LEVELS OF STRUCTURAL ORGANIZATION

Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: subatomic particles, atoms, molecules, organelles, cells, tissues, organs, organ systems, organisms and biosphere.

To study the chemical level of organization, scientists consider the simplest building blocks of matter: subatomic particles, atoms and mnolecules. All matter in the universe is composed of one or more unique pure substances called elements, familiar examples of which are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these pure substances (elements) is an atom. Atoms are made up of subatomic particles such as the proton, electron and neutron. Two or more atoms combine to form a molecule, such as the water molecules, proteins, and sugars found in living things. Molecules are the chemical building blocks of all body structures.

A cell is the smallest independently functioning unit of a living organism. Even bacteria, which are extremely small, independently-living organisms, have a cellular structure. Each bacterium is a single cell. All living structures of human anatomy contain cells, and almost all functions of human physiology are performed in cells or are initiated by cells.

A human cell typically consists of flexible membranes that enclose cytoplasm, a water-based cellular fluid together with a variety of tiny functioning units called organelles. In humans, as in all organisms, cells perfor all functions of life. A tissue is a group of many similarcells (though sometimes composed of a few related types) that work together to perform a specific function. An organ is an anatomically distinct structure of the body composed of two or more tissue types. Each organ performs one or more specific physiological functions. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body.

1. UNICELLULAR, COLONIAL AND MULTICELLULAR FORMS

The structural and functional units of all living organisms are cells. Cells are the building blocks of life. All living organisms are made up of cells. The cells were discovered by Robert Hooke in 1665. In the human body there are about 100 trillion to 1014 cells. The size of the cells is about 10 micrometer. The cell contains cellular organelles that control the activity of the cell.

Organisms can be classified as unicellular and mutlicellular organisms. Unicellular organisms are made up of a single cell like most bacteria, while the multicellular organisms include plants and animals. The number of cells varies between species. The size of most plant cells and animal cells is between 1 to 100 micrometers; hence they are visible under microscope. The cell theory developed by Schwann and Schleiden states that all organisms are made up of one or more cells. Cells emerged on Earth about 3.5 billion years ago.

Unicellular Organisms

Unicellular organisms are known as single-celled organisms. They are made up of a single cell. Organisms like the amoeba, Paramecium are single-celled organisms, they are the oldest forms of life, they existed about 3.8 million years ago. Bacteria, archaea, protozoa,unicellular algae and unicellular fungi are the main groups of unicellular organisms. The single-cell regulates all the activity of the organism. Unicellular organisms are small are mostly invisible to the naked eye.

There are two general categories of unicellular organisms: prokaryotic and eukaryotic organisms. Prokaryotic unicellular organisms are protists and some fungi. Some of these unicellular prokaryotes live in colonies. They live together and all the cells of the colony is the same. All the process of life is carried out in each cell in order for the cell to survive. Simplest multicellular organisms are made of cells that dependent on each other for their survival. Most of the multicellular organism are microscopic and are known as microscopic organisms.

Unicellular organisms vary in size. The smallest organism a bacteria is only 300 nanometers and range upto 20cm. These organisms usually posses cilia, flagella or pseudopidia that help them in locomotion. They have simple body with basic features. Reproduction is both by asexual and sexual means. Nutrition is usually by the process of phagocytosis, where the food particle is engulfed and stored in vacuoles present in the organism.

 

Examples of Unicellular Organisms

Unicellular organisms are of two types- Unicellular prokaryotic organisms and unicellular eukaryotic organisms

Unicellular prokaryotic organisms- they are unicellular in nature and they do not have membrane-bound nucleus and membrane bound cellular organelles. These organisms are usually bacteria and cyanobacteria. Example: E.coli, Salmonella, Nostoc, etc.

Unicellular eukaryotic organisms – these organisms are unicellular and are eukaryotes. They have membrane bound true nucleus and other membrane bound organelles. These are mainly free living or aquatic parasites like the protozoans, some fungi and algae or some protists.

Multicellular Organisms

Organisms that consist of more than one cell are known as multicellular organisms. Multicellular organisms are made up of more than one cell. These cells identify and attach to each other to form a multicellular organism. Most of the multicellular organisms are visible to the naked eye. Organisms like plants, animals and some algae arise from a single cell and they grow up into a multi-celled orgainsm. Both prokaryotes and eukaryotes show multicellularity. True multicellular organisms regenerate a whole organism from germ cells.

There are three theories to discuss the mechanisms by which multicellularity could have evolved.

Symbiotic Theory– Symbitoic theory states that the first multicellular organism arose from symbiosis behaviour of diferent species of a single-celled organism, each performing different functions. Such symbitoic relation ship is seen between clown fish and Riterri sea anemones.

The Cellularization Theory or The Syncytial Theory – The cellularization theory states that a unicellular organism would have developed from membrane boundaries/partitions around each nuclei from a single celled organism with multiple nuclei. Protists like ciliates and slime molds have multiple nuclei supporting this theory.

The Colonial Theory- The Colonial Theory of Haeckel, 1874, proposes that the symbiosis of many organisms of the same species (unlike the symbiotic theory, which suggests the symbiosis of different species) led to a multicellular organism. At least some, it is presumed land-evolved, multicellularity occurs by cells separating and then rejoining (e.g. cellular slime molds) whereas for the majority of multicellular types (those that evolved within aquatic environments), multicellularity occurs as a consequence of cells failing to separate following division. The mechanism of this latter colony formation can be as simple as incomplete cytokinesis, though multicellularity is also typically considered to involve cellular differentiation.

The advantage of the Colonial Theory hypothesis is that it has been seen to occur independently in 16 different protoctistan phyla. For instance, during food shortages the amoeba Dictyostelium groups together in a colony that moves as one to a new location. Some of these amoeba then slightly differentiate from each other. Other examples of colonial organisation in protista are Volvocaceae, such as Eudorina and Volvox, the latter of which consists of up to 500-50,000 cells (depending on the species), only a fraction of which reproduce. For example, in one species 25-35 cells reproduce, 8 asexually and around 15-25 sexually. However, it can often be hard to separate colonial protists from true multicellular organisms, as the two concepts are not distinct; colonial protists have been dubbed “pluricellular” rather than “multicellular”.

Advantages of Multicellularity in organism are that multicellularity allows the organism to exceed the size limits. Multicelluarity also permits in increasing the complexity of the organism by allowing differentiation cellular lineages in an organism. Reproduction in multicelluar organism is by sexual means.

Examples of Multicellular Organisms

Multicellular organisms are of two types- multicellular prokaryote and multicellular eukaryote organisms Multicellular prokaryotes are mostly multicelluar bacterial species like myxobacteria. Some cyanobacteria like Chara, Spirogyra, etc are also multicellular prokaryotes. Sometimes these bacteria are considered as colonial instead of multicellular.

Multicellular eukaryotes-Most of the eukaryotic organisms are multicellular. These organisms have a well-developed body structure and they have specific organ to perform specific function. Most of the well developed plants and animals are multicellular. Examples are almost all species of gymnosperm and angiosperm plants and almost all animals are eukaryotic and mutlicellular.

Unicellular Organisms	Multicellular Organisms
Body of the organism is made up of a single cell.	The body of multicellular organism is made up of numerous cells.
Body organization is simple.	Organization is complex.
The function of the whole organism is carried out bya single cell.	Specialized functions are performed by different cells, tissues, organs or organ systems.
Division of labor in the organism is at organelle level.	Division of labor in the organic may be at cellular level, tissue level, organs and organ syster level.
Usually prokaryotic in nature.	They are mostly eukaryotic in nature.
The body of the cell is exposed to the environment on all sides.	Outer cells face the environmentg
Any injury to cell can cause death of the organism.	Injury or death of some cells does not affect the organisms, the affected cells are replaced
A limit is imposed to the size of the cell by the surface area to volume ratio and hence it can attain large size.	Due to multicellularity, the organism can attain large size
Lifespan of the organism is usually short.	Organisms have a longer lifespan.
Reproduction is by vegetative/asexual methods.	Reproduction is sexual type.
Genome has a few introns.	High introns are present in the genome.
Has good capacity of regeneration and power of division.	Capacity of regeneration decreases with increase in specialization and certain cel that are specialized loose the power of division.
There is no cell differentiation process,	Cell differentiation is evident
Nutrition is by engulfing food.	Nutrition is by specific orga by food production. They cal autotrophs or heterotrophs.
They are microscopic in nature.	They are macroscopic nature

LEVELS OF ORGANIZATION OF TISSUES. ORGANS AND SYSTEMS

Cells are the basic building blocks of multicellular animals. When cells with the same characteristics or specializations are grouped together, they form a tissue. There are four basic types of tissues-epithelial, connective, muscle, and nervous-but there are variations on each basic type. An organ is usually made up of several different tissue types.

Epithelial tissues cover the body and line organs

Epithelial tissues are sheets of densely packed, tightly connected cells that cover inner and outer body surfaces. They form the skin and line hollow organs of the body, such as the gut . Some epithelial cells have secretory functions; examples are the groups of epithelial cells that secrete hormones, milk, mucus, digestive enzymes, or sweat. Other epithelial cells have cilia to help substances move over surfaces or through tubes. Since epithelial cells create boundaries between the inside and the outside of the body and between body compartments, they frequently have protective as well as absorptive and transport functions. Epithelial cells can also form receptors that provide information to the nervous system. Smell and taste receptors, for example, are epithelial cells that detect specific chemicals.

Epithelial tissues have distinct inner and outer surfaces. The outer surface faces the air. as in the case of the skin and lungs, or a fluid-filled organ cavity, such as the lumen of the gut. These outer surfaces are the apical ends of the epithelial cells, which may have cilia or may be highly folded to increase their surface area. The inner surfaces of an epithelium are the basal ends of the epithelial cells, Which rest on an extracellular matrix called a basal lamina.

The skin and the lining of the gut are examples of epithelial tissues that receive much wear and tear. Accordingly, cells in these tissues have a high rate of cell division to replace cells that die and are shed. Dandruff consists of discarded skin cells.

Connective tissues support and reinforce other tissues

In contrast to densely packed epithelíal tissues, connective tissues consist of dispersed populations of cells embedded in an extracellular matrix that they secrete. The composition and properties of the matrix differ among types of connective tissues. An important component of the extracellular matrix secreted by connective tissue cells is protein fibers. The dominant protein in the extracellular matrix is collagen. Collegen is, in fact, the most abundant protein in the human body, representing 25 percent of total body protein.

Collagen fibers are strong. They give the connective tissue of skin, tendons, and ligaments resistance to stretch. Similarly, collagen fibers provide a netlike framework for organs, giving them shape and structural strength. Connective tissue that fills spaces between organs has a low density of collagen fibers.

Another type of protein fiber in the extracellular matrix of connective tissues is the stretchable protein elastin. It can be stretched to several times its resting length and then recoil. Fibers composed of elastin are most abundant in tissues that are regularly stretched, such as the walls of the lungs and the large arteries. Gradual loss of elastin fibers with age causes gradual loss of resiliency of the skin.

Cartilage and bone are connective tissues that provide rigid structural support. In cartilage, a network of collagen fibers is embedded in a flexible matrix consisting of a protein-carbohydrate complex. Cartilage, which lines the joints of vertebrates, is resistant to compressive forces. Since it is flexible, it provides structural support for flexible structures such as external ears and noses. The extracellular matrix in bone also contains many collagen fibers, but it is hardened by the deposition of the mineral calcium phosphate.

Adipose tissue is a form of loose connective tissue that includes adipose cells, which form and store droplets of lipids. Adipose tissue, or “fat,” is a major source of stored energy. It also serves to cushion organs, and layers of tissue under the skin can provide a barrier to heat loss.

Blood is a connective tissue consisting of cells dispersed in an extensive extracellular matrix: the blood plasma. The blood plasma is much more liquid than the extracellular matrices of the other connective tissues, but it too contains an abundance of proteins.

Muscle tissues contract

Muscle tissues consist of elongated cells that can contract and cause movement. Muscle tissues are the most abundant tissues in the body, and when animals are active, they use most of the energy produced in the body.

Nervous tissues process information

There are two basic cell types in nervous tissues: neurons and glial cells. Neurons, which are extremely diverse in size and form, communicate Via electrochemical signals. These nerve impulses can be conducted via long extensions of the neurons to other parts of the body, where they are communicated to other neurons, muscle cells, or secretory cells. Neurons are involved in controlling the activities of most organ systems to achieve homeostasis.

Glial cells do not generate or conduct electrochemical signals, but they provide a variety of supporting functions for neurons. There are more glial cells than neurons in our nervous systems.

Organs consist of multiple tissues

Adiscrete structure that carries out a specific function in the body is called an organ. Examples are the stomach, the heart, the liver, and the kidney. Most organs include all four tissue types. The wall of the stomach is a good example. The inner surface of the stomach that contacts food is lined with a sheet of epithelial cells. Some of the epithelial cells secrete mucus, enzymnes, or stomach aciid,Beneath the epithelial lining is Connective tissue. Within this connective tissue are nerves, glands (cluster secretory epithelial cells), and blood vessels. Concent layers of smooth muscle tissue enable the stomach contract to mix food with digestive juices.

A network of neurons between the muscle laver controls these movemnents and also influences the secretions of the stomach. Surrounding the stomach is sheath of connective tissue.

An individual organ is usually part of an organ system a group of organs that function together. The stomach is part of the digestive system, which also includes the food tube (esophagus), the small and large intestines, the pancreas, which secretes digestive enzymes, and the liver, which secretes bile.

COMPARATIVE ANATOMY:

There are many forms of evidence for evolution. One of the strongest forms of evidence is comparative anatomy; comparing structural similarities of organisms to determine their evolutionary relationships. Organisms with similar anatomical features are assumed to be relatively closely related evolutionarily, and they are assumed to share a common ancestor. As a result of the study of evolutionary relationships, anatomical similarities and differences are important factors in determining and establishing classification of organisms.

Some organisms have anatomical structures that are very similar in embryological development and form, but very different in function. These are called homologous structures. Since these structures are so similar, they indicate an evolutionary relationship and a common ancestor of the species that possess them. A clear example of homologous structures is the forelimb of mammals. When examined closely, the forelimbs of humans, whales, dogs, and bats all are very similar in structure. Each possesses the same number of bones, arranged in almost the same way. While they have different external features and they function in different ways, the embryological development and anatomical similarities in form are striking. By comparing the anatomy of these organisms, scientists have determined that they share a common evolutionary ancestor and in an evolutionary sense, they are relatively closely related.

Other organisms have anatomical structures that function in very similar ways, however, morphologically and developmentally these structures are very different. These are called analogous structures. Since these structures are so different, even though they have the same function, they do not indicate an evolutionary relationship nor that two species share a common ancestor.

The Major Organ Systems of Mammals

SYSTEM	TISSUES AND ORGANS	FUNCTIONS
Nervous system	Brain, spinal cord, sensory organs, peripheral nerves	Receives, integrates, stores information, and controls muscles and glands
Endocrine System	Glands: pituitary, thyroid, Parathyroid, Pineal, adrenal, Testes, ovaries, pancreas	A system of glands releases chemical messages(hormones) that control and regulate other tissues and organs
Muscle System	Skeletal muscle, Smooth muscle, cardiac muscle	Produces forces and motion
Skeletal  System	bones	Provides Structural support for the body
Reproductive System	Female: ovaries, Oviducts, uterus, Vagina, mammary glands Male: testes, sperm ducts, accessory gland, penis	Produces sex cells and hormones necessary to procreate and nurture offspring
Digestive System	Mouth, esophagus, stomach, intestines, liver, pancreas, rectum, anus	Acquires and digests food, absorbs and stores nutrients, then makes them available to the cells of the body
Respiratory System	Airways, lungs, diaphragm	exchanges respiratory gases with the environment
Circulatory System	Heart and blood vessels	Transports respiratory gases, nutrients, and heat around the body
Lymphatic system	Lymph and lymph vessels, lymph nodes, spleen	Brings extracellular fluids back into the circulatory system; helps the immune system fight invading organisms
Immune System	many types of white blood cells	fights invading organisms and infections
skin System	Skin, sweat glands, hair	Protects the body from invading organisms and harsh physical conditions, helps regulate body temperature
Excretory System	Kidneys, bladder, ureter, urethra	Regulates the composition of the extracellular fluids; excretes waste products

For example, the wings of a bird and a dragonfly both serve the same function; they help the organism to fly. However, when comparing the anatomy of these wings, they are very different. The bird wing has bones inside and is covered with feathers, while the dragonfly wing is missing both of these structures. They are analogous structures. Thus, by comparing the anatomy of these organisms, scientists have determined that birds and dragonflies do not share a common evolutionary. ancestor, nor that, in an evolutionary sense, they are closely related. Analogous structures are evidence that these organisms evolved along separate lines.

Vestigial structures are anatomical features that are still present in an organism (although often reduced in size) even though they no longer serve a function. When comparing anatomy of two organisms, presence of a structure in one and a related, although vestigial structure in the other is evidence that the organisms share a common evolutionary ancestor and that, in an evolutionary sense, they are relatively closely related. Whales, which evolved from land mammals, have vestigial hind leg bones in their bodies. While they no longer use these bones in their marine habitat, they do indicate that whales share an evolutionary relationship with land mammals. Humans have more than 100 vestigial structures in their bodies.

Comparative anatomy is an important tool that helps determine evolutionary relationships between organisms and whether or not they share common ancestors. However, it is also important evidence for evolution. Anatomical similarities between organisms support the idea that these organisms evolved from a common ancestor. Thus, the fact that all vertebrates have four limbs and gill pouches at some part of their development indicates that evolutionary changes have occurred over time resulting in the diversity we have today.

ADAPTIVE RADIATION AND MODIFICATIONS

The earth contains an incredible diversity of life. Literally millions of species are known to man, with more discovered every day. And yet scientists estimate that the species alive today make up less than 0.1% of those that have ever lived. We know the ways in which new species are formed. But all those mechanisms of speciation involve the creation of one or two species from another species over a long period of time. This ratio of at most 2 species emerging from 1 original species hardly seems enough to account for the extreme diversity we see today and throughout history

The mechanism of adaptive radiation helps explain this diversity. An adaptive radiation is a burst of evolution, creating several new species out of a single parent species. As when we discussed species richness, it is useful here to think of uninhabited “islands” of habitat, though in this case, the islands merely need to be uninhabited by the species in question. A population of given species, which we’l imaginatively name species 1, moves into a new habitat and establishes itself in a niche, or role, in the habitat. In so doing, it adapts to its new environment and becomes different from the parent species. If a new population of the parent species, 2, moves into the area, it too will try to occupy the same niche as 1. However, the niche rule states that only one of a group of closely related species may occupy the same niche in a given habitat. Competition between species 1 and 2 ensues, placing pressure on both groups to adapt to separate niches, further distinguishing them from eachother and the parent species. As this happens many times in a given habitat, several new species may be formed from a single parent species in a relatively short period of time. Darwin’s finches are an excellent example of adaptive radiation.

An adaptive radiation generally means an event in which a lineage rapidly diversifies, with the newly formed lineages evolving different adaptations. Different factors may trigger adaptive radiations, but each is a response to an opportunity.

1.The evolution of a key adaptation

A key adaptation usually means an adaptation that allows the organism to evolve to exploit a new niche or resource. A key adaptation may open up many new niches to an organism and provide the opportunity for an adaptive radiation. For example, beetle radiations may have been triggered by adaptations for feeding on flowering plants.

2.Release from competition/vacated niches

Lineages that invade islands may give rise to adaptive radiations because the invaders are free from competition with other species. On the mainland, other species may fill all the possible ecological niches, making it impossible for a lineage to split into new forns and diversify. On an island, however, these niches may be empty. Extinctions can also empty ecological niches and make an adaptive radiation possible. For example, open niches vacated by dinosaur extinctions may have allowed mammals to radiate into these positions in the terrestrial

3.Specialization

Specialization may subdivide a single niche into many new niches. For example, cichlid fishes have diversified in East African lakes into more than 600 species. This diversification may have been possible because different fish lineages evolved to take advantage of different foods (including insects, algae, mollusks, small fish, large fish, other fishes’ scales, and even other fishes’ eyes!).