Cell

Bodies of all organisms are made up of cells. Cell is the basic, structural and functional unit of life. In bodies of living organisms, group of cells are organized to form tissues.

A group of tissues forms organ. A group of organs form organ system.

Cell → Tissue → Organ → Organ system→ Organism

Cell number: On the basis of cell number, organisms can be of 2 types:

(i) Unicellular organisms: Are those which are made up of a single cell. In these organisms, all essential life functions (digestion, respiration, excretion etc.) are carried out by single cell.

Examples: Amoeba, Chlamydomonas, Paramecium and bacteria.

(ii) Multicellular organisms: Are those whose bodies are made up of a few to billions of cells.

Examples: Most plants and animals.

Differences between unicellular and multicellular organisms:

1.0Parts of Cell

Cell Membrane

It is a very thin membrane making the outer boundary that surrounds the cytoplasm of a cell. It is also called plasma membrane. It is porous and thus allows the movement of substances or materials both inwards and outwards.

Plasma membrane is a flexible and selectively permeable membrane.

Plasma membrane performs the following functions:

(i) It protects the internal components of the cell.

(ii) It helps to maintain the shape of the cell.

(iii) It allows materials to enter and leave the cell through the tiny holes called pores.

2.0Cell Wall

In addition to the cell membrane, plant cells have an outer rigid thick layer called cell wall. It is protective in function and mainly determines the shape of cell.

Besides plants, cell wall forms the outer covering in the cell of bacteria and fungi. Animals do not have cell wall.

3.0Cytoplasm and Cell Organelles

A transparent, jelly like protoplasmic mass of cell inner to plasma membrane and excluding nucleus is called cytoplasm.

It contains a number of minute living structures known as the cell organelles.

Living substances of the cell is called protoplasm. It includes cytoplasm and nucleus.

Cell organelles and their functions 

An organelle is a specialized subunit within a cell that has a specific function. Individual organelles are usually separately enclosed within their own membrane.

4.0Mitochondria

Singular mitochondrion 

It is also called ‘Power house of the cell’. 

It performs the function of respiration, provide energy to the cell in the form of ATP. ATP (Adenosine triphosphate) is the energy currency of the cell.

Plastid

Plastids are found in plant cells only. They are of different colours. The green coloured plastids are known as chloroplasts, which provide green colour to the leaves.

5.0Chloroplast

It is present only in plant cells. 

It is also called "Kitchen of the cell".

It contains a green pigment called chlorophyll which helps in food manufacturing (photosynthesis), also provides green colour to the leaves.

6.0Endoplasmic Reticulum

ER is of two types -

(i) RER (Rough endoplasmic reticulum) 

(ii) SER (Smooth endoplasmic reticulum)

It is a network of membranes.  

It provides a large surface area for life functions to take place.  

Its functions are synthesis of proteins (RER) and synthesis of lipids (SER).

7.0Golgi Complex

It collects and distributes the substances made in the cell (for example, proteins); synthesis and secretion of many materials.

Lysosomes : It contains hydrolytic enzymes which help in breaking down or destroying the various materials.

Ribosomes : Its function is protein synthesis.

Vacuoles : In plant cells, large non-cytoplasmic spaces enclosed inside a membrane are seen. These spaces are called vacuole. Vacuoles are either absent or smaller in size in animal cells while they are large in size and mostly single in plant cells. 

Nucleus

Nucleus is also called headquarter of the cell or brain of the cell. It is the most important part of the living cell. It is usually spherical or oval in shape. It controls all the vital functions of the cell. 

Nucleus contains a network of fine thread like structures called chromatin network that form chromosome.

Chromosomes are made up of DNA (Deoxyribonucleic acid ) and proteins. Chromosomes contain units of heredity known as gene. They are responsible for the transmission of characters from parents to the offsprings.

8.0Prokaryotic and Eukaryotic Cells

(i) Prokaryotic cells

These are cells having primitive type of nucleus without nuclear membrane. Organisms with primitive nucleus are known as prokaryotes. e.g. Bacteria and Blue green algae.

It lacks all membrane bound cell organelles (Golgi bodies, ER, lysosomes), but it has ribosomes.

(ii) Eukaryotic cells

These are cells having a well-organized nucleus with nuclear membrane. Organisms with true nucleus are known as eukaryotes. e.g. Animal cell and Plant cell. 

It has membrane bound cell organelles (Golgi bodies, ER, lysosomes).

9.0Also Read