▶️ Answer/Explanation
(a)(i) Identification of structures:
A: Trachea
B: Bronchus (or Bronchi)
C: Bronchiole(s)
Explanation: The trachea (windpipe) is the main tube with cartilage rings (A) leading from the mouth/nose. It splits into two bronchi (B) which enter the lungs, and further subdivides into smaller tubes called bronchioles (C).
(a)(ii) Function of cartilage:
The function is to keep the airway open or prevent the collapse of the airway.
Explanation: The C-shaped rings of cartilage provide structural support so that the trachea does not collapse inward when the pressure drops during inspiration.
(a)(iii) Mechanism of inspiration:
During inspiration (breathing in):
- The diaphragm contracts and flattens (moves downwards).
- The external intercostal muscles contract (while internal intercostal muscles relax), causing the ribs to move up and out.
- This action increases the volume of the thorax (chest cavity).
- Consequently, the pressure decreases inside the thorax/lungs.
- Air flows into the lungs to equalise the pressure difference.
(b)(i) Protection of airways:
Two common cell types are:
- Goblet cells: They secrete mucus. This sticky mucus traps dust, particles, and pathogens (bacteria/viruses).
- Ciliated cells: They have hair-like structures called cilia. These beat/move to sweep the mucus (containing trapped pathogens) up and out of the airways towards the throat to be swallowed.
Note: Lymphocytes (produce antibodies) and Phagocytes (engulf pathogens) are also acceptable answers.
(b)(ii) Effect on exercise:
Thicker mucus creates a blockage or narrowing of the airways. This leads to:
- Reduced surface area for gas exchange or reduced rate of diffusion.
- Less oxygen ($O_2$) reaches the blood and muscle cells (and less $CO_2$ is removed).
- Muscle cells perform less aerobic respiration and must rely more on anaerobic respiration.
- This results in less energy/ATP being released/produced for muscle contraction, causing fatigue more quickly.
(c) Genetic Diagram:
Since both parents are heterozygous, they carry one dominant and one recessive allele.
Parental genotypes: $Ff$ x $Ff$
Gametes: ($F$), ($f$) x ($F$), ($f$)
Offspring genotypes: $FF$, $Ff$, $Ff$, $ff$
Offspring phenotypes: No cystic fibrosis, No cystic fibrosis, No cystic fibrosis, Cystic fibrosis
Probability: $0.25$ or $25\%$ or $1:3$ or $\frac{1}{4}$
Explanation: The cross yields one homozygous dominant ($FF$), two heterozygous carriers ($Ff$), and one homozygous recessive ($ff$). Since cystic fibrosis is recessive, only the $ff$ genotype presents the condition.