Chapter 3 - Cells: The Living Units

INTRODUCTION - CELL THEORY

• A cell is the basic structural and functional unit of living things
• Activity of an organism depends on the activities of its cells.
• Biochemical activities of a cell are dictated by their organelles.
• Continuity of life has a cellular basis.
• Characteristics of Cells
  • Vary greatly in their size, shape, and function.
  • All are composed primarily of CHONS
  • All have the same basic parts and some common functions.
  • A generalized human cell contains the plasma membrane, the cytoplasm, and the nucleus

The Plasma Membrane: Structure

• The Fluid Mosaic Model
  • The plasma membrane is composed of a double layer of phospholipids embedded with small amounts of cholesterol and proteins.
  • The phospolipid bilayer is composed of two layers of phospholipids lying tail to tail, with their polar heads exposed to water inside and outside the cell.
  • The inward-facing and outward-facing surfaces of the plasma membrane differ in the kinds and amounts of lipids they contain.
    • Glycolipids are found only in the outer membrane.
    • Lipid rafts are also found only in the outer membrane, and are assumed to function in cell signaling.
      • Composed of sphingolipids and cholesterol
      • Are concentrating platforms for cell-signaling molecules
  • Integral proteins are firmly inserted into the plasma membrane.
    • Most integral proteins are transmembrane proteins that span the entire width of the membrane and are involved with transport as channels or carriers.
  • Peripheral proteins are not embedded in the plasma membrane, but attach to integral proteins or to phospolipids.
    • Peripheral proteins may function as enzymes or in mechanical functions of the cell.
  • The glycocalyx is the fuzzy, sticky, carbohydrate-rich area surrounding the cell.
• Specializations of the Plasma Membrane
  • Microvilli are fingerlike extensions of the plasma membrane that increase the surface area of the cell.
  • Most body cells are bound together using glycolipids, specialized interlocking regions, or specialized membrane junctions.
    • Tight junctions are a type of membrane junction in which integral proteins on adjacent cells fuse together to form an impermeable junction in order to prevent molecules from passing through the extracellular space between cells.(Blood Brain Barrier)
    • Desmosomes are mechanical couplings that are scattered along the sides of adjoining cells that prevent their separation and reduce the chance of tearing when a tissue is stressed. (Layers of the skin)
    • Gap junctions are a communication junction between cells that allows substances to pass between adjacent cells.(Cardiac Conduction System)
• Plasma Membrane: Functions
  • Separates intracellular fluids from extracellular fluids
  • Membrane Transport - selectively permeable
  • Generating & Maintaining a Resting Membrane Potential - Nerve impulse transmission
  • Functions of Membrane Proteins
    • Transport
    • Enzymatic activity
    • Receptors for signal transduction
    • Intercellular adhesion
    • Cell-cell recognition
    • Attachment to cytoskeleton and extracellular matrix

CYTOPLASM COMPOSITION

• The cytoplasm is the cellular material between the cell membrane and the nucleus, and is the site of most cellular activity.
• Contents of Cytoplasm
  • Cytosol - largely water with dissolved protein, salts, sugars, and other solutes
  • Inclusions - chemical substances such as glycosomes, glycogen granules, and pigment
  • Cytoplasmic organelles - metabolic machinery of the cell
    • Membranous - mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus
    • Nonmembranous - cytoskeleton, centrioles, and ribosomes.

Organelles

• Mitochondria
  • Double membrane structure with shelflike cristae (internal matrix)
  • Provide most of the cell's ATP via aerobic cellular respiration
  • Contain their own DNA and RNA
    • Self replicating
    • Protein syntheses of respiratory proteins
    • Passed to offspring by female parent
    • Mitochondrial myopathies are inherited muscle disorders resulting from faulty mitochondrial genes. As a result muscles become weak and fatigue easily.
• Ribosomes
  • Granules containing protein and rRNA
  • Site of protein synthesis
  • Free ribosomes synthesize soluble proteins
  • Membrane-bound ribosomes synthesize proteins to be incorporated into membranes
  • Synthesized by the nucleolus in the nucleus
• Rough Endoplasmic Reticulum
  • External surface studded with ribosomes
  • Manufactures all secreted proteins
  • Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes
• Smooth Endoplasmic Reticulum
  • Tubules arranged in a looping network
  • Catalyzes the following reactions in various organs of the body
    • In the liver - lipid and cholesterol metabolism, breakdown of glycogen and, along with the kidneys, detoxification of drugs
    • In the testes - synthesis of steroid-based hormones
    • In the intestinal cells - absorption, synthesis, and transport of fats
    • In skeletal and cardiac muscle - storage and release of calcium
  • One of the functions of smooth ER is to detoxify drugs. Repeated exposure to certain drugs produces changes to the smooth ER in the liver which results in tolerance to the drug.
• Golgi Apparatus and Endomembrane system
  • Stacked, flattened membranous sacs
  • Functions in modification, concentration, and packaging of proteins
  • Transport vessels from the ER fuse with the cis face of the Golgi apparatus
  • Proteins then pass through the Golgi apparatus to the trans face
  • Secretory vesicles Golgi stack and move to designated parts of the cell
  • Vesicle Pathways
    • Secretion vesicles
    • New membrane vesicles
    • Lysosome vesicle
• Lysosomes
  • Spherical membranous bags containing digestive enzymes, low pH
  • Digest ingested bacteria, viruses, and toxins
  • Degrade nonfunctional organelles
  • Breakdown glycogen and release thyroid hormone
  • Breakdown nonuseful tissue
  • Breakdown bone to release Ca2+
  • Secretory lysosomes are found in white blood cells, immune cells, and melanocytes
  • Tay Sachs Disorder
    • Affects children of eastern European-Ashkenazi descent
      • seizures, muscle rigidity, blind, demented and dead before the age of 5
    • Genetic disorder caused by absence of single lysosomal enzyme
      • enzyme normally breaks down glycolipid commonly found in nerve cells
      • as glycolipid accumulates, nerve cells lose functionality
      • chromosome testing now available
• Peroxisomes - abundant in liver and kidney cells
  • Detoxifies harmful chemicals -alcohol, drugs etc.
  • Membranous sacs containing oxidases and catalases
    • Catalase breaks down H₂O₂ --->H₂O + O₂
  • Neutralize dangerous free radicals
    • Free radicals - highly reactive chemicals with unpaired electrons (i.e., O2-)
  • Lack of enzyme causes ALD
• Cytoskeleton - structure and movement
  • Consists of microtubules, microfilaments, and intermediate filaments
  • Microfilaments --Dynamic strands of the protein actin (muscle contraction)
    • Attaches cytoplasmic side of the plasma membrane
    • Braces and strengthens the cell surface
  • Intermediate filaments
    • scaffolding of the cell
    • Tough, insoluble protein fibers with high tensile strength
  • Microtubules - Determines overall shape of cell & distribution of organelles
    • Make up centrioles which function in mitosis
    • Make up cilia and flagella
  • Motor Molecules
    • Attach to receptors on organelles and transport organelles
    • Motion accomplished by Motor Molecules (dynein & kinesin) and cytoskeleton
      • Protein complexes that function in motility
      • Powered by ATP
    • Cause Cilia and Flagellum Action
      • Cilia - hair like extensions that move substance through passageways
      • Flagella -long whip like extension that moves a whole cell
  • Centrioles
    • Small barrel-shaped organelles located in the centrosome near the nucleus
    • Pinwheel array of nine triplets of microtubules
    • Organize mitotic spindle during mitosis
    • Form the bases of cilia and flagella

CELL NUCLEUS

• Contains the genetic library with blueprints for nearly all cellular proteins
• Dictates the kinds and amounts of proteins to be synthesized
• Nuclear Envelope - Selectively permeable double membrane barrier containing pores
  • Outer membrane is continuous with the rough ER and is studded with ribosomes
  • Pore complex regulates transport of large molecules into and out of the nucleus
• Nucleoli - Dark-staining spherical bodies within the nucleus
  • Site of ribosome production
• Chromatin - Threadlike strands of DNA and histones
  • Arranged in fundamental units called nucleosomes
  • Form condensed, barlike bodies of chromosomes when the nucleus starts to divide

• Nuclear envelope separates the nucleus from the cytoplasm
  • Double membrane with pores
• Nucleolus
  • spherical body in nucleus
  • synthesizes ribosomes which move out through the pores into the cytoplasm
• Chromatin
  • Made up of DNA and histones
  • Carry the code for cellular proteins

Web Link

• Inner Cell Animation Harvard Multimedia

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