Structure of the Cell

The components of cells are molecules,  non _ living structures formed by the union of atoms. Small molecules serve as building  blocks for large molecules. Proteins, nucleic acids, carbohydrates and lipids, which include fats and oil, are the four major molecules that underline cell structure  and also participate  in cell functions. The deoxyribonucleic acid ( DAN) contains the hereditary  information  of cells, and ribonucleic acid ( RNA) works with the DNA to build the thousands of proteins  which the cell needs.

Because all organisms are made of cell,  the cell is as fundamental to an understanding of Zoology as the atom is to an understanding of chemistry. 

□ Cell are the Functional units of life 

In which all the chemical reactions necessary for the maintenance and reproduction of life take place. 

● Anton von Leeuwenhoak first saw and described a live cell.Robert Brown later discovered the nucleus.

□ A division of labor among cells allows specialization into higher levels of organization ( tissues, organs, and organ systems)

□ Cells are characteristically microscopic in size.

Although there are exceptions, a typical  Eukaryotic cell is 10 to 100 micrometer ( ųm) in diameter, while most   Prokaryotic cells only 1 to 10 ųm in diameter. 

□ Cell differ greatly in size, shape, and activities. 

■ For example, Mycoplasmas, the smallest cells; are 0.3 ųm in length while bacteria could be 3 to 5 ųm. 

● The largest isolated single cell is the egg of an ostrich. 

□ The shape and size of cell are related to the special function they perform. 

 ● Small cells like Paramecium Amocba and Euglena frequently change their shape.


The cell theory was proposed to explain the observation that all organisms are composed of cell. Cell theory was given by schleiden and Schwann.

□ In its modern form, the cell theory includes the following three Principles 

● All organisms are Composed of one or more cells and the life processes of metabolism and heredity occur within these cells.

● Cells are the Smallest living things the basic units of organization of all organisms.
● Cells arise only by division of a previously existing cell.


The two types of cells are: prokaryotic and Eukaryotic cell

Based on complexity, cells can be classified as Eukaryotic cells and prokaryotic cells.

□ Eukaryotic cells contains well _ defined membrane bound nuclei, whereas prokaryotic cells do not have well_ defined membrane _ bound nuclei. 


●The Prokaryotic lack nuclei and other membrane bound organelles. 

● They consist of cytoplasm surrounded by a plasma membrane and are encoded within a rigid cell wall. 

● Prokaryotic are very important in the ecology of living organisms,.Some harvest light by Photosynthesis,  others break down dead organisms and recycle their components. 

● These simple cells are classified into two domains:
Archara and Eubacteria
 ● The three _ domain system is a Biological classification introduced by Carl Woese in 1990

● It divides cellular life forms into archaeabacteria and eukaryote domains

 ■ The Archaea have unique characteristics and share features with Eubacteria and the third domain Eukarya.


Eukaryotic cells are larger and more complex than prokaryotic cells. 

□ All Eukaryotic ("true nucleus ") have cells with a membrane _ bound nucleus containing DNA.

□ In addition,.Eukaryotic cells contain many other structures called organelles ("little organs") that performs specific functions.

 □ Eukaryotic cells also have a network of specialized structure called micro filaments and microtubules organized into the cytoskeleton which gives shape to the cell and allows intracellular movement. 

Generalized Structure of cell:

Every  cell is surrounded  by a membrane or a living covering through which the cell takes in what it needs from the atmosphere and discharge  what is superfluous, without  the membrane is the protoplasm. 

Parts of the Protoplasm : The nucleus  controls and directs the activities of all the other parts of the cell. The remainder  is  known  as Cytoplasm where  many vital activities of the cell take place.The diagram shows the following  structure  under an ordinary  microscope. 


This general plan of cellular organization varies between different organisms,but despite these modifications,all cells resemble one another in certain fundamental ways. 
But four major features all cells have in common. 
1. A plasma membrane
3.a nucleotide or nucleus where genetic material is located, and
 4.ribosomes to synthesize proteins 

1. Plasma membrane 

The plasma membrane encloses a cell and separates its contents from its surroundings.

● The plasma membrane is flexible and it is made up of phospholipids and proteins

● It is selectively permeable and regulates the transport of molecules in and out of the cell.

□ Chalesterol is present in the Plasma membrane and organelle membranes of eukaryotic cells.

1) Cholesterol molecules are embedded in the interior of the membrane and help make the membrane less permeable to water Soluble substances 

2) Cholesterol molecules help stabilize the membrane.

Functions of cell Membranes 

1) Separate various organelles within the cell.

2) Separate the inside of the cell from the outside. 

3) Selective permeability: The ability of the plasma membrane to let some substances in and keep others out.


1) A Semifluid matrix called the cytoplasm fills the interior of the cell.

2) The cytoplasm contains the various organelles and micro and macro molecules which help in the functioning of the cell.

3) In Eukaryotic cells, cytoplasm refers only to the region between the nucleus and the plasma membranes. 

4) The part of the Cytoplasm that contains organic molecules and ions in solution is called cytosol.

3.A nucleoid and nucleus 

Every cell contains DNA, the hereditary molecule. 

1) In Prokaryotes, the simplest organisms, most of the genetic material lies in a single circular molecule of DNA.

● It typically resides near the center of the cell in an area called the nucleoid. 

2) The DNA of Eukaryotes is contained in the nucleus, which is surrounded by a double _ membrane structure called the nuclear envelope.

 3) Within the nucleus, the DNA is organized into discrete Units called Chromosomes,  structures that carry the genetic information. 

Major Components of the Nucleus: 

1) The Nucleoplasm is the control area in the cell that contains the genetic material.

2) The nucleolus is the distinct structure present in the nucleus of Eukaryotic cells.

(i) Primarily, it participates in assembling the ribosomes alteration of transfer RNA and sensing cellular stress.

(ii) The nucleolus is composed of RNA and proteins,  which form around specific chromosomal regions.

3) Chromatin : Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus of Eukaryotic cells.


The ribosome is a complex molecule mode of ribosomal RNA molecules and proteins that form a factory for protein synthesis in cells.

1) The Eukaryotic ribosomes are the 80s while the prokaryotic ribosomes are 70s .

2) Here 'S'( Svedberg's unit) stands for the sedimentation coefficient; it is indirectly a measure of density and size.

3) Ribosomes are not membrane bounded and thus are not considered organelles.

4) cells with rates of protein Synthesis have particularly large numbers of ribosomes. 


Organelles are specialized Structures that perform various jobs inside cells.The term literally means " little organs" .

(1) Organelles serve specific functions to keep a cell alive.For example, Mitochondria produce ATP which acts as the energy currency of cells.


Each Mitochondria is a double membrane _ bound structure with the outer membrane and the inner membrane. 

(1) Mitochondria are the sites of aerobic respiration. They produce Cellular energy in the form of ATP, hence they are called 'power houses' of the cell.

(2) They also possess a single circular DNA molecule,a few RNA molecules,ribosomes (70s) and the components required for the synthesis of proteins. 


(1) The Peroxisome is a specialized metabolic Compartment bounded by a single membrane.

(2) Peroxisomes contain enzymes that remove hydrogen atom from various substrates and transfer them to oxygen (o2) , producing hydrogen peroxide as a by_ product.

(3) Peroxisomes in the liver detoxify alcohol and other harmful compounds. 

(4) Specialized Peroxisomes called glyoxysome are found in the fat_ storing tissues of plant seeds.

● These organelles contain enzymes that initiate the Conversion of fatty acids to sugar,which the emerging seedlings uses as a source of energy and carbon unit it can produce it's own sugar by photosynthesis. 

Endomembrane System :

Many of the Organelles are considered together as an endomembrane system because their functions are coordinated.

 ■ The endomembrane system includes endoplasmic reticulum ( ER), golgi complex lysosomes and vacuoles:

(1) Endoplasmic reticulum 

Endoplasmic reticulum is a network of membranes inside a cell through which proteins and other molecules move.

Types of ER

(1) Rough ER: It has ribosomes attached to its surface and it is an active site for protein Synthesis.

 (2) Smooth ER: In the absence of ribosomes they appear smooth and are called smoth endoplasmic reticulum (SER).

(3) The smooth ER functions in diverse metabolic processes like; Synthesis of lipids metabolism of carbohydrates, detoxification of drugs and poisons and storage of calciumions.

■ The smooth endoplasmic reticulum is the major site for Synthesis of lipids.In animals cells lipids _ like steroidal hormones are synthesized in SER.

■ Few enzymes of the smooth ER help detoxify drugs and poisons, especially in liver cells.

2. Golgi Apparatus:

i) After leaving the ER, many transport vesicles travel to the Golgi apparatus.

ii) A Golgi body, also known as a Golgi apparatus, is a cell organelle that helps process and package proteins and lipid molecules. 

iii) Here, products of the ER, such as proteins, are modified and stored and then sent to other destinations.

iv) The Golgi  apparatus also manufactures some macromolecules, Many polysaccharides  secreted by cells are Golgi  products.

v) For example, pectin and certain other non cellulose polysaccharides are made in the Golgi of plant cells and then incorporated along with cellulose into their cell walls.

3)  Lysosomes 

A lysosome is a membrane _ bound cell organelle that contains various hydrolytic enzymes. 

1) These enzymes help in Destruction and breakdown of various worn_ out cell parts of the cell. Thus, they help in detoxification of the cell.

2) They also destroy pathogens, such as bacteria and viruses and play a role in immunity.

3) They are also called as Suicidal bag of the cell because they help in self _ destruction of the cells.

4)  Vacuoles 

The vacuole is the membrane _ bound space found in the Cytoplasm. 

i) It contains water, sap, excretory products and other materials not useful for the cell.

ii) It vacuole is bound by a single membrane called tonoplast.

iii) A big Central vacuole that forms instore plants cells often comes from the agglomerates of smaller vacuoles.

iv) In plant cells the vacuoles can occupy up to 90% of the volume of the cell.

v) The solution inside, the central vacuole called cell sap, is the plant cell's main repository of inorganic ions, including potassium and chloride. 

Based on components,Eukaryotes are divided into Animal cells and plant cells.

i) plant cells have a cell.wall outside the cell membrane. 

ii) plant cells often have a large membrane _ bound sac called a central vacuole which stores proteins pigments, and wastes materials. 


1) The cell wall is an extracellular structure of plant cells

■ This is one of the features that distinguishes plants cells from animals cells. 

■ The wall protects the plant cell, maintains its shape, and prevents excessive uptake of water.

■ At the level of the whole plant the strong wall of specialized cells hold the plant up against the force of gravity. 

■ Prokaryotes some protists, and fungi also have cell walls.

Q.1 Write down the differences between plants cell and Animal cell

Plant cell: 

1) Usually they are larger than animals cells.

2). Cell wall present in addition to plasma membrane and consists of middle lamellae, primary and secondary walls.

3) plasmodesmata present 

4) Chloroplasts present 

5) vacuale large and permanent 

6) Tonoplast present around vacuole 

7) Centrioles absent except motile cells of lower plants.

8) Necleus present along the periophery of lower plants 

9) Nucleus present along the periphery of the cell.

10) Lysosomes are rare

11) storage material is starch grains. 

Animals cells:

1) Usually smaller then plant cells.

2) cell wall absent.

3) plasmodesmata absent. 

4) Chloroplast absent. 

5) Vacuole small and temporary. 

6) Tonoplast  absent. 

7) Centrioles present. 

8). Centrioles present. 
9) Nucleus at the centre of the cell.

10)  Lysosomes present. 

11) Storage material is a glycogen granules.

Cell organelles specific only to the plant cells. 


● Plastids are found in all plant cells and in Euglenoids.

● They bear some specific pigments thus imparting specific colors to the plants.

● Based on the types of Pigments, plastics can be classified into chloroplasts, chloroplasts and leucoplasts.

■ Amylopast , is a colorless organelles that stores starch ( amylose) .

■ Chromoplast, which has pigments that give  fruits and flowers their organelles and yellow hues.

2. Chloroplasts:

1) Chloroplasts contain the green pigment Chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar. 

2)  Like Mitochondria, the chloroplasts are also double membrane _ bound. 

3) It also contains small, double _ stranded circular, DNA molecules and ribosomes. 


Popular posts from this blog



Nucleic Acids