Strucrure and function of organelles

• Eukaryotic cells


  Basic unit of structure and function in animals and plants
• Animal cells


  • Membrane-bound organelles: nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.
  • Lack a cell wall
  • Lack chloroplasts (with rare exceptions)
  • Highly specialized for various functions like movement, nerve signalling, and hormone production

  
• Plant cells


  • Membrane-bound organelles: nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, etc.
  • Have a rigid cell wall made of cellulose
  • Contain chloroplasts for photosynthesis
  • Have large central vacuoles filled with cell sap
  • Support plant structure, produce energy through photosynthesis, and store nutrients and waste products

  
• Organelle


  Specialized structures that perform specific jobs in the cell
• Organelles are found only in eukaryotic cells
• Many organelles are "membrane-bound" (a membrane surrounds the organelle)

  
• Cytosol


  The stuff in between the organelles
• Cytoplasm


  Everything in a cell except the nucleus
• Cell wall


  The rigid, permeable protective layer in some cell types
• Protozoans and animals do not have a cell wall
• Composition of plant cell wall

  • Middle lamella
  • Primary cell wall
  • Secondary cell wall

  
• Middle lamella


  First layer formed during cell division, rich in pectin, outermost layer that joins together adjacent plant cells
• Primary cell wall


  Formed after the middle lamella, composed of pectin compounds, hemicellulose and glycoproteins, a framework of cellulose micro-fibrils in a gel-like matrix, thin, flexible and extensible
• Secondary cell wall


  Thick layer formed inside the primary cell wall, extremely rigid and provides strength, composed of cellulose, hemicellulose and lignin
• Plasmodesmata


  Microscopic channels that traverse the cell walls of plant cells, creating cytoplasmic connections between adjacent cells, facilitate communication and transport of materials

  
• Pits, Plasmodesma, and Plasmodesmata


  • Pits: Small indentations or depressions in the cell wall that play a role in the transport of water, ions, and small molecules
  • Plasmodesmata: Microscopic channels that traverse the cell walls of plant cells, creating cytoplasmic connections between adjacent cells
  • Plasmodesma: Singular form of Plasmodesmata

  
• Functions of cell wall

  • Support
  • Withstand turgor pressure
  • Regulate growth
  • Regulate diffusion
  • Communication
  • Protection
  • Storage
• Plasma membrane


  Contains cell contents, double layer of phospholipids & proteins

  
• Fluid mosaic model


  Describes the structure of biological membranes as a fluid lipid bilayer with embedded proteins

  
• Key points of fluid mosaic model

  • Lipid bilayer: Continuous lipid bilayer made up of phospholipids, fluid and dynamic
  • Protein mosaic: Integral membrane proteins embedded within the lipid bilayer, vary in size, shape, and function, can move laterally
  • Fluidity: Biological membranes are fluid structures, allowing for various biological processes

  
• Cell membrane lipids


  • Phospholipids: Major component
  • Cholesterol: Found in animal cell membranes, helps maintain fluidity
  • Glycolipids: Located on cell membrane surfaces, have a carbohydrate sugar chain attached

  
• Cell membrane protein types and functions


  • Transport proteins: Facilitate passage of specific ions or molecules
  • Receptor proteins: Bind to signaling molecules to initiate cellular responses
  • Enzymatic proteins: Catalyze specific chemical reactions
  • Structural proteins: Provide structural support and maintain membrane integrity
  • Cell recognition proteins: Serve as markers for cell identification and recognition
  • Adhesion proteins: Mediate cell-cell and cell-matrix adhesion
• Cytoplasm


  Viscous fluid containing organelles and other components
• Components of cytoplasm


  • Interconnected filaments & fibers
  • Fluid = cytosol
  • Organelles (not nucleus)
  • Storage substances

  
• Cytoskeletal elements


  • Microfilaments (actin)
  • Intermediate filaments
  • Microtubules

  
• Functions of cytoskeleton


  • Mechanical support
  • Anchor organelles
  • Help move substances
• Cilia and flagella


  • Cilia: Short, hair-like structures that move substances outside human cells
  • Flagella: Long, whip-like extensions found on sperm cells
• Cilia and flagella structure

  • Axoneme: Contains a "9+2" arrangement of microtubules
  • Basal body: Originates from a basal body, similar in structure to a centriole
• Plant cells do not have centrioles, instead they use spindle pole bodies (SPBs) to organize spindle fibers
• Centromere


  The region of a chromosome where the two sister chromatids are joined, acts as a binding site for kinetochore proteins, essential for the attachment of spindle fibers
• Ribosome


  Cellular organelles responsible for protein synthesis, found in both prokaryotic and eukaryotic cells
• Microtubule Formation


  Microtubules originate from multiple sites within the nuclear region where SPBs are located
• Chromosome Capture


  Microtubules attach to chromosomes at kinetochores, guiding their alignment during cell division
• Cell Plate Formation


  In cytokinesis, microtubules guide the formation of a new cell wall, called the cell plate, between daughter cells
• Centromere


  • The region of a chromosome where the two sister chromatids are joined
  • It acts as a binding site for kinetochore proteins
  • Centromeres ensure proper alignment and segregation of chromosomes during cell division (mitosis and meiosis)
  • They are essential for the attachment of spindle fibers, facilitating chromosome movement
• Ribosome


  • Cellular organelles responsible for protein synthesis
  • They can be found in both prokaryotic and eukaryotic cells, either free-floating in the cytoplasm or attached to the endoplasmic reticulum
  • Composed of two subunits: large and small, each made up of ribosomal RNA (rRNA) and proteins
  • Ribosomes facilitate translation, the process of converting mRNA (messenger RNA) into a specific sequence of amino acids to form a protein
• Ribosomes "read" the genetic information encoded in mRNA and link together amino acids to form polypeptide chains according to the sequence specified by the mRNA
• Ribosomes play a crucial role in cell growth, development, and maintenance by synthesizing proteins required for various cellular functions, including enzyme activity, structural support, and signaling pathways
• Membranous Organelles


  • Functional components within cytoplasm
  • Bound by membranes

  
• Nucleus


  • The control centre of eukaryotic cells
  • Enclosed by the nuclear envelope, a double membrane structure
  • Contains nucleoplasm, a gel-like substance
  • Houses chromosomes, consisting of DNA and proteins
  • Includes the nucleolus, responsible for making ribosomes
  • Regulates cell activities by controlling gene expression
  • Ensures accurate distribution of genetic material during cell division