What is an Ecosystem?

An ecosystem includes all of the living things (plants, animals and organisms) in a given area, interacting with each other, and also with their non-living environments (weather, earth, sun, soil, climate, atmosphere). 
In an ecosystem, each organism has its' own niche, or role to play.
Consider a small puddle at the back of your home. In it, you may find all sorts of living things, from microorganisms, to insects and plants. These may depend on non-living things like water, sunlight, turbulence in the puddle, temperature, atmospheric pressure and even nutrients in the water for life.
Anytime a ‘stranger’ (living thing(s) or external factor such as rise in temperature) is introduced to an ecosystem, it can be disastrous to that ecosystem. This is because the new organism (or factor) can distort the natural balance of the interaction and potentially harm or destroy the ecosystem.

Cell Cycle Control System

Regulation of the cell cycle is a crucial process to the survival of the cell. Cell regulation includes the detection and repair of genetic damage and also prevention of uncontrolled cell division. The molecular events that control the cell cycle occurs in a sequential fashion and is impossible to reverse the cycle. 

The major event in the cell cycle is the replication of DNA, which occurs in the S phase, and separation of the duplicated chromosomes and the constituents of the cell which occurs in the M phase. Regulation and the initiation and completion of S and M phases ensures the genetic information and other cellular components are duplicated and divided equally between the daughter cells with each cycle.  

Cell cycle checkpoints are the regulatory pathways that control the order and the timing of the transitions of the cell cycle. The checkpoints also ensures that critical events such as replication of DNA and segregation of chromosomes are completed before the cell progresses further through the cycle. The cell-cycle checkpoints respond to the cellular damage by slowing the cycle to provide time for repair and it also induces transcription of genes that facilitate the repair. The loss of the checkpoints results in instability of chromosomes and it can result in the transformation of normal cells into cancer cells.

 

Proteins like the cyclin dpendent kinase, kinases and cyclins control the switches for the cell cycle causing the cell to move from G1 to S or G2 to M.Regulatory molecules are of two classes cyclins and cyclin-dependent kinases. The genes encoding cyclins and CDKs are conserved among all eukaryotes. Proteins like the p27 and p53 prevent the cells from passing check points. They are also knwon as protein suppressors. P27 protein binds to cyclin and CDK blocking the entry into the S phase. P53 protein blocks the cell cycle at the M checkpoint if the DNA is damaged. P53 mutation is the most frequent mutation found in cancer cells. P53 fucntions by blocking the cycle giving the cell time to repair its DNA. If there is severe damage in the DNA the protein causes the cell to apoptosis.

Phases of Cell Cycle

There are four distinct phases in the cell cycle - G₁ phase, S phase (synthesis), G₂ phase and M phase. The G₁, S phase and G₂ phase together are known as interphase. The M phase or the mitotic phase is of two processes, one where the chromosomes of the cell is divided between two sister cells and the other is cytokinesis where the cell's cytoplasm divides into half forming two distinct cells.

 

The cells that have stopped dividing temporarily or reversibly are said to be in the state of quiescence called G₀ phase. Progression from phase to another depends on the proper completion of the previous one. After the process of cell division, the daughter cells begin the interphase of the new cycle. The stages of interphase are not morphologically distinguishable, yet each phase has a distinct biochemical process that prepares the cell for initiation of cell division. 

G₀ Phase:

Sometimes the cells in the quiescent and senescent cells are referred to as post mitotic. The cells which are indivisible in  multicellular eukaryotes generally enter the quiescent G₀ state from G_1. They may remain in the quiescent state for long periods of time. This state can be for indefinite like in neurons and is very common in cells that are fully differentiated. Death of the cells in response to damage of DNA or degradation would make the progeny of the cells nonviable. Some cells like the cells of liver and kidney enter the G₀ phase semi-permanently. 

Interphase:

Earlier to to the cell division process, the cell needs to accumulate nutrients. During the interphase all the preparations are done. In interphase of a newly formed cell, a series of changes takes place in the cell and the nucleus, before it is capable of division. This phase is also known as intermitosis. Earlier this stage was known as resting stage because no remarkable activiyt realated to cell division takes place here. Interphase proceeds in a series of three stages, G₁,S, and G₂. Division of cell operates in a cycle, hence the interphase of the cycle is preceded by the previous cycle of M phase and cytokinesis. Interphase is also called the preparatory phase. In the interphase stage the division of nucleus and cytosol does not occur. The cell prepares for division. This is a stage between the end of mitosis and start of the next phase. Many events occur in this stage and most significant event that occurs is the replication of genetic material.   

G₁ Phase:

This is the first phase in the interphase. From the end of the previous M phase till the beginning of the DNA synthesis in the next cycle is called the G₁ phase, here G indicates gap. This phase is also called growth phase. In this phase the biosynthetic activities of the cell, which shows a considerable slow down during the M phase of resumes it activities at a high rate. In this phase there is a marked production of proteins by the use of 20 amino acids. Also enzymes that are required in S phase needed during DNA replication. The duration of the G₁ phase is highly variable, also among different cells of the same species. The G₁ phase is under the control of the p53 gene. 

S  phase:

The start of the S phase is when the DNA replication commences. When the phase completes all the chromosomes have been replicated. Each chromosome has two sister chromatids. During this phase, the amount of DNA in the cell is doubled but the ploidy of the cell remains unchanged. In this phase the synthesis is completed as soon as possible as the exposed base pairs are sensitive to external factors like drugs or mutagens. 

G₂ phase: 

It is again the gap phase which happens during the gap between the DNA synthesis and mitosis. During this phase the cell will continue to grow. The G₂ checkpoint mechanism controls to ensure that the cell is ready to enter the M (mitosis) phase and divides. 

Mitosis or M phase:

The M phase consists of karyokinesis - nuclear division. The M phase is of several distinct phases, known as 

Prophase,

Metaphase,

Anaphase,

Telophase,

Cytokinesis.

The process of mitosis takes place only in eukaryotes, the chromosomes in the nucleus of the cell into two identical nucleus. This stage is followed by cytokinesis. In cytokinesis the cell, nuclei, cytoplasm, organelles and cell membrane is divided into two equal shares. Mitosis and cytokinesis together make the mitotic (M) phase of the cell cycle. The mother cell divides into two daughter cells that are genetically identical to each other. Mitosis is seen only in eukaryotic cells, but it occurs in different ways in different species. The process of mitosis is a sequence of events divided into three stages - prophase, metaphase, anaphase and telophase. During this process of mitosis the chromosome pairs condense and they attach to fibres that pull sister chromatids to opposite sides of the cell. The cell with the process of cytokinesis produces two identical daughter cells. 

What is Cell Cycle?

The cell cycle is the sequence of events that takes place in cells. It leads to cell division and replication (duplication). In prokaryotes, the cell cycle is through the process of binary fission. In eukaryotes, the cell cycle can be divided into two phases - interphase and mitotic phase. Interphase is the stage during which the cell prepares, grows and accumulates nutrients needed for mitosis and also duplicates the DNA.  In the mitotic phase, the cell splits itself into two distinct cells known as 'daughter cells' and the final phase is cytokinesis, where the newly formed cells are completely divided. The cycle of cell division is a vital process by which a fertilized egg that is single-celled develops into a mature organism. It is by this process by which skin, hair, blood cells and some internal organs are renewed. 

Cell Cycle

All cells arise from pre-existing cells by division. Every living cell today is said to be descended from a single ancestral cells that lived 3-4 billion years ago. In the vast time period, evolution of cells and organisms was seen, thus continuing the success of life on Earth. The genetic information has been preserved through cell division.

Reproduction of cells is fundamental to the development and function of all life. In single-celled organisms, cell division creates are a entire new organism. In multicellular organisms cell division transform a single founder cell into different communities of cells that constitute the tissues and organs. In adult organisms cell division replaces cells those die from natural causes or those that are lost to environmental changes.

Prokaryotic Cell Structure

Prokaryotic Cell Structure

They are the first organisms to be present on our planet earth. Organisms, with this cell type are known by the term prokaryotic organisms (or) prokaryotes. Bacteria, blue green algae and E.coli are few examples of this category. Prokaryotic cells are single-celled organisms, with the absence of nucleus and comprises of capsule, cell wall, cell membrane, cytoplasm, nucleiod, ribosome, plasmids, pili and flagella.

Prokaryotic Cell General Features

• The size of a cell ranges from 1-10 microns. Few prokaryotic cells vary in their size.
• They are single-celled (unicellular), which forms a colony or filamentous.
• The shape of the cell includes spherical, rod and flat shaped organisms.
• Mode of nutrients-- few organisms are photosynthetic (performing food with the help of sunlight), feed on living things and dead things.
• They reproduce asexually by the process called binary fission, transformation, conjugation, transduction.

Structure and Functions of a Prokaryotic Cell

Capsule: It is the slimy outer coating of the cell wall. It is composed of the polypeptide. The main function of the capsule is to protect the cell from getting dry and also helps in protecting cells from external pressures.

Cell wall: It is the tougher and a rigid structure, which provides the shape and protects the internal organelles of a cell. It is the middle layer, which is present in between the capsule and cell membrane.

Cell membrane: It is the inner delicate structure, which plays a vital role in regulating the entry and exits of materials in the cell. It acts a permeable membrane and separates the cell from its environment. It is of about 5-10nm in thickness, which helps in the secretion of proteins and elimination of waste products. It is also called by a name plasma membrane.

Cytoplasm: It is the liquid membrane, which is present in between the cell membrane and nucleiod. It plays a vital role in storing all types of materials, which are required for an organism to sustain the life.

Nucleiod: It is the cytoplasm region containing genetic material. The DNA of a prokaryotic organism is one big loop or a circular, which is located inside the nucleiod. It plays a vital role in cell division.

Ribosome: It  comprises of both RNA and proteins. It helps in protein synthesis in the cell. They are smallest membrane present inside the cytoplasm.

Plasmids: They are smallest membrane of a cell with double stranded DNA. Plasmids are rarely present in prokaryotic organisms. The main role of plasmids is it helps in DNA exchanging between the bacterial cells.

Pilli: It is the thinnest membrane of a prokaryotic cell. They are composed of protein complex called pilin and are mainly involved in sticking to the objects especially during sexual reproduction.

Flagella: It is the helical shaped membrane, whose sizes ranges from 19-20nm in diameter and plays a vital role in motility of an organism from one place to another place. It also helps in swimming, gliding, spinning and rotating both in clockwise and anti clockwise directions.

Eukaryotic Cell Structure

They are the cells with the presence of true nucleus. Organisms, with this cell type are known by the term eukaryotic organisms (or) eukaryotes. Animals, plants and other organisms excluding bacteria, blue green algae and E.coli have been grouped into this category. Eukaryotic cells are more complex than prokaryotic cells. These organisms have membrane bound nucleus with many cell organelles to perform several cellular functions within the system.

Eukaryotic Cell General Features

• The size of a eukaryotic ce

• ll ranges from 10-100 microns. Few eukaryotic cells vary in their size.
• They are large, advanced, multicellular and have membrane bound organelles.
• They reproduce both by sexually and by asexually.
• Mode of nutrients - Autotrophic and heterotrophic.
• Kingdom protozoa, algae, fungi, Plantae and Animalia are organisms with eukaryotic cell.

Structure and Functions of a Eukaryotic Cell

Plasma membrane: They are semi permeable membrane that acts as a boundary of a cell, which protects and separates the cell from the external environment.

Nucleus: It is surrounded by double layered of Phospho lipid bilayer called as nuclear envelope.  

They are the storehouse for the cell’s genetic materials in the form of DNA and store all the necessary information, which are required for a cell to control all types of activities. 

Nuclear membrane: It is the double membrane layer that surrounds the nucleus and it plays a role of entry and exits of materials within the nucleus.

Nucleolus: It is the non membrane bound organelles, which is present within the nucleus and is mainly involved in controlling all types of cellular activities including cellular reproduction.

Mitochondria: They are the double smooth membrane, which are present in all eukaryotic cells. They are the powerhouse of the cell. It plays a vital role in the synthesis of ATP and converts glucose to ATP.

Endoplasmic reticulum: They are the double membrane organelle, which divides the cell into compartments. It is connected to the nuclear membrane of the cell. It plays a vital role in protein synthesis, biosynthesis of lipids and steroids, stores and regulates calcium and metabolism of carbohydrates.  Endoplasmic reticulum is of two types – rough and smooth Endoplasmic reticulum.

Ribosome: It is present in the cytoplasm. They are the site for cell’s protein synthesis, which are composed of ribosomal RNA and proteins.

Golgi Bodies: It is the flattened membrane, which are mainly used to store the substances made by the cell. This membrane also helps in preserving, transporting materials within the cell. Hence it is also called as the post office of a cell.

Lysosomes: They are the membrane bound organelles, which contains digestive enzymes to break down macromolecules. Lysosome plays a vital role in protecting cell by engulfing or destroying foreign bodies entering the cell.

Cytoplasm: They are the jelly types of organelles, which are present in the inner region of a cell. It plays a vital role in keeping a cell in a stable and keeps the cell organelles separate from each other.

Chromosomes: The rod shaped structures, which are composed of proteins and DNA. Chromosomes also play a vital role in determining a sex of an individual. All human cells contain 46 numbers of chromosomes.

Plant Cell Structure

A plant cells are eukaryotic cells, with the presence of true nucleus, multicellular large and advanced membrane bound organelles. These plant cells are quite different from animal cells like in shape and other few organelles which are only found in animal cells but are absent in plant cells. Based on structure and functions, plant cells comprise of:

Cell wall: It is the outer layer of a plant cell, which helps a providing the shape and strength to the entire plant. A cell wall is composed of cellulose that protects and supports the plant to grow.

Cell membrane: It is a biological membrane that separates living cell organelles from non living structures. This membrane plays a vital role in helping a cell to communicate with the external environment and also in transporting proteins and other molecules throughout the cell.

Chloroplasts: They are green coloured oval shaped double membrane organelles, which are the sites of photosynthesis. The green colour pigment (chlorophyll) present in the leaves helps plants in absorbing solar energy to prepare food.

Cytoplasm: A jelly types double membrane organelles, which are present in the inner region of a cell. It helps by keeping a cell in stable and protects the cell organelles by separating them from each other.

Lysosomes: They are single membrane round organelles, which helps in digesting proteins, fats and carbohydrates. It also helps in excretion of undigested materials from the plant cells. It also helps in cell renewal and breakdown of large cells into smaller cells, old and dead cells.

Golgi Bodies (or) Gogi complex: The sac like structures, which are present in a cell to manufacture, store, packing and shipping the materials throughout the cell.

 Endoplasmic reticulum: The network of membrane, which helps in transporting materials around the cell. It forms a connection between nuclear envelope and the cell membrane of a cell.

Mitochondria: They are rod shaped organelles, plays an important role in releasing energy and they are the powerhouse of a cell. In plant cell, they are the sites of cellular respiration.

Nucleus: They are large, oval shaped organelle that contains one or more nucleoli with DNA. The main role of nucleus in plant cell is it controls all types of cellular activities.

Nucleolus: They are the spherical membranes, present inside the nucleus. This membrane contains RNA, which is used to build proteins. They are the sites of RNA synthesis.

Vacuole: They are the fluid sacs, which are present in large numbers in plant cells. The main function is this membrane is to store food and other waste materials. Vacuoles are the largest organelles present in the plant cells.

Animal Cell Structure

Animal cells are eukaryotic cells, with the presence of true nucleus; multicellular large and advanced membrane bound organelles. Like plant cells, animal cells have same organelles except the cell wall, chloroplasts, number of vacuoles and many more. Due to the absence of cell wall the shape of an animal cell is irregular. Based on structure and functions, animal cells comprise of:

Cell membrane: They are semi-permeable membrane surrounding the cell. It helps in holding the cell together and allows entry and exits of nutrients into the cell.

Nucleus: The largest organelle in the cell, which contains DNA and other cell's hereditary information. The main role of nucleus in animal cell is it controls all cellular activities.

Vacuole: They are the fluid sacs, which are present in less numbers in animal cells compared to plant cells. The main function is this membrane is to store food and other waste materials.

Golgi Bodies (or) Gogi complex: The sac like structures, which are present in a cell to manufacture, store, packing and shipping the selected particles throughout the cell.

Ribosome: It is present in the cytoplasm. They are the site of protein synthesis, which are composed of ribosomal RNA and proteins.

Endoplasmic reticulum: The network of membrane, which helps in transporting materials around the cell and also helps in the synthesis of lipids and proteins. It forms a connection between nuclear envelope and the cell membrane of a cell.

Mitochondria: They are rod shaped organelles, plays an important role in releasing energy and they are the powerhouse of a cell.

Cytoplasm: A jelly types double membrane organelles, which are present in the inner region of a cell. It helps by keeping a cell in stable and protects the cell organelles by separating them from each other.

Lysosomes: They are round single membrane round organelles, which helps in recycling of cell organelles. Lysosome of an animal cell contains some digestive enzymes that help in the digesting out the cell debris.

All types of cells eukaryotic and prokaryotic cell, animal and plant cells have many similarities and also differences in them, which all depends on its structure and functions of each cell organelles.

Cell Structure

  

A cell is the smallest working unit of all living organisms on our planet earth, which is capable of performing life functioning. Hence it can also be defined as a fundamental unit of life. The term cell was first observed and identified by an English physicist Robert Hook in the year 1665.There were many theories developed for cell. Later in the year 1839 a two German scientist – Schwann and Schleiden provided few basic principles of cell.

Cell Structures

There are many cells in an individual, which performs several functions throughout the life. The different types of cell include- prokaryotic cell, plant and animal cell.  The size and the shape of the cell range from millimeter to microns, which are generally based on the type of function that it performs. A cell generally varies in their shapes. A few cells are in spherical, rod, flat, concave, curved, rectangular, oval and etc. These cells can only be seen under microscope.

Cell Theory

• Every living organism is made up of a single cell (unicellular) (or) many cells (multicellular) and all types of cells have certain structures in common like: genetic material and plasma membrane.
• Cell is the smallest living thing.
• Each cell arises only from pre-existing cells.

• Cell Structure and Function

  Cell wall: It helps in protecting the plasma membrane and plays a vital role in supporting and protecting the cells.    It is a thick outer layer made of cellulose. 
  
  
  Cell membrane: It is a double layered, thin barrier, surrounding the cell to control the entry and exit of certain substances.
  
  
  Cytoplasm: It is a membrane, which protects the cell by keeping the cell organelles separate from each other. This helps to keep a cell in stable. Cytoplasm is the site, where many vital biochemical reactions take place.
  Nucleus: They are the membrane bound organelles, which are found in all eukaryotic cells. It is the very important organelle of a cell as it controls the complete activity of a cell and also plays a vital role in reproduction.
  
  
  Nuclear membrane: The bilayer membrane, which protects the nucleus by surrounding around it and acts as a barrier between the cell nucleus and other organs of a cell.
  
  
  Nucleolus: It is an important membrane found inside the nucleus. It plays a vital role in the production of cell's ribosome.
  
  
  Chromosomes: It is made up of DNA and stored in the nucleus, which contains the instructions for traits and characteristics. 
  
  
  Endoplasmic reticulum: It helps in the movement of materials around the cell. It contains an enzyme that helps in building molecules and in manufacturing of proteins. The main function of this organelle is storage and secretion. 
  
  
  Ribosome: It plays a vital role in protein synthesis.
  Mitochondria: They are double membrane, filamentous organelles, which play a vital role in generating and transforming the energy. Mitochondria play a vital role in various functions of the cell metabolisms including oxidative phosphorylation.
  
  
  Golgi Bodies: It helps in the movement of materials within the cell.
  
  
  Lysosomes: It is also called as suicidal bags as it helps in cell renewal and break down old cell parts.
  
  
  Vacuoles: It helps plants in maintaining its shape and it also stores water, food, wastes, etc.
  
  
  Chloroplast: They are the site of photosynthesis, which are present in chlorophyll bacteria, blue-green algae, etc

Cell Theory

                     

Fundamental functional units called cells are present deep inside our body, on the surface and in all parts. These fundamental units are busy keeping the body in working condition. Numerous activities takes place in the cell at all times, our nervous system handles the behavioral activity of the cells. This is an example the of the structural and functional composition of our body. Plant cells are similar to animal cells. The plant cells have cell wall which is completely absent in animal cell. There are a few fundamental characteristics that are particular to every cell. cellular modifications make the cell different from each other. In diameter most plant cells are approximately 0.002 inches, most bacteria are smaller at 10 to 50 nanometers long, they are impossible to be seen without magnification. The size of the cell is limited due to the inability of very large cells to provide nutrients and water and also remove cellular wastes efficiently.

What is Cell Theory

Cell theory is a doctrine that states all organisms have organizations that are composed of similar units called cells. This theory was put forth in 1839 by Scheliden and Shwann. This theory remains till date the foundation of modern cell theory. The foundation of this theory are taken from the great frameworks of biology including Darwin's theory of evolution (1859), Mendel's laws of Inheritance (1865) and so on.

The cell theory states :

• All living things are made of cells and their products.
• New cells are  created by division of old cells into two.
• The basic building units of life are cells.

Parts of the Cell Theory

There are three parts of cell theory as described below:

• All living things or organisms are composed of one or more cells.
• The cell is the basic unit of structure, function and organization in all organisms. 
• Cells come from pre-existing living cells.

History of Cell Theory

The development of magnifying lenses and light microscopes made the observation and description of living cells possible. Many scientists contributed to the formulation of the cell theory. 

• In the late 1500's Hans and Zacharias Janssen, father and son, two Dutch lens grinders produced first compound microscope with 2 lenses.
•  In 1665 Robert Hooke used light microscope to observe at thin slices of plant tissues - cork. He observed empty, monk's chambers. He called these tiny chambers as "cells". 
• Antony von Leeuvenhoek invented the first microscope. He was a Dutch scientist and the first to see bacteria and protists. He made powerful magnifying microscopes out of magnifying glasses. In the year 1673 Anthony von Leeuvenhoek observed pond water organisms. He was the first person to see living microscopic organisms. He made careful sketches of the observed organisms and called them 'animalcules' meaning "little animals".
• Robert Brown in the year 1831 a botanist. He concluded from his studies that all plants had cells with a nucleus. 
• Matthias Schleiden a German botanist studied plants. He viewed plant parts under a microscope. He discovered that plant parts are made of cells. In the year 1838 "all plants are made of cells."
• Theodore Schwann a German scientist studied animals. He observed that all animals he studied were made of cells. Hence, he concluded in the year 1839 that "all animals are made of cells." 
• Rudolf Virchow another German physician studied reproduction in cells. He stated in the year 1855 that "where a cell exists, there must have been a pre-existing cell."
• Brown, Theodore and Schwann hence put forth the cell theory.

Modern Cell Theory

General accepted parts of modern cell theory are as follows:

1. All living things are made up of one or more cells. 
2. All living cells come from per-existing cells, by division.
3. The cell is the fundamental, structural and functional unit of all living organisms.
4. The activity of an organism is the total activity of the independent cells of the organism. 
5. All energy flow of life occurs within the cell.  
6. The cells contain hereditary information and is passed from one to another during cell division. 
7. The chemical composition of cell are basically the same in organisms of similar species

Differences between Animal Cells and Plant Cells

CELL ORGANELLES	ANIMAL CELL	PLANT CELL
Cell wall	Absent	Present
Cell shape	Usually round or spherical.  (irregular in shape - absence of cell wall )	Rectangular (fixed shape -  presence of cell wall)
Nucleus	Present	Present
Plasma membrane	Present	Present
Nucleolus	Present	Present
Endoplasmic reticulum	Present	Present
Golgi apparatus	Present	Present
Lysosomes	Present	Present
Vacuoles	Present (Small and more in number)	Present (Large and few in number)
Mitochondria	Present	Present
Plastids	Absent	Present
Chromosomes	Present	Present
Ribosome	Present	Present
Centrosomes	Present	Absent
Chloroplast	Absent	Present
Cytoplasm	Present	Present

Prokaryotic and Eukaryotic Cells

Differences between Prokaryotic cells and Eukaryotic cells

Prokaryotic Cells	Eukaryotic Cells
They are very minute in size.	They are comparatively larger in size.
Nuclear region (nucleoid) is not enveloped by a nuclear membrane.	Nucleus is surrounded by a double membrane layer.
Single chrmosome present.	More than one chromosome are present.
Nucleolus is absent.	Nucleolus is present.
Membrane bound organelles are absent.	Membrane bound organelles are present.
Multiplication of cell is by fission or budding.	Cell division by mitosis or meiosis.
Cell Walls presnt, which are chemically complex.	Cell walls seen in only plant cells, which are chemically simpler.
Cell type is usually unicellular.	Usually multicellular cells.

Eukaryotic Cell

Eukaryotic cells are those cells, which are complex and larger than the prokaryotic cells. The term eukaryote is derived from the Greek word- “eukaryote” meaning true or good nuclei. This cell includes all life kingdoms except monera. Eukaryotic cells can be easily distinguished through a membrane-bound nucleus. The life, which is present and visible by our naked eye, is all made up of these cells. Eukaryotic cells are membrane-bound organelles, which have a multiple membrane-bound organelles to carry out specific cell tasks. They have different internal membranes, which are known as organelles. These organelles play a vital role in cell maintenance and other functions. These organelles generally consist of cell wall, plasma membrane, nucleus, mitochondria, chloroplasts (plastids), endoplasmic reticulum, ribosome, Golgi apparatus, lysosomes, vacuoles, cytoplasm and chromosomes.

Parts of Eukaryotic Cell and their Functions

Cell wall: It helps in protecting the plasma membrane and plays a vital role in supporting and protecting the cells. It is a thick outer layer made of tough cellulose. Cell walls are present in plant cells and are absent in animal cells.

Plasma membrane: The plasma membrane is present in animal cells, plant cells and even in eukaryotic cells. It is a double layered, thin barrier, surrounding the cell that controls the entry and exit of certain substances. It also refers to a thin, fluid entity that manages to be very flexible and it is stable. It is also called as cell membrane. It is the living ultra thin biological membrane ranging from 6 to 8nm and composed of a dynamic layer that chemically comprises a molecule of lipids and proteins that are arranged in a fluid mosaic pattern. It acts as a protective barrier. This membrane plays a vital role in:

It acts as a boundary and separates the internal and external organelles of a cell. 

• Transportation of materials.
• Cell to cell recognition.
• Enzymatic activity.
• Signal transduction.

Nucleus: It is present both in animal cell and in plant cells. It is large and present in the center of a cell. It contains DNA and stores all the necessary information, which is required to control all the activities within the cell. Hence it is also called as a brain of the cell.

Nuclear membrane: It is a double layered, which surrounds the nucleus and helps in the entry and exits of material into the nucleus. It also separates the nucleus from the other parts of the cell.

Nucleolus: It is present in nucleus of both plant cell and animal cell. It plays a vital role in the synthesis of RNA and in the formation of the ribosome.

Mitochondria: The organelles that convert energy into usable forms, which are used by the cell to perform their cellular functions. They are double membranes, semi-autonomous organelles. This organelle plays a vital role in generating and transforming the energy. Albert von Kolliker recognized the structure of the mitochondria in the year 1880. It is a powerhouse of the cell, which produces energy by breaking down fats and carbohydrates. Mitochondria play a vital role in:

• The most important function of the mitochondria is to produce energy. 
• It converts glucose to ATP.
• Helps in cellular respiration.
•  It synthesizes ATP from the breakdown of sugars, fats and other fuels in the presence of oxygen.
• It plays a vital role in oxidative phosphorylation.

Chloroplast: They are the sub cellular sites of photosynthesis. Chloroplasts were discovered early in the 17th century and were identified by a scientist named Antony van Leeuwenhoek and by another scientist named Nehemiah Grew. Chloroplasts are important because, if there were no chloroplasts, plants cannot produce oxygen, sugars and starches, which other animals use and eat. They also produce energy in the daylight.  They are present only in plant cells and are absent in animal cells. Chloroplasts are also found in chlorophyll bacteria, blue-green algae, etc. It is a site for photosynthesis.  

Endoplasmic reticulum: It helps in movement of materials around thecell. Its main functions are storage and secretion. It is of two types:-

Rough endoplasmic reticulum - it manufactures proteins.

Smooth endoplasmic reticulum - it contains an enzymes that helps to build molecules.

Ribosome: They are biological molecule, which are composed of proteins and RNA. It is complex and smallest organelle in the cell. It plays a vital role in synthesis millions of protein, which are required for cells to perform several activities. These organelles are present in all animal cells and absent in plant cells.

Golgi Bodies: They are sac like structures, which are specifically used for storing or preserving all the substances made by the cell. It helps in the movement or transportation of materials within the cell and in synthesis of plant cell wall; hence it is also called as the post office of the cell. It also plays a vital role in the modification, transportation and processing of macromolecules, which includes proteins and lipids. These organelles are present in all animal cells and absent in plant cells.

Lysosomes: They are spherical organelles, which contains enzymes that help in maintaining the physiologic turnover of cellular constituents. These organelles are present in all animal cells and absent in plant cells. They play a vital role in breaking the food materials and making it easier to digest. The size of these Lysosomes varies from 0.1 to 1.2 µm. It is also called as suicidal bags as it helps in cell renewal and break down old cell parts. Lysosomes play a vital role in:

• Removal of dead cells, hence they are named as a suicide bags.
• They protect the cell by ingestion of other dying cells or larger extracellular material like-foreign invading microbes.
• Lysosomes attacks the foreign or disease-causing agents such as bacteria, viruses, fungi, food and old organelles and break them into small pieces that can hopefully be used again.
• Lysosomes play a vital role in protecting the cell from being digested by surrounding the cell membrane. 

Vacuoles: They are vesicle that helps in the digestion. They are present both in plant cells and in animal cells. In plant cells it helps in maintaining its shape and it also stores water, food, enzymes, wastes, etc.

Cytoplasm: It refers to the jelly like material with organelles in it. It is present both in plant and in animal cells. They consist of inner region of the plasma membrane and also the outer region of DNA. The cytoplasm is made of components, which benefits the cell by keeping the organelles separate from each other. This helps to keep a cell in stable. Cytoplasm also contains some important organelles like endoplasmic reticulum, lysosomes, mitochondria and   Golgi apparatus. Along with these organelles, it also contains chloroplast in plant cells. Every organelle is bound by a fatty membrane, which have some specific functions. A cytoplasm plays a vital role in:

• Storage and manufacturing of energy.
• Maintains the cells shape and its consistency and provides suspension to the organelles.
• All types of cell functions like: cell expansion, growth and replication are carried out in the cytoplasm of a cell.
• All types of cellular activities take place in this cytoplasm.
• Cytoplasm also helps in the movement of different elements or molecules present within the cell.

Chromosomes: The term chromosome is derived from the Greek word–“Chroma” meaning colour and “Somes” meaning the body. Chromosomes are small, coloured thread like structures present in the nucleoplasm of living cells, which helps in the inheritance or transmission of characters in form of Genes from one generation to another generation.  It is made up of DNA and stored in the nucleus, which contains the instructions for traits and characteristics. A chromosome plays a vital role in:

• Self-duplication.
• They help in transmitting or transferring the characters from one generation to another generation (or) from parents to offspring.
• Controls the  biological processes in the body of an organism.
• They control cell metabolism by directing the formatting of necessary proteins.
• They help in cell differentiation during development.
• A chromosome also helps in determining a sex of an individual.

Centrosomes: They are the small hollow cylindrical shaped organelles, which are composed of nine bundles of micro tubules.  They play a vital role in cell division or in the cell cycle. Centrosomes are present only in animal cells and are absent in plant cells. Centrosomes are also called by centrioles.

Prokaryotic Cell

Prokaryotic cells are simpler and smaller than the eukaryotic cells. The term prokaryote is derived from the Greek word- “prokaryote” meaning before nuclei. These cells lack membrane bound organelles. Prokaryotic cells are unicellular organisms, which reproduce through binary fission. In some cases few prokaryotic organisms also reproduce by budding. Prokaryotic cells have a cell envelope, which generally consists of a capsule, cell wall, cytoplasm, plasma membrane, cytoplasm region or nucleiod region, ribosome, plasmids, pili and flagella.

Example: Bacteria, blue green algae, E.coli, etc.

Parts of Prokaryotic Cell and their Functions

Capsule:  It is composed of a thick polysaccharide. It is a kind of slime layer, which covers the outside of the cell wall. It is used to stick cells together and works as a food reserve and it also protects the cell from dryness and from chemicals.

Cell wall: It is made from the glycoprotein murein. Cell wall provides strength and rigidity to the cell and it is permeable to solutes.

Cytoplasm:  It helps in cellular growth, metabolism and replication. Cytoplasm is the storehouses for all types of chemicals and components that are used to sustain the life of a bacterium.

Plasma membrane: It is also known as a cell membrane.It is mainly composed of proteins, phospholipids and carbohydrates, which forms into a fluid-mosaic. Plasma membrane surrounds the bacteria and it is a most important organelle and plays a vital role in controlling the movement of substances in the cell.

Cytoplasm region (or) nucleiod region: An area of the cytoplasm that contains the single bacterial DNA molecule.

Ribosome: They are the smallest part of cell organelle. Ribosome plays a vital role in protein synthesis as they consist of protein and RNA. They are located freely in the cytoplasm of attached to the rough endoplasmic reticulum.

Mesosomes: They are the folding, present inside the plasma membrane. Mesosome plays a vital role in cellular respirations, replication of DNA, cell division, separation of chromosomes during cell division and also performs the role of Golgi bodies and mitochondria.

Plasmids: They are a small circle of DNA. Plasmid plays a vital role in exchanging DNA between the bacterial cells. Bacterial cells have many plasmids.

Pili: They are short protein appendages, which fixes bacteria to surfaces. These pili are smaller than those flagella and are used in conjugation to exchange the genetic information.

Flagella: They are rigid rotating tail. The clockwise rotation moves the cell forward and anticlockwise rotation helps the cell to spin. The rotation is powered by H+ gradient across the cell membrane.

Cell

The basic structural and functional unit of life.

The Cell                 

The cell can be defined as a basic functional unit of life. The term cell is derived from the Latin word and was first observed by a scientist named Robert Hook in the year 1665.  All living organisms are composed of one or many cells to perform their individual functions. A cell is a smallest unit of a life, which is able to control and perform several functions in all living organisms. All unicellular organisms (single cell) including bacteria and archae, which are composed of a single cell, are examples of prokaryotic cells. All multicellular organisms (many or more than one cell) including humans, which are composed of complex or many cells, are examples of eukaryotic cells. Both prokaryotic cells and eukaryotic cells have cytoplasm, cell membrane and genetic material in common.

Is a cell necessary for a life? 

Yes, all living organisms on planet earth require a cell. Cell plays a vital role in keeping us alive by controlling all types of biochemical functions inside an organism. A new cell produces through cell division of pre-existing cells.

Types of Cells

 There are two primary types of cells.

Examples of Vertebrates

Below are the examples of vertebrates:

Pisces: Sharks, trouts, eels, tunas, seahorse, pirahnas, salmons, etc.

Amphibians: Salamanders, frogs, toad, newts, caecilians, etc.

Reptiles: Snakes, lizards, tortoise, turtles, crocodiles, gharials, alligators, etc.

Birds: Penguins, emu, eagles, ostrich, parrot, crow, pigeon, ducks, owls, kites, robins, woodpeckers, albatross etc.

Mammals: Kangaroo, echidna, platypus, bats, mice, tigers, moles, dolphins, whales, deer, gorrilas, lemurs, rabbits, wolves, lions, leopards, jackals, etc.

Vertebrate Classification

                                                             

     
                                                              Classes of Vertebrates

Class - Cyclostomata

The living members of this class are all ectoparasites on some fishes. They have a elongated body. They bear 6-15 pairs of gills through which they respire. The mouth of the cyclostomes is sucking circular mouth without jaws. They do not have body scales and paired fins. The vertebral column and the cranium is cartilaginous. Circulation is closed type. These are marines organisms but  they migrate to fresh water for spawning. After metamorphosis their larvae returns to the ocean.

Example: Petromyzon (Lamprey), and Myxine (Hagfish).

Class - Chondrichthyes

These organisms are marine and have streamlined body. The endoskeleton is cartilaginous. Mouth is located ventrally and the notochord is present throughout life. The gills are separate and are not covered by the operculum. The skin contains minute placoid scales. The placoid scales are modified as teeth and the jaws are powerful. They are predaceous animals. Air bladder is absent in these animals, hence, they have to swim constantly to avoid shrinking.

Example: Scolidon, Pristis, Trygon.



Class - Osteichthyes

This class includes animals that are both marine and fresh water fishes with bony endoskeleton. Their body is streamlined. Mouth is terminal in position. The gills are covered with operculum on each side and are in four pairs.The skin is covered by ctenoid/cycloid scales. Air bladder is present, it regulates buoyancy. Two-chambered heart is present, with one auricle and one ventricle. They are poikilothermic animals. The sexes are separate, fertilization takes place externally. Most of them are oviparous animals and development is direct, with no larval stages.

Example: Hippocampus, Clarias.

Class - Amphibia

The name Amphibia indicates - from Greek, Amphi meaning dual and bios meaning life. 

Ampibians can live both in aquatic and terrestrial habitats. These animals have two pairs of limbs. Body is divisible into head and trunk and tail is present in some animals. The skin of these animals are scaleless and moist. The eyes have eyelids, and the ears are represented by a tympanum. Cloaca is a opening to the exterior, it is a common chamber for the alimentary canal, urinary and reproductive tracts.Gills, lungs and skin aids in respiration. Heart is three-chambered. They are cold-blooded animals. Sexes are separate, fertilization is external. They are oviparous animals and development is indirect. 

Example: Toad, frog, salamander.

Class - Reptilia

In Latin repere or reptum means to creep or crawl, hence, the class name refers to locomotion that is of creeping or crawling mode. 

These animals are mostly terrestrial and their body is covered dry and cornified skin, epidermal scales or scutes. External ear opening is absent, tympanum represents the ear. Limbs, if present are of two pairs. Usually heart is three-chambered, but is four chambered in crocodiles. They are poilkilothermic animals. Some animals like the snake and the lizards shed their skin. Sexes are separate, internal fertilization takes place. They are oviparous and development is direct. 

Example: Turtle, Chameleon, crocodile.

Class - Aves

Most of the members can fly, except the flightless birds. The characteristic feature of birds is the presence of feathers. The forelimbs are modified as wings. The hind limbs are modified for walking, swimming or clasping and generally have scales. The skin is dry and does not have glands, except a oil gland at the base of the tail. Endoskeleton is bony, and the bones are hollow with air cavities known as pneumatic bones. Heart is four-chambered completely. They are warm-blooded animals. Lungs are the organs of respiration. Sexes are separate, fertilization is internal. They are oviparous animals and development is direct. 

Example: Crow, Pigeon, Parrot etc.

Class - Mammalia

Mammals are present in almost all habitats - polar ice caps, deserts, mountains, forests, and grasslands.The unique characteristic of the class mammalia is the presence of milk producing glands (mammary glands), by which the young ones are nourished. The limbs are of two pairs. The skin is covered with hairs. External ear 'pinna' is present. Heart is four-chambered and they are homeothermic animals. Respiration is through lungs. Sexes are separate and fertilization is internal. They viviparous animals, with direct development. 

Example: Kangaroo, Tiger, Lion, Platypus etc.