Cytoplasmic Membrane Systems
The following questions are a guide to your preparation for Examination II.
1. What are the major organelles in a eukaryotic cell? What do we mean by organelle?
What organelles belong to the endomembrane system?
How would you describe this
Be familiar with these terms: exocytosis/endocytosis, anterograde and
retrograde trafficking, constitutive and regulative secretion, sorting and targeting of
vesicles. Understanding the difference between cytosolic and exoplasmic leaflets of
The nucleus, er, golgi apparatus, lysosomes, endosomes, mitochondria and peroxisomes
are distinct compartments separated from the cytosol by atleast one selectively permeable
membrane. Organelles mean membrane-enclosed compartments of eukaryotic cell.
Nucleus, ER, Golgi, lysosomes, endosomes, vesicles are part of endomembrane system.
Function of endomembrane system: protein (and lipid) synthesis, processing, sorting and
packaging for specific target sites. It is dynamic, integrated, ordered membrane bound
Anterograde - Moving forward or extending forward. Moving from nucleus to ER to
Golgi to plasma membrane
Retrograde - From outside of cell towards nucleus
Constitutive secretion - In
, proteins are packaged in vesicles in the
Golgi apparatus and are secreted immediately via exocytosis, all around the cell. They
secrete continuously and unlike the regulated pathway, no external signal is needed to
stimulate the process. Cells that secrete constitutively have many Golgi apparatus
scattered throughout the cytoplasm. Fibroblasts, osteoblasts and chondrocytes are some
of the many cells that use this pathway.
Regulated Secretion - In
, proteins are packaged as described in the
constitutive pathway, but they are only secreted in response to a specific signal, such as
neural or hormonal stimulation. Cells that use the regulated secretory pathway are usually
apical or polarized. The Golgi apparatus are found in a supranuclear position (between
the nucleus and the secretory surface). Example cells that use regulated pathway are:
goblet cells (secrete mucus), beta cells of the pancreas (secrete insulin) and odontoblasts
2. Be able to describe some of the properties of the nuclear pore. What types of transport
take place here? What is being transported?
Review the basic steps by which the nuclear
protein lamin would be transported from the cytoplasm to the nucleus.
should include an understanding of: NLS, importin, GTP-RAN.
The nuclear envelope in all eucaryotic cells is perforated by nuclear pores that form the
gaes through which all molecules enter or leave the nucleus. Traffic occurs in both
directions through the pores: newly made proteins destined for the nucleus enter from the
cytosol; RNA molecules, which are synthesized in the nucleus, and ribosomal subunits,
which are assembled in the nucleus are exported. mRNA molecues that are incompletely
spliced are not exported from the nucleus, suggesting that nuclear transport serves a final