Structure of Cell

• Besides the difference in shape, size and activities, the Structure of Cell have three basic components which are

1. 1. The Cell Membrane or Plasma Membrane and Cell Wall (in Plant Cells only)
   2. The Nucleus
   3. The Cytoplasm with Cytosol and Cell Organelles

• The Outer membrane of the Cell is called Plasma Membrane.
• Inside the Plasma Membrane, lies the Cytoplasm in which various cell Organelles and Nucleus is suspended.
• All the Functions of the Cell is performed by these cell Organelles which are the part of Structure of Cell.

1. Cell Membrane

(also known as Plasma Membrane)

Main Characteristic

• • Very thin, Delicate, elastic, flexible, living membrane
  • Made up of 75% phospholipids. Also contains Proteins, cholesterol and Polysaccharides.
  • Outermost in animal cells, Lies next to cell wall in plant cells and determines the Structure of Cell.
  • Selectively-permeable.
  • In 1972, Singer and Nicolson suggested Fluid Mosaic Model which explain the ultrastructure of Plasma membrane.

Chief Functions

• • Separate the contents of cell from its surroundings
  • Regulates the entry of certain solutes and ions
  • Maintains shape of the cell( in animal cells only)
  • Performs physical activities such as diffusion, Osmosis, Active Transport etc.

Structure of Plasma Membrane

• • Composed of Phospholipids (75%), Also contains Proteins, Cholesterol and Polysaccharides.
  • Fluid Mosaic Model – proposed by Singer and Nicolson in 1972.
  • According to this model, membrane contain bi-molecular lipid layers, surface of which is interrupted by proteins.
  • Two types of proteins are present in lipid layers namely – Intrinsic Proteins and Extrinsic Proteins.
  • Intrinsic Proteins – completely covered by Lipid bi-layer
  • Extrinsic Proteins – Occurs on outer or inner surface of the lipid membrane
  • Proteins serves as –
    • • Enzymes – catalyse chemical reactions within the membrane
      • Transport proteins or Permeases – for movement of water soluble ions
      • Pumps – active transport
      • Receptor Proteins – for endocytosis

1a. Cell Wall (Plant cells only)

Main Characteristic

• • Outermost in plant cells which determine the rigid Structure of Cell in plants.
  • Non living rigid layer surrounding the plasma membrane
  • Freely permeable
  • Mainly composed of fibrous polysaccharides called cellulose

Chief Functions

• • Gives rigidity and shape to the plant cells
  • Allows substances In solution to enter and leave the cell without any hindrance
  • Provides protection
  • Provides Mechanical Strength

Difference Between Cell Wall & Plasma Membrane

Cell Wall	Plasma Membrane
Occurs in Plant cells	Found both in Plant and animal cells.
Lies outside of Cells.	Lies outside of animal cells and inner to cell wall in plant cell.
Non-living and thick in plants.	Living and thin.
Rigid	Flexible
Permeable	Selectively-Permeable
Formed of Cellulose, Pectin and Hemicellulose.	Formed of Lipids and Proteins
Major Function – Protection and Mechanical Strength	Major Functions – to hold cellular content and control the passage of materials in and out of cell.

2. Nucleus

Nature and Occurrence

• • Nucleus is the main component of Structure of Cell.
  • Large centrally placed spherical cellular component.
  • It contains the following components –
    • Nuclear Membrane also known as Nuclear Envelope
    • Nucleoplasm
    • Nucleopores or Nuclear pores
    • Nucleolus
    • Genetic Material or Chromatin Fibres

Chief Characteristics

• • Bounded by two nuclear membrane, forming Nuclear Envelope.
  • Nuclear Envelope separates the Nucleus from Cytoplasm.
  • Inside the Nuclear envelope a liquid is filled which is known as Nucleoplasm.
  • Nuclear envelope contains many pores called Nuclear Pores which allows the transfer of materials between nucleoplasm and cytoplasm.
  • Nucleolus are non-membranous body rich in Protein and RNA and acts as the site for Ribosome formation.
  • Chromatin Material are thin, thread like structure which are composed of DNA and Proteins. Chromatin fibres forms the Chromosomes during the time of cell Division.

Functions

• • Controls all metabolic Activities of the cell.
  • Regulates Cell Cycle.
  • Concerned with the transmission of Hereditary traits from Parents to Offsprings.

3. Cytoplasm

Main Characteristic

• • Occurrence – between plasma membrane and nuclear envelope
  • All the organelles together with aqueous ground substance, cytosol inside the plasma membrane excluding nucleus is called Cytoplasm.
  • Contains a mixture of water and soluble inorganic and organic compounds, and various organelles.
  • Cytosol is about 90% water and forms solution which contains all biochemical of life.

Parts of Cytoplasm

• • Cytosol

• • • Cytosol is about 90% water and forms solution which contains all biochemical of life.
    • Soluble part of cytoplasm

Functions

• • • acts as a vital chemicals such as amino acids, vitamins, ions etc.
    • Site of certain metabolic pathways eg. Glycolysis. Synthesis of various compounds etc.

• • Cell Organelles

    • Cell performs its various functions with the help of various organelles such as – mitochondria, ribosomes, endoplasmic reticulum, plastids etc.

Functions 

• • • Synthesis of various substances
    • Secretion of cell products
    • Digestion
    • Generation of energy etc.

Chief Functions

• • Different organelles contained in it performs different functions.
  • All metabolic activities occur in it.
  • Medium for initial steps of respiration (production of pyruvic acid) (anaerobic respiration).

Cell Organelles

Occurrence and Nature

• • Cell Organelles are the Various organs of the cell which performs various functions of the cell.
  • These Cell Organelles are responsible for the synthesis, secretion, digestion, respiration and various other processes of the cell.

Types of Cell Organelles

• • • Endoplasmic Reticulum
    • Ribosomes
    • Golgi Apparatus
    • Lysosomes
    • Mitochondria
    • Plastids
    • Vacuoles
    • Peroxisomes
    • Centrosomes etc.

A. Endoplasmic reticulum (ER)

Main Characteristic

• • • Irregular membranous network of double membraned tubules.
    • continuous with plasma membrane on the outside & nuclear membrane on the inside.
    • It is of two types –
    • Smooth ER(without ribosomes)
    • Rough ER (with ribosomes)

Difference Between Smooth Endoplasmic Reticulum and Rough Endoplasmic Reticulum

Rough Endoplasmic Reticulum	Smooth Endoplasmic Reticulum
Contains flattened sacs called Cisternae.	Mainly formed of  Vesicles and tubules.
Ribosomes attached to outer surfaces.	Do not contains Ribosomes.
Specialised in Protein Synthesis.	Specialised in Lipid and Steroids synthesis.
Found in abundance in Exocrine pancreatic cells and plasma cells.	Abundant in liver and testicular cells.

Chief Functions

• • • Supportive skeletal framework for cell.
    • Synthesis and transport of proteins & fats.

Some Special Functions –

• • • Helps in detoxification – metabolizes various toxins which enters through food, air or water.
    • SER helps in synthesis of glycolipids, phospholipids and cholesterol.
    • RER helps in synthesis of various secretory proteins such as mucus, Digestive enzymes and hormones.
    • SER helps in synthesis of Steroid hormones such as estrogens, testosterone and cortisol.
    • Helps in formation of Plasma membrane.

B. Ribosomes

Main Characteristic

• • • Small, dense, spherical granules.
    • Either scattered in cytoplasm or attached to the outside of Endoplasmic Reticulum.
    • Mainly composed of RNA and Proteins.
    • Absent in Mammalian RBC.

Chief Functions

• • • Protein Synthesis

C. Golgi Apparatus (in animal cells)

(called Dictyosomes in plant cells)

Main Characteristic

• • • Consists of set of membrane bounded Fluid filled vesicles, vacuoles and flattened cisternae.
    • Cisternae are formed at cis face of Golgi.
    • Vesicles are formed at trans face of Golgi.
    • It arises from Smooth ER.
    • Absent in Bacteria, Blue-green algae, mature sperms and RBC of mammals and other animals.

Chief Functions

• • • Acts as a assembly area for storage, processing and packaging of various cellular secretions.
    • Synthesis and secretion of enzymes and hormones etc.
    • Involved in synthesis of Cell wall, Plasma Membrane and Lysosomes.
    • Formation of acrosome of sperm.

D. Lysosomes

Main Characteristic

• • • Membranous sacs budded off from Golgi bodies.
    • Small single membrane vesicles
    • Contains 40 different types of powerful enzymes.

Chief Functions

• • • Intracellular digestion
    • Destroy foreign material or body eg. Bacteria.
    • Helps in removal of poorly working cell organelles so replacement can be done. Hence, called scavengers and cellular housekeepers.
    • When cell is old or injured, these rapidly destroy cell organelles, hence called as “SUICIDE BAGS”.
    • Digest cartilages during formation of Bones.

E. Mitochondria

Main Characteristic

• • • Tiny bodies of various shapes but usually sausage like.
    • Double walled, inner walls thrown into folds(cristae).
    • Cristae are studded by F1 Particles or Oxysomes.
    • Have their own DNA (containing several genes).
    • Also contain their own ribosomes.
    • Filled with a Proteinaceous matrix
    • Absent in Bacteria and RBCs of Mammals.

Chief Functions

• • • Synthesis of respiratory enzymes.
    • Site of Cellular respiration
    • Release of energy from pyruvic acid produced in cytoplasm in the form of ATP.
    • Hence called Power House of the Cell.
    • This ATP is used for synthesis of Chemical Compounds and for mechanical work.

F. Plastids (only in Plant cell)

Main Characteristic

• • • Double Membraned, Proteinaceous matrix, contains own DNA and Ribosomes.
    • Self replicating organelles.

Types of Plastids

• • • Plastids are of following types –
    • Chromoplast – Coloured Plastids
    • Chloroplast – Green coloured Plastids
    • Leucoplast – Colourless Plastids

Chief Functions

• • • Chloroplasts (green) trap solar energy for photosynthesis by the help of pigment called chlorophyll.
    • Leucoplast – Store Starch.
    • Chromoplast – impart color to flowers and fruits, contain the pigments Xanthophyll (yellow coloured pigment) carotene (Orange-red pigment)

G. Chloroplast

Main Characteristic

• • • Disc-like structure called thylakoids contains chlorophyll.
    • Chlorophyll helps in Photosynthesis.
    • Double membrane structure.
    • Has two distinct regions –
      • Grana – stacks of membrane bounded discs called thylakoids which contains chlorophyll.
      • Stroma – homogeneous matrix. Contains photosynthetic enzymes, starch grains, DNA and Ribosomes.

Chief Functions

• • • Trap Solar energy and use it to produce food.
    • Hence, called Kitchen of the Cell.

Difference between Leucoplast and Chromoplast

Leucoplast	Chromoplast
Colourless	Range from brownish to reddish in color
Cylindrical or rounded in shape	Irregular in shape
Found in unexposed cells or where light is not available	Found in both exposed or unexposed cells
Can change to other type of plastids	Do not changes to other types of plastids.
Takes part in storage of Food Eg.  Amyloplast form carbohydrates, elaioplast for lipids	Provides colour to organs to attract pollinators and disseminators such as bees.
Found in Parenchymatous cells, embryonic cells, sex cells and meristematic cells.

Difference between Chloroplast and Chromoplast

Chloroplast	Chromoplast
Green Plastids	Non-green coloured plastids.
Contains chlorophyll and carotenoids	Chlorophyll absent Carotenoids
Lamella are present	Lamella are absent
It is site for Photosynthesis	They add color to the organ are attracting animals for pollination.
Found in Mesophyll cells of leaf and Chlorenchyma cells in stem

 

Difference between Chloroplast and Mitochondria

Mitochondria	Chloroplast
Occurs in cells of aerobic organisms (exception- RBCs of Mammals)	Occurs in the cells of green photosynthetic parts of plants
Colourless	green in colour
Shape is rod like or sausage like.	Generally disc like in outline
Inner membrane thrown into folds called Cristae.	Inner membranes forms flattened sacs called thylakoids or lamellae.
Liberate energy	Traps solar energy and convert it into chemical energy.
Performs oxidation of food	Synthesise food by photosynthesis
Consume Oxygen and liberate Carbon dioxide	Consume Carbon dioxide and liberate oxygen

 

H. Vacuoles

Main Characteristic

• • • Clear spaces filled with water or other substances.
    • Covered by covering known as Tonoplast.
    • Animal cell has small and temporary vacuoles which stores water, glycogen and proteins.
    • Plant cell has large, distinct and permanent vacuoles.
    • Vacuoles are filled with Cell sap, a watery solution rich in sugar, proteins, minerals, amino acids etc. in plants cell.

Chief Functions

• • • Act as a Storage Sacs by Storage of Water and other Substances Such as food, Waste Materials etc.
    • Give Turgidity and rigidity to Plant cells.
    • Contains pigments like anthrocyanins etc.
    • Maintains osmotic pressure of the cell
    • Stores toxic metabolic by-products of plant cell.

I. Centrosomes

Main Characteristic

• • • Found only in Animal Cells.
    • Not bounded by any membrane.
    • Region surrounding the centrioles, located near nucleus.
    • Consists of one or two pair of centrioles.
    • Centrioles are hollow and cylindrical structure made up of Microtubules.
    • Plant cells have Polar Caps in place of Centrioles.

Chief Functions

• • • Initiates and regulates cell division
    • Forms Spindle Fibers with help of asters.

J. Peroxisomes

Main Characteristic

• • • Small spherical organelles containing powerful oxidative enzymes.
    • Bounded by single membrane.
    • Found in Kidney and Liver Cell.
    • Contains crystallized proteins called Catalase Enzyme.

Chief Functions

• • • Specialised for carrying oxidative reactions such as detoxification.

K. Flagellum and Cilium

Main Characteristic

• • • It is a whip like structure present outside the cell.
    • It is a tiny projections which arises from the cytoplasm and extends to outside by a pore in cell membrane or cell wall.

Chief Functions

• • • Helps in the movement of the cell.
    • Eg. Sperm

 

So, this was all about the Structure of Cell – Basic Components and Cell Organelles.

In the Next Post (Click Here), we will discuss about the Cell Cycle and Cell Division : Types of Division (Interphase).