Biological Molecules and Enzymes

Study flashcards covering molecular bonding, carbohydrate types, lipid characteristics, protein hierarchy, and enzyme mechanisms based on the lecture notes.

How do covalent, ionic, and hydrogen bonds form and differ in strength according to the text?

Covalent bonds form when atoms share a pair of electrons in their outer shells to form a stable molecule. Ionic bonds form when oppositely charged ions attract one another (weaker than covalent bonds, e.g., $Na^+$ and $Cl^-$). Hydrogen bonds form between polarised molecules with an uneven distribution of charge; individually weak, they collectively alter physical properties significantly.

What is the distinction between a condensation reaction and a hydrolysis reaction?

A condensation reaction attaches monomer sub-units together with the formation and release of a molecule of water. A hydrolysis reaction uses the addition of water to break the bonds linking sub-units of a polymer, splitting the molecule into its constituent parts.

What chemical reaction takes place in the Benedict's test for reducing sugars to create a color change?

Reducing sugars donate electrons to Benedict's reagent (an alkaline solution of Copper(II) sulfate). When heated, this reduces the reagent to form an insoluble red precipitate of Copper(I) oxide.

Based on this chart of Benedict's test results, what precipitate colour and sugar percentage correspond to Traceable, Moderate, and High levels of reducing sugar?

Traceable corresponds to a green precipitate ($0.5\text{--}1\,\text{g\%}$), Moderate corresponds to an orange-red precipitate ($1.5\text{--}2\,\text{g\%}$), and High corresponds to a brick-red precipitate ($>2\,\text{g\%}$).

What specific reaction and products are depicted in this diagram?

The hydrolysis of the disaccharide maltose, where the addition of a water molecule breaks the glycosidic bond to yield two monosaccharide molecules of $\alpha$-glucose.

Which pairs of monosaccharides combine via condensation to form maltose, sucrose, and lactose?

Glucose joined to glucose forms maltose; glucose joined to fructose forms sucrose; glucose joined to galactose forms lactose.

How is a test for non-reducing sugars carried out if the initial Benedict's test is negative?

Add dilute hydrochloric acid to a fresh sample and place in a gently boiling water bath for 5 minutes to hydrolyse disaccharides. Slowly add sodium hydrogen carbonate solution to neutralise the acid, check with pH paper that it is alkaline, and re-heat with Benedict's reagent for 5 minutes. An orange-brown colour indicates non-reducing sugar was present.

How does the structure of starch suit its primary role as an energy storage molecule in plants?

Starch is insoluble so it does not affect water potential or draw water into cells by osmosis, and does not diffuse out of cells. It is compact, winding into a tight coil to store large amounts in small spaces. When hydrolysed, it releases $\alpha$-glucose readily for respiration, and its branched form has many ends for simultaneous enzyme action.

How does glycogen differ structurally from starch, and where is it stored?

Glycogen has shorter chains and is more highly branched than starch, allowing enzymes to break it down even more rapidly into glucose monomers. It is found in animal and bacterial cells (never plant cells) and is mainly stored in animal muscles and the liver.

What polysaccharide structure is shown in this diagram, and how do its monomers and bonding create its physical properties?

Cellulose. It is composed of straight, unbranched chains of $\beta$-glucose running parallel to each other. Hydrogen bonds form cross-linkages between adjacent chains, giving collective structural strength. The chains group into microfibrils, which assemble into fibres.

What are the key biological roles of lipids mentioned in the text?

Lipids act as an energy source (providing more than twice the energy of carbohydrates when oxidised), waterproofing material (waxy cuticles in plants and insects, sebaceous secretions in mammals), thermal and electrical insulation (retaining body heat and surrounding the myelin sheath), and physical protection around delicate organs such as kidneys.

What is the structural difference between a triglyceride and a phospholipid?

A triglyceride consists of three fatty acids each forming an ester bond with glycerol. In a phospholipid, one fatty acid is replaced by a phosphate molecule, creating a hydrophilic head that interacts with water and hydrophobic fatty acid tails that repel water.

What laboratory test is illustrated in this diagram, and what observation indicates a positive result?

The emulsion test for lipids. A sample is dissolved in ethanol in a dry tube, shaken, and decanted into water; the formation of a milky-white emulsion indicates the presence of a lipid.

How do fibrous and globular proteins differ in shape, solubility, and primary function?

Fibrous proteins form long, rope-like molecules, are insoluble in water, highly stable, and serve structural support and protective roles (e.g., collagen, keratin). Globular proteins have a compact spherical shape, are soluble in water with hydrophilic groups on the exterior, and perform metabolic, transport, catalytic, and immune roles.

What are the four structural levels of proteins shown in this diagram?

Primary: sequence of a chain of amino acids; Secondary: hydrogen bonding of the peptide backbone forming alpha helices or pleated sheets; Tertiary: 3D folding pattern driven by side chain interactions (hydrogen, ionic, and disulfide bonds); Quaternary: protein consisting of more than one amino acid chain assembled together.

What are the steps and color changes for the Biuret test for proteins?

Make the solution alkaline by adding a few drops of sodium hydroxide solution, then add copper(II) sulfate solution. The solution turns purple if protein is present; if no protein is present, it stays blue.

What historical hypothesis of enzyme specificity is shown in this diagram?

The lock and key model, which posits that the enzyme's active site has a rigid, pre-determined shape (the lock) into which only a complementary substrate molecule (the key) can precisely fit.

How does the 'induced fit' model shown here explain enzyme-substrate interaction?

The active site does not start with a fully complementary shape; the binding substrate induces a slight conformational change in the enzyme's active site, allowing it to mold tightly around the substrate to form an enzyme-substrate complex and strain its bonds.

Why does the rate of reaction decline rapidly after exceeding the optimal temperature shown on this curve?

Excessive thermal energy causes bonds maintaining the enzyme's tertiary structure to break, altering the active site shape (denaturation) so the substrate can no longer fit to form enzyme-substrate complexes.

How do changes in pH reduce the rate of an enzyme-catalysed reaction?

A change in pH alters charges on the amino acids constituting the active site, preventing the substrate from attaching. If significant, the pH change breaks ionic and hydrogen bonds holding the tertiary structure, denaturing the active site.

Based on this diagram, how do competitive and non-competitive inhibitors differ in their binding sites and mechanisms?

Competitive inhibitors have a similar shape to the substrate and compete directly for binding at the active site; their effect decreases as substrate concentration increases. Non-competitive inhibitors bind to an alternate site, changing the active site shape so the substrate cannot fit regardless of substrate concentration.