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Unit 1
8–11% of examEvery living thing is built from a handful of elements arranged into a few kinds of large molecules. In this unit you'll see how water's shape makes life possible, how cells build big molecules and break them down, and how a molecule's structure decides its job. Almost everything later in the course, from membranes to DNA, builds on these ideas.
Longer videos that cover the whole unit. Good for a first pass or a final review.
Oxygen pulls shared electrons harder than hydrogen does, so a water molecule is polar, with a slightly negative end and two slightly positive ends. Those ends form hydrogen bonds with neighboring molecules, which gives water cohesion, adhesion, surface tension and a high specific heat that helps living things hold a steady temperature.
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Living things take atoms in from their surroundings to build new molecules, and carbon, hydrogen and oxygen make up most of them. Nitrogen is part of proteins and nucleic acids, phosphorus is part of nucleic acids and phospholipids, and sulfur is found in some amino acids.
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Large biological molecules are polymers: long chains of smaller repeating units called monomers. Dehydration synthesis links two monomers by removing the equivalent of a water molecule, and hydrolysis breaks that bond by adding water back.
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Simple sugars (monosaccharides) such as glucose are the building blocks of carbohydrates, and they link into long straight or branched chains called polysaccharides. Plants store energy as starch and build their cell walls from cellulose, while animals store energy as glycogen.
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Lipids are mostly nonpolar, so they don't mix with water. They include fats for energy storage, phospholipids that form membranes, and steroids such as cholesterol and some hormones. A saturated fatty acid has only single bonds between its carbons, while an unsaturated one has at least one double bond that kinks the chain, so unsaturated fats tend to be liquid at room temperature.
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DNA and RNA store information in the order of their nucleotides, each made of a five-carbon sugar, a phosphate and a nitrogenous base, and new nucleotides are always added to the 3′ end of a strand. DNA is a double helix whose two strands run in opposite directions, with A pairing with T and C with G; RNA is usually single-stranded, uses the sugar ribose, and has uracil instead of thymine.
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Proteins are chains of amino acids joined by peptide bonds, and each amino acid's R group (side chain) can be nonpolar, polar or charged. The amino acid sequence (primary structure) drives local folding into helices and sheets, an overall 3D shape, and sometimes a complex of several chains, and that final shape determines what the protein does.
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