Biology 12

Appendix 5 • MHR 561

Figure A5.4 Interaction of R-groups

Quaternary Structure

When two or more polypeptides join and
intertwine to form a protein, the resulting
complex structure is called a quaternary
structure. Part D of Figure A5.3 shows a
quaternary structure made up of two
polypeptides.
Why do proteins fold into these complicated
structures? In fact, a folded protein is in a
stable position of lowest energy. Hydrophobic
groups are near other hydrophobic groups,
while polar and ionic groups are interacting
with other polar groups, or with polar water
molecules. Because the structure of a protein
is stable, the protein retains its specific shape
unless the bonds between amino acids and
R-groups are broken.
This specific shape allows a protein to
perform complex functions. When a protein is
unfolded, by heating or by some other process,
the protein can no longer perform its function.
If a protein is mutated, by substituting one
amino acid in the chain of the protein for
another, the shape and function of the protein
is also disrupted. For example, sickle cell
hemoglobin has a single amino acid
substitution (valine replacing glutamic acid)
near the outer edge of the protein. The
placement of a non-polar amino acid on the
surface of the hemoglobin protein creates a
region that enables similarly mutated proteins
to stick together, forming long chains that are
unable to move through cell membranes.

Part B: The Cell Membrane and

Passive Transport of Molecules

All cells are surrounded by a structure, the cell

membrane, composed largely of a framework of

lipid molecules, with proteins embedded in it.

The cell membrane is so thin that it would take

about 10 000 of these membranes, stacked on

top of one another, to equal the thickness of a

sheet of paper. Nevertheless, the fine, delicate

structure of the cell membrane is essential to

the life of the cell. Not only does it provide a

structure for defining the shape and contents of

the cell, but also it serves as a selective barrier,

permitting only certain molecules and ions to

enter and leave the cell.

The cell membrane consists largely of

phospholipid molecules, each of which is made

up of a phosphate functional group and two

fatty acid chains that are bonded to a glycerol

molecule by means of a condensation synthesis

reaction. The resulting molecule has a non-polar,

hydrophobic “tail” (the fatty acids) and a polar,

hydrophilic “head” (the glycerol and phosphate

group). Due to natural forces of repulsion and

attraction with polar water molecules, the tail

of the phospholipid molecule faces inward,

away from water molecules, and the head faces

outward, towards the water molecules. When

large numbers of phospholipids interact with

water, they spontaneously form a double-layered

structure — a bilayer — with a non-polar interior.

The phospholipid bilayer is not a solid

structure. Rather, its texture is more like that

of thick olive oil; the phospholipid molecules

are in constant motion, moving and undulating

with the movement of the surrounding water

molecules.

One method by which small molecules may

pass through the cell membrane is by diffusion

— the net movement of molecules from a

region in which they are more concentrated to

one in which they are less concentrated. The

diffusion of a solvent, such as water, across a

membrane that separates two solutions is called

osmosis. The difference in concentrations of

any molecules separated by the cell membrane

is called a concentration gradient. When

molecules are distributed equally on both sides

of the membrane, there is no concentration

gradient.

hydrophobic and

van der Waals

interactions

polypeptide

backbone

CH 2

disulfide bridge

S S CH 2

CH 2 NH 3 +−O

ionic bond

CH 2 CH 2 CH 2 C

O

CH 2

H 3 C CH 3

H 3 C CH 3

CH

CH

CH 2

hydrogen
bond

C OH

O

O

H

CH 2