Cell Ultrastructure

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

Table 1-10-2: Microscopes (1)

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