Quiz Me!

Cell structure and function

  Lecture Index
  Course Index
Last revised: Tuesday, October 19, 1999
Reading: Ch. 7 in text
Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!
Copyright 1999. Thomas M. Terry

Microscopy

Units of Measurement

Size and Scale

Resolving Power

Optical Instrument Resolving Power RP in Angstroms
Human eye 0.2 millimeters (mm) 2,000,000 A
Light microscope 0.20 micrometers (µm) 2000 A
Scanning electron microscope (SEM) 5-10 nanometers (nm) 50-100 A
Transmission electron microscope (TEM) 0.5 nanometers (nm) 5 A

Light Microscopes

  1. Bright-field Light Microscope
    • Theoretical limit of R.P. for light scope is 0.2 um.
    • Most readily available microscope, found in every biology lab, hospital
    • Most biological materials lack contrast; need to fixed and stained
    • Thick materials must also be embedded in paraffin or plastic and sectioned
  2. Phase-Contrast Light Microscope
    • Phase scope converts slight diffs. in refractive index and cell density into variations.
    • Advantage: can see live material w/o staining
  3. Fluorescence Microscope
    • Fluors are chemicals that adsorb light to produce excited electrons, later reradiate light = flourescence.
    • To use, need special type of microscope. Illuminate with ultraviolet or violet light (--> excited fluor). Need filters to remove this light from light traveling to ocular lens; only fluoresced light emitted from object will then appear to eye. Need dark field condenser to create dark background.

Electron Microscopes

PROKARYOTES & EUKARYOTES

Cell Structure and Cell Components

CELL MEMBRANE

  1. like a soap bubble. Flexible, thin, permeability barrier, separates IN &OUT
  2. View TEM of membrane structure
  3. View animation of membrane structure (from the Biology Place)
  4. depends on phospholipids: form lipid bilayer. Hydrophilic & hydrophobic domains.
  5. modified & strengthened by membrane proteins. Allow selective passage of materials. (Basic process = diffusion)

NUCLEUS

  1. Largest compartment in most cells.
  2. Control center. Contains chromosomes . If removed, cell works for a while, peters out
  3. not found in prokaryotes
  4. Bounded by nuclear membrane = double layered structure. Contains many nuclear pores , allow material to move in and out of nucleus.
  5. View nuclear pores
  6. Pores have octagonal "doors" made of protein; open and close on either side depending on specific signals.
  7. View diagram of nuclear pore structure
  8. Nucleus contains chromatin = DNA + proteins. Chromatin can condense into chromosomes during cell division. Contains DNA complexed to histone proteins =chromatin. During division, chromatin condenses by tight coiling into chromosomes, may be visible in light microscope. Machinery can handle large numbers of chromosomes, from 2 to over 1000. Very different from bacterial DNA separation (no spindle involved, no chromosome condensation, only 1 circular DNA molecule/cell (unless replicated prior to division).)
  9. View TEM of nucleus showing condensed chromatin ( heterochromatin ) and dispersed chromatin ( euchromatin )
  10. DNA remains in nucleus; information is exported in form of messenger RNA. mRNA is synthesized in nucleus, processed, and exported into cytoplasm to direct protein synthesis.
  11. Nucleolus = assembly plant for ribosomes. Ribosomal proteins are made in cytoplasm, must be transported back into nucleus. Ribosomal RNA made in nucleus. These two elements are integrated inside nucleolus to create ribosomal subunits. These are then exported out of nucleus through nuclear pores.

CYTOPLASM

  1. Fluid compartment. Filled with water, thousands of different cell molecules.Most metabolism occurs here (chemical transformations): food ---> energy,building blocks for growth.
  2. View TEM of a cell ; the clear zones inside the membrane are the cytoplasm (cytosol)
  3. Contains many ribosomes = sites of protein assembly.

RIBOSOMES

  1. Ribosome = assembly factory to make many PROTEINS in cytoplasm. Proteins direct most interesting cell processes.
  2. View ribosomes attached to endoplasmic reticulum membrane system (rER)
  3. Ribosome size measured in Svedberg (S) units; derived from sedimentation inultracentrifuge (used before electron microscopes were available)
  4. Prokaryotes: ribosomes made of 30S and 50S subunits, assemble into 70S ribosome
  5. Eukaryotes: ribosomes made of 40S and 60S subunits, assemble into 80S ribosome
  6. View schematic diagram of ribosome subunit structure
  7. In bacteria, ribosomes occupy 25% of cell volume, use 90% of cell energy. Less in many specialized eukaryotic cells, but still the dominant activity of almost all cells.

Endomembrane system

CYTOPLASMIC ORGANELLES

Endosymbiont theory:

All organelles seem to share many properties with bacteria: contain 70S ribosomes (whereas rest of eukaryote cells contain 80S ribosomes), divide by binary fission, contain circular DNA without nucleus, etc. Lynn Margulis proposed endosymbiont hypothesis: that organelles derived from ancient colonization of large bacteria (became the eucaryotic cell) by smaller bacteria(became the mitochondria, chloroplast, etc.) Symbiosis = "living together".Eventually, organelles lost ability to exist as separate organisms, cannot be separated from cell. Recent evolutionary taxonomy by comparing ribosomal RNA shows that this idea has lots of merit. Mitochondrial and plastid ribosomes are very similar to current bacteria, very different from eukaryotes.

CYTOSKELETON

CELL WALLS

  1. Found in plants, fungi, bacteria -- not in animal cells
  2. Allow cells to survive in plain water, rigid structure (tree towering 150 feet high!)
  3. Thicker than cell membrane. Made from cellulose (plants and fungi), other polysaccharides (bacteria).
  4. View plant cell wall ( protected)
  5. Cells maintain contact by plasmodesmata -- thin cytoplasmic connections, lined by membrane, that pass across cell wall junctions.

EXTRACELLULAR MATRIX (ECM)

  1. Animal cells don't have walls, but do have ECM = meshwork of macromolecules outside plasma membrane. Consists mainly of glycoproteins(proteins with oligosaccharide chains), especially collagen.
  2. Some cells attached directly to ECM by bonding to collagen or fibronectin.
  3. View diagram of ECM ( protected)

INTERCELLULAR JUNCTIONS (see text for examples)

  1. Gap junctions (found in animals): formed by two connecting protein rings embedded in cell membrane of adjacent cells. Allows passage of water, small solutes, but not macromolecules (proteins, nucleic acids).
    View animation of gap junctions (at The Biology Place)
  2. Tight junctions (found in animals): specialized "belts" that bind two cells tightly to each other, prevent fluid from leaking into intracellular space.
    View animation of tight junctions (at The Biology Place)
  3. Desmosomes (found in animals): intercellular "rivets" that create tight bonds between cells, but allow fluids to pass through intracellular spaces.
    View animation of desmosomes (at The Biology Place)
  4. Plasmodesmata (found in plants): channels connecting cells; allow free passage of water and small solutes, but not macromolecules (proteins, nucleic acids).
    View figure showing plasmodesmata (at The Biology Place)

Take a self-quiz on this lecture
Return to Lecture Index
Return to Biology 107 Index page