Cell Structure and Function I

21 August 2006
Cell Structure and Function I
Dr. Joana Chakraborty, Ph.D.



The Cell


Definition

  • Basic functional unit
  • has plasma membrane, fluid suspension (cytosol), nucleus, organelles (mitochondria, lysosomes, golgi apparatus, endoplasmic reticulum), inclusions and cytoskeleton.
  • Cell contains 2 compartments
    • Nucleus and cytoplasm compartments
    • Cytoplasm compartment contains organelles and cytosol
    • Cytosol – made up of micromolecules that perform different cellular functons.

Cellular functions

  • Summary of cell function: motility, contractility, conductivity, absorption, synthesis/assimulation, respiration, secretion, excretion, growth, reproduction, death.
    • Movement – phagocytes (contains lysosomes and bacteria) and muscle cell
    • Contraction – Muscle cell
    • Conductivity – Nerve cell
    • Synthesis/Secretion of enzymes – pancreatic acinar cells (exocrine)
    • Synthesis/Secretion of mucous – mucous-gland cells
    • Steroids – adrenal, testes and hormones (secretory cells have lots of smooth ER)
    • Ion transport – kidney and salivary gland ducts
    • Intracellular digestion – macrophages, WBC
    • Tranform stimuli to nerve impulse – sensory cell
    • Metabolite absorption – intestinal and kidney cells

Dimensions

  • 1 mm = 1,000 μm
  • 1 μm = 1,000 nm
  • 100 μm – visible via human eye.
  • 0.5-0.22 μm – visible via light microscope, need to cut sections
  • 1 nm – visible via electron microscope, requires very thin sections
  • Approximate sizes:
    • Cells – μm to cm
    • Organelles – nm to μm
    • Macromolecules – 50-500 nm
    • Small molecules (amino acids) – 1nm
    • Atoms – Å (angstrom)

Location of Organelles

  • Centrioles are usually closer to nucleus, however, in motile cells, the centrioles can be found near the edge.
  • Microenvironment of cell is compartmentalized

Selected Techniques


  • Resolving power – smallest distance between 2 particles at which they can be seen as separate particles

Light microscopy

  • max resolving power = 0.22 μm
    • Lens – Objective lens (4x, 10x, 40x), and Ocular lense (10x) condenser lens
    • Magnification: 40-1500x
    • Tissue Prep:
      • Tissue fixed, placed in parafilm blocks, and sectioned before staining
      • Longitudinal sections
      • Transverse (cross) sections
      • Oblique sections
      • Grazing sections
      • Tangential sections

Electron microscopy

  • max resolution power = 1 nm (0.1 nm, theoretical)
    • 1000-100,000x magnification
    • Tissue Prep:
      • Thin sectioning (~100 nm)
      • Stain w/ heavy metal
      • Uses electromagnetic lenses
      • Like an upside-down light microscope

Differential Centrifugation

  • Different fractions from each centrifugation precipitate different types of cellular organelles for study
    • Homogenize tissue and centrifuge precipitate
    • Take supernatant and centrifuge, and repeat
    • Ribosomes are too small to see on light microscope -- must use electron microscope

Cell culture

  • for study of animal cells growth, differentiation, gene structure and functions
    • Primary culture – homogenize tissue and plate culture
    • Passage of primary culture to produce secondary cultures
    • Stem cells – idea is to control the direction of stem cell development.

Plasma Membrane


  • Boundary of cell membrane
  • Components are in fluid condition:
    • Phospholipids:protein ratio can vary depending on the type of cell
    • Carbohydrates
    • Lipids and many proteins are laterally mobile
  • Organized into a lipid-bilayer with imbedded proteins
    • 5 nm in thickness
  • Organelles in eurkaryotic cells are membrane-bound – internal membrane surface area >>> plasma membrane
    • Nuclear membrane (nuclear envelope) and mitochondria has a double membrane
    • Other organelles have a single membrane

Proteins in plasma membrane

  • Trans-membrane proteins (integral proteins) can extend from cytosolic and ectoplasmic faces
  • Peripheral proteins (do not cross membrane)
  • Used for receptors, recognition and binding units, pores and channels, and enzymes
  • Freeze fracture technique can separate membrane bilayer
    • Two sides of membranes are called inner (E-face) and outer (P-face) leaflets

Function of Plasma Membrane

  • Structural integrity
  • Interface cytoplasm and external environment
  • Control movement of substances – control internal environment
  • Cell-cell interactions (tight junction, etc.)
  • Recognition and cellular identity (via carbohydrates and glycolipids)

Endocytosis

  • Two types:
    • Phagocytosis – cell eating via evagination
      • Occurs in macrophage and neutrophil
      • When microbe in extracellular fluid contacts macrophage, macrophage will extend plasma membrane to engulf microbe.
      • Plasma membrane pinches off to form vesticle and combines with lysosome to become phagolysome
      • Microbe becomes digested and end-products either taken up by the cell or excreted
    • Pinocytosis – cell drinking via invagination
      • Plasma membrane forms a depression to intake macromolecules
      • Fluid phase endocytosis – plasma membrane invaginates and takes in macromolecules
      • Receptor-mediated endocytosis – specific receptors on plasma membrane binds to specific macromolecules and internalize macromolecules (example: LDL binding). Very specific macromolecular uptake
      • Pinched off plasma membrane called “endosome.”

Intracellular Organelles



Endoplasmic Reticulum (ER)

  • Largest membrane bound system in cell
  • Rough and smooth sides
  • Site of protein (rough ER) and lipid (smooth ER) synthesis

Smooth ER

  • No ribosomes attached (hence, smooth)
  • Active synthesis of steroids, cholesterol, and triglycerides
  • Detoxification of drugs
  • Cells synthesizing steroid hormones have lots of smooth ER

Rough ER

  • Studded with ribosomes on outer surface
  • Site of protein synthesis
  • Protein secretory cells have more rough ER
  • Continuous with nuclear envelope

Golgi Apparatus

  • Protiens synthesized and packaged in rough ER are transported to Golgi for modification and final packaging for secretion
  • Flattened, slightly curved membrane-bound cisternae
    • Cis-face (close to rough ER; convex in shape, entry phase)
      • ERGIC – Endoplasmic reticulum golgi interface cisternae
    • Medial Face
    • Trans-face (concave, exit phase)
  • Secreted granules can be smooth or coated vesticles

Mitochondria

  • 0.5-1 μm long (sometimes 7 μm long)
  • Produces ATP
  • Smooth outer membrane and folded inner membrane (cistae)
  • Has inner membrane space and matrix space
  • Matrix contains mitochondrial DNA and enzymes
    • ATP synthase on inner surface of inner membrane
    • Contains granules in matrix, but function unknown
  • Mitochondrial DNA:
    • have own separate DNA
    • Circular DNA with multiple copies
    • Human mitochondrial genome is 16,569 base pairs
    • Germline mutations in mitochondrial DNA are transmitted by the mother
      • Recent research suggests that mitochondrial DNA from father may be integrated somehow to the genome of children

Peroxisomes

  • 0.1 to 1 μm
  • Contains 40-50 oxidative enzymes
    • urate oxidase
    • catalase – destroys hydrogen peroxide generated by peroxisome
  • Catabolism of long chan fatty acids

Lysosome

  • 0.3-0.8 μm diameter
  • Contains 40 types of enzymes
    • acid hydrolases such as sulfatases, proteases, nucleases, etc.
  • Actively transports H+ ions to lysosome
  • Lumen is acidic (pH = 0.5) for enzymes to function
    • Rough ER produces lysosomal enzymes
    • Golgi produces vesticles containing lysosomal enzymes - primary lysosome
    • Endocytosed vesticle combines with primary lysosome, producing secondary lysosome
    • After phagocytosed material is digested, secondary lysosome becomes a residual body and contents are either taken up by the cell or excreted to the external environment
    • Lysosomes can also digest old cellular components such as secretory granules from the Golgi

Transport of Macromolecules

  • Serveral methods of macromolecule transport
    • Gated Transport
      • Cytosol to nucleus
    • Transmembrane Transport
      • Cytosol to mitochondria, ER, membrane, and peroxisomes
    • Vesicular Transport
      • ER to Golgi
      • Golgi to lysosome, endosome, secretory vesticles
      • Secretory vesticles to cell surface
      • Cell surface to endosome

Cytoplasmic Incusions

  • Glycogen
    • Store glucose
    • Mostly in liver and muscle cells
    • Varies from 10-40 nm
  • Lipid
  • Pigments
    • possess some color
    • Hemoglobin, melanin, lipfuscin
  • Crystals
    • Crystal of Charcot Bottcher in Sertoli cells of testes

Abnormal Functions


  • Abnormalities:
    • Changes in morphology
    • Defects in storage of macromolecules
    • Vacuolization - cell produces many vacuoles as a result of recent cell injury
    • Cell death characterized by:
      • typical nuclear changes:
        • Condensed Nucleus
        • Fractured Nucleus
        • Nuclear materal coated on nuclear membrane
      • cell membrane rupture
  • Apoptosis
    • Apoptosis is required for cell function
    • Inability for cell to correctly undergo apoptosis at a certain time may be abnormality.

Objectives