Biology 107
Cell II
September 23, 2005
Cell II
Student Objectives: As a result of this lecture and the assigned
reading, you should understand the following:
1.
Ribosomes are non-membranous organelles that are the sites of
synthesis of proteins in the cell. Ribosomes consist of two
subunits, the large and the small, comprised of proteins and
specific RNAs, called rRNAs (ribosomal RNAs). The mRNA and
the amino acids with their carriers (tRNAs or transfer RNAs) bind
to ribosomes, and polypeptide chains are formed by dehydration
synthesis.
2.
Ribosomes may be attached to membrane or "free" (nonmembrane attached) in the cytoplasm.
Cell II
3.
Endoplasmic reticulum is a membranous organelle that may be
subdivided into two different types with different functions.
a. Rough endoplasmic reticulum (RER) is so named because it is
studded with ribosomes. RER functions to make proteins for
secretion by the cell or for incorporation into cellular membranes. The
RER also begins the compartmentalization and processing of some
proteins so they can be shipped out of the cells or distributed to
specific subcompartments.
b. Smooth endoplasmic reticulum (SER) is a series of interconnected
membrane tubules that lack ribosomes. SER and RER are
continuous. One of the functions of ER lacking ribosomes is the
synthesis of lipids, including phospholipids, fats and steroids. SER
also contains enzymes that are involved in detoxification, and in some
cells SER is involved in calcium storage and release.
Cell II
4.
The Golgi apparatus is a stack of flattened membrane sacs, but
they are not interconnected like ER stacks. Golgi receives,
modifies and segregates different proteins.
a. Some of the proteins modified and sorted by the Golgi are
parts of membranes (such as channel proteins or receptor
molecules), some of the proteins are secreted, and some of the
proteins are routed to other organelles (e.g., lysosomes).
b. Within the Golgi are enzymes to add sugar molecules, sulphate
groups and phosphate groups to proteins.
c. Specific chemical modifications help tag and sort molecules
through the Golgi into different destination streams.
Cell II
5.
Vesicles are relatively small membrane sacs that function to
shuttle materials around the cells and between the cell surface
and the membranous organelles. For those membranous
organelles that do not have direct membranous continuity,
vesicles provide indirect, functional continuity through pitching off,
movement to a new site, fusion and transfer of materials. This
shuttling system has directionality and provides points of control
of passage. When vesicles fuse with a new membrane system
they are adding not only contents but also membrane.
6.
The lysosomes are membrane-bounded sacs (vesicles) that
contain hydrolytic (digestive) enzymes necessary for the normal
turnover of cell components. The membrane enclosure of the
hydrolytic enzymes keeps these enzymes from destroying the
cell.
Ribosomes Consist of Large
and Small Subunits
Ribosomes May be “Free” or
Bound to ER Membrane
RER Contains Bound
Ribosomes
RER Synthesizes Integral Membrane
Proteins, Lysosomal Proteins, and
Proteins for Secretion from the Cell
Endoplasmic
Reticulum
(ER)
Rough endoplasmic reticulum
(RER) is involved in protein
synthesis
Smooth endoplasmic
reticulum (SER) is involved in
lipid synthesis, breakdown of
certain toxins, and calcium
sequestration
Golgi Apparatus
Golgi Apparatus Processes and Sorts
Materials it Receives from the RER
Lysosomes Contain Hydrolytic
Enzymes
Example of Pathway for Proteins
Synthesized in RER and
Processed/Sorted in the Golgi
Synthesis and Processing of
Lysosomal Proteins
Cellular Trafficking
Cellular Trafficking