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Chapter 5 - The Fundamental Unit of Life
What are Living organisms made up of?
All living organisms are made up of cells. Cell is thebasic structural andfunctional unit ofcomplex organisms.
History of cell: Cells were first discovered by Robert Hooke in 1665 with the help of a primitive microscope. Leeuwenhoek, in 1674, with the improved microscope, discovered free-living cells in pond water for the first time. Robert Brown in 1831 discovered the nucleus in the cell. Purkinje in 1839 coined the term 'protoplasm' for the fluidpart of thecell. Schleiden in 1838and Schwann in 1839 proposed the cell theory which stated thatall plants andanimals are composed of cells. Rudolf Virchow in 1855 further expanded the celltheory by suggesting that all cellsarise from pre-existing cells. The invention of magnifying lenses led to thediscovery of themicroscopic world. Unicellular organisms are theorganisms in whicha single cellperforms all thefunctions like nutrition, respiration, excretion and reproduction. Eg. Amoeba, Chlamydomonas, Paramecium and Bacteria possess single cells constituting the whole organism. Multicellular organisms are theorganisms which possess many cells to perform different functions. Multicellular organisms represent themselves as a member of a group of cells or as an individual. Eg. Fungi, plants and animals have many cells that group together to form tissues. Every multi cellular organism hascome from a single cell. All cells thuscome from pre existing cell. Some organisms canalso have cells of different kinds.
The shape and size of cell are related to the specific function they perform. Some cells change their shapes. Eg. Amoeba. In some cases the cell shape could be more or less fixed and the peculiar for a particular type of cell. Eg: nerve cells Each living cell has the capacity to perform certain basic functions that are characteristic of all living forms. There is a division of labour in multicellular organism such as human beings. This means that different parts of the human body perform different functions. Similarly division of labour is also seen within a single cell. In fact each such cell has got certain specific components within it known as cell organelles. Each kind of cell organelle performs a special function. A cell is able to live and perform all its functions because of these organelles. These organelles together constitute the basic unit calledthe cell. What is a cell made up of? What is the structural organization of a cell? Every cell wouldhave three features- plasma membrane, nucleus and cytoplasm. Allactivities inside thecell and interactions of the cellwith its environment are possible dueto these features. Plasma membrane orcell membrane: This is the outermost covering of the cell that separates the contents of the cell from its external environment. It is flexible and made up of organic molecules called lipids and proteins. 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 asendocytosis. Eg: Amoeba It allows the movement of some substances into and out of the cell. It also prevents movement of some other materials. Therefore it is called a selectively permeable membrane. Movement of substances through this semi-permeable membrane can be by the process of diffusion, osmosis etc.
Difference between diffusion and osmosis
If we put an animal cell or a plant cell into a hypotonic solution the cell is likely to swell up. The cell will stay in the same size if it kept it in isotonic solution. If the solution is hypertonic then the cell willshrink.
Unicellular freshwater organism andmost plants tendto gain water through osmosis.
Cell wall: It is present only in plant cells. The cell wall is composed of cellulose and is permeable. It separates the contents of the cell from the surroundings. It gives shape and protection to the cell. Cell walls permit the cells of plants, fungi and bacteria to withstand very dilute external media without bursting.
Plasmolysis: It is theprocess in whichcells lose water in a hypertonic solution.
Nucleus:
The nucleus has a double layered covering called nuclear membrane. The nuclear membrane has pores whichallow the transfer of material frominside to outside. The nucleus contains chromosomes which arecomposed of Deoxyribonucleic acid(DNA) and proteins. Nucleus controls all the activities of the cell.
As the nucleus carries genetic information in the form of DNA, it plays a major role in cell division and cell development. The functional segments of DNA are called genes. Nucleus plays an important role in protein synthesis and transmission of characters from one generation to another generation. It plays a central role in cellular reproduction. In some organisms nuclear membrane is absent and nuclear region contains only nucleic acids called nucleoid. Such organisms called prokaryotes. Eg. Bacteria.
Organisms with cells having a nuclear membrane are called eukaryotes.
Cytoplasm: The cytoplasm is the fluidcontent inside theplasma membrane. It is a jelly like viscous substance occupying entirecell except thenucleus. It alsocontains many specialized cell organelles thatperform a specific function for thecell.
Cell organelles:
Cell organelles include endoplasmic reticulum, Ribosomes, Golgi apparatus, Mitochondria, Plastids, Lysosomes, and Vacuoles. They are important because they carry out some very crucialfunctions in cells.
Endoplasmic reticulum (ER):
The ER is a large network of membrane bound tubes and sheets. It serves as channels for thetransport of materials especially proteins between various organs ofthe cytoplasm orbetween the cytoplasm and nucleus. It also functions as a cytoplasmic framework providing a surface for some of the biochemical activities of the cell. There are two types of ER- Rough endoplasmic reticulum and smooth endoplasmic reticulum. RER: These are rough atsurface and areassociated with ribosomes. These are responsible for the synthesis of proteins. SER: These aresmooth at surface andare not associated with ribosomes. It helpsin the manufacture of fat molecules or lipids. It also plays a crucial role in detoxifying many poisons nd drugs. Membrane biogenesis: Someof the proteins and lipids synthesized by EF helpin building thecell membrane. Thisprocess is known as membrane biogenesis. Golgi Apparatus: These cell organelles are named after the biologist, Camillo Golgi, who first described it. TheGolgi consists of a stackof membrane-bound cisternae. These membranes oftenhave connections with the membranes of ER and therefore constitute another portion of a complex cellular membrane system. Its functions include the storage, modification and packaging of products in vesicles. It is alsoinvolved in theformation of lysosomes.
Lysososmes: Lysosomes are membranous sacs filled with enzymes. These enzymes are made by RER. They are a kind of waste disposal system of the cell. They help to keep the cell clean by digesting any foreign material as well as worn out cell organelles. Lysosomes contain hydrolytic enzymes which arecapable of digesting cellular macromolecules. When thecell gets damaged, the lysosome may burst and its enzymes may digest thecell itself. Hence, lysosomes are called as ‘suicidal bags’.
Mitochondria: These are cellular organelles termed as‘power houses ofthe cells’. These are bounded bya double membrane. The outer membrane is smooth while the innermembrane is thrown into folds
called as cristae. The cristae increase the area of cellular respiration. Mitochondria releases energy in the form of ATP molecules. ATP is known as the “energy currency of the cell”. Mitochondria have its own DNA DNA ribosomes and are able to make some of their own proteins. Plastids: Plastids are present only in plant cells. These are of two types- chromoplasts (coloured plastids) and leucoplasts (white or colourless plastids). Plastid contains pigment called chlorophyll are known as chloroplasts. These are important for photosynthesis in plants. Chromoplasts are the organelles which provide bright colours to the plant structures like buds, flowers etc. Leucoplasts are the organelles which store starch, oils and protein granules. Plastids consist of numerous membrane layers embedded in a material called the stroma. Plastids also have their own DNA andribosomes. Vacuoles: Vacuoles are membrane bound compartments present in both plant and animal cells. These are storage sacs for solid or liquid contents. These are small sized in animal cells while bigger in plant cell. In plant cells vacuoles are full of sap and provide turgidity and rigidity to the cell. These organelles store water, waste products, and substances like amino acids, sugars and proteins. In some unicellular organisms specialized vacuoles also play important roles in expelling excess water and somewastes from the cell.
Difference between plant cells andanimal cells
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