II. INTERNAL ORGANIZATION OF EUKARYOTIC CELLS

NUCLEOLUS

Cell nuclei and nucleoli in the liver. Light micrographCell nuclei and nucleoli in the liver. Light micrographHaematein-eosin stainingHaematein-eosin staining

Spherical structures within the nucleus, ~1-2 m.

In interphase cells: nucleolus is normally not readily recognisable or absent, except in neurons, where a large, prominent nucleolus is present.

The nucleolus in electron micrographs

MAIN PARTS OF THE NUCLEOLUS:MAIN PARTS OF THE NUCLEOLUS:

NO:NO: nucleolar organizer regionof those chromosomes possessingnucleolar genesPF:PF: pars fibrosa, dense fibrillarycomponentPG:PG: pars granulosa, or nucleolonema: ribonucleo-protein particlesNAC:NAC: nucleolus-associatedchromatin

FUNCTION OF THE NUCLEOLUSFUNCTION OF THE NUCLEOLUS::

It produces the ribosomes.

Main steps:Main steps:- transcription of nucleolar genes code for ribosomal RNA occurs at the nucleolar organizernucleolar organizer - ribosomal proteins are syntesized in the cytoplasmcytoplasm- RNA-protein assembly takes place in pars fibrosapars fibrosa- ribonucleoprotein particles accumulate in pars granulosapars granulosa and undergo maturation process- completed ribosomal subunits are transported through the nuclear pores into the cytoplasm where they carry out their function

ENDOPLASMIC RETICULUMENDOPLASMIC RETICULUM

Nissl staining of neuronal cell bodies visualizesNissl staining of neuronal cell bodies visualizesstacks of rER at light microscopic levelstacks of rER at light microscopic level

Endoplasmic reticulum (ER) in EM

SER RER

Nucleus

Nuclear membran

Three-dimensional view of the rough endoplasmic reticulumThree-dimensional view of the rough endoplasmic reticulum

Small subunit Large subunit

mRNA

Exit of new peptide

Region of peptide synthesis

Structure of a ribosomeStructure of a ribosome

ER membrane

pool of ribosomal subunits in cytosol

mRNA encoding a cytosolic protein

mRNA encoding a protein targeted to ER

Free and ER-bound ribosomesFree and ER-bound ribosomes

Peptide synthesis on ER-bound ribosomesPeptide synthesis on ER-bound ribosomes

SER in a steroid-producing Leydig cell in the testisSER in a steroid-producing Leydig cell in the testis

The sarcoplasmic reticulum: the special form of SER The sarcoplasmic reticulum: the special form of SER in skeletal musclein skeletal muscle

Function: Ca ion trafficFunction: Ca ion trafficfor the contractionfor 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 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 SERSER 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 PROCESSESGOLGI COMPLEX: THE CENTER OF SECRETORY PROCESSES

GOLGI COMPLEX: GOLGI COMPLEX: RECONSTRUCTION FROM EMRECONSTRUCTION FROM EM

Three-dimensional structure of the Golgi complexThree-dimensional structure of the Golgi complex

FUNCTIONAL CONNECTION FUNCTIONAL CONNECTION BETWEEN THE RER AND GOLGI MEMBRANESBETWEEN THE RER AND GOLGI MEMBRANES

LYSOSOMES: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:PEROXISOMES, GLYOXISOMES:Small membrane-limited organelles. Contain enzymes that degradefatty acids and amino acids. A product of these reactions is hydrogenperoxide, a corrosive substance. Catalase of peroxisomes degrades the dangerous molecule.

Primary and secondary lysosomesPrimary and secondary lysosomes

PEROXISOME WITH CRYSTALLOIDPEROXISOME WITH CRYSTALLOID

Modification of proteins in the Golgi apparatus: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 :Types of secretion :

- continuous: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: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:MITOCHONDRION:

Mitochondrial types

according to the form of the inner membrane

Mitochondria with cristae:Mitochondria with cristae: Cristae: thin folds which project into the interior of the mitochondrion.

Occurence: most cell types.

Tubular mitochondria:Tubular mitochondria:the inner membrane projections are relatively broad tubes. Occurence: steroid hormonesynthesizing cells (adrenal cortex,Leydig cells in testis)

Sacculus-type mitochondria:Sacculus-type mitochondria: the inner membrane tubuli are decorated with pearl-like broadenings (adrenal cortex)

Mitochondria with prisms:Mitochondria with prisms:some of the cristae form triangulartublike 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 translocator-protein 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:Negative staining of the inner mitochondrial membrane:cristae contain elementary particlescristae contain elementary particles

STRUCTURE OF THE INNER MITOCHONDRIAL MEMBRANESTRUCTURE OF THE INNER MITOCHONDRIAL MEMBRANE

1. Outer membrane2. Inner membrane3. Crista4. Head of elementary particle5. Stalk of elementary particle

1. Enzymatic coupling factors3. Enzymes of respiratory chain4. Cytochrome C molecules5. Outer chamber6. 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