the biological system.
- Solvent density: The rate of diffusion is inversely proportional to the solvent density. That means the higher the solvent density, the lower the rate of diffusion of molecules. Higher solvent density makes it difficult for molecules to move from one side to the other side of the system. However, when the solvent is less dense, the molecules face less resistance in their movement and readily cross the permeable barrier of the system.
- Solubility: The non-polar and lipid-soluble materials easily pass through the plasma membrane (have a faster diffusion rate) than polar and non-lipid materials.
- Surface area and thickness of the plasma membrane: The surface area is directly proportional to diffusion rate whereas the thickness of the membrane has an inversely proportional relationship. That is, the larger the surface area, the higher the diffusion rate across the membrane, and the thicker the membrane is, the lower the diffusion rate will be.
- Distance traveled: The greater the distance that a substance must travel, the slower the diffusion rate. So, smaller-sized cells or flattened cells facilitate faster transport of molecules through passive diffusion.
2. Facilitated Diffusion
1. Channel-mediated transport
Channel-mediated transport is the spontaneous passage of molecules or ions across the biological membrane passing through specific transmembrane integral proteins. These integral proteins are collectively known as transport proteins.
Channel proteins are specific for the materials they transport across the membrane. Structurally, they have hydrophilic domains exposed to the intracellular and extracellular fluids and a hydrophobic channel through their core that provides a hydrated opening through the membrane layers.
The passage created by both domains prevents the polar molecules from coming in contact with the non-polar central layer of the membrane. An example of channel-mediated transport is the passage of water through aquaporins.
Channel proteins are also of two types:
- Gated channels: Here, channel gates control the transport of molecules, and they require signals in the form of voltage change, mechanical stress, or binding of a ligand before opening.
- Non-gated channels: They are not regulated by any signal and are always open to transport molecules.
In the kidney, both gated and non-gated channels are found in different parts of the renal tubules. In contrast, nerve cells and muscle cells involved in the transmission of electric impulses have gated channels for sodium, potassium, and calcium in their membranes.
2. Carrier-mediated transport
The carrier protein bind to the molecules to be transferred that eventually triggers a change in its shape. Then, based on the concentration gradient, the molecule moves across the membrane.
An example of carrier-mediated transport is a group of carrier proteins called glucose transport proteins, or GLUTs, which transport glucose and other hexose sugars through plasma membranes within the body.
Factors Affecting Facilitated Diffusion:
The process of facilitated diffusion depends on several factors like:
- Temperature: The rate of facilitated diffusion is directly proportional to the temperature of the system. For example, an increase in temperature leads to an increase in the energy of the molecules, leading to faster transfer of molecules.
- Concentration: The concentration of the molecules on both sides of the membrane determines the direction of the movement.
- Diffusion distance: The rate of diffusion is inversely proportional to the distance of diffusion. The longer the distance of diffusion, the lesser the diffusion rate of the molecule.
- Size of molecules: Smaller molecules move faster than heavier molecules. So, the diffusion rate has a direct relationship to the size of the molecule.
Differences Between Simple Diffusion and Facilitated Diffusion
The simple diffusion and facilitated diffusion process might seem similar, but there are four significant differences among both techniques. They are:
- The transport through facilitated diffusion requires channel or carrier proteins. But, in simple diffusion, no such assistance is needed.
- The transport rate through facilitated diffusion is saturable because of its dependence on the channel or carrier proteins. Only one molecule is transferred at a time. However, simple diffusion is linear in the concentration difference. It means that the higher the concentration gradient on both sides of the membrane, the higher the diffusion rate of molecules. The process never reaches the saturation stage because of its dependence on carrier proteins.
- The rate of diffusion of materials through carrier-mediated transport is lower than that of channel-mediated transport. Channel proteins facilitate diffusion at a rate of tens of millions of molecules per second. In contrast, carrier proteins work at a rate of a thousand to a million molecules per second.
- The facilitated transport is more dependent on temperature due to an activated binding event than simple diffusion – the temperature has a mild effect on simple diffusion.
Other Processes Involving the Mechanism of Passive Transport
Other than simple and facilitated diffusion, osmosis and filtration are two other techniques that work on the principle of passive transport. They are also involved in the transfer of certain molecules across biological membranes based on concentration gradient and without any energy expenditure.
It’s a process of transporting solvent molecules from an area of high water potential (lower solute concentration) to lower water potential (higher solute concentration) through a selectively permeable membrane. Any type of gases and supercritical liquids like CO2 can cross the membrane or any other system through the process of osmosis.
The osmotic solutions are of three types:
- Hypotonic: This is when a higher solute concentration is present inside the cell than outside.
- Hypertonic: It’s a solution that has a higher solute concentration outside the cell than inside.
- Isotonic: This is when the concentration of solutes is equal on both sides of the cell.
Image: An illustration of the effect of blood cells when placed in solutions of different tonicity.
Depending on the physiological mechanism occurring in a cell when placed in an osmotic solution, the process of osmosis is of two types:
- Endosmosis: The movement of the solvent molecules into the cell when placed in a hypotonic solution is termed endosmosis. In this condition, the cell becomes turgid or undergoes deplasmolysis.
- Exosmosis: The movement of the solvent molecules out of the cell when placed in a hypertonic solution is termed exosmosis. In this situation, the cell becomes flaccid or undergoes plasmolysis.
Stomatal opening in plant cells is an example of osmosis. Here, water enters the cell through osmosis, the guard cells swell up, and the stomata open for the gaseous exchange. Another example is the absorption of water from the soil.
Filtration is the process of separating solids from liquids and gases. It doesn’t require energy expenditure and takes place along the concentration gradient. An example includes the selective absorption of nutrients in the human body. The glomerulus filters the blood in the kidney, and the body reabsorbs the necessary molecules.
Passive transport is a physiological mechanism of transporting molecules across the membrane that favors the concentration gradient. Without any expenditure of energy, the process transfers essential molecules, nutrients, and gases to the organism’s body required for their living. However, channel and carrier proteins present in the membrane also facilitate this transport.
The transport rate depends on the permeability of the cell membrane, which, in turn, depends on the organization and characteristics of membrane lipids and proteins.
Researchers are currently digging up more hidden properties of the membrane and studying their utilization for drug transport during disease treatment. So, despite being an old matter, the area has the novel potential for breakthroughs in health and medicine.
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