Nucleus

The nucleus, a large and rounded organelle, is the control center of the cell.  In a cell that is not in the process of dividing, the nucleus contains loosely coiled material called chromatin.  When a cell prepares to divide, the chromatin becomes more tightly coiled and condenses to form rod-shaped bodies, called the chromosomes.  Each chromosomes contains hundreds or thousands of genes which are the units of hereditary information that govern the structure and activity of the cell.  The genes, which are arranged in a specific linear order, are composed of the chemical compound DNA.  We can think of the chromosomes as a chemical cookbook for the cell and each gene as a recipe for making a specific protein.

The complete set of genes that make up the human genetic material is the human genome.  The Human Genome Project, completed in 2003, mapped the 30,000 or so genes that make up the human genome.  Researches continue to study the proteins coded by the genes.  Genome research is providing knowledge that can be applied to prevention and treatment of many human disorders.

The nucleolus is a specialized region within the nucleus.  The nucleolus assembles ribosomes, organelles that help manufacture proteins.

Function of Cell membranes and Structure of the Plasma membrane

Many organelles are surrounded by membranes that form compartments.  These compartments allow the cell to separate various processes.

Most cells are bounded by a thin plasma membrane that protects the cell and regulates the passage of materials into and out of the cell.

The plasma membrane consists of a double layer of lipids in which proteins are embedded.  Many of the lipids are phospholipids, lipids that contain phosphorus.  The positions of the proteins change as they move about like icebergs in a fluid sea of phospholipids.  Some of the proteins form channels that selectively permit the passage of certain substances through the membrane.

General characteristics of Cells

The cell is an amazingly complex structure.  It has a control center, internal transportation system, power plants, factories for making needed materials, and packaging plants.

Cells, the building blocks of the body, have specialized organelles that carry out specific functions.

Most organelles are dispersed within the cytoplasm which is the jellylike material of the cell.

Most cells are very small and must be studied with microscope.  The microscope is one of the biologist's most important tools for studying the internal structure of cells.  Most cell structures were first identified with an ordinary light microscope, which uses visible light as the source of illumination.  The development of the electron microscope, which came into widespread use in the 1950s, enabled researches to study the fine detail (ultra structure) of cells and their parts.  The electron microscope uses an electron beam of energized electrons.  Photographs taken with the light microscope are referred to as light micrographs (LMs) and photographs taken with the electron microscope are referred to as electron micrographs (EMs) .

The size and shape of the cell are related to the specific functions it must perform.  For example, sperm cells are tiny cells with long, whiplike tails (called flagella) .  The tail is used to move towards the ovum or egg.  The ovum is one of the largest cells in the human body.  Epithelial cells which look like little building blocks cover body surfaces and line body cavities.  Muscle cells are elongated and specialized for contraction.  Nerve cells have long extensions that permit them to transmit messages over long distances within the body.  Lymphocytes, a type of white blood cells, change their shape as they move through the tissues of the body, destroying invading bacteria.

Homeostasis and Feedback Mechanisms

Metabolic activities are carefully regulated to maintain homeostasis which is an appropriate internal environment, or steady state.

Homeostatic mechanisms are the self-regulating control systems that maintain homeostasis.  Homeostatic mechanisms are remarkably sensitive and efficient.  Homeostasis must be maintained even though conditions may continuously change in the external environment.

Stressors are the stimuli that disrupt homeostasis, cause stress that activate homeostatic mechanisms.  For example eating a bag of candy is a stressor that raises the blood sugar concentration above the homeostatic level.  This stress activates homeostatic mechanisms that bring the blood sugar concentration back to the normal range.  When homeostatic mechanisms are unable to manage stress, steady state is not restored.  The stress may then lead to a malfunction, which can cause disease or even death.

Many homeostatic mechanisms are negative feedback systems, in which the response of the regulator (control center) is opposite to the change.  Examples of negative feedback mechanisms are

• Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount of water available for cells to absorb.
• Osmoreceptors that are capable of detecting water concentration are situated on the hypothalamus next to the circulatory system.
• The hypothalamus sends chemical messages to the pituitary gland next to it.
• The pituitary gland secretes anti-diuretic hormone (ADH) which targets the kidney responsible for maintaining water levels.
• When the hormone reaches its target tissue, it alters the tubules of the kidney to become more/less permeable to water.
• If more water is required in the blood stream, high concentrations of ADH make the tubules more permeable.
• If less water is required in the blood stream, less concentrations of ADH make the tubules less permeable.

Many homeostatic mechanisms are positive feedback systems, in which variation from the steady state sets off a series of events that intensify the change.  Examples of positive feedback mechanisms are

• One example of a biological positive feedback loop is the onset of contraction in childbirth.  When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions.  This results in contractions increasing in amplitude and frequency.
• Lactation involves positive feedback so that the more the baby suckles, the more milk is produced.
• In most cases, once the purpose of the feedback loop is completed, counter-signals are released that suppress or break the loop.  Childbirth contractions stop when the baby is out of the mother's body.  Lactation stops when the baby no longer nurses.

Metabolism, Catabolism, and Anabolism

The chemical activities that take place in the body are its metabolism.  Two phases of metabolism are catabolism and anabolism.

Catabolism, the breaking down phase of metabolism, provides energy required for anabolism which is the building phase of metabolism.  For example, catabolism generates the energy needed for muscle contraction and for growth.  Cellular respiration is the catabolic process in which nutrients are broken down and their energy packaged within a special storage molecule called ATP (adenosine triphosphate) for use by the cell.  Cellular respiration requires oxygen as well as nutrients.

Anabolism is the building, or synthetic phase of metabolism.  During anabolism energy is used to make chemical compounds and structures needed by the cell.  These chemical compounds are needed for growth, maintenance and repair of the body.

Human body consists of Inorganic and Organic compounds

The human body is made up of atoms, ions and molecules.  Life processes depend on the organization and interaction of these chemical units.  An atom is the smallest amount of the chemical element that has the characteristic properties of that element.  An ion is an electrically charged atom or a group of atoms.  For example electrically charged hydrogen atom is also called hydrogen ion.  Because of their electric charges, ions are very important in many life processes including energy transformation, transmission of nerve impulses and muscle contraction.

A chemical compound is a molecule that consists of two or more than two different atoms combined in a fixed proportion.  For example water is a chemical compound consisting of two atoms of hydrogen chemically combined with one atom of oxygen.

Chemical compounds can be classified into two broad groups, organic and inorganic compounds.  Inorganic compounds are relatively simple, small compounds such as water, salt, simple acid like hydrochloric acid, and simple base like ammonia.  These substances are required for fluid balance and for many activities of cell such as transporting materials through the cell membrane.  Organic compounds are large, complex compounds containing carbon.  These are the chemical building blocks of the body and also serve as fuel molecules that provide energy for body activities.  Organic compounds also regulate and participate in thousands of chemical reactions necessary for life.

Four important group of organic compounds are carbohydrates,  lipids, proteins, and nucleic acid.

• Carbohydrates are sugars and starches.  They are used as fuel by the body and to store energy.  Glucose is a simple sugar that is a main source of energy for the body.  The liver and muscles can link glucose molecule into long chains, producing glycogen, a large molecule that can be stored.
• Lipids include fats, compounds that store energy; phospholipids, which are components of cell membrane; and steroids, which include several hormones.
• Proteins are complex, and large compounds composed of subunits called amino acids.  Some proteins serve as enzymes, catalyst that regulate chemical reactions.  Other proteins are the structural components of cells and tissues.  The kind and amount of proteins in a cell determine to a large extent what a cell looks like and how it functions.  For example, muscle cells have large amounts of the proteins myosin and actin, which are responsible for their appearance and their ability to work.  
• Nucleic acid are also large and complex compounds.  Two very important nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) .  DNA makes up genes (the hereditary material) .  It contains the instructions for making all the proteins needed by the cell.  RNA is important in the process of manufacturing proteins.

Principal organ system

The human organism is made up of 11 main organ systems that work together to maintain life.

• The integumentary system consists of skin,hair,nails,sweat glands.It provides a protective covering for the body,helps to regulate body temperature and receives information about touch,pressure,temperature and pain.
• The skeletal system consists of bones,cartilage and ligaments.This provides support ,protects the body,stores calcium,produces blood cells and works with the muscular system to carry out movement.
• The muscular system consists of skeletal muscle,cardiac muscle,smooth muscle.It moves parts of skeleton like locomotion,pumps blood and moves internal materials.
• The nervous system consists of nerves and sense organs;brain and spinal cord.It is the principal regulatory system.It receives stimuli from the external and the internal environment and transmits impulses to muscles and glands.
• The endocrine system consists of pituitary,adrenal,thyroid and other ductless glands.This system works with the nervous system to regulate metabolic activities and body chemistry.It also secretes chemical messengers called hormones.
• The digestive system consists of mouth,esophagus,stomach,intestine,liver,pancreas and salivary glands.This system ingests and digests food,breaks down food and absorbs nutrients into the blood.
• The respiratory system consists of lungs,trachea and other air passageways.This system delivers oxygen to the blood and removes carbon dioxide from the body,maintains appropriate oxygen content and helps to regulate acid-base balance of blood.
• The cardiovascular system consists of heart,blood vessels and blood.It transports nutrients,oxygen,hormones and other substances to the body cells and carries wastes from the cells to the excretory organs,helps in maintaining fluid balance.
• The lymphatic(immune) system consists of lymph,lymphatic vessels,lymph nodes and other lymph structures.This systems collects excess tissue fluid and returns it to the blood,absorbs lipids from digestive tract and transports them to cardiovascular system and defends the body against organisms that cause disease.
• The urinary system consists of kidneys,bladder and associated ducts.This functions to excrete metabolic wastes and helps to regulate fluid balance and acid-base balance.
• The reproductive system consists of testes,ovaries and associated structures,The male reproductive system produces and delivers sperms ans the female reproductive system produces ova and incubates the developing offspring.Both systems release hormones that establish and maintain sexuality.