molecules apply questions Flashcards
(10 cards)
A student adds Benedict’s reagent to two sugar solutions: glucose and sucrose. Only glucose gives a positive result after heating. Explain why.
Benedict’s test detects reducing sugars. Glucose is a reducing sugar and can donate electrons to the copper(II) ions in Benedict’s solution, reducing them to red copper(I) oxide, which forms a brick-red precipitate. Sucrose is a non-reducing disaccharide and does not react unless first hydrolysed by acid or an enzyme to break it into glucose and fructose, both of which are reducing sugars.
Starch and cellulose are both polymers of glucose. Explain how their structures differ and how this relates to their functions.
Starch is made of α-glucose, forming amylose (a coiled, helical chain) and amylopectin (branched chains). These compact, insoluble structures make starch ideal for energy storage. In contrast, cellulose is made of β-glucose, with every other glucose flipped, forming straight, unbranched chains that link via hydrogen bonds to form strong microfibrils. This gives cellulose structural strength in plant cell walls.
A student compares the energy content of equal masses of carbohydrate and lipid. Lipids provide more energy per gram. Explain why.
Lipids have a higher proportion of C–H bonds and are more reduced than carbohydrates, so they release more energy when oxidised during respiration. Also, lipids are hydrophobic and stored anhydrously, whereas carbohydrates associate with water, making lipids more energy-dense by mass.
A change in a single amino acid in a protein leads to loss of function. Explain how this can happen in terms of protein structure.
A single amino acid substitution can affect the primary structure, which determines the folding and final 3D shape of the protein. This may alter hydrogen bonding, ionic bonds, or disulfide bridges in the tertiary structure, changing the shape of the active site (if an enzyme) or disrupting the protein’s function (e.g., haemoglobin’s O₂-binding site).
A protein loses its function when placed in an acidic solution. Explain why.
Acidic conditions increase H⁺ concentration, disrupting ionic bonds and hydrogen bonding between R-groups in the tertiary structure. This causes the protein to denature, altering its 3D shape and therefore its ability to function, such as loss of enzyme activity due to active site distortion.
A student adds an enzyme to two solutions with different pH levels. Enzyme activity is much lower in one. Explain the likely cause
Each enzyme has an optimum pH. A change in pH alters the concentration of H⁺ or OH⁻ ions, disrupting bonds that maintain the enzyme’s tertiary structure. This changes the shape of the active site, preventing substrate binding and reducing the rate of enzyme-substrate complex formation.
In a competitive inhibition experiment, increasing substrate concentration overcomes the inhibitor’s effect. Explain why.
Competitive inhibitors have a similar shape to the substrate and bind to the enzyme’s active site. As substrate concentration increases, it becomes more likely that a substrate, rather than an inhibitor, will bind to the active site. This reduces the effect of the inhibitor and allows the normal reaction to proceed.
Water has a high specific heat capacity. Explain the importance of this property for organisms.
A high specific heat capacity means water resists rapid temperature changes, allowing aquatic environments to remain stable. It also helps maintain a stable internal temperature in organisms, protecting enzyme function and maintaining homeostasis.
Phosphate ions are essential in biological systems. Explain two roles they play.
Phosphate forms part of the phosphodiester backbone in DNA and RNA, linking nucleotides in long chains.
Phosphate groups are found in ATP, where breaking a phosphate bond releases energy used for metabolic processes.
Iron ions are a component of haemoglobin. Explain their role and why a deficiency causes fatigue.
Each haem group in haemoglobin contains an iron ion (Fe²⁺), which binds reversibly to oxygen for transport in the blood. A deficiency in iron means fewer functional haem groups, so less oxygen is transported, reducing aerobic respiration and leading to fatigue due to less ATP production.
