Exercises: Structures of nutrition

List the four processes of animal nutrition and explain in one sentence the function of each, indicating what would happen to a cell if any of them failed.

Order the path of a mouthful of food through the human digestive tract, from the mouth to the anus, naming all the organs it passes through.

Differentiate between mechanical digestion and chemical digestion, and give an example of each in the mouth and another in the stomach.

Compare branchial (gill), tracheal, and pulmonary respiration: in which animal group each occurs, what exchange surface it uses, and to which environment (aquatic or terrestrial) it is adapted.

Explain the difference between an open and a closed circulatory system, and between single and double circulation, citing a representative animal group for each case and one advantage of double circulation.

Observe the image of the human digestive system. a) Describe what happens to food in structures 1, 4, and 8. b) Indicate which numbered structures are accessory glands, that is, those that are not part of the digestive tract, and what each secretes into the tract. c) Order structures 1, 3, 4, 8, 9, and 11 according to the path of the bolus.

A paleontology team analyzes the remains of two extinct mammals of similar size (~50 kg). Species A possesses worn molars with pronounced enamel ridges (lophodont) and an abdominal cavity volume that suggests a large-capacity digestive system; species B presents molars with sharp cutting cusps (secodont) and a remarkably reduced abdominal cavity.

1. a) Predict and justify in a comparative manner the relative length of the digestive tract and the food retention time in both species based on the chemical constraints of degrading cellulose versus proteins.
2. b) If a live animal with a digestive anatomy analogous to species A is given prolonged treatment with broad-spectrum antibiotics, it suffers drastic loss of body mass despite continuing to consume abundant forage. Justify this phenomenon at the biochemical and physiological levels.
3. c) Why has natural selection not favored a hypertrophied digestive tract with microbial fermentation in obligate carnivores if this would allow them to extract nutrients from plant material when prey is scarce?

Analyze the following two physiological scenarios regarding nitrogen excretion:

• Case 1: Amphibian tadpoles are obligate ammonotelic organisms, but during metamorphosis into terrestrial adults they activate the genes for the enzymes of the ornithine cycle and become ureotelic.

• Case 2: The placental mammal embryo eliminates urea across the placenta, whereas the embryo of birds and reptiles encloses its nitrogenous wastes inside the shelled (cleidoic) egg.

1. a) Justify the switch in excretion during amphibian metamorphosis by evaluating the trade-off among cellular toxicity, environmental water availability, and energy cost in ATP molecules.
2. b) Predict the physiological and osmotic consequences for the development of a bird embryo if it were to carry a mutation preventing it from synthesizing uric acid and causing it to excrete only urea inside the egg. Compare its viability with that of a mammal embryo.
3. c) A student states: "Uric acid is the best evolutionary strategy for any animal because it allows water conservation at no biological cost." Refute this statement using thermodynamic and metabolic arguments.

A volunteer is orally administered glucose labeled with a radioactive isotope ($^{14}\text{C}$) and is made to inhale air enriched with heavy oxygen ($^{18}\text{O}$). During an intense muscular exercise session involving the arms, the metabolism of their tissues is analyzed.

1. a) Trace chronologically the pathway of the radioactive glucose from the lumen of the small intestine to the sarcoplasm of a biceps muscle fiber, specifying the four organ systems involved in nutrition, the biological membranes or barriers it crosses, and the type of transport it uses.
2. b) A classmate claims: "The $\text{CO}_2$ expelled by the athlete through the lungs directly contains the inhaled $^{18}\text{O}$, and the nitrogen from muscle protein catabolism is converted into urea inside the muscle itself before passing into the blood." Refute both statements by justifying the biochemical fate of $^{18}\text{O}$ in the respiratory chain and the role of the glucose-alanine cycle and the liver in nitrogen excretion.
3. c) Predict what homeostatic alterations would occur in blood pH and composition if a transient failure of renal perfusion occurred during exertion, and what compensatory respiratory mechanism will be immediately activated.