IB DP Biology- C1.3 Photosynthesis- IB Style Questions For HL Paper 2 -FA 2025
▶️ Answer/Explanation
(a)
A codon is a three-base sequence on mRNA. During translation, each codon corresponds to a specific amino acid. Because the codons appear in a particular order along the mRNA strand, they determine the exact sequence of amino acids that will be joined to form the polypeptide’s primary structure. The genetic code is degenerate since multiple codons can specify the same amino acid, and stop codons (UAA, UAG, UGA) signal termination of polypeptide synthesis.
(b)
Photosystems are arranged within the thylakoid membrane as complexes containing chlorophyll and accessory pigments. Their structural design allows them to capture light efficiently across a broad range of wavelengths. Energy absorbed by pigment molecules is transferred toward a reaction center, which releases an excited electron to begin the light-dependent reactions of photosynthesis. This architecture ensures optimal light absorption and energy transfer.
(c)
Causes of reduced blood pH:
A drop in blood pH is commonly due to rising levels of carbon dioxide (CO₂), which combines with water to form carbonic acid. This dissociates into H⁺ ions, lowering pH. Intense exercise may also cause accumulation of lactic acid, further contributing to increased hydrogen ion concentration.
Effects on ventilation rate:
Chemoreceptors located in the aorta, carotid arteries, and the medulla sense the rise in H⁺ concentration. These receptors stimulate the respiratory center, which sends signals to the diaphragm and external intercostal muscles. The result is an increase in the rate and depth of ventilation, speeding up CO₂ removal and helping return blood pH to its normal range.