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Compound Lipids

Compound lipids contain fatty acids and glycerol as well as elements other than carbon, hydrogen, and oxygen. Some of the most important members of this group in biology are the phospholipids, which contain a phosphate molecule in addition to the fatty acids and glycerol (figure 2.29). The phosphate is further linked to a variety of other polar molecules, such as an alcohol, a sugar, or one of certain amino acids. This entire group is often referred to as a polar head group and is soluble in water.

Phospholipids occur as a double layer, termed a bilayer or unit membrane, in the cytoplasmic membrane, a structure that separates the outside of the cell from its internal contents (see figure 2.29). The chemical structure of the bilayer produces the essential properties of the cytoplasmic membrane. This membrane acts as the major barrier to the entry and exit of substances from the cell. The structure of the phospholipids is responsible for this barrier. A phospholipid consists of two parts, each with unique properties. The end with the phosphate bonded to the polar head group is hydrophilic and therefore soluble in water. The long fatty acid chains consisting of only C and H atoms are hydrophobic and therefore water insoluble. The hydrophilic regions orient themselves toward the external or internal (cytoplasmic) environment, with its high concentration of water. The long-chain fatty acids orient themselves away from the aqueous areas and inward interacting hydrophobically with the long hydrocarbon chains of other phospholipid molecules (see figure 2.29). Water-soluble substances, which are the most common and important in the cell's environment, cannot pass through the hydrophobic portion. Therefore, the cell has special mechanisms to bring these molecules into the cell. These will be discussed in chapter 3.

Other compound lipids are found in the outer covering of bacterial cells and will be discussed in chapter 3. These include the lipoproteins, covalent associations of proteins and lipids, and the lipopolysaccharides, molecules of lipid linked with polysaccharides through covalent bonds.

Some of the most important properties of macromole-cules of biological importance are summarized in table 2.3.

Chapter 2 The Molecules of Life

Table 2.3 Structure and Function of Macromolecules

Name

Subunit

Some Functions of Macromolecules

Protein Nucleic acids Polysaccharide Lipids

Amino acid

Nucleotide

Monosaccharide

Varies—No similar subunits

Catalysts; structural portion of many cell components

RNA—Various roles in protein synthesis; DNA—Carrier of genetic information

Structural component of plant cell wall; storage products

Important in structure of cell membranes

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