What happens to Large Molecules after Digestion?

The Mosaic Nature of a Cell Membrane

Have you ever eaten a peanut butter and mayonnaise sandwich? A cell membrane is built sort  of like a sandwich.

It is a double layer of fat sandwiched between two water soluble layers. There are a variety of things that facilitate movement back and forth across this selectively permeable membrane.

The by-products of digestion are monosaccharides (little molecules of sugar) , amino acids (little protein parts) and fats (fatty acids and glycerol.) These little guys have to make it through the cell membrane somehow. 

If the molecule is fairly simple, a concentration gradient is enough. If you have a large number of molecules on one side of a membrane and fewer molecules on the other side the flow of the molecules goes from a high concentration of the molecules to a concentration that is lower.

This is a definition of Simple Diffusion across a selectively permeable membrane. The membrane uses simple diffusion based on concentration gradients.

What happens if one or more of the following prevents or slows down diffusion?

• The molecule is charged. (like an ion.)
• The molecule is too large.
• The molecule is shaped wrong. (a square peg in a round hole.)
• The molecule is water soluble but the membrane has a double lipid layer (think fat). (they are not miscible)
• The molecule is soluble in fats but will not mix with water or water soluble molecules.
• There isn't a concentration gradient or you try to move a molecule against a concentration gradient if a gradient exists.

Since in Diabetes lets see what happens to Glucose, a monosaccharide breakdown product of a complex polysaccharide. 

Glucose is a large molecule that needs the presence of a transporter protein that is unique to glucose. The transporter acts as a signal mechanism that opens an area either on the outside or inside of a cell membrane. 

Imagine a V-shaped structure that opens on the outside of a cell membrane (V) and the glucose molecule, with the assistance of the protein transporter, helps the glucose wedge into the open end of the (V). As soon as the glucose is wedged in the (V) is reversed and the open end of the (V) is now pointing the other way. The glucose molecule is released into the inside of the cell. It is a modified protein tube that passes completely through the mosaic like cell membrane.

It by-passes several of the hinderances to the passage of glucose mentioned above.

By-passes charge, shape, chemical nature, and size (up to a point). Since digestion keeps a steady supply of glucose coming into the system, the concentration gradient is higher on the outside of the mosaic membrane. The glucose molecule follows is concentration gradient.

In fact the cell membrane has a bunch of these holes that make it look like a piece of swiss cheese.

In summary, this is how a large molecule, like glucose, makes its way into a cell or out of a cell.