1st year · General cytology

Under an electron microscope (thin section technique with osmium tetroxide fixation), the plasma membrane appears trilamellar, with a thickness of approximately 75 Å, formed of two dark layers separated by a light layer.

Within the plasma membrane, membrane lipids are represented by 55% phospholipids, 25% cholesterol and 20% glycolipids.

Cholesterol is only present in the membranes of animal cells; it is absent from plant cells and bacteria.

Cholesterol reinforces the strength of the membrane because by placing itself between the phospholipid molecules, it immobilizes neighboring hydrocarbon chains, making the membrane more rigid.

Glycophorin is a monotopic intrinsic transmembrane protein (making a single pass through the lipid bilayer), unlike band 3 which is polytopic.

Singer and Nicholson proposed in 1972 a model of molecular architecture which defines the membrane as a fluid and asymmetric mosaic.

Cells have enzymes called flippases that actively move certain phospholipids from one leaflet to another, playing a role in membrane lipid asymmetry.

Mitochondria have two membranes: the outer membrane and the inner membrane, which separates the mitochondrial matrix from the intermembrane space.

The inner membrane forms invaginations in the mitochondrial matrix called cristae, which are the locus of phosphorylative oxidation reactions and ATP synthesis.

The inner membrane is very rich in proteins: it is composed of 80% proteins and 20% lipids, and contains numerous transporters, enzymatic protein complexes and TIM translocases.

Mitochondrial DNA contains 37 genes that encode 13 proteins (subunits of oxidative phosphorylation complexes), 22 transfer RNAs and 2 ribosomal RNAs.

The internal membrane of the mitochondria carries a family of transmembrane enzymes, the cytochromes P450, whose active site is immersed in the matrix and which hydroxylate cholesterol to pregnenolone, the initial step in steroid synthesis.

The cytoskeleton has three types of fibers: microfilaments (actin, ~7 nm), microtubules (~25 nm) and intermediate filaments (~8-10 nm).

Microfilaments are long fibers with a diameter of about 7 nanometers, composed of two intertwined protein chains of globular actin.

Microtubules are hollow tubes with a diameter of approximately 25 nm, formed of 13 protein protofilaments arranged in a crown, each protofilament being composed of α and β tubulin dimers.

Keratin is a class of intermediate filaments found in epithelial cells (cells lining organs and cavities in the body) as well as associated structures such as hair and nails.

Each centriole is composed of nine triplets of microtubules. Centrioles occur in pairs, usually arranged at right angles to each other, close to the nucleus.

The ER membrane protein kinectin attaches vesicles to the motor protein kinesin, which drives the vesicle along microtubules toward the periphery of the cell, using ATP as an energy source.

The REG is a set of flattened bags communicating with each other, whose internal reticular face is always smooth but whose cytosolic external face is lined with ribosomes, playing a role in proteosynthesis and glycosylation.

The REL is formed by a network of small vesicles and tubules without ribosomes; it plays a role in the synthesis of lipids and steroid hormones.

N-glycosylation begins in the REG by transferring a bulk oligosaccharide (via dolichol) to the asparagine of the protein being synthesized. It continues and is completed in the Golgi apparatus.

Labeling of lysosomal hydrolases occurs in cis saccules of the Golgi apparatus by phosphorylation of mannose (addition of a phosphate on the 6th carbon of a mannose), creating the mannose-6-phosphate signal recognized by specific receptors at the trans-Golgi network.

Nuclear pores are protein complexes organized in the form of channels whose role is to ensure molecular exchanges between the cytoplasm and the nucleoplasm (RNA, enzymes, nucleotides, etc.).

The nucleolus is a non-membrane-limited nuclear compartment responsible for the synthesis of ribosomal RNAs (rRNAs) and the assembly of ribosomal subunits.

Euchromatin is the decondensed form of chromatin during interphase (very weakly colored); it corresponds to active areas of DNA with strong gene transcription.

The S phase (Synthesis) is the phase during which DNA replication takes place. The amount of nuclear DNA is doubled at the end of this phase.

During metaphase, the genetic material is condensed as much as possible in the form of chromosomes, aligned on the equator of the cell (middle part). This is the ideal phase for the study of karyotype.

The G0 phase is a phase of “mitotic quiescence” or “mitotic rest” which does not belong to the classic phases of the cell cycle; the cell enters a non-proliferative resting state and stops its multiplication activity.

The discovery of the Golgi apparatus is due to the Italian biologist Camillo Golgi, Nobel Prize winner in 1906.

The cis face (external face) is in the vicinity of the RE; it is at this level that the ER transition vesicles bud to fuse with the Golgi saccules. The trans side is the opposite and gives rise to secretory vesicles towards lysosomes or the cell surface.

The nucleus is present in all cells of the body with the exception of red blood cells (red blood cells), keratinocytes (cells of the surface layer of the skin) and thrombocytes (platelets).

The shape of the nucleus differs depending on the cell type: spherical in cubic epithelial cells, ovoid in smooth muscle cells, and polylobed in polynuclear cells.

Mitochondria, along with the smooth endoplasmic reticulum, are the main reservoir of intracellular calcium; they are capable of capturing calcium, storing it in the matrix, then releasing it into the cytosol via ion channels in the inner membrane.

The signal peptide is an N-terminal hydrophobic residue that acts as a transfer initiation signal: it is recognized by the signal recognition particle (SRP), which directs the ribosome to the ER membrane where the protein is co-translationally translocated into the ER lumen.

The plasma membrane of red blood cells is composed of 40% lipids, 52% proteins and 8% carbohydrates.

The human diploid genome contains approximately 200 identical copies of the gene coding for the 3 nucleolar rRNAs, distributed over the short arms of 5 pairs of acrocentric chromosomes: chromosomes 13, 14, 15, 21 and 22. Each chromosome includes 20 copies of these genes.

The dense outer layer of the plasma membrane is lined with a thin glycoprotein film (glycocalyx or cell-coat) whose fibrils are perpendicular to the membrane. The cell-coat is made up of felting of carbohydrate fibrils.

Glypiation (GPI anchoring) takes place on an amino acid in the C-terminal position of the integrated protein anchored by a lipid.

The inter-membrane space contains H+ protons playing an important role in phosphorylation, cytochrome C molecules and molecules generally less than 10 kDa.

The intermediate filaments of nerve cells are called neurofilaments. Vimentin is present in many non-epithelial cells, and keratin in epithelial cells.

A, C and D are correct. Phospholipids are amphiphilic (hydrophilic head, hydrophobic tail). Carbohydrates impart a negative charge via sialic acid. Membrane proteins have diverse functions (receptors, transporters, enzymes, adhesion molecules, messengers). Cholesterol is ABSENT from bacteria (B is false).

A, B and C are correct. The mitochondrion synthesizes ATP via oxidative phosphorylation, cooperates with the REL for the synthesis of steroid hormones (via cytochromes P450), and stores intracellular calcium. The synthesis of immunoglobulins (D) is carried out by plasma cells via ribosomes and the REG.

A, B and C are correct. The cytoskeleton maintains cell shape and anchors organelles; microtubules serve as rails for vesicular transport; the shortening of microtubules during mitosis allows chromosome migration. The synthesis of ribosomal proteins (D) is ensured by the nucleus (gene transcription) and cytoplasmic ribosomes.

A, B and D are correct. The Golgi apparatus ensures the continuation of N-glycosylation, O-glycosylation (binding on serine/threonine in the median and trans saccules) and the labeling of lysosomal hydrolases by Man-6P in the cis saccules. Mitochondrial DNA replication (C) takes place in the mitochondrial matrix.

A, B and D are correct. Heterochromatin is the condensed form of chromatin (D). It is subdivided into: constitutive (repetitive DNA never transcribed, centromeres and telomeres - A) and facultative (repressed genes that can be active in other cell types - B). Proposition C describes euchromatin, not heterochromatin.

A, B and C are correct: Hsp70 unfolds the protein, the outer membrane receptor recognizes the targeting sequence, and translocation takes place via TOM (outer membrane) then TIM (inner membrane). The protein is then matured by cleavage of the targeting sequence and folding by Hsp60. Proposition D is false: these proteins are synthesized in the cytosol under the control of the nuclear genome.

The interphase brings together phases G1, S and G2. M phase (mitosis) is the actual cell division phase and is not part of interphase.

A, B and C correspond to the modifications mentioned in the course: myristoylation, palmitylation and glypiation (GPI). Isoprenylation (farnesyl or geranylgeranyl) is also mentioned. Ubiquitination (D) is a mark for degradation by the proteasome, not a membrane anchor.

A, B and C are correct. PRS recognizes the signal peptide (A), the complex binds to the ER membrane anchor protein (B), and then a translocation pore opens (C). Proposition D is false: the signal peptide is cut by peptidases and degraded by ER proteases after translocation.

A, B and C correspond to telophase: equatorial constriction giving two daughter cells (cytodieresis), decondensation of the chromatin and reformation of the nuclear envelope. Proposition D describes the metaphase.

The answer must mention the trilamellar aspect to the ME, the proportions of the constituents and the two categories of membrane proteins.

The answer should describe the two membranes, their respective characteristics, the ridges and the contents of the matrix.

The answer must compare the 3 types, mentioning diameter, proteins and distinctive functions for each.

The answer should mention the key steps of vesicular trafficking from the REG to the plasma membrane.

The answer must describe the 4 phases in order with the precise chromosomal events of each.

The answer must link structure (peaks, H+ gradient) to function (ATP synthesis by ATP synthase).

The answer must distinguish the two types of chromatin by their condensation and transcriptional activity, and subdivide heterochromatin.

The answer should explain the concept of membrane fluidity, types of lipid movements and the role of flippases.

The answer should describe mannose phosphorylation, the role of Man-6P receptors, clathrin vesicles and lysosomal maturation.

The answer should describe the 3 phases of interphase with their distinctive characteristics.