Arthropod and Chordate Biology: Digestion, Absorption, and Transport

Arthropod Characteristics

Arthropods exhibit bilateral symmetry and a ventral nervous system. Their heart is positioned dorsally, and they have an open circulatory system. They possess an external chitinous exoskeleton, which they shed and replace with a new one during growth, a process called molting. The sensory organs of arthropods are highly developed, primarily located in the head and legs.

Chordate Features

• The alimentary canal includes glands that aid digestion, such as salivary glands, the pancreas, and the liver.
• The cerebrospinal nervous system consists of a thick dorsal nerve cord, forming the brain and spinal cord, with nerves branching out.
• Most chordates have separate sexes. Development typically occurs in eggs, although mammals and some other chordates are viviparous.
• Respiration can occur through gills or lungs, both of which are connected to the blood pharynx.
• Circulation is closed; blood remains within vessels and does not leave the heart. The heart is ventrally positioned.

Digestion of Carbohydrates

Carbohydrate digestion begins in the mouth, where salivary amylase breaks down starch into smaller fragments: dextrin, maltose, and glucose. In the stomach, amylase is denatured, but hydrolysis continues slowly due to hydrochloric acid. In the small intestine, pancreatic amylase completes the work started by salivary amylase.

Intestinal juice contains enzymes such as maltase, lactase, and sucrase, which break down disaccharides. There are also three enzymes that convert these into glucose, which is then absorbed by the intestinal villi.

Digestion of Proteins

Protein digestion begins in the stomach with the action of pepsin, facilitated by hydrochloric acid. In the small intestine, pancreatic juice releases trypsin, chymotrypsin, and carboxypeptidases, which further degrade peptides. Aminopeptidases in the intestinal juice complete the breakdown of peptides into amino acids, which are absorbed by the intestinal villi.

The intestinal juice also secretes enteropeptidase, which converts trypsinogen into trypsin.

Digestion of Fats

Fat digestion begins in the small intestine and requires the prior action of bile, which disperses fats into small droplets suspended in water, allowing digestive enzymes to act upon them. This process is slow, and to assist, the pancreas secretes a hormone that slows the release of chyme. Pancreatic lipase and intestinal enterolipase hydrolyze fats into glycerol and fatty acids, which are absorbed by the intestinal villi. In intestinal epithelial cells, glycerol and fatty acids are recombined.

• Amylase: Starch to Maltose, Dextrin
• Renin: Peptides
• Trypsin: Tripeptides to Amino Acids
• Enterolipase: Fat to Glycerol, Fatty Acids
• Lactase: Starch to Lactose and Galactose

Absorption and Transport of Water and Mineral Salts

The absorption of water and dissolved mineral salts, known as raw sap, occurs in the roots via two pathways:

1. Through the intercellular spaces of cortex cells by diffusion.
2. Through the living root bark, penetrating the root hairs and then moving from cell to cell. Water enters by osmosis, and salts by active transport.

The raw sap reaches the endodermis, where a waterproof band (Casparian strip) forces water to enter via osmosis. Past this, the central cylinder is reached, where the xylem is located.

Xylem Transport

Xylem walls are reinforced with lignin deposits in the form of rings and spirals, forming wood vessels. Two forces, transpiration and root pressure, along with a property of water, molecular cohesion, are responsible for the ascent of raw sap. Transpiration occurs through the stomata. The molecular structure of water provides high cohesion, which, combined with the narrowness of the xylem tubes, prevents rupture. Root pressure is generated by the constant osmotic absorption of water. It is effective when water is available, and the stomata are open during transpiration.

Transportation of Elaborated Sap

Carbohydrates, proteins, and other biomolecules constitute elaborated sap. Phloem is responsible for its transport. Phloem is a system of long, thin tubes formed by the union of living columnar cells called sieve cells, due to their perforated bases. Each sieve cell has an accompanying companion cell, which takes over its function when the sieve cell dies. The transport of elaborated sap from photosynthetic cells to non-photosynthetic cells is called translocation, driven by pressure differences.