Biomolecules and Macromolecules

A macromolecule is a large molecule built by joining many smaller repeating units together, the same way a long train is built by linking many individual train cars. The small repeating unit is called a monomer. A chain of monomers linked together is called a polymer. Living things are built almost entirely from four families of these large molecules: carbohydrates, lipids, proteins, and nucleic acids. Each family uses its own kind of monomer and does its own job in the cell, from storing energy to carrying genetic information.

Cells build and break down polymers using two opposite reactions. Dehydration synthesis (also called a condensation reaction) joins two monomers together by forming a covalent bond between them and releasing a water molecule (H₂O) as a byproduct. "Dehydration" means water is removed. This is how a cell builds a polymer one monomer at a time. Hydrolysis breaks a polymer apart by adding a water molecule across a bond, splitting it back into two monomers. "Hydro-" means water, "-lysis" means to break apart. This is how digestion works, enzymes in your digestive system use hydrolysis to break the polymers in food back down into monomers small enough to absorb.

Example: Two glucose molecules join through dehydration synthesis to form the disaccharide maltose, releasing one water molecule: glucose + glucose → maltose + H₂O. Example: During digestion, an enzyme uses hydrolysis to break maltose back into two glucose molecules by adding a water molecule across the bond: maltose + H₂O → glucose + glucose.

Carbohydrates are built from sugar monomers called monosaccharides, the most common of which is glucose (C₆H₁₂O₆). Carbohydrates are the cell's quick energy source and, in some organisms, a structural material. When two monosaccharides join by dehydration synthesis, the product is a disaccharide. When many monosaccharides join into a long chain, the product is a polysaccharide.

Lipids include fats, oils, phospholipids, and steroids. Unlike the other three families, lipids are not built from one single repeating monomer, so they are not true polymers. Most lipids are built from a glycerol molecule (a small 3-carbon backbone) bonded to one or more fatty acids (long hydrocarbon chains with a carboxyl group at one end). A triglyceride (a fat or oil) forms when glycerol bonds to three fatty acids through dehydration synthesis, forming three bonds and releasing three water molecules. Triglycerides are the body's long-term energy storage molecule.

A phospholipid replaces one of the three fatty acids with a phosphate group. This gives the molecule two different ends: a phosphate head that is polar and attracts water (hydrophilic), and two fatty acid tails that are nonpolar and repel water (hydrophobic). Because of this two-sided structure, phospholipids arrange themselves into the bilayer that forms every cell membrane, heads facing the water inside and outside the cell, tails facing each other in the middle. A steroid is a different kind of lipid built from four fused carbon rings instead of a glycerol-fatty acid structure. Cholesterol is a steroid found in animal cell membranes, and it is also the starting material the body uses to build hormones such as testosterone and estrogen.

Proteins are built from monomers called amino acids. Every amino acid has the same basic core, a central carbon bonded to an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable R group (side chain). There are 20 different amino acids used in living things, and the R group is the only part that changes from one to the next, giving each amino acid its own size, shape, and chemical properties. Amino acids link together through dehydration synthesis, forming a peptide bond between the carboxyl group of one amino acid and the amino group of the next, releasing a water molecule. A chain of amino acids linked by peptide bonds is called a polypeptide. Example: Two amino acids join through dehydration synthesis to form a dipeptide (a two-amino-acid chain), releasing one water molecule: amino acid + amino acid → dipeptide + H₂O.

Nucleic acids are built from monomers called nucleotides. Each nucleotide has three parts, a phosphate group, a five-carbon sugar, and a nitrogenous base. There are two nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). DNA's sugar is deoxyribose, and its four bases are adenine (A), thymine (T), cytosine (C), and guanine (G). RNA's sugar is ribose, and it swaps thymine for uracil (U), so its four bases are A, U, C, and G. Nucleotides link into a chain through their phosphate and sugar groups, forming a sugar-phosphate backbone with the bases sticking out to the side. DNA is normally double-stranded, twisted into a double helix, with bases on one strand pairing to bases on the other strand by complementary base pairing: A always pairs with T (2 hydrogen bonds), and C always pairs with G (3 hydrogen bonds). RNA is normally single-stranded.

Example: Given the DNA strand 5′-A T G C C T-3′, complementary base pairing (A with T, C with G) gives the paired strand 3′-T A C G G A-5′.

Using food-test-results.csv, answer using only the four test results given for each sample: 1. Which sample(s) test positive for starch? 2. One sample tests negative on all four indicators. Which sample is it, and what does that tell you about what it contains? 3. Which macromolecule(s) are present in sample E, and which are absent? 4. Sample B tests positive on Benedict's but negative on iodine. What does that combination tell you about sample B, compared to a sample that is positive on both? 5. Two samples both test positive for starch. What is the one other difference between their results, and which macromolecule does it point to?