The Fundamental Unit of Life

Cell

Cell is the fundamental structural and functional unit of life.

All living organisms are made up of cells. And all of the basic functions required for the life are performed at the cellular level.

Cell Theory

The three tenets of the Cell Theory are:

(1) All living organisms are composed of one or more cells.

(2) The cell is the basic unit of structure and organization in organisms.

(3) Cells arise from pre-existing cells.

Measurement of the size of Cells

Cells are very small in size and cannot be seen by the naked eye. The size of a cell of an organism is measured in micrometer because of its small size.

The shape and size of cells

Cells of some organisms have different sizes and shapes.

Example:

Nerve cells are cylindrical in shape with a tail. Nerve cells are the longest cell in the human body.

Blood cells are spherical in shape.

Ovum cells are spherical in shape but bigger than blood cells.

Bone cells have star like shape.

Smooth muscle cells have long cylindrical shapes pointed at both ends.

The Largest Cell

The egg of an ostrich is the largest cell. The length of the cell of an ostrich is about 15 cm long and 13 cm wide. The longest cell in a human is neuron. The smallest cell is Mycoplasma gallicepticum which is a bacterium and causes disease in chickens and other some birds.

Cell – Tissue – Organs

Many cells combined together form tissue and many tissues combined together form organs. Many organs combined together and form an organ system, such as the digestive system, circulatory system, nervous system, skeletal system, and excretory system.

Cell → Tissue → Organs → Organ System

Discovery of Cell

Robert Hooke an English scientist discovered the cell in the year 1665. He examined the cork using his own designed microscope and found that the cork has a similar structure to a honeycomb which has little compartments. He called these boxes "cells".

The Structure of an Onion Peel

An onion peel is made up of many tiny compartment like structures. These structures resemble a honeycomb. All structures look similar to each other. Each cell of an onion peel has a dot like structure in the middle. These dot like structures are called nuclei.

Unicellular Organisms

Organisms having only one cell are called Unicellular Organisms. Here "Uni" means "one or single". For example, Amoeba, Chlamydomonas, Paramoecium, and Bacteria. All functions that require for the life processes are performed by only a single cell in unicellular organisms.

Multicellular Organisms

Organisms having more than one cell are called Multicellular Organisms, for example, fungi, plants, and animals. In Multicellular Organisms, many cells group together in a single body and assume different functions in it to form various body parts.

Basic Structure of a Cell

Plasma Membrane

Plasma membrane is the outer covering of a cell. The plasma membrane is made up of a double layer of protein and lipids. This double layer of the plasma membrane is called phospholipid bilayer Plasma Membrane are known as Cell Membrane also.

The plasma membrane separates a cell and its components from its external environment. The plasma membrane is a semi permeable membrane.

The exchanges of substances from inside to outside and vice versa take place from a cell through the plasma membrane by the process called Diffusion and Osmosis.

Diffusion

The Diffusion is a process in which the movement of substance takes place from a region of high concentration to a region of low concentration.

In the cell, carbon dioxide is a waste produced during respiration and needs to be expelled. The expelling of carbon dioxide out from the cell takes place by the process of diffusion. Similarly, when the concentration of oxygen decreases inside the cell, it moves into the cell by the process of diffusion through the plasma membrane.

Osmosis

Osmosis is a process in which the molecules of a liquid enter through a semi permeable membrane from a high concentration to a low concentration.

Plasma membrane allows required substances especially liquid, such as water to enter and expel from a cell using a process called Osmosis.

Since osmosis takes place from the region of high concentration to the region of low concentration, thus osmosis is a special case of diffusion through a selectively permeable membrane.

Endocytosis

An Amoeba ingests (takes in) food by changing its shape. This process of taking other materials from the external environment by a cell, like Amoeba, is known as Endocytosis.

Cell Wall

In the plant cell, an additional layer of covering is found over the plasma membrane, called Cell Wall. The Cell Wall provides rigidity and structural strength to the plants.

The cell wall is made up of cellulose.

It is the cell wall that enables the cells of plants, fungi, and bacteria to withstand very dilute solutions without bursting.

Hypotonic Solution

The solution having a lower concentration of the solutes than another solution is known as Hypotonic solution.

Hypertonic Solution

The solution having a higher concentration of the solutes than another solution is known as Hypertonic solution.

Plasmolysis

This phenomenon of shrinkage or contraction of living plant cells by losing water due to the presence of concentrated solution in the external environment (media) is known as Plasmolysis.

Deplasmolysis (Cytolysis)

Swelling of plant cells by the gain of water due to the presence of dilute solution in the external environment is called Cylolysis. Cytolysis is known as Deplasmolysis also.

Deplasmolysis or Cytolysis is the reverse process of Plasmolysis.