Cell structural and functional unit of life

Cell is called the structural unit because:- Cells are arranged in proper and systematic order to form

a body of living organism. In multicellular organisms cells are first organized into

tissues, each with a specific function, then into organs. Organs are grouped into organ systems each with a major function. A living organism has a number of organ systems.

Cell is also called functional unit of life because:- All the metabolic activities of life take place at cell level.

Respiration, digestion, excretion and reproduction occur at cell level.

Cells contain genetic material which regulate cell functions and pass on all information to the next generation.

HistoryDiscovery of a cell ( Latin word “cellula”= Little room)

Robert Hooke( 1635-1703) discovered and named the cells .

Anton Von Leewenhoek in 1674 discovered the free

/living cells in the form of bacteria. He also described

the sperm cells of humans, dogs, rabbit, frogs, fish and insects.

Robert Brown (1773- 1858) discovered and named the nucleus in plant cells.

J.E. Purkinje (1787-1869) in 1839 gave the term protoplasm for the living fluid substance present inside the cell.

Haeckel- in 1866 established that the nucleus was responsible for storing and transmitting hereditary characters.

Cell theory- Joint findings of Jakob Matthias Schleiden (Jacob

Schleiden) and Theodor Schwann forms the basis of cell theory.

The cell theory was redefined by R. Virchow in 1855. ( stated that all cells arise from pre- existing cells)

Postulates of cell theory:-All organisms are composed of cells and cell products. cells are structural and functional units of life.New cells arise from pre- existing cells.

Instruments for studying cells. (a) Light or Compound microscope (b) Electron Microscope.

Light/compound microscope Electron microscope1. It uses glass lenses for magnification.

1. It uses electromagnets for magnification.

2. It uses beam of light to illuminate the object.

2. It uses beam of electrons instead of light.

3. Internal Vacuum is not required.

3. Internal vacuum is essential.

4.Magnification power range from 300 to 1500 times,

4. Magnification is 100,000 to 500,000 times.

Basic similarities among all cells of higher organisms – They have plasma membrane, a cytoplasm with organelles and a nucleus.

Classification of living organismsAll living organisms present on earth can be classified into following two types:-

1. Non cellular organism- which do not contain any cell in their body organization. Example:- Viruses- lack any membrane and hence do not show characteristics of life until they enter in a living body to use its cell machinery to reproduce.

2. Cellular organisms- contain either one or many cells in their bodies. Examples:- bacteria , plants and animals

Types:- Prokaryotes and eukaryotes.

Prokaryotic cell Eukaryotic cell1. Size of cell is generally small. ( 1-10 mm)

1. Size of the cell is generally large ( 5-100 mm)

2. Nuclear region or nucleoid is not surrounded by a nuclear membrane.

2. Nuclear material is surrounded by a nuclear membrane.

3. It contains single chromosomes.

3. It contains more than one chromosome.

4. Nucleolus is absent. 4. Nucleolus is present.5. Membrane bound cell organelles are absent.

5. Membrane bound cell organelles such as mitochondria, plastids, endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, etc are present.

6. Cell division takes place by fission or budding.

6. cell division takes place by mitotic or meiotic cell division.

Cell shapeThe basic shape of eukaryotic cell is spherical but the

shape of cell is ultimately determined by the specific function of the cell.

The shape of the cell may be variable or fixed.Variable or irregular shape occurs in Amoeba and WBC.Fixed shape or cells occurs in most plants and animals.Eg:- spherical (eggs of many cells), spindle shaped

( smooth muscle fibres), elongated ( nerve cells) ,

branched (chromatophores or pigment cells of skin), discoidal ( RBC) etc.

Cell sizeThe size of different cells ranges between broad limits.The size of cells varies from the very small cells of

bacteria( 0.2-5.0 micrometer) to the very large eggs of the ostrich( 18cm)

Some nerve cell of human beings has a meter long tail or axon.

Cell volumeThe volume of a cell is fairly constant for a particular

cell type and is independent of the size of the organisms.For example, kidney or liver cells are about the same

size in the bull, horse and mouse.The difference in the total mass of the organ or organism

depends on the number, not on the volume of the cells.Cell numbers

The number of cells in multicellular organisms usually is correlated with the size of the organisms.

Thus, small sized has less number of cells in comparison to large sized organisms.

Structure of cellAll cells have following 3 major functional regions:-

1.The cell membrane or plasma membrane, and cell wall.

2.The nucleus.3.And the cytoplasm

Plasma membrane

Nature and occurrenceThe plasma membrane is the outer covering of each cell.It is present in cells of plants, animals and

microorganisms.Structure

Plasma membrane is a living, thin, delicate, elastic, selective membrane.

made up of 75% lipid bilayer (phospholipids) along with proteins ( intrinsic and extrinsic) cholesterol and polysaccharide ( a kind of carbohydrate)

Intrinsic protein - within lipid bilayer.Extrinsic protein -on the outer surface or on the inner

surface of the lipid membrane.Functions

Plasma membrane is called selective permeable membrane because it allows the entry of certain

substances, exit of some other substances while preventing passage to remaining substances.(a) Selective permeability:- The membrane determines

what substances are to be allowed to enter or exist the cell.

(b) Shape:- It provide a definite shape to semifluid contents of the cell.

(c) It functions as mechanical barrier that protects the internal contents of the cell.

(d) Osmosis- it is the passage of water from a region of high water concentration through a semi permeable membrane to a region of low water concentration.

(e) Diffusion:-it is the spontaneous movement of molecules from a region of high concentration to one of lower concentration, leading finally to uniform concentration.*It is faster in the gaseous phase than in liquid and solids.*Some substances (molecules, ions) such as carbon dioxide, oxygen , water etc can move across the plasma membrane by diffusion.

(f) Endo-cytosis:- Mechanisms of taking a substance inside.

(g) Exocytosis:- Mechanism of removing waste materials from the cell.

Cell wallIt is non living and freely permeable.

It is secreted by cell itself for the protection of its plasma membrane and cytoplasm.

It determines the shape of plant cell and prevents desiccation of cells.

It is made up of cellulose( Fibrous polysaccharide- carbohydrate)

Functions1.Strength:-It provides mechanical strength to support the

cell.2.Shape:- It provides shape to the cell.3.Protection:- It protects the cell against pathogens and

mechanical injury.4.Environmental variation:- because of cell wall, plant cell

can withstand a lot of variations in the surrounding medium. Eg- temperature variation.

5.Limited endo osmosis:- Cell wall prevents bursting of cell or endo –osmosis. As water enters living cell, their protoplasm swells up and builds pressure against the cell wall. The cell wall exerts equal and opposite pressure against the swelling protoplasm. As a result further entry of water inside the cell is stopped.

PlasmolysisWhen a living plant cell loses water through osmosis

there is shrinkage or contraction of the contents of the cell away from the cell wall.

This phenomenon is known as plasmolysis.

Cell wall Plasma membrane1. It occurs in plant cells 1. It is found in both plant and

animal cells2. It lies outside of the cells.

2. It lies on the outside of animal cells and inner to the cell wall in plant cells.

3. It is non living and quite thin in plant cells.

3. It is living and quite thin.

4. It is rigid. 4. It is flexible.5. It is generally permeable.

5. It is selective permeable.

6. It is formed of cellulose, hemi cellulose and pectin.

6. It is formed of lipids and proteins and small number of carbohydrate.

7. Its major function is to provide protection and strength of the cell.

7. Its major function is to hold cellular contents and control passage of materials into out of the cell.

Diffusion OsmosisMovement of substance from area of higher concentration to lower concentration.

Movement of solvent molecules from area of higher concentration to lower concentration through selective semi permeable membrane.

Can take place in solid, liquid and gases.

Can take place ion only liquid medium.

May or may not involve semi permeable membrane.

Semi permeable membrane is required.

It equalizes the concentration of substance throughout the available space.

Doesnot equalizes concentration of solvent on the two sides of membrane.

Different substances diffuse independently one another as per their concentration differences.

Only solvent undergoes diffusion across semi permeable membrance, solute does not.

Nucleus

Nature and occurrenceIt is a major centrally located spherical cellular

component.It is bounded by two nuclear membrane, both forming a

nuclear envelope.

The nuclear envelope separates the nucleus from the cytoplasm.

Nucleopore:- allow transfer of materials between the nucleoplasm and the cytoplasm.

Nuceloplasm:- Fluid content of nucleus.Within nuceloplasm nucleolus (factory of ribosome) and chromatin material are present.

Nucleolus :- rich in protein and RNA molecules and is the site of ribosome formation.

The chromatin material is a thin thread like intertwined mass of chromosome material and compose of the genetic substance DNA(deoxyribonucleic acid) + Protein ( histones)

DNA stores al the information necessary for the cell to function, to grow and to reproduce further cells of the next generation.

functional segments of DNA are called genes.The chromatin is condensed into two or more thick

ribbon like chromosomes during the division of cell.

Cells which lack the nucleus.RBC of human beings and other mammals lose their

nuclei- this enables them to carry more hemoglobin and so pick up more oxygen.

Phloem sieve tubes provide the transport system for sucrose in plants.

Functions

1.Control centre:-The nucleus controls al metabolic activities of the cell. If the nucleus is removed from a cell, the protoplasm ultimately dries up and dies.

2.It regulates the cell cycle3.Genetic information:-It is concerned with the

transmission of hereditary traits from the parent to offspring.

4.Ribosome:- They are formed by nuceloplasm.5.Division:- division of nucleus is essential for cell

division or cell reproduction.6.Variation:- Reshuffling of chromosomes and changes in

gene location produce variation among organism.

Cytoplasm

Nature and occurrenceThe part of the cell which occurs between the plasma

membrane and nuclear envelope is called the cytoplasm.Cytoplasm consists of an aqueous ground substance, the

cytosol- containing variety of cell organelles, insoluble waste and storage products ( starch , glycogen, lipid etc)

FunctionsIt acts as a store of vital chemicals such as amino acids,

glucose, vitamins, ions etc.It is the site of certain metabolic pathway (glycolysis).Synthesis of fatty acid, nucleotides and some amino

acids takes place in the cytosol.Living cytoplasm is always in state of movement.

Cell organelles

1.Endoplasmic reticulum ( ER)

Nature and occurrence1.It is large network of membrane bound tubes and sheets.2.On the one end ER is connected to the outer membrane

of nucleus and on the other end to the plasma membrane.3.ER occurs in three forms.- Cisternae, vesicles and

tubules.4.The ER is absent in the red blood cells of mammals.5.It is of two types:-

a)Rough endoplasmic reticulum- ( RER) with ribosomes attached on its surface for synthesizing protein.

b) Smooth endoplasmic reticulum (SER) which is without ribosomes and meant for secreting lipids.

Functions1.It forms supporting skeletal framework of the cell.2.It provides a pathway for the distribution of nuclear

material from one cell to the other.3.Certain enzymes present in smooth ER synthesis fats

( lipids) , steroids and cholesterol.4.RER is concerned with the transport of proteins which

are synthesized by ribosomes on their surface.5.IT serves as channels for the transport of materials

(especially proteins) between various regions of the cytoplasm or between cytoplasm and the nucleus.

6.It functions as a cytoplasmic framework providing surface for some of the biochemical activities of the cell.

Some special functions of ER1.SER of liver of vertebrates is involved in the process of

detoxification.2.SER plays an important role in manufacture of fat

molecules( lipids) which is use in the formation of plasma membrane/cell membrane and various hormones( steroid type)

3.SER synthesize hormones such as estrogen, testosterone and cortisol.

4.Digestive enzymes of lysosomes are produced by RER.

Digestive enzymes ( synthesized on ribosome)-------RER------Golgi apparatus5.Plasma membrane and other cellular membranes are also

formed by endoplasmic reticulum ( by proteins and lipids formed by ER). The formation of plasma membrane called membrane biogenesis, involve the following organelles, all forming the endomembrane system.

RER---- SER---- Golgi apparatus----- Secretary vesicles--- plasma membrane

6.Proteins which are synthesized by the cell and then are released into outer medium of the cell, are called secretary proteins. Examples of secretary proteins

include mucus, digestive enzymes and hormones these proteins are synthesizes by RER.

Rough endoplasmic reticulum

Smooth endoplasmic reticulum

1. It contains flattened sacs called cisternae.

1. it is mainly formed of vesicles and tubules.

2. Ribosomes are attached to outer surface of its membrane.

2. it doesn’t contain ribosomes.

3. It is specialized to synthesized proteins.

3. It is specialized to synthesized lipids and steroids.

4. It is abundant in exocrine pancreatic cells and antibodies secreting plasma membrane.

4. it is abundant in liver and the testicular cells synthesizing steroid hormones.

RibosomesRibosomes are dense, spherical and granular particles

which occur freely in the matrix (cytosol) or remain attach to the RER.

Chemically the major constituents of ribosomes are RNA (ribonucleic acid) and proteins.

Ribosomes are not bounded by membrane.

They are present in both prokaryotic and eukaryotic cells except mammalian RBC.

Function:- Ribosomes play an important role in the synthesis of proteins.

Golgi Apparatus ( Golgi Body or Golgi Complex)

Nature and OccurrenceGolgi apparatus consists of a set of membrane

bounded, fluid filled vesicles, vacuoles and flattened cisternae.

Cisternae are usually stacked together in parallel rows.It exists as an extensive network near the nucleus in

the animal cells.

However, the plant cells contain many freely distributed subunits of Golgi apparatus, called dictyososmes.

Cisternae are formed at one end of the stalk, called cis face of Golgi.

They are budded off as vesicles at the other face of Golgi apparatus. Called Trans face of Golgi.

Golgi apparatus is absent in bacteria, blue green algae, mature sperms and RBC of mammals and other animals.

The Golgi apparatus are arises from the membrane of the SER , which in turn originates from the rough endoplasmic reticulum.

The golgi saccules are constantly and rapidly renewed.

Functions:-The main function of the Golgi apparatus is secretary.

Golgi apparatus acts an assembly area for the storage, processing and packaging of various secretions. It packages material synthesized in the cell and dispatched them either intracellular target such as plasma membrane and lysosomes or extra cellular targets (Zymogens)

It produces vacuoles or secretary vesicles which contain cellular secretions e.g, enzymes, proteins, cellulose, melanin pigment, lactoprotein or milk.

Golgi apparatus is also involved in the synthesis of cell wall, plasma membrane and lysosomes.

It helps in storage , modification and packaging of products in vesicles.

In some cases complex sugar is made from simple sugar.

Lysosomes

Nature and OccurrenceLysosomes are simply tiny spherical sac like structures

evenly distributed in the cytoplasm.Each lysosome is a small vesicle surrounded by a single

membrane and contains powerful enzymes.These enzymes are capable of digesting or breaking

down all organic materials.Lysosomal enzymes are made by RER.

Functions:-Defence:-. They destroy any foreign material which

enters the cell such as bacteria and virus. In this way they protect the cells from bacterial infection.

Digestion:- They help in intracellular digestion of food particles.

Cleaning:- Lysosomes also remove worn out and poorly working cellular organelles by digesting them .

Significance of Lysosomes:- In WBC and Leucocytes- cells of leucocytes digest

foreign proteins, bacteria and viruses.During starvation, the lysosomes digest stored food

contents such as proteins, fats and glycogen of the cytoplasm and supply the necessary amount of energy of the cell.

In metamorphosis – during transformation of a tadpole into frog, the embryonic tissues such as gills and tail are digested by the lysosomes and utilized by other body cells.

In fertilization:- the lysosomal enzymes present in the acrosomes of sperm cells digest the limiting membrane of the ovum. Thus, the sperm is able to enter the ovum and start the fertilization.

Mitochondria

Nature and occurrence1.The mitochondria are tiny bodies of varying shape and

size, distributed in the cytoplasm.2.Each mitochondria is bounded by a double membrane

envelope.3.Outer membrane is porous.4.The inner membrane is thrown into folds, and therefore,

have an area several times the surface of area of the outer membrane.

5.These folds are called cristae and are dotted with small rounded bodies known as oxysomes.

6.The interior cavity of the mitochondria is filled with a proteinaceous matrix which contains a few small sized ribosome, a circular DNA molecule and phosphate granules.

7.Mitochondria are absent in bacteria and the RBC of mammals.

Functions

1.Mitochondria are sites of cellular respiration.They use molecular oxygen from air to oxidize the

carbohydrate and fats present in the cell to carbon dioxide and water vapour.

Oxidation releases energy, a portion of which is used to from ATP (Adenosine triphosphate).

Since the mitochondria synthesize, energy rich compounds (ATP) they are known as “Power house” of the cell.

The energy stored in ATP is used by the cell.ATP is generally known as energy carrier or energy

currency of the ell. It is common cellular fuel i.e. it is used to drive many

energy requiring processes of the ell.The body of organisms uses energy stored in ATP for

synthesis of chemical compounds ( DNA replication, synthesis of protein, carbohydrate and lipids) and for mechanical work, such as contraction of muscles, movement of cilia and flagella, conduction of nerve impulses and production of heat, electricity and ;light.

Mitochondria are able to make some of their own proteins, so they are regarded as semiautonomous organelles.

2.They provide intermediate substance for the systhesis of various types of chemicals like fatty acids, steroids, amino acids, chlorophyll etc.

Plastids

Nature and occurrence:-Plastids occur in most plant cells and are absent on

animal cells.Like the mitochondria, the plastids also have their own

genome (DNA) and ribosomes.They are self replicating organelles like the mitochondria

i.e. they have the power to divide.Plastids are of following three types:-

1.Chromoplast:- coloured plastids ( except green colour)- Give colour to flowers, fuits and seeds. Helps in pollination.

2.Chloroplast:- Green colour plastids. Helps in photosynthesis.

3.Leucoplasts:- The colourless plastids. Helps in storage of food. Amyloplast ( storage of carbohydrate) , Elaioplast ( storage of oils and fats) Alleuroplast ( storage of proteins)

Leucoplasts Chromoplasts1. They are colourless. 1. They are brownish or

reddish in colour.2. They are cylindrical or rounded in shape.

2. They are irregular in shape.

3. They are found in unexposed cells.

3. They are found in both exposed and unexposed cells.

4. They can change to other type of plastids.

4. They do not change to other types of plastids.

5. They take part in storage of food, e.g. amyloplasts(carbohydrates) elaioplasts (lipids) aleuroplasts (proteins)

5. They provide colouration to organs for attracting pollinators.

Chloroplasts Chromoplasts1. They are green plastids 1. They are non green

coloured plastids.2. They contain chlorophylls and carotenoids.

2. Chlorophylls are absent. Only caratenoides are present.

3. Lamellae are present. 3. Lamellae are absent.4. Chloroplasts are site of photosynthesis.

4. Thy colour in the organs ( flowers/ fruits) for attracting animals to perform pollination and fruit dispersal.

Chloroplasts

Nature and occurrence

Chloroplasts are present in green algae and higher plants.

They have the green pigment called chlorophyll and they are involved in the photosynthesis of food.

So chloroplasts are the kitchens of the cells.Each chloroplasts is bounded by two unit membranes

like mitochondria.It shows two distinct regions:- (a) Grana are stacks of

membrane bounded, flattened discoid sacs containing the molecules of chlorophyll. They are the main functional unit of chloroplasts. (b) Stroma is the homogeneous matrix in which grana are embedded.

Stroma contains a variety of photosynthetic enzymes, starch grains, DNA and ribosomes.

Grana is the site of light reaction during photosynthesis, while stroma is the site of dark reaction during photosynthesis.

Functions- Plastids perform the following functions:-1.Chloroplasts trap solar energy and utilize it to

manufacture food for the plant.2.Chloroplasts impart various colours to flowers to

attract insects for the plant.3.Leucoplasts store food in the form of

carbohydrates (starch), fats and protein.

Mitochondria Chloroplasts

1. they occur in cells of aerobic organisms(plants and animals) with the exception of mammalian RBCs.

1. They occur in the cells of green photosynthetic parts of plants (leaves).

2. They are colourless. 2. They are green in colour.3. The shape is rod like or sausage shaped.

3. they are generally disc like in outline.

4. Inner membrane of each mitochondria is thrown into folds called cristae.

4. Their inner membrane forms fattened sacs called thylakoids.

5. They liberate energy. 6. They trap solar energy and convert it into chemical energy.

6. they perform oxidation of food.

6. They synthesize food by photosynthesis.

7. They consume oxygen and liberate carbon dioxide.

7. They consume carbon dioxide and liberate oxygen.

Vacuoles.

Nature and occurrence

Vacuoles are fluid filled or solid and membrane bounded spaces.

They are kind of storage sacs.In animal cells, the vacuoles if present are small and

temporary.They store water, glycogen and proteins.The vacuolar membrane is typically a single unit

membrane and is often associated with the maintenance of water balance.

Thus, food vacuole of a single celled organism such as Anoeba or Paramoecium contains the food item that the animal has consumed.

In plant cells, the vacuoles are large, distinct and permanent.

In mature plant cells the vacuoles occupies almost the entire volume of the cell.

Because of central position of vacuole. The nucleus and other cell organelles in plant cells are pushed near the boundary wall.

The vacuole is bounded by a membrane, called tonoplast.

The vacuole is filled with cell sap which is a watery solution rich in sugar, amino acids, proteins, minerals and metabolic wastes.

Functions:-

1.Osmoregulation:-Vacuoles help to maintain balance of water inside the cell i.e the osmotic pressure in cell.

2.Excretion:-They remove the waste from the cell.3.Storage:- It stores a no. of substances like salt, sugar,

amino acids, organic acids and some proteins.4.Cellular waste:- Vacuoles function as dump house of

waste products in plant cells. It stores various types of waste.

5.They provide turgidity and rigidity to the plant cells.

PeroxisomesNature and occurrence

Peroxisomes are small and spherical organelles containing powerful oxidative enzymes.

They are bounded by a single membrane.Peroxisomes are mostly found in kidney and liver cells.Inner contents of Peroxisomes are finely granular, but

sometimes a crystalline is visible by electron microscope in the centre of Peroxisomes.

Functions:-Peroxisomes helps in detoxification of toxic substances from the cell.

Centrosome

Nature and occurrenceIT is found only in animal cells.It is not bounded by any membrane but consists of two

granular like centrioles.Centrioles are hollow and cylindrical structures which

are made up of microtubules.In plant cells, polar caps perform the function of

centrioles.

Functions:-Centrosome helps in cell division in animal cells.During cell division centrioles migrate to the poles of

animal cells and are involves in the formation of the spindle.

In plant cells, cell division involves polar caps for the spindle formation.

Plant cell Animal cell1. Plant cells are larger than animal cells.

1. Animal cells are generally small in size.

2. It is enclosed by a thick, rigid cell wall in addition to plasma membrane.

2. It is enclosed by a thin , flexible, living plasma membrane only.

3 .Plastids are usually present. Parts exposed to sunlight contain chlorophyll.

3. Except the protozoan Euglena , no animal cell possesses plastids. So is chlorophyll.

4. Most mature plant cells have a permanent and large central sap vacuole.

4. Vacuoles in animal cells are many, small and temporary.

5. Its nucleus is often pushed to one side in the peripheral cytoplasm by the sap vacuole.

5. Its nucleus is generally near the centre.

6. Plant cells lack centrosome and centriole.

6. Animal cells have centrosome and centriole.