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For a cell to be able to live, there must be communication (=transport) between the intracellular fluid (=the cytoplasm) and the extracellular fluid (= interstitial fluid).
These are the four major passive transport systems:
These are the three major active transport systems:
These are proteins located in the cell membrane that ‘actively’ pump ions and molecules in or out of the cell.
‘Actively’ means that this pumping requires energy, usually in the form of ATP.
The most famous pump is the Sodium-Potassium pump (=Na+-K+). This Na+-K+ pumps sodium ions out of the cell and, at the same time, potassium ions into the cell.
Other pumps are the H+-Ca2+ pump (used in muscles) and the H+-K+ pump (in the stomach).
The Sodium-Potassium pump is extremely important in the body. It is located in every cell membrane. And its main task is to produce a significant difference in the sodium and potassium concentrations inside and outside a cell.
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As we will see later (in the Nerves chapter), this concentration differences for sodium and for potassium is crucial for a proper functioning of the nerves and the muscle cells.
Primary Active Transport is transport that is made possible with energy from the breakdown of ATP (or other high-energy phosphate compound).
Secondary Active Transport is made possible by the ‘energy’ that is created by the differences in sodium concentration across the membrane; a difference which was made possible by the primary active transport in the first place!
Examples of Primary active transport systems are the sodium-potassium pump, the hydrogen-potassium pump and the calcium pump (as discussed in panel B).
In Secondary active transport systems, specialized proteins in the membrane use the concentration difference of, for example, the sodium ions across the membrane to “co”-transport another molecule.
For example, in the case of a symporter, every time a sodium ion goes back into the cell (based on its concentration gradient), it takes another molecule with it, such as glucose, amino acids etc. into the cell.
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In the case of an antiporter, the situation is the opposite; every time a sodium ion flows into the cell, another molecule, often a calcium or a hydrogen ion, is pumped out of the cell. (Q: what happens to the sodium ion that just went into the cell?).
In this transport system, vesicles are formed that either transport particles or molecules into the cells or moves them out of the cell.
This pit then gradually invaginates and finally forms a vesicle that separates itself from the plasma membrane to disappear into the machinery of the cell.
In exocytosis (‘exo’ = out of the cell), the opposite occurs. Vesicles that contain certain molecules, for example produced by the Golgi apparatus, migrate towards the plasma membrane of the cell and are then expelled out of the cell.
At the plasma membrane, the vesicle then fuses with the membrane and releases its contents into the extracellular space.
Phagocytosis (‘engulfing’) is like endocytosis in the sense that something is picked up and transported into the cell.
These pseudopodia fuse to each other thereby creating a vesicle, also called a phagosome, that contains the particle, and is moved into the cell.
Phagocytosis is performed by special cells such as macrophages, to ‘eat’ extracellular material such as bacteria, pathogens or cellular debris (as when cleaning up a wound).
Often, after the phagosome has been created, it fuses with neighboring lysosomes that contain destructive enzymes.
Answer to question B1:
The sodium ion is simply pumped back to the extracellular space by the sodium-potassium pump.
