UnivBio Lecture4 Essay

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Chapter 6
A Tour of the Cell

Cellular level of organization
• Cell theory:
• A unifying concept in biology
• Originated from the work of biologists Schleiden and Schwann in 1838-9
• States that:
– All organisms are composed of cells
– All cells come only from preexisting cells
– Smallest unit of life
• German botanist Matthais Schleiden in 1838
• German zoologist Theodor Schwann in 1839
• German physician Rudolph Virchow in 1850’s

Organisms and Cells


Cell Size
• Most much smaller than one millimeter (mm)
• Some as small as one micrometer (m)
• Size restricted by Surface/Volume (S/V) ratio
– Surface is membrane, across which cell acquires nutrients and expels wastes
– Volume is living cytoplasm, which demands nutrients and produces wastes
– As cell grows, volume increases faster than surface
– Cells specialized in absorption modified to greatly increase surface area per unit volume

Surface to Volume Ratio

96 cm2
192 cm2
64 cm3
64 cm3

384 cm2
64 cm3

Compound light microscope
• Light passed through specimen
• Focused by glass lenses
• Image formed on human retina
• Max magnification about 1000X
• Resolves objects separated by 0.2 m, 500X better than human eye

Transmission Electron Microscope
• Abbreviated T.E.M.
• Electrons passed through specimen
• Focused by magnetic lenses
• Image formed on fluorescent screen
– Similar to TV screen
– Image is then photographed

• Max magnification 1000,000s X
• Resolves objects separated by 0.00002 m, 100,000X better than human eye

Scanning Electron Microscope
• Abbreviated S.E.M.
• Specimen sprayed with thin coat of metal
– Electron beam scanned across surface of specimen
– Metal emits secondary electrons

• Emitted electrons focused by magnetic lenses
• Image formed on fluorescent screen
– Similar to TV screen
– Image is then photographed

Immunofluorescence Light Microscope
– Antibodies developed against a specific protein
– Fluorescent dye molecule attached to antibody molecules
– Specimen exposed to fluorescent antibodies

• Ultra-violet light (black ligt) passed through specimen
– Fluorescent dye glows in color where antigen is located
– Emitted light is focused by glass lenses onto human retina

• Allows mapping distribution of a specific protein in cell

Confocal Microscopy
• Narrow laser beam scanned across transparent specimen
• Beam is focused at a very thin plane
• Allows microscopist to optically section a specimen
– Sections made at different levels
– Allows assembly of 3d image on computer screen that can be rotated Microscopy
Video-enhanced Contrast Microscopy
• Great for specimens with low contrast, like living cells
• Image is captured by TV camera instead of eye
• Image is then “tweaked” by adjusting contrast
– Darkest part of image is made black
– Lightest part of image is made white
– All parts in between made shades of gray

• Also allows various shades to be converted to different colors for more contrast

Phase contrast microscopy
• Great for transparent specimens with low contrast, like living cells • Some organelles have higher density than others
– Speed of light is affected by density
– Light passes more slowly through high density than low density
– Light waves entering a specimen “in phase” exit some parts of the specimen out of phase

• Microscope shows only light that is slower or faster
• Causes transparent organelles to “glow”



Cell Fractionation

• Cell fractionation takes cells apart and separates the major organelles from one another
• Centrifuges fractionate cells into their component parts • Cell fractionation enables scientists to determine the functions of organelles
• Biochemistry and cytology help correlate cell function with structure

© 2011 Pearson