Cell : The Unit Of Life

• Cell theory emphasises the unity underlying the diversity of living organisms, highlighting the cellular organisation of all life forms
• Cell structure and cell growth by division are described in the chapters of this unit
• Cell theory created a sense of mystery around living phenomena, requiring integrity of cellular organisation for living processes to be observed
• A physico-chemical approach using cell-free systems can be used to study and understand physiological and behavioural processes in living organisms
• This approach enables the description of various processes in molecular terms and helps identify the types of organic compounds present in living organisms
• The physico-chemical approach to studying living organisms is known as 'Reductionist Biology', applying concepts and techniques of physics and chemistry to understand biology
• G.N. Ramachandran was an influential figure in the field of protein structure, known for his discovery of the triple helical structure of collagen and the 'Ramachandran plot'
• All living organisms are composed of cells, with some being unicellular and others multicellular
• Cell is the fundamental structural and functional unit of all living organisms, as observed by Anton Von Leeuwenhoek and Robert Brown
• Matthias Schleiden and Theodore Schwann formulated the cell theory, stating that all living organisms are composed of cells and products of cells, and all cells arise from pre-existing cells
• Eukaryotic cells have membrane-bound organelles like the endoplasmic reticulum, golgi complex, lysosomes, mitochondria, microbodies, and vacuoles, while prokaryotic cells lack such organelles
• Ribosomes are non-membrane bound organelles found in all cells, responsible for protein synthesis
• Prokaryotic cells have a cell wall, cytoplasm, and lack a well-defined nucleus, with genetic material that is not enveloped by a nuclear membrane
• Most prokaryotic cells have a cell envelope consisting of glycocalyx, cell wall, and plasma membrane, classified as Gram positive or Gram negative based on staining procedures
• Prokaryotic cells may have inclusions for storing reserve materials and may exhibit motility through flagella, pili, and fimbriae
• Ribosomes in prokaryotes are associated with the plasma membrane and are the site of protein synthesis, forming polysomes for translating mRNA into proteins
• Inclusion bodies in prokaryotic cells store reserve materials in the cytoplasm and are not bound by a membrane system
• Eukaryotic cells include protists, plants, animals, and fungi
• Eukaryotic cells have membrane-bound organelles
• Eukaryotic cells have an organised nucleus with a nuclear envelope
• Eukaryotic cells have complex locomotory and cytoskeletal structures
• Genetic material in eukaryotic cells is organised into chromosomes
• Plant cells have cell walls, plastids, and a large central vacuole
• Animal cells have centrioles
• Cell membrane structure:

  • Composed of lipids and proteins
  • Major lipids are phospholipids arranged in a bilayer
  • Lipids have polar heads towards outer sides and hydrophobic tails towards inner part
  • Membrane also contains cholesterol
  • Membrane proteins can be integral or peripheral
• Fluid mosaic model of plasma membrane:

  • Proposed by Singer and Nicolson in 1972
  • Lipid nature enables lateral movement of proteins within the bilayer
  • Membrane fluidity is important for cell functions like growth, junction formation, secretion, and division
  • Selectively permeable membrane allows passive transport of molecules
  • Passive transport includes simple diffusion and osmosis
  • Active transport requires energy, e.g., Na+/K+ Pump
• Cell wall:

  • Forms outer covering for plasma membrane in fungi and plants
  • Gives shape, protection, and cell-to-cell interaction
  • Plant cell walls made of cellulose, hemicellulose, pectins, and proteins
  • Middle lamella mainly of calcium pectate glues neighbouring cells together
• Endomembrane system:

  • Includes endoplasmic reticulum (ER), golgi complex, lysosomes, and vacuoles
  • Mitochondria, chloroplasts, and peroxisomes are not part of the endomembrane system
• Endoplasmic Reticulum (ER):

  • Rough ER has ribosomes, smooth ER is for lipid synthesis
• Golgi apparatus:

  • Stacked disc-shaped sacs for packaging materials
  • Modifies proteins before release
• Lysosomes:

  • Membrane-bound vesicles with hydrolytic enzymes
  • Digest carbohydrates, proteins, lipids, and nucleic acids
• Vacuoles:

  • Membrane-bound space in cytoplasm
  • Contains water, sap, and excretory products
  • Plant vacuoles can occupy up to 90% of cell volume
• Mitochondria:

  • Double membrane-bound structure with inner matrix and cristae
  • Sites of aerobic respiration, produce ATP
  • Possess DNA, RNA, ribosomes, and enzymes for protein synthesis
• Plastids:

  • Found in plant cells and euglenoides
  • Classified into chloroplasts, chromoplasts, and leucoplasts
  • Chloroplasts contain chlorophyll for photosynthesis
• Chloroplasts are double membrane bound organelles
• The inner chloroplast membrane is relatively less permeable
• The space limited by the inner membrane of the chloroplast is called the stroma
• Thylakoids are organised flattened membranous sacs present in the stroma
• Thylakoids are arranged in stacks called grana or the intergranal thylakoids
• Stroma lamellae are flat membranous tubules connecting the thylakoids of different grana