Cell Theory or Cell Hypothesis

After the discovery of cell by Robert Hook in 1665, many scientists throughout the world had extensively worked on cell. Two German scientists, namely M. J. Schleiden (1838) and Theodor Schwann (1839), independently worked on plant and animal cells respectively. Their findings were later compiled together and became known as cell theory.

Characteristics of Cell Theory

• All living organisms are composed of small living units called cells.
• All cells are fundamentally similar in chemical composition and metabolic activities.
• All cells arise from pre-existing cells.
• The functions of an organism as a whole are the outcome of the activities and interactions of the cells consisting the body of that organism.
• The cells are structural and functional unit of life.
• The growth of an organism occurs by cell division and cellular growth in multicellular organisms, while by cellular growth in unicellular organisms.

Exception to the Cell Theory

• The cells of organisms like bacteria and blue-green algae do not have a true nucleus.
• The matured RBC of mammals and sieve tube cells of phloem do not have nucleus.
• In organisms like Mucor, Vaucheria, etc. there are many nuclei in their protoplasm.

Cellular Totipotency

The term cellular totipotency was coined by Haberlandt (1902) and was experimentally proved by F. C. Steward and his workers (1950).

The concept of plant tissue culture was developed due to cellular totipotency.

Cell Inclusions or Cytoplasmic Inclusions

During the metabolism of cells, wide ranges of non-living substances are produced within the cell. Such non-living substances found within the cytoplasm are known as cell inclusions.

The cell inclusions are categorized into the following categories:

1. Reserve materials
2. Secretory materials
3. Excretory materials

Reserve Materials

Reserve materials are materials stored in the form of reserve food. They may be nitrogenous or non-nitrogenous compounds. Examples given in the scan include proteins, amino acids, starch, glycogen, insulin, sugar, fats, oils and cellulose.

Secretory Materials

These are materials formed during metabolism of plants. They play important roles in physiological activities of plants. Some secretory materials are pigments, nectar, enzymes, etc.

Excretory Materials

These are materials formed during the metabolism of plants and are usually produced as by-products of metabolism. The scan notes that they do not have any use to plants but may have economic importance for human beings.

Examples written in the scan include organic acids, alkaloids, tannins, latex, resins, essential oils, minerals, crystals and gums.

Types of Cells on the Basis of Complexity of Cell Structure

1. Prokaryotic cell: cell with primitive or incipient nucleus.
2. Eukaryotic cell: cell with true or well-organized nucleus.

Difference Between Prokaryotic and Eukaryotic Cells

Animal Cell and Plant Cell

Difference Between Plant Cell and Animal Cell

Cell Wall

The cell wall is the outermost protective covering of a plant cell. It was discovered by Robert Hooke in 1665. It is absent among animal cells.

It is made up of cellulose, hemicelluloses and pectic substances like calcium and magnesium pectate.

The ultrastructure of cell wall shows the following:

1. Primary wall
2. Secondary wall
3. Middle lamella
4. Plasmodesmata

A) Primary Wall

It is the first or initially formed wall. It lies towards the outer side of the cell. It is thin, elastic and soft in nature. It is made up of microfibrils of cellulose.

B) Secondary Wall

It is a later formed wall. It lies towards the outer side. It is thick, elastic and hard in nature, as written in the scan. It is made up of microfibrils of cellulose and hemicelluloses. There is also deposition of cell wall materials like lignin, pectin and suberin.

C) Middle Lamella

In between the primary walls of adjacent cells, there is presence of middle lamella. It is made up of pectic substances like magnesium and calcium pectate. It acts as connecting material as it joins the adjacent cells.

D) Plasmodesmata

The primary and secondary walls of the cell are discontinued in certain regions so as to form minute pores. The protoplasm of one cell remains connected with another cell through such pores. The protoplasmic connection between adjacent cells along with the pores is called plasmodesmata. It helps in exchange of materials between adjacent cells.

Functions of Cell Wall

• It provides mechanical strength to the cells and to the plant as a whole.
• It protects inner contents of the cell from mechanical injuries and checks entry of germs.
• It maintains shape of the cells.
• It prevents undue expansion of the cells.
• The cell wall of root hairs helps in absorption of water and minerals from the soil.
• Suberin and cutin deposited on cell wall prevent evaporation of water.
• It is involved in many enzymatic activities.
• It helps in keeping a balance between intracellular osmotic pressure and that of the environment and thus prevents osmotic bursting of the cell.

Cell Membrane (Plasma Membrane)

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

Structural Models of Plasma Membrane

After the discovery of plasma membrane many scientists proposed structural models. The universally accepted models discussed in the scan are:

1. Sandwich model
2. Fluid-Mosaic model

A) Sandwich Model

This model of plasma membrane was proposed by G. Danielli and H. Davson in 1935 A.D. According to this model, plasma membrane is lipoproteinous and trilaminar or tripartite. It consists of two monolayers of proteins and a bilayer of phospholipids. The phospholipid bilayer is sandwiched in between two monolayers of protein. Protein molecules show regular pattern.

B) Fluid-Mosaic Model

This model of plasma membrane was proposed by Singer and Nicolson in 1972 A.D. According to this model, plasma membrane is lipoproteinous and trilaminar or tripartite. The phospholipid molecules are fluid in nature, while the protein molecules are solid in nature.

There are two types of proteins, including extrinsic and intrinsic proteins. The proteins and lipid molecules are arranged in such a way that it appears as “protein icebergs in the sea of lipids”. The protein molecules show mosaic pattern.

Functions of Plasma Membrane

• It gives definite shape to the animal cell.
• It protects inner content of the cell.
• Being semipermeable, it selectively allows entry of molecules through it.
• Endocytosis: during acute shortage of food materials, the cell membrane binds with extracellular food materials and takes them inside the cell.
• Exocytosis: dead remains of the cell are expelled outside of the cell.
• Pinocytosis: during acute shortage of water, the cell membrane binds with extracellular water molecules and takes them inside the cell.
• Phagocytosis: cells like WBC engulf foreign toxins and destroy them.
• Locomotion: cell membrane also helps in locomotion by forming pseudopodia in Amoeba.

Protoplasm

The entire fluidal and granular mass surrounded by cell membrane is called protoplasm. The term protoplasm was coined in 1839 by Johannes Purkinje and was described by Huxley in 1868 as physical basis of life.

Chemical Composition of Protoplasm

Mitochondria

Mitochondria were first discovered by Kolliker (1880) in the voluntary muscles of insects. The number of mitochondria per cell varies depending upon the metabolic state of the cells.

Structure

Mitochondria are granular or filamentous cytoplasmic structures. They are double membrane-bound cell organelles surrounded by outer and inner membrane. Both membranes are unit membranes, i.e. lipoproteinous and trilaminar or tripartite.

The two membranes remain separated by a space called perimitochondrial space, filled with perimitochondrial fluid. The outer membrane is smooth, while the inner membrane has finger-like infoldings called cristae. These increase surface area for aerobic respiration.

Inner to the inner membrane there is a fluid-filled space called matrix. The composition of matrix is similar to cytoplasm. There is also presence of single circular DNA, RNA and 70S type ribosomes. Due to presence of DNA, RNA and ribosomes, it is known as a self-replicating cell organelle.

On the inner surface of the inner membrane there are numbers of stalked particles called F₁ particles or oxysomes. Necessary respiratory enzymes are situated on oxysomes.

Functions of Mitochondria

• Mitochondria are the site for aerobic respiration.
• It is called powerhouse of the cell because it synthesizes, stores and releases energy in the form of ATP.
• Due to presence of DNA, RNA and ribosomes it is called a self-replicating cell organelle.
• It also regulates calcium ion concentration inside the cell.
• It helps in yolk formation during development of ovum.
• The scan states: “From middle piece of sperm during sperm maturation.”
• It is the site of synthesis of haeme of haemoglobin and myoglobin.
• It is also the site of thermogenesis.

Plastids

Plastids are the largest cell organelles among plant cells. They are absent among animal cells, fungi and prokaryotes. On the basis of presence or absence of pigments and their types, plastids are of the following types:

1. Chloroplast – green plastids
2. Chromoplast – coloured plastids other than green
3. Leucoplast – colourless plastids

A) Chloroplast

Chloroplast is a green plastid found on green parts of plants, especially leaves. The scan states that it was discovered by Antony van Leeuwenhoek in 1679.

Shape

In higher plants chloroplasts are generally biconvex or plano-convex. In different plant cells they may have various shapes such as filamentous, saucer-shaped, ovoid, discoid, spheroid, star-like, girdle-shaped, spiral ribbon-like, reticulate or cup-shaped.

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

Chloroplast is a double membrane-bound cell organelle. It is surrounded by outer and inner membranes. Both membranes are unit membranes, lipoproteinous and trilaminar or tripartite. The space between the two membranes is called periplastidial space and is filled with periplastidial fluid.

Both membranes are smooth. Inside the inner membrane is a matrix or stroma. The composition of matrix is similar to cytoplasm. There is also presence of single circular DNA, RNA and 70S type ribosomes.

Within the stroma there are membranous and filamentous structures called grana lamellae. Grana remain interconnected with stroma lamellae. The light reaction of photosynthesis takes place on grana, while dark reaction occurs on stroma.

Functions of Chloroplast

• It is the site for photosynthesis.
• It is known as “kitchen of the cell”.
• It gives greenery to the surroundings.
• It maintains O₂ and CO₂ balance in the atmosphere.
• It also helps to reduce global warming.

B) Chromoplast

Chromoplast is a coloured plastid other than green. It occurs on colourful parts of plants, like petals of flowers and ripe fruits.

Its structure is similar to chloroplast but there is lack of grana-stroma lamellae. If lamellae are present, they are represented by few degenerating lamellae. There is also presence of minute vacuole-like structures called vesicles.

Coloured pigments written in the scan include fucoxanthin, phycocyanin, phycoerythrin and anthocyanin. They are found on vesicles. Chromoplast gives attractive colours to petals of flowers to attract insects for pollination and also helps in ripening of fruits.

C) Leucoplast

Leucoplast is a colourless plastid. It usually occurs on underground and unexposed parts of plants. Its structure is similar to chloroplast but there is lack of grana. It is mainly concerned with storage of reserve food materials such as carbohydrates, lipids and proteins.

Endoplasmic Reticulum (ER)

It is a well-developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm, especially in the endoplasm. The term endoplasmic reticulum was given by Porter (1953).

Occurrence

The endoplasmic reticulum is present in all eukaryotic cells except germinal cells and matured mammalian RBC. It is also absent in prokaryotes. Occurrence and development of ER depends upon the metabolic state or differentiation of cells; the scan gives it as absent in embryonic cells, less developed in spermatocytes, and well developed in fully differentiated and metabolically active cells.

Structure

The endoplasmic reticulum consists of the following structures:

1. Cisternae
2. Vesicles
3. Tubules

A) Cisternae

Cisternae are long, flattened, sac-like, narrow, two-layered and unbranched tubules near the nucleus. They are placed one above another and remain interconnected with each other. The space between cisternae is called inter-cisternal space. They usually occur near the nuclear membrane. On the outer surface of cisternae there are ribosomes.

B) Vesicles

Vesicles are oval, membrane-bound vacuolar structures made up of lipoproteinous membrane. They are found associated with cisternae or freely scattered in cytoplasm.

C) Tubules

Tubules are wider, tubular and branched structures forming a reticular system along with cisternae and vesicles. They usually occur near the plasma membrane.

Types of Endoplasmic Reticulum

1. Smooth or agranular endoplasmic reticulum (SER)
2. Rough or granular endoplasmic reticulum (RER)

Difference Between RER and SER

Golgi Complex (Golgi Body)

Golgi body was discovered by Camillo Golgi (1898). It is also known as Golgi complex or Golgi apparatus.

Occurrence

It is absent in prokaryotic cells and present in eukaryotic cells except matured mammalian RBC, antherozoids of bryophytes and pteridophytes, and sieve tube cells of phloem of angiosperms, as listed in the source.

Shape

Its shape varies and depends upon the functional state of the cell. It is thus called a pleomorphic organelle.

Size

The size of Golgi body also varies. It is large in functional and secretory cells such as nerve cells, pancreatic cells, liver cells and germinal cells. The scan then notes it as small-sized in secretory cells such as muscle cells.

Number

The number of dictyosomes in plant cells is highly variable; the scan gives examples ranging from one in lower algal forms to about 25,000 dictyosomes per rhizoid of Chara.

Structure

The Golgi body is a complex structure made up of:

• Cisternae
• Vesicles
• Tubules
• Vacuoles

Cisternae

Cisternae are flat, elongated, sac-like structures made up of lipoproteinous membrane. They are single membrane-bound and placed one above another, remaining interconnected. They are slightly curved so as to give concave and convex faces. The ends of cisternae are slightly swollen.

Vesicles

Vesicles are minute, spherical, oval or rounded structures made up of lipoproteinous membrane. They are single membrane-bound and associated with cisternae. They usually occur towards the convex face of cisternae.

Tubules

The scan describes tubules as minute, spherical, oval or rounded structures made up of lipoproteinous membrane and single membrane-bound; it further states they are associated with cisternae and occur towards the swollen end of cisternae.

Vacuoles

Vacuoles are fluid-filled spacious structures made up of lipoproteinous membrane and are single membrane-bound. They usually lie towards the concave end of cisternae and are usually formed by detaching the swollen ends of cisternae. They are also known as Golgian vacuoles.

Functions of Golgi Body

• It secretes cell wall materials like cellulose, hemicelluloses, lignin, pectin and suberin.
• During cell division it takes part in formation of plasma membrane and cell plate.
• It helps in formation of primary lysosomes.
• It helps in formation of hormones in endocrine cells, e.g. thyroxin in thyroid gland cells.
• It participates in transformation of membranes and recycling of plasma membrane.
• It takes part in biosynthesis of glycoproteins.
• Golgi complex is also known as “dictyosome” having single cisternae, as written in the source.

Ribosomes

Ribosomes are nucleoprotein bodies because they consist of RNA and protein. Ribosomes were first isolated by Claude (1943) and were named by G. Palade (1955).

Occurrence

Ribosomes are found among prokaryotic and eukaryotic cells. They occur in cytoplasm, nucleoplasm, chromatin reticulum, matrix of mitochondria and plastids, and also on the membrane of endoplasmic reticulum.

Structure

Ribosomes are not surrounded by any unit membrane. They have naked structure and consist of a larger sub-unit and a smaller sub-unit. The two sub-units are held together with the help of Mg⁺⁺ ions. They remain in cytoplasm in inactive stage and are activated only at the time of protein synthesis.

Types

On the basis of sedimentation coefficient, ribosomes are of two basic types: 70S and 80S. The letter “S” refers to Svedberg unit.

• 70S type consists of larger 50S type and smaller 30S type.
• 80S type consists of larger 60S type and smaller 40S type.

Functions

• It is known as “protein factory” of cells.
• It is the site of protein synthesis.
• It can synthesize protein by receiving the message of DNA through messenger RNA (m-RNA).
• Free ribosomes produce enzymes for extracellular use.
• Ribosomes also store proteins temporarily.

Lysosomes

Lysosomes are electron microscopic, vesicular structures of cytoplasm involved in intracellular digestive activities. Lysosomes were discovered by Christian de Duve (1955) in liver cells of rat.

These are found among animal cells except matured mammalian RBC. They are usually absent in plant cells and prokaryotes, as written in the scan.

Structure

Lysosomes are single membrane-bound structures. The membrane is lipoproteinous. They contain about 40 different types of hydrolytic or digestive enzymes required for intracellular and extracellular digestion.

The enzymes are categorized into six categories:

1. Proteases
2. Lipases
3. Glycosidases
4. Nucleases
5. Sulphatases
6. Phosphatases

Forms of Lysosomes

Lysosomes are also known as polymorphic cell organelles as they occur in various forms:

1. Primary lysosomes: contain only hydrolytic enzymes in inactive form and are also known as storage granules.
2. Secondary lysosomes: primary lysosomes containing food vacuoles are known as secondary lysosomes.
3. Residual bodies (tertiary lysosomes): after digestion, the remains of digested materials remain as residue in lysosomes.
4. Autophagosomes or autolysosomes: during shortage of food, lysosomes fuse with cell organelles and digest the organelle by a process called autolysis or autophagy.

Functions of Lysosomes

• It is concerned with intra- and extracellular digestion of food materials.
• Lysosomes of WBC destroy foreign toxins or pathogens.
• It takes part in digestion of dead remains of the cells.
• Lysosomes are known as “suicidal bag” of the cell because during lack of food it may digest cell organelles so as to cause death of cells; such a process is called autolysis.

Cilia and Flagella

Cilia and flagella are microscopic hair- or thread-like motile structures present extracellularly but originating intracellularly from basal bodies. They help in movement, locomotion, feeding, circulation, etc.

Structure

Each flagellum or cilium consists of 9 peripheral tubules made up of protein called tubulin. There are two central tubules. The central tubules and peripheral tubules remain interconnected with each other. The bundle of tubules is known as axoneme or axial filament. These tubules are mainly enclosed by cell membrane.

Functions

• Cilia and flagella help in locomotion.
• Cilia also help in catching prey.
• Cilia of wind pipe prevent entry of dust particles into the lungs.
• Cilia of kidney nephron move nephric filtrate.

Difference Between Cilia and Flagella

Vacuoles

Vacuoles are non-cytoplasmic areas found within the cytoplasm. Vacuoles are found in both plant and animal cells. In plant cells, vacuoles are fewer in number and larger. In animal cells they are small or almost absent.

Structure

Vacuoles are surrounded by a single lipoproteinous membrane called tonoplast. Inner to the tonoplast there is fluid called cell sap. Cell sap contains water, minerals, sugars, amino acids, lipids, pigments and gases like O₂ and CO₂.

Types of Vacuoles

• The heading line includes the phrase “that stores food”.
• Food vacuoles → “that stores gases”.
• Gas vacuoles → “that stores water”.
• Contractile vacuoles → no additional description is clearly attached in the scan.
• Sap vacuoles → “that stores fluid”.

Functions of Vacuoles

• Vacuoles can store water and minerals; one word between “and” and “minerals” is obscured in the scan.
• They also store food as well as gases.
• They also store waste materials of the cells.
• Gas vacuoles provide buoyancy to organisms in water.
• Contractile vacuoles take part in osmoregulation of organisms.

Nucleus

Nucleus was discovered by Robert Brown in 1831 A.D. in orchid cells. The scan also states that it was first observed by Anton van Leeuwenhoek.

Occurrence

Nucleus is found in eukaryotic cells except matured mammalian RBC and sieve tube cells of phloem, etc. Among prokaryotes there is absence of true nucleus; such a nuclear region is known as nucleoid.

Shape

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

Size

The size of nucleus also varies widely. It depends upon the nucleo-cytoplasmic index and the number of chromosomes present in the nucleus. In a young cell nucleus occupies 25% of cell volume, while in a mature cell it occupies about 10% of cell volume.

Number

Usually a cell contains a single nucleus, but the number may vary in different cells. On the basis of number of nuclei, the scan recognizes:

1. Uninucleate (monokaryotic) cells: mostly the cells are monokaryotic.
2. Binucleate cells: cells containing two nuclei, e.g. Paramecium, liver cells and cartilage cells, as written in the scan.
3. Polynucleate cells: cells containing many nuclei, e.g. osteoblasts, Rhizopus and Vaucheria, as written in the scan.

Ultrastructure of Nucleus

Nucleus is composed of the following four components:

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

1) Nuclear Membrane

The nucleus remains enveloped by a thin transparent membrane known as nuclear membrane. There are two membranes: outer membrane and inner membrane. Both are lipoproteinous in nature.

The outer and inner nuclear membranes are separated by a perinuclear space filled with perinuclear fluid. It acts as a shock absorber. The outer membrane is rough due to presence of endoplasmic reticulum and ribosomes, while inner membrane is smooth.

Nuclear membranes are discontinuous in certain regions so as to form minute pores called nuclear pores. Exchange of materials between nucleus and cytoplasm takes place through nuclear pores.

2) Nuclear Sap or Nucleoplasm

The entire fluidal and granular mass surrounded by nuclear membrane is known as nucleoplasm. It is transparent, homogeneous, semi-solid mass. The chemical composition of nucleoplasm is similar to cytoplasm. There is also presence of RNA, ribosomes and nuclear proteins.

3) Chromatin Fibres

In nucleoplasm there are numerous fine thread-like, highly coiled, indistinguishable masses called chromatin reticulum. During cell division, the scan states that they become uncoiled and distinguishable as chromosomes. Chromosomes are chemically made up of DNA, RNA and proteins.

4) Nucleolus

Nucleoplasm contains a spherical, densely stained structure called nucleolus. It is a naked structure, i.e. without any limiting membrane. It is chemically composed of RNA and proteins. It is found to be attached with one chromosome.

Functions of Nucleolus

• It is considered as brain of the cell.
• It controls and regulates all the activities of the cell.
• It is the site for synthesis and storage of ribosomes.
• It contains genetic materials which can carry hereditary information from one generation to another.
• The nucleolus forms spindle fibre during cell division in plant cells, as written in the scan.

Difference Between Heterochromatin and Euchromatin

Chromosomes

Chromosomes are DNA-protein hereditary structures which are formed by condensation of chromatin fibres during cell division or reproduction. The size and number 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. The supplied scan then shows the following labelled forms: