Biology is the study of living things. We share the planet with over a million plants and animals. A Swedish scientist named Carl Linné, also known as Linnaeus, devised the classification system used in modern biological science. Every organism is grouped according to seven basic levels of classification, which are, from broadest to most specific: kingdom, phylum, class, order, family, genus, species.
Classification
Linnaeus's system describes organisms that have shared physical traits with a two-word, or binomial, name. The scientific name of an organism consists of a genus name and a species name. A genus name, always capitalized, precedes the species name, which is in lowercase. Both genus and species names are underlined or italicized.
- A human belongs to genus Homo, species sapiens, so it is Homo sapiens.
- A common frog belongs to genus Rana, species temporaria, so it is Rana temporaria.
- An African violet belongs to genus Saintpaulia, species ionantha, so it is Saintpaulia ionantha.
Most biologists divide all living things into five major types, each forming a kingdom: animals, plants, fungi, protists, and monerans. This chapter focuses on animals and plants because those kingdoms contain the majority of life. It is useful, however, to know a little about the other three kingdoms.
Monerans, such as blue-green algae and bacteria, are single-celled organisms containing no nuclei. Blue-green algae produce their own food through photosynthesis (defined later in this chapter). Many bacteria are parasites that cause diseases, or they are decomposers, meaning that they absorb food from decaying material.
Protists, such as protozoa and algae, are single-celled organisms that contain cell nuclei. Fungi, such as molds and mushrooms, are multiple-celled organisms that form spores and decompose other organic matter. Yeasts are unicellular fungi that form colonies.
Plants
Plants contain many cells and make their own food through photosynthesis. The two phyla, or large groupings within the plant kingdom, are the Bryophyta, such as mosses and hornworts, and the Tracheophyta, including flowering plants and pine trees. Bryophytes are tiny, grow on surfaces, and reproduce by spores. They are simply organized and lack the structural support of true roots, stems and woody tissue. Tracheophytes, or vascular plants, are the plants that we encounter every day. Almost all have roots, stems of woody tissue—which allow them to grow to great heights and in soil with a dry surface—and leaves.
The largest class of the Tracheophyta is composed of the following divisions:
- Filicophytes, or ferns
- Angiosperms (Magnoliophyta), flowering plants that produce seeds with protective coverings
- Gymnosperms (encompassing four divisions), which produce seeds without protective covering, though some produce seed cones, such as the pine cone
Angiosperms are further divided into monocots and dicots. Monocots bear seeds with only one cotyledon, a leaf within the embryo. Monocots, such as onions, tulips, and palms, are characterized by parallel leaf veins and flowers in groups of threes. Dicots bear two cotyledons. Dicots, such as potatoes, roses, and oaks, are characterized by net-like leaf veins and flower parts in fours, fives, or multiples of either four or five.
Animals
Animals are many-celled, mobile organisms that cannot produce their own food. The animal kingdom is divided into approximately 26 phyla. Some of the major animal phyla are shown in the table on this page.
Vertebrates
The vertebrates are a subphylum in the chordate phylum and include birds, fish, mammals, and reptiles. (All other animals are invertebrates.) Vertebrates have a spinal cord enclosed in a flexible, bony column that extends down the long axis of the body, providing skeletal support. There are eight classes of vertebrates. Four of the vertebrate classes are fish: agnatha (lamprey), chondrichthyes (shark), osteichthyes (trout), and the extinct placodermi. The other four classes of vertebrates and some of their characteristics are listed in the table on the next page.
Only the aves and mammalia are warm-blooded. Birds and humans generate and regulate their own body heat. Feathers on birds and fur on mammals help them retain body heat, and sweating (yes, birds sweat) helps them cool down. All of the other vertebrates are cold-blooded. This means that their body temperature is determined by the temperature of their surrounding environment.
Mammals
All mammals share certain characteristics. They are warm-blooded, have a hair or fur covering for insulation, have a four-chambered heart, and breathe with lungs. In addition, females produce milk for their young. Mammals are divided further by body structures into 17 orders containing a total of only approximately 4,250 species. Some of the more important mammal orders are shown on the table at the bottom of this page.
Viruses
Viruses are difficult to classify because scientists do not agree on the definition of a virus. Some scientists believe that a virus is nonliving because, alone, a virus is incapable of reproducing. However, viruses consist of a DNA or RNA core encapsulated in a protein coat (a capsid), which causes many to argure for its status as a life form. A virus has no true cell structure, and it isincapable of independent metabolism and reproduction without the aid of a host cell. Once inside another cell, a virus takes over and uses the host cell's resources to replicate viral DNA. Eventually the host cell dies and the (many) replicated viruses are released to infect new cells. Viral diseases in animals include the common cold, influenza, herpes, measles, polio, and rabies.
Basic Life Principles
All living organisms perform certain biochemical and biophysical activities to achieve homeostasis—a balanced internal environment. The life functions are as follows:
- Circulation: the transport of materials such as oxygen and nutrients throughout an organism
- Excretion: the elimination of metabolic waste products from an organism
- Growth: cell division and/or enlargement
- Nutrition: getting nutrients, or food molecules, from the environment via eating, absorption, or photosynthesis
- Regulation: the chemical control and coordination of life activities
- Reproduction: the production of new individuals
- Respiration: organic substances are broken down to simpler products with the release of energy, which is used to fuel other metabolic processes (in animals and plants)
The Cell
Cells are the basic structural and functional unit of living things. One cell, alone, is the smallest unit of matter that is considered living. In general, plant and animal cells are similar, except that plant cells contain chloroplasts and cell walls. Chloroplasts contain chlorophyll, a food-generating substance. Cell walls, containing cellulose and other compounds, give plant cells a rigid structure and prevent desiccation, or drying out.
The size of cells varies, but most are microscopic (an average of 0.01–0.1 mm in diameter). They may exist independently, or they may form colonies or tissues—like those in plants and animals. Each cell contains a mass of protein, called protoplasm, that consists of jelly-like cytoplasm and a nucleus. The nucleus, in turn, contains deoxyribonucleic acid, or DNA, which is the genetic material of most organisms. The protoplasm is bound by a cell or plasma membrane, which controls the materials that pass in and out of the cell.
There are two types of cells, distinguished from one another by a number of characteristics, one being the way in which they reproduce. Bacteria are one example of prokaryotic cells. The nuclear material in prokaryotic cells is not bound by a membrane, and cell reproduction occurs by fission—asexual cell cleavage—the cell breaks apart to form another, identical cell. The other type of cell, found in most plants and animals, is a eukaryotic cell, in which the nucleus is separated from the cytoplasm by the nuclear membrane and there are separate organelles. In eukaryotic plant and animal cells, the major cell organelles are as follows:
- Cell membrane: partially permeable membrane that regulates flow of materials in and out of the cell and holds the structure of the cell together
- Cytoplasm: jelly-like material that encompasses the other cell structures
- Endoplasmic reticulum: a network of membranes extending from the nucleus into the cytoplasm, responsible for making lipids, proteins (in association with ribosomes), and transporting these products throughout the cell
- Golgi body/apparatus: stores and transports secretory products within the cell
- Lysosome: contains and releases enzymes within the cell
- Mitochondrion: the largest organelle and site of energy production, known as cellular respiration, in the cell (there are several mitochondria in each cell)
- Nucleus: contains genetic material and functions as the control center of the cell
- Ribosome: site of protein synthesis (there are many ribosomes in each cell)
Plant cells additionally have chloropoasts, where photosynthesis takes place, and a cell wall.
Genetics
Genetics is the study of heredity and variation. Heredity is the transmission of characteristics from parents to offspring via chromosomes. Gregor Mendel, a European monk in the late 18th century, developed some of the basic laws of heredity which have been updated, but are still applicable today:
- Law of Segregation: each hereditary characteristic is controlled by alleles that separate in the reproductive cells
- Law of Independent Assortment: each characteristic is inherited independent of other characteristics
- Law of Dominance: when two different alleles for the same characteristic are present in one cell, only one allele will be expressed, or dominant, and the other will be masked, or recessive.
The structure and function of every cell in the human body is predetermined by units of heredity called genes, located in specific positions on the chromosomes in a cell nucleus. Genes are made up of DNA. Genes give the cell instructions about how to function, when to reproduce, and even when to die. The process of cell reproduction through cell division is called mitosis. During mitosis, the genes of the parent cell are copied. Then, when the parent cell divides, it becomes two identical daughter cells.
Most cells in the human body have 46 chromosomes—23 chromosomes from the mother (female, egg cell) and 23 chromosomes from the father (male, sperm cell). However, human sex cells (egg or sperm) have 23 chromosomes each. When the sex cells, or gametes, undergo the process of fertilization—the union of female and male sex cells—the resulting fertilized cell has 46 chromosomes. This fertilized cell will multiply to form a new individual consisting of a combination of chromosomes from the mother and the father. This fusion of two gametes during fertilization to produce offspring is referred to as sexual reproduction.
The sex of a human embryo is determined by the 23rd chromosome in the sperm. Human females have the chromosome pair XX. Males have the pair XY. Females, therefore, always give offspring an X chromosome, whereas males give either an X or a Y chromosome to the offspring.
Structure and Function of Human Systems
The human body can be divided into 11 systems, each of which is discussed below.
Integumentary System
The integument—the skin—is the outermost covering of the body. It consists of the epidermis (the thinner outermost layer) and the dermis (the thicker innermost layer). It also includes the hair and nails. Beneath the skin is the subcutaneous tissue.
The integumentary system has the following functions:
- protecting the body from injury, dehydration, and invasion by harmful agents such as bacteria
- providing sensitivity to pain, temperature, and pressure
- regulating body temperature
Skeletal System
The skeleton provides the structure of the body. Comprised of 206 bones, along with cartilage and ligament, the skeletal system is rigid yet flexible. Joints are points where bones meet and may or may not move against each other. The cartilage is the flexible but strong substance found in joints, nose, and ears. The ligaments, made of softer, flexible tissue, attach bones to each other.
The skeletal system has the following functions:
- providing mechanical support
- protecting body organs
- making body movement possible (along with the muscles)
- storing calcium in the bones, which contain marrow for production of red and white blood cells and platelets
Muscular System
Muscles are made of sheets or bundles of cells. Muscles can do work only by contracting; expansion is passive. Therefore, skeletal muscles are usually attached to a bone in opposing pairs—one to contract while the other expands. There are three major types of muscles:
- Voluntary (or striated) muscles can be controlled by conscious thought.
- Involuntary (or smooth) muscles cannot be controlled by the will.
- Cardiac (or heart) muscles exist only in the heart. They contract spontaneously without needing nervous stimulation.
Tendons attach muscles to the skeleton.
Circulatory System
The circulatory system consists of the cardiovascular and lymphatic systems, including:
- the heart
- blood components such as red blood cells and platelets
- blood vessels, including arteries, veins, and capillaries
- lymphatic vessels and nodes
- lymph
The circulatory system circulates blood throughout the body, making the body's other functions possible by bringing oxygen and other materials to the cells and carrying away waste products and other secretions.
Immune System
The immune system is the body's protective mechanism. It consists of the lymphatic system; the white cells of the blood and bone marrow; antibodies; the thymus gland; and the skin.
The basic characteristics of the immune system include the concepts of:
- Specificity: the capacity to recognize and get rid of antigens by producing lymphocytes and antibodies. An antigen (literally meaning "antibodygenerating") can include anything "foreign" to the body, such as viruses, bacteria, pollen, and, unfortunately, tissue that has been transplanted.
- Diversity: the capacity to respond to millions of kinds of invaders.
- Self/nonself recognition: the ability to distinguish the body's own molecules ("self ") from antigens ("nonself ").
- Memory: the capacity to "remember" previously encountered antigens and react more quickly when exposed again. This process is called acquired immunity.
The immune system's basic function is to protect the body from disease and injury.
Respiratory System
The respiratory system is responsible for taking oxygen into the body and eliminating carbon dioxide. It includes the lungs, nose, pharynx, larynx, trachea, bronchi, and diaphragm.
Digestive (or Gastrointestinal) System (GI)
The digestive system includes the gastrointestinal (GI) tract (also called the alimentary canal). This is basically a tube with two openings—the mouth and anus—for intake of food and elimination of waste. The parts of the GI tract are the mouth, the esophagus, the stomach, the small intestine, and the large intestine. Also included in this system are structures such as teeth, tongue, liver, pancreas, and gallbladder.
The digestive system breaks down food for energy, reabsorbs water and nutrients, and eliminates waste.
Urinary or Excretory System
The urinary system consists of:
- Kidneys: two compact, bean-shaped organs through which blood is cycled for removal of waste
- Nephrons: excretory tubes in the kidneys
- Blood vessels that serve the kidneys
- Urinary structures that carry waste out of the body—the ureters, bladder, and urethra
The kidneys remove waste or toxic byproducts from the blood and maintain homeostasis of blood and body fluids.
Nervous System
The nervous system is made up of the nerves, brain, spinal cord, and sense organs for sight, sound, smell, and taste. The brain is the nervous system's main control center and consists of three parts:
- The cerebral hemispheres are responsible for the higher functions, such as speech and hearing.
- The cerebellum is responsible for subconscious activities and some balance functions.
- The brain stem is responsible for basic functions such as breathing and circulation
The nervous system controls the flow of information between the sensory and motor cells and organs.
Endocrine System
The endocrine system controls communication between systems in the body. It consists of:
- Hormones are substances that regulate the growth or function of a specific tissue or organ. Hormones include insulin, sex hormones, adrenaline, and serotonin.
- Glands that secrete hormones include the pituitary, adrenal gland, thyroid, ovary, testis, and part of the pancreas.
Together with the nervous system, the endocrine system regulates and balances bodily fluids and chemicals. Hormones affect the body's growth, the development and functioning of reproductive organs and sexual characteristics, the development of higher nervous functions such as personality, and the ability of the body to handle stress and resist disease.
Reproductive System
Reproduction in humans involves two sets of organs, the internal reproductive organs and the external genitalia. In reproduction, the female ovum and the male spermatozoon fuse to form a zygote, which eventually develops into a fetus.
The reproductive system functions to create new individuals from existing ones and propagate the species.
Nutrition
The process by which organisms obtain energy from food for growth, maintenance, and repair is called nutrition. Food provides nutrients, such as carbohydrates, proteins, and fats. In humans, energy in the form of calories is gleaned chiefly from carbohydrates. A calorie is actually a unit of heat. It is the unit of heat necessary to raise the temperature of one gram of water one degree Celsius.
Carbohydrates are starches or sugars that fuel all of the cells in the body. Common sources of carbohydrates are potatoes, beans, cereals, fruit, and milk. Both starches and sugars ultimately yield glucose. Cells convert glucose to energy or store excess glucose, called glycogen, in the liver and muscles. If the excess glucose exceeds a certain point, the excess glucose is converted to fat and stored.
Proteins are abundant in all living organisms. Structural proteins make up the fingernails, skin, bones, and teeth in humans. Other proteins regulate metabolism, produce movement, and transport oxygen and other substances in and out of body cells. Common sources of protein are meat, cheese, beans, nuts, and fish.
Fats are also essential for human nutrition. Fats function in cellular structure, composition, and transport. Containing approximately twice the caloric value of carbohydrates, fats are found in butter, cream, eggs, and most cheese.
Vitamins are other organic substances required in small amounts to maintain normal health. Most animals cannot manufacture many vitamins themselves and must have adequate amounts in the diet. The table above shows the major vitamins.
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