AQA A Level Biology Topic 1: Biological Molecules

Practice flashcards covering the key concepts of AQA A Level Biology Topic 1: Biological Molecules, including monomers, polymers, carbohydrates, lipids, proteins, enzymes, nucleic acids, ATP, water, and inorganic ions.

What are monomers?

Smaller, repeating molecules / units from which larger molecules / polymers are made.

Describe the difference between a condensation reaction and a hydrolysis reaction.

A condensation reaction joins 2 molecules together by forming a chemical bond and releasing a $H_2O$ molecule, while a hydrolysis reaction separates 2 molecules by breaking a chemical bond using a $H_2O$ molecule.

Why are lipids not considered polymers?

Lipids are not made from repeating monomers.

Identify the three common examples of monosaccharides.

Glucose, fructose, and galactose.

What is the structural difference between $\alpha$-glucose and $\beta$-glucose?

They are isomers; the $OH$ group is below carbon 1 in $\alpha$-glucose but above carbon 1 in $\beta$-glucose.

Name the monosaccharides that form maltose, sucrose, and lactose.

Maltose is glucose + glucose; Sucrose is glucose + fructose; Lactose is glucose + galactose.

Describe the relationship between the structure of starch (amylose) and its function.

It is a helical polysaccharide of $\alpha$-glucose, making it compact for storage; it is also large and insoluble, so it cannot cross the cell membrane or affect the water potential of the cell.

How does the structure of cellulose provide strength to plant cell walls?

It consists of long, straight, unbranched chains of $\beta$-glucose (with every other molecule inverted) linked in parallel by many hydrogen bonds which form strong microfibrils.

Outline the biochemical test for a non-reducing sugar.

1. Perform the Benedict's test (result stays blue). 2. Heat a new sample in a boiling water bath with acid to hydrolyse. 3. Neutralise with alkali. 4. Heat with Benedict's solution. 5. A positive result is a green, yellow, orange, or red precipitate.

What is the biochemical test for starch?

Add iodine dissolved in potassium iodide; a positive result is a change from orange/brown to blue-black.

Describe the structure of a triglyceride.

One glycerol molecule and three fatty acids joined by three ester bonds via three condensation reactions.

What is the difference between saturated and unsaturated fatty acids?

Saturated fatty acids have no $C=C$ double bonds in the hydrocarbon chain (carbons are fully saturated with hydrogen), whereas unsaturated fatty acids have one or more $C=C$ double bonds (creating a bend or kink).

How do phospholipids differ from triglycerides in structure?

In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group.

Describe the positive result for the emulsion test for lipids.

A milky white emulsion forms after adding ethanol, shaking, and then adding water.

Explain the four levels of protein structure.

Primary: sequence of amino acids in a polypeptide chain. Secondary: folding (alpha helix/beta pleated sheet) due to hydrogen bonding. Tertiary: 3D folding due to R-group interactions (hydrogen, ionic, and disulfide bridges). Quaternary: interactions between more than one polypeptide chain.

Describe the induced-fit model of enzyme action.

The substrate binds to the active site, which is not initially fully complementary; the active site changes shape slightly to become complementary, forming an enzyme-substrate complex and distorting bonds in the substrate to lower activation energy.

How do non-competitive inhibitors affect enzyme activity?

They bind to an allosteric site (other than the active site), changing the enzyme's tertiary structure and the shape of the active site so it is no longer complementary to the substrate, preventing enzyme-substrate complexes from forming.

What is the purpose of using a pH buffer or a thermostatically controlled water bath in enzyme experiments?

A buffer solution maintains a constant pH, and a water bath maintains a constant temperature, ensuring these control variables do not affect the rate of reaction.

Contrast DNA and RNA nucleotides.

DNA nucleotides contain deoxyribose and can have the base thymine, while RNA nucleotides contain ribose and can have the base uracil.

Describe the process of semi-conservative DNA replication.

1. DNA helicase breaks hydrogen bonds to unwind the double helix. 2. Strands act as templates. 3. Free DNA nucleotides join by complementary base pairing. 4. Hydrogen bonds form. 5. DNA polymerase joins adjacent nucleotides by condensation reactions, forming phosphodiester bonds.

Name the scientists who validated the Watson-Crick model of DNA replication.

Meselson and Stahl.

Describe the structure and breakdown of ATP.

ATP consists of ribose, adenine, and 3 phosphate groups; it is broken down into ADP and $P_i$ (inorganic phosphate) via a hydrolysis reaction catalysed by ATP hydrolase.

List five biologically important properties of water.

1. Metabolite. 2. Solvent. 3. High specific heat capacity. 4. Large latent heat of vaporisation. 5. Strong cohesion between molecules.

What is the biological role of Iron ions ($Fe^{2+}$)?

They are a component of the haem group in haemoglobin, allowing oxygen to bind for transport as oxyhaemoglobin.

How do sodium ions ($Na^{+}$) function in biological systems?

They are involved in the co-transport of glucose and amino acids into cells, action potentials in neurons, and affecting the water potential of cells.