Cell: The Fundamental Unit of Life

1. CELL THE FUNDAMENTAL UNIT OF LIFE

INTRODUCTION

• Both living and non-living things are composed of molecules made from chemical elements such as Carbon, Hydrogen, Oxygen, and Nitrogen.

• The organization of these molecules into cells is one feature that distinguishes living things from all other matter.

• The cell is the smallest unit of matter that can carry on all the processes of life.

• Not all cells are alike.

• Even cells within the same organism show enormous diversity in size, shape, and internal organization.

• Our body contains around 10¹³ to 10¹⁴ cells of around 300 different cell types.

CELL

• The word cell was derived from a Greek word “Cellulae” which means small room.

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

• The term protoplasm was coined by Purkinje in 1839.

• Protoplasm was discovered by “Felix Dujardin” and named as sarcode.

• 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 organic molecules like proteins, carbohydrates, fats, nucleic acids, vitamins etc.

CELL THEORY

Two biologists, “Schleiden and Schwann” gave the “Cell theory” which was later on expanded by “Rudolf Virchow”. Cell theory states that

(i) All plants and animals are composed of cells.

(ii) Cell is the basic unit of life.

(iii) All cells arise from pre-existing cells.

(iv) Viruses are the exceptions of cell theory.

TYPES OF CELLS (CLASSIFICATION)

On the basis of number of cells living organisms can be divided into

On the basis of complexity of organization, cells can be divided as Prokaryotes and Eukaryotes.

1. Prokaryotic cells: these are primitive and incomplete cells. They have less developed nucleus without nuclear membrane & nucleolus. e.g. Bacteria. Check Out- CBSE Class 9 Science Notes

2. Eukaryotic cells: these are well developed cells. They have advanced nucleus with nunclear membrane and nucleolus. e.g. Plants & animals.

Difference between Prokaryotic cells and Eukaryotic cells

SHAPE, SIZE, NUMBER, VOLUME OF CELL

The size, shape, number and volume of the cell is different among unicellular and multicellular organisms.

Cell Shape

(a) Cell shape can be variable (e.g. Amoeba) or fixed (Euglena and Paramoecium).

(b) In multicellular organisms, the shape depends on functional adaptations and partly on the surface tension, viscosity of protoplasm, the mechanical action exerted by adjoining cells and rigidity of cell membrane.

(c) The various shapes are

Amoeba showing its irregular body shape

CELL SIZE

Cell size vary. Some plant and animal cells are visible to the naked eyes but mostly seen with microscope since they are only few micrometers in diameter. The cells can be as small as 0.2 to 5 μm e.g. bacteria.

Different cells showing diversity in size and shape

CELL VOLUME

The volume of a cell

(a) Is fairly constant for a particular cell type.

(b) Is independent of the size of an organism.

Difference between Animal and Plant Cell:

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.

There is division of labour present in multicellular organisms.

• This means that different types of cells present in the body of the organism carry out different functions.

For Example: lungs perform the function of exchange of gases, stomach is for digestion of food.

• Division of labour is also seen within a cell.

• Specific structures that is cell organelles present inside the cell perform specific functions.

For Example mitochondria synthesize ATP or ribosome carry out protein synthesis.

• Interestingly, different types of cells have same organelles though they may differ in their functions.

• The three basic components of all the cells are

(i) Plasma Membrane (ii) Nucleus (iii) Cytoplasm

DETAILED STRUCTURE OF CELL

Flow chart of cell

CELL MEMBRANE

• Cell membrane is also called as plasma Membrane or Plasma lemma.

• It is the limiting boundary of each cell which separates the cytoplasm from its surroundings.

• It is found in both plant as well as animal cells.

• It is the outer most covering of a cell in case of animals and lies below the cell wall in case of plants.

• It is made up of proteins and lipids where proteins are sandwiched between bilayer of lipids.

• Plasma membrane name was given by Nageli.

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

• Singer and Nicholson gave the fluid mosaic model of plasma membrane according to him it consists of a protein layer sandwiched between two layers of lipids. It is in quasifluid state. It is 75A thick.

• It is flexible and can be folded, broken and reunited.

•  

(i) 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:

(A) 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.

(B) 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 solvent or 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”.

• Endosmosis: Movement of solvent into the cell is called as endosmosis.

• Exosmosis: Movement of solvent outside the cell is called as exosmosis.

(iii) Types of solution on the basis of concentration:

1. Isotonic solution: When the concentration of the solution outside is equal to the concentration of cytoplasm of the cell it is called as isotonic solution.

2. Hypertonic solution: When the concentration of the solution outside the cell is more than that of inside the cell. Due to this cell looses water and becomes plasmolysed.

3. 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.

4. Active Transport : It is the movement of molecules or ions from their lower concentration to higher concentration across the plasma membrane, i.e., it occurs against the concentration gradient. It often results in the accumulation of substances within the cell in higher concentrations than the outside concentration. It requires the utilization of energy in the form of ATP and also some membrane proteins acting as carrier molecules within the plasma membrane.

(a) Advantages of active transport

• It helps the cell to absorb many selective ions or molecules inside it against concentration gradient.

• It is a very rapid process as compared to passive transport.

• It helps to maintain ionic and water balance between the cells and outside fluid.

• It helps to maintain action potential inside and outside the nerve membranes to enable the flow of stimulus across them.

(b) Types of Active Transport

Active transport may occur by the following 2 modes

(i) Endocytosis (ii) Exocytosis

(i) Endocytosis : It is a process of intake of materials inside the cell through the plasma membrane. According to the nature of substances taken in, it is of following two types.

(a) Phagocytosis : It is the process by which the cell takes in solid food material through its plasma membrane. Therefore, the process is also called as cell-eating. All protozoans take in their food by this process.

Phagocytosis in Amoeba : A closer view

Different stages of phagocytosis in Amoeba

(b) Pinocytosis : It is the process by which the cells takes in the fluid matter through its plasma membrane, therefore the process is called cell-drinking. In this process an invagination occurs at any point in the plasma membrane near the fluid source and small-pocket like structure is formed. The fluid is then engulfed inside the cell via this invagination and it forms a pinosome. The digestion of pinosome is then brought about by various enzymes of the cell that are present in the lysosomes.

Pinocytosis in Amoeba : A closer view

• This phenomenon is called as plasmolysis

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 hemicellulose. Cell walls of two adjacent cells are joined by a layer called middle lamellae. It is made up of calcium and magnesium pectate.

• It has narrow pores, called pits, through which fine strands of cytoplasm, called plasmodesmata, are able to pass. These intercellular connections allow exchange of materials between living cell contents.

Functions of cell wall:

• It provides definite shape to the cell.

• It provides strength to the cell.

• It is permeable and allows entry of molecules of different sizes.

• It is antigen specific.

• It has the characteristics of repair and regeneration.

Difference between cell wall and plasma membrane :

PROTOPLASM

(a) It is a colourless, transparent, viscous, base liquid in which all cell organelles are suspended. Every cell is made up of protoplasm.

(b) The chemical structure of protoplasm was discovered in 1880 by Huxley in 1898 discovered that it is the physical basis of life.

(c) Protoplasm consists of

(i) Nucleus

(ii) Cytoplasm

Nucleus:

Figure: 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 contain a 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 nuclear membrane nucleus also contains nucleolus and chromatin material and the substance filled inside the nucleus is nucleolus or karyolymph.

• Chromosomes or chromatin material consists of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.

CYTOPLASM

• Cytoplasm was discovered by Kolliker in 1862.

• It is the site of both biosynthetic and catabolic pathways.

• It can be divided into two parts:

(i) Cytosol: Aqueous soluble part contains various fibrous proteins forming cytoskeleton.

(ii) Cell organelles: Living part of the cells having definite shape, structure and function bounded by Plasma membrane.

ENDOPLASMIC RETICULUM

Structure of Endoplasmic Reticulum

Nature and occurrence

• It is the network of membranes present in the cytoplasm.

• It was discovered by Porter, Claude and Fullam.

• These are present in all cells except prokaryotes and mammalian erythrocytes.

• They are made up of three components:

1. Cisternae: These are long, flattened, parallely arranged, unbranched tubules. These form successive layers of nucleus. These are found in cells which are active in protein synthesis and are 40-50 µm in diameter.

2. Vesicles: These are around or spherical they are found in synthetically active cells.

3. Tubules

Various parts of Endoplasmic Reticulum

1. Types :

Endoplasmic reticulum is of two types

Smooth ER Rough ER

-Made of tubules mainly. -Made of cisternae and vesicles.

-Helps in steroid, lipids and. –helps in protein synthesis.

Polysaccharide synthesis. Contains ribosomes on its surface.

-Ribosomes are absent.

-Helps in membrane biogenesis.

1. Function of ER:

   1. 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.

   2. It also function as a cytoplasmic framework to provide space for some of the biochemical activities. It forms endoskeleton of cell.

   3. It helps in synthesis of fats, steroids, cholesterol etc.

   4. It contains secretory proteins.

   5. SER plays a crucial role in detoxification of drugs and poisonous by-products.

RIBOSOMES

Nature and occurrence

• Discovered by Palade hence called Palade granules.

• Smallest cell organelle (230 Å in diameter).

• They are dense, spherical, granular particles occuring freely in cytosol (matrix) remain attached to ER.

• The main constituents of it are RNA and proteins.

• Two types of ribosomes are found 70S and 80S. 70S is found in prokaryotes which has two subunits (50S and 30S). 80S is found in eukaryotes which has two subunits (60S and 40S). Both the subunits of the ribosome remain attached when Mg²⁺ concentration increases but are separated when concentration decreases.

In this way each mRNA molecule brings about polymerization of specific protein molecules, with the help of ribosomes from amino acid molecules found in the Cytosol.

Function :

Ribosomes are site of protein synthesis and hence are called protein factories or engine of cells.

Golgi Apparatus

Golgi body / Golgi Complex / Golgisome / Lipochondria

Nature and Occurrence

• First discovered by Camello Golgi described by the name internal reticular apparatus.

• The apparatus lies near the nucleus. It is found in all eukaryotic cells except mammalian RBC’s, bacteria, blue-green algae and mature sperms.

• In plants Golgi complex are known as dictyosomes.

• They are composed of

• Cisternae : They are flattened, plate like tubules.

• Vesicles : They are spherical tubules

• Vacuoles : Large spherical peripherally occurring vesicles.

• Golgi vacuoles are constantly and rapidly renewed.

• Composed of lipids and fats mainly and that is why called lipochondria.

Functions

(i) The main function of Golgi apparatus is secretory.

(ii) It packages materials synthesized in the cell and dispatches from cell across the plasma membrane.

(iii) It produces vacuoles or secretory vesicles which contain cellular secretions e.g. proteins, cellulose, melanin pigment, lactoprotein of milk, enzymes etc.

(iv) It is also involved in the synthesis of cell wall, plasma membrane and lysosomes.

(v) Responsible for the development and growth of the reproductive cells in mammals.

(vi) Anterior part of sperm i.e. acrosome is formed by them.

Golgi apparatus

Lysosomes (Enzymes packets)

Nature and occurrence

• Discovered by Christian de Duve (1955) in rat liver cells.

• Are tiny spherical sac-like structures surrounded by a single, thin membrane of lipoproteins which contain digestive enzymes for intracellular digestion and waste disposal.

• Lysosomal enzymes are made by RER.

• It is abundant in digestive glands.

• The most important enzyme in lysosome is acid phosphates.

Functions

(i) They serve as intracellular digestive system, hence, called digestive bags.

(ii) Destroy any foreign material in the cell.

(iii) Digestion of food, foreign material etc. by lysosome is called heterophagy.

(iv) Remove the worn out and poorly working cell organelles by digesting them, removing cell debris and are hence known as demolition squad, scavengers, cellular housekeepers. Thus, forming a garbage disposal system of the cell.

(v) During breakdown of cell structure, when the cell gets damaged, lysosomes may burst and eat up their own cells. Therefore also called suicidal bags of cell.

Structure of Lysosome

Mitochondria

Nature and occurrence

• ‘Mitochondrion’ was first coined by Benda.

• Are tiny bodies of varying shapes and size (0.2 μm to 2 μm).

• Each mitochondria is bound by a double membrane.

• Outer membrane is porous, the inner membrane is thrown into folds called cristae and are studded with small round bodies known as F₁ particles or oxysomes.

• The interior is filled with gel like matrix. Matrix contains small sized ribosomes, circular DNA molecule and phosphate granule.

• Mitochondria are absent in bacteria, RBC’s of mammals.

• Oxysomes – generate ATP by oxidative phosphorylation.

Functions

• Are sites of cellular respiration.

• They synthesize energy-rich compounds (ATP) hence called ‘power house of cell’.

• ATP is known as energy-carrier or energy-currency of the cell. It is cellular-fuel.

• ATP is used for the synthesis of chemical compounds e.g. (DNA replication, transcription, protein, carbohydrates and lipid synthesis) mechanical work (contraction of muscles, movement of cilia and flagella).

• Mitochondria makes some of their proteins therefore it is a semiautonomous organelle.

Internal structure of mitochondria

Plastid

Nature and occurrence

• It is double membranous discoidal structure, found only in plant cells.

• Term plastid was given by Haeckel.

• Chloroplast was discovered by A.V. Leeuwenhoek and named by Schimper.

• 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.

Chloroplast have Following Two Parts :

(i) Grana: It constitutes the lamellar system. These are found layered on top of each other, these stacks are called as 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 contain photosynthetic pigment chlorophyll. In each thylakoid Quantasomes are present which are called as Photosynthetic units. Each quantansome possesses 230 chlorophyll molecules.

(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.

Chloroplast : inner structure

SOMETHING SPECIAL

Mitochondria and Chloroplasts are semi autonomous, self replicating, they divide to increase in number. They have a circular chromosome, resembling the chromosomes of prokaryotes but much smaller. They contain their own genetic system which is separate and distinct from the nuclear genome.

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

Cell Inclusions:

• These are non-living structures of the cytoplasm. These include reserve food. In the form of grains (e.g. starch grains of potato tuber cells) or granules (e.g. glycogen in liver and muscle cells); or droplets (e.g. oil droplets in fat cells).

Vacuoles

• These are small sized but many in number of animal cells while plant cells have single large central-vacuole (which occupies 50-90% of the cell volume), so that the nucleus and cell organelles are pushed near the cell wall.

• A vacuole is formed of outer limiting membrane, called tonoplast.

Functions of Vacuole

• In animal cells they store water, glycogen and proteins.

• In unicellular animals like Amoeba and Paramecium they contain food material and maintain osmotic balance.

• Plant vacoules are filled with cell sap containing sugars, amino acids, some proteins, minerals and metabolic wastes.

• In plant cells they provide turgidity and rigidity.

Peroxisomes

Nature and occurrence

• They are small (0.3 to 1.5 μm in diameter) and spherical organelles containing powerful oxidative enzymes.

• They are found in photosynthetic cell of plants, liver and kidney.

• They are bound by single membrane. There is a crystalline core in the centre of peroxisomes.

• The crystalline core is a crystallized protein called catalase.

Centrosome

Nature and occurrence

• Found only in animal cell.

• It consists of two granules called centrioles.

• Centrioles are hollow, cylindrical structures which are made up of microtubules.

• In plant cells, polar caps perform the function of centrioles.

Structure of Centrosome

Functions

(i) Centrosome helps in cell division in animal cells as they are involved in the formation of spindle fibers.

(ii) The microtubules of cilia and flagella originate and are borne by basal bodies or kinetosome formed by them.

(iii) It develops the axial fibers of sperms.