# What Is Life? The Chemistry of Living Things Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/what-is-life-the-chemistry-of-living-things ## Built From Four Families Life is assembled from four classes of macromolecule, each a polymer of small repeating units. | Class | Monomer | Bond | Main role | |---|---|---|---| | Carbohydrates | Monosaccharides | Glycosidic | Energy, structure | | Lipids | (not true polymers) | Ester | Membranes, storage | | Proteins | Amino acids | Peptide | Catalysis, structure | | Nucleic acids | Nucleotides | Phosphodiester | Information | Every one of these bonds is made by **condensation**, releasing one water molecule per bond, and broken by **hydrolysis**, consuming one. So joining $n$ monomers into a single chain forms $n-1$ bonds and releases $n-1$ waters, a small piece of arithmetic that recurs throughout biochemistry. **Proteins** are the workhorses, and their function follows their shape through four levels: the **primary** sequence of amino acids, **secondary** helices and sheets held by hydrogen bonds, the **tertiary** three-dimensional fold, and the **quaternary** assembly of several chains. Denature a protein and the sequence survives while the function does not, which is exactly why the higher levels matter. **Enzymes** are catalytic proteins that lower a reaction's activation energy without being consumed. They do not change the equilibrium position, only the speed of arrival, the same rule that governs any catalyst. **Membranes** are lipid bilayers, and they form spontaneously because phospholipids are **amphipathic**: a hydrophilic phosphate head and two hydrophobic fatty-acid tails. No machinery assembles them; water does, by excluding the tails. Finally, a constraint that shapes all of cell biology. A sphere's surface area goes as $r^2$ while its volume goes as $r^3$, so $$\frac{SA}{V} = \frac{3}{r}$$ Doubling the radius halves the surface available per unit of contents. Since everything a cell needs must cross that surface, cells stay small, and large organisms are built from many small cells rather than from few large ones. > **Common pitfall:** thinking a big cell simply needs a bigger membrane. It has one, but volume outgrows area, so the supply per unit of demand keeps falling. This is why the answer to being large is more cells, not bigger ones. ## Practice questions 7 of this lesson's 11 practice questions, with answers. The full set is in the app. ### 1. A protein chain is assembled from 150 amino acids joined into one continuous strand. How many water molecules are released in forming it? **Answer:** 149 **Why:** Joining 150 amino acids into a single chain forms $150 - 1 = 149$ peptide bonds, and condensation releases one water per bond, so **149** water molecules. The off-by-one is the point: bonds count the gaps, not the units. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/a-protein-chain-is-assembled-from-150-amino-acids-joined-into-one ### 2. Why do phospholipids form a bilayer spontaneously when placed in water? A. They link together through covalent ester bonds between their hydrocarbon tails B. They form condensation bonds that release water as their polar heads align C. They are amphipathic, so water excludes the tails while accepting the heads D. They carry identical charges on their heads that repel them into two flat sheets **Answer:** C. They are amphipathic, so water excludes the tails while accepting the heads **Why:** A phospholipid has a water-loving head and water-fearing tails, so the only arrangement satisfying both is a sheet with tails buried inside. Water does the organising by exclusion, no machinery is needed, which is why membranes could arise before any protein existed to build them. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/why-do-phospholipids-form-a-bilayer-spontaneously-when-placed-in ### 3. Match each level of protein structure to what holds it together. **Answer:** - Primary structure → Peptide bonds in a fixed sequence - Secondary structure → Hydrogen bonds forming helices and sheets - Tertiary structure → Side-chain interactions folding the whole chain - Quaternary structure → Association of several separate chains **Why:** Only the primary level is covalent along the backbone; the higher levels are held by weaker interactions, which is exactly why heat or pH can destroy the fold while leaving the sequence intact. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/match-each-level-of-protein-structure-to-what-holds-it-together ### 4. Sort each statement by whether it describes condensation or hydrolysis. **Answer:** - Condensation: A water molecule is released, Two monomers are joined into a polymer - Hydrolysis: A water molecule is consumed, A polymer is broken into its subunits, The reaction digestion relies on **Why:** Condensation builds and releases water; hydrolysis breaks and consumes it. They are exact reverses, which is why the same bond count governs both directions. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/sort-each-statement-by-whether-it-describes-condensation-or ### 5. Arrange these levels of biological organisation from smallest to largest. **Answer:** 1. Atom 2. Macromolecule 3. Organelle 4. Cell 5. Tissue 6. Organism **Why:** Biology is organised hierarchically, and each level has properties the level below does not, a property biologists call emergence. The cell is the first level that is unambiguously alive. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/arrange-these-levels-of-biological-organisation-from-smallest-to ### 6. Denaturation destroys a protein's biological function while leaving its primary covalent backbone intact. What explains this loss of activity? A. Denaturation hydrolyses peptide bonds between adjacent residues B. Secondary and tertiary folding establish the catalytic geometry C. Unfolding prevents the polypeptide chain from forming aggregates D. The covalent backbone can no longer interact with water molecules **Answer:** B. Secondary and tertiary folding establish the catalytic geometry **Why:** Hydrolysis breaks primary bonds, whereas denaturation disrupts higher-order folding. Biological activity depends on precise three-dimensional conformations; once these collapse, catalytic sites lose the specific geometries needed to bind substrates. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/denaturation-destroys-a-proteins-biological-function-while-leaving ### 7. Which statements about enzymes are correct? A. They shift the equilibrium toward the products B. They speed the forward and reverse reactions equally C. They are not consumed by the reaction they catalyse D. They lower a reaction’s activation energy **Answer:** B. They speed the forward and reverse reactions equally; C. They are not consumed by the reaction they catalyse; D. They lower a reaction’s activation energy **Why:** Enzymes lower the barrier, emerge unchanged, and accelerate both directions equally, which is exactly why they cannot move the equilibrium. Only the *rate* of arrival changes, a rule that holds for every catalyst. Page: https://tryals.app/practice/biology-i/what-is-life-the-chemistry-of-living-things/which-statements-about-enzymes-are-correct