Biological Physics: Energy, Information, Life

2.2. The molecular parts list[[Student version, December 8, 2002]] 47

H

N

N

O

O

O

O

O

H

H

H

H

H

C

C

C

C

C

C

N

N

R N

R

1

2

3

4

5

6

Figure 2.19:(Artist’s sketches of molecular structure.) The alpha helix structure. An oxygen atom in each amino
acid forms a hydrogen bond with a hydrogen located four units farther down the chain, helping to stabilize the
ordered structure against thermal motion. Chapter 9 will discuss the formation and loss of ordered structures like
this one under changes in environmental conditions. The structure shown is “right-handed” in the following sense:
Choose either direction along the helix axis, for example upward in the figure. Point your right thumb along this
direction. Then as you proceed in the direction of your thumb, the ribbon in (b) rotates around the axis in the same
direction as your fingers point (and oppositely to the direction you’d have gotten using your left hand). [Copyrighted
figure; permission pending.]

can be thought of as polymers made up of monomers which are themselves proteins. Two examples
will be of particular interest in Chapter 10: microtubles and F-actin.
The organelles mentioned in Section 2.1.1 are suspended within the eukaryotic cell’s cytosol. The
cytosol is far from being a structureless, fluid soup. Instead, a host of structural elements pervade
it, both anchoring the organelles in place and conferring mechanical integrity upon the cell itself.
These elements are all long, polymeric structures; collectively they are called thecytoskeleton.
The most rigid of the cytoskeletal elements are themicrotubules(Figure 2.20). Microtubules
are 25nmin diameter and can grow to be as long as the entire cell. They form an interior network of
girders, helping the cell to resist overall deformation (Figure 2.23). Another function of microtubules
is to serve as highways for the transport of cell products from one place to another (see Figure 2.21
and Section 2.3.2).