Tissue Class 9 Notes
The cell (Latin word means ‘little room‘) is the basic structural, functional and biological unit of all known living organisms. Cells may be compared to the bricks which are assembled to make a building. Cells are also assembled to make the body of every organism.
Discovery of the Cell
Cells were first discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a primitive microscope. Hooke observed that the cells in the cork were actually dead cells. Leeuwenhoek firstly observed living cells in pond water in 1674 with the help of improved microscope.
Shape and Size of Cells
Cells can be of different shape and size. It can be different for different organisms and may also vary from the cell of one organ to other organ in the same organism. Generally, cells are round, spherical or elongated in shape but some cells are long and pointed at both ends (a spindle shape) whereas, some are quite long. The shape and size of cells are related to the specific functions that they perform. In some cases, the cell shape could be more or less fixed and peculiar for a particular type of cell, e.g., nerve cells have a typical shape.
Number of Cells in Living Organisms
All organisms are composed of cells. Some cells, which can exist as an independent organism are called unicellular organisms or single celled organism, e.g., Amoeba, Chlamydomonas, Paramecium and bacteria, whereas multicellular organisms are those that consist of more than one cell, e.g., fungi, plants and animals. Every multicellular organism has come from a single cell. Cells also divide to produce cells of their own kind and their functions are performed by different organs, e.g., blood cell, nerve cell, fat cell, sperm, bone cell, ovum, smooth muscle cell, etc.
Characteristics of Cells
The various properties exhibited by a cell are as follows
- Cells are capable of independent existence and they have the ability to replicate independently.
- They contain hereditary information which passes from one generation to another.
- Each living cell has the capacity to perform to certain basic functions depending upon its location.
- Number of cells present, depends on the size of an organ or body organisation.
- They are lesser in number in small organisms and large animals like beer, whale, etc have greater number of cells.
Cell theory is one of the most important generalisations of biology. It was put forward by Matthias Schleiden (German botanist) in 1838 and Theodore Schwann (British zoologist) in 1839 in their paper i.e., “Microscopic investigations on the similarities of structure and growth in animals and plants”.
According to this theory, all living things are composed of minute units, called cells which are smallest entities that can be called living. It also describes the properties of cells and also states that it is the basic unit of reproduction. The study of cells is called cell biology.
This theory however, did not explain as to how new cells were formed. Rudolf Virchow in 1955 first explained that the pre-existing cells divide and new cells are formed from them. He modified the hypothesis of Schleiden and Schwann to give the cell theory a final shape. Cell theory as described today is as follows
- All organisms are composed of one or more cells.
- Cell is the most basic unit of structure, function and organisation in all organisms.
- All cells arise from pre-existing living cells.
With the discovery of an electron microscope in 1940, it is possible to observe and understand the complex structure of the cell. Structurally, the cell constitutes the following three basic components
Cell membrane or plasma membrane
- All these components interact with its environment to perform various functions.
The cell wall is a rigid (non-living) but flexible covering of the cell which is located outside the cell membrane and provides shape and structural support to the cells. It also protects the cells from the outside attack and infection. Cell walls are particularly found in plants, fungi, bacteria, algae and some archaea. Animals and protozoans do not have cell walls.
Cell walls are made up of various materials depending on the species like
- Bacterial cell wall is made up of peptidoglycan.
- Fungi’s cell wall is made up of the glucosamine polymer chitin.
- Algae’s cell wall is made up of glycoproteins and polysaccharides.
- Plant’s cell wall is mainly made up of cellulose, which provides structural strength to it.
Structure of Cell Wall
Cell wall constitutes following three layers in its organisation
Also called middle lamella, rich in pectins, forms interface between adjacent plant cells and glue them together.
Primary Cell Wall
Thin flexible layer, forms when the cells grow, it is single layered.
Secondary Cell Wall
Thick layer, forms when cell is fully grown, not found in all cells, it is 2-3 cell thick
Functions of Cell Wall
- Cell wall permits the cell of plants, fungi and bacteria to withstand without bursting by acting as a pressure vessel.
- It also helps cell to tolerate much greater changes in the surrounding medium as compared to animal cells.
Cell Membrane or Plasma Membrane
This is the outermost covering of cell, separating the contents of the cell from its external environment. Plasma membrane is made up of lipids (major content is phospholipid) and proteins. Structurally, it is a thin, flexible and semi-permeable or selectively permeable membrane. The cytoplasm and nucleus are enclosed within the cell membrane. Animal cells, plant cells, prokaryotic cells and fungal cells all have cell membranes as their outer covering except viruses which are devoid of cell membrane.
Functions of Cell Membrane
- Its major function is to maintain shape of the cell by providing support to it.
- It protects the interior components of the cell by selectively allowing the movement of certain substances (ions, organic molecules) in and out of the cell. This function is regulated by selectively permeable nature of the membranes.
- The flexible nature of the cell membrane allows the cell to engulf food and other substances from the surrounding environment and this process of intake is known as endocytosis, e.g., in Amoeba.
- It also serves as a base for the attachment of cytoskeleton in some organisms and the cell wall in others.
- It performs various other functions in a cell such as cell adhesion, ion conductivity and cell signaling.
Nucleus was discovered by Robert Brown in 1831. It is an important component of the living cell. It is generally spherical or elliptical in shape and is located in the centre of the cell. It acts as a major controlling centre of the cell and also called as brain of the cell.
Various components of nucleus are as follows
Nucleus is separated from the cytoplasm by a membrane, called nuclear membrane. It is a double-layered structure made up of protein and fat which encloses the contents of the nucleus. A fluid-filled space or perinuclear space is present between the two layers of a nuclear membrane. The nuclear membrane contains micropores, allowing the transfer of material from inside the nucleus to outside (to the cytoplasm). The outer layer of the nuclear membrane is connected through the endoplasmic reticulum of the cell.
As compared to the protoplasm inside the cell, nucleoplasm is the protoplasm present inside the nucleus of the cell. It is little acidic and also known as karyoplasm or nuclear sap. It is composed of nucleolus and chromatin. It is made up of abundant protein, phosphorus and nucleic acid.
The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. There are generally one or more spherical nucleoli inside the nucleus of the cell, but some eukaryotic organisms have nucleus that contains upto four or more nucleoli. The main function of nucleolus is to synthesise rRNA and to store genetic information in its DNA.
During the cell division, chromatin shrinks (compressed) and gets divided into various smaller thick and consolidated form, known as chromosomes. These are thread-like, rod-shaped structures and are made up of a combination of DNA (Deoxyribonucleic Acid) and protein, found in the nuclei of cells.
Chromosomes consist of a thick jelly like substance, called matrix and chromonemata (two mutually thin interconnected coil shaped structure). As every chromonemata is called chromatid hence, every chromosome is composed of two chromatids. They also carry genes that act as a unit of inheritance in living organisms. Genes control the transfer of characters from the parents to next generation. Thus, chromosomes are also called hereditary vehicles in an organism.
Cytoplasm is a fluid-like substance of viscous nature filled in the cell. In eukaryotic cells, it can also be defined as the part of the cell present between the cell membrane and the nuclear envelope. Protoplasm’ is a term given by Purkinje in 1839, which is the living content of a cell that is surrounded by a plasma membrane. It is basically composed of a mixture of small molecules such as nucleic acids, proteins, lipids and polysaccharides In eukaryotes the protoplasm surrounding the cell nucleus is known as the cytoplasm and that inside the nucleus as the nucleoplasm. While, in prokaryotes the material inside the plasma membrane is the bacterial Cytoplasm.
Cytoplasm contains the following components
- Cytosol ::- Cytosol is the cytoplasmic matrix (liquid) found inside the cells. It is separated into compartments by membranes. e.g., the mitochondrial matrix separates the mitochondrion into compartments i.e., outer and inner compartments.
- Cell Organelles ::- Each cell has certain specific components within it, known as cell organelles. Various cell organelles perform special functions, such as making new material in the cell, cleaning up the waste material from the cell, etc. Cell organelles constitute various components like endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, plastids and vacuoles, which perform various functions in the cell.
These organelles are described below in detail
- EndoPlasmic Reticulum (ER) It was discovered by Albert Claude and Keith Porter in 1945. ER is basically structured as a network of tubules or flattened sac, called vasicles or oblong bags. These are all interconnected by a single continuous membrane.
There are two types of endoplasmic reticulum
- Rough Endoplasmic Reticulum (RER) ::- Synthesises and secretes serum proteins (e.g., albumin), hormones (e.g., insulin) and other substances (e.g., milk) in the glands because it possesses ribosomes at the cytoplasmic side of the membrane.
- Smooth Endoplasmic Reticulum (SER) ::- Synthesises lipids and steroids, metabolises carbohydrates and helps in regulating calcium concentration, drug detoxification and attachment of receptors on the cell membrane.
Golgi Apparatus (Golgi Body)
It was first discovered by Camillo Golgi in 1898 in the nerve cells of barn owl and cat. It is basically composed of flat sacs known as cisternae. The sacs are attached/stacked in a bent or semicircular shape. Each stacked grouping has a membrane that separates it inside from the cell cytoplasm. It is also composed of tubules (arising from the periphery of cisternae) and vesicles. These membranes get link to the membrane of ER and forms another complex membrane system.
Functions of Golgi Apparatus
- The main function of Golgi apparatus is to carry out the processing of proteins generated in ER.
- It helps in the transportation of lipids around the cells .
- In Golgi bodies, some complex sugars are formed from the simple sugars.
- It also helps in the formation of cell wall (in plant cells) and lysosomes.
These were discovered by De Duve in 1955 and are also known as suicidal bags (digestive bags) or atom bomb of the cell, due the presence of powerful hydrolytic enzymes which are able to digest every organic substance in the cell. It has complexly arranged lumen (internal space) in the form of packet that can serve a variety of function in a cell. Lysosomes occur practically in all animal cells and protozoans.
- Intracellular and extracellular digestion is perfomed by lysosomes, i.e., in intracellular digestion, individual cells may obtain food through phagocytosis and extracellular digestion occur when lysosomes release enzymes in the external enivironment through exocytosis.
- It helps in disposing useless cells and replace them with new cell.i.e., breakdown of ageing and dead cell.
- It helps in body defence system.
It was discovered by Richard Altman in 1880 and was named as bioblast’ by Benda. It acts as the cell’s power producer and also called as power house or power plant of the cell.
Mitochondria is elliptical in shape and bounded by two membrane coverings, i.e., an outer one which is porous and smooth and an inner one, which is deeply folded. The inner membrane form finger shaped matrix called tubuli (cristae) at various places. Mitochondria have their own DNA and ribosomes therefore they can make some of their proteins.
It has the ability to convert energy into such forms that are usable by the cell, i.e., in the form of ATP (Adenosine Triphosphate). It acts as the major site of cellular respiration, which ultimately generates fuel for the cell’s activities. Mitochondria is also involved in other cell processes such as cell division and growth, as well as in cell death (apoptosis).
However, mitochondria.are not fully autonomous as both structure and function are partially controlled by nucleus of the cell and they depend upon the availability of material from cytoplasm. Hence, they are considered as semiautonomous in nature.
- It provides energy for energy requiring processes of the cell like muscle contraction, nerve impulse, conduction etc.
- It may store and release calcium when required.
The term plastids was introduced by E.’ Hackel in 1866. It is a major organelle found in the cells of plants and algae. It has numerous membrane layers embedded in a part called stroma. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Like mitochondria plastids also have their own DNA and ribosomes. Based on the types of pigments present, plastids can be classified into following three types
- Leucoplast ::- Colourless, occurs near the nucleus, in non-green cells like roots, underground stems, etc. They may be amyloplast (store starch), elaioplast (store fat) or proteinoplast (store proteins).
- Chromoplast : :- Yellow or reddish in colour due to the presence of carotenoid pigments, found in coloured parts like flowers, fruits, etc, various forms are found in tomato (lycopene), carrot (carotene), beet root (betamin), etc.
- Chloroplast ::- Greenish plastids due to the presence of chlorophyll and carotenoids, takes part in photosynthesis. Chloroplasts of algae other than green ones are called chromatophores.
These are granular structures first observed under the electron microscope as dense particles by George Palade (1953). These are basically composed of ribonucleic acid (RNA) and proteins. Ribosomes are not surrounded by any membrane. It is the smallest cell organelle and was earlier known as microsomes before the contribution of Palade, who later called it ribosomes. The eukaryotic ribosomes are 80 S type while prokaryotic are 70 S type.
Function of Ribosomes
Its major function is to synthesise proteins in the cell, due to which it is also known as factory of protein in the cell.
It is an organelle usually containing two cylindrical structures, called centrioles. Both centrioles lie perpendicular to each other forming an organisation like that of a cartwheel. Centrosome is mainly found in animal cells and was first discovered by T Boveri in 1888.
Function of Centrosomes
It is helpful in cell division and formation of spindle fibre.
It is a membrane bound organelle present in all plants, fungal cells, some
protists, animals and bacterial cells. They enclose compartments, which are filled with water containing inorganic and organic molecules including enzymes in solution. In certain cases, they may also contain solids, which have been engulfed e.g., in Amoeba.
Vacuoles are formed mainly by the fusion of multiple membrane vesicles. The organelle has no basic shape and size, its structure varies according to the needs of the cell. It is generally large in plants (confined within the nucleus) to provide turgidity and rigidity to its cell but are less in number and smaller in size in animals.
In some unicellular organisms, specialised vacuoles are present for releasing excess water and some waste from the cell.
Sap vacuoles They are fluid filled vacuoles which are separated from the cytoplasm by a semi-permeable membrane called tonoplast.
Gas vacuoles. They store gases and provide buoyancy, mechanical strength and protection from harmful radiation.
These are basically considered as the non-living components present in the cell that do not possess metabolic activities and are not bounded by membranes as well. The most common inclusions are glycogen, lipid droplets, crystals and pigments.
Types of Cells
On the basis of structure, cells are of two basic types
The cells having nuclear material without nuclear membrane, are termed as prokaryotic cells. These are generally smaller and not very complex in structure. These are essentially infoldings of cell membrane. In prokaryotes, the nuclear region (containing only nucleic acids), called nucleoid of the cell may be poorly defined due to the absence of a proper nuclear membrane. Prokaryotic cells also lack most of the other cytoplasmic organelles present in eukaryotic cells.e.g., prokaryotic cells are present in bacteria like E.coli, Clostridium, bluegreen algae, etc.
The cells, like onion cells and cheek cells, having well-organised nucleus with a nuclear membrane, are termed as eukaryotic cells. These cells are more complex than the prokaryotic cells. The nucleus in eukaryotes is separated from the cytoplasm by double-layered membrane and it directs the ne processes of the cell.e.g., eukaryotic cells are present in protists, fungi, plant and animals, etc.