membrane will be the focus of this lesson.
What is Transport?
The selectively permeable nature of the plasma membrane is due in part to the chemical
composition of the membrane. Recall that the membrane is a double layer of phospholipids
(a ”bilayer”) embedded with proteins (Figure4.1). A single phospholipid molecule has a
hydrophilic, or water-loving, head and hydrophobic, or water-fearing, tail. The hydrophilic
heads face the inside and outside of the cell, where water is abundant. The water-fearing,
hydrophobic tails face each other in the middle of the membrane. At body temperature,
the plasma membrane is fluid and constantly moving, like a soap bubble; it is not a solid
structure.
Water and small non-charged molecules such as oxygen and carbon dioxide can pass freely
through the membrane by slipping around the phospholipids. But larger molecules and
charged molecules cannot pass through the plasma membrane easily. Therefore, special
methods are needed for transporting some molecules across the plasma membrane and into
or out of the cell.
Figure 4.1: The plasma membrane is made up of a phospholipid bilayer with embedded
proteins. ( 9 )
Since atoms have an equal number of protons and electrons, they have no net charge.
The negative charges of the electrons balance out the positive charges of the protons.
Many molecules have an equal number of electrons and protons, so we call them non-polar
molecules. However, some atoms can lose or gain electrons easily, giving them a positive or
negative charge. These charged particles are calledions. If an atom loses an electron, it
becomes a positively charged ion, such as the sodium ion Na+. If an atom gains an electron,
it will be a negatively charged ion, such as the chloride ion, Cl-. Na+and Cl-readily form
NaCl, or common table salt. Since Na+and Cl-are charged, they are unable to pass freely
through the plasma membrane.