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Transcellular transport involves the transportation ofsolutes by acellthrough a cell.[1] Transcellular transport can occur in three different ways: active transport, passive transport, and transcytosis.
Active transport is the process of movingmolecules from an area of lowconcentrations to an area of high concentration. There are two types of active transport,primary active transport andsecondary active transport.[citation needed] Primary active transport usesadenosine triphosphate (ATP) to move specific molecules and solutes against its concentration gradient. Examples of molecules that follow this process arepotassium K+,sodium Na+, andcalcium Ca2+. A place in the human body where this occurs is in the intestines with the uptake ofglucose. Secondary active transport is when one solute moves down theelectrochemical gradient to produce enough energy to force the transport of another solute from low concentration to high concentration.[citation needed] An example of where this occurs is in the movement of glucose within the proximal convoluted tubule (PCT).
Passive transport is the process of moving molecules from an area of high concentration to an area of low concentration without expelling any energy. There are two types of passive transport, passive diffusion andfacilitated diffusion. Passive diffusion is the unassisted movement of molecules from high concentration to low concentration across apermeable membrane.[2] One example of passive diffusion is thegas exchange that occurs between theoxygen in theblood and thecarbon dioxide present in the lungs.[3] Facilitated diffusion is the movement of polar molecules down the concentration gradient with the assistance ofmembrane proteins. Since the molecules associated with facilitated diffusion are polar, they are repelled by the hydrophobic sections of permeable membrane, therefore they need to be assisted by the membrane proteins. Both types of passive transport will continue until the system reachesequilibrium.[4] One example of facilitated diffusion is the movement glucose from small intestineepithelial cells into theextracellular matrix of the blood capillaries.[5]
Transcytosis is the movement of large molecules across the interior of a cell. This process occurs by engulfing the molecule as it moves across the interior of the cell and then releasing the molecule on the other side. There are two types of transcytosis are receptor-mediated transcytosis (RMT) and adsorptive-mediated transcytosis (AMT). An example where both types of transcytosis occur is the movement of macromolecules across the blood-brain barrier (BBB) into the central nervous system (CNS).[citation needed]
In contrast,paracellular transport is the transfer of substances across an epithelium by passing through an intercellular space between the cells.
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