Biological Mechanisms: Digestion, Catabolism, and Synapses

Types of Digestion

Intracellular Digestion

Intracellular digestion involves engulfed food particles within a vesicle inside the cell. Lysosomes pour hydrolytic enzymes within this vesicle, forming a digestive vacuole where the chemical digestion of food is conducted. Once these gradients are established, the nutrients pass through the vacuole membrane and are incorporated into the hyaloplasm. Cellular waste products are expelled outside the vacuole by means of stool.

Mixed Digestion

Mixed digestion runs in two stages: one intracellular and the other extracellular. It occurs in animals such as cnidarians and platyhelminthes. In cnidarians, digestion begins in the gastrovascular cavity where the cells lining the walls secrete protein-degrading enzymes. Digestion continues inside cells located in the gastrodermis, the digestive layer of cells lining the gastrovascular cavity, where nutrients pass from the cells. Undigested food is expelled through the mouth via diffusion.

Extracellular Digestion

Extracellular digestion takes place outside the cells in the inner cavities that form the digestive tube. During this digestion, food undergoes transformations of two types:

• Mechanical transformations: The food is broken down into small portions with the aid of specialized organs, favoring chemical transformations. In most vertebrates, the teeth are responsible for fragmentation.
• Chemical transformations: These consist of enzymatic hydrolysis of large organic molecules to transform them into simpler ones.

Cellular Catabolism

Living things use two different types of catabolic pathways for organic compounds and energy production: cellular respiration, which is an aerobic process that releases much energy, and fermentation, an anaerobic process in which little energy is released. It is necessary to degrade nutrients; this is called catabolism, which requires oxygen and produces energy and waste substances.

Surfaces of Exchange

The element of any respiratory system is the respiratory surface, which is the region through which gases are exchanged. For gas exchange to occur efficiently, the respiratory surface must have these characteristics:

• Thin walls: Favoring diffusion.
• Moisture: In both aquatic and terrestrial animals, O₂ and CO₂ spread through respiratory surfaces after being dissolved in water.
• Vascularization: It is lined internally by a great number of blood vessels to facilitate gas exchange between the outer environment and the transport liquid.

Synapses

Once the nerve impulse reaches the end zone of the axon, it is transmitted to another neuron or an effector organ through functional contact. When the impulse reaches the presynaptic element, neurotransmitters are released into the synaptic cleft and bind to receptors on the postsynaptic membrane, causing an increase in Na⁺ permeability. The entry of sodium depolarizes the membrane and makes the nervous momentum spread again through the second neuron.