Download Ribosomes (20-30nm)

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
o Subunits bind with mRNA in the cytoplasm
o This starts translation of mRNA for protein synthesise
(assembly of amino acids into proteins)
 Free ribosomes make proteins used in the cytoplasm.
Responsible for proteins that
o go into solution in cytoplasm or
o form important cytoplasmic, structural elements
 Ribosomal ribonucleic acid (rRNA) are made in nucleus of
cell
Endoplasmic Reticulum (ER)
 Rough ER
o Have ribosomes attached to the cytosolic side of their
membrane
o Found in cells that are making proteins for export
(enzymes, hormones, structural proteins, antibodies)
o Thus, involved in protein synthesise
o Modifies proteins by the addition of carbohydrates,
removal of signal sequences
o Phospholipid synthesis and assembly of polypeptides
 Smooth ER
o Have no ribosomes attached and often appear more
tubular than the rough ER
o Necessary for steroid synthesis, metabolism and
detoxification, lipid synthesis
o Numerous in the liver
Golgi apparatus
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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
Endocytosis and Exocytosis
 Substances are transported across plasma membrane in
bulk via small vesicles
 Endocytosis
o Part of the plasma membrane sinks into the cell
o Forms a vesicle with substances from outside
o Seals back onto the plasma membrane again
o Phagocytosis: endocytosis brings solid material into the
cell
o Pinocytosis: endocytosis brings fluid materials into the
cell
 Exocytosis
o Vesicle is formed in the cytoplasm //May form from an
edge of the Golgi apparatus
o Moves towards plasma membrane and fuses with
plasma membrane
o Contents are pushed outside cell
o Insulin is secreted from cells in this way
Mitochondria (1µm in diameter and 7µm in length)
 Mostly protein, but also contains some lipid, DNA and RNA
 Power house of the cell
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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