Physical Structure of Cell
The cell contains highly organized physical structures, called intracellular
organelles. The physical nature of each organelle is as important as the cell’s
chemical constituents for cell function.
The Cell Membrane:
The cell membrane (also called the plasma membrane) envelops the
cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is
composed almost entirely of proteins and lipids. The approximate composition is
proteins (55 percent); phospholipids (25 percent); cholesterol (13 percent); other
lipids (4 percent); and carbohydrates (3 percent).
Its basic structure is a lipid bilayer, which is a thin, double-layered
film of lipids—each layer only one molecule thick—that is continuous over the
entire cell surface. Interspersed in this lipid film are large globular proteins. The
basic lipid bilayer is composed of three main types of lipids: phospholipids,
sphingolipids, and cholesterol. Phospholipids are the most abundant of the cell
membrane lipids. One end of each phospholipid molecule is soluble in water; that
is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic.
The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion
is hydrophobic.
Integral and Peripheral Cell Membrane Proteins
These membrane proteins are mainly glycoproteins. There are two types of cell
membrane proteins: integral proteins that protrude all the way through the
membrane and peripheral proteins that are attached only to one surface of the
membrane and do not penetrate all the way through.
Integral Proteins: Many of the integral proteins provide structural
channels (or pores) through which water molecules and water-soluble substances
can diffuse. Other integral proteins act as carrier proteins for transporting
The cell contains highly organized physical structures, called intracellular
organelles. The physical nature of each organelle is as important as the cell’s
chemical constituents for cell function.
The Cell Membrane:
The cell membrane (also called the plasma membrane) envelops the
cell and is a thin, pliable, elastic structure only 7.5 to 10 nanometers thick. It is
composed almost entirely of proteins and lipids. The approximate composition is
proteins (55 percent); phospholipids (25 percent); cholesterol (13 percent); other
lipids (4 percent); and carbohydrates (3 percent).
Its basic structure is a lipid bilayer, which is a thin, double-layered
film of lipids—each layer only one molecule thick—that is continuous over the
entire cell surface. Interspersed in this lipid film are large globular proteins. The
basic lipid bilayer is composed of three main types of lipids: phospholipids,
sphingolipids, and cholesterol. Phospholipids are the most abundant of the cell
membrane lipids. One end of each phospholipid molecule is soluble in water; that
is, it is hydrophilic. The other end is soluble only in fats; that is, it is hydrophobic.
The phosphate end of the phospholipid is hydrophilic, and the fatty acid portion
is hydrophobic.
Integral and Peripheral Cell Membrane Proteins
These membrane proteins are mainly glycoproteins. There are two types of cell
membrane proteins: integral proteins that protrude all the way through the
membrane and peripheral proteins that are attached only to one surface of the
membrane and do not penetrate all the way through.
Integral Proteins: Many of the integral proteins provide structural
channels (or pores) through which water molecules and water-soluble substances
can diffuse. Other integral proteins act as carrier proteins for transporting
