* Size: Proteins are large molecules, too big to pass through the cell membrane's phospholipid bilayer.
* Hydrophobicity: The cell membrane is primarily hydrophobic, while proteins are often hydrophilic (water-loving). This makes it difficult for them to cross the membrane barrier.
Here's how proteins get into cells:
* Breakdown into Amino Acids: Proteins are broken down into their constituent amino acids through digestion in the gut. These smaller amino acids are then absorbed into the bloodstream.
* Amino Acid Transport: The amino acids travel through the bloodstream and are taken up by cells via specific protein transporters located in the cell membrane. These transporters facilitate the movement of amino acids across the membrane.
Therefore, it's not the whole protein that enters the cell, but the individual amino acids that are used to build new proteins within the cell.
