Animal Cell – A Definitive Guide into Animal Cell

Animal Cell is basic structural and functional unit of life of organism belongs to kingdom Animalia.

Ultra Structure of Animal Cell

The ultra-structure of animal cell is shown in the figure below. The cell is the unit mass of protoplasm surrounded by a plasma membrane.

When the cell is observed by the compound microscope under high magnification, a prominent structure called the nucleus is seen in the center. The protoplasm surrounding the nucleus is known as the cytoplasm. In addition to the nucleus, the cell contains various organelles and inclusions in the cytoplasm.

The organelles are the living structures having definite functions whereas the inclusion is lifeless and without definite functions. The cell minus nucleus is known as cytosome.

The detailed structure of the cell and its components were studied only after the invention of the electron microscope by Max Knoll and E. Ruska in the year 1932.

When the cell is studied under an electron microscope, several cellular structures are visible with remarkable details. This is called the ultrastructure of the cell.

The electron microscope is a much more sophisticated and expensive instrument in comparison to a compound light microscope. The electron microscope utilizes a stream of high-speed electrons instead of light waves.

This microscope is based on the response of electrons to electromagnetic fields. The resolving power of the human eye is 100 microns.

What is Plasma Membrane?

Introduction of plasma membrane 

Definition of plasma membrane: Plasma Membrane forms the outer boundary of all living cells. 

Its structure has been studied under electron microscopy. 

It cannot be seen under a microscope. The term unit membrane was coined by Robertson (1959) to denote the membrane covering cell organelles of eukaryotic cells.

 Now the term biomembrane is used to denote all these membranes or membrane systems of eukaryotic cells. 

Basically, all types of biomembranes have a similar structure, chemical composition and function. 

Structure of Plasma membrane

Danielli and Davson (1935) proposed a triamellar structure of plasma membrane. According to them, the plasma membranes consist of three layers of molecules. 

The outermost layer is formed by protein molecules which are about 20 Å and an inner layer of 20 Å thickness is made up of protein molecules.

In between these two protein layers lies a bio-molecular layer of phospholipids having 35 Å of thickness. 

Roderick Capaldi (1974) has indicated the middle layer of lipid is a bio-molecular layer. The lipid molecules are so arranged that they have hydrophilic ends directed towards the outer surface and their hydrophobic ends are directed towards the inner side.

Various models of plasma membrane

From time to time many attempts have been made to explain the molecular structure of plasma membranes. The following are important ones.

Trilamellar Model of Plasma Membrane:

It was presented by Danielli and Davson (1935).

Its diagram has been given below:

Micellar Model:

According to this concept, the plasma membrane is made up of globular micelles or subunits.

Fluid Mosaic Model: 

According to Singer and Nicholson (1972), the membrane is a two-dimensional solution of oriented globular proteins and lipids. 

The lipid molecules form a double continuous layer representing a mobile fluid. It contains intrinsic proteins appearing as a mosaic work within the fluid lipid layer.

Lipid molecules are phospholipids with outer polar hydrophilic ends and inner polar hydrophobic ends. 

On the outer side, few lipids have attached oligosaccharides (Sugar) forming glycolipids. 

These proteins are of two types depending on their position.

  1. Extrinsic: That is, lying on the outer or inner surface.
  2. Intrinsic: Embedded partially or whole in the lipid layer. The proteins projecting on the outer surface may be linked with sugar molecules forming glycoproteins. 

Specialization of plasma membrane

In specialized cells the plasma membrane may have the following modification:

  1. Microvilli: In some cells e.g, the mucosa cells of ileum have microscopic folding on the free inner surface of the cells. These microvilli increase the absorptive surface of the cell.
  2. Interdigitation: Some cells have finger-like projections of the plasma membrane by which they are attached with the corresponding interdigitation of adjacent cells. 
  3. Desmosome: Adjacent cells adhere to one another by means of specialized surfaces on plasma membranes. These are termed desmosome

Properties of Plasma Membrane

Permeability of plasma membrane: It is their elastic and semi-permeable membrane. It forms an outer limiting membrane. It regulates the inflow and outflow of molecules, exercising its control through these processes. 

  1. Passive transport and diffusion: It involves the movement of molecules or ions from higher concentration towards lower concentration. Thus, the movement of molecules or ions along the pressure of gradient is called passive diffusion. No energy is required for this movement.
  2. Active transport: It is the process in which molecules or ions move against the concentration gradient, that is the molecules move from lower concentration to higher concentration. Here energy is required in the form of ATP.
  3. Osmosis: It is the movement of water molecules or solvent molecules through a semipermeable membrane.
  4. Pinocytosis: Certain substances of high molecular weight, such as protein etc. which can not diffuse through the plasma membrane, are taken into the cytoplasm by the process of pinocytosis. In this process, the plasma membrane sinks in the form of small vacuoles. The vacuoles are known as pinocytic vesicles or pinosomes; the process is called pinocytosis. These vacuoles are filled with foreign material along with water and disappear in the cytoplasm after breaking off from the plasma membrane.

With electron microscopes, many small ridges are seen in the plasma membranes of some cells, especially absorptive cells. These structures are called microvilli. It is probable that plasma membrane forms the cells’ organelles by sinking at several places.

  1. Phagocytosis: Under this process, the plasma membrane ingests solid particles, e.g, food.

Functions of plasma membrane

These are the functions of plasma membrane:

  1. It provides an external shape to the cell.
  2. Protects the cytoplasm from the exterior
  3. Regulates the flow of material to and from the cell.
  4. It checks the cytoplasm from flowing out and saves it from external damage.
  5. The chemical exchange between the cytoplasm and outside materials takes place through it by diffusion, osmosis, pinocytosis and permeability. 

Cytoplasm

Region between cell membrane and nuclear membrane is called cytoplasm.

Cyto = cell, plasm = fluid. Fluid of the cell is called cytoplasm.

Protoplasm forms the ground substance on the matrix of the cell and is enclosed within the cell membrane. 

Generally, protoplasm and cytoplasm are taken as one and the same but the two are really different. Except the cell membrane whole of the cell is called protoplasm while cytoplasm is the fluid that is present between the cell membrane and nucleus.

It is colorless, transparent and viscous.

It is composed of protein, lipids, carbohydrate, water and some inorganic compounds. Electron microscopy reveals areas of different density in cytoplasm.

The outermost peripheral clear part is called ectoplasm and the inner granular part is known as endoplasm. Cytoplasm is referred to as holoplasm in the basis of homogeneity.

The macromolecular structures found in cytoplasm are divided into two groups on the basis of their living or nonliving nature. The living structures are known as organelles, whereas the non-living substances are called inclusions and metaplasts. 

Components of Cytoplasm

  1. Cytosol: It is the liquid portion of cell.
  2. Cytoskeleton with motor protein: Microtubules, Microfilament and intermediate filament present
  3. Cytoplasmic Inclusion: (Biochemicals)- Protein, Pigments, Lipids, Carbohydrates etc.
  4. Organelles (little organs inside cell)- Mitochondria, ER, Golgi Bodies etc.

Organelles of Cytoplasm

The followings are cytoplasmic organelles:

  1. Endoplasmic reticulum
  2. Mitochondria
  3. Golgi Complex
  4. Lysosome
  5. Ribosomes
  6. Nucleus and Nucleolus
  7. Chromosomes
  8. DNA and RNA

We can divide these cell organelles into three types:

  1. Living cell organelles:
    1. Endoplasmic Reticulum
    2. Mitochondria
    3. Golgi Complex
    4. Lysosome
    5. Centrosome
    6. Ribosomes
    7. Plastids
    8. Cilia and Flagella
  2. Non-living cell inclusions
    1. Vacuoles
    2. Crystals and oil droplets
  3. Other Sub-cellular Structures
    1. Microsomes

Function of Cytoplasm

  1. It is the site of metabolic activities. For example, Glycolysis is done here.
  2. Storehouse for biochemicals. For example Amino acids, Sugar, Nucleotide
  3. Because of the cytoskeleton, it maintains cell shape and helps in the movement of molecules.
  4. It provides a suitable environment for organelles to function.
  5. Most proteins are synthesized in the cytoplasm.

Let us know all the cell organelles one-by-one.

A. LIVING CELL ORGANELLES

1. Endoplasmic Reticulum

Electron microscope shows that there is an irregular network of very fine double-walled canals in

cytoplasm. The presence of this network was established by K.R. Porter in 1953.

These structures are called endoplasmic reticulum or ergastoplasm. They are not present in mature erythrocytes because of the absence of nucleus in them. The endoplasmic reticulum is part of the plasma membrane with which they are connected.

According to its nature, endoplasmic reticulum is of two types. Those on whose surface ribosomes are present are called rough endoplasmic reticulum while those without them are known as smooth endoplasmic reticulum.

The endoplasmic reticulum is a system of tubules, cisternae and vesicles forming an interconnected reticulum like structure.

Functions of Endoplasmic Reticulum

  • It forms the supporting framework of the cell and is the device of distribution of various enzymes.
  • It helps in transportation and circulation of the substances formed by synthesis between nucleus and cytoplasm and cytoplasm to outside liquid.
  • Some enzymes filled in it take part in the synthesis of fats, steroids, cholesterol etc.
  • Rough endoplasmic reticulum provides surface for the attachment of ribosome and helps in protein synthesis.
  • It forms the nuclear membrane of newly formed cells.

2. Mitochondria

They are commonly known as power houses of the cell. They are minute structures present in cytoplasm and supply energy to the cell. Mitochondria may be filamentous, granular, club-shaped or rod-like vesicles.

Their size varies from 0.5 to 1.0 micron and number may be around 1,50,000 in a cell.

Their number and shape are definite for a particular cell, but vary from cell to cell.

Brief History of Mitochondria

Flemming in 1882 observed thread-like structures in nerve cells fixed in acetic acid and called them Filia. The name mitochondria was first given by Benda in 1897 and Porter and Palade studied their ultrastructure.

Ultrastructure of Mitochondria

Each mitochondrion is a double-walled structure. Each of its walls is about 60 Å thick and separated from the other by 60 Å to 80 Å wide intermembranous space or outer chamber. This space is filled with a fluid which is rich in enzymes.

The outer wall is elastic and semipermeable while the inner extends into inner chamber filled with matrix in the form of irregular ridges which are called cristae.

Minute granules are dispersed on the inner wall and cristae. These granules are elementary units of mitochondria through which chemical reactions take place. Further studies showed that the granules present on the inner wall are made up of three parts:

  1. spherical head,
  2. cylindrical basal part,
  3. cylindrical stalk.

Chemical Composition of Mitochondria

Both the inner and outer walls of mitochondria have molecular

organisation like plasma membrane. Besides the granules, the walls are mainly formed of protein and lipids. The dry weight of protein is about 4/5 parts while lipids are about 1/5 parts in the

phospholipids. Matrix contains many enzymes and mitochondrial DNA.

Functions of Mitochondria

  • Mitochondria are the respiratory centers of the cell.
  • The energy is stored in them in the form of ATP and is liberated during respiration.
  • According to Horming’s view, proteolytic enzymes are present in them to control lytic and synthetic activities.
  • Mitochondria take part in yolk formation in some eggs and also form the middle piece in mature sperms.
  • According to Bensley and Hoerr (1935) mitochondria are associated with fat metabolism. Thus, they are called chemical factories or powerhouses of the cell.
  • Mitochondria have mitochondrial DNA and take part in protein synthesis and contain genetic information for mitochondrial synthesis.

3. Golgi Bodies or Golgi Apparatus

In 1898 Italian physician and histologist Camillo Golgi discovered it and named it as Golgi body which later on became known as Golgi complex.

This Golgi complex is found near the nucleus and formed of groups of thin-walled plates called cisternae, minute vesicles and vacuoles.

Since there is a large amount of lipids or fat in Golgi complex, they are also known as lipochondria. In plants these structures are called dictyosomes.

They are made up of lipoproteins, phosphates, phospholipids. Their wall is similar to plasma membrane in structure.

Functions of Golgi bodies

  • Condensation and packing of substances takes place in Golgi complex.
  • Since they are found in large numbers in secretory cells, they are associated with the secretory activity.
  • Formation of condensation membranes which absorb various substances used in synthetic functions.
  • Formation of acrosome of spermatozoa during spermato-genesis.
  • The continuous changes taking place in them are associated with the development of mammalian germ cells.
  • Secretion of enzymes.

4. Lysosomes

Christian de Duve in 1955 observed groups of round membrane-bound structures and called them lysosomes. Their diameter varies from 0.2 to 0.8 microns. Lysosomes are of different shapes.

They are small bag-like structures with a single layered membrane formed of lipoprotein. Different types of hydrolytic enzymes are stored in lysosome for breaking down and digestion of fats, carbohydrates, nucleic acids and proteins.

On the basis of their phagocytic activity, they are of the following forms:

  • Prelysosomes
  • Primary lysosomes
  • Autophagosomes
  • Secondary lysosomes
  • Post lysosomes

Functions of Lysosome

  • They are helpful in digestion of engulfed bacteria and cellular debris.
  • Lysosome saves the cells from autolysis.
  • It secretes hydrolytic enzymes which break larger molecules of protein so that they can be oxidised easily by mitochondria.
  • Their enzymes can come out to destroy and digest the foreign substances around them.
  • They are mainly responsible for intracellular digestion because enzymes formed by ribosomal activities are stored in them.
  • Lysosome help in taking out the dead cells from the tissues. In these cells the surface membrane of lysosomes bursts and is digested by the enzymes.

5. Ribosomes

In cytoplasm some dense oblique particles are present. In 1955, Palade first observed these particles which are known as Eukaryotic ribosomes or Ribonucleo particles-RNA.

The electron microscope has revealed that these particles are round and flat structures disposed around the endoplasmic reticulum and also in the cytoplasm. They measure 150 to 220 Å.

The ribosomes of all eukaryotic cells are made up of 60 S and 40 S subunits which are held together by bonds. 60 S unit has an RNA molecule and its molecular weight is double that of 40. Protein is synthesized only by larger units. Ribosomes are often seen in clusters or arranged in rows.

These clusters or rows of ribosomes are called polyribosomes. Ribosomes have 40 to 60% RNA and the rest is protein. The central core is formed of RNA which is surrounded by proteins.

Protein and RNA are held together by electrostatic force and bonds. Their function is to provide site for protein synthesis. They are often called as protein factories.

6. Centrosome

The centrosome is a rounded body situated near the nucleus. It consists of one or two spherical bodies called centrioles which remain bounded by a zone of clear cytoplasm called centrosphere.

The centrioles are visible only during cell division. Electron microscopy reveals that a centriole is in the form of a cylinder measuring 150 mμ, in diameter and 300 to 500 mμ in length.

It is made up of nine rod-like filaments (called triplet fibers) arranged in a ring. Each rod-like filament is a triplet composed of three microtubules.

Each microtubule is about 150-200 Å in diameter and embedded in the matrix. The cylinder is closed at one end and open at the other.

Centrioles are generally double, one lying at right angles to the other. Centrosome has got the power of self-replication.

Function of Centrosome

  1. The centrosome and the centrioles play an important role in initiating and regulating cell division and in the spindle formation during cell division.
  2. They help in determining the pores of mitotic apparatus during cell division.

7. Plastids

The plastids are common in plants cell but occurs in some animal cell like Euglena.

The size of the plastids vary from about 4-5 micra in length to 1-3 micra in thickness.

The plastids arise from the per-exiting ones and are passed on to the new cells during cell division.

Plastids are of three different kinds, namely chloroplast, chromoplast and leucoplast.

The chloroplasts help in the synthesis of carbohydrates.

The leucoplast is the storage center of starch.

The chromoplast imparts varius colors.

8. Cilia and Flagella

Many cells have hair or whip-like processes which are capable of lashing movement.

The smaller hair-like processes are termed as cilia and the longer thread-like structure are known as flagella.

Both cilia and flagella are similar in structure. Each consisting of two filaments in the center and nine filaments arranged in aring around the two central filaments.

They arise from the basal bodies in the cell and are capable of duplicating themselves.

Generally, cilia and flagella are found in the protozoans and they help in the process of locomotion.

B. NON-LIVING CELL INCLUSIONS

The cell inclusions are lifeless contents of the cell and they include the vacuoles, crystals, oil droplets and other cell inclusions.

  1. Vacuoles: The vacuoles are fluid-filled spaces found inside the cytoplasm. They contain water, inorganic salts and various organic materials. The vacuoles help in regulating the amount of water in the cell, store various substances and help in the digestion of food in the unicellular organisms.
  2. Crystals and Oil Droplets: Crystals f food, waste materials and droplets of fat are also found in the cytoplasm of the cell. The oil droplets serve as reserve supply of energy rich fuel. Other cell inclusion include crystals of inorganic salts, grains of starch and glycogen, most of which are surplus food stuffs meant for storage.

C. SUB-CELLULAR STRUCTURE

Among sub-cellular structure, micro-some are notable. Microsomes are also called spherosomes. They are not cell organelles. They consists of pieces of endoplasmic reticulum in the forms of tubules, vesicles and cisternae.

Ribosomes are attached to their walls. Other ruptured plasma membranes, Golgi membranes, and cell fragments are also fixed. They possess a high lip content also along with 50-6- of the RNA of the cell.

Spherosomes are rich in hydrolytic enzymes and capable of synthesizing oils and fats.

What is Nucleus?

The nucleus is generally located in the center of the cell and governs all the vital activities. For this reason, it is known as the “control room or control chamber” of the cell. The position, shape, size and the number of nucleus are variable.

Normally one nucleus is found in a cell, but sometimes two or even more nuclei may be present. If the number of nuclei is two or more in a cell, the latter is said to be syncitial. The shape of the nucleus may be rounded, spherical, elongated, lobed, or ribbon-shaped.

Chemical composition of a nucleus

Details of the chemistry of the nucleus show that it contains basic proteins such as protamines and histones and acid proteins such as nuclear phosphoproteins.

Certain complex proteins, including enzymes such as nucleoside phosphorylase, adenosine diaminase, and nucleic acids (DNA and RNA) are present in the nucleus.

Lipids are also present and comprise about 3 to 10% of the nucleus. They occur mainly as lipoproteins and phospholipids.

In addition to the above, certain compounds of phosphorus, potassium, sodium, calcium, and magnesium are also present in the nucleus.

General functions of the nucleus

  • It acts as controlling centre of the cell.
  • During cell division, it takes an active part, i.e., it divides first and then is followed by cytoplasmic division.
  • It is directly connected with sexual reproduction i.e., in gamete formation and fertilization.
  • As the nucleus contains chromosomes, it determines the hereditary characters of the animal.
  • It is mainly concerned with the synthesis of various types of enzymes that catalyze many metabolic reactions.

Structurally, the nucleus is composed of four different parts, namely (a) Nuclear membrane (b) Nucleoplasm (c) Nucleolus, and (d) Chromosomes.

a. Nuclear Membrane:

Nuclear membrane also known as Karyotheca or nuclear envelope.

The nucleus is surrounded by a thin, elastic and semipermeable nuclear membrane which acts as the barrier between the nucleoplasm and the cytoplasm.

The nuclear membrane is formed of two membranes that are separated from each other by a perinuclear space.

Each membrane is about 90 Å in thickness and the width of the perinuclear space is about 100 to 150 Å.

The nuclear membrane is perforated at different places by roughly circular or polygonal nuclear pores, each pores measuring about 400-600 Å in diameter.

The nuclear pores permit an easy passage of materials to and from the cytoplasm. The outer membrane of the nuclear membrane is continuous with the endoplasmic reticulum so that the nuclear membrane joins with the plasma membrane by endoplasmic reticulum at certain places.

The outer surface of the nuclear membrane may even contain ribosome like granules while the inner surface is smooth.

The chemical composition of the nuclear membrane shows that it is formed of bio-molecular layers of lipid and protein molecules like plasma membrane.

Function:

The nuclear membrane protects the nucleus and also regulates the passage of substances entering and leaving the nucleus.

b. Nucleoplasm:

The protoplasm of the nucleus is known as the nucleoplasm or karyolymp or nuclear sap.

It is homogeneous, granular and denser than the cytoplasm. It contains more protein and phosphorous in comparison to cytoplasm.

The nucleoplasm contains nucleic acids and a number of hydrolytic enzymes like ribonuclease, alkaline phosphatase and dipeptidase.

c. Nucleolus:

The nucleus (pl: nucleoli) is seen as a spherical, ball-like structure inside the nucleus.

Generally, one nucleolus is seen in the nucleus of a cell, but in some cells the number may be more than one.

The nucleolus becomes prominent particularly at the time of high rate of protein synthesis in a cell.

Vacuoles are also frequently seen in nucleoli. There is no limiting membrane surrounding the nucleolus.

Chemically, it is composed of about 5 to 10% RNA, the rest being protein.

The nucleolus helps in the synthesis of protein and performs an important role in the synthesis of ribsomal RNA and in the transfer of genetic information.

During cell division, the nucleolus disappears altogether.

Functions of Nucleolus

  1. It has an important role during cell division.
  2. It synthesizes RNA and stores it.
  3. It transmits hereditary information from one cell generation to another cell generation.
  4. It coordinates the activities between cytoplasm and the nucleus.

d. Chromosome

During the interphase or resting stage (ie, a stage before cell division) chromosomes are represented by fine knotted thread-like structures, called the chromatin materials. These are found in the nucleoplasm.

They exhibit little coiling and appear as twisted filaments forming a sort of network or reticulum. Sometimes chromatin network is not visible but only chromatin granules can be seen instead.

Each chromatin filament measures 20 Å to 40 Å in diameter. The chromatin consists of DNA (Deoxyribonucleic acid), RNA (Ribonucleic acid), histones, and non-histone proteins.

During nuclear division, the chromatin network condenses and coils, so that the long threads gradually become short and thick. These rod-like structures are then called chromosomes.

The term chromosome (Gk: Chroma- color, Soma-body) was given by Waldeyer in 1888.

The chromosomes have got the power of self-duplication. Generally, the size of a chromosome is relatively constant for a particular species.

The size varies from 0.1 to 30 μ in length and from 0.2 to 2 μ, in diameter. The number of chromosomes is constant for a particular species of animal. Thus in the frog (Rana spp.) the chromosome number is 26, in the toad (Bufo spp) the number is 22 and in man, it is 46.

Structure of chromosome

During cell division, chromosomes are visible clearly and their structure can be studied well as a definite organelle. Chromosomes appear long and thin or short and thick depending on their coiling and condensation.

Each chromosome is long and consists of two chromatids. The chromatids are joined together at the centromere.

The chromatids along with centromere are embedded in the viscous fluid, called matrix. The matrix is bounded by an outer sheath or pellicle

Each chromosome possesses two arms which are demarcated due to the presence of centromere. Each chromatid is composed of two longitudinal coiled filaments called chromonemata.

Each chromonema appears like beaded string due to the presence of several small bead-like structures along the length.

These beaded structures are termed chromomeres which contain genes. Gene is the smallest unit of the chromosome and is the unit of inheritance.

Chemical composition of chromosome:

Chromosomes are composed of about 40% DNA, 50% histones and other basic proteins, 1.5 RNA, and 8.5% acidic proteins. More amounts of DNA are localized in the chromosomes of the nucleus.

Types of Chromosome:

Chromosomes are of four types according to their shape and position of the centromere. They are as follows:

  1. Telocentric: In this type, centromere is present at the end of the chromosome.
  2. Acrocentric: Rod-like chromosome having a very large arm and a too small arm. Centromere is almost terminal in position.
  3. Sub-metacentric: Here arms are unequal and chromosome resembles L in shape.
  4. Metacentric: Chromosome are V shaped and have almost equal arms.

The function of Chromosomes:

  1. Chromosomes carry genetic information about the organism. That is, they carry the genes which are responsible for the inheritance of characters from generation to generation.
  2. Chromosomes control the physiology of an organism with the help of genes.

What is Nuclear reticulum?

The dark staining network, which can readily be stained with basic dyes is called nuclear reticulum. The threads of this reticulum are made up of chromatin.

This chromatin network can be seen in the interphase nuclear stage. During interphase stage, chromosomes exhibit minimum degree of condensation or coiling.

At the time of cell division (Metaphase) the chromosomes appear as cylindrical bodies. A primary constriction has a centromere. One or more secondary constrictions known as nucleolar organizer region, a satellite with a rounded body, the telomere.

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