Cell | Class 11 Biology Notes — Structure and Organelles

Cell theory or cell hypothesis

After the discovery of cell by Robert Hooke in 1665. Many scientists throughout the world had extensively worked on cell. They had put forward their findings, which were later on compiled together to be known as cell theory.

Statement of cell theory

1. All living organisms are composed of small living units called cells.
2. All cells are fundamentally similar in chemical compositions and metabolic activities.
3. All cells arise from pre-existing cells.
4. The cells are structural and functional units of life.

Cellular totipotency

The term cellular totipotency was coined by Hebert lands (1902) and was experimentally proved by PC Steward and his workers (1950).

All the vegetative cells of the plants have potentially to develop into new individual. Such a potentiality of plant cells is known as cellular totipotency. The concept of plant tissue culture was developed due to cellular totipotency.

Prokaryotic cell

A cell in which the nuclear (genetic) material is not enclosed by a definite nuclear membrane to form an organised nucleus and also lacks the membrane bound cell organelles is called prokaryotic cell.

Characteristics of prokaryotic cell

1. They lack nuclear membrane.
2. Plasma membranes forms intrusions into the cytoplasm called mesosomes.
3. Cytoplasmic streaming does not occur in prokaryotic cell.
4. Respiration occurs through cytoplasmic membranes.

Eukaryotic cell

A cell which passes membrane bound organized nucleus and all other cell organelles is called eukaryotic cell.

Characteristics of eukaryotic cell

1. Mitotic apparatus is present.
2. Cell division occurs by mitosis, meiosis or mitosis
3. Cytoplasmic streaming occurs in the eukaryotic cell.

There is a great variation in the shapes, sizes, volumes and types of cells.

1. Shape of cells: The shape of a cell is related to its function.
2. Size of cells: The cell of size can from very very small to very large.

Difference between unicellular and multicellular organism

Difference between plant cell and animal cell

Cell organelles

These are living sub-cellular structure of the cytoplasm having definite structures and functions and also known as protoplasmic bodies or organoids.

Cell inclusions

These are non-living bodies of the cytoplasm formed as a result of metabolism in the cell and are also known as deutoplasmic or ergastic substances.

Cell wall

It is the outermost, rigid, protective and supportive layer found in all the plant cells, bacteria, cyanobacteria, fungi and some protists.

Structure

• Middle lamella
• Primary cell wall
• Secondary cell wall
• Plasmodesmia

Functions

• It provides mechanical strength to the cells.
• It maintains the shape of the cells.
• It prevents undue expansion of the cells.
• It is involved in many enzymatic activities.

Cell coat

The layer is further strengthened by the deposition of calcium salts, silicon or other material is called cell coat.

Functions

• It protects the underlying plasma membrane.
• It provides a shape to the cell.
• It makes the cells sensitive to foreign cells and helps to build immunity against them.

Protoplasm

It refers to the substance of which the cell is made up of and includes all parts of the cell. It is considered to be a living substance. It is divided into nucleoplasm or protoplasm of nucleus and cytoplasm or extra-nuclear protoplasm.

The entire fluidal and granular mass surrounded by cell membrane is called protoplasm.

Chemical composition of protoplasm

• Water = 75% – 90%
• Proteins = 7% – 20%
• Carbohydrates = 1%
• Lipids = 2–3%
• Inorganic materials = 1%
• RNA = 0.7%
• DNA = 0.4%

Cell membrane / plasma membrane

It is the outermost protective covering of animal cell, while in plant cell it lies inner to the cell wall. It is the thin transparent, elastic, regenerative and semi-permeable membrane that surrounds the protoplasm. It is popularly known as plasma membrane as its surrounds protoplasm.

Structural models of plasma membrane

After the discovery of plasma membrane many scientist throughout the world have proposed the structural models of plasma membrane.

1. Sandwich model

According to this model, the plasma membrane is lipoproteinous and trilaminar or tripartite. It consists of two mono layers of proteins and a bilayer of phospholipids. The lipid and protein are arranged in such a way that phospholipid bilayer is sandwich in between two mono layers of proteins.

2. Fluid–mosaic model

According to this model plasma membrane is same as above. The phospholipid molecules are fluid in nature. While the protein molecules are solid in nature. There are two types of proteins i.e. Extrinsic or peripheral proteins and Intrinsic or integral proteins.

Functions of plasma membrane

• It gives definite shape to the animal cell.
• It protects inner content of the cell.

Endocytosis: During acute shortage of food material the cell membranes bind with extra cellular food materials and are taken inside the cell, such process is called endocytosis.

Exocytosis: The dead remains of the cell are expelled outside of the cell, is called exocytosis.

• Pinocytosis: During acute shortage of water the cell membranes binds with extra cellular water molecules and are taken inside the cell, is called pinocytosis.
• Phagocytosis: The cells like WBC engulf the foreign toxins and destroy them, such process is called phagocytosis.

Locomotion: Cell membrane also helps in locomotion by forming pseudopodia in Amoeba.

Mitochondria

It was first discovered by Kolliker (1880) in the voluntary muscles of insects. They are granular or filamentous cytoplasmic structure. It is double membrane bound cell organelles, surrounded by outer and inner membrane. The two membranes are remained separated by a space called perimitochondrial space. The outer membrane is smooth, while the inner membranes have numbers of fingers like infolding called cristae.

Functions of mitochondria

• They are the site of aerobic respiration
• Due to presence of DNA, RNA and ribosomes it is called self replicating cell organells
• It also regulates the calcium ion concentration inside the cell.
• It helps in yolk formation during the development of ovum
• Also the site of thermogenesis

Plastids

The plastids are the largest cell organells among the plant cells. It is absent among the animal cells, fungi and prokaryotes.

1) Chloroplast (green plastids)

It is a green plastids found on the green parts of the plants, especially on the leaves. It was discovered by Antony Von leowenhoef in 1679.

Shape: In higher plants chloroplast are generally biconvex or plano-convex. However in different plant cell it may have various shape such as filamentous, sauer shaped, ovid, discoid, spheroid, star-like.

Size: The size of chloroplast varies from species to species. It generally measures 2–3 µm in thickness and 5–10 µm in diameter.

Structure: It is double membrane bound cell organelle. It is surrounded by two membranes i.e. an outer membrane and inner membrane. There is a space in between the two membrane called periplastidial space. Both the membranes are smooth. There is presence of a space inside to the inner membrane called matrix or stroma.

Functions of chloroplast

1. It is the site for photosynthesis
2. It is known as kitchen of the cell
3. It gives greenery to the surroundings
4. It maintain O₂ and CO₂ balance in the atmosphere
5. It also helps to reduce global warming

2) Chromoplast

It is coloured plastids other than green. The structure of chromoplast is similar with chloroplast but there is lack of grana-strama lamellae. There is also presence of minute vacuoles like structure called vesicles. The colored pigments like anthocyanins are found on vesicles.

3) Leucoplast

It is a colourless plastids. It usually occurs on the underground and unexposed parts of the plants. It is mainly concerned with storage of various kinds of reserve food materials such as carbohydrates, lipids & proteins.

Endoplasmic reticulum (ER)

It is a well developed electron microscopic network of interconnected cristernae; tubules and vesicles present throughout the cytoplasm, especially in the endoplasmic. The endoplasmic is present in all the eukaryotic cells except germinal cells and matured mammalian RBC. It is also absent in prokaryotes.

Structure

It is a complex structure consists of the following structures.

a) Cisternae

They are long, flattened, narrow, two layered and unbranched tubules near the nucleus. They are placed one above another and remain inter-connected with each other. There are presence of space in between the cristome called inter-cisternal space.

b) Vesicles

They are oval, membrane-bound vacuolar structures made up of lipoproteinous membrane. They are found to be associated with cisternal or freely scattered on the cytoplasm.

c) Tubules

They are wider, tubular and branched structures forming the reticular system along with the cristernae and vesicles.

Types and their difference

Golgi complex

It was discovered by Camillo Golgi. It is absent in prokaryotic cells and present in all the eukaryotic cells except mature mammalian RBC. It varies and depends upon the functional state of the cell. The number of dictyosomes in plant cells is highly variable eg: One in lower algal forms to about 25000 dictyosomes per rhizoidal of Chara. Its structures:

1. Cisternae: They are flat, elongated, made up of lipoproteinous membrane. They are single bound structure. The cisternae are placed one above another and remain interconnected with each other.
2. Vesicles: They are minute, spherical oval or rounded structure made up of lipoproteinous membrane. They are also single membrane bound structure.
3. Tubules: They are elongated, tubular-and much branched structure made up of lipoproteins membrane. They are also single membrane bound structure.
4. Vacuoles: They are fluid filled spacious structure, made up of lipoproteinous membrane. They are also single membrane bound structure. They usually lie towards concave end of the cristernae. They are also known as golgian vacuoles.

Functions of golgi body

• It helps in the formation of primary lysomes
• It helps in the formation of hormones in the endocrine cells e.g. thyroxin in thyroid gland cells.
• It takes part in the biosynthesis of glycoproteins.
• Golgi complex is also known as "Dictyosome" having single cisterna.

Ribosomes

They are nucleoprotein body because they consist of RNA and protein. They are found among the prokaryotic and eukaryotic cells. They are not surrounded by any unit membrane. They have naked structure. It consists of a larger sub-unit and a smaller sub-unit. On the basis of sedimentation co-efficient the ribosomes are of two basic types: 70S and 80S. The letter 'S' refers to Svedberg unit. The 70S type consists of larger 50S type and smaller 30S type. Where as 80S type consists of larger 60S type and smaller 40S type. They remain on cytoplasm in inactive stage and activated only at the time of protein synthesis. They occur on cytoplasm, nucleoplasm, chromatin reticulum, matrix of mitochondria and plastids and also on the membrane of endoplasmic reticulum.

Functions

1. It is known as protein factory of cells.
2. It is the site for protein synthesis.
3. Free ribosomes produce enzymes for extracellular use.
4. Ribosomes also store proteins temporarily.

Lysosomes

They are electron microscopic, vesicular structures of the cytoplasm which are involved in intra-cellular digestive activities. They are single membrane bound structure. The membrane is lipoproteinous. They contains 40 different types of hydrolic enzymes required for intracellular and extracellular digestion.

The 40 different enzymes are categorized into 6 categories. Such as proteases, lipases, glycosidases, nucleases, sulphatases and phosphatases.

Lysosomes are also known as polymorphic cell organelle as it occurs in various forms. They are as follows:

1. Primary lysosomes — It contains only hydrolytic enzymes in inactive form and is also known as storage granules
2. Secondary lysosomes — The primary lysosomes containing food vacuoles are known as secondary lysosomes.
3. Residual bodies (tertiary lysosomes) — The secondary lysosomes digest food materials. The remains of digested materials are remained as residue on the lysosomes.
4. Autophagosomes or autolysosomes — During shortage of food, the lysosome fuses with cell organelles. Thus it destroy or digest the cell organelle by the process called autolysis or autophagy.

Functions of lysosomes

1. It is concerned with intra and extracellular digestion of food materials.
2. The lysosome of WBC destroys foreign toxins or pathogens.
3. It takes part in digestion of dead remains of the cell.
4. Lysosomes are known as "suicidal bag" of the cell because during lack of food it digests cell organelles so as to cause death of the cell. Such a process is called autolysis.

Cilia and flagella

They are microscopic, hair or thread like motile structures present extracellularly but originate intra-cellularly from the basal bodies and help in movement, locomotion, feeding, circulation etc.

Each flagellum and cilium consists of nine peripheral tubules made up of proteins called tubuline. There are presence of two central tubules. The bundle of tubules is known as axoneme or axial filament.

Functions

1. The cilia and flagella help in locomotion
2. The cilia also help in catching the prey
3. The cilia of wind pipe prevent the entry of dust particles into the lumen
4. Cilia of kidney - nephrons move nephric filrate.

Difference between cilia and flagella

Vacuole

They are non-cytoplasmic area found within the cytoplasm. Vacuoles are found on both plants and animal cells. In plant cells the vacuoles are fewer in number and larger. In animal cell they are small or almost absent.

Types of vacuoles

1. Food vacuoles — that stores food
2. Gas vacuoles — that stores gas
3. Contractile vacuoles — that stores water
4. Sap vacuoles — that stores fluid

Functions of vacuoles

1. They can store water and minerals
2. They also store foods as well as gases
3. They also store waste materials of the cells
4. Gas vacuoles provide buoyancy to the organisms in water.

Nucleus

It is found in all the eukaryotic cell except mature mammalian RBC; sieve tube cell of phloem etc. Among the prokaryotes, there is absence of true nucleus. Such nucleus is known as nucleoid.

The shape of nucleus varies from cell to cell. The shape of the nucleus may be spherical, cuboidal, discoidal or even irregular.

On the basis of number of nucleus, following types of cells have been recognized.

1. Uninucleate (monokaryotic) cells — Mostly the cells are monokaryotic
2. Binucleate cells — The cells which contain two nuclei are known as binuclei cell eg: Paramecium, Liver cell etc
3. Polynucleate cells — The cells which contain many nuclei are known as polynucleate cells. eg: Osteoblasts, Rhizopus etc.

Ultrastructure of nucleus

Nucleus is composed of following four components:

1. Nuclear membrane (karyotheca)
2. Nuclear sap (nucleoplasm)
3. Chromatin fibers
4. Nucleolus

Functions of nucleolus

1. It is considered as brain of the cell
2. It controls and regulates all the activities of the cell
3. It is the site for the synthesis and storage of
4. The nucleolus forms spindle fiber during cell division in plant cells.

Difference between heterochromatin and euchromatin

Chromosome

They are DNA protein hereditary structures which are formed by condensation of chromatin fibres during cell division or reproduction. The size of chromosomes vary from species to species and the shape changes with the phase of cell division. The shape of chromosome depends upon the position of centromere which is follows:

1. Metacentric
2. Sub-metacentric
3. Acrocentric
4. Telocentric