Notes Of Fundamental Unit Of Life : CELL

fundamental unit of cell

Study Key Points :

Discovery of Cell, Types of Cell & Organism, Unicellular & Multicellular, Prokaryotic & Eukaryotic, Plant Cell & Animal Cell, Cell Shape, Cell Size, Cell Membrane, Cell wall, Diffusion & Osmosis, Nucleus, Cytoplasm, Endoplasmic Reticulum, Golgi Apparatus, Mitochondria, Ribosomes, Plastid, Vacuoles, Lysosomes.

plant and animal cell

★ All living forms are composed of microscopic units called as ‘Cells’.

★ A cell is the basic structural and functional unit of all life forms.

★ Study of structure and composition of cell is called as ‘Cytology’.

★ Cell was first observed by Robert Hooke in a dead cork slice in the year 1665.

★ First living cell was discovered by A. V. Leeuwenhoek.

★ Its consistency differs under different condition. It exists in sol-gel states.

★ Protoplasm is an aggregate of various chemicals such as water, ions, salts and other organicmolecules like proteins, carbohydrates,fats, nucleic acids, vitamins etc.

Cell Theory

Two biologists, Schleiden and Schwann gave the Cell theory which states that :

  • All plants and animals are composed of cells.
  • Cell is the basic unit of life.
  • All cells arise from pre-existing cells.

★ Viruses are the exceptions of cell theory.

Types of Cell & Organism :

Types of Cell Organisms

On the Basis of Number of Cells :

CharacteristicsUnicellular organismMulticellular organism
Cell numberSingle cellLarge number of cells
FunctionAll functions are performed by single cellDifferent cells perform different specific functions.
Division of labourNot performedCells specified to perform different functions.
ReproductionInvolves the same single cellSpecialised cells, germ cells take part in reproduction.
Life spanShortLong

On the Basis of Type of Organization

Prokaryotic CellsEukaryotic Cells
Very minute in size.Fairly large in size.
Nuclear region (nucleoid) not surrounded by a nuclear membrane.Nuclear material surrounded by a nuclear membrane.
Single chromosome present.More than one chromosome present.
Nucleolus present.Nucleolus absent.
absent.Present.
Membrane bound cell organelles areMembrane bound cell organelles are

Cell division by fission or budding Cell division by mitosis or meiosis. (no mitosis).

Cell division by fission or budding Cell division by mitosis or meiosis. no mitosis

Cell Shape : Cells are of variable shapes and sizes. Their shape is according to the function. Generally cells are spherical but they may be elongated (nerve cell), branched (pigmented), discoidal (RBC),spindle-shaped(muscle cell) etc

Cell Shape

Different kinds of cell found in the human body

Cell Size : Size of cell is variable depending upon the type of organism. Some are microscopic while some are visible with naked eyes. Their size may vary from 0.2 mm to 18 cm.

  • Size of a typical cell in a multicellular organism ranges from 20-30 mm.
  • The largest cell is ostrich egg (15 cm in diameter with shell & 8 cm in diameter without cell).
  • The longest cell is nerve cell (upto 1 m or more).
  • Smallest cells so far known are PPLOs e.g., mycoplasma (0.1 mm in diameter).
  • Human egg is 0.1 mm in diameter.

Components of Cell

There is an occurrence of division of labour within a cell as they all got certain specific components called ‘Cell organelles’. Each of them perform a specific function. The three basic components of all the cells are :

  1. Plasma membrane
  2. Nucleus
  3. Cytoplasm
Plasma membrane edited

Cell Membrane :

(a) Cell membrane is also called as plasma membrane or plasma lemma.

(b) It is the limiting boundary of each cell which separates the cytoplasm from its surroundings.

(c) It is found in both plant as well as animal cells.

(d) It is the outermost covering of a cell in case of animals and lies below the cell wall in case of plants.

(e) It is made up of proteins and lipids where proteins are sandwiched between bilayer of lipids.

(f) Plasma membrane is selectively permeable in nature. It allows or permits the entry and exit of some materials in and out of the cell.

(g) Singer and Nicholson gave the fluid mosaic model of plasma membrane. According to them, it consists of a protein layer sandwiched between two layers of lipids. It is in quasifluid state. It is 75A thick.

(h) It is flexible and can be folded, broken and reunited.

Functions of Plasma Membrane :

(a) It regulates the movement of molecules inside and outside the cell.

(b) It helps in maintaining the distinct composition of the cell.

(ii) Transportation of molecules across the Plasma Membrane :

This can be done by following ways :

Transportation of molecules across the Plasma Membrane

Diffusion : Movement of solutes or ions from higher concentration to lower concentration is called as diffusion. It does not require energy therefore, it is called as passive transport.

Osmosis : The movement of solvent or water from higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called as osmosis Or the movement of solventor water from lower concentration to higher concentration of solution through a semipermeable membrane is called as osmosis.

Osmosis can also be called as ‘Diffusion of solvents’.

  • Endomosis : Movement of solventinto the cell is called as Endomosis.
  • Exosmosis : Movement of solventoutsidethe cell is called as Exomosis.

Types of Solutions on the Basis of Concentration

Screenshot 249

(a) Isotonic Solution : When the concentration of the solution outside the cell is equal to the concentration of cytoplasm of the cell, it is called as isotonic solution.

(b) Hypertonic Solution : When the concentration of the solution outside the cell is more than the inside the cell. Due to this, cell looses water and becomes plasmolysed.

(c) Hypotonic Solution : When the concentration of the solution outside the cell is lesser than that of cytoplasm of cell. Due to this, cell swells up and bursts.

Cell Wall

  • It is the outermost covering of the plant cells.
  • It is absent in animal cells.
  • Cell wall is rigid, strong, thick, porous and non-living structure. It is made up of cellulose and hemicelluloses.

Cell walls of two adjacent cells are joined by a layer called middle lamellae.

Cell Wall

Functions of Cell Wall :

(a) It provides definite shape to the cell.

(b) It provides strength to the cell.

(c) It is permeable and allows entry of molecules of different sizes.

(d) It has the characteristics of repair and regeneration.

Nucleus

Nucleus

Nucleus is the most important cell organelle which directs and controls all its cellular activities.

  • It is called as ‘Headquarter of the cell’.
  • It was discovered by Robert Brown in 1831. In Eukaryotes, a well-defined nucleus is present while in Prokaryotes, a well-defined nucleus is absent.
  • Prokaryotes containa primitive nucleus.
  • It has double layered covering called as nuclear membrane.
  • Nuclear membrane has pores which regulate the movement of materials in & out of the cell.
  • Besides nuclearmembrane, nucleus also contains nucleolus and chromatin material and the substance filled inside the nucleus is nucleolus.
  • Chromosomes or chromatin material consists of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.

Functions of Nucleus :

(a) It controls all the metabolic activities of the cell and regulates the cell cycle.

(b) It helps in transmission of hereditary characters from parents to off springs.

Cytoplasm

Cytoplasm 1 edited
  • Cytoplasm was discovered by Kolliker in 1862.
  • It is the site of both biosynthetic and catabolic pathways.
  • It can be divided into two parts :
    1. Cytosol : Aqueous solublepart contains variousfibrous proteins forming cytoskeleton.
    2. Cell organelles : Living part of the cells having definite shape, structure and function bounded by plasma membrane.

Endoplasmic Reticulum

  • It is the network of membranes presentin the cytoplasm.
  • It was discovered by Porter, Claude and Fullam.
  • These are presentin all cells except prokaryotes and mammalian
Endoplasmic Reticulum
Smooth ERRough ER
Made of tubules mainly.Made of clstemae and vesicles.
Helps in steroid, lipids and polysaccharide synthesis.Helps in Proteins synthesis.
Ribosomes are absent.Contains ribosomes on its interface.

Functions of ER :

(a) It is the only organelle which can move within a cell so it serves   as a channel for the transport of materials between various regions of cytoplasm and between cytoplasm and nucleus.

(b) It also functions as a cytoplasmic  framework to provide space for    some of the biochemical activities.It forms endoskeleton of cell.

(c) It helps in synthesis of fats, steroids, cholesterol etc.

(d) SER plays a crucial role in detoxification of  drugs and poisonous byproducts.

Golgi Apparatus

Golgi apparatus  consists of a system of membrane bounded vesicles arranged parallel to each other in stacks called Cisternae along with  some large and spherical vacuoles. It was discovered by Camilo Golgi. In plants Golgi membrane is bounded. It is single membrane bounded.

 It is absent in prokaryotes, mammalian RBC’s &  sieve cells.

Golgi Apparatus

Functions of Golgi apparatus :

(a) It helps in formation of lipids.

(b) It helps in formation of middle lamellae.

(c) It is secretary in nature.

(d) It helps in melanin synthesis.

(e) Lipids and proteins synthesized in endoplasmic reticulum are packed at Golgi complex. They provide the site for assembly of new membrane material.

Mitochondria

Mitochondria

It is a rod shaped structure found in cytoplasm of all eukaryotic cells except mammalian RBC’s.

  • These are also absent in prokaryotes. It was first seen by Kolliker in insect cells.
  • Maximum mitochondria are found in metabolically active cells.
  • It is also called as ‘PowerHouse of the Cell’ or the ‘Storage Battery’.
  • It is double membranous structure where outer membrane has specific proteins while inner membrane is folded inside to form chambers called Cristae.

Functions of Mitochondria :

(a) Its main function is to produce and store the energy in the form of ATP.

(b) It is the site of Kreb cycle of respiration.

Ribosomes

Ribosomes

Ribosomes are the sites of protein synthesis. All structural and functional proteins (enzymes) coded by the nuclear DNA are synthesized upon cytoplasmic ribosomes. The DNA codes are transcripted into messenger RNA (mRNA) molecules in the chromosomes of the nucleus.

Functions of Ribosomes :

(a) Ribosomes are the main site of protein synthesis. Synthesized proteins is transported by endoplasmic reticulum.

Plastid

Plastid
  • It is double membranous discoidal structure, found only in plant cells.
  • Besides being discoidal of rhombic in plant cells, they occur in variable shapes like in algae. They can be ‘U’-shaped, spiral, coiled, ribbon- shaped etc.

Depending upon the type of pigment present in them, they are of following three types :

(i) Leucoplast – White, found in underground parts

(ii) Chloroplast – Red, brown

(iii) Chloroplast – Green in colour, found in aerial parts of plants

These are found only in plant cell. It helps in the process of photosynthesis so it is called the ‘Kitchen of Plants’.

Chloroplast have following two parts :

(i) Grana : It constitutes the lamellar system. These are found layeredon top of each other. These stacks are called Grana. Each granum of the chloroplast is formed by superimposed closed compartments called Thylakoids.

Function : They are the sites of light reaction of photosynthesis as they containphotosyntheticpigment chlorophyll. In each thylakoid, Quantasomes are present which are called as Photosynthetic units.

(ii) Stroma : It is a granular transparent substance also called as matrix.Grana are embedded in it. Besides Grana they also contain lipid droplets, starch grains, ribosomes etc.

Function : This is the site of dark reaction of photosynthesis. Also helps in protein synthesis due to presence of ribosomes.

Vacuoles

Vacuoles
  • These are membrane bounded regions in the cytoplasm containing water and other substances.
  • They are bounded by a single membrane called Tonoplast.
  • In animal cells vacuoles are smaller in size and numerous while in plant cells a single large vacuole is found which occupies about 90% of the volume of cell.

Functions : It helps in maintaining osmotic pressure in a cell & stores toxic metabolic products of plant cell.

Lysosome

Lysosome
  • They are tiny sac-like granules containing enzymes of intracellular digestion.
  • They are bounded by a single membrane.
  • They occur in animal cells and a few plant cells.
  • They do not have a definite shape or size.

Functions :

(a) Their main function is phagy = digestion.

(b) They are kind of waste disposal system.

(c) They help in digesting foreign materials & cells.

Suicidal Bag : During disturbances in cellular metabolism i.e., in case of cell damage, lysosomes burst and their enzymes are released into the cytoplasm and they digest their own cell. So they are also called ‘Suicidal Bag’.

Plant CellAnimal Cell
Contain chloroplasts for photosynthesis.No cell wall.
Have a cell wall to maintain structure and rigidity.No chloroplasts.
Usually do not contain lysosomesContain cilia and/or flagella   Peroxisomes.
Cells are square and rigid or geometric Cells are fluid and flexible, shaped. Many shapes.
Limited movement. Cells can move around.
Have one large central vacuole.Has small vacuoles.