Biology is the science that studies the processes fundamental to all forms of life. Biology strives to answer such questions as: What is life and when does it begin? How do organisms grow and reproduce? How and why do pollutants threaten certain life forms? What makes one life form unique and at the same time changeable by genetic engineering or natural selection? And how can human life be sustained and lengthened by new medical advances?
An understanding of the biological sciences and their basic principles can help you make informed decisions about biological issues that affect your own life and life in general. Biology as a major is a stepping stone from which one may go on to research, graduate school, medicine or various health professions, teaching, or work in business or service in government or independent agencies.
All exhibit certain characteristics of life:
Metabolism: Energy-rich materials are broken down; the energy is used or stored, and low-energy wastes are discarded.
Selective response (irritability):
Growth and biosynthesis:
1. THE CHEMICAL BASIS OF LIFE
For many centuries, biology was the study of the natural world. Biologists searched for and classified plants and animals and studied their anatomy and how they acted in nature. Then in the 1700s, scientists discovered the chemical and physical basis of living things. They soon realized that the chemical organization of all living things is remarkably similar. All living systems are built of matter composed of atoms and molecules. Biologically important molecules include lipids, carbohydrates, proteins, and vitamins.
1. Chemical principles
Elements. All living things on earth are composed of one or more fundamental building blocks of matter called elements. More than one hundred elements are known to exist, including some made by scientists. An element is a substance that cannot be decomposed by chemical means. Oxygen, iron, calcium, sodium, hydrogen, carbon, and nitrogen are examples of elements.
Atoms. Each of the elements is composed of one particular kind of atom. An atom is the smallest part of an element that can enter into combinations with atoms of other elements. Atoms consist of positively charged particles called protons surrounded by negatively charged particles called electrons. A third particle called the neutron has no electrical charge; it has the same weight as a proton. Protons and neutrons adhere tightly to form the dense, positively charged nucleus of the atom. Electrons spin around the nucleus. The arrangement of electrons in an atom plays an essential role in the chemistry of the atom. Atoms are most stable when their outer shell of electrons has a full quota. The first electron shell has a maximum of two electrons. The second and all other outer shells have a maximum of eight electrons. Atoms tend to gain or lose electrons until their outer shells have a stable arrangement. The gaining or losing of electrons, or the sharing of electrons, contributes to the chemical reactions in which an atom participates.
Molecules. Most of the compounds of interest to biologists are composed of units called molecules. A molecule is a precise arrangement of atoms of different elements, and a compound may be a collection of molecules. The arrangements of the atoms in a molecule account for the properties of a compound. The molecular weight is equal to the atomic weights of the atoms in the molecule. For example, the molecular weight of water is 18. A molecule may also be composed of two or more atoms of the same element, as in oxygen gas, O2. But oxygen gas is not a compound. The atoms in molecules may be joined to one another by various linkages called bonds. One example of a bond is an ionic bond. An ionic bond is formed when the electrons of one atom transfer to a second atom. This creates electrically charged atoms called ions. The electrical charges cause the ions to be attracted to one another, and the attraction forms the ionic bond. A second type of linkage is called a covalent bond. A covalent bond forms when two atoms share one or more electrons with one another. For example, oxygen shares its electrons with two hydrogen atoms, and the resulting molecule is water (H2O). Nitrogen shares its electrons with three hydrogen atoms, and the resulting molecule is ammonia(NH3). If one pair of electrons is shared, the bond is a single bond; if two pairs are shared, then it is a double bond.
Acids and bases. Certain chemical compounds release hydrogen ions (H+) when the compounds are placed in water. These compounds are called acids. For example, when hydrogen chloride is placed in water, it releases its hydrogen ions and the solution becomes hydrochloric acid. Certain chemical compounds attract hydrogen atoms when they are placed in water. These substances are called bases. An example of a base is sodium hydroxide (NaOH). When this substance is placed in water, it attracts hydrogen ions, and a basic (or alkaline) solution results as hydroxyl (-OH) ions accumulate.
2. Organic Compounds
The chemical compounds of living things are known as organic compounds because of their association with organisms. Organic compounds, which are the compounds associated with life processes, are the subject matter of organic chemistry.
Carbohydrates. Among the numerous types of organic compounds, four major categories are found in all living things. The first category is the carbohydrates. Carbohydrates are used by almost all organisms as sources of energy. In addition, some carbohydrates serve as structural materials. Carbohydrates are molecules composed of carbon, hydrogen, and oxygen; the ratio of hydrogen atoms to oxygen atoms is 11. The simple carbohydrates are commonly referred to as sugars. Sugars can be monosaccharides if they are composed of single molecules, or disaccharides if they are composed of two molecules. The most important monosaccharide is glucose, a carbohydrate with the molecular formula C6H12O6. Glucose is the basic form of fuel in living things. It is soluble and is transported by body fluids to all cells, where it is metabolized to release its energy. Glucose is the starting material for cellular respiration, and it is the main product of photosynthesis (see the chapters "Photosynthesis" and "Cellular Respiration"). Three important disaccharides are also found in living things. One disaccharide is maltose, a combination of two glucose units covalently linked. Another disaccharide is sucrose, the table sugar formed by linking glucose to another monosaccharide called fructose. A third disaccharide is lactose, composed of glucose and galactose units. Complex carbohydrates are known as polysaccharides. Polysaccharides are formed by linking innumerable monosaccharides. Among the most important polysaccharides are the starches, which are composed of hundreds or thousands of glucose units linked to one another. Starches serve as a storage form for carbohydrates. Much of the world's human population satisfies its energy needs with the starches of rice, wheat, corn, and potatoes. Another important polysaccharide is glycogen. Glycogen is also composed of thousands of glucose units, but the units are bonded in a different pattern than in starch. Glycogen is the form in which glucose is stored in the human liver. Still another important polysaccharide is cellulose. Cellulose is used primarily as a structural carbohydrate. It is also composed of glucose units, but the units cannot be released from one another except by a few species of organisms. Wood is composed chiefly of cellulose, as are the cell walls of all plants. Cotton fabric and paper are commercial cellulose products.
Lipids. Lipids are organic molecules composed of carbon, hydrogen, and oxygen atoms. In contrast to carbohydrates, the ratio of hydrogen atoms to oxygen atoms is much higher. Lipids include steroids (the material of which many hormones are composed), waxes, and fats. Fat molecules are composed of a glycerol molecule and one, two, or three molecules of fatty acids. A glycerol molecule contains three hydroxyl (-OH) groups. A fatty acid is a long chain of carbon atoms (from four to twenty-four) with a carboxyl (-COOH) group at one end. The fatty acids in a fat may be all alike or they may all be different. They are bound to the glycerol molecule by a process that involves the removal of water. Certain fatty acids have one or more double bonds in their molecules. Fats that include these molecules are called unsaturated fats. Other fatty acids have no double bonds. Fats that include these fatty acids are called saturated fats. In most human health situations, the consumption of unsaturated fats is preferred to the consumption of saturated fats. Fats stored in cells usually form clear oil droplets called globules because fats do not dissolve in water. Plants often store fats in their seeds, and animals store fats in large, clear globules in the cells of adipose tissue. The fats in adipose tissue contain much concentrated energy. Hence, they serve as a reserve energy supply to the organism. The enzyme lipase breaks down fats into fatty acids and glycerol in the human digestive system.
Proteins. Proteins are among the most complex of all organi compounds. They are composed of units called amino acids, which contain carbon, hydrogen, oxygen, and nitrogen atom Certain amino acids also have sulfur atoms, phosphorous, or othe trace elements such as iron or copper. Many proteins are immense in size and extremely complex. However, all proteins are composed of long chains of the relatively simple amino acids. 'nere are twenty kinds of amino acids, and each has an amino (-NH2) group, a carboxyl (-COOH) group, and a group of atoms called an -R group (-R for radical). The amino acids differ depending on the nature of the -R group. Examples of amino acids are alanine, valine, glutamic acid, tryptophan, tyrosine, and histidine. Amino acids are linked to form a protein by the removal of water molecules. The process is called dehydration synthesis, and a byproduct of the synthesis is water. The links forged between'the amino acids are called peptide bonds, and small proteins are often called peptides. All living things depend upon proteins for their existence. Proteins are the major molecules from which living things are constructed. Certain proteins are dissolved or suspended in the watery substance of the cells, while others are incorporated into various structures of the cells. Proteins are also found as supporting and strengthening materials in tissues outside of cells. Bone, cartilage, tendons, and ligaments are all composed of protein. One essential use of proteins is in the construction of enzymes. Enzymes catalyze the chemical reactions that take place within cells. They are not used up in a reaction; rather, they remain available to catalyze succeeding reactions. Every species manufactures proteins unique to that species. The information for synthesizing the unique proteins is located in the nucleus of the cell. The so-called genetic code specifies the sequence of amino acids in proteins. Hence, the genetic code regulates the chemistry taking place within a cell. Proteins also can serve as a reserve source of energy for the cell. When the amino group is removed from an amino acid, the resulting compound is energy rich.
Nucleic acids. Like proteins, nucleic acids are very large molecules. The nucleic acids are composed of smaller units called nucleotides. Each nucleotide contains a carbohydrate molecule, a phosphate group, and a nitrogen-containing molecule that because of its properties is called a nitrogenous base. Living organisms have two important kinds of nucleic acids. One type is called deoxyrihonueleic acid, or DNA. The other is ribonucleic acid, or RNA. DNA is found primarily in the nucleus of the cell, while RNA is found in both the nucleus and the cytoplasm of the cell. DNA and RNA differ from one another in their components. DNA contains the carbohydrate deoxyribose, while RNA has ribose. In addition, DNA contains the base thymine, while RNA has uracil.
3. Basic Concepts
1. PROTEINS: Formed of sulfur (S), hasphorus (P), carbon (C), oxygen (0), hydrogen (H), nitrogen (N); built into simple compound. called amino acids and used for growth and repair of protoplasm.
2. FATS: composed of carbon, hydrogen, oxygen; energy is released when fats are oxidized (burned with oxygen).
3. CARBOHYDRATES: Formed from carbon, hydrogen and oxygen; release energy quickly. (a) Starches - (C6H10O5). (b) Sugars - Simple - glucose, fructose and galactose (C6H22O6) Double - sucrose, lactose, and maltose (C12 H22 O11) (c) Cellulose - found in cell walls.
4. WATER: (H2O). Aids metabolism and in transportation of material within body; makes up 2/3 of body volume.
5. SALT: (Sodium chloride, NaCl). Regulates water balance.
1. IRON:(Fe). Part of hemoglobin in red blood cells; found in liver and vegetables.
2. MAGNESIUM: (Mg). Part of chlorophyll in green plants.
3. CALCIUM: (Ca). Builds bones, teeth: found in milk, vegetables and meat.
4. IODINE: (I). Part of thyroxin secreted by the thyroid gland; found in sea food and iodized salt.
5. TRACE ELEMENTS: Needed in only minute quantities; consist of copper (Cu), zinc (Zn). cobalt (Co) and fluorine (F). Fluorine reduces tooth decay.
2. Cells and Tissues
Living organisms are all made of cells. Each cell contains several types of organelles and each is surrounded by a membrane. Most cells have a central nucleus containing the chromosomes, which carry hereditary information. Normal cell division (mitosis) ensures that each new cell carries all the hereditary information of its parent cell. A special form of cell division called meiosis gives rise to sex cells that then combine during fertilization. Most multicellular organisms are conducted of groups of similar cells called tissues.
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- Cells: Basic structure
- Membrane organelles
- Other organelles
- Nucleus and mitosis
- Meiosis and fertilization