Ch 08 - Cell : The unit of Life || Class 11 ||

Chapter 08 - Cell : The unit of Life
|| Class 11 ||
NCERT
CBSE

1. CHAPTER 08 CELL – THE UNIT OF LIFE

2. Discovery of the cell In 1965 robert hooke observed slices of cork from the bark of a tree under a simple magnifying glass. He noticed small boxes or compartments in the cork slice. The boxes were separated from each other by a wall or partition. He named each box as a cell. the cells which hooke observed were actually dead cells of plants.

3. Cell Theory Cell Theory was formulated by Scheleiden and Schwann, and was modified by Rudolf Virchow . The cell is the smallest unit of life. Cell is basic structural and functional unit of living organisms. New cells arise from pre-existing cells. (Omnis cellula -e cellula ) All organisms are composed one or more cells .

4. 1. Prokaryotic cell In Greek, pro : primitive; karyon : nucleus. No Nucleus No membrane-bound organelles like mitochondria or chloroplasts. Bacteria and Blue-Green algae Reproduce asexually by budding and fission First type of cell on the earth Very small (1 - 10 μ m ) DNA without histone covering Eg . Bacteria, Mycoplasma, Blue green Algae

5. 2 . Eukaryotic cell In Greek, Eu : True; karyon : nucleus. True Nucleus Membrane bound organelles All other organisms except Bacteria and Blue-Green algae Cell size generally 10 - 100 μ m Include fungi, protists , plant, and animal cells. Sunday, December 10

6. Cells are the basic structural units of living organisms . Some organisms are made up of a single cell and some are made up of many cells. The number of cells in a tall tree or an animal like an elephant or human being are billions and trillions of cells. Organisms made up of many cells are called multicellular organisms. Eg : a tree, an elephant. Organisms made up of only one cell are called unicellular organisms. Eg : amoeba, paramaecium . The Cell

7. Shape of cells Cells are of different shapes. Some cells keep changing their shape. Eg : amoeba, white blood cells in human beings. Other common shapes – spherical, round , elongated , spindle shaped, branched .

8. Size of cells Cells in living organisms are of different sizes. They may be as small as millionth of a meter ( micrometre or micron) or large as a few centimetres . The smallest cell is the cell of bacteria PPLO(0.1 to 0.5 micrometre ). The largest cell is the egg of an ostrich (17cm x 13cm).

9. Cell Membrane (plasma membrane ) Outer membrane of cell that controls the entry and exit of substances according to the requirement of cell the cell. Sanger and Nicolson (1972) gave fluid mosaic model’ . According to this the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer two types of proteins (Peripheral and integral proteins ) with cholesterol, glycolipids and glycoporteins . Erythrocyte membrane has 52% protein and 40% lipids . Function - — It is semi permeable and helps in transport of molecule across it.

10. Passive transport Transport of molecules from higher concentration It do not utilise energy (ATP ). e.g ., diffusion Active transport Transport of molecules from lower to higher to lower concentration. It utilises energy (ATP ) e.g ., Na+/k+ ATPase Pump.

11. Mesosomes Formed by extension of plasma membrane into cell . In the form of vesicles, tubules and lamella. Help in cell wall formation, DNA replication and distribution to daughter cells. Also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content

12. Cell Wall It provide rigidity, Supports, & shape to cell. Non-living material Found in plants, fungi, bacteria. Surrounds plasma membrane Protects cell from mechanical damage and infection, helps in cell-to-cell interaction provides barrier to undesirable macromolecules. Cell wall of algae is made of cellulose, galactans , mannans and minerals like calcium carbonate. Plant cell wall consists of cellulose, hemicellulose, pectins and proteins.

13. Layers of cell wall Lamella Outermost Made up of mainly calcium pectate . Holds or glues the different neighbouring cell together. 2. Primary wall Capable of growth. Present in young cell. Gradually diminishes as cell matures. Madeup of cellulose, hemicelluloses. Present in meristem, pith, cortex etc.

14. 3 . Secondary wall Innermost layer. Lignified (in sclerenchyma, vesels , tracheids ), suberinised ( casparian strips, endodermis) Suberin , lignin make cell wall impermeable. Present in sclerenchyma, collenchyma, and vessels, tracheids . Cell wall and middle lamella maybe traversed by plasmodesmata which connects the cytoplasm of neighbouring cells.

15. Modification of cell envelope Slime layer : Clycocalyx in form of loose sheath. Capsule : Glycocalyx in form of thick and tough sheath. Mesosomes : Extension of plasma membrane. These can be in the form of vesicles , tubules and lamellae.

16. Cytoplasm The portion of the protoplasm lying inner to cell membrane, but outside the nuclear membrane. It comprises of Gel-like substances that fills the inside of the cell. Contains hereditary material

17. Nucleus first described by Robert Brown . the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming . The interphase nucleus has nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called the nuclear envelope is consists of two parallel membranes with a space inbetween called perinuclear space. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it. At a number of places the nuclear envelope is interrupted by minute pores. These nuclear pores provide passages for movement of RNA and protein molecules .

18. Normally, there is only one nucleus per cell.Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants . The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. It is non-membrane bound. It is a site for active ribosomal RNA synthesis. During cell division, chromatin network condenses into c Chromatin contains DNA and some basic proteins called histones, some non-histone proteins and also RNA. Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present.

19. Nuclear Membrane Surrounds nucleus Made of two layers Openings allow material to enter and leave nucleus Chromosomes It is made of DNA and present in nucleus It is responsible for the inheritance of characters (genes) from one generation to other

20. Cell Organelles

21. Mitochondria They are rod shape or spherical structures. Double membraned structure. Outer membrane smooth and inner membrane forms a number of infoldings called cristae The inner compartment is called matrix. The cristae increase the surface area. Responsible for cellular respiration & produces energy for different activities. So , it is called power house of cell Controls level of water and other materials in cell Recycles and decomposes proteins, fats, and carbohydrates Matrix possesses single circular DNA molecule , a few RNA molecules, ribsomes (70S). It divides by binary fission .

22. Plastids : Plastid are large organelles & characteristic of plant cell. Chloroplasts , chromoplasts and leucoplasts are 3 types of plastids depending on pigments contained. CHLOROPLAST - green coloured plastids, contains chlorophyll, Ds DNA and 70s Ribosomes CHROMOPLAST - carotenoid (fat souble carotens (give colour to plant parts) Xanthophylls . Others (Yellow, red, orange ) LEUCOPLAST - Amyloplast (Starch), Eliaoplasts (oil + fat), Alecuroplast (store proteins ) Function : Site of photosynthesis, and imparts colours to fruits and flowers .

23. Endoplasmic Reticulum It is network of membranes . It provides a skeletal framework to the cell. Moves materials around in cell. ER divides the intracellular space into two distinct compartments – luminal (inside ER) and extra luminal (cytoplasm). Rough Endoplasmic Reticulum (RER) :Ribosomes attached to outer surface. Function : Involved in protein synthesis and secretion. 2. Smooth Endoplasmic Reticulum (SER) : Lack ribosomes. Function : Site for synthesis of lipid.

24. Ribosomes Composed of RNA and proteins; without membrane. Eucaryotic ribosomes are 80S (60+40) S = Swedberg ’s unit Prokaryotic ribosomes 70S ( 5 0+30 ) Function : Site of protein synthesis. Present in the cytoplasm & float throughout the cell

25. Golgi Complex Sac-like structure stacked one above the other Store the material produced by cell So they are called store house of the cell First observed by Camillo Golgi (in 1898) Consist of cisternae stacked parallel to each other. Two faces of the organelle are convex/ cis or forming face and concave/trans or maturing face. Functions : Performs packaging of materials, to be delivered either to the intra-cellar targets or secreted outside the cell. Important site of formation of glycoproteins and glycolipids.

26. Lysosomes : Membrane bound vesicular structures formed by the process of packaging in the golgi apparatus. Contain hydrolysing enzymes (lipases, proteases, carbohydroses ) which are active in acidic pH. Also called ‘Suicidal Bag’. Function : Intracellular digestion.

27. Vacuoles Membrane-bound space found in the cytoplasm. Membrane known as tonoplast . It contains water, sap, excretory product and other materials not useful for the cell. In plant cells the vacuoles are very large. In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole. In Amoeba the contractile vacuole is important for excretion. In many cells food vacuoles are formed by engulfing the food particles.

28. Cilia and flagella They are hair like outgrowths of cell membrane responsible for locomotion and movement of cell . Cilia are small structures which work like oars, causing the movement of either the cell or the surrounding fluid. Flagella are comparatively longer. Eukaryotic cilium and flagellum are covered with plasma membrane. Their core called the axoneme , possesses a number of microtubules running parallel to the long axis. The axoneme usually has nine pairs of doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. (9+2) Both the cilium and flagellum emerge from centriole-like structure called the basal bodies.

29. Centrosome and Centrioles Centrosome contains two cylindrical structures called centrioles. Surounded by amorphous pericentriolar material. It Has 9 + 0 arrangement. Centrioles form the basal body of cilia or flagella and spindle fibres for cell division in animal cells. They produces spindle apparatus during cell divison . Nucleus : With double membrane with perinuclear space and nuclear pores; has Chromatin, nuclear matrix and nucleoli (site for rRNA synthesis). ( Named by Robert Brown – 1831) Chromatin DNA + nonhistone proteins. (Named by Flemming ) Nucleoplasm – Nucleolus + Chromatin

30. Nuclear membrane -—It is with perinuclear space and nucleopores . Centromere : Primary constriction –disc is known as kinetochores. Chromosomes (on basis of position of centromere) : Metacentric : Middle centromere. Sub-metacentric : Centromere nearer to one end of chromosomes. Acrocentric : Centromere situated close to its end. Telocentric :Has terminal centromere. Satellite : Some chromosomes have non-staining secondary constructions at a constant location, which gives the appearance of small fragment called satellite.

31. Cytoskeleton Microtubules – Present in cilia , flagella, centrioles , and basal bodies Hollow unbranched25nm in diameter with hollow core of 15nm 13 proto-filaments , which is made up of alpha and beta subunit of tubulin protein (non contractile ) GTP and calcium requires for assembly and disassembly 2. Microfilaments – Solid , un-branched rod like fibrils of indefinite length. globular actin and filamentous myosin are contractile

32. 3 . Intermediate filaments – non contractile hollow filament made up of acidic proteins Forms scaffolds for chromatin Basket around nucleus

33. Functions Microtubules – Spindle and astral rays formations. Cytoskeleton of cilia and flagella shape rigidity and motility Anaphasic movement of chromosome Intra cellular transports Position of future cell plate

34. 2. Microfilaments – Support to plasma membrane forms contractile system of the cell and involve in cytoplasmic streaming Pseudopodia formation Formation of cleavage and furrow during cell division

35. Structure of an animal cell

36. Comparison of Plant cell and Animal cell Sl.No . Part Plant cell Animal cell 1 Cell membrane Present Present 2 Cell wall Present Absent 3 Nucleus Present Present 4 Nuclear membrane Present Present 5 Cytoplasm Present Present 6 Plastids Present Absent 7 Vacuole Present (smaller in size Present (larger in size)

37. Levels of organization of living being

38. CELL : cell is the smallest unit of life. TISSUE : Group of similar cells that perform specific function. ORGAN : Group of different tissues form an organ. Ex – liver, kidney, stomach ORGAN SYSTEM: Group of organ that work together to perform specific functions. Ex- Digestive System , circulatory system etc. ORGANISM : Different organ systems work together to form organism.

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