Nutrition Science and Everyday Application - beta v 0.1

326 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
it is negatively charged it balances the many positively charged molecules circulating in the
blood, such as hydrogen protons (H + ), calcium, potassium, and magnesium. Albumin acts as
a buffer against abrupt changes in the concentrations of these molecules, thereby balancing
blood pH and maintaining homeostasis. The protein hemoglobin also participates in acidbase
balance by binding hydrogen protons.
TRANSPORT
Proteins also play vital roles in transporting substances around the body. For example,
albumin chemically binds to hormones, fatty acids, some vitamins, essential minerals, and
drugs, and transports them throughout the circulatory system. Each red blood cell contains
millions of hemoglobin molecules that bind oxygen in the lungs and transport it to all the
tissues in the body. A cell’s plasma membrane is usually not permeable to large polar
molecules, so to get the required nutrients and molecules into the cell, many transport
proteins exist in the cell membrane. Some of these proteins are channels that allow
particular molecules to move in and out of cells. Others act as one-way taxis and require
energy to function.
Figure 6.13. Molecules move in and out of cells through transport proteins, which are either
channels or carriers.