CLASS 9 Science CHAPTER 5 The Fundamental Unit Of Life (NCERT Notes)
Cell is basic ,structural and fundamental unit of life.
Robert Hooke saw that the cork resembled the structure of a honeycomb consisting of many little compartments. Cork is a substance which comes from the bark of a tree. This was in the year 1665 when Hooke made this chance observation through a self-designed microscope. Robert Hooke called these boxes cells. Cell is a Latin word for ‘a little room’.
Robert Hooke first discoved the dead cell.
Leeuwenhoek (1674), with the improved microscope, discovered the free living cells in pond water for the first time.
Robert Brown in 1831 who discovered the nucleus in the cell.
The cell theory ,said that all the plants and animals are composed of cells and that the cell is the basic unit of life, was presented by two biologists, Schleiden (1838) and Schwann (1839).
The cell theory was further expanded by Virchow (1855) by suggesting that all cells arise from pre-existing cells.
Every multi-cellular organism has come from a single cell. Cells divide to produce cells of their own kind. All cells come or arises from pre-existing cells.
Unicellular Organisms
Unicellular organisms are made up of only one cell that carries out all of the functions needed by organisms. Cell are not specific.
Examples Amoeba, Chlamydomonas, Paramoecium and bacteria. etc .Cell are specific.
Multicellular Organisms
Multicellular organisms are made up of more than one type of cell.
Examples fungi, plants and animals
What is a Cell Made Up of? What is the Structural Organisation of a Cell?
PLASMA MEMBRANE OR CELL MEMBRANE
The outermost covering of the cell that separates the contents of the cell from its external environment is plasma membrane. The plasma membrane allows or permits the entry and exit of some materials in and out of the cell. It also prevents movement of some other materials. The cell membrane, therefore, is called a selectively permeable membrane.
How does the movement of substances take place into the cell? How do substances move out of the cell?
Some substances like carbon dioxide or oxygen can move across the cell membrane by a process called diffusion. Diffusion is spontaneous movement of a substance from a region of high concentration to a region where its concentration is low.
Some substance like CO2 (which is cellular waste and requires to be excreted out by the cell) accumulates in high concentrations inside the cell. In the cell’s external environment, the concentration of CO2 is low as compared to that inside the cell. As soon as there is a difference of concentration of CO2 inside and outside a cell, CO2 moves out of the cell, from a region of high concentration, to a region of low concentration outside the cell by the process of diffusion. Similarly, O2 enters the cell by
the process of diffusion when the level or concentration of O2 inside the cell decreases. Thus, diffusion plays an important role in gaseous exchange between the cells as well as the cell and its external environment.
Water also obeys the law of diffusion. The movement of water molecules through such a selectively permeable membrane is called osmosis. The movement of water across the plasma membrane is also affected by the amount of substance dissolved in water. Thus, osmosis is the passage of water from a region of high water concentration through a selectively permeable membrane to a region of low water
concentration till equilibrium is reached.
If we put an animal cell or a plant cell into a solution of sugar or salt in water. One of the following three things could happen:
1. If the medium surrounding the cell has a higher water concentration than the cell, meaning that the outside solution is very dilute, the cell will gain water by osmosis. Such a solution is known as a hypotonic solution.
Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.
2. If the medium has exactly the same water concentration as the cell, there will be no net movement of water across the cell membrane. Such a solution is known as an isotonic solution. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
3. If the medium has a lower concentration of water than the cell, meaning that it is a very concentrated
solution, the cell will lose water by osmosis. Such a solution is known as a hypertonic solution.
Unicellular freshwater organisms and most plant cells tend to gain water through osmosis. Absorption of water by plant roots is also an example of osmosis. Thus, diffusion is important in exhange of gases and water in the life of a cell. Cell also obtains nutrition from its environment. Different molecules move in and out of the cell through a type of transport requiring use of energy.
The plasma membrane is flexible and is made up of organic molecules called lipids and proteins. We can observe the structure of the plasma membrane only through an electron microscope.
The flexibility of the cell membrane also enables the cell to engulf in food and other material from its external environment. Such processes are known as endocytosis. Amoeba acquires its food through such processes.
CELL WALL
In addition to the plasma membrane ,plant cells have another rigid outer covering called the cell wall. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance and provides structural strength to plants.
When a living plant cell loses water through osmosis there is shrinkage or contraction of the contents of the cell away from the cell wall. This phenomenon is known as plasmolysis.
Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute (hypotonic) external media without bursting. In such media the cells tend to take up water by osmosis. The cell swells, building up pressure against the cell wall. The wall exerts an equal pressure against the swollen cell.
Because of their walls, such cells can withstand much greater changes in the surrounding medium than animal cells.
NUCLEUS
Nucleus is darkly coloured, spherical or oval, dot-like structure near the centre of each cell. The nucleus has a double layered covering called nuclear membrane. The nuclear membrane has pores which allow the transfer of material from inside the nucleus to its outside, that is, to the cytoplasm . The nucleus contains chromosomes, which are visible as rod-shaped structures only when the cell is about to divide.
Chromosomes contain information for inheritance of characters from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules. Chromosomes are composed of DNA and protein. DNA molecules contain the information necessary for constructing and organising cells. Functional
segments of DNA are called genes.
In a cell which is not dividing, this DNA is present as part of chromatin material. Chromatin material is visible as entangled mass of thread like structures. Whenever the cell is about to divide, the chromatin material gets organised into chromosomes.
The nucleus plays a central role in cellular reproduction, the process by which a single cell divides and forms two new cells. It also plays a crucial part, along with the environment, in determining the way the cell will develop and what form it will exhibit at maturity, by directing the chemical activities of the cell.
In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane. Such an undefined nuclear region containing only nucleic acids is called a
nucleoid. Such organisms, whose cells lack a nuclear membrane, are called prokaryotes. Organisms with cells having a nuclear membrane are called eukaryotes. Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells. Many of the functions of such organelles are also performed by poorly organised parts of the cytoplasm. The chlorophyll in photosynthetic prokaryotic bacteria is associated with membranous vesicles (bag like structures) but not with plastids as in eukaryotic cells.
Prokaryotic Cell
1. Size : generally small ( 1-10 µm) 1 µm = 10–6 m
2. Nuclear region: Nuclear membrane is absent.
3. Chromosome: single chromosome
4. Membrane-bound: cell organelles absent
Exception ribosomes are present in both.
Eukaryotic Cell
1. Size: generally large ( 5-100 µm)
2. Nuclear region: well defined and surrounded by a nuclear membrane
3. Chromosome: More than one
4. Membrane-bound: cell organelles are present
CYTOPLASM
The cytoplasm is the fluid content inside the plasma membrane. It also contains many specialised cell organelles. Each of these organelles performs a specific function for the cell.
Cell organelles are enclosed by membranes. In prokaryotes nuclear membrane is absent. Membrane bound cell organelles are also absent.
In the eukaryotic cells have nuclear membrane as well as membrane-enclosed organelles.
Viruses lack any membranes and hence do not show characteristics of life until they enter a living body and use its cell machinery to multiply.
CELL ORGANELLES
Every cell has a membrane around it to keep its own contents separate from the external environment. Large and complex cells, including cells from multicellular organisms, need a lot of chemical activities to support their complicated structure and function. To keep these activities of different kinds separate from each other, these cells use membrane-bound little structures (or ‘organelles’) within themselves. This is one of the features of the eukaryotic cells that distinguish them from prokaryotic cells. Some
of these organelles are visible only with an electron microscope.
Some important examples of cell organelles which we will discuss now are: endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria and plastids. They are important because they carry out some
very crucial functions in cells.
ENDOPLASMIC RETICULUM (ER)
The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets. It looks like long tubules or round or oblong bags (vesicles). The ER membrane is similar in structure to the plasma membrane. There are two types of ER– rough endoplasmic reticulum (RER) and smooth endoplasmic
reticulum (SER).
RER looks rough under a microscope because it has particles called ribosomes attached to its surface. The ribosomes, which are present in all active cells, are the sites of protein manufacture. The manufactured proteins are then sent to various places in the cell depending on need, using the ER.
The SER helps in the manufacture of fat molecules, or lipids, important for cell function. Some of these
proteins and lipids help in building the cell membrane. This process is known as membrane biogenesis. Some other proteins and lipids function as enzymes and hormones.
ER varies greatly in appearance in different cells, it always forms a network system. Function of the ER is to serve as channels for the transport of materials (especially proteins) between various regions
of the cytoplasm or between the cytoplasm and the nucleus. The ER also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. In the liver cells of the group of animals called vertebrates , SER plays a crucial role in detoxifying many poisons and drugs.
GOLGI APPARATUS
The Golgi apparatus, first described by Camillo Golgi, consists of a system of membrane-bound vesicles (flattened sacs) arranged approximately parallel to each other in stacks called cisterns. These membranes often have connections with the membranes of ER and therefore constitute another portion
of a complex cellular membrane system. The material synthesised near the ER is packaged and dispatched to various targets inside and outside the cell through the Golgi apparatus. Its functions include the storage, modification and packaging of products in vesicles. In some cases, complex sugars may be made from simple sugars in the Golgi apparatus. The Golgi apparatus is also involved in the formation of lysosomes
LYSOSOMES
Lysosomes are membrane-bound sacs filled with digestive enzymes. These enzymes are made by RER. Lysosomes are a kind of waste disposal system of the cell. These help to keep the cell clean by digesting any foreign material as well as worn-out cell organelles. Foreign materials entering the cell,
such as bacteria or food, as well as old organelles end up in the lysosomes, which break complex substances into simpler substances. Lysosomes are able to do this because they contain powerful digestive enzymes capable of breaking down all organic material. During the disturbance in cellular
metabolism, for example, when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell. Therefore, lysosomes are also known as the ‘suicide bags’ of a cell.
MITOCHONDRIA
Mitochondria are known as the powerhouses of the cell. Mitochondria have two membrane coverings. The outer membrane is porous while the inner membrane is deeply folded. These folds increase surface area for ATP- generating chemical reactions. The energy required for various chemical activities needed
for life is released by mitochondria in the form of ATP (Adenosine triphopshate) molecules. ATP is known as the energy currency of the cell. The body uses energy stored in ATP for making new chemical compounds and for mechanical work.
Mitochondria have their own DNA and ribosomes. Therefore, mitochondria are able to make some of their own proteins
PLASTIDS
Plastids are present only in plant cells and euglenoides. There are two types of plastids – chromoplasts
(coloured plastids) and leucoplasts (white or colourless plastids).
Chromoplasts containing the pigment chlorophyll are known as chloroplasts. Chloroplasts are important for photosynthesis in plants.
Chloroplasts also contain various yellow or orange pigments in addition to chlorophyll.
Leucoplasts are primarily organelles in which materials such as starch, oils and protein granules are stored.
The internal organisation of the Chloroplast consists of numerous membrane layers embedded in a material called the stroma. These are similar to mitochondria in external structure. Like the mitochondria, plastids also have their own DNA and ribosomes.
VACUOLES
Vacuoles are storage sacs for solid or liquid contents. Vacuoles are small sized in animal cells while plant cells have very large vacuoles. The central vacuole of some plant cells may occupy 50-90% of the cell volume.
In plant cells vacuoles are full of cell sap and provide turgidity and rigidity to the cell. Many substances of importance in the life of the plant cell are stored in vacuoles. These include amino acids, sugars, various organic acids and some proteins.
In single-celled organisms like Amoeba, the food vacuole contains the food items that the Amoeba has
consumed. In some unicellular organisms, specialised vacuoles also play important roles in expelling excess water and some wastes from the cell.
Each cell acquires its structure and ability to function because of the organisation of its membrane and organelles in specific ways. The cell is a basic structural organisation. This helps the cells to perform
functions like respiration, obtaining nutrition, and clearing of waste material, or forming new proteins.
Thus, the cell is the fundamental structural unit of living organisms. It is also the basic functional unit of life.
Cell Division
New cells are formed in organisms in order to grow, to replace old, dead and injured cells, and to form gametes required for reproduction.
The process by which new cells are made is called cell division. There are two main types of cell division: mitosis and meiosis.
The process of cell division by which most of the cells divide for growth is called mitosis.
It is process in which each cell called mother cell divides to form two identical daughter cells. The daughter cells have the same number of chromosomes as mother cell. It helps in growth and repair of tissues in organisms.
Specific cells of reproductive organs or tissues in animals and plants divide to form gametes, which after fertilisation give rise to offspring. They divide by a different process called meiosis which involves two consecutive divisions. When a cell divides by meiosis it produces four new cells instead of just two . The new cells only have half the number of chromosomes than that of the mother cells.
( From NCERT Book )