Cells

cells - found in all organisms  

• genetic material - found in central nucleoid area of prokaryotes or nucleus (surrounded by nuclear envelope) of eukaryotes
• DNA has the genes that code for the proteins made by the cell
• cytoplasm - semifluid substance within the cell containing sugars, amino acids, proteins, and organelles (specialized structures in eukaryotes)
• plasma membrane - phospholipid bilayer separating the cell from its surroundings
  • proteins in membrane determine how cell interacts w/ the environment
  • transport proteins - help molecules/ions move across the membrane
  • receptor proteins - sends messages to the cell when in contact w/ certain molecules
  • markers - identify to the cell as a particular type

cell theory - cell size ranges from 1 micrometer to 5 centimeters  

• cells couldn't be observed until microscopes invented in 17th century
• Robert Hooke - 1st to describe cells when he examined cork; named what he saw after the "small rooms" of monks
• Antonie van Leeuwenhoek - 1st to examine living cells; named them "animalcules"
• Matthias Schleiden - stated in 1838 that plants were combinations of tiny/independent cells
• Theodor Schwann - stated in 1839 that all animal tissue were also made of cells
• 3 principles of the cell theory
  • all organisms contain cells, where metabolic/hereditary functions take place
  • cells are the smallest living things, basic units of life
  • cells are produced only from other pre-existing cells

cell size - usually not large for practical purposes  

• most protein processes involve diffusion of substances at some point
• larger cell >> longer time for substances to diffuse from membrane to cell center
• smaller cells >> more efficient than larger cells
• surface area-to-volume ratio - volume increases faster than surface area; larger ratio increases efficiency of the cell
• muscle cells have more than 1 nucleus to allow genetic information to spread around the larger cell
• neurons are extremely skinny to ensure that cytoplasm remains close to the membrane

visualizing cells - other than egg cells, most cells very hard to see  

• resolution - min distance 2 points can be apart and still be seen as separate points
• human eye can only distinguish points over 100 micrometers apart
• modern microscopes (compound microscopes) use 2 magnifying lenses to make things appear much larger (resolves objects 200 nms apart)
  • dark-field microscope - only light reflected from the specimen is seen
  • bright-field microscope - light transmitted through the specimen; provides very little contrast
  • phase-contrast microscope - bring light waves out of phase, producing contrast/brightness differences
  • differential-interference-contrast microscope - uses 2 light beams traveling close together to produce more contrast than phase-contrast microscopes
  • fluorescense microscope - filters only shows light emitted by stained molecules
  • confocal microscope - laser focused on a point and scanned in 2 directions
• light beams reflecting off of objects start to overlap when within a few hundred nms
• transmission electron microscopes - uses electron beams instead of light beams; can resolve objects only 0.2 nms apart
• scanning electron microscope - analyzes substance by looking at the electrons that bounce off the surface of the substance
• immunocytochemistry - uses stains/antibodies to make certain substances more easily seen under a light microscope