GEN BIO: Cells

• Cell is the structural and functional unit of life.
• Robert Hook


  An English scientist who observed a thin slice of cork under the microscope in 1665 and described small spaces surrounded by wall, naming them cells
• Robert Brown


  Discovered the nucleus of the cell
• The cell theory put forth by two scientists Matthias Scheiden and Theodore Schwann, with modification by Rudolf Virchow in 1855:
• Cell theory


  • All living organism are made up of cell and their products
  • Cell is the structural and functional unit of organism
  • New cells are formed by division of the pre existing cell
• Cell types


  • eukaryotes
  • prokaryotes
• Cell has two major parts the nucleus and the cytoplasm. The nucleus is separated from the cytoplasm by nuclear membrane, and the cytoplasm is separated from the surrounding fluids by the cell membrane, also called the plasma membrane
• Basic constituents of the cytoplasm

  • Water
  • Ions
  • Proteins
  • Lipids
  • Carbohydrates
• Cell membrane


  Envelops the cell in a thin, pliable, elastic structure only 7.5-10nm in thickness, composed mainly of proteins and lipids
• Approximate composition of cell membrane

  • Proteins 55%
  • Phospholipids 25%
  • Cholesterol 13%
  • Other lipids 4%
  • Carbohydrates 3%
• Membrane carbohydrates


  Occur in the form of glycoprotein and glycolipid, many other carbohydrate compounds are called proteoglycans. The entire outside surface of the cell has a loose carbohydrate coat called the glycocalyx. Functions: provide negative charge, help in cell attachment, act as receptor substances, enter into immune reactions
• Major organelles in the cytoplasm

  • Endoplasmic reticulum
  • Golgi apparatus
  • Mitochondria
  • Nucleus
  • Lysosome
• Granular endoplasmic reticulum


  Contains several minute granular particles called ribosomes, which are composed of RNA and proteins and function to produce new proteins in the cell
• Smooth or agranular endoplasmic reticulum


  Functions for production of lipid substances
• Golgi apparatus


  • Prominent in secretory cells, located on the side of cell from which secretory substances are extruded. Works in association with the endoplasmic reticulum to process and transport substances produced in the ER
• The Golgi apparatus has the capability of producing certain carbohydrates that are not produced by the endoplasmic reticulum, including hyaluronic acid and chondroitin sulphate. These are major components of proteoglycans, ground substances, and the organic matrix of bone and cartilage.
• Lysosomes


  Serve as intracellular digestive system that allows the cell to digest damaged cellular substances, food particles, and unwanted matter like bacteria. Surrounded by a lipid bilayer membrane and filled with hydrolase enzymes.
• Peroxisomes


  Similar to lysosomes but formed by self-replication and contain oxidases rather than hydrolases
• Secretory vesicles


  Storage vesicles formed by the endoplasmic reticulum-Golgi apparatus system and released from the Golgi to contain and release secretory substances
• Mitochondria


  The powerhouse of the cell, composed of inner and outer membranes with the inner membrane having many infoldings (cristae) that contain the enzymes necessary for extracting energy from nutrients to synthesize ATP
• Microtubules


  • Stiff filaments composed of polymerized tubulin molecules that act as a cytoskeleton, providing rigid physical structures for certain parts of the cell
• Centrosome and centrioles


  The centrosome is a round, dense mass of cytoplasm near the nucleus that contains a pair of centrioles. During cell division, the centrioles divide and move to opposite poles, forming the spindle that separates the chromosomes.
• Nucleus


  Contains large quantities of DNA, which are the genes that determine the characteristics of the cell's proteins and control the rate of cell reproduction. The nuclear membrane is also known as the nuclear envelope.
• Nucleoli


  Highly staining structures within the nucleus that are accumulations of RNA and protein that form ribosomes
• Types of intercellular junctions


  • Occluding (tight) junctions
  • Adhesive junctions (cell-to-cell, cell-to-matrix)
  • Communicating (gap) junctions
• Intercellular junctions


  • Consist of three components: a transmembrane adhesive protein, a cytoplasmic adapter protein, and a cytoskeletal filament
• Tight junctions


  Hold opposing cell membranes in close contact, essentially obliterating the intercellular space, using transmembrane adhesive proteins like occludin and claudins arranged in anastomosing strands
• Adhesive junctions


  Hold cells together or anchor cells to the extracellular matrix, maintaining an intercellular space of approximately 20 nm. Important for cellular signaling and regulating cell shape, motility, polarity, and proliferation.
• Desmosomes


  Adhesive junctions where the transmembrane cadherins are desmoglein and desmocollin, with a dense plaque of desmoplakin and plakoglobin on the cytoplasmic side that attaches to intermediate filaments
• Cell-matrix junctions (focal adhesions)

  Use integrins as the transmembrane adhesive proteins, which bind to extracellular matrix proteins like collagen, laminin, and fibronectin, with cytoplasmic adapter proteins like alpha-actinin, vinculin, and talin linking to the actin cytoskeleton
• Desmosome


  Attachment site for the cytoskeletal components, which in the case of the desmosome are intermediate filaments
• Desmosome components


  • Desmoplakin
  • Plakoglobin
  • Membrane
  • Intermediate filaments
  • Desmocollin
  • Desmoglein
• Cell matrix junctions


  Have a structural organization similar to that of cell-cell adhesive junctions, but they use different molecular components and attach the cell to the extracellular matrix
• Focal adhesions


  The transmembrane component is a member of the integrin family of adhesion molecules
• Integrins


  Heterodimers of different alpha and beta subunits with specificity for various extracellular matrix molecules
• Integrin subunits


  • 18 known alpha subunits
  • 8 beta subunits
  • 24 different combinations
• Cytoplasmic adapter proteins


  Include the actin-binding proteins alpha-actinin, vinculin, and talin, which link the transmembrane integrins to the actin cytoskeleton
• Binding of the integrin to collagen, laminin, fibronectin, and other extracellular matrix proteins


  Results in recruitment and remodeling of the actin cytoskeleton
• Hemidesmosomes


  Link the cell to the basal lamina, and through additional extracellular molecules, to the rest of the extracellular matrix
• Hemidesmosome components


  • Integrin alpha 6, beta 4
  • Collagen XVII (BP180)
  • BP230
  • Plectin
  • Intermediate filaments