Water is by far the most commonly used solvent in biology because it is the major component of all living organisms. Most known biochemical reactions take place in an aqueous environment, and water is frequently a reactant in, or a product of, biochemical reactions. Biologically important macromolecules, organelles, cells, and organs all are designed to function in an aqueous environment.

Water quality is highly variable, and for any task an appropriate grade of water must be chosen. For example, tap water is fine for washing dishes, but it is not recommended for making solutions because the quality of such water is unknown. Tap water typically contains sediments (suspended particles), metal and other ions, deliberately added chemicals such as chlorine or fluoride, and/or traces of organic solvents. Although tap water is generally safe for drinking and other personal uses, materials in tap water can be toxic to some cells or may interfere with assays or biochemical reactions. Therefore, tap water is inappropriate for making solutions. Also, it is recommended that glassware that has been washed and rinsed in tap water be thoroughly rinsed with a higher quality water before use in the laboratory.

Distilled water, obtained from the condensation of steam, is of better quality because distillation eliminates all the sediment and most of the inorganic solutes. Organic contaminants and some of the inorganic contaminants remain.

Deionized water is produced by running tap water through a resin cartridge, or series of cartridges. A home deionizing system might simply replace divalent cations with sodium ions, producing what is commonly known as "soft" water. Laboratory deionized water is usually treated to remove both cations and anions, which are exchanged for hydrogen and hydroxyl ions, respectively. Deionized water often is of better quality than distilled water. However, on the downside, the resins used in the cartridges may release organic contaminants into the water.

The highest grade of water is called 18 megohm water. Eighteen megohms is 18 million ohms, which are units representing resistance to the flow of electricity. Eighteen meghoms is more than a million times the electrical resistance of a typical household electric circuit. Very pure water does not conduct electricity as well as contaminated water because it contains no inorganic ions with which to carry electric current. Eighteen megohm water is usually produced in multiple steps, including reverse osmosis and the passage of product through ion exchange resins, activated carbon beds and filters.

Pure water is somewhat acidic, with pH close to 5. It is also what we call an aggressive reagent, meaning that it will leech ions from plastic or glass containers. It does so because of the polar nature of water molecules. Ions dissolve most readily in 18 megohm water because the system (water plus dissolved ions) is more stable than when pure water is separated from soluble materials. Because very pure water accumulates contaminants during storage, it should be freshly prepared. Avoid use of plastic tubing, funnels, and especially metal containers.

 

Cells are the fundamental structural and functional units within living organisms. All living organisms consist of one or more cells. With the exception of bacteria, all organisms are made of eukaryotic cells, which have a membrane-enclosed nucleus and organelles (e.g., mitochondria, endoplasmic reticulum, and ribosomes). The nucleus within each cell contains the hereditary information for the entire organism, encoded within DNA.

In multi-cellular organisms, cells differentiate and specialize. Specialized cells organize into tissues (e.g., muscle, blood, bone, fat, nerve), which make up organs (e.g., kidneys, heart, stomach, lung), which, in turn, comprise organ systems (e.g., respiratory, digestive, excretory). Genes that do not pertain to the functioning of each individual cell become inactive, or "turn off." For example, a kidney cell uses only the DNA needed to be a kidney cell. The remaining information is "turned off," but it is still present. There are more than 200 different types of cells (nerve cells, muscle cells, epithelial cells, blood cells, bone cells, etc.) among the human body's estimated 100,000,000,000,000 total cells.

Current research is focusing on how to "program" stem cells to become other cell types, such as insulin-producing pancreas cells, heart muscle cells, or nerve cells. Through cloning technology (SCNT), an individual could, in theory, produce all the material needed to repair his or her own body using his or her own cells. This would eliminate tissue rejection problems and long waiting lists for replacement tissues and organs.

Glucose is used by many organisms as fuel, but it is vital that glucose levels be tightly regulated. Too little glucose will lead to starvation, while too much is toxic. Glucose homeostasis is accomplished through highly complex mechanisms involving many different molecules, cell types, and organs.

Briefly, when glucose enters the bloodstream (after the digestion of food), it is detected by specialized cells in the pancreas, called β-cells. These cells respond to the rising blood-glucose concentration by releasing the enzyme, insulin. Insulin then signals to other tissues in the body (i.e., muscle cells and adipose tissue) to take in glucose to be used as energy (in muscle cells) or stored for later use (in adipose tissue). The result is a lowering of blood-glucose concentration to non-toxic levels. 

In times of low glucose intake (between meals or in cases of starvation) the α-cells of the pancreas release the enzyme, glucagon. This enzyme directs the liver to break down stored glycogen into glucose and release this glucose into the bloodstream, thereby raising blood-glucose concentration to a desired level. The glucose transporters expressed on the β- and α-cells that bind glucose are the receptors of this homeostatic system. The β- and α-cells, themselves, are the control centers. They process information from the receptors and respond to it in a way that will maintain a constant internal environment. Insulin and glucagon are the effectors. This system is complex; the β- and α-cells working continuously to achieve the optimal, homeostatic blood-glucose concentration.

There are numerous ways to disrupt homeostasis, and the results of this disruption can vary in severity. Failure to achieve or restore homeostasis can result in death, which can be considered the ultimate disruption of homeostasis.

Injuries can have severe homeostatic consequences. A punctured lung, for example, will disrupt the flow of oxygen to the body. Cells in the brain cannot be deprived of oxygen for extended periods of time without dire consequences.

Illness also will cause a temporary disruption of homeostasis. Fever, a common symptom of a cold or flu, is a disruption of the body's constant internal temperature.  It usually is a sign that our body is fighting an infection of some type and, therefore, might be considered a good sign. After the illness subsides, the fever breaks and the normal, constant body temperature is re-achieved. However, an unchecked fever can damage neurons and organs, or even result in death..

Some disruptions in homeostasis are genetic. For example, a disease such as diabetes, to which some people have a genetic predisposition, can disrupt homeostasis. In addition, lifestyle factors such as obesity, lack of exercise, and a fatty diet-which also can disrupt homeostasis-have been shown to enhance one's chances of becoming diabetic. Not surprisingly, an individual with poor health habits is more likely to have problems maintaining homeostasis than a person with good health habits.

All vertebrates share the same basic body plan, with tissues and organs functioning in a similar manner. Here, we will focus on the human body, studying form (anatomy) and function (physiology). The two go hand-in-hand and are an extension of one of biology's central themes, evolution.

Animals are made of complex systems of cells, which must be able to perform all of life's processes and work in a coordinated fashion to maintain a stable internal environment. Early in a human's development, groups of cells specialize into three fundamental embryonic or germ layers: endoderm, mesoderm, and ectoderm. These embryonic layers differentiate into a number of specialized cells and tissues. Tissues are groups of cells similar in structure and function and may be held together by some sort of matrix. There are four primary groups of tissues: epithelial, connective, muscular, and nervous.

Different tissues functioning together for a common purpose are called organs (eg, stomach, kidney, lung, heart).

Organ systems are composed of individual organs working together to accomplish a coordinated activity. For example, the stomach, small intestine and large intestine all play a role in digestion.

Groups of organs working together to perform major activities of the body are called organ systems.

• The skeletal system, made of bones, cartilage and joints, is the framework of the body. It protects internal organs, stores minerals and provides a place for muscles to attach.
• The human muscular system is composed of smooth, cardiac and skeletal muscle tissue. Skeletal muscle, attached to the skeleton with dense strips of connective tissue called tendons, is responsible for the movement of body parts. Smooth muscle, sometimes called visceral muscle, is found in internal organs (eg, lines the walls of many blood vessels, makes up the iris of the eye and forms the wall of the gut). Cardiac muscle forms the bulk of the heart which controls blood circulation.
• The circulatory system-blood, blood vessels and the heart-is the body's transportation system, moving oxygen, carbon dioxide, nutrients, wastes, hormones, vitamins, minerals and water throughout the body. It also aids in regulation of temperature.
• The digestive system converts foods to simple substances that can be absorbed and used by the cells of the body. It is composed of the mouth, pharynx, esophagus, stomach, small intestine and large intestine and is aided by several accessory organs (liver, gall bladder, and pancreas).
• Made up of the skin, lungs, sweat glands and the kidneys, the excretory system removes metabolic wastes from the body. The kidneys are responsible for eliminating the bulk of wastes from the human body.
• The reproductive system generates reproductive cells (gametes) and provides a mechanism for them to be fertilized and maintained until the developing embryo can survive outside the body. The primary reproductive organs are the ovaries (female) and the testes (male).
• The nervous system regulates and coordinates the body's responses to changes in the internal and external environment. Major structures of the nervous system are the brain, spinal cord and nerves.
• The endocrine system consists of the hypothalamus, pituitary, thyroid, parathyroid and adrenal glands, as well as the pancreas, ovaries and testes. This system helps to maintain homeostasis, regulate temperature, and control growth, development, metabolism and reproduction by secreting and releasing hormones.
• The first line of defense in protecting the body is the integumentary system, which is composed of the skin, hair, nails, sweat and oil glands. It protects against injury, infection and fluid loss and also aids in temperature regulation.

The skeleton forms a sturdy internal framework of 206 bones and associated tissues - cartilage, tendons, and ligaments. Bones provide the base to which muscles attach and also the leverage required to accomplish external movement. The skeleton protects vital organs such as the brain, spinal cord, heart and lungs. As a living, dynamic tissue, bone stores vitamins and minerals (especially calcium and phosphorus) and houses red bone marrow, which produces blood cells.

Anatomists divide the skeleton into two parts, axial and appendicular. The axial skeleton (upright, or core of the body) includes the skull, ribs, sternum and vertebral column. Comprised of the shoulders, arms, hips and legs, the appendicular skeleton forms the appendages that attach to the axial skeleton.

Approximately four times as strong as concrete, bone is one of the strongest materials produced by nature. It is a connective tissue composed of cells called osteocytes, which are embedded in a hard, calcified matrix. Bones are made of a dense outer layer of compact material that surrounds a core of loosely structured spongy bone. The compact layer of bone is covered by a fibrous membrane called the periosteum. Cavities within each bone contain red bone marrow (blood-forming tissue) or yellow bone marrow (fat storage).

Movement of the skeleton occurs at the joints where two or more bones meet. There are three categories of joints. Slightly movable joints allow some movement but function mainly as a cushion (eg, joints between the vertebra). Freely movable, or synovial, joints allow a range of movement determined by the structure of the joint.  Examples of movable joints are the ball and socket (shoulder), hinge (elbow), pivot (between radius, ulna and humerus), and saddle joint (thumb). A few joints found in the skull are non-movable (sutures). Ligaments are inelastic connective tissues which hold bones together in a joint.

The primary role of the muscular system is to produce movement. As muscle tissue contracts, energy is used and heat is generated. Muscles also maintain body positions and postures, such as supporting your head or sitting.

Muscle is a unique tissue in its ability to contract (shorten). The functional unit of skeletal muscle tissue is a sarcomere, composed of actin and myosin protein filaments. When a muscle contracts, the sarcomere is shortened by actin filaments "sliding" over myosin filaments. Since a muscle fiber moves by shortening (it pulls and cannot push), muscles must work in antagonistic pairs. For example, a flexor contracts (shortens) and decreases the angle of joint while an extensor is stretched, increasing the angle of the joint. One muscle group contracts while an opposing muscle group extends. Muscle tissue is controlled and coordinated by stimuli from the nervous system.

The three types of muscle tissue are skeletal, smooth and cardiac. Skeletal muscle is attached to the skeleton with tendons and is controlled consciously. Skeletal muscle cells are long, fiber-like and multinucleated. The number of muscle fibers is fixed at birth, but protein can be added to increase the size the fiber with use and exercise. Lack of use causes muscle fibers to atrophy. Movement of smooth muscle tissue, found in internal organs, is usually involuntary. The cells of smooth muscle tissue are spindle-shaped and contain a single nucleus. Cardiac muscle, also involuntary muscle, is found only in the heart. Cardiac muscle tissue contains "gap" junctions that allow the diffusion of ions and the spread of electrical impulses from one cell to another.

The circulatory system distributes materials to and from all regions of the body and plays a role in regulating temperature. Blood transports oxygen from the lungs to cells of the body while metabolic wastes, including carbon dioxide, are removed from body cells and delivered to organs that eliminate them from the body. Nutrients are picked up from the digestive system and distributed throughout the body, along with hormones, vitamins and minerals. The circulatory system facilitates distribution of heat to maintain a relatively constant body temperature.

The human circulatory system consists of the heart, a series of blood vessels and blood. Made primarily of cardiac muscle, the heart serves as two separate pumps, one for pulmonary circulation and the other for systemic circulation. Valves control the directional flow of blood through the heart and prevent backflow. The average heart rate ranges from 60 to 70 beats per minute initiated by the sinoatrial node, or pacemaker, which is found in the wall of the heart. William Harvey (an English physician) demonstrated the function of the heart and complete circulation of the blood in 1628, laying the foundation for modern medicine.

Blood vessels of the circulatory system consist of arteries, veins and capillaries. The largest artery is the aorta and the smallest arteries are arterioles. Arteries are muscular, transport blood away from the heart, and, with the exception of pulmonary arteries, contain oxygenated blood. Arteries expand and recoil under pressure (creating what we call "pulse") because of the elastic connective tissue in the walls. Veins are thinner and less muscular than arteries and return blood to the heart. With the exception of the pulmonary veins, veins transport blood low in oxygen. Many veins contain valves to prevent backflow of blood. Capillaries are one cell in thickness. This allows for the essential exchange of materials between the blood and cells of the body.

Blood is a connective tissue consisting of a liquid matrix, called plasma, and a solid, cellular portion. Plasma is approximately 90% water and 10% solutes (nutrients, wastes, vitamins, hormones, gases, ions, and plasma proteins, which maintain osmotic pressure). The cellular portion of the blood is made of red blood cells (erythrocytes), which transport oxygen; white blood cells (leukocytes), which aid in defending the body from infection; and platelets, which are important in blood-clotting reactions. In humans, erythrocytes do not have a nucleus as mature cells but do contain hemoglobin, an iron-containing pigment which binds and transports oxygen.  Normally, less than 1% of the cells in human blood are white blood cells. They have nuclei and are larger than red blood cells. Neutrophils, eosinophils, and basophils are granular leukocytes, while monocytes and lymphocytes are non-granular. Platelets are pieces of cytoplasm, which are pinched off cells called megakaryocytes.

Some of the fluid in blood, along with several plasma proteins, moves into the tissues and is returned back to normal circulation by the lymphatic system. The lymphatic system consists of a series of vessels and "nodes," which filter out bacteria and other microorganisms.