The movement of materials across the cell membrane is broadly divided into active and passive transport. During active transport, a substrate moves against its concentration gradient (uphill) through the action of energy expenditure. In passive transport, no energy is required.
The Semi-permeability of the Cell Membrane
The semi-permeability of the cell membrane is a defining characteristic of living organisms. It allows the cells to maintain a high level of internal order yet interact with the environment continuously.
The phospholipid membrane is semi-permeable; this means that it allows the free passage of some molecules across its lipid bilayer while restricting others. This property allows for the expression of proteins on both sides of the membrane. It also cells to grow, divide, and respond to environmental stresses.
The semi-permeability of the cell membrane is further attributable to the polar and non-polar nature of most molecules. Polar molecules, such as water, are relatively small in size and carry a charge (i.e., they have an uneven distribution of electrons). This results in unequal attraction to the polar and non-polar regions of the lipid membrane.
Meanwhile, non-polar molecules are large and carry no charge, allowing them to diffuse across the cell membrane in either direction freely.
Active transport refers to the uphill movement of a substrate against its concentration gradient. It is actively transported from an area of high concentration to one of low concentration. This movement requires the expenditure of energy, indicating that it is not a spontaneous process. There are two types of active transport: Primary and secondary.
Primary Active Transport
Primary active transport involves the direct movement of a solute across a cell membrane via specific carrier proteins embedded in the lipid bilayer. This method involves moving one molecule against its concentration gradient. The molecule transitions from a region of high concentration to low concentration.
One of the most widely known examples is how sodium ions are moved up the concentration gradient and out of a cell. This mechanism is known as the Sodium-Potassium Pump.
As opposed to secondary, primary active transport is a primary mechanism for enabling cells to maintain stability. It is also the predominant mechanism for regulating concentrations of various ion species.
For example, primary active transport via sodium-potassium pumps is responsible for maintaining the concentration of sodium and potassium ions. These two ions are essential components of blood.
The Sodium-potassium pump
The sodium-potassium pump (Na+/K+ ATPase) is a protein located in the membrane of all animal cells. It is responsible for moving three sodium ions out of the cell and two potassium ions into the cell. During this process, the sodium-potassium pump also hydrolyzes one adenosine triphosphate (ATP) molecule.
This process is only possible as a result of primary active transport. The energy required to maintain this mechanism leads to the internalization and degradation of ATP molecules.
One side effect of this process is that it generates a concentration gradient. Though this is an essential feature of the pump, it is also its weakness. Primary active transport relies on energy expenditure to overcome concentration gradients. Metabolic inhibitors can inhibit the activity of this protein.
This mechanism is essential for maintaining homeostasis in the body. Without ATP available for hydrolysis, sodium-potassium pumps would not have enough power to sustain their activity.
Potassium accumulates in the cytosol, and sodium accumulates in both the cytosol and extracellular fluid. This accumulation results in a high concentration of each in their respective compartments.
The change in ionic concentrations would be highly toxic to the body. Potassium ions are essential for proper muscle function, while excess sodium contributes to hypertension.
A higher potassium concentration in the cytosol and excess sodium in both the cytosol and extracellular fluid would depolarize the membrane. This depolarization will cause an increase in permeability across cell membranes throughout different regions of the body.
The results would be the eventual destabilization of cellular membranes and an influx of sodium and potassium ions into the cytosol. This influx could cause severe swelling and even eventual cell lysis.
Secondary Active Transport
Secondary active transport refers to the movement of a molecule against its concentration gradient through a secondary ion gradient.
Certain types of carrier proteins have both charged and non-charged sub-units. These carrier proteins are known as ion pairs. They can move ions either up or down their concentration gradient depending on the relative charges of the sub-unit.
When a molecule is bound to an ion pair by a process known as ion-binding, the resulting complex will move against its concentration gradient. This process is known as secondary active transport. An example of this process can be observed in the absorption of vitamin B12 by the small intestine.
The cell membrane has the potential to transport both ions and molecules against their concentration gradient. This means that the two types of active transport (primary and secondary) are not mutually exclusive mechanisms.
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Passive transport refers to the movement of substances across cell membranes in a non-spontaneous manner. It is mediated by the cell membrane’s lipid bilayer and does not require any external input of energy. Passive transport also does not necessarily need any specialized membrane proteins to be functional.
There are two main types of passive transport, and they rely on the permeability of lipid membranes. These are simple and facilitated diffusion.
A lipid membrane’s polar head groups allow it to be permeable to molecules that are themselves polar. Therefore, solute movement across the membrane occurs by a process called simple diffusion. This is the passive diffusion of compounds across a lipid membrane without input from ATP.
Simple diffusion, also known as random or non-specific transport, occurs via an equal probability of moving across a membrane in any direction. The movement of molecules through simple diffusion occurs at rates dependent upon the solubility of the compound and its temperature.
Solubility in water at equilibrium is usually taken as an indication of the rate of simple diffusion. Those compounds with the highest solubility in water at equilibrium (e.g., ethanol) also tend to diffuse the fastest across membranes.
This type of diffusion is highly dependent on the size of a given molecule. Larger compounds cross lipid bilayers more easily than small molecules. This is because larger molecules have a lower density and higher free energy of diffusion.
However, this need not always be the case, as cases exist in which smaller molecules diffuse more rapidly than larger ones. This happens because smaller molecules often have higher polarizability. The higher polarizability enables small molecules to move faster across the lipid membrane.
In addition to size and polarity, a high concentration gradient of an ion is another example of a non-specific driving force for simple diffusion. The high concentration of sodium ions inside the cell aids the passive diffusion of sodium. Similarly, the high concentration of chloride ions inside the cell is a non-specific driving force. It helps in the passive diffusion of chloride out of the cell.
While passive diffusion across the lipid bilayer is egalitarian and straightforward, this may not be the case for specific compounds. Some molecules, including vitamins and the majority of proteins, are too large to diffuse across lipid bilayers by simple diffusion. For molecules such as these to cross membranes, specific proteins must be synthesized to form channels through which the molecule can diffuse. This process is known as facilitated diffusion.
Facilitated diffusion occurs when a small water-soluble or lipid-insoluble molecule (i.e., glucose) diffuses in the presence of a membrane protein. The movement of molecules through facilitated diffusion is usually directed. This process is dependent on the concentration of the solute as well as its energy state.
The movement of Glucose through Glucose Transport (GLUT) proteins is an example of cotransport. Both G- and Na+ ions are simultaneously transported in the same direction through a channel in the membrane.
The use of cotransport is extremely energetically favorable, as the movement of Glucose and Na+ ions through the channel allows for the hydrolysis of ATP to occur, as discussed below.
Role of ATP
The movement of substances across cell membranes is called transport, and concentration gradients drive it. The active transport of substances across membranes occurs via the use of adenosine triphosphate (ATP).
The binding of ATP to a transport protein called cytoplasmic solute-binding proteins (SV) causes the protein to undergo conformational changes. This change in conformation results in a movement of the transported compounds, for example, glucose, directly across the membrane.
Using transport proteins allows for specific, unidirectional movement of substances out of or into cells. To examine the use of these proteins, we can look at glucose transport. The transport proteins move along with the glucose and transport it directly across the membrane.
A cytosolic protein, called cytoplasmic solute binding proteins (SV), binds to glucose and transports it across the membrane. Glucose moves from a high concentration outside the cell to a low concentration inside the cell. This movement happens through the membrane protein channel.
How the Membrane Protein Works
Glucose + NS across membrane <=> Glucose inside cell + Na+ outside cell + NS. If the concentration of glucose is high enough, translocation occurs through a facilitated transport protein. Sugar moves from the higher concentration to the lower concentration across a membrane protein channel.
The protein facilitates transport for both the sugar and Na+ ions in the same direction. In eukaryotes, amino acids move by protein transport.
Transport proteins bind to and transport glucose while simultaneously transporting sodium out. Cotransport is used to help move glucose across the membrane, as it is a much more energetically favorable process than simple diffusion.
Sodium ions are moved out of the cell through a combination of concentration and electrical gradients. These ions are pushed out by the large concentration gradient formed between the higher concentration and lower concentration of glucose.
An electrical gradient also moves sodium ions out as the cell is depolarized. The sodium ions are then pumped out of (or into) the membrane via sodium-potassium ATPases.
There is an electrochemical gradient across the cell membrane. The Na+ moves towards the negative end while the glucose moves towards the positive end of the gradient. The movement of these substances is facilitated by a concentration and an electrical gradient across the membrane.
The two Pathways by Which Glucose Enters a Cell
The first pathway is active transport, in which a protein binds glucose and transports it across the membrane. In this pathway, intracellular glucose concentration is high while extracellular glucose concentration is low. The difference in concentration causes a net movement of glucose across the membrane.
The second pathway involves simple diffusion. This process occurs when there is no net movement of substances due to the lack of a concentration gradient.
Compare and Contrast Active and Passive Transport
Active and passive methods of transport have some similarities, as discussed below:
- Both processes involve the use of solute-binding proteins to transport substances across a membrane. Solute binding proteins bind glucose molecules and help them cross the membrane.
- The two transport methods involve the use of electrochemical gradients to facilitate movement across the membrane.
- All of these processes are facilitated and controlled by enzymes. The enzymes catalyze reactions that either produce or consume ATP to help facilitate the reactions.
The main difference between active and passive transport is the energy expenditure.
- Active transport involves the use of ATP to provide power for movement across the membrane. The active transport of glucose across the membrane utilizes proteins that contain membranes bound ATPases, which utilize energy from ATP to pump sodium ions out of the cells and allow glucose to enter the cell.
Passive transport does not require ATP to move substances across a membrane. It only involves the movement of substances down a concentration gradient. A protein facilitates movement without the utilization of ATP.
- Active transport is applicable for eukaryotes only, while eukaryotes, viruses, and bacteria use passive transport.
- Active transport can be either unidirectional or bidirectional, while passive transport is always unidirectional.
During active transport, glucose is actively transported out of a cell in what is known as the “SLC2” transporter. When moving out of a cell, this protein is a “uniporter.” When moving into a cell via the same protein, this is known as a “diporter.”
- Using active transport to move substances across a membrane takes longer than passive transport.
Glucose transport across the membrane for bacteria is facilitated by a protein that utilizes sodium to pump glucose into the cell. Lipid-soluble substances are typically not transported into a cell by active transport. They pass through the membrane via passive diffusion and are immediately used for other purposes in the cell.
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Both active and passive methods of transport are essential to the survival of cells. The cell membrane controls what can enter and leave a cell by either being permeable or impermeable.
Active transport is used when the cell needs specific molecules for survival. Passive diffusion occurs with substances going out of the cell without any help from an external force. The differences between the two methods are complementary. This characteristic makes the movement of molecules across the cell membrane more effective.
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