After nutrients enter the bloodstream through the small intestinal wall, they travel to each cell and permeate its membrane via either passive transport, active transport or endocytosis. Passive and active transport utilize small proteins embedded in the cell membrane to allow a nutrient's passage, whereas endocytosis involves engulfing a nutrient.
Nutrients must first be broken down through chewing and enzymes located in both the stomach and small intestine. Cells in the small intestine wall are the point where most nutrients enter the bloodstream. Enzymes work to modify the molecular structure of cells for absorption in the small intestine itself and in the cells which form its wall.
The membrane of a cell contains both an entry and an exit side, and both sides employ the use of proteins as nutrient doorways. This system of nutrient transportation is either passive or active, although sometimes passive transport concerns nutrients that enter directly through the membrane. Active transport, conversely, requires energy to enable a transport protein to act as a pump.
Different types of nutrients use varying methods of access to a cell. Carbohydrates end up as glucose and use both active and passive transport depending on the type of cell being entered/exited. Amino acids, the final conversion stage of proteins, uses active transport. Fats, or triglycerides, experience a chain of transport methods until they are eventually converted in the bloodstream to enter cells through passive transport.