Nutrients are absorbed from the bloodstream (serum) into the cellular compartment through a process called cellular uptake. This process involves various mechanisms that facilitate the transportation of nutrients across the cell membrane and into the interior of the cell, and this absorption can occur through five mechanisms: (1) active transport, (2) passive diffusion, (3) facilitated diffusion, (4) , and (5) endocytosis [ref].
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Passive Diffusion: Some small molecules, such as water and lipid-soluble substances, can diffuse directly across the cell membrane from an area of higher concentration (in the serum) to an area of lower concentration (inside the cell). This is a passive, energy-independent process.
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Facilitated Diffusion: Certain molecules, like glucose and amino acids, require specific transporter proteins to facilitate their movement across the cell membrane. These transporters assist in moving these nutrients into the cell along their concentration gradient, without the need for energy.
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Active Transport: Some nutrients, such as ions (e.g., sodium, potassium) and larger molecules, need energy to move against their concentration gradient from an area of lower concentration to a higher concentration inside the cell. This process is called active transport and often involves ATP (adenosine triphosphate) as an energy source.
- Co-transport (or secondary active transport) - a cellular process in which the movement of one substance across the cell membrane is coupled to the movement of another substance, utilizing the energy stored in the concentration gradient of the latter to transport both substances in the same direction or opposite directions.
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Endocytosis: In this process, the cell engulfs molecules or particles from the extracellular environment by forming vesicles. These vesicles are pinched off from the cell membrane and transport the nutrients into the cell.
