Cell Structure and Function

• Microscopes were invented

  1590
• Microscopes were improved throughout

  1600s
• Robert Hooke observed dead cells from oak tree bark through a microscope

  1665
• Hooke visited van Leeuwenhoek and was introduced to the world of microorganisms

  1674
• Light microscopes (LM)

  Use visible light that passes through a specimen and glass lenses, which refract the light and magnify the specimen for observation
• Microscopy parameters

  • Magnification
  • Resolution
  • Contrast
• Magnification

  The ratio of the image size to the actual size of the specimen
• Light microscopes can magnify up to 1,000 times the actual size of the specimen, but beyond this limit, additional details may not be clear
• Resolution

  The measure of image clarity, defined as the minimum distance between two points that can be distinguished as separate
• Contrast


  The difference in brightness between light and dark areas in an image
• Techniques to enhance contrast

  • Staining
  • Labeling
• Microscopy methods


  Used to study cells, each with its own advantages and applications
• The resolution barrier in cell biology has been a challenge since the 1950s, when standard light microscopy was introduced to study organelles, membrane-enclosed structures within eukaryotic cells
• Electron microscope (EM)


  Introduced in the 1950s, allowed for a 100-fold improvement over the standard light microscope
• How EM works


  1. Focuses a beam of electrons through the specimen or onto its surface

  2. Resolution inversely related to the wavelength of the light used
• Capabilities of modern electron microscopes

  • Can theoretically achieve a resolution of about 0.002 nm
  • Usually cannot resolve structures smaller than about 2 nm across
• Scanning electron microscope (SEM)


  Particularly useful for detailed study of the topography of a specimen, as the electron beam excites electrons on the surface, which are detected by a device that translates the pattern of electrons into an electronic signal sent to a video screen
• Transmission electron microscope (TEM)


  Used to study the internal structure of cells, using electromagnets as lenses to focus the image onto a monitor for viewing
• Electron microscopes have revealed many subcellular structures that were impossible to resolve with the light microscope
• Electron microscopes also have disadvantages, such as the introduction of artifacts in specimen preparation that were not present initially
• In recent decades, light microscopy has been revitalized by major technical advances, such as labeling individual cellular molecules or structures with fluorescent markers, producing sharper images of three-dimensional tissues and cells, and breaking the resolution barrier to distinguish subcellular structures even as small as 10-20 nm across
• Cells


  • All cells are bounded by a selective barrier, the plasma membrane
  • Inside cells is a semifluid substance called cytosol, where subcellular components are suspended
  • All cells contain chromosomes carrying genes in the form of DNA
  • All cells have ribosomes, tiny complexes that make proteins according to instructions from the genes
• Eukaryotic cells


  Have most of the DNA in an organelle called the nucleus, bounded by a double membrane
• Prokaryotic cells


  Have the DNA concentrated in a region not membrane-enclosed, called the nucleoid
• Cytoplasm


  The interior of either type of cell, refers only to the region between the nucleus and the plasma membrane
• Eukaryotic cell cytoplasm


  • Within the cytoplasm of a eukaryotic cell, a variety of organelles of specialized form and function are suspended in cytosol
• Eukaryotic cells


  • Are generally larger than prokaryotic cells due to the logistics of carrying out cellular metabolism
  • The plasma membrane functions as a selective barrier that allows passage of enough oxygen, nutrients, and wastes to service the entire cell
  • A sufficiently high ratio of surface area to volume is especially important in cells that exchange a lot of material with their surroundings, such as intestinal cells
• A eukaryotic cell has extensive internal membranes that divide it into compartments, or organelles, which provide different environments for specific metabolic functions. These compartments can cause incompatible processes to occur simultaneously in a single cell.
• Nucleus


  The central organelle in eukaryotic cells, containing most genes and being the most conspicuous organelle
• Nucleus


  • Enclosed by a double membrane, a lipid bilayer with associated proteins, and perforated by pore structures
  • At the lip of each pore, the inner and outer membranes of the nuclear envelope are continuous
  • An intricate protein structure called a pore complex lines each pore and plays an important role in the cell by regulating the entry and exit of proteins and RNAs, as well as large complexes of macromolecules
• Nuclear envelope


  Lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope
• Nuclear matrix


  A framework of protein fibers extending throughout the nuclear interior
• Intermediate filaments


  • Found not only in the nucleus as part of the nuclear lamina but also in the cytoplasm of animal cells
  • A component of the cytoskeleton, a dynamic network of protein filaments that provides structural support to the cell, helps maintain cell shape, enables cell movement, and facilitates intracellular transport
• Cytoplasmic intermediate filaments


  • Contribute to the structural integrity of the cell and are involved in various cellular processes such as cell migration, mechanical strength, and signaling
  • Particularly abundant in cells subjected to mechanical stress, such as muscle cells and skin cells, where they provide resilience and stability to withstand tension and stretching
• Primary function of nuclear lamina

  To provide structural support to the nucleus, helping to maintain its shape and integrity
• Chromosomes


  Discrete units that carry genetic information, each containing one long DNA molecule associated with many proteins
• Chromatin


  The complex of DNA and proteins making up chromosomes
• When a cell is not dividing, stained chromatin appears as a diffuse mass in micrographs
• As a cell prepares to divide, the chromosomes coil further, becoming thick enough to be distinguished under a microscope
• Nucleolus


  A prominent structure within the nondividing nucleus, appearing as a mass of densely stained granules and fibers adjoining part of the chromatin