Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

with a partially bony endoskeleton and bony, armored plates. Another class of fishes is Chondrichthyes. These fishes have endoskeletons composed entirely of cartilage. This class includes the sharks, skates, and rays. Gas exchanges are made at several prominent, vertical gill openings on either side of the throat.

 

Bony fishes belong to the class Osteichthyes. Most of the familiar fishes living today are bony fishes. They live in the oceans (for example, tuna, mackerel, and herring) and in freshwater (for example, striped bass, trout, and goldfish). Bony fishes prospered during the Devonian Period, which is also called the Age of Fishes. Bony fishes have gills, as well as fleshy pectoral and pelvic fins. The ray-finned fishes are the predominant type of bony fishes living today. These fishes have a swim bladder, a gas-filled sac near the gut that permits a fish to change its buoyancy.

 

Amphibians

 

Amphibians are animals that live both on land and in water. The members of the class Amphibia are believed to have evolved from the lobe-finned fishes about 370 million years ago, taking advantage of the higher concentration of oxygen in air than in water. A fossil called Tiktaalik, discovered in 2006, was an amazing model of the intermediate form between early tetrapods and their fish ancestors. Tiktaalik had qualities of a fish (fins, gills), but unlike a fish, also possessed a full set of ribs, a neck, and shoulders. The bones of the front fin also had the basic pattern (humerus, radius, ulna, wrist bones) of all limbed animals.

 

Today’s amphibians are represented by frogs, toads, and salamanders. Amphibians live on land and breathe air to meet their oxygen demands. Amphibians are also able to exchange gases through their lungs, their skin, and the inner lining of their mouths. Gas exchange is enhanced by an efficient circulatory system.

 

Amphibians remain in moist environments or water to avoid dehydration. Amphibians also lay their eggs in water because the eggs would quickly dry out on land. Sperm cells are released into the water, where they fertilize the egg mass. The early-stage tadpoles lead an aquatic existence and later emerge onto land as adult amphibians.

 

Reptiles

 

Fossils indicate the earliest reptiles lived about 310 million years ago and resembled today’s lizards. In ancient times, the predominant reptiles were dinosaurs. The modern survivors of the Age of Reptiles include lizards, snakes, crocodiles, alligators, turtles, and birds (which are included in the reptile clade). Reptiles belong to the class Reptilia.

 

Reptiles display a number of adaptations that support their life on land. They have a dry, scaly skin that retards water loss. The structural makeup of their limbs provides better support and allows them to move more quickly than any amphibian.

 

Reproduction in reptiles occurs exclusively on land. The male places sperm into the body of the female, and the embryo develops within an egg, which is laid on dry land. Other reptilian characteristics pertain to the respiratory and circulatory systems. The lungs have a greater surface area than that of the amphibians and permit more air to be inhaled. The circulatory system includes a three-chambered heart that separates oxygen-rich and oxygen-poor blood.

 

Birds

 

Though they are also classified as reptiles, birds belong to their own class, Aves. Birds have many structural adaptations for flight. For instance, the body is streamlined to minimize air resistance, and the endoskeleton bones are light and hollow. Many of the bones are also fused to provide compact strength. To enable flight, birds have feathers, which are lightweight adaptations of reptilian scales. Feathers also insulate against the loss of body heat and water.

 

Birds are homeothermic, meaning they are able to maintain a constant body temperature. The rapid pumping of their four-chambered heart and a high blood-flow rate contribute to this characteristic. Insulating feathers also help maintain a constant body temperature.

 

Mammals

 

Members of the class Mammalia have hair and nourish their young with milk produced by mammary glands. The presence of body hair or fur helps maintain a constant body temperature in the homeothermic mammals.

 

Several types of mammals exist: the monotremes, marsupials, and placentals. Monotremes are egg-laying mammals that produce milk. The duck-billed platypus and the spiny anteater are examples. They are both found in Australia and probably developed during the geographic isolation of that continent.

 

Marsupials are mammals whose embryos develop within the mother’s uterus for a short period of time before birth. After birth, the immature babies crawl into the mother’s abdominal pouch, where they complete their development. Animals such as the kangaroo, opossum, and koala bear are marsupials.

 

The placental mammals include many familiar animals, such as rabbits, deer, dogs, cats, bats, whales, monkeys, and humans. These mammals have a placenta: a nutritive connection between the embryo and the mother’s uterine wall. Embryos are attached to the placenta, and they complete their development within the mother’s uterus.

 

Mammals have spread to virtually all environments on Earth, ranging from the oceans to the deserts. They live underground, on the ground surface, in trees, and in the air. Mammals have a highly developed nervous system, and many have acute senses of smell, hearing, taste, vision, and touch. Mammals rely on memory and learning to guide their activities. They have been able to develop numerous appropriate responses to different environmental situations. They are considered the most successful animals on Earth today.

Chapter 22: Nutrition and Digestion

  Introduction to Nutrition and Digestion

 

Nutrition refers to the activities by which living things obtain raw materials from the environment and transport them into their cells. The cells metabolize these raw materials and synthesize structural components, enzymes, energy-rich compounds, and other biologically important substances. All the elements and compounds taken into a living thing are nutrients.

 

Animals, including humans, are heterotrophic organisms, and their nutrients consist of preformed organic molecules. The organic molecules rarely come in forms that are readily useful, so animals must process the foods into forms that can be absorbed. This processing is called digestion.

 

Human Digestive System

 

In the human digestive system, large organic masses are broken down into smaller particles that the body can use as fuel. This is a complex process. The breakdown of the nutrients requires the coordination of several enzymes secreted from specialized cells within the mouth, stomach, intestines, and liver. The major organs or structures that coordinate digestion within the human body include the mouth, esophagus, stomach, small and large intestines, and liver.

 

Mouth

 

In the human body, the mouth (oral cavity) is a specialized organ for receiving food and breaking up large organic masses. In the mouth, food is changed mechanically by biting and chewing. Humans have four kinds of teeth: incisors are chisel-shaped teeth in the front of the mouth for biting; canines are pointed teeth for tearing; and premolars and molars are flattened, ridged teeth for grinding, pounding, and crushing food.

In the mouth, food is moistened by saliva, a sticky fluid that binds food particles together into a soft mass. Three pairs of salivary glands—parotid, submaxillary, and sublingual—secrete saliva into the mouth. The saliva contains an enzyme called amylase, which digests starch molecules into smaller molecules of the disaccharide maltose.

 

During chewing, the tongue moves food about and manipulates it into a mass called a bolus. The bolus is pushed back into the pharynx (throat) and is forced through the opening to the esophagus.

 

Esophagus

 

The esophagus is a thick-walled muscular tube located behind the windpipe that extends through the neck and chest to the stomach. The bolus of food moves through the esophagus by peristalsis: a rhythmic series of muscular contractions that propels the bolus along. The contractions are assisted by the pull of gravity.

 

Stomach

 

The esophagus joins the stomach at a point just below the diaphragm. A valvelike ring of muscle called the cardiac sphincter surrounds the opening to the stomach. The sphincter relaxes as the bolus passes through and then quickly closes.

The stomach is an expandable pouch located high in the abdominal cavity. Layers of stomach muscle contract and churn the bolus of food with gastric juices to form a soupy liquid called chyme.

The stomach stores food and prepares it for further digestion. In addition, the stomach plays a role in protein digestion. Gastric glands called chief cells secrete pepsinogen. Pepsinogen is converted to the enzyme pepsin in the presence of hydrochloric acid. Hydrochloric acid is secreted by parietal cells in the stomach lining. The pepsin then digests large proteins into smaller proteins called peptides. To protect the stomach lining from the acid, a third type of cell secretes mucus that lines the stomach cavity. An overabundance of acid due to mucus failure may lead to an ulcer.

 

Small intestine

 

The soupy mixture called chyme spurts from the stomach through a sphincter into the small intestine. An adult’s small intestine is about 23 feet long and is divided into three sections: the first 10 to 12 inches form the duodenum; the next 10 feet form the jejunum; and the final 12 feet form the ileum. The inner surface of the small intestine contains numerous fingerlike projections called villi (the singular is villus). Each villus has projections of cells called microvilli to increase the surface area.

Most chemical digestion takes place in the duodenum. In this region, enzymes digest nutrients into simpler forms that can be absorbed. Intestinal enzymes are supplemented by enzymes from the pancreas, a large, glandular organ near the stomach. In addition, bile enters the small intestine from the gallbladder to assist in fat digestion.

 

The enzymes functioning in carbohydrate digestion include amylase (for starch), maltase (for maltose), sucrase (for sucrose), and lactase (for lactose). For fats, the principal enzyme is lipase. Before lipase can act, the large globules of fat must be broken into smaller droplets by bile. Bile is a mixture of salts, pigments, and cholesterol that is produced by the liver and stored in the gallbladder, a saclike structure underneath the liver.

 

Protein digestion is accomplished by several enzymes, including two pancreatic enzymes: trypsin and chymotrypsin. Peptides are broken into smaller peptides, and peptidases reduce the enzymes to amino acids. Nucleases digest nucleic acids into nucleotides in the small intestine also.

 

Most absorption in the small intestine occurs in the jejunum. The products of digestion enter cells of the villi, move across the cells, and enter blood vessels called capillaries. Diffusion accounts for the movement of many nutrients, but facilitated diffusion is responsible for the movement of glucose and amino acids. The products of fat digestion pass as small droplets of fat into lacteals, which are branches of the lymphatic system.

Absorption is completed in the final part of the small intestine, the ileum. Substances that have not been digested or absorbed then pass into the large intestine.

 

Large intestine

 

The small intestine joins the large intestine in the lower-right abdomen of the body. The two organs meet at a blind sac called the cecum and a small fingerlike organ called the appendix. Evolutionary biologists believe the cecum and appendix are vestiges of larger organs that may have been functional in human ancestors.

The large intestine is also known as the colon. It is divided into ascending, transverse, and descending portions, each about one foot in length. The colon’s chief functions are to absorb water and to store, process, and eliminate the residue following digestion and absorption. The intestinal matter remaining after water has been reclaimed is known as feces. Feces consist of nondigested food (such as cellulose), billions of mostly harmless bacteria, bile pigments, and other materials.