Notes Cell The Unit of Life Class 11 Biology

Notes Class 11

Please refer to the Cell The Unit of Life Notes Class 11 Biology given below. These revision notes have been designed as per the latest NCERT, CBSE, and KVS books issued for the current academic year. Students will be able to understand the entire chapter in your class 11th Biology book. We have provided chapter-wise Notes for Class 11 Biology as per the latest examination pattern.

Revision Notes Chapter 8 Cell The Unit of Life Class 11 Biology

Students of Class 11 Biology will be able to revise the entire chapter and also learn all important concepts based on the topic-wise notes given below. Our best teachers for Grade 11 have prepared these to help you get better marks in upcoming examinations. These revision notes cover all important topics given in this chapter.

1. Basic unit of life → the cell in all living organisms.

2. Unicellular organisms are capable of →
o (i) independent existence and
o (ii) performing the essential functions of life.

3. Anything less than a complete structure of a cell does not ensure independent living.
o Cell → fundamental structural & functional unit of all living organisms.
4. Anton Von Leeuwenhoek → first saw and described a live cell.
5. Robert Brown → discovered the nucleus.
6. CELL THEORY 1838 – 1839 → Schleiden & Schwann
o Matthias Schleiden, a German botanist
o Theodore Schwann , a British Zoologist,
7. Rudolf Virchow (1855) → Omnis cellula-e cellula
(New cells are formed from pre-existing cells)

8. Cell theory :
o All living organisms are composed of cells and products of cells.
o All cells arise from pre-existing cells.

9. Type of Cells
o Membrane bound nuclei are called eukaryotic
o Lack a membrane bound nucleus are prokaryotic.

10. The cytoplasm → main arena of cellular activities
o Various chemical reactions occur in it to keep the cell in the ‘living state’.

11. The prokaryotic cells lack membrane bound organelles.
o Ribosomes are non-membrane bound organelles found in all cells

12. Within the cell, ribosomes are also found → in the two organelles –
o Chloroplasts (in plants) and
o Mitochondria

13. Animal cells contain → Another non-membrane bound organelle → centriole
o Helps in cell division.

14. Size of Cells
o Mycoplasmas, the smallest cells, (0.3 μm in length)
o Bacteria (3 to 5 μm)
o The largest isolated single cell → Egg of an ostrich
o Human red blood cells → 7.0 μm in diameter.
o Nerve cells are the longest cells.

15. The Prokaryotic cells →
o Bacteria,
o Blue-green algae,
o Mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).

16. Prokaryotes → Multiply more rapidly than the eukaryotic cells

17. The four basic shapes of bacteria
o Bacillus (rod like),
o Coccus (spherical),
o Vibrio (comma shaped) and
o Spirillum (spiral).

18. All prokaryotes have a cell wall ; except mycoplasma.

19. Prokaryotes → no well-defined nucleus. ( genetic material naked, single circular DNA)

20. Bacteria have small circular DNA outside the genomic DNA. → Plasmids.
o The plasmid DNA confers Resistance to antibiotics.

21. Nuclear membrane is found in eukaryotes.

22. Mesosome → Characteristic of prokaryotes.
o Infoldings of cell membrane.
o Help in cell wall formation, DNA replication and distribution to daughter cells.
o Also help in respiration, secretion processes, to increase the surface area of the plasma membrane and enzymatic content.

23. Most bacterial cells → Have a chemically complex cell envelope.
o Tightly bound three layered
o Outermost glycocalyx then cell wall and then the plasma membrane.
o Glycocalyx → If loose sheath (slime layer) or If thick and tough (Capsule)

24. Bacteria can be classified into two groups
o On the basis of the differences in the cell envelopes and the manner
o Gram positive and Gram negative bacteria.

25. The cell wall functions in Bacterial cell
o determines the shape of the cell
o Provides a strong structural support (prevent bursting or collapsing)

26. The plasma membrane →
o semi-permeable
o structurally similar in both prokaryotes & eukaryotes.

27. Chromatophores → present in prokaryotes like cyanobacteria,
o Membranous extensions into the cytoplasm which contain pigments.

28. Bacterial cells motility
o May be motile or non-motile.
o Thin filamentous extensions from their cell wall called flagella.
o Bacterial flagellum is composed of three parts – filament, hook and basal body.
o The filament is the longest portion and extends from the cell surface to the outside.

29. Pili and Fimbriae →
o Surface structures of the bacteria
o Do not play a role in motility.
o The pili are elongated tubular structures made of a special protein.
o The fimbriae are small bristle like fibres sprouting out of the cell.
o Fimbriae are known to help attach the bacteria to surface

30. Ribosomes in Prokaryotes → 70s Type
o Associated with the plasma membrane of the cell.
o About 15 nm by 20 nm in size
o Made of two subunits – 50S and 30S units
o Site of protein synthesis.
o Several ribosomes may attach to a single mRNA → Polyribosomes or polysome.
o The ribosomes of a Polysome translate the mRNA into proteins.

31. Inclusion bodies in Prokaryotic cells →
o Reserve material in the cytoplasm
o Not bound by any membrane
o Phosphate granules, Cyanophycean granules and Glycogen granules.

32. Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.

33. The Eukaryotes includes →
o All the Protists, Plants, Animals and Fungi (4 Kingdom out of 5 are eukaryotic)

34. Eukaryotic cells characteristics →
o Extensive compartmentalisation of cytoplasm (Membrane bound organelles)
o An organised nucleus with a nuclear envelope,
o Variety of complex locomotory and cytoskeletal structures.
o Their genetic material is organised into chromosomes.

35. Cell walls, plastids and a large central vacuole → Absent in animal cells.

36. Centrioles → Absent in almost all plant cells.

37. Most studies on the cell membrane → Human red blood cells (RBCs

38. Cell membrane features →
o Lipid bilayer. (Polar head outer sides & hydrophobic tails inner part)
o The lipid component → Mainly phosphoglycerides.
o Also possess protein and carbohydrate.
o Membrane of the erythrocyte → 52 % protein and 40 % lipids.
o Peripheral proteins → On surface of membrane
o Integral proteins → partially or totally buried in the membrane.

39. Most accepted model of cell membrane →
o Proposed by Singer & Nicolson in 1972 (fluid mosaic model)

40. Advantage of quasi-fluid nature of lipid in membrane →
o Enables lateral movement of proteins within the overall bilayer.
o This ability to move within the membrane is measured as its fluidity.
o The fluid nature of the membrane → important for cell growth, formation of intercellular junctions, secretion, endocytosis, cell division etc.

41. Most important functions of the plasma membrane
o Transport of the molecules across it.
o The membrane is selectively permeable.

42. Transport across Membrane
o Without any requirement of energy → Passive transport
o Neutral solutes → Simple diffusion along the concentration gradient,
o Movement of water by diffusion is called osmosis.
o Polar molecules cannot pass through the nonpolar lipid bilayer so they require a arrier protein of the membrane to facilitate their transport across the membrane.
o A few ions or molecules are transported against their concentration gradient & energy dependent process → Active transport, e.g., Na+/K+ Pump.

43. Cell wall features →
o Non-living rigid structure
o Outer covering for the plasma membrane of fungi and plants.
o Gives shape, protects the cell , helps in cell-to-cell interaction and provides barrier to undesirable macromolecules.
o Algae cell wall → Cellulose, Galactans, Mannans and Minerals like calcium carbonate,
o Other plants wall → cellulose, hemicellulose, pectins and proteins.
o Primary wall is capable of growth
o Secondary wall is formed on the inner side of the cell.

44. The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together.

45. Plasmodesmata → Connect the cytoplasm of neighbouring Plant cells.

46. The endomembrane system → include endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles.

47. Mitochondria, chloroplast and peroxisomes are not considered as part of the endomembrane system.

48. Endoplasmic Reticulum (ER) →
o Network or reticulum of tiny tubular structures
o Scattered in the cytoplasm
o Divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
o If bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).
o In the absence of ribosomes they appear smooth ( smooth endoplasmic reticulum) 

49. RER is frequently observed in the cells actively involved in protein synthesis and secretion.
o They are extensive and continuous with the outer membrane of the nucleus.

50. The smooth endoplasmic reticulum is the major site for synthesis of lipid.
o In animal cells lipid-like steroidal hormones are synthesised in SER.

51. Golgi Bodies →
o Camillo Golgi (1898) first observed Golgi bodies
o Consist of many flat, disc-shaped sacs → Cisternae [0.5μm to 1.0μm diameter]
o The Golgi cisternae are concentrically arranged near the nucleus
o Have Polarity → Distinct convex cis or the forming face and concave trans or the maturing face.
o The cis and the trans faces of the organelle are entirely different, but interconnected.
o Principally performs the function of packaging materials, to be delivered either to the intra-cellular targets or secreted outside the cell.
o Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the golgi apparatus and move towards the maturing face. This explains, why the golgi apparatus remains in close association with the endoplasmic reticulum.

52. Protein Modification in Cell →
o Proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its trans face.
o Golgi apparatus is the important site of formation of glycoproteins and glycolipids.

53. Lysosomes
o Membrane bound vesicular structures
o Formed by the golgi apparatus.
o Very rich in almost all types of hydrolytic enzymes
o Optimally active at the acidic pH.
o Enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.

54. The vacuole
o Membrane-bound space found in the cytoplasm.
o It contains water, sap, excretory product and other materials not useful for the cell.
o The vacuole is bound by a single membrane called tonoplast.
o In plant cells the vacuoles can occupy up to 90 per cent of the volume of the cell.
o In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole, hence their concentration is significantly higher in the vacuole than in the cytoplasm.
o In Amoeba → the contractile vacuole is important for excretion.
o In many cells, as in protists, food vacuoles are formed by engulfing the food particles.

55. Mitochondria features →
o Unless specifically stained, are not easily visible under the microscope.
o The number of mitochondria per cell → depending on the physiological
o In terms of shape and size also, considerable degree of variability is observed.
o Typically it is sausage-shaped or cylindrical
o Having a diameter of 0.2-1.0μm (average 0.5μm) and length 1.0-4.1μm.
o Double membrane-bound structure
o The outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the outer compartment and the inner compartment.
o The inner compartment is called the matrix.
o The outer membrane forms the continuous limiting boundary of the organelle.
o The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix .
o The cristae increase the surface area.
o The two membranes have their own specific enzymes associated with the mitochondrial function.
o Mitochondria are the sites of aerobic respiration.
o They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.
o The matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins.
o The mitochondria divide by fission.

56. Plastids are found in all plant cells and in euglenoides.
o Easily observed under the microscope
o Bear specific pigments → imparting specific colours to the plants.
o Based on the type of pigments → chloroplasts, chromoplasts and leucoplasts.
o The chloroplasts contain chlorophyll and carotenoid pigments → Photosynthesis.
o In the chromoplasts fat soluble carotenoid pigments like carotene, xanthophylls and others are present. This gives the part of the plant a yellow, orange or red colour.
o The Leucoplasts → the colourless plastids ; stored nutrients:
o Leucoplast types → Amyloplasts store carbohydrates (starch), e.g., potato; elaioplasts store oils and fats whereas the aleuroplasts store proteins.

55. Chloroplasts of the green plants
o found in the mesophyll cells of the leaves.
o Length (5-10μm) and width (2-4μm).
o 1 per cell of the Chlamydomonas, a green alga to 20-40 per cell in the mesophyll.
o Double membrane bound; inner chloroplast membrane is relatively less permeable.
o The space limited by the inner membrane of the chloroplast is called the stroma.
o A number of organised flattened membranous sacs called the thylakoids, are present in the stroma
o Thylakoids are arranged in stacks like the piles of coins called grana
o In addition, there are flat membranous tubules called the stroma lamellae connecting the thylakoids of the different grana.
o The membrane of the thylakoids enclose a space called a lumen.
o The stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins.
o It also contains small, double-stranded circular DNA molecules and ribosomes.
o Chlorophyll pigments are present in the thylakoids.
o The ribosomes of the chloroplasts → 70S type

56. Ribosomes
o Granular structures observed → under electron microscope by George Palade (1953).
o Composed of ribonucleic acid (RNA) and proteins
o Not surrounded by any membrane.
o The eukaryotic ribosomes are 80S while the prokaryotic ribosomes are 70S.
o Here ‘S’ (Svedberg’s Unit) stands for the sedimentation coefficient; it indirectly is a measure of density and size.
o Both 70S and 80S ribosomes are composed of two subunits.

57. Cytoskeleton
o Network of filamentous proteinaceous structures
o Present in the cytoplasm
o Involved in many functions such as mechanical support, motility, maintenance of the shape of the cell.

58. Cilia and Flagella
o Hair-like outgrowths of the cell membrane.
o The prokaryotic flagella ; structurally different from that of the eukaryotic flagella.
o Cilium or the flagellum → Covered with plasma membrane.
o Their core called the axoneme, possesses a number of microtubules running parallel to the long axis.
o Arrangement of axonemal microtubules is referred to as the 9+2 array
o Both the cilium and flagellum emerge from the basal bodies.

59. Centrosome and Centrioles
o Centrosome is an organelle
o Containing two cylindrical structures called centrioles.
o Surrounded by amorphous Pericentriolar materials.
o Both the centrioles lie perpendicular to each other
o Organisation like the cartwheel.
o Made up of nine evenly spaced peripheral fibrils of tubulin protein.
o Each of the peripheral fibril is a triplet.
o The central part of the centriole → proteinaceous ; called the hub
o Hub is connected with tubules of the peripheral triplets by radial spokes made of protein.
o The centrioles form the basal body of cilia or flagella, and spindle fibres that give rise to spindle apparatus during cell division in animal cells.

60. Nucleus
o First described by Robert Brown
o Term chromatin given by Flemming.
o The interphase nucleus has highly extended nucleoprotein fibres → Chromatin
o Nuclear envelope → Porous double membranes with a space ; perinuclear space
o The outer membrane → continuous with the ER and also bears ribosomes
o Nuclear pores → Passages for movement of RNA and protein in both directions; between the nucleus and the cytoplasm.
o Mature cells lack nucleus, e.g., erythrocytes and sieve tube cells
o The nuclear matrix or the nucleoplasm contains nucleolus and chromatin.
o The nucleoli are spherical structures present in the nucleoplasm.
o The content of nucleolus is continuous with the rest of the nucleoplasm → Not a membrane bound structure.
o Nucleolus is a site for active ribosomal RNA synthesis.
o Larger and more numerous nucleoli → In cells actively carrying out protein synthesis.

61. Interphase nucleus has →
o Loose and indistinct network of nucleoprotein fibres called chromatin.

62. During cell division
o Cells show structured chromosomes (Condensed chromatin)

63. Chromatin contains →
o DNA, Basic proteins histones, Non-histone proteins and also RNA.

64. A single human cell has approximately two metre long DNA distributed among its forty six (twenty three pairs) chromosomes.

65. Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present

66. Based on the position of the centromere, the chromosomes can be classified into four types
o The metacentric chromosome has middle centromere (equal arms)
o The sub-metacentric chromosome has centromere slightly away from the middle of the chromosome (one shorter arm and one longer arm)
o The acrocentric chromosome the centromere is situated close to its end forming one
extremely short and one very long arm
o The telocentric chromosome has a terminal centromere.

67. Few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite.

68. Microbodies
o Many membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells.