Bio1151 Chapter 6 A Tour of the Cell
  1. All organisms are made of        , which are either              or             .

      Both prokaryotic and eukaryotic cells are bound by a membrane called the plasma membrane.

      The interior is an aqueous cytosol.

      The cytosol and various macromolecules and organelles immersed in it comprise the cytoplasm.

      Prokaryotes do not possess membrane-bound organelles.

      Prokaryotes lack a nucleus or other membrane-bound organelles.

      DNA is located in a nucleoid region, and the cell is encased in a rigid cell wall.

  2. Cells are generally              and can only be visualized through microscopes.

      Most cells are too small to see with the naked eye.

      Eukaryotic cells typically range from 10 - 100 Ám in diameter.

      Prokaryotic cells are even smaller, usually less than 10Ám.


      Light microscopes:

    • brightfield
    • brightfield stained
    • phase-contrast
    • differential-contrast
    • fluorescence
    • confocal

      Electron microscopes

      Brightfield passes light directly through specimen.

      Unless cell is naturally pigmented, image has little contrast.

      Staining with dyes can enhance contrast, but this usually kills the cells.

      Phase-contrast uses a transparent phase-plate to enhance contrast by amplifying variations in density within specimen;.

      Differential interference contrast (Nomarski) works by separating a polarised light source into two beams taking slightly different paths through the sample.

      When recombined the two beams interfere and enhance the variation of optical density of the sample.

      Nomarski microscopy:

      Fluorescence microscopy can tag specific molecules with fluorescent dyes or antibodies.

      These fluorescent substances absorb ultraviolet light and emit visible light to reveal the desired structures.

      Confocal uses lasers to focus on a single plane of a specimen.

      A computer cam regenerate a 3-D view by combining images of several sections of the specimen.

      The top photo shows in-focus nerve cells and support cells of nerve tissue.

      The bottom photo is a standard fluorescence micrograph.

      Electron microscopes.

    • The scanning electron microscope (SEM) shows a 3D image of the surface of a specimen.
    • The transmission electron microscope (TEM) profiles a thin section of a specimen.
  3. Their small size enables them to have a high          area to         ratio, to facilitate the exchange of materials into and out of the cell.

      The surface area to volume ratio of an object decreases when its size increases.

      Thus small size facilitate the exchange of material into and out of the cell.

  4. Eukaryotic cells have membrane-bound             and can be grouped into 2 main types: animal cells and plant cells.

      A typical animal cell comprises numerous organelles, including a DNA-containing nucleus.

      The organelles are embedded in a semifluid cytoplasm and enclosed in a plasma membrane.

      Organelles found in animal cells but not in plant cells include lysosome and centrosome.

      Most plant cells contain several structures not found in animal cells.

      Chloroplasts perform photosynthesis.

      A central vacuole stores material.

      A rigid cell wall made of cellulose enclose the plasma membrane.

      Plasmodesmata provide cytoplasmic connections through openings in the cell wall.

    Cells exercise:
  5. The          contains most of the          material in the cell, and is bounded by a nuclear           with many nuclear        .


      The nucleus is surrounded by a double membrane called nuclear envelope, which is perforated with nuclear pores.

      Within the nucleus can be found:

    • chromatin (DNA and proteins)
    • nucleolus, which synthesizes ribosomes.
  6. The endoplasmic            (ER) is continuous with the nuclear           .

      The Endoplasmic reticulum (ER) is a membranous system continuous with the nuclear envelope.

      Rough ER is studded on its outer surface with ribosomes to produce proteins.

      Smooth ER lacks ribosomes and is involved in lipid synthesis.

      Transport vesicles bud off and carry ER products to the Golgi apparatus and other destinations.

      Ribosomes carry out protein synthesis and are composed of two subunits. Ribosomes may be found free in the cytosol or bound to the ER.
  7. The        apparatus is the cell's "Shipping and Receiving Center".

      The Golgi apparatus consists of stacks of flattened membranes (cisternae) and is the cell's "Shipping and Receiving Center". Molecules produced in the ER enter this organelle via transport vesicles and, after processing, are delivered to other parts of the cell. If the final destination is the plasma membrane, the processed molecules are secreted from the cell.
  8. The           contains enzymes to carry out intracellular digestion by               (cell eating ), and breaks down damaged organelles by            (self eating ).

      Phagocytosis (cell eating).

      Lysosomes digest (hydrolyze) materials taken into the cell and recycle intracellular materials.

      Molecules can be engulfed by infolding of the plasma membrane, yielding a food vacuole.

      The food vacuole then fuses with a lysosome for digestion - phagocytosis.

      Autophagy (self eating).

      Top: a lysosome engulfs 2 disabled organelles, a mitochondrion and a peroxisome. Bottom: a lysosome fuses with a vesicle containing a damaged mitochondrion.

  9. The membrane-bound organelles involved in cellular transport form the               system.

      The Endomembrane System:
  10.               are the cell's "power plant" and perform cellular              .

      The mitochondrion, site of cellular respiration. A mitochondrion has an outer membrane and an inner membrane folded into cristae enclosing the matrix. This organelle contains its own circular DNA and ribosomes.
  11.               perform                 in some plant cells.

      A chloroplast has a double membrane: the inner membrane encloses a compartment containing the fluid stroma as well as ribosomes and DNA. A third membrane called thylakoids is stacked to form structures called grana (singular, granum).
  12. Plant cells also contain a large central          , which stores organic compounds and water.

      The central vacuole, surrounded by a membrane called the tonoplast, holds organic compounds and water in plant cells and is not found in animal cells.

      It is usually the largest organelle seen under the microscope.

  13. The               is a network of fibers which provides          for the cell, and enables           .

      The cytoskeleton provides support for the cell and enables cellular motility.
    •               help maintain cell        , and guide the movement of cell components.

        Microtubules in fibroblast cells. Microtubules are made of tubulin subunits and help maintain cell shape and guide the movement of cell components.

        Vesicles can travel inside the cell by attaching to motor proteins.

        The complex then moves along "monorails" of microtubules.

      •        and           use microtubules for motion in some cells.

        Cilia have a back-and-forth motion that moves the cell. A dense nap of cilia covers this Colpidium, a freshwater protist. Videos: protists humans

        A cell may have one flagellum or a few flagella. Propulsion of a human sperm cell is an example of flagellate locomotion. Videos: protists sperm

        Eukaryotic cilia and flagella. Both cilia and flagella have a "9 + 2" arrangement of microtubules: a ring of 9 doublets surrounding 2 central microtubules. The 9 doublets are moved by dynein proteins.

        Dynein movement of microtubules.

      • The dynein arms of one microtubule doublet grip the adjacent doublet, push it up, release, and then grip again.
      • The doublet do not slide far past each other since they are restrained by cross-linking proteins, so they bend.
      • Repeated bending yields a wavelike motion.
    •                 called        and         are protein filaments that function in cellular motility, such as the contraction in         fibers.

        Microfilaments in fibroblast cells. Microfilaments are made of actin and myosin filaments that slide past each other to cause muscle contraction.

        The myosin arms "walk" along the actin fibers, causing the filaments to slide past each other. As the actin filaments approach each other in the middle, the muscle cell contracts. Tutorial:
      • Contraction of actin and myosin            , together with sol-gel reversals, also result in           crawling of a cell through the movements of              .

          Amoeboid movement.

          The cytoplasm can switch between a fluid state called sol, and a stiffer state called gel.

          Breakdown of the actin network squeezes the cytoplasm into the sol, forming a pseudopodium.


      • Cytoplasmic            in        cells also involve microfilaments to circulate nutrients among organelles.

      Cytoplasmic streaming.

      A layer of cytoplasm cycles around a plant cell, moving over a carpet of parallel actin filaments.


    Review: Animal Cell Structure and Function. Cell types.
    Review: Plant Cell Structure and Function. Differences.