Cell the unit of life

• When you look around, you see both living and non-living things. You must have wondered and asked yourself – 'what is it that makes an organism living, or what is it that an inanimate thing does not have which a living thing has' ? The answer to this is the presence of the basic unit of life – the cell in all living organisms.
• All organisms are composed of cells. Some are composed of a single cell and are called unicellular organisms while others, like us, composed of many cells, are called multicellular organisms.
• Unicellular organisms

  Capable of (i) independent existence and (ii) performing the essential functions of life
• Anything less than a complete structure of a cell does not ensure independent living. Hence, cell is the fundamental structural and functional unit of all living organisms.
• Cell


  • First seen and described by Anton Von Leeuwenhoek
  • Nucleus discovered by Robert Brown
  • Structural details revealed by microscope and electron microscope
• Cell theory


  • Proposed by Matthias Schleiden and Theodore Schwann
  • All living organisms are composed of cells and products of cells
  • All cells arise from pre-existing cells
• Cells that have membrane bound nuclei are called eukaryotic whereas cells that lack a membrane bound nucleus are prokaryotic.
• Components of eukaryotic cells

  • Nucleus
  • Cytoplasm
  • Endoplasmic reticulum
  • Golgi complex
  • Lysosomes
  • Mitochondria
  • Microbodies
  • Vacuoles
  • Ribosomes
  • Centrosome
• Prokaryotic cells lack membrane bound organelles.
• Components of prokaryotic cells

  • Cell wall
  • Cell membrane
  • Cytoplasm
  • Genetic material (circular DNA)
  • Plasmids
  • Ribosomes
  • Inclusions
  • Mesosome
• Prokaryotic cells


  • Generally smaller and multiply more rapidly than eukaryotic cells
  • Four basic shapes: bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral)
• Cell envelope of prokaryotes

  • Consists of glycocalyx, cell wall and plasma membrane
  • Gram positive and Gram negative bacteria based on cell envelope
• Bacterial cell structures


  • Flagella for motility
  • Pili and fimbriae for attachment
  • Mesosome for cell wall formation, DNA replication and distribution, respiration, secretion
• Ribosomes in prokaryotes


  • Associated with plasma membrane
  • 70S ribosomes made of 50S and 30S subunits
  • Site of protein synthesis
• Inclusion bodies in prokaryotes

  Stores reserve material like phosphate granules, cyanophycean granules, glycogen granules, gas vacuoles
• Eukaryotic cells possess an organised nucleus with a nuclear envelope, and a variety of complex locomotory and cytoskeletal structures. Their genetic material is organised into chromosomes.
• Plant and animal eukaryotic cells differ in the presence of cell walls, plastids and a large central vacuole in plant cells, and centrioles in animal cells.
• Cell membrane


  • Composed of lipids (phospholipids, cholesterol) and proteins
  • Phospholipids arranged in a bilayer with polar heads outward and non-polar tails inward
• Cell membrane


  • Mainly composed of lipids and proteins
  • Major lipids are phospholipids arranged in a bilayer
  • Lipids arranged with polar heads towards outer sides and hydrophobic tails towards inner part
  • Also contains cholesterol
• Integral proteins


  Partially or totally buried in the membrane
• Peripheral proteins


  Lie on the surface of membrane
• Fluid mosaic model


  Quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer
• Membrane fluidity


  • Important for functions like cell growth, formation of intercellular junctions, secretion, endocytosis, cell division
• Passive transport

  1. Molecules can move briefly across the membrane without any requirement of energy

  2. Neutral solutes may move by simple diffusion along concentration gradient

  3. Water may move by osmosis from higher to lower concentration
• Active transport

  Molecules transported against concentration gradient using energy from ATP, e.g. Na+/K+ Pump
• Cell wall

  • Rigid structure forming outer covering for plasma membrane in fungi and plants
  • Gives shape, protects from damage and infection, facilitates cell-cell interaction, provides barrier
• Composition of plant cell wall

  • Cellulose
  • Hemicellulose
  • Pectins
  • Proteins
• Primary cell wall

  Capable of growth, diminishes as secondary wall forms
• Middle lamella

  Layer of calcium pectate holding neighbouring cells together
• Components of endomembrane system

  • Endoplasmic reticulum
  • Golgi complex
  • Lysosomes
  • Vacuoles
• Endoplasmic reticulum (ER)

  • Network of tubular structures dividing intracellular space into luminal and extra-luminal compartments
  • Rough ER has ribosomes attached, Smooth ER lacks ribosomes
• Rough ER

  Involved in protein synthesis and secretion
• Smooth ER

  Major site for lipid synthesis, including steroidal hormones in animal cells
• Golgi apparatus


  • Consists of stacked, disc-shaped cisternae
  • Functions in packaging and modification of materials from ER before delivery or secretion
• Lysosomes


  Membrane-bound vesicles containing hydrolytic enzymes for digestion of carbohydrates, proteins, lipids and nucleic acids
• Vacuoles


  • Membrane-bound spaces containing water, sap, excretory products and other materials
  • In plants, facilitate transport of ions and materials against concentration gradients
• Mitochondria


  • Double membrane-bound organelles with outer membrane, inner membrane forming cristae, and matrix
  • Sites of aerobic respiration, producing ATP, hence called 'power houses'
  • Contain their own DNA, RNA, ribosomes for protein synthesis
• Plastids


  • Found in plant cells and euglenoids, bear specific pigments
  • Classified as chloroplasts, chromoplasts and leucoplasts based on pigments
• Chloroplasts

  Contain chlorophyll and carotenoid pigments for photosynthesis
• Chromoplasts

  Contain fat-soluble carotenoid pigments, giving yellow, orange or red colour