Download Compartimentation, biological membranes

II. INTERNAL ORGANIZATION
OF EUKARYOTIC CELLS
NUCLEOLUS
Cell nuclei and nucleoli in the liver. Light micrograph
Haematein-eosin staining
The nucleolus in electron micrographs
Spherical structures within the nucleus, ~1-2 mm.
In interphase cells: nucleolus is normally not readily
recognisable or absent, except in neurons, where a
large, prominent nucleolus is present.
MAIN PARTS OF THE NUCLEOLUS:
NO: nucleolar organizer region
of those chromosomes possessing
nucleolar genes
PF: pars fibrosa, dense fibrillary
component
PG: pars granulosa, or
nucleolonema: ribonucleoprotein particles
NAC: nucleolus-associated
chromatin
FUNCTION OF THE NUCLEOLUS:
It produces the ribosomes.
Main steps:
- transcription of nucleolar genes code for ribosomal RNA
occurs at the nucleolar organizer
- ribosomal proteins are syntesized in the cytoplasm
- RNA-protein assembly takes place in pars fibrosa
- ribonucleoprotein particles accumulate in pars granulosa
and undergo maturation process
- completed ribosomal subunits are transported through
the nuclear pores into the cytoplasm where they carry out
their function
ENDOPLASMIC RETICULUM
Nissl staining of neuronal cell bodies visualizes
stacks of rER at light microscopic level
Endoplasmic reticulum (ER) in EM
SER
RER
Nucleus
Nuclear
membran
Three-dimensional view of the rough endoplasmic reticulum
Structure of a ribosome
mRNA
Region of peptide
synthesis
Large subunit
Small subunit
Exit of new
peptide
Free and ER-bound ribosomes
mRNA encoding a
cytosolic protein
pool of ribosomal
subunits in cytosol
mRNA encoding a
protein targeted to ER
ER membrane
Peptide synthesis on ER-bound ribosomes
SER in a steroid-producing Leydig cell in the testis
The sarcoplasmic reticulum: the special form of SER
in skeletal muscle
Function: Ca ion traffic
for the contraction
Functions
The endoplasmic reticulum plays key role in the synthesis of
several types of molecules. ER and the Golgi complex exert
correlated, coordinated activity.
Proteins are synthesized on RER-bound ribosomes (signal
sequences!). After completion of the synthesis several types
of proteins are submitted to post-translational modifications.
Proteins are then further transported to the Golgi
apparatus.
Phospholids and fatty acids are synthesized and metabolised
in the SER
Further roles:
Liver: glycogen synthesis, detoxication
Adrenal gland: synthesis of steroid hormons
Skeletal and heart muscle: storage of calcium ions.
GOLGI COMPLEX: THE CENTER OF SECRETORY PROCESSES
GOLGI COMPLEX:
RECONSTRUCTION FROM EM
Three-dimensional structure of the Golgi complex
FUNCTIONAL CONNECTION
BETWEEN THE RER AND GOLGI MEMBRANES
LYSOSOMES:
Acidic organelles that contain a battery of degradative enzymes.
Membrane-limited organelles, degrade proteins, particles
taken up by the cell, or the cell’s own degenerating organelles.
Types:
- primary lysosomes: spherical, do not contain particles
or membrane debris, do contain acid hydrolases (phosphatases,
nucleases, proteases, etc.)
- secondary lysosomes: larger, irregularly shaped, do contain
debris that is being digested.
PEROXISOMES, GLYOXISOMES:
Small membrane-limited organelles. Contain enzymes that degrade
fatty acids and amino acids. A product of these reactions is hydrogen
peroxide, a corrosive substance. Catalase of peroxisomes degrades
the dangerous molecule.
Primary and secondary lysosomes
PEROXISOME WITH CRYSTALLOID
Modification of proteins in the Golgi apparatus:
- alteration of amino acid side chains
- addition of saccharide residues
- remodeling of oligosaccharides
- specific proteolytic cleavages
- formation of disulphide bonds
- assembly of multiprotein complexes
Types of secretion :
- continuous: proteins are sorted in the trans-Golgi
reticulum into vesicles that immediately fuse with
the cell membrane (example: collagen secretion
of fibroblast cells in connective tissue)
- regulated: the release of these proteins is
initiated by different neural and hormonal
stimuli. The exocytosis is triggered by a
rise in the intracellular calcium level (example:
hormone production of endocrine glands).
MITOCHONDRION:
Mitochondrial types
according to the form of the inner membrane
Mitochondria with cristae:
Cristae: thin folds which
project into the interior of
the mitochondrion.
Occurence: most cell types.
Tubular mitochondria:
the inner membrane projections
are relatively broad tubes.
Occurence: steroid hormone
synthesizing cells (adrenal cortex,
Leydig cells in testis)
Sacculus-type mitochondria:
the inner membrane tubuli are
decorated with pearl-like
broadenings (adrenal cortex)
Mitochondria with prisms:
some of the cristae form triangular
tublike structures (astrocytes)
Origin of mitochondria: symbiosis of prokaryotes
Mitochondria arise exclusively by division, no de novo genesis!
Mitochondria contain DNA, RNA and ribosomes: they can divide
by themselves.
Nuclear DNA codes for 600-1000 mitochondrial proteins, while
mtDNA contains the code for 8-13 ( ~ 1 %).
Import of proteins synthesized in the cytosol : by a translocatorprotein complex in the mitochondrial membrane.
Mitochondria are always inherited from the mother
(the mitochondria of the sperm are
unable to get into the oocyte during
fertilization - determination of
female site pedigree).
Negative staining of the inner mitochondrial membrane:
cristae contain elementary particles
STRUCTURE OF THE INNER MITOCHONDRIAL MEMBRANE
1. Outer membrane
2. Inner membrane
3. Crista
4. Head of elementary particle
5. Stalk of elementary particle
1. Enzymatic coupling factors
3. Enzymes of respiratory chain
4. Cytochrome C molecules
5. Outer chamber
6. Lipid bilayer
Functions
Main function: „power plant” of the cell: synthesis of ATP by the
enzymatic oxidation of amino acids, fatty acids and glucose.
Functions of the intramitochondrial compartments:
Outer membrane
outer cover, protein import, pores
Intermembrane space
cytochromes, apoptosis factors
Inner membrane
Respiratory chain, ATP-synthesis,
protein import, metabolite transport
Matrix
Citrate cycle, β-oxidation of fatty acids,
mtDNA replication, proteine biosynthesis,
haem synthesis, urea cycle