the fundamental unit of life. Flashcards
write a short note on cells.
innumerable kinds of organisms live on our planet earth. all these organisms look different from each other and are broadly classified as bacteria, protists, fungi, plants and animals. however, all of them are made up of microscopic units, called cells. in some cases, single cell functions as an entire organism. such organisms are called unicellular organisms e.g, bacteria, amoeba, paramecium. in others, many cells group together and acquire different functions to form various body parts. such organisms are termed as multicellular organisms e.g some fungi, plants and animals. organisms have cells to perform various metabolic reactions in a delicately balanced environment to sustain life. cells, in living organisms act as life supporting chambers and provide them such a special balanced environment. in other words, every living cell keeps its chemical composition steady within its boundary and, thus, is capable of performing the activities in a desired manner. cell is regarded as the structural and functional unit of living organisms.
how was the cell discovered?
the term cell was introduced by an english scientist, robert hooke in his book micrographia, published in the year 1665. he examined thin slices of cork under his self designed primitive microscope. the cork is a substance obtained from the bark of a spanish oak tree, Quercus. he was suprised to see that the cork resembled the structure of a honey comb and consisted of many small compartments. in fact, he saw the dead cells of plants ( only cell walls which looked like empty rooms. these dead cells had lost their living contents. he called these rooms as cellulae, now termed as cells. cellulae is a latin word which means a little room.
this simple observation which seems to be a very small and insignificant, was an extremely important discovery in the history of biological sciences. it was the first observation which showed that organisms were made up of small units, called cells. robert hooke subsequently used his primitive microscope to observe many other plant cells.
explain the history of cell in detail.
- robert hooke coined the tern cell in 1665.
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anton von leeunwenheok ( 1632-1723 ) designed improved microscope and described, for the first time, free living cells in 1674. later, he also described protozoans, red blood corpuscles and human sperm cells.
( with the advent of more sophisticated tools and techniques, cell study developed rapidly in the nineteenth century ) - in 1831, a scotch botanist robert brown ( 1773-1858 ) discovered the nucleus in an orchid root cell. By this time, it had also been found that the cells were surrounded by some sort of limiting structure, the cell membrane.
- a French zoologist dujardin, discovered in 1835 a semi fluid living material in certain protozoans and he named it as sarcode.
- in 1840, __ johannes E. purkinje noted similar material in plant cells and named it protoplasm, the first substance.
- a German botanist M.J schleiden, in 1838, announced that all plants were composed of cells. a year later, german zoologist, T. schwann stated that all animals are also formed of cells. schwann found that, except for the cell wall, the animal cells more or less resembled the plant cells in having a nucleus located in a clear substance bounded by a cell membrane. These findings formed the basis of cell theory.
- in 1858, another German biologist rudolph virchow ( 1821- 1920 ) presented the idea omnis cellula e cellula which means that all living cells arise from the pre existing cells. this led to the modification of cell theory.
- haeckal, in 1866 reported that nucleus stores and transmits hereditary information.
- electron microscope was discovered by knoll and ruska in 1940. Discovery of electron microscope made it possible for us to observe and understand the complex structure of the cell and its various cell organelles, i.e, it’s ultrastructure
why is the term cell considered a misnomer?
We now know that the really important parts of the cell are its contents and the cell may or may not have a cell wall. thus, the term cell is a misnomer. misnomer is a wrong or inaccurate name. The term cell is considered a misnomer because the literal meaning of cell is a hollow space or chamber. however, a cell is not hollow. the word cell is still used to describe the structural and functional unit of life.
what is the instrument that is used for studying cells?
Cells are microscopic in structure and cannot be seen with a naked eye. They are studied with the help of instruments, called microscopes. These provide enlarged images of cells and the structures they contain.
microscope is the most common optical instrument used to observe cellular organisation of living organisms.
there are different kids of microscopes which can be broadly divided into two categories:
1. light microscopes.
2. electron microscopes.
what is a simple microscope?
Simple microscopes are just magnifying glasses. In fact, human eye works as an optical instrument and the eye lens acts as a simple lens. The resolving power of a healthy young human eye is approximately 0-1mm at 25cm viewing distance. any object smaller than this cannot be viewed by the naked eyes. a microscope is, therefore, used for enlargement and magnifying the smaller object so that it becomes visible by the human eye.
what is a compound microscope? explain in detail.
a compound microscope is one in which magnification takes place is two stages. it consists of two lens units __ the first, called the objective, produces a primary magnified image. and the second lens unit, called eye piece or ocular magnifies the first image. such microscopes employing two lens units in working position at one time are called monocular monoobjective microscopes.
The simple student’s microscope, generally used in the biological laboratory to observe tiny cells is called compound microscope. Such a microscope uses light, generally sunlight to illuminate the object or specimen under study, and hence it is also called light microscope. it has a magnification of 100-150. to magnify the object, power of objective ( 10x, 40x, 100x ) and eye piece ( 10x, 15x ) is used.
The light or compound microscope is a **strong, heavy metal instrument which comprises of u shaped base having two vertical pillars. a curved arm is movably joined to the pillars to hold the microscope. The arm can be bent over the pillars at inclination joint to suit the viewer. The upper part of the arm holds the movable body tube. The other parts of the microscope are reflector, condenser lens, stage, objective lens, ocular lens ( eye piece ), and adjustment screws ( coarse and fine ).
a glass slide, containing the object or specimen under study, is kept on a stage under an objective lens. Light is then passed through the object or the specimen with the help of a mirror ( reflector ) and the condenser from below the stage. A magnified, sharp image of the object/specimen can thus be seen through the ocular lens/eye piece by focusing the coarse and fine adjustment knobs properly. We can increase or decrease the magnification of the image by changing the objective lens of high or low power.
what is a electron microscope?
it is a very large instrument that helps in observing sub-cellular structures which cannot be seen with a light microscope. it uses very high voltage electricity. For its working, internal vacuum is essential. It uses electromagnets instead of glass lenses and beam of electrons instead of light. Ultra thin and dry section of material ( under study ) is impregnated with some metal to enhance the contrast and the image is obtained on a photographic plate or fluorescent screen. Magnification is 1,00,000 to 5,00,000 times.
what are unicellular organisms?
organisms wherein a single cell constitutes the whole organism. *amoeba, paramecium, chlamydomonas ( all protists ), bacteria ( prokaryotes ) and yeasts ( fungi ) are few examples of such organisms. these single celled organisms live independently in nature and their single cells are capable of performing all the life processes such as obtaining food, respiration, metabolism, excretion, growth and reproduction
what are multicellular organisms?
These organism are formed of many cells. in fact, number of cells group together and acquire different functions to form various body parts in multicellular organisms. Examples are fungi, plants and animals including human beings. In fact, the multi cellular organism has come into existence from a single cell. for example, fertilization of eggs with sperm results in the formation of a single celled zygote. This single cell divides and redivides in various planes to produce a multicellular body. The cells further divide to produce different organs of a multicellular organism. thus, all cells arise from pre existing cells and this confirms the theory of cell lineage proposed by virchow ( 1858 ). according to the theory, all cells arise from pre existing cells.
All cells, whether they exist as independent unicellular organisms or as part of a multicellular organism, have certain structures in common to carry out basic functions essential for the survival of the cells. Every living cell has the capacity to perform certain basic functions that are characteristic of all living organisms.
how does a living cell perform these basic functions?
All multicellular organisms show division of labour. it means that different parts of a multicellular organism perform different functions. Each cell possesses specific components within it known as cell organelles. Each kind of cell organelle performs a special function. For instance:
1. Making of new material in the cell such as Protein synthesis by ribosomes and food synthesis ( carbohydrate ) by chloroplasts.
2. Energy generation in the form of ATP ( adenosine triphosphate ) by mitochondria.
3. Clearing up the waste substances from the cell by lysosomes
these organelles together constitute the basic building block called the cell and the cell is able to live and perform its functions because of these organelles.
Differentiate between unicellular and multicellular organisms.
UNICELLULAR ORGANISMS:
1. consists of a single cell.
2. the single cells performs all the life activities.
3. there is no division of labour as the single cell performs all the activities.
4. single cell is involved in the production of new organisms during reproduction.
MULTICELLULAR ORGANISMS:
1. It consists of a large number of cells
2. a single cell performs one or a few activities
3. There is division of labour as cells are _ specialized_ to perform different functions of the body
4. only germ cells take part in reproduction.
Write a short note on the shape, size and number of cells.
cells vary in shape, size and number. Unicellular organisms are identified on the basis of their shape. However, some unicellular organisms, e.g, amoeba, are irregular in shape. In multicellular organisms, the shape of cells depends upon their position in the body, specific function which they perform and their interaction with neighbouring cells.
Cells also vary in size. Their dimensions are usually expressed in microns or micrometers ( um ) and angstrom ( Å ). Most human cells typically range in diameter from 8-25 um. They are observed under the microscope. The small objects which can be seen only with the help of microscope are called microscopic in size. However, there are some cells which are either extremely small or extremely large in size.
1um = 10^-3mm or 1/1000nm
1nm = 10^-3um or 10^-6mm
1Å = 10^-1nm or 10^-7mm or 10^-10m
what are prokaryotic and eukaryotic cells?
Prokaryotic cell
The cells of some most primitive organisms such as bacteria and blue green, i.e, cyanobacteria lack nuclear membrane around their genetic materials and hence are called prokaryotic cells or prokaryotes ( pro_before, karyon_nucleus ). The genetic material in these cells lies in direct contact with the cytoplasm and is called nucleod. prokaryotes also do not contain membrane bound organelles in the cytoplasm. ribosomes are, however, present.
eukaryotic cell
The cells of other organisms possess true nucleus bounded by nuclear membrane. suck organisms are called eukaryotes. The eukaryotes may be unicellular or multicellular organisms. Examples include protists, fungi, plants and animals. Every Eukaryotic cell contains a plasma membrane, membrane bound nucleus containing the genetic material and other membrane bound subcellular organelles in the cytoplasm. The organelles divide the cytoplasm into compartments to faciliate specific metabolic functions
give the detailed structure of a eukaryotic cell.
no cell is completely unspecialized so as to be considered a typical cell. most of the cells have certain common subcellular structure. a eukaryotic cell has three main components:
1. plasma membrane/cell membrane.
every cell is enclosed on all sides by a distinct covering, called plasma membrane or cell membrane. it is living, ultra thin structure made of lipid and proteins. it keeps the cell contents seperated seperated from the external environment. the plasma membrane allows/permits the entry and exit of only selected materials and hence, is called a selectively permeable membrane.
2. nucleus.
Each eukaryotic cell has a deeply stained rounded structure, called nucleus. It is located centrally in animal cells but pushed to one side due to large vacuole in plant cells. It is the most conspicuous and the largest organelle of an Eukaryotic cell. Nucleus contains genetic material.
3. cytoplasm.
The fluid and semi fluid matrix filled in between the nucleus and the plasma membrane is called the cytoplasm. It contains various specialized cell organelles. Each cell organelle performs specific function for the cell. All the components of a cell taken together keep it living. No part can survive is separated from the cell.
Explain plasma membrane.
each cell, prokaryotic as well as eukaryotic, is surrounded by a covering called the plasma membrane or plasmalemma or cell membrane. Also, most cell organelles in Eukaryotic cell ( e.g mitochondria, plastids, golgi apparatus, lysosomes, endoplasmic reticulum, peroxisomes, vacuoles etc. ) are enclosed by subcellular unit membranes. These membranes, thus, compartmentalise the cell. however, neither the cell nor the compartments in it are totally isolated from the surrounding medium. The membrane, in fact, allow continuous flow of selected materials across them as required from time to time.
what are the different kinds of permeability?
1. impermeable. if a membrane does not allow both solvent and solute particles to pass through it, it is called impermeable membrane, e.g, cuticle layer.
2. permeable. if a membrane allows both the solvent and solute particles to pass through it freely, the membrane is called permeable, e.g, primary cell wall.
3. semi permeable. if a membrane allows penetration of only solvent molecules but not the solute particles, it is called semi permeable, e.g, artificial vapour membrane.
4. selectively permeable. if a membrane allows penetration of solvent freely but selects the passage of specific solute particles, it is called selectively permeable membrane, e.g plasma membrane
explain the molecular structure of cell membrane
Plasma membrane is a living, ultra thin, elastic, selectively permeable membrane that appears as a mere line under the light microscope. However, its detailed molecular structure can be studied under an electron microscope. Basically all the plasma membrane are composed of proteins, lipids and small fractions of carbohydrates. fine structure of plasma membrane, as revealed by electron microscopic studies, appeared to be three layered ( trilaminar membrane ).
lamellar model
this model was proposed by danielli and davson in 1935 and J. D. robertson in 1959 to explain the structural organization of plasma membrane. according to these scientists, the plasma membrane has three layers ( dark-light dark ). A central light lipid bilayer is sandwitched between two protein monolayers ( P-L-L-P )
fluid mosaic model
This model is the most recent and accepted model proposed by singer and nicolson in 1972. according to this model, the cell membrane is made up of a lipid bilayer and two types of protein molecules. the lipid bilayer forms a highly viscous liquid in which the two types of protein molecules ( intrinsic proteins and extrinsic proteins ) are organized in a mosaic manner. intrinsic proteins are embedded in the lipid bilayer incompletely or completely, and the extrinsic proteins occur superficially.
Fluid mosaic model provides satisfactory explanation of the structure and functions of plasma membrane
what are the function of cell membrane/plasma membrane?
- it gives form to the cell. (shape )
- it maintains individuality of the cell.
- it keeps the cell contents in place and prevent mixing with the extracellular materials.
- protects the cell from injury.
- it is selectively permeable i.e it regulates the flow of selected materials into an out of the cell.
- it forms organelles with the cytoplasm.
- its junctions ( point where two or more things are joined. ) keep the cells together.
explain the transport of materials across cell membrane
in order to perform various life activities, all cells must take up and turn out materials through the cell membrane. The plasma membranes acts as physical barriers between the cell and its surrounding environment and between cell organelles and the surrounding cytoplasm.
the cell membranes are not freely permeable, i.e., they do not allow movement of all kinds of substances across the. instead, they are selectively permeable, i.e., they allow the entry or exit of only selected materials. thus, selective permeability of plasma membrane enables the cell to maintain homeostasis, i.e., a constant internal environment despite the changes outside it. cell membranes allow the moment of different materials across them differently.
