The basic unit that constitutes a living organism and facilities life functions is called a cell. All living organisms comprise cells, and millions of microorganisms like bacteria are nothing but a cell capable of fully functioning on its own. Further, for complex multi-cellular organisms like plants and animals, these cells are organized into tissues, organs, and organ systems. Cells are categorized as eukaryotic cells and prokaryotic (primitive) cells based on the presence or absence of the nucleus and other membrane-bound organelles.
Tissues are formed by cells of similar functions being grouped together. The tissues further develop into organs with specific functions. Tissues are hence similar to cells but more organized and specialized to perform a particular function. In plants, there are broadly two types of tissues- meristematic tissues that contain actively dividing cells, and permanent tissues that are differentiated to do specific functions.
In animals, the tissues are divided into specialized categories as follows.
Epithelial Tissues - They form the outer surfaces, membranes, and linings of organs and organ systems.
Muscle Tissues - They form the skeletal muscles, voluntary muscles, involuntary muscles, and cardiac muscles. They account for movement inside and outside the body.
Connective tissues constitute blood, bone, ligaments, tendons, and adipose tissues, connecting between other tissues.
Nervous tissues are the tissues in the brain, spinal cord, and peripheral nervous system that transmit neural messages for control and coordination.
One can differentiate between cell and tissue as follows.
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The ovum or the mature egg released by a female human's ovaries is the largest cell in the body. Its diameter is 120µm. On the contrary, the smallest cell in humans is the head of a spermatozoon, which is 5µm.
About 95% of the cells inside a human body are bacteria, and most of these bacteria live in the digestive tract. Human skin also hosts billions of bacteria. There are more bacterial cells in the human body compared to human cells. These bacteria are all not bad for us either. Unlike disease-causing bacteria, many bacteria within the human body are useful and are called 'probiotics.' These bacteria protect the body from infections and provide several essential nutrients for the healthy functioning of our system.
Cells suicide when they become infected! Whenever a cell is damaged or infected, the lysosomes within a cell break. The potent digestive enzymes destroy the whole cell. This is why lysosomes are called ‘suicidal bags’ of the cell. This process of self-destruction by cells is called autophagy. Apoptosis is a similar process for programmed cell death. These self-destruction processes are natural, beneficial, and keep the body's functioning and mitosis regulated. Whenever our body cannot self-destruct unnecessary cells, it can lead to conditions like tumors or cancer. Most anti-tumor drugs work by inducing apoptosis among cells to tackle cancerous cells.
Conversely, when there is a defect in these self-destruction systems, situations like autoimmune diseases occur. Thereby, normal cells are also attacked and destroyed by the immune system of mistaking it as a foreign body. The latest researches are looking into the genetic and environmental reasons behind defective apoptosis and autoimmune diseases.
Q1. What are the Lifespan Differences Between Cells and Tissue?
Answer: The cells in the human body are renewed continuously. Except for a few types of cells like nerve cells and cardiac muscle cells. Most cells die within a few days and are replaced by new cells. These lifespans, therefore, apply to the tissues they make up as well.
Cells in connective tissues such as blood are renewed continuously. While red blood cells last for 120 days, different white blood cells have very short life spans like 3-4 days (Neutrophils).
The lifespan of tissues is also affected by the functions they perform. The epithelial lining of digestive organs such as the stomach lasts for five days and is replaced after that. On the other hand, the outermost layer of the body- skin is renewed every two weeks. The tissues making up the human skeleton renew themselves every ten years in adults. One has to give their body adequate nutrition and exercise so that cells and tissues can renew and repair effectively.
Q2. How do Cells Specialize in Functions to Form a Tissue?
Answer: Cell specialization begins right from the embryo state of an organism. The embryonic stem cells signal cytoplasm of different cells to develop differently. Therefore the stem cells in an embryo broadly split into ectoderm, mesoderm, and endoderm. Furthermore, these broad groups get specialized and become tissues dedicated to performing particular functions like lung cells, muscles of the digestive tract, cardiac muscles, etc.
The embryonic stem cells regulate the development of cells based on genetic expression. In the adult stages, these specializations are maintained by specific gene transcription factors. These genetic expressions are developed, preserved, and passed on across generations. The latest research on biochemistry suggests that these genetic expressions cause differences in calcium messenger signals sent to different cells.
Simply put, genetic information causes embryonic stem cells to send different signals to different cells. Based on these signals, they specialize in various functions and become tissues and organs in an organism. This genetic information maintains these specializations further in adult stages as well.