The Fundamental unit of life Class 9 Short Notes | Chapter 5
Introduction
Cell is basic unit of life and also known structural and functional unit of life.
- it`s discovered by Robert Hook in 1665 in wood of cork slice with help of primitive microscope.
- With a better microscope, Leeuwenhoek observed free-living cells in pond water for the first time in 1674.
- The nucleus of the cell was found by Robert Brown in 1831.
- Purkinje created the name "protoplasm" for the cell's fluid portion in 1839
The Cell theory
Two biologists, Schleiden and Schwann (1838) gave the Cell theory which states that :-
- All plants and animals are composed of cells.
- Cell is the basic unit of life.
- All cell arise from pre-existing cells.
- Viruses are the exceptions of cell theory.
Cell Formation
Cellular material is a total of different synthetics, for example, water, particles, salts and other natural atoms like proteins, sugars, fats, nucleic acids, nutrients and so on alongside cell organelles and core that comprise a cell. It exists in sol-gel states.
Type of Cells
There are two types of cell :
- Prokaryotic Cells
- Eukaryotic Cells
Prokaryotic
- Size: generally small ( 1-10 µm) Where 1 µm = 10 - 6 m.
- The nucleus is not well defined and known as a nucleoid.
- Single, circular chromosome.
- Membrane-bound organelles absent. Eg: Bacterial cell
Eukaryotic
- Size: generally large ( 5-100 µm).
- The nucleus is well defined and surrounded by a nuclear membrane.
- Possess more than one and linear chromosome.
- Membrane-bound cell organelles are present. Eg: Human cell.
Difference between Animal cell and Plant cell
| Plant Cell | Animal Cell |
|---|---|
| Contain chloroplasts for phtosynthesis. | No chloroplasts |
| Have a cell wall to maintain structure and rigidity. | No cell wall |
| Usually do not contain lysosomes and Peroxisomes. | May Contain cilia and/or flagella |
| May Contain cilia and/or flagella | Cells are fluidic and flexible, many shapes. |
| Have one large central vacuole. | Cells can move around. Has small or no vacuoles. Have lysosome |
Type of Organism
Organism are of Two Type
Unicellular Organism
These organisms are single celled which perform all the functions. Example :- Amoeba, paramecium, bacteria.
Multicellular Organism
Many cells grouped together to perform different function in the body and also form various body parts. Example :- fungi, plants, animals.
Cell Shape
The shape and size of cells are related to the specific function they perform. Some cells like Amoeba have changing shapes. In some cases the cell shape could be more or less fixed and peculiar for a particular type of cell
for example, nerve cells have a typical shape.
Cell Size
Size of cell is variable depending upon the its position & function Some are microscopic while some are visible with naked eyes. Their size may vary from 0.2 um to 18 cm.
- Size of typical cell in a multicellular organism ranges from 2-120 micron.
- The largest cell is ostrich egg (15 cm long 13cm wide & weight 1.4 kg)
- The longest cell is nerve cell (upto 1m).
- Smallest cells so far known are PPLOS e.g., mycoplasma
Components of Cell
There is an occurrence of division of labour within a Eukaryotic cell as they all got certain specific components called ‘Cell organelles’. Each of them perform a specific function.
The three basic components of all the cells are :-
- Plasma membrane
- Nucleus
- Cytoplasm
Plasma membrane/Cell membrane
This is outermost covering of the cell that separates the contents of the cell from its external environment. The plasma membrane allows or permission the entry and exit of some materials in and out of the cell. it also prevents movement of some other materials.it is made up of protein and lipid.
Cell Wall
Cell wall is outer rigid outer covering in addition to the plasma membrane found in plant cell. The cell wall lies outside the plasma membrane. The plant cell wall is mainly composed of cellulose. Cellulose is a complex substance which provides structural strength to plants.
Nucleus
Nucleus is brain of the cell as it controls all the activities of cell. it is plays a central role in cellular reproduction. It is the process by which a single cell divides and forms two new cells.
Nucleus membrane
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 the cytoplasm.
The nucleus contains chromosomes, Which are visible as rod-shaped structures only when the cell is about to divide.
Chromosomes
Chromosomes contain information for inheritance of feature from parents to next generation in the form of DNA(Deoxyribo Nucleic Acid) molecules .
Chromosomes = DNA + Protein
Nucleoid
In some organisms like bacteria, the nuclear region of the cell may be poorly defined due to the absence of a nuclear membrane.
Nucleoid has containing only nucleic acids
Cytoplasm
The cytoplasm is the fluid content inside the plasma membrane.
- it helps in exchange of material between cell organelles.
- it`s act as store of vital chemicals such as amino acid, glucose, vitamins and iron etc.
- it is the site of certain metabolic pathways such as glycolysis
it also contains many specialised cell organelles. Each of these organelles perform a specific function for the cell.
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
- endoplasmic reticulum,
- Golgi apparatus
- lysosomes
- mitochondria
- plastids.
They
are important because they carry out some
very crucial functions in cells.
Endoplasmic Reticulum(ER)
The Endoplasmic Reticulum is a large network of membrane-bound tubes and sheets.
it looks like long tubules or round. the ER membrane is similar in structure to the plasma membrane. it is also made up of lipid and proteins
There are two type of ER:-
- Rough Endoplasmic Reticulum
- Smooth Endoplasmic Reticulum
Rough Endoplasmic Reticulum
It's 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
Smooth Endoplasmic Reticulum
It helps in the manufacture of fat molecules, or lipids , important for cell functions
Golgi Apparatus
Golgi Apparatus 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.
Functions
- 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 a kind of waste disposal system of the cell.
- its helps to keep the cell clean by digesting any foreign material as well as worn out cell organelles.
- Lysosomes have membrane-bounded structure whose sacs are filled with digestive enzymes.
- its also known as the 'suicide bags' of a cell
Functions
- lysosomes break foreign materials entering the cell, such as bacteria or food as well as old organelles into small pieces
- during the disturbance in cellular metabolism such as when the cell gets damaged, lysosomes may burst and the enzymes digest their own cell.
Mitochondria
Mitochondria are know as the powerhouse of the cell
Structure of mitochondria
→ Structure of Plastids
→ Function of Plastids
- Mitochondria have two membrane coverings.
- The outer membrane is very porous while the inner membrane is deeply folded.
- These folds create a large 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.- There are three types of plastids:
- Chromoplasts (coloured plastids).
- Leucoplasts (white or colourless plastids).
- Chloroplasts (contains the pigment chlorophyll).
→ Structure of Plastids
- The internal organisation of the plastids consists of numerous membrane layers embedded in a material called the stroma.
- Plastids also have their own DNA and ribosomes like mitochondria and similar to its structure.
→ Function of Plastids
- 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.
Vacuoles
Vacuoles are storage sacs for solid or liquid contents.They are small sized in animal cells while plant cells have very large vacuoles.
Function of 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 important substance in the life of the plant cell are stored in vacuoles which 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.
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
- meiosis
Mitosis
In this process, 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.The process of cell division by which most
of the cells divide for growth is called mitosis.
Meiosis
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.
6 comments