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

and uterus into the fallopian tubes, where fertilization takes place.

 

In the human female, egg cell production begins before birth, when about 2 million primitive cells known as oogonia accumulate in the ovaries. These oogonia are formed in the early stages of meiosis. After the age of puberty, the oogonia develop into primary oocytes and then into egg cells at a rate of one per month. Egg cell production occurs by the process of meiosis.

 

The egg cells develop within the ovary in a cluster of cells called the Graafian follicle, which secretes the female hormone estrogen that regulates the development of secondary female characteristics. Egg cell development within the follicle requires approximately 14 days. The development is controlled by two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both hormones are secreted by the anterior lobe of the pituitary gland.

 

During the 14 days of egg cell development, the endometrium increases its supply of blood and nutrients in anticipation of a fertilized egg cell. On about the 14th day, the release of the egg cell from the follicle takes place. This process is called ovulation. The egg cell is swept into the fallopian tube and begins to move toward the uterus. Meanwhile, the follicle is changed into a mass of cells known as the corpus luteum. The LH stimulates the conversion. The corpus luteum then secretes the hormone progesterone, which together with estrogen continues to regulate the buildup of tissue in the endometrium and inhibit contractions of the uterus.

 

The egg cell remains alive in the fallopian tubes for 24 to 72 hours. If fertilization by a sperm cell fails to occur, the egg cell moves toward the uterus, and the corpus luteum begins to degenerate. This degeneration causes the level of progesterone and estrogen to drop off. Within two weeks, the hormone levels decline to a point where they cannot inhibit contractions of the uterus. Uterine contractions then occur, and the endometrium is released in the process of menstruation. Follicle development begins again, but in the opposite ovary.

 

Fertilization and Development

 

For fertilization to occur, sperm cells must be released in the vagina during the period that the egg cell is alive. The sperm cells move through the uterus into the fallopian tube, where one sperm cell may fertilize the egg cell. The fertilization brings together 23 chromosomes from the male and 23 chromosomes from the female, resulting in the formation of a fertilized egg cell with 46 chromosomes. The fertilized cell is a zygote.

 

 

The zygote undergoes mitosis to form two identical cells that remain attached. This takes place about 36 hours after fertilization. Mitosis then occurs more frequently. Soon a solid ball of cells, a morula, results. Morula formation occurs about six days after fertilization. During that time, the cells are moving through the fallopian tube. Within the next two days, a hollow ball of cells called a blastocyst forms. The blastocyst enters the uterus. At one end of the blastocyst, a group of cells called the inner cell mass continues to develop.

 

About eight days after fertilization, the blastocyst implants itself in the endometrium of the uterus. During implantation, the outer cells take root in the endometrium. This outer layer of cells, called the trophoblast, gives rise to projections that form vessels. These vessels merge with the maternal blood vessels to form the placenta. The trophoblast also develops into three membranes: the amnion, the chorion, and the yolk sac membrane.

 

The inner cell mass undergoes changes to form three germ layers known as the ectoderm, the mesoderm, and the endoderm. The ectoderm becomes the skin and nervous system, the mesoderm becomes the muscles and other internal organs, and the endoderm becomes the gastrointestinal tract. The embryo is formed at about the fourth week when all the organs of the body have taken shape.

 

Birth

 

The birth process, which is complex, is regulated by various hormones. When diminished levels of progesterone remove the inhibition on uterine contractions, the uterine muscles contract. The posterior pituitary gland releases the hormone oxytocin, which stimulates further contractions.

 

During the first stage of labor, the cervix opens and the baby descends into the birth canal, or vagina. By this time, the amniotic sac has broken. In the second stage of labor, the baby passes through the birth canal, assisted by painful uterine contractions. The baby’s head normally appears first, but in a breech birth the buttocks may appear first. In the third stage, the placenta (afterbirth) is delivered.

 

Fetal Development

 

During the third month, the fetus definitely resembles a human, but the head is relatively large. During the ensuing months, the remainder of the body increases in size proportionally. Cartilage is replaced by bone, and the reproductive organs develop.

 

During the fourth month, the length of the fetus increases to about 6 inches. The heartbeat can be heard through the mother’s abdominal wall, and the fetus moves about. Distinctive movements can be felt at the fifth month, and by the sixth month the fetus weighs almost 2 pounds. By the end of six months, the fetus might be able to survive outside the mother’s body, but it would have little fat in its skin, so temperature control would be a problem. By the end of the ninth month, the fetus has an average length of about 20 inches and a typical weight of 6 to 8 pounds.

 

Male Reproductive System

 

The male reproductive organs are the testes (or testicles). The testes are two egg-shaped organs located in a pouch called the scrotum outside the body. In the scrotum, the temperature is a few degrees cooler than body temperature. The testes develop in the abdominal cavity before birth and then descend into the scrotum.

 

Sperm production in the testes takes place within coiled passageways called seminiferous tubules. Within the walls of these tubules, primitive cells called spermatogonia undergo a series of changes and proceed through meiosis to yield sperm cells. Each human sperm cell has 23 chromosomes.

 

The sperm cells mature in a tube called the epididymis, which is located along the surface of the testes. The hormone that stimulates sperm cell production is follicle-stimulating hormone (FSH). A second hormone important in reproduction is interstitial cell-stimulating hormone (ICSH), which acts on interstitial cells located between the seminiferous tubules. The interstitial cells secrete male hormones, including testosterone. The male hormones regulate the development of secondary male characteristics.

 

The organ responsible for carrying the sperm cells to the female is the penis. Within the penis, the sperm cells are carried in a tube, the urethra. During periods of sexual arousal, the penis becomes erect as blood fills its spongelike tissues. The sperm cells are mixed with secretions from the prostate gland, seminal vesicles, and Cowper’s glands. These secretions and the sperm cells constitute the semen.

 

Embryonic Stage

 

At the age of four weeks, the embryo is about the size of a pea. A primitive heart is beating, the head is defined with rudimentary eyes and ears, and tiny bumps represent arms and legs. The embryo also contains a primitive nervous system, and the head has begun to enlarge. A cartilage skeleton has appeared, and muscles have taken shape.

 

By the end of eight weeks, the embryo is somewhat human looking. Facial features are evident, and most of the organs are well developed. From this point onward, development consists chiefly of growth and maturation. The embryo is about 1.5 inches in length. Henceforth it is known as a fetus.

 

Nourishment of the embryo, and then the fetus, is accomplished through the placenta. The maternal and embryonic blood supplies meet at this organ, but the blood does not mix. Instead, diffusion accounts for the passage of gases, nutrients, and waste products across the membranous barriers. The placenta is also an endocrine gland because it secretes estrogen and progesterone to continue to inhibit follicle development and maintain the integrity of the endometrium. As the embryo becomes a fetus, it moves away from the placenta, and a length of tissue called the umbilical cord becomes its source of attachment to the maternal blood supply.

Chapter 30: Ecology

  Communities

 

Communities of plants, animals, and other organisms may be found in such places as a desert, a salt marsh, or a forest. A community is defined as multiple populations interacting with each other at the same time and in the same place. Within a community, each population of organisms has a habitat and may occupy one or more niches. The habitat is the physical place where the organisms live, while the niche is the role or roles that the population plays in the life of the community. The niche is the population’s function and position in the ecosystem, and it reflects the population’s relationship to other populations. A population’s niche is defined by how and when it reproduces, what time of the day and year it is most active, what climatic factors it can withstand, what it eats, and so forth. The competition exclusion principle suggests that two species cannot occupy the same niche in the same place at the same time indefinitely.

 

Two populations living together in a community in a close and permanent association is called symbiosis. If the symbiotic relationship is mutually beneficial, it is known as mutualism. Lichens represent an example of mutualism. If one population receives a benefit from an association, while the other is neither benefited nor harmed, the symbiosis is commensalism. Humans and the bacteria of their intestines exist commensally. Another type of symbiosis is parasitism, in which one population benefits while the other is harmed. The microorganisms that cause human disease are considered parasites. A final type of symbiosis is synergism. In this instance, two populations accomplish together what neither population could accomplish on its own.

 

Populations within a community may interact; indeed, one population may capture and feed on the other. Such a relationship is known as predation. Predators have more than one prey species, but they normally feed upon the most abundant prey species available. As a prey population decreases, the predator switches to a more abundant species. This change causes fluctuations in population sizes.

 

Natural selection favors the most effective predator, while also favoring the prey that can escape predation. Among the adaptations that help prey to escape are poisonous toxins, chemical adaptations, warning coloration, camouflage, and mimicry.

 

In a community, the orderly and predictable replacement of populations over a given period of time in a given area is called succession. Primary succession occurs in lifeless areas where soil has not yet formed. An area covered by a lava flow would undergo primary succession. In secondary succession, communities are established on a site previously occupied by a population. In each succession, certain populations dominate and then decline, to be superseded by new dominant populations. A community at the last stage of succession is called the climax community.

 

Ecosystems

 

Interactions between communities and their abiotic physical environments form systems known as ecosystems. One of the major phenomena underlying an ecosystem is the flow of energy. Because photosynthesizing organisms trap the energy in an ecosystem, they are producers. Because certain organisms in the community meet their energy needs by feeding on these producers, they are consumers. Producers are autotrophs, while consumers are usually heterotrophs. Primary consumers feed directly on plants, while secondary consumers (carnivores) feed on the animals that eat the plants. The energy flow forms a one-way pattern; the energy is