Class 9th Science: Chapter 5 The Fundamental Unit of Life
Introduction to Cell
All living organisms are made up of cells. Cell is the basic structural and functional unit of complex organisms. Cells perform all the metabolic activities taking place at cellular level. The discovery of cells was possible with the invention of microscope.
History of cell
• Marcello Malpighi, in 1661, proposed that plants are made of tiny structural units called 'Utricles'.
• Robert Hooke, in 1665, observed many tiny, hollow, room like structures in a thin slice of cork through a compound microscope and called them ‘cells’.
• 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 fluid part of the cell.
• Schleiden in 1838 and Schwann in 1839 proposed the cell theory which stated that all plants and animals are composed of cells.
• Rudolf Virchow in 1855 further expanded the cell theory by saying ‘omniscellula-e-cellula’, which means all cells arise from pre-existing cells.
• Cell is derived from the Latin word "Cellula" which means "a little room".
The two types of microscopes used to visualise the cell are compound microscopes and electron microscopes.
a) Compound microscopes are also called as light microscopes.
• Compound microscope is an instrument used to magnify the specimen up to 4,000 times.
• Compound microscope consists of a stage on which the specimen is placed under an objective piece.
• The light reflected from the mirror is used to illuminate the specimen.
• From the eye piece, a magnified image of the specimen is seen.
• The organisms which cannot be seen by naked eye are selected as specimen.
b) Electron microscope are used for very large magnification of the specimen.
• It uses a beam of electrons to magnify the specimen. Electrons are spread by electromagnets on to a photographic plate to obtain enlarged image of the specimen.
• It magnifies the specimen upto 1,00,000 times.
• It is used to observe very small structures of the size of 10 Ao.
M.J.Scleiden and Theodre Schwann proposed cell theory for the first time.The cell theory can be stated as follows.
• Cell is the basic structural unit of all the organisms.
• Preexisting cells only gave rise to new cells.
• An organism has its body composed of cells.
• A single cell transfers life from one generation to another generation.
Microscopic examination of cells
Plant and animal cells are only visible only under a microscope.
• The microscopic examination of a plant cell enables us view a prominent vacuole, nucleus, and cytoplasm.
• For the microscopic examination of an animal cell, spread the specimen on a glass slide, and add a drop of water and methylene blue. Cells with darkly stained spherical nuclei at their centre can be observed.
Shape of cells
• Cells differ in their shape. The shape of a cell is related to the specific function that it performs. Cells can be round, spherical, elongated, pointed and even branched.
• Shape of the cell also depends on age, pressure, internal skeleton, external skeleton etc.
• Amoeba is irregular in shape.
• Neuron, the nerve cell is a branched structure.
• White Blood Corpuscle is the only animal cell that changes its shape.
• Red Blood Corpuscles are round and flattened.
• Muscle cells are spindle-shaped.
• Plant cell has definite shape.
Motile or Non-motile
• Cells which change their shape from time to time in order to perform locomotion are termed to motile cells. Cells like amoeba (motile) change their shape for motility.
• Cells whcih have fixed shape and do not exhibit any change during locomotion are termed to be non-motile cells. Cells like nerve cells have a fixed shape (non-motile) that suits their function of transmitting nerve impulses.
Unicellular and multicellular organisms
Unicellular organisms are the organisms in which a single cell performs all the functions like nutrition, respiration, excretion and reproduction.
• These are also called as acellular organisms.
e.g. Amoeba, Chlamydomonas, Paramecium and Bacteria possess single cells constituting the whole organism.
Multicellular organisms are the organisms which possess many cells to perform different functions.
• Multicellular organisms represent themselves as a member of a group of cells or as an individual.
e.g. Fungi, plants and animals have many cells that group together to form tissues.
• Division of labour in multicellular organims increases the efficiency of the organism for its better survival
Level of organisation
• A cell is a basic unit of life.
• A group of cells that have similar structure and function constitute a tissue. e.g. A group of hepatic cells form hepatic tissue.
• A group of tissues aggregate to form an organ. e.g. hepatic tissues aggregate to form the organ, liver.
• A group of organs together constitute an organ system. e.g. Buccal cavity, oesophagus, stomach, intestine collectively form the organ system, digestive system.
• A group of organ systems aggregate to form an organism. e.g. Digestive system, respiratory system, circulatory system, muscular system, nervous system reproductive system etc. work together to form a complete organism.
Prokaryotic and eukaryotic cells
Prokaryotic cells are the cells which have indefinite nucleus called as nucleiod. A single chromosome represents the genetic material. Genetic material is not covered by a nuclear membrane e.g Bacteria.
Eukaryotic cells are the cells which possess a definite nucleus. e.g. Plant cell. Eukaryotic cells have nucleus with a distinct nuclear membrane. Chromosomes are considered to be vehicles of inheritance and are seen in nucleoplasm. Eukaryotes can be unicellular such as trypanosoma, euglena and paramecium. They can be multi-cellular such as fungi, plants and animals.
Structure of a prokaryotic cell
• A typical bacterial cell is a prokaryotic cell which possesses following characters.
• The cell wall is a non-living layer composed of polysaccharides and proteins.
• The plasma membrane is a living membrane made up of lipids. It is selectively permeable. Therefore, it transports ions, nutrients and wastes into and out of the cell.
• The cytoplasm is a clear, thick, jelly-like material that forms the medium for all cellular functions.
• The cytoplasm of bacteria contains the nucleoid, ribosome and flagella.
• Nucleoid is a single circular DNA molecule considered as the genetic material.
• Ribosome transfers genetic messages into proteins.
• Flagella are extracellular appendages which help in locomotion.
• Prokaryotes do not have vacuoles.
Differences between a prokaryotic and a eukaryotic cell
Very small cells each measuring from 1µm to 10µm.
Large cells but microscopic measuring from 5µm to 100µm.
Absence of specific nucleus. Genetic material is dispersed as nucleoid.
Presence of nucleus with well defined nuclear membrane.
A single chromosome is present.
Chromosomes are many in number.
Membrane bound cell-oganelles are absent.
Membrane bound cell organelles are present.
The cell is the fundamental and structural unit of life.
Eukaryotic cells are the cells which possess a definite nucleus with a distinct nuclear membrane. Let us observe the contents of a eukaryotic cell. The living structures of a cell are called as cell organelles.
Cell wall: It is the non-living outermost covering of the cell. 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. It is present only in plant cells.
Functions of cell wall
• Cell wall provides rigidity and shape to the cell.
• Cell wall is a dead structure but permeable to substances.
• Cell wall is made mainly of cellulose.
• Cell wall is thick and protective in function.
Cell membrane: It is also called as plasma membrane. It is present both in animal and plant cells. It is a thin living membrane made of lipoproteins. It comprises three layers, a lipid layer sandwiched between two protein layers. It is a selectively permeable membrane.
Functions of cell membrane
• It maintains the shape of the cell. It separates the cellular contents from the external environment.
• It helps in absorption of mechanical shocks and protects the cell from injury.
• It allows the movement of some substances into and out of the cell. Movement of substances through this semi-permeable membrane can be by the process of diffusion, osmosis etc.
• It forms the membrane for various cell organelles.
• It keeps the adjacent cells in contact.
• Its infolds help in absorption of materials by the process of pinocytosis or phagocytosis.
• Its outfolds increase the area for absorption.
Difference between diffusion and osmosis
It is the process by which molecules spread from a region of higher concentration to a region of lower concentration.
It is a process by which solvent molecules move from a region of low concentration to theregion of high concentration through a semipermeable membrane.
Diffusion occurs in all the mediums. Diffusing particles can belong to either one of the states, solid, liquid or a gas.
Osmosis takes place only in liquid medium and only the solvent molecules can move from one place to another.
Semipermeable membrane may not be present.
Presence of semipermeable membraneseparating the two liquids is necessary.
Different types of diffusion may include Brownian motion, facilitated diffusion etc.
Forward osmosis and reverse osmosis are the types of osmosis.
Nucleus: A true nucleus acts as the control centre of the cell. It is covered by a two layered, perforated nuclear membrane allowing substances to enter and leave the nucleus. Nucleus is filled with semifluid colloidal substance called as nucleoplasm in which float the nucleolus and the chromatin fibres. The nucleus contains chromosomes, which are composed of Deoxyribonucleic acid (DNA) and proteins.
Functions of nucleus
• 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.
Cytoplasm: It is a jelly like viscous substance occupying entire cell except the nucleus. The cytoplasm supports and protects the cell organelles that perform different metabolic functions. Cell organelles include endoplasmic reticulum, Ribosomes, Golgi apparatus, Mitochondria, Plastids, Lysosomes, and Vacuoles. Inclusions are the non-living substances present in the cytoplasm.
It is a jelly like homogeneous substance.
It is a transparent fluid present in the nucleus.
Protoplasmic part which lies outside the nucleus.
Protoplasmic part which is present inside the nucleus.
Cell organelles and inclusions are suspended in the cytoplasm.
Chromatin fibres and nucleolus are suspended.
Both inorganic and organic molecules are present.
Composition is rich in nucleotides.
Endoplasmic reticulum: It is a network of tubules and flattened sacs perform various activities in the cell. The space inside the endoplasmic reticulum is called as lumen. Endoplasmic reticulum serves as a channel for the transport of proteins between various regions of the cytoplasm. Two types of endoplasmic reticulum are Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).
ROUGH ENDOPLASMIC RETICULUM
SMOOTH ENDOPLASMIC RETICULUM
These are rough at surface and are associated with ribosomes.
These are smooth at surface and are associated with Golgi bodies.
These are responsible for the synthesis of proteins and enzymes.
These are responsible for the synthesis of glycogen, lipids etc.
These provide structural framework for the cell.
These take part in the synthesis of fats in adipose cells.
These also act as channels for quick transport.
These can also detoxify drugs and some poisons.
These help in the synthesis of membranes.
These help in the formation of rhodopsin from vitamin A.
Ribosomes: These are naked granules with no membrane. They are found scattered in the cytoplasm or attached to the outside of the endoplasmic reticulum. They are the biofactories of proteins. Ribosomes can be free ribosomes scattered in the cytoplasm or bound ribosomes seen attached to endoplasmic reticulum. Ribosomes are made up of two subunits, the larger subunit and the smaller subunit. 70s ribosomes are present in prokaryotes and 80s ribosomes are present in eukaryotes.
Functions of ribosomes
• Ribosomes are considered to be the biofactories since they are the sites of protein synthesis.
• They are the lodging sites for many enzymes participating in the process of protein synthesis.
Golgi apparatus: These cell organelles are named after the biologist, Camillo Golgi, who first described it. The Golgi consists of a stack of membrane-bounded cisternae located between the endoplasmic reticulum and the cell surface.
Functions of Golgi apparatus
• It synthesises certain biopolymers.
• It also consists of some processing enzymes which alter some proteins and phospholipids synthesised by endoplasmic reticulum.
Mitochondria: These are cellular organelles termed as ‘power houses of the cells’. These are bounded by a double membrane. The outer membrane is smooth while the inner membrane is thrown into folds called as cristae. The cristae increase the area of cellular respiration. Cristae on their surface possess structures called oxysomes. Oxysomes are rich in ATP synthetase enzyme. Both the membranes are separated by intermembrane space. Mitochondrial matrix is rich in respiratory enzymes. Mitochondria have their own DNA, RNA and ribosomes to synthesise respiratory enzymes. The enzymes in the mitochondria oxidise glucose molecules to produce energy in the form of Adenosine Triphosphate or ATP.
Functions of mitochondria
• These synthesise energy rich ATP molecules.
• These help in the synthesis of fatty acid, amino acids, steroids by providing them with biological intermediates.
Lysosomes: Lysosomes are membranous sacs filled with enzymes. The enzymes are hydrolytic enzymes which are capable of digesting cellular macromolecules. When the cell gets damaged, the lysosome may burst and its enzymes may digest thecell itself. Hence, lysosomes are called as ‘suicidal bags’.
Functions of lysosomes
• Help in killing foreign cells referred to as pathogens.
• Help in destroying the diseased cells.
Vacuoles: Vacuoles are membrane bound compartments present in both plant and animal cells. These organelles store water, waste products, and substances like amino acids, sugars and proteins. The fluid in the vacuoles is called ‘cell sap’. A vacuole is covered by a living membrane called “tonoplast”. Vacuoles also provide buoyancy to the cell.
Functions of vacuoles
• They help the cell to maintain its buoyancy and turgidity.
• They play an important role in the growth of the cell
Plastids: These are major organelles found only in the cells of plants and algae. These are of three types namely, Chloroplasts, Chromoplasts and Leucoplasts.
a) Chloroplasts are one kind of plastids present mainly in plant cells. Chloroplasts contain green pigment called as chlorophyll. Chlorophyll absorbs energy from the sunlight necessary for photosynthesis.
b) Chromoplasts are the organelles which provide bright colours to the plant structures like buds, flowers etc.
c) Leucoplasts are the organelles which store starch, oils and protein granules.
Functions of plastids
• Plastids are responsible for synthesisof food.
• Plastids are responsible for colouration of different parts of the plant.
• Plastids are responsible for storage of food molecules.
Difference between plant cells and animal cells
Plant cells possess cell wall.
Animal cells do not possess cell wall.
Chloroplasts are present in plant cells.
Animal cells do not possess chloroplasts.
Plant cells are almost rectangular in shape.
Animal cells are round in shape.
Plant cells possess large vacuoles.
Animal cells have many small vacuoles.
Cilia rarely occur in plant cells.
Cilia are present in animal cells.
Higher plants do not possess centrioles.
Animal cells do contain centrioles.
Plant cells have no or less number of lysosomes.
Animal cells possess many number of vacuoles.