All organisms in the animal kingdom are multicellular, however, they do not show the same pattern of organization in cells. Based on the patterns of cellular organization, levels of organization in animals are classified into:

The Cellular Level of Organization: Animals with this type of cell organization have cells arranged in the form of loose cell aggregates. This type of organization can be observed in sponges.
Tissue Level of Organization: The cells of the animals show divisions in cell activities. The cells which perform the same functions are arranged as tissues. Example: Coelenterates.
Organ Level of Organization: Tissues in a certain group of animals that have the same function are grouped to form an organ. Each organ has a specific function. Example: Platyhelminthes
Organ System Level of Organization: In animals where organs have associated to form functional systems where each system is concerned with a certain physiological function are observed to exhibit organ system level of organization. Example: Annelids, Arthropods, Echinoderms, Mollusks, and Chordates.

What is Structural Organization in a Cell?

The cell is the structural and functional unit of any animal or plant. It consists of various cellular components that carry out the functions of the cell. These structures are called cell organelles and they make up the structural organization of a cell.

Cell Organelles and its Functions

Cell Organelle	Function
Nucleus	It contains the hereditary material, the DNA, and is surrounded by a nuclear envelope that separates the DNA from the rest of the cell.
Endoplasmic Reticulum	It plays a major role in the metabolism of carbohydrates, synthesis of lipids, proteins, and steroids.
Cytoplasm	It’s a thick jelly-like substance that houses various cell organelles. Many chemical reactions take place within the cytoplasm.
Golgi Apparatus	Its major function is the transportation of materials within the cell.
Mitochondrion	ATP- the energy currency of the cell is produced here.
Ribosome	Known as the protein factory of a cell, all proteins are synthesized here.
Lysosome	It engulfs foreign bodies that enter the cell and helps in cell renewal.
DNA	Deoxyribonucleic acid- the DNA is the genetic material of the cell.
Cell Membrane	Supports and protects the cell and regulates the movement of substances in and out of the cell. It is a porous membrane and is a wall-like structure between two cells.

Tissues are organic material that makes up organs and other bodily structures. Tissues are formed from cells and have similar structures and functions. There are four types of tissues in animal bodies:

Tissue

Epithelial Tissue	This type of tissue consists of tightly packed sheets of cells that cover surfaces that include the outside of the body. For example, the outside layer of the human skin. The cells are polarized which means they have a top and a bottom side. The top side faces the inside cavity or the outside which is open to the air and the outside environment whilst the bottom side faces the underlying cells. Because the cells are tightly packed, it lets them act as barriers to the movement of fluids and harmful microbes. Often the cells are joined by specialized junctions that hold them tightly together.
Connective Tissue	This type of tissue consists of cells suspended in an extracellular matrix. The matrix is made of protein fibers like fibrin, collagen, and solid, liquid, or jelly-like substance. The most common type of connective tissue is the loose connective tissue which is found throughout the body and it supports blood vessels and other organs. This type of tissue also links epithelial tissue to muscles that lie underneath. Connective tissue is found in ligaments and tendons which connect muscles to bones and one bone to another. Special forms of connective tissue include adipose tissue- body fat, blood and cartilage. In these tissues, the extracellular matrix is liquid and is called plasma.
Muscle Tissue	Muscle tissue allows the body to move, pump blood and move food through the digestive system. Muscle cells contain actin and myosin proteins which allow them to contract. The three types of muscles seen are Skeletal Muscle: Also called striated, striped muscles, and are attached to tendons. It allows us to consciously control our movement. Cardiac Muscle: Found in the walls of the heart and are striated and striped like skeletal muscles. They cannot be controlled. The individual fibers are connected to intercalated disks which allows them to contract in sync. Smooth Muscle: Found in the walls of blood vessels and the uterus, the digestive tract, the urinary bladder, and many internal structures. They cannot be controlled and are not striped or striated.
Nervous Tissue	This type of tissue consists of neurons and neuroglial cells and functions as processing and transmitting information. Neurons generate electrical signals called nerve impulses that allow them to convey information rapidly across long distances. Neuroglia’s act to support neuronal function.

Organ Level of Organization

Human body organs like the lungs, heart, stomach, etc are made of two or more types of tissues that serve a particular function. The lungs bring in oxygen and eliminate CO2, the heart pumps blood through the body- just to name a few examples. Most of the organs have all four types of tissues and they make up the whole organ system.

Organs are grouped into organ systems and they work together to carry out a particular function. The major organ systems of the human body are listed below:

Organ System Level of Organization

Organ System	Organ Tissues and Structures Involved	Function
Cardiovascular	Heart, blood, blood vessels.	Transports blood, oxygen, nutrients to all parts of the body and transports wastes away from cells. Stabilizes body temperature
Lymphatic	Lymph, lymph vessels, lymph nodes	Defends our body against diseases and infections. Transfers lymph between the blood and other tissues.
Digestive	Mouth, salivary glands esophagus. Stomach, liver, gallbladder, pancreas, small and large intestine.	Processes food and absorbs minerals, vitamins, nutrients, and water.
Endocrine	Pituitary, pineal, thyroid and parathyroid, adrenals, ovaries, testes, endocrine pancreas (all glands)	Secrete various hormones which have specific functions and assist in many bodily activities. ADH secreted from the adrenal glands controls the passing of urine.
Muscular	Skeletal, smooth, and cardiac muscles.	Provides movement and support to the body as well as heat production.
Nervous	Brain, spinal cord, sensory organs- eyes, ears, skin, tongue and nose, nerves	Collects, transfers and processes information
Integumentary	Nails, skin, hair	Protects the body from injury and fluid loss. Defends the body from infectious microbes.
Reproductive	Uterus, vagina, ovaries, mammary glands, testes, vas deferens, seminal vesicles, prostate, penis, fallopian tubes.	Produces gametes and sex hormones and assists in reproduction.
Respiratory	Nose, pharynx, larynx, trachea, bronchi, lungs, diaphragm, and mouth.	Respiration
Skeletal	Bones, cartilage, tendons, joints, ligaments, tendons, and joints.	Provides movement at joints Supports and protects the soft tissues of the body Produces blood vessels Acts as mineral storage
Immune	Leukocytes, thymus, adenoids, spleen, tonsils	Defend against pathogens and disease-causing agents.
Urinary	Kidneys, ureters, urinary bladder, urethra	Removes excess water from the body, as well as salts and waste products from the blood.

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FAQs on Structural Organization in Animals

1. What are the Levels of Structural Organization and Body Systems?

The levels of structural organization and body systems in animals are

The cellular level of organization

Tissue level of organization

Organ level of organization

Organ system level of organization,

However, there are arguments in the scientific community that there are three additional levels of structural organization.

These are

Atomic or chemical level of organization

Cell organelle level of organization

Organism level of organization

2. What is adipose tissue? Where does it lie in the human body? Give its important functions.

Adipocytes are specialized fat cells that reside in the matrix of adipose tissue. They are large, rounded, contain large fat droplets, and their nucleus and cytoplasm are located in the outer regions of the cell. This fat can be stained by Sudan III or osmic acid easily. The fat cells are supported by a loose framework of areolar tissue that contains fibroblasts, macrophages, small collagen fibers, elastic fibers, as well as lymphatics, and blood vessels.

The lactating tissue of a human is found in the subcutaneous tissue, the mesentery, the perinephric tissue, and the subperiosteal tissue. The lactating mammary gland also contains a significant quantity of adipose tissue, but these tissues are scarce in the penis, scrotum, and eyes of humans.

Its main function is to synthesize, store, and metabolize fat and to form an insulating layer under the skin. It forms around the viscera, particularly the kidneys, and protects them from shock and injury.

3. What is the Chemical Level of Organisation?

It is the simplest level in the structural organization hierarchy. This level includes the smallest building blocks of matter such as atoms that combine to form molecules of water and various organic products. Afterward, all molecules combine to form cell organelles and eventually the organs of a body. The chemical organization is the simplest level of organization. It includes the building blocks that make life possible atoms. Atoms bind together in different combinations to form molecules, which perform several essential functions for life. Cellular organelles and organelles constitute the fundamental structural and functional unit of life, while atoms contain protons, electrons, and neutrons that make up subatomic particles.

4. What is connective tissue? Give its important functions.

Connective tissue is composed of matrix cells, fibers, and a ground substance composed of extracellular elements that fill up the intercellular spaces between cells.

Functions of connective tissue: The connective tissue performs a variety of functions, including

The structure serves as packing material for nearly all organs.

It serves as a strong elastic rope that binds one organ to another.

It provides skeletal support and sculpts the body.

It protects the vital organs in the cranial and thoracic cavities.

Adipose connective tissue is a store of fat and energy. It is also a barrier to foreign particles, such as bacteria.

The phagocytes of leukocytes ingest the bacteria and germs, protecting the body against infection.

5. What is a gland? Differentiate between simple and compound exocrine glands.

A gland is a structure that secretes a specific substance. Glands are developed from the epithelium tissues and can be either cubical, short columnar, or polygonal in shape.

Simple Exocrine Gland: Simple exocrine gland has no branches, but is composed of an unbranched duct lined by epithelial cells. The secretory part of the gland is made up of epithelial cells arranged in a simple tube, a coiled tube in a flask that sends secretions to the bladder.

Thus, simple tubular glands, simple coiled tubular glands, and simple alveolar glands are all described in this way.

Compound Exocrine Gland: Compound exocrine gland is made up of a long, branched system of ducts. Secretory tissue consists of tubules-compound tables, many alveoli-compound alveolar glands, or a combination of tubules and alveoli-compound tubuloalveolar glands.

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