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Cells & Molecules > Cell Ultrastructure
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Cell Ultrastructure

  • Magnification → increases the size of an object
  • Resolution/resolving power → ability to distinguish between adjacent points

Table 1-10-2: Microscopes (1)


Feature

Optical microscope

Electron microscope

Radiation

Light

Electrons

Magnification

400x (max1500)

≈500 000x

Resolution

2µm

1nm / 0,001µm
Electrons have a small wavelength
\ Higher resolution

Vacuum in microscope

Absent

Present

Specimen is

- Alive or dead
- Stained

- Dead (vacuum!)

Transmission microscope:
Electrons pass through internal
structure of specimen

Scanning microscope:
Beams of electrons are reflected
off specimens surface. Allows a
three dimensional view

Cell wall (plant cells only)

  • Made up of cellulose fibres which provide strength
  • Cell does not burst if surrounding solutions become dilute

Nucleus (5µm)

  • Contains chromosomes (genes made of DNA which control cell activities)
  • Separated from the cytoplasm by a nuclear envelope
  • The envelope is made of a double membrane containing small holes
  • These small holes are called nuclear pores (100nm)
  • Nuclear pores allow the transport of proteins into the nucleus

Rough Endoplasmic Reticulum (rough ER)

  • Have ribosomes attached to the cytosolic side of their membrane
  • Found in cells that are making proteins for export (enzymes, hormones, structural proteins, antibodies)
  • Thus, involved in protein synthesis
  • Modifies proteins by the addition of carbohydrates, removal of signal sequences
  • Phospholipid synthesis and assembly of polypeptides

Smooth Endoplasmic Reticulum (smooth ER)

  • Have no ribosomes attached and often appear more tubular than the rough ER
  • Necessary for steroid synthesis, metabolism and detoxification, lipid synthesis
  • Numerous in the liver

Ribosomes (20-30nm)

  • Small organelles often attached to the ER but also found in the cytoplasm
  • Large (protein) and small (rRNA) subunits form the functional ribosome
    • Subunits bind with mRNA in the cytoplasm
    • This starts translation of mRNA for protein synthesis (assembly of amino acids into proteins)
  • Free ribosomes make proteins used in the cytoplasm. Responsible for proteins that
    • go into solution in cytoplasm or
    • form important cytoplasmic, structural elements
  • Ribosomal ribonucleic acid (rRNA) are made in nucleus of cell

Golgi apparatus

  • Stack of flattened sacs surrounded by membrane
  • Receives protein-filled vesicles from the rough ER (fuse with Golgi membrane)
  • Uses enzymes to modify these proteins (e.g. add a sugar chain, making glycoprotein)
  • Adds directions for destination of protein package - vesicles that leave Golgi apparatus move to different locations in cell or proceed to plasma membrane for secretion
  • Involved in processing, packaging, and secretion
  • Other vesicles that leave Golgi apparatus are lysosomes

Vacuole and vesicles

  • Membranous sacs of liquid which store substances - vacuoles are storage areas

Lysosomes (0.05 to 0.5 micron)

  • Performs intracellular digestion - more numerous in cells performing phagocytosis
  • Limiting membrane keeps digestive enzymes separate from the cytoplasm
  • Lysosomal enzymes digest particles
    • They function optimally at pH 5 and are mostly inactive at cytosolic pH
    • Lysosomal enzymes are synthesized on rough ER
    • Transferred to the Golgi apparatus for modification and packaging
  • Primary lysosomes are small concentrated sacs of enzymes (no digestion process)
    • Primary lysosomes fuse with a phagocytic vacuole
    • Become secondary lysosomes
    • Digestion begins
    • Nutrients diffuse through lysosomal membrane into the cytosol

Mitochondria (1µm in diameter and 7µm in length)

  • Mostly protein, but also contains some lipid, DNA and RNA
  • Power house of the cell
    • Energy is stored in high energy phosphate bonds of ATP
    • Mitochondria convert energy from the breakdown of glucose into adenosine triphosphate (ATP)
    • Responsible for aerobic respiration
  • Metabolic activity of a cell is related to the number of cristae (larger surface area) and mitochondria
  • Cells with a high metabolic activity (e.g. heart muscle) have many well developed mitochondria

Chloroplast (4-6µm in diameter and 1-5µm in length)

  • Only in photosynthesising cells (plants)
  • Light energy, CO2, and H2O are converted to produce carbohydrates and O2
  • Inner membrane has folds, called lamellae (where chlorophyll is found), which surround a fluid, called stroma