Essential Cell Biology 5th edition

64 CHAPTER 2 Chemical Components of Cells
ESSENTIAL CONCEPTS
• Living cells obey the same chemical and physical laws as nonliving
things. Like all other forms of matter, they are made of atoms, which
are the smallest unit of a chemical element that retains the distinctive
chemical properties of that element.
• Cells are made up of a limited number of elements, four of which—C,
H, N, O—make up about 96% of a cell’s mass.
• Each atom has a positively charged nucleus, which is surrounded by
a cloud of negatively charged electrons. The chemical properties of
an atom are determined by the number and arrangement of its electrons:
it is most stable when its outer electron shell is completely
filled.
• A covalent bond forms when a pair of outer-shell electrons is shared
between two adjacent atoms; if two pairs of electrons are shared, a
double bond is formed. A cluster of two or more atoms held together
by covalent bonds is known as a molecule.
• When an electron jumps from one atom to another, two ions of opposite
charge are generated; these ions are held together by mutual
attraction, forming a noncovalent ionic bond.
• Cells are 70% water by weight; the chemistry of life therefore takes
place in an aqueous environment.
• Living organisms contain a distinctive and restricted set of small,
carbon-based (organic) molecules, which are essentially the same
for every living species. The main categories are sugars, fatty acids,
amino acids, and nucleotides.
• Sugars are a primary source of chemical energy for cells and
can also be joined together to form polysaccharides or shorter
oligosaccharides.
• Fatty acids are an even richer energy source than sugars, but their
most essential function is to form lipid molecules that assemble into
sheet-like cell membranes.
• The vast majority of the dry mass of a cell consists of macromolecules—mainly
polysaccharides, proteins, and nucleic acids (DNA
and RNA); these macromolecules are formed as polymers of sugars,
amino acids, or nucleotides, respectively.
• The most diverse and versatile class of macromolecules are proteins,
which are formed from 20 types of amino acids that are covalently
linked by peptide bonds into long polypeptide chains. Proteins constitute
half of the dry mass of a cell.
• Nucleotides play a central part in energy-transfer reactions within
cells; they are also joined together to form information-containing
RNA and DNA molecules, each of which is composed of only four
types of nucleotides.
• Protein, RNA, and DNA molecules are synthesized from subunits by
repetitive condensation reactions, and it is the specific sequence of
subunits that determines their unique functions.
• Four types of weak noncovalent bonds—hydrogen bonds, electrostatic
attractions, van der Waals attractions, and the hydrophobic
force—enable macromolecules to bind specifically to other macromolecules
or to selected small molecules.
• Noncovalent bonds between different regions of a polypeptide or RNA
chain allow these chains to fold into unique shapes (conformations).