Chapter 2: The Composition of Cells
Chapter Summary
THE MOLECULES OF CELLS
Carbohydrates: Carbohydrates include simple sugars and polysaccharides. Polysaccharides serve as storage forms of sugars, structural components of cells, and markers for cell recognition processes.
Lipids: Lipids are the principal components of cell membranes, and they serve as energy storage and signaling molecules. Phospholipids consist of two hydrophobic fatty acid chains linked to a hydrophilic phosphate-containing head group.
Nucleic Acids: Nucleic acids are the principal informational molecules of the cell. Both DNA and RNA are polymers of purine and pyrimidine nucleotides. Hydrogen bonding between complementary base pairs allows nucleic acids to direct their self-replication.
Proteins: Proteins are polymers of 20 different amino acids, each of which has a distinct side chain with specific chemical properties. Each protein has a unique amino acid sequence, which determines its three-dimensional structure. In most proteins, combinations of α helices and β sheets fold into globular domains with hydrophobic amino acids in the interior and hydrophilic amino acids on the surface.
CELL MEMBRANES
Membrane Lipids: The basic structure of all cell membranes is a phospholipid bilayer. Membranes of animal cells also contain glycolipids and cholesterol.
Membrane Proteins: Proteins can either be inserted into the lipid bilayer or associated with the membrane indirectly, by protein-protein interactions. Some proteins span the lipid bilayer; others are anchored to one side of the membrane.
Transport across Cell Membranes: Lipid bilayers are permeable only to small uncharged molecules. Ions and most polar molecules are transported across cell membranes by specific transport proteins, the action of which can be coupled to the hydrolysis or synthesis of ATP.
PROTEOMICS: LARGE-SCALE ANALYSIS OF CELL PROTEINS
Identification of Cell Proteins: Characterization of the complete protein complement of cells is a major goal of proteomics. One well-established method for proteomic studies is two-dimensional gel electrophoresis, which can be used to separate thousands of cell proteins. Mass spectrometry provides a powerful tool for protein identification, which can be used to analyze protein samples separated by two-dimensional gel electrophoresis or to identify proteins in unfractionated cell extracts.
Global Analysis of Protein Localization: Isolated subcellular organelles can be analyzed by mass spectrometry to determine their protein constituents. Alternatively, a large fraction of yeast proteins have been tagged with green fluorescent protein for global studies of protein localization.
Protein Interactions: A variety of large-scale approaches are being applied to systematically identify protein-protein interactions and complexes with the goal of elucidating the complex networks of protein interactions that regulate cell behavior.
