TEAS 7 · Science · 6 min read
Biomolecules and Microorganisms
- Differentiate the chemical structures, building blocks, and bonding patterns of carbohydrates, lipids, proteins, and nucleic acids.
- Contrast dehydration synthesis and hydrolysis reactions in the assembly and breakdown of biological macromolecules.
- Distinguish cellular microorganisms from acellular infectious agents based on cellular organization, structural components, and pathogenicity.
Foundations of Macromolecules and Carbohydrates
Biological macromolecules are large organic molecules primarily composed of carbon, hydrogen, and oxygen, often containing additional elements like nitrogen, phosphorus, or sulfur. Many of these large macromolecules exist as polymers, which are structures formed by covalently linking smaller, repeating subunits called monomers. Monomers join together to build long chains, and cells assemble or disassemble these structures depending on whether they need to store resources or liberate smaller molecules.
Cells control macromolecular assembly and breakdown through two fundamental chemical processes: dehydration synthesis and hydrolysis. Dehydration synthesis creates covalent bonds between individual monomers by removing a molecule of water, linking the subunits together into a larger polymer. Conversely, hydrolysis reactions break covalent bonds between monomers by adding a molecule of water. For example, when an organism digests stored nutrient polymers, it relies on hydrolysis reactions to break those polymers down into usable monomers.
Carbohydrates are essential biological macromolecules composed of carbon, hydrogen, and oxygen. While many monosaccharides display a hydrogen-to-oxygen ratio of 2:1, this ratio does not apply to all carbohydrates. Monosaccharides represent the monomer units of carbohydrates, with glucose, fructose, and galactose serving as prime examples. When two monosaccharides join through a glycosidic bond, they create a disaccharide. Common dietary disaccharides formed by these bonds include sucrose, lactose, and maltose.
Polysaccharides are long carbohydrate polymers that function either in energy storage or in maintaining structural integrity. Starch serves as the primary carbohydrate energy-storage molecule in plants. In animals, glycogen functions as the branched energy-storage polysaccharide, stored primarily in the liver and skeletal muscles. Other polysaccharides provide structural strength: cellulose is a structural polysaccharide found in plant cell walls that humans cannot digest due to lacking the cellulase enzyme, whereas chitin is a structural polysaccharide found in fungal cell walls and arthropod exoskeletons.
Common mistakes
- Confusing dehydration synthesis with hydrolysis: dehydration synthesis removes water to create covalent bonds, while hydrolysis consumes water to break covalent bonds.
- Assuming all biological catalysts are proteins: while most enzymes are catalytic proteins, some RNA molecules also act as enzymes.
- Believing lipids are polymers: unlike carbohydrates, proteins, and nucleic acids, lipids are not composed of repeating chains of monomers.
- Thinking prions contain genetic material: prions are infectious, abnormally folded proteins that lack nucleic acids entirely.
Key points
- Polysaccharides, proteins, and nucleic acids are assembled from monomers by dehydration synthesis and broken down by hydrolysis; lipids are not true monomer-based polymers.
- Carbohydrates include monosaccharide monomers (glucose, fructose, galactose), disaccharides joined by glycosidic bonds (sucrose, lactose, maltose), and polysaccharides for energy storage (starch, glycogen) or structure (cellulose, chitin).
- Lipids are hydrophobic non-polymers, including triglycerides (glycerol and fatty acids linked by ester bonds), amphipathic phospholipids that form cell membrane bilayers, and four-ring steroids like cholesterol.
- Proteins are amino acid polymers joined by peptide bonds; their structure can be described at primary, secondary, and tertiary levels, with a quaternary level when multiple polypeptide subunits assemble.
- Nucleic acids are polymers of nucleotides joined by phosphodiester bonds; DNA stores genetic information in a double-stranded helix, while RNA participates directly in protein synthesis.
- Bacteria are unicellular prokaryotes lacking a membrane-bound nucleus and organelles, with peptidoglycan walls that are thick in Gram-positive bacteria and thin with an outer membrane in Gram-negative bacteria.
- Acellular pathogens include obligate intracellular viruses (capsid containing DNA or RNA, sometimes an envelope) and prions (infectious abnormally folded proteins lacking nucleic acids).
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