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We know the human body as an entire organism that works and functions as a single unit, despite having countless component parts. Billions of smaller sub-units align and arrange themselves in various groups to perform different tasks within our body, all of which are required to occur in tandem for our continued existence.
In this way, our body can be compared to a building being constructed from scratch. A brick is the smallest unit of this under-construction building, and many such bricks are needed to build the complete structure. Some bricks are used to line the main load-bearing walls, while others are used to construct regular walls, hallways, storage spaces or connection points within the building.
An architectural plan is being followed in this case, one that decides which bricks will be a part of which area of the building. Very similar to this, an architectural plan is followed in our body, with the “cell” being the most basic unit of the body (much like a brick), as well as “tissues”, collections of bricks that come together to perform specialized functions. These tissues then collaborate and form organs, which do much of the load-bearing work in our body that is specially designated for them, e.g., the lungs carry out respiration, the stomach carries out digestion etc.
However, organs like the stomach and lungs alone cannot carry out all their designated functions, but instead need other organs to fully carry out these functions, thus forming organ systems. The respiratory system includes organs like the nose, mouth, pharynx, larynx, trachea and lungs, while the digestive system includes organs like the mouth, salivary glands, esophagus, stomach, liver, gallbladder, pancreas, small and large intestines. With this overview in mind, let’s get back to the basics and talk briefly about the cellular hierarchy of our body.
A cell is the smallest and most fundamental unit of a living organism. It is entirely capable of functioning independently. A good example of that independence can be seen in good ol’ bacteria.
A bacterium is a unicellular (single-celled) organism, and we all know that such a single-celled organism is not only capable of surviving, but also multiplying and flourishing! Just as cells can exist as unicellular organisms, they are also present in multicellular organisms like plants, animals, and humans. Plants are made of cells that carry out functions like photosynthesis and respiration that are necessary for their survival.
Basically, all living things are made up of cells and their products. These cells divide and give rise to new cells, which differentiate according to their designated roles; thus, it can be said that cells are the functional and structural units of all living things. Another distinct characteristic of cells is that they are not visible to the naked eye and can only be observed under a microscope, unlike larger structures like tissues and organs.
A cell is generally enclosed by a cell wall, which protects its internal environment from the external environment. Inside the cell, smaller organelles that help in the functioning of the cell are present; these organelles include the nucleus, lysosomes, mitochondria, Golgi apparatus, etc.
The main functions of a cell in any living organism are growth, metabolism, and reproduction, which occurs by the cell undergoing meiosis and mitosis.
Groups of similar cells come together to perform specific tasks and are referred to as a tissue. The cells within a particular tissue type share a common embryonic origin and also resemble each other morphologically.
It’s interesting to note that tissues exist in more complex organisms, but not in ancient eukaryotic organisms. Four broad categories of tissues exist in our body that perform different functions. Epithelial tissue lines our body externally and is found within internal cavities existing as tightly packed sheets of cells. The main function of this tissue is to provide a protective barrier against harmful microbes.
Connective tissue like fat, cartilage, blood and other soft padding tissues connect and bind the organs and cells of the body, thereby providing integration and support to all parts of our body. This tissue consists of cells dispersed in an extracellular matrix.
Muscle tissue comes in three types: skeletal muscle, smooth muscle, and cardiac muscle. This tissue responds to stimuli, can involuntarily contract and help in bringing about bodily movement, or it can enable the functioning of our visceral organs and heart.
Nervous tissue picks up clues from the external or internal environment, converts it to electrochemical signals and transmits these signals in the form of nerve impulses in order to generate an appropriate response to the stimuli. In the animal kingdom, these four types of tissues are found evenly distributed throughout the body.
Next up the ladder in the hierarchy of human architecture are the organs and organ systems. An organ is a structure made of two or more types of tissues that carry out specific functions, e.g., the stomach, heart, kidneys, lungs, etc.
These organs carry out tasks designated to be performed only by them; the stomach can only aid in digestion, but it cannot pump blood. Similarly, the heart can only pump blood, but cannot carry out reproductive tasks. As organs are made up of specialized tissues, any repair to organs is brought about by repair being achieved in the tissues. When a group of organs comes together to meet the physiological needs of the body, organ systems are formed. There are several such organ systems in our body, namely:
- Skeletal system – the bones, joints, tendons, ligaments, muscles, and cartilages.
- Nervous system – the brain, spinal cord, and peripheral nerves
- Integumentary system – the hair, skin, and nails
- Endocrine system – the thyroid gland, pituitary gland, pancreas, adrenal glands, hypothalamus, pineal gland, parathyroid gland, etc.
- Cardiovascular system – the heart and blood vessels
- Lymphatic system – the spleen, thymus, lymph nodes, lymphatic vessels
- Digestive system – the stomach, liver, gall bladder, large and small intestine
- Respiratory system – the nose, trachea, lungs, bronchi, and diaphragm
- Excretory system – the kidneys, urinary bladder and urethra
- Male and female reproductive system – the male reproductive system consists of the epididymis, seminal vesicles, prostate, penis and testes, while the female reproductive system consists of the mammary glands, vulva, vagina, ovaries and uterus.
From the above descriptions of cells, tissues, organs and organ systems, it can be clearly understood that our body is meant to work in a systematic, organized and hierarchical way. A cell is truly the most basic fundamental unit of the living world, but that is only the beginning. As far as plants and animals are concerned, if there were no cells, there would be no tissues, organs, organ systems or life as we know it today!