THE FUNDAMENTAL UNIT OF LIFE

LIVING ORGANISMS 

 

Living organisms are living things that can grow, reproduce, can react to stimuli, and have an organised structure. Living organisms are made up of cells.

 

CELL

• Cells are the small structures that are the basic building units of life.
• All living organisms are made up of cells.
• A cell is considered the functional unit of life because the cell provides structure to our body and all the body function takes place at the cell level. 
• Cell was discovered by Robert Hooke an English scientist in 1665 and coined the term cell, and the nucleus of cell was discovered by Robert Brown in 1831.
• Most cells are between 1-100 µm in diameter which can be seen by a light microscope.
• Robert Brown discovered the nucleus in the cell in 1831. 

CELL THEORY

Cell theory states that 

1.  Cell is the fundamental unit of life.

2.  All cells arise from pre-existing cells.

3.  All living organisms are composed of cells.

 

TYPES OF ORGANISMS

There are two types of organisms.

1. Unicellular Organisms - Single-celled organisms are called unicellular organisms.

Example: Amoeba, Chlamydomonas, Bacteria, and Paramecium are unicellularorganisms.

2. Multicellular Organisms - The organisms made up of more than one cell are called multicellular organisms.

In multicellular organisms, there are different organs in the body that perform different functions and these organs are made up of different cells. These cells have different shapes and sizes depending on their functions. 

The shape of cells may be spherical (blood cells), spindle (mussels cells), or irregular shaped (nerve cells).

[Note : Multicellular or unicellular, both organisms have some the same functions: uptake and processing of nutrients, response to environmental stimuli excretion of wastes, and reproduction among others] 

Basic features of cells  

• All cells consist of plasma membranes. It is also called cytoplasm.
• The semi-fluid substance within the membrane is called cytoplasm which contains the organelles. like mitochondria, ribosomes, lysosomes, Golgi apparatus, and endoplasmic reticulum. (the living structures of a cell are called cell organelles)
• All cells contain chromosomes that carry genes in the form of DNA. 
• All cells have ribosomes that make proteins using the instructions contained in genes.

Types of Cells

There are two types of cells, Prokaryotic and Eukaryotic:

1. Prokaryotic Cells – Cells in which the true nucleus is absent are called prokaryotic cells. All prokaryotes are unicellular organisms. Eg. Bacteria, blue-green algae, etc.

2. Eukaryotic Cells – Cell in which the true nucleus is present, called a eukaryotic cell. Eukaryotic cells are much bigger than prokaryotic cells.

Cell Organelles

1. Cell Wall - Cell wall is an outer rigid cover lying outside the plasma membrane.

• It is mainly composed of cellulose. 
• Cellulose is a complex substance and provides structural strength to plants.
• Cell-wall helps in plasmolysis.

[ Plasmolysis - It is a process in which a living plant cell
loses water through osmosis which is shrinkage or contraction of the contents of the cell away from the cell wall. ]
loses water through osmosis which is shrinkage or contraction of the contents of the cell away from the cell wall. ]
 

2. The nucleus

• The nucleus is enclosed by a nuclear envelope which is a double membrane nucleus membrane that allows large macro-molecules and particles to pass through.
• It acts as the control center of the cell.
• In the nucleus, the DNA and proteins are organized into fibrous material called chromatin.
• It plays an important role in cell division and cell development. 
• When the cell prepares to divide, the chromatin is seen as coiled separate structures called chromosomes. 

3. Cytoplasm :

• It is a semi-fluid liquid inside the plasma membrane.
• It is enclosed by the cell membrane
• It contains many specialized cell organelles. Each of these organelles performs a specific function for the cell.

4. Cell organelles:

• Cell organelles are enclosed by membranes in eukaryotic, in prokaryotic cells the membrane-bound cell organelles are absent.
•  Cell organelles are: Endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles. 
• They carry out some very crucial functions in cells. 

a.  Endoplasmic reticulum,

• It is a large network of membrane-bound long tubules or oblong vesicles. 
• There are two types of ER– rough endoplasmic reticulum (RER)  and  smooth endoplasmic reticulum (SER).  
• RER has particles called ribosomes attached to its  surface.  
• It is the site of protein manufacture.

b.  Golgi apparatus

• It is first described by Camillo Golg
• It is consists of a system of membrane-bound vesicles which are arranged in parallel to each other in stacks called cisterns. Its functions include the storage, modification and packaging of products in vesicles.
• The Golgi apparatus helps in the formation of lysosomes

C. Lysosomes 

• It is a waste disposal system in cells 
• Lysosomes help to keep the cell clean by digesting any foreign material like bacteria, old organelles, etc., and worn-out cell organelles.
• Lysosomes contain powerful digestive enzymes capable of breaking down all organic material. 

d. Mitochondria

• It is the site of cellular respiration
• It provides energy in the form of ATP to the cell to perform various chemical activities, so it is known as a powerhouse of cells.
• It is a double membrane organelle found in eukaryotes
• It has its own DNA and proteins 

e. Plastids

• Plastids are present only in plant cells
• Plastids are three types: Chloroplast - contains a green pigment called chlorophyll which helps in photosynthesis Chromoplasts - provides colours to the plants' leaf and fruits, and Leucoplasts - colourless plastids present in roots, seeds, and tubes, help in store foods.
• It has its own DNA and proteins like mitochondria. 

f. Vacuoles

• Vacuoles are the storage sacs for solid or liquid contents.
• Vacuoles are small sized in animal cells large sized in plant cells.
• In plant cells, vacuoles are full of cell sap and provide turgidity and rigidity to the cell.
• It also stores many substances like amino acids, sugars, various organic acids.  
• It expels excess water and wastes.