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

3

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

TotalSurfaceArea
(HeightWidthNumberOfSidesNumberOfCubes)
96 cm2
192 cm2
TotalVolume
(HeightWidthLengthXNumberOfCubes)
64 cm3
64 cm3
SurfaceAreaPerCube/VolumePerCube
(SurfaceArea/Volume)
1.5/1
3/1

384 cm2
64 cm3
6/1

Microscopy
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

Microscopy
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

Microscopy
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

Microscopy
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

Microscopy
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

Microscopy
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”

Microscopy

Microscopy

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

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