Structural Organization in Animals

What is the Structural Organisation in Animals?

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

  • 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 the Structural Organization of 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 Organelle



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. 


It’s a thick jelly-like substance which 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. 


ATP- the energy currency of the cell is produced here. 


Known as the protein factory of a cell, all proteins are synthesised here. 


It engulfs foreign bodies that enter the cell and helps in cell renewal.


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. 

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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:


Epithelial Tissue

This type of tissue consists of tightly packed sheets of cells that cover surfaces which includes the outside of the body. For example, the outside layer of the human skin. The cells are polarised 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 fibres 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 fibres 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 the 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.

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Organ Level of Organisation

Human body organs like the lungs, heart, the 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 Organisation

Organ System

Organ Tissues and Structures Involved



Heart, blood, blood vessels.

  • Transports blood, oxygen, nutrients to all parts of the body and transports wastes away from cells. 

  • Stabilises body temperature


Lymph, lymph vessels, lymph nodes

  • Defends our body against diseases and infections.

  • Transfers lymph between the blood and other tissues.


Mouth, salivary glands oesophagus. Stomach, liver, gallbladder, pancreas, small and large intestine.

Processes food and absorbs minerals, vitamins, nutrients and water.


Pituitary, pineal, thyroid and parathyroid, adrenals, ovaries, testes, endocrine pancreas (all glands)

Secretes various hormones which have specific functions and assist in many bodily activities. 

ADH secreted from the adrenal glands controls the passing of urine. 


Skeletal, smooth and cardiac muscles.

Provides movement and support to the body as well as heat production.


Brain, spinal cord, sensory organs- eyes, ears, skin, tongue and nose, nerves 

Collects, transfers and processes information 


Nails, skin, hair

  • Protects the body from injury and fluid loss

  • Defends the body from infectious microbes.


Uterus, vagina, ovaries, mammary glands, testes, vas deferens, seminal vesicles, prostate, penis, fallopian tubes.

Produces gametes and sex hormones and assists in reproduction.


Nose, pharynx, larynx, trachea, bronchi, lungs, diaphragm and mouth.



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


Leukocytes, thymus, adenoids, spleen, tonsils

Defends against pathogens and disease-causing agents.


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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FAQ (Frequently Asked Questions)

Q: What are the Levels of Structural Organization and Body Systems?

A: The levels of structural organization and body systems in animals are:

  1. The cellular level of organisation

  2. Tissue level of organisation

  3. Organ level of organisation

  4. Organ system level of organisation

However, there are arguments in the scientific community that there are three additional levels or structural organisation.

These are:

  1. Atomic or chemical level of organisation

  2. Cell organelle level of organisation

  3. Organism level of organisation

Q:  What is the Chemical Level of Organisation?

A: It is the simplest level in the structural organisation 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.