All of these must pass through the cell membrane. Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vescicle attaches to the cell membrane from the inside and then opens to form a pocket, expelling its contents to the outside. This may be called exocytosis.ll of these must pass through the cell membrane. Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vescicle attaches to the cell membrane from the inside and then opens to form a pocket, expelling its contents to the outside. This may be called exocytosis.
Many enzymes that occur in certain tissues are found in blood plasma at higher levels when the cancer involves that tissue. Enzymes are usually measured by determining the rate at which they convert a substrate to an end product, while most tumor markers of other types are measured by a test called an immunoassay.
Enzymes are best measured not by their weight but by the work they are capable of doing. This is usually expressed in some form of activity unit as a measure of potency. Many enzymes can be assayed using methods that are internationally recognized. Use of these recognized assay procedures ensures that the methodology utilized can be reproduced to verify accuracy and legitimacy of the assay.