The Molecular Composition of Cells

 Figure 2.1 

Characteristics of Water  

(A) Water is a polar molecule, exhibiting a slight negative charge (δ-) on the oxygen atom and a slight positive charge (δ+) on the hydrogen atoms. This polarity enables water molecules to form hydrogen bonds (dashed lines), which are crucial for many biological interactions.

In addition to water, the inorganic ions present in cells—such as sodium (Na+), potassium (K+), magnesium (Mg²+), calcium (Ca²+), phosphate (HPO₄²-), chloride (Cl-), and bicarbonate (HCO₃-)—make up 1% or less of total cell mass. Despite their small proportion, these ions play vital roles in various aspects of cell metabolism and function.

However, it is the organic molecules that are the distinctive components of cells. Most organic compounds fall into four major classes: carbohydrates, lipids, proteins, and nucleic acids. Proteins, nucleic acids, and most carbohydrates (polysaccharides) are macromolecules formed by the polymerization of smaller precursors—amino acids, nucleotides, and simple sugars, respectively. Together, these macromolecules constitute 80 to 90% of the dry weight of most cells. Lipids represent another key component, while the remaining cell mass consists of a variety of small organic molecules, including precursors to these macromolecules. Understanding the basic chemistry of cells involves exploring the structures and functions of these four major classes of organic molecules.

Carbohydrates 

Carbohydrates include both simple sugars and polysaccharides. Simple sugars, such as glucose, serve as the primary nutrients for cells; their breakdown provides energy and serves as starting materials for synthesizing other cellular constituents. Polysaccharides function as storage forms of sugars and are also essential structural components of the cell. Additionally, polysaccharides and shorter sugar polymers act as markers in various cell recognition processes, facilitating cell adhesion and the proper transport of proteins within the cell.

The structures of representative simple sugars, or monosaccharides, are depicted in Figure 2.2. These molecules have a basic formula of (CH₂O)ₙ, from which the term "carbohydrate" is derived (C for "carbo" and H₂O for "hydrate"). Glucose (C₆H₁₂O₆), a six-carbon sugar (n = 6), is particularly significant for cells as it serves as the primary source of cellular energy. Other simple sugars typically contain between three and seven carbon atoms, with three- and five-carbon sugars being the most prevalent. Sugars with five or more carbons can cyclize, forming ring structures that are the predominant forms found within cells. As shown below, these cyclized sugars can exist in two alternative forms (α or β) based on the configuration of carbon 1.

Figure 2.2

Structure of Simple Sugars

Illustration of sugars with three, five, and six carbons (triose, pentose, and hexose sugars, respectively). Sugars with five or more carbons can form ring structures that exist in two alternative forms.

Monosaccharides can be linked together through dehydration reactions, where water (H₂O) is removed, forming glycosidic bonds between their carbon atoms. When only a few sugars are connected, the resulting polymer is known as an oligosaccharide. In contrast, when hundreds or thousands of sugars are linked, the polymers are termed polysaccharides.

Figure 2.3

Formation of a Glycosidic Bond

Two simple sugars are joined through a dehydration reaction, as illustrated by the bond formation between two glucose molecules in the α configuration, linking carbon 1 of one to carbon 4 of another.

Common polysaccharides such as glycogen and starch serve as storage forms of carbohydrates in animal and plant cells, respectively. Both glycogen and starch consist entirely of glucose molecules in the α configuration (Figure 2.4). The primary linkage is between carbon 1 of one glucose and carbon 4 of another. Additionally, glycogen and one form of starch, amylopectin, contain occasional α (1→6) linkages, connecting carbon 1 of one glucose to carbon 6 of another, resulting in branched structures. While both glycogen and amylopectin have these branches, another form of starch, amylose, is unbranched.

Figure 2.4

Structure of Polysaccharides

Polysaccharides are macromolecules formed from hundreds or thousands of simple sugars. Glycogen, starch, and cellulose are composed entirely of glucose residues linked by α (1→4) glycosidic bonds.

The structures of glycogen and starch are fundamentally similar, serving the same function of glucose storage. In contrast, cellulose plays a distinct role as the main structural component of plant cell walls. Interestingly, cellulose is also made entirely of glucose molecules, but these residues are in the β configuration, making cellulose an unbranched polysaccharide (see Figure 2.4). The β (1→4) linkages between glucose residues allow cellulose to form long, extended chains that align side by side, creating fibers with remarkable mechanical strength.

Beyond their roles in energy storage and structural integrity, oligosaccharides and polysaccharides are vital in various cell signaling processes. Oligosaccharides often attach to proteins, serving as markers that guide proteins for transport to the cell surface or integration into specific subcellular organelles. Additionally, they play crucial roles in cell recognition and interactions among cells in the tissues of multicellular organisms.