3B Scientific Animal cell model - Manual (Page 4)

®
4
The Animal Cell
and thus detoxifies them. In certain types of cell (e.g. muscle cells) the smooth ER also serves as a store for
calcium ions.
The Golgi apparatus (GA) consists of groups of flat sac-like membranes (sacculi) und numerous small vesi-
cles. Here proteins newly synthesized in the ER are further modified. Specific sugar groups are synthesized
and coupled onto proteins, and also some proteins are cleaved. In addition polysaccharides with appropri-
ate sulfatization are synthesized and glycolipids are synthesized and joined together. These mature proteins
are packed into vesicles in the GA and made ready for transport. Thus the GA plays an important role in the
sorting and delivery of proteins (“Post Office of the cell“).
The GA is also connected with exocytotic events (release of vesicles) and fagocytosis (protrusion of the
plasma membrane with subsequent “pinching off” of the membrane together with the components that it
surrounds). Vesicles can also be actively enveloped and captured (endocytosis) and also receptors acting as
mediators can be brought in (pinocytosis). Membrane protrusions such as the microvilli (small finger-like
protrusions with little mobility), stereocilia (larger protrusions with relatively little mobility) and kinocilia
(flagellum-shaped protrusions for active cilia movement) will only be discussed in the course reading.
Vacuoles can be formed both endocytotically and by cell components that digest the contents of vesicles
(lysosomes).
Alongside the lysosomes in a cell are small organelles (peroxisomes), which principally oxidize fatty acids,
amino acids and uric acid and can detoxify potentially cytotoxic organic compounds.
Mitochondria are bacteria that entered unicellular organisms in primeval times and developed into so-
called symbionts. They possess their own genetic material and also ribosomes for the synthesis of their own
proteins. They have however become so intertwined with the genome of the host cell that they can no lon-
ger live independently. The benefit of this symbiosis is that the mitochondria can utilize oxygen (a principle
toxic substance for living cells, that first arose in the primeval atmosphere from the evolution of plants).
Thus carbohydrates, fatty acids and amino acids are oxidized to CO2 and H2O by consumption of elemental
oxygen. In this way ATP, the universal energy carrier, is obtained for the cell. The mitochondrion possesses
a double membrane wall. ATP synthesis and the processes of the respiratory chain take place on the inter-
nal wall itself. Fatty acid oxidation and the citric acid cycle take place within the mitochondria matrix. Thus
the mitochondria can be described as the powerhouse of the cell.
The cell nucleus is the information centre for the cell. The information itself is distributed over 46 deoxy-
ribonucleic acid molecules (DNA). They are accommodated in the cell nucleus together with the histones
(capping proteins). The nucleus is typically more densely packed than the cytoplasm and is surrounded by
a nuclear double membrane (cistern of the ER) with defined channels (nuclear pores). The information for
protein synthesis is taken to the ribosomes by the messenger ribonucleic acid (mRNA), which is synthesized
on the gene segments of the DNA. This process is called transcription and produces copies of the DNA. The
ribosomal RNA is synthesized on specialized segments in the nucleoli (aggregations in the cell nucleus). In
addition there is close communication between the cytoplasm and the membrane receptors, so the cell
nucleus represents the central information and control unit of the cell.
Author: Prof. Dr. R.H.W. Funk, Institute of Anatomy, Dresden University of Technology
English