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The Cell: Theory & Structure

Updated on January 6, 2019
Mamoona Rasheed profile image

Mamoona is Qualified author in HSC Pre Medical.She is interested in such topics.



Cell as Basic Unit of Life:

The term cell was first used by Robert Hooke (1665) to describe his investigations on the structure of cork.

Later Robert Brown (1831) discovered a spherical body called Nucleus.

The fact that living organism has a cellular organization was emphasized by Matthias Schneider (1838)and Schwinn (1839) observed that the nucleus was surrounded by a fluid in the cell. His observation changed the definition of cell, and he described the Cell as a structure which consists of a nucleus surrounded by a semi fluid substance enclosed by a membrane. Later it was point out that the structure of a cell in animals and plants is similar with only one difference, that Plant cell has a Cell wall in addition to the cell membrane.

Definition of Cell:

Cell is defined as the structural and functional unit of living organisms made up of protoplasm containing nucleus surrounded by cytoplasm and bounded by cell membrane.

Cell Theory

In 1838 Schleiden (German botanist), concluded that despite difference in structure of various tissues, plants were made of cells. In 1839 a (German zoologist, T.Schwann, published a comprehensive report on the cellular basis of animals. Both of them proposed the cell theory which states that:

i) All organisms are composed of one or more cells.

ii) The cell is the structural and functional unit of life.

In 1855 Rudolf Virchow a German pathologist, had made a convincing case and added a third point to the cell theory.

iii) Cells can arise only by division of preexisting cell.

Cell theory brought a great revolution in the field of biology which established that the function of an organism is the result of activities and interaction of the cell units.

Cell Structure

Difference Between Eukaryotes and Prokaryotes

Eu=true karyon=nucleus
Pro=early karyon=nucleus
Eukaryotes are those organisms having a true nucleus in their cells. They may be unicellular and multicellular organisms.
Prokaryotes are those organisms which do not contain membrane bound nucleus in their cells,their nucleic material is usually coiled and concentrated in a region of the cell called the nucleoid.They are unicellular organisms like bacteria and cynobacteria.
They also contain a chromosome and a variety of membrane bound organelles like mitochondria,golgi apparatus,lysosomes,plastids etc in their cells.
They only contain mesosomes,which are simple infoldings of the Plasma membrane responsible for respiration.

Eukaryotic Cell

The typical eukaryotic cells contain three major parts, but 4th component i.e cell wall is only found in plant cell.

1. Plasma membrane

2. Nucleus

3. Cytoplasmic Organelles

4. Cell Wall

1. Plasma membrane

All cells are enclosed in a membrane that serves as the outer boundary, separating the cytoplasm from the external environment. This membrane is known as plasma membrane.

It allows the cell to take up and retain certain substances while excluding others.

The phospholipid molecules in the plasma membrane are arranged in two parallel layers. Their non polar hydrophobic ends face each other. Plasma membrane also contains several types of lipids like cholesterol. In certain animal cell's cholesterol may constitute up to 50 percent of the lipid molecules in plasma membrane.

2. Nucleus

Nucleus was discovered by Robert Brown in 1831. It is the most important and prominent part of the cell which controls all its activities. It is commonly spherical or oval in shape, but may be lobed or elongated and is surrounded by a membrane called nuclear membrane. It is double membraned structure. Usually cells could have one some have two or more nuclei. The nuclear membrane is not a complete barrier. It is perforated by nuclear pores. Certain substances may pass freely through would pore between the nucleus and the surrounding cellular substances. The nucleus is filled with a protein rich substance called the nucleoplasm or Karyolymph. In the nucleoplasm are numerous fine strands in the form of net work called chromatin net work or nuclear reticulum, which is composed of nucleic acid, Deoxyribo-nucleic acid (DNA) and protein.


Cytoplasmic Organelles and Membrane System

Their are two types of Endoplasmic reticulum: a) A granulated or Smooth endoplasmic reticulum (SER) and b)Rough or granulated endoplasmic reticulum.

A granulated or Smooth Endoplasmic reticulum (SER)

1▪They are not associated with ribosomes.

2▪ It is found in steroid producing cells like adipose cells(fat cells), interstitial cells, glycogen storing cells(liver) and the muscle cells.

Rough or Granular Endoplasmic Reticulum(RER)

They are heavily coated with ribosomes on its outer surface to towards cytoplasmic surface.

2▪They occur mostly in protein synthesising cells. Such as those of the mammalian salivary glands and pancreas.

(Smooth endoplasmic reticulum in the skin converts cholesterol into the lipid compounds called vitamin D whenever sunlight strikes the skin:this vitamin helps to maintain strong, healthy bones.

Functions of Endoplasmic Reticulum

The harmful de endoplasmic reticulum has many important function. Primarily, it serves as a supporting platform for the ribosomes. The endoplasmic reticulum, forms a structural framework of the cell with increase surface for various metabolic reactions, and they themselves take an active part by means of attached enzymes. It also helps in detoxification of drugs, storage and release of Ca+2 ions and manufacture lipids.


They are center of aerobic respiration. All mitochondrion are approximately about 0.2 to 1.0 nm in diameter and about 10 micrometers long. There are two ruin membranes which form the boundary of the mitochondrion. Both membranes are formed of lipids and proteins. The inner membrane forms irregular, incomplete partitions called Cristae. On the cristae are located enzymes and co enzymes by means of which carbohydrate (starch) fatty acids (lipids) and amino acids (proteins) are metabolized to CO 2 and H 2 O.

Energy in the form of ATP is released in this process which is stored within mitochondria.Adenosine triphosphate (ATP) is energy rich compound and it provides energy to the cell of organs for various activities. Hence, mitochondria are known as “Power House” where energy is stored and released wherever and whenever required by a living body.

Golgi Apparatus(Dictyosome)

After the name of its discovered Golgi, it was named as Golgi body. The Golgi complex of the plants and lower invertebrates cells is usually referred as dictyosome. The Golgi bodies of plant cells and Golgi complex of animal cells basically have same morphology. Each of them is disc shaped and consists of central, flattened plate likes compartments called cisternae.

Usually in animal cells single Golgi apparatus are found in each cell, in plant cell, they may be more.

They perform the function of collection, packaging and distribution. In addition, It's finishing work, the Golgi apparatus manufacture certain macromolecules by itself.


These are spherical bodies a few micrometers in diameter surrounded by a single membrane originated by Golgi apparatus and containing digestive enzymes. They occur only in cytoplasm of animal cells, and function in the digestion of material taken into the cell by phagocytosis, as bacteria are ingested by white blood corpuscles. Normally they function as destroyers of foreign particles and worn out cellular components. When the membrane of lysosomes is ruptured, the cell undergoes chemical breakdown, or lysis. Since release of enzymes, cause a cell to destroy itself by digesting its own proteins, lysosomes have been referred as “suicide sacs” and this process is called autophagy.


They are special protoplasmic double membrane bounds organelles which function as chemical synthesizers and storage bodies. Plastid occur in the greatest number in cells of plants and in the primitive single celled organisms, the protoctists. There are three types of plastids e.g Chloroplast, Chromoplast and Leucoplast


The most common type of plastid is chloroplast containing chlorophyll which gives plant their green color and is of great importance in the manufacture of food by the process of photosynthesis.

Chloroplast contains a substantial amount of DNA and are capable of programming synthesis of some others new structural components. It has its own DNA and RNA.


They have pigments like xanthophyll, and carotene. The chromoplasts are responsible for the various color combinations found in flowers, fruits and other colored parts except green.


These are colorless plastids which develop in the absence of sunlight and are thus commonly found in all underground part of the plants. They store the food material as carbohydrate, lipids and proteins.

Proplastids :

They are immature,colorless plastids occurring in cells of meristematic tissues. They consist of double membrane enclosing granular stroma. They multiply by division. In mature cells proplastids develop into chloroplast or chromoplast or leucoplast.

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