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Academic Session: 201_-201_
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Basic structural and functional unit of life.
Often called Building block of life.
Discovered by Robert Hooke in 1665.
Named from the Latin word “Cellula”.
Can be differentiated by structure, number, size, shape, etc.
Discovery of Cell
Discovered by Robert Hooke.
He saw that cork resembled the structure of honeycomb.
In 1665, he discovered cell with the help of self designed microscope.
He called these boxes cell. Cell is a Latin word for a “little room”.
His discovery was important as it indicated for the first time that living organisms consisted of a smaller structures or units.
In 1838, Jacob Matthias Schleiden, a German botanist, first proposed the idea that all plants consist of cell.
In 1839, Theodor Schwann, a German zoologist, asserted that all animals and plants are made up of cell.
This joint finding forms the basis of cell theory.
The cell theory was refined further in 1855, when another German Biologist, R. Virchow presented the idea that all cells arise from pre-existing cells(‘Omnis cellulae e cellulae’).
Cell theory II
Thus cell theory comprise the following postulates :
All organisms are composed of cell and cell secretion.
All metabolic reactions take place in cells. Thus, cells are structural and functional unit of life.
All cells arise from pre-existing cells. No cell can originate spontaneously but comes into being only by division of already existing cells.
Every organism starts its life as a single cell. Viruses are an exception to cell theory.
Instruments for Studying Cells
Cells are too small to be seen with naked eyes.
Cells are studied with the help of microscopes.
Microscopes are high resolution instruments that are used for observing fine details of very fine objects.
Two common types of microscope are light microscope and electron microscope.
Cells in living organisms
All living organism on earth can be divided into 2 types :
Non-cellular organisms which do not contain any cell in their body organization, e.g., viruses. Viruses lack any membrane and hence do not show characteristics of life until they enter a living body.
Cellular organisms which contain either one or many cells in their bodies, eg., bacteria, plants and animals.
Eukaryotes and Prokaryotes.
Cellular organisms can be divided into following two main types :
These organisms have primitive and incomplete cells.
Prokaryotes have all 3 basic structures of a typical cell but lack nuclear membrane around their genetic substances (DNA).
Undefined nuclear region in the cytoplasm is called nucleoid, i.e., there is no nuclear membrane.
Cell organelles like ER are absent except Ribosome.
They include archaebacteria, bacteria and cyanobacteria.
These have advanced and complete cells.
These cells contain membrane bound nuclei and other cell organelles and are called eukaryotes.
They are found in unicellular and multicellular plants and animals and contain plasma membrane, nucleus, DNA and cytoplasm with ribosomes and cellular organelles such as mitochondria.
Difference Prokaryotes and Eukaryotes
Size of the cell is small.
Nucleus is absent.
Contains single chromosome.
Nucleolus is absent.
Membrane bound cell organelles are absent.
Size of the cell is large.
Nucleus is present.
Contains more than one chromosome.
Nucleus is present.
Membrane bound cell organelles are present.
Division of Labour
Same organism like humans can have cell of different type, e.g., sperm, WBC, Bone cells, muscle cells, nerve cells, fat cells, etc.
This is due to the fact that there is a division of labour in multicellular organism.
This means that different parts of human body perform different functions.
This is only possible if some cells or tissues are specialized to any specific function.
The basic shape of eukaryote is spherical but is determined by specific function of the cell.
Shape of cell is variable (Amoeba, WBC) or fixed (most plants and animals, Euglena, Paramecium).
Some cell shapes are :
The size of different cells ranges between broad limits.
Only a few cells are visible with naked eyes while most cells are visible by microscope only.
The size of cell varies from very small cells of bacteria (0.2 to 5µm) to very large eggs of the ostrich (18 cm).
The prokaryotes usually range from 1 to 10µm.
The eukaryotes usually range from 10 to 100µm.
Amoeba proteus is biggest (60 µm) among the unicellular organisms.
The volume of a cell is approx. constant for a particular cell type and is independent of the size of the organism.
For example, kidney or liver cells are about the same size in the bull, horse and mouse.
The difference in the total mass of the organ or organism depends on the number, not on the volume of the cells.
Thus, the cells of elephants may always not be larger than the cells of tiny organism.
The no. of cells in most multicellular organism is indefinite.
The no. of cells in an organism may be genetically fixed. This phenomenon is called eutely. It is seen is Ascaris, a type of roundworm.
No. of cells in large size animals like elephant may be countless.
No. of cells in humans are estimated to be around 100 trillion.
They are Eukaryotes.
Differ from other cells because of some different features.
There wall is made up of cellulose.
They are Eukaryotes.
They are the most common cell found.
Structure of Cell
Though their shape size and activities vary, all cells have following three major functional regions.
The cell membrane or plasma membrane and cell wall.
The plasma membrane (cell membrane) is made of two layers of phospholipids.
The membrane has many proteins embedded in it.
The plasma membrane regulates what enters and leaves the cell.
It is called semi-permeable membrane.
Many molecules cross the cell membrane by diffusion and osmosis.
Plasma Membrane II
Plasma membrane contains 75% phospholipids.
It also contains proteins, cholesterol and polysaccharides.
In 1972, Singer and Nicholson suggested a model, called fluid mosaic model, for explaining ultrastructure of the plasma membrane.
The fluid mosaic model of the membrane has been described as ‘a number of protein icebergs floating in the sea of lipids”.
Cell walls made of cellulose are only found around plant cells.
Cell walls are made of specialized sugars called cellulose.
Cellulose provides a protected framework for a plant cell to survive
Cell walls also help a plant keep its shape
The nucleus is a major centrally located spherical cellular component.
It is bounded by 2 nuclear membranes, both forming a nuclear envelop.
It has a ground substance named nucleoplasm.
2 Type of nuclear structures-the nucleolus and the chromatin network are embedded in nucleus.
Nucleus may be 1 or more and it is not bound by any membrane.
Nucleolus and Chromatin Material
Nucleus is 1 or more and is not bound by any membrane.
It is rich in protein and RNA and is the site of ribosome formation.
It is known as factory of ribosomes.
Chromatin material is a thin, thread like intertwined mass of chromosomes material.
It is composed of genetic substance DNA and proteins.
It is condensed into 2 or more thick ribbon-like chromosomes during division of cells.
They are backbone of cells.
Various life processes are performed by them.
They may be single or double membrane bounded, or membrane may be absent.
They are essential for functioning of cell.
They may contain DNA in them.
eg. Mitochondria, Chloroplast, Lysosomes, etc.
The endoplasmic reticulum (ER) is repsonible for the production of the protein and lipid components of most of the cell's organelles.
This internal space is called the ER lumen. The ER is additionally responsible for moving proteins and other carbohydrates to the Golgi apparatus, to the plasma membrane, to the lysosomes, or wherever else needed.
There are two types of ER - rough, which is coated with ribosomes, and smooth, which isn't.
Rough ER is the site of protein synthesis. The smooth ER is where the vesicles carrying newly synthesized proteins (from the rough ER) are budded off.
They are dense, spherical and grannular particles which occur freely in the matrix.
They may remain attached to RER.
The 2 main constituents of ribosomes are RNA and proteins.
They are not bounded by a membrane.
They are both found in eukaryotes and prokaryotes.
It consists of fluid filled vacuoles and flattened cisternae.
It is single membrane bound cell organelle.
It is absent on bacteria, blue green algae, mature sperms and RBC of mammals.
Golgi apparatus originates ER, which in turn originates the former.
It stores, processes and packages various cellular secretions.
They are tiny spherical sac-like structures evenly distributed in the cytoplasm and contain powerful enzymes.
It is single membrane bounded cell organelle.
They are capable of breaking down all the organic matter.
They remove worn out and poorly working cellular organelles by digesting them to make way for their new replacement.
They are also called demolition squads, scavengers, etc.
Significance of Lysosomes
In WBC : Cells of WBC digest foreign proteins, bacteria and viruses.
In Autophagy : During starvation, the lysosomes digest stored food contents such as proteins, fats and glycogen of the cytoplasm and supply the necessary amount of energy to cell.
In metamorphosis : During the transformation of a tadpole into frog, the embryonic tissues such as gills and tails are digested by the lysosomes and utilized by other body cells.
It is known as power house of the cell.
ATP synthesis take place here.
It is double membrane bounded cell organelle.
It has their own genome.
It is absent in bacteria and the red blood cell of mammals.
It contains cristae, F1 particles and matrix.
It can make their own protein, so they are regarded as semiautonomous organelles.
Plastid occur in most plant cells and are absent in animal cells.
They have their own genome.
They are self replication organelles.
Plastids are of 3 types :
Chloroplasts are present in green algae and higher plants.
They have the green pigment called chlorophyll that are involved in photosynthesis of food. So, chloroplasts are called “kitchen of the cells”.
Chloroplast is a double membrane bounded cell organelle.
Chloroplasts trap solar energy and utilize it to manufacture food for plant.
Vacuoles are fluid filled or solid filled and membrane bounded spaces. They are a kind of storage sacs.
In plant cells, the vacuoles are large, distinct and permanent. In mature plant cells the vacuole occupies almost the entire (90%) volume of the cell.
The vacuole is bounded by a membrane, called tonoplast.
Vacuole help to maintain the osmotic pressure in a cell. They store toxic metabolic by-product or end products of plant cells.