All cells in the body require a constant supply of essential substances, such as nutrients, water, and electrolytes. Simultaneously, cells must eliminate waste products and carbon dioxide. These processes are achieved through various transport mechanisms that occur across the cell membrane.
Basic Mechanisms Of Transport
PASSIVE TRANSPORT
Passive transport refers to the movement of substances along a chemical, electrical, or electrochemical gradient. This type of transport, also known as diffusion or downhill movement, does not require energy. Passive transport encompasses the following:
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Simple diffusion through the lipid layer: Lipid-soluble substances can diffuse directly through the lipid bilayer of the cell membrane. The rate of diffusion is proportional to the substance’s lipid solubility.
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Simple diffusion through the protein layer: Substances can diffuse through protein channels embedded within the cell membrane. These channels can be either gated or ungated:
- Gated channels: These channels remain closed until a specific condition triggers their opening. There are three main types of gated channels:
- Mechanically-gated channels: Opened by mechanical stimuli, such as pressure changes.
- Voltage-gated channels: Opened by changes in the electrical potential across the membrane.
- Ligand-gated channels: Opened by the binding of specific signaling molecules (ligands), such as hormones.
- Ungated channels: These channels are continuously open, allowing for the constant passage of specific substances.
- Gated channels: These channels remain closed until a specific condition triggers their opening. There are three main types of gated channels:
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Facilitated or carrier-mediated diffusion: Larger, water-soluble molecules require the assistance of carrier proteins to cross the cell membrane. These proteins bind to the substance and facilitate its movement down the concentration gradient.
FACTORS AFFECTING THE RATE OF DIFFUSION THROUGH THE CELL MEMBRANE:
- Permeability of the cell membrane
- Temperature
- Concentration gradient or electrical gradient of the substance across the cell membrane
- Solubility of the substance
- Thickness of the cell membrane
- Size of the molecules
- Size of the ions
- Charge of the ions
SPECIAL TYPES OF PASSIVE TRANSPORT:
- Bulk flow: Movement of large quantities of substances along a pressure gradient. An example is the exchange of gases across the respiratory membrane in the lungs.
- Filtration: Movement of water and solutes due to hydrostatic pressure differences. This occurs at the arterial end of capillaries and in the glomeruli of the kidneys.
- Osmosis: Movement of water across a semi-permeable membrane to equalize osmotic pressure differences.
ACTIVE TRANSPORT
Active transport involves moving substances against their concentration gradient, requiring energy in the form of ATP. This process typically utilizes carrier proteins.
TYPES OF ACTIVE CARRIER PROTEINS:
- Uniport: Transports a single substance in one direction.
- Symport: Simultaneously transports two different substances in the same direction.
- Antiport: Simultaneously transports two different substances in opposite directions.
TYPES OF ACTIVE TRANSPORT:
- Primary active transport: Energy is directly derived from ATP hydrolysis. Examples include the sodium-potassium pump (Na+/K+ pump), calcium ion pumps, and hydrogen ion pumps.
- Secondary active transport: Utilizes the energy stored in an electrochemical gradient, often created by primary active transport. There are two types:
- Co-transport: The substance is transported in the same direction as the ion (usually sodium) moving down its gradient. Examples include Na+/glucose co-transport and Na+/amino acid co-transport.
- Counter-transport: The substance is transported in the opposite direction to the ion (usually sodium) moving down its gradient. Examples include Na+/Ca2+ counter-transport and Na+/H+ counter-transport.
SPECIALIZED TRANSPORT MECHANISMS
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Endocytosis: Uptake of macromolecules into the cell through membrane invagination. There are three types:
- Pinocytosis: "Cell drinking" – involves the uptake of small molecules and fluids.
- Phagocytosis: "Cell eating" – involves the engulfment of larger particles, such as bacteria.
- Receptor-mediated endocytosis: Highly specific uptake of molecules that bind to specific receptor proteins on the cell surface.
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Exocytosis: Expulsion of substances from the cell by fusion of vesicles containing the substances with the cell membrane.
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Transcytosis: Transport of macromolecules across a cell layer. The macromolecule enters one side of the cell, traverses the cytoplasm, and exits the opposite side.
Clinical Correlates
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Abnormalities of Na+/K+ pump (number or function):
- Reduced Na+/K+ pump activity in the myocardium is associated with heart failure.
- Excessive sodium reabsorption in renal tubules contributes to hypertension.
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Channelopathies (Ion Channel Diseases):
- Sodium channel diseases: Dysfunction of sodium channels can lead to muscle spasms and Liddle’s syndrome (a kidney disorder characterized by hypertension).
- Potassium channel diseases: Potassium channel dysfunction can cause heart disorders, inherited deafness, and epileptic seizures in newborns.
- Chloride channel diseases: Dysfunction of chloride channels can result in renal stone formation and cystic fibrosis (a multi-organ disorder affecting the lungs, pancreas, biliary system, and immune system).
