Oh

a -D-Glucose ch2oh ch2oh

Level 4: The cell and its organelles

Level 3: Supramolecular complexes

Level 2: Macromolecules

Level 4: The cell and its organelles

Level 3: Supramolecular complexes

Level 2: Macromolecules

Nucleotides

Sugars p&off

Cell wall

FIGURE 1-11 Structural hierarchy in the molecular organization of cells. In this plant cell, the nucleus is an organelle containing several types of supramolecular complexes, including chromosomes. Chro-

Level 1: Monomeric units

Nucleotides

Amino acids

Amino acids oh h h3n-c-coo-ch3

Cell wall

Sugars p&off n oh h

FIGURE 1-11 Structural hierarchy in the molecular organization of cells. In this plant cell, the nucleus is an organelle containing several types of supramolecular complexes, including chromosomes. Chro-

with the light microscope. Figure 1-11 illustrates the structural hierarchy in cellular organization.

The monomeric subunits in proteins, nucleic acids, and polysaccharides are joined by covalent bonds. In supramolecular complexes, however, macromolecules are held together by noncovalent interactions—much weaker, individually, than covalent bonds. Among these noncovalent interactions are hydrogen bonds (between polar groups), ionic interactions (between charged groups), hydrophobic interactions (among nonpolar groups in aqueous solution), and van der Waals interactions—all of which have energies substantially smaller than those of covalent bonds (Table 1-1). The nature of these noncovalent interactions is described in Chapter 2. The large numbers of weak interactions between macromolecules in supramolecular complexes stabilize these assemblies, producing their unique structures.

In Vitro Studies May Overlook Important Interactions among Molecules

One approach to understanding a biological process is to study purified molecules in vitro ("in glass"—in the test tube), without interference from other molecules present in the intact cell—that is, in vivo ("in the living"). Although this approach has been remarkably revealing, we must keep in mind that the inside of a cell is quite different from the inside of a test tube. The "interfering" components eliminated by purification may be critical to the biological function or regulation of the molecule purified. For example, in vitro studies of pure mosomes consist of macromolecules of DNA and many different proteins. Each type of macromolecule is made up of simple subunits— DNA of nucleotides (deoxyribonucleotides), for example.

enzymes are commonly done at very low enzyme concentrations in thoroughly stirred aqueous solutions. In the cell, an enzyme is dissolved or suspended in a gellike cytosol with thousands of other proteins, some of which bind to that enzyme and influence its activity.

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