Cell Membrane Structure, Composition, and Transport Mechanics

Classification and General Overview of Cell Membranes

• Organismal Domains and Kingdoms:

  • Living organisms are divided into Prokaryotes (Archéo-Bactéries and Eubactéries) and Eukaryotes (Plantes, Protistes, Animals/Champignons).

• Membrane System Differences:

  • Prokaryotes: Lack an internal endomembrane system.

  • Eukaryotes: Possess an extensive internal membrane system (endomembrane system).

• Eukaryotic Cell Compartmentalization:

  • An animal eukaryotic cell features a plasma membrane enclosing the cytoplasm (cytosol containing organelle structures) and various membranous organelles:

    1. Nucleolus (nucléole)

    2. Nucleus (noyau)

    3. Ribosome

    4. Vesicle (vésicule)

    5. Rough Endoplasmic Reticulum / Ergastoplasm (ergastoplasme)

    6. Golgi Apparatus (appareil de Golgi)

    7. Cytoskeleton (cytosquelette)

    8. Smooth Endoplasmic Reticulum (réticulum endoplasmique lisse)

    9. Mitochondrion (mitochondrie)

    10. Vacuole

    11. Cytosol

    12. Lysosome

    13. Centriole

  • Organelles are enclosed by single membranes, except for the nucleus and mitochondrion, which have double membrane systems.

• Prokaryotic Membrane Envelope Architecture:

  • Cytoplasmic Membrane: Inner phospholipid bilayer surrounding the cytoplasm.

  • Cell Wall (Paroi): Outer protective structure composed of peptidoglycan.

  • Gram-Positive Bacteria:

    • Thick, homogeneous peptidoglycan layer.

    • No outer membrane.

    • Stain purple during Gram staining.

    • Example: Staphylococcus aureus (Gram-positive cocci).

  • Gram-Negative Bacteria:

    • Thin peptidoglycan layer.

    • Outer membrane containing lipopolysaccharides (LPS) and porins.

    • Heterogeneous and irregular wall structure.

    • Stain pink/red during Gram staining.

    • Example: Escherichia coli (Gram-negative bacillus).

• Gram Staining Procedure Steps:

  1. Application of Crystal Violet: Initial primary staining.

  2. Application of Iodine (Lugol): Acts as a mordant to intensify crystal violet complexing.

  3. Alcohol Wash: Decolorization step (decolorizes thin-walled Gram-negative cells).

  4. Application of Safranin / Fuchsin: Counterstaining step (stains Gram-negative cells pink).

Structure and Chemical Composition of Biological Membranes

• General Chemical Composition:

  • Typical plasma membrane composition: $38\%$ lipids, $56\%$ proteins, and $6\%$ carbohydrates.

  • Composition varies significantly depending on cell type, specific organelle, and inner versus outer membrane leaflets.

• Structural Organization:

  • Amphiphilic lipids spontaneously organize into a phospholipid bilayer in aqueous environments.

  • Proteins (integral, anchored, or peripheral) are embedded in or associated with the bilayer.

  • Carbohydrate chains (oligosaccharides) attach to extracellular domains of proteins (glycoproteins) and lipids (glycolipids), forming the glycocalyx.

Membrane Lipids: Types, Structures, and Distribution

• Amphiphilic Nature:

  • Membrane lipids contain a hydrophilic (polar) head oriented toward the aqueous extracellular or intracellular media, and hydrophobic (non-polar) fatty acid tails facing the core of the bilayer.

• Phospholipids:

  1. Glycerophospholipids:

     • Composed of a glycerol backbone, two fatty acid chains (typically one saturated and one unsaturated with a double bond producing a kink), a phosphate group, and a polar head group (alcohol or amino acid).

     • Fatty acid carbon chains in natural lipids have an even number of carbon atoms, ranging from $\text{C}_{14}$ to $\text{C}_{24}$.

     • Phosphatidylethanolamine (PE): Neutral net charge.

     • Phosphatidylcholine (PC): Neutral net charge.

     • Phosphatidylserine (PS): Negative net charge.

     • Phosphatidylinositol (PI): Negative net charge.

  2. Sphingophospholipids:

     • Contain a sphingosine backbone instead of glycerol, attached to an unsaturated fatty acid chain, a phosphate group, and a polar head group.

     • Example: Sphingomyelin (contains a choline head group).

• Glycolipids:

  • Formed by esterification or amidification of fatty acids by sugars or amino sugars.

  • Represent less than $5\%$ of total membrane lipids.

  • Structurally similar to phospholipids but lack a phosphate group.

  • Synthesized in the Golgi apparatus (unlike phospholipids and cholesterol, which are synthesized in the Endoplasmic Reticulum).

  • Main Classes: Glyceroglycolipids and Sphingoglycolipids.

  • Functions:

    • Cellular recognition and cell-cell interaction.

    • Determine ABO blood groups on erythrocyte membranes.

    • Glycosylphosphatidylinositol (GPI) anchors anchor proteins to the outer leaflet.

    • Cerebrosides (e.g., galactocerebroside) mediate interactions between neural cells.

• Steroids (Cholesterol):

  • Exclusively present in animal cell membranes; absent in plant cells and bacteria.

  • Constitutes approximately $25\%$ ($\frac{1}{4}$) of animal plasma membrane lipids.

  • Structure: Hydrophobic steroid ring system, hydrophobic hydrocarbon tail, and a single hydrophilic hydroxyl group ($\text{-OH}$) serving as the polar head.

  • Function: Intercalates between phospholipids to stabilize the membrane, restrict fatty acid chain motion, and prevent excessive membrane fluidity.

• Lipid Composition Across Cellular Membranes:

  • Phosphatidylethanolamine (PE): Plasma Membrane ($16\%$), Outer Mitochondrial Membrane ($23\%$), Endoplasmic Reticulum (16\).

  • Phosphatidylserine (PS): Plasma Membrane ($6\%$), Outer Mitochondrial Membrane ($2\%$), Endoplasmic Reticulum ($3\%$).

  • Phosphatidylcholine (PC): Plasma Membrane ($17\%$), Outer Mitochondrial Membrane ($50\%$), Endoplasmic Reticulum ($55\%$).

  • Phosphatidylinositol (PI): Plasma Membrane ($1\%$), Outer Mitochondrial Membrane ($0\%$), Endoplasmic Reticulum ($0\%$).

  • Sphingomyelin: Plasma Membrane ($17\%$), Outer Mitochondrial Membrane ($5\%$), Endoplasmic Reticulum ($3\%$).

  • Glycolipids: Plasma Membrane ($2\%$), Outer Mitochondrial Membrane ($0\%$), Endoplasmic Reticulum ($0\%$).

  • Cholesterol: Plasma Membrane ($45\%$), Outer Mitochondrial Membrane ($5\%$), Endoplasmic Reticulum ($6\%$).

Membrane Proteins: Anchoring, Types, and Modifications

• Functions of Membrane Proteins:

  • Protein and ion transporters/channels.

  • Cellular adhesion and anchoring to the extracellular matrix or adjacent cells.

  • Reception of extracellular chemical signals and signal transduction.

  • Enzymatic activity support.

• Protein Structural Properties:

  • Polymers with an amino-terminal end ($\text{NH}_2$) and a carboxy-terminal end ($\text{COOH}$).

• Types of Association with the Bilayer:

  1. Integral (Intrinsic) Proteins:

     • Transmembrane: Cross the bilayer once or multiple times.

       • Single $\alpha$-helix (e.g., cell surface receptors).

       • Multiple $\alpha$-helices (e.g., transport proteins, enzymes, GPCRs).

       • $\beta$-barrel / $\beta$-sheets (e.g., pore-forming channels/porins).

     • Non-transmembrane Lipid-Anchored: Located entirely outside the lipid bilayer but covalently attached to lipid molecules in the membrane.

  2. Peripheral (Extrinsic) Proteins:

     • Do not enter the hydrophobic core of the membrane; associated via weak non-covalent electrostatic or hydrogen interactions with integral proteins or lipid polar heads on either the extracellular or intracellular side.

• Covalent Lipid Modifications (Co- or Post-translational):

  1. Isoprenylation (Prenylation / Farnesylation): Addition of a farnesyl or geranylgeranyl isoprenoid lipid group to a cysteine residue near the C-terminus.

  2. Myristoylation: Attachment of myristic acid (a saturated fatty acid) to an N-terminal glycine residue.

  3. Palmitoylation (S-acylation): Attachment of palmitic acid to any cysteine residue.

  4. Glypiation: Covalent attachment of a Glycosylphosphatidylinositol (GPI) anchor to the C-terminal amino acid, projecting the protein on the extracellular face.

Membrane Carbohydrates and the Glycocalyx

• Linkages:

  • Carbohydrates do not exist in a free state in membranes.

  • Bound to proteins via N-glycosidic linkages (small glycoproteins) or O-glycosidic linkages (proteoglycan complexes).

  • Bound to lipids as glycolipids.

• The Glycocalyx (Cell Coat):

  • A fibrous outer layer consisting of all oligosaccharide side chains of membrane glycoproteins and glycolipids.

  • Functions:

    • Provides structural rigidity to the membrane surface.

    • Protects the cell against mechanical and chemical damage.

    • Facilitates specific cell-cell recognition, communication, and adhesion.

• ABO Blood Group System Antigens:

  • Differentiated by specific glycoprotein/glycolipid surface antigens on red blood cell membranes:

    • Antigen A: Contains N-Acetyl-Galactosamine (NAGA).

    • Antigen B: Contains Galactose (GAL).

    • Antigen H (Group O): Lacks the terminal NAGA or GAL sugar, exposing the core fucose-containing carbohydrate chain.

  • Carbohydrate Components:

    • $\text{GAL}$ = Galactose

    • $\text{NAGA}$ = N-Acetyl-Galactosamine

    • $\text{NAGL}$ = N-Acetyl-Glucosamine

    • $\text{FUC}$ = Fucose

Key Properties of the Plasma Membrane

• Spontaneous Lipid Self-Assembly:

  • Depending on concentration and physical conditions, amphiphilic lipids form:

    • Bilayer: Extended double layer enclosing an internal compartment.

    • Micelle: Spherical monolayer aggregate with hydrophobic tails inside.

    • Liposome: Synthetic spherical vesicle consisting of a lipid bilayer surrounding an aqueous core.

• Transverse Lipid Asymmetry:

  • Asymmetrical distribution of lipids, proteins, and carbohydrates between the outer and inner leaflets.

  • Sphingomyelin and Phosphatidylcholine are enriched in the outer leaflet.

  • Phosphatidylserine (PS) and Phosphatidylinositol (PI) are localized predominantly to the inner (cytosolic) leaflet.

  • This lipid asymmetry confers a net negative electric charge on the cytosolic face of the plasma membrane.

• Membrane Fluidity and Lipid Dynamics:

  • Types of Lipid Movement:

    1. Lateral Diffusion: Rapid sideways movement within the monolayer.

    2. Rotation: Very rapid spinning around the long molecular axis.

    3. Swing: Flexing motion of hydrocarbon chains.

    4. Flexion: Contraction/bending of fatty acid tails.

    5. Transverse Diffusion ("Flip-Flop"): Migration from one leaflet to the other; extremely rare and slow spontaneously; catalyzed by flippase enzymes.

  • Factors Regulating Fluidity:

    • Temperature: Higher temperatures increase kinetic energy and fluidity.

    • Fatty Acid Composition: Unsaturated fatty acids with double bonds increase fluidity due to chain kinks; saturated fatty acids increase packing and rigidity.

    • Cholesterol Content: Buffers fluidity; restrains phospholipid movement at warm temperatures and prevents tight packing at cold temperatures.

    • Protein Density: High concentration of embedded proteins reduces lateral lipid mobility.

Membrane Epithelial Differentiations

• Epithelial cells exhibit structural polar differentiations across distinct membrane domains:

  • Apical Pole:

    • Microvilli: Isolated projections or dense arrays ("brush border") increasing surface area for absorption.

    • Vibratile Cilia: Motile structures propelling lumenal fluid.

  • Basal Pole:

    • Invaginations: Deep infoldings of the basal plasma membrane involved in active water and solute transport.

  • Lateral Pole:

    • Interdigitations: Interlocking cell membrane folds enhancing intercellular adhesion and junctional contact.

Transmembrane Transport: Passive Transport Mechanisms

• Classification Overview:

  • Simple Diffusion: No protein involved, non-saturable.

  • Facilitated Diffusion: Protein-mediated (channels, uniports, porins), saturable ($\text{V}_{\max}$).

• Simple Diffusion:

  • Applies to lipid-soluble (hydrophobic) molecules and small uncharged polar molecules:

    • Respiratory gases ($\text{O}_2$, $\text{CO}_2$, $\text{N}_2$).

    • Steroid and thyroid hormones.

    • Fat-soluble vitamins (Vit A, D, E, K).

    • Urea, ethanol, glycerol.

  • Molecules cross directly through the phospholipid bilayer down their electrochemical or concentration gradient (from higher to lower concentration).

  • Requires no membrane transport proteins and no energy.

  • Non-saturable linear kinetics; rate is directly proportional to concentration gradient and hydrophobicity, and inversely proportional to molecular size.

• Facilitated Diffusion:

  • Applies to non-liposoluble (hydrophilic) species: charged ions ($\text{Na}^+$, $\text{K}^+$, $\text{Ca}^{2+}$, $\text{Cl}^-$), polar molecules (water, glucose), amino acids, nucleotides.

  • Occurs down an electrochemical gradient without cellular energy input.

  • Faster than simple diffusion but exhibits saturable kinetics ($\text{V}_{\max}$) due to a finite number of transporter proteins.

  • Stereospecific: D-glucose diffuses significantly faster via GLUT transporters than its enantiomer L-glucose.

• Intracellular vs. Extracellular Ion Concentrations:

  • $\text{Na}^+$: Intracellular $5\text{--}15\,\text{mM}$, Extracellular $145\,\text{mM}$.

  • $\text{K}^+$: Intracellular $145\,\text{mM}$, Extracellular $5\,\text{mM}$.

  • $\text{Mg}^{2+}$: Intracellular $0.5\,\text{mM}$, Extracellular $1\text{--}2\,\text{mM}$.

  • $\text{Ca}^{2+}$: Intracellular $0.0001\,\text{mM}$, Extracellular $1\text{--}2\,\text{mM}$.

  • $\text{Cl}^-$: Intracellular $5\text{--}15\,\text{mM}$, Extracellular $110\,\text{mM}$.

• Mechanisms of Facilitated Diffusion:

  1. Ion Channels (Protéines Tunnels):

     • Multimeric transmembrane proteins forming continuous aqueous pores.

     • Extremely high transport rate ($10^6\,\text{ions/sec}$ per channel; up to $2 \times 10^9\,\text{molecules/sec}$).

     • Highly selective for specific ions.

     • Leak Channels: Permanently open pores.

     • Gated Channels: Opening/closing regulated by stimuli:

       • Voltage-Gated Channels: Opened by membrane potential alterations (depolarization).

       • Ligand-Gated Channels: Opened by binding of a specific neurotransmitter/messenger (e.g., Nicotinic Acetylcholine Receptor on post-synaptic membranes).

  2. Aquaporins (Water Channels):

     • Specialized transmembrane channels enabling high-speed, bidirectional passive water flux across membranes.

     • Highly abundant in renal tubule cells, erythrocytes, and lacrimal gland cells.

     • Osmosis Principle: Passive net movement of water across a semipermeable membrane from a hypotonic solution (lower solute concentration) to a hypertonic solution (higher solute concentration).

     • Osmotic Response in Red Blood Cells:

       • Isotonic Solution ($320\,\text{mOsm}$): Equal water flux in and out; normal erythrocyte morphology.

       • Hypotonic Solution: Massive water influx leading to cell swelling (turgescence) and osmotic lysis (hemolysis).

       • Hypertonic Solution: Massive water efflux leading to cell shrinkage and crenation (plasmolysis).

     • Renal Physiology: Aquaporin-2 (AQP2) in the nephron collecting duct regulates urine concentration under anti-diuretic hormone (ADH / vasopressin) control. Deficiency in vasopressin or AQP2 gene mutations leads to nephrogenic diabetes insipidus (excretion of up to $30\,\text{liters/day}$ of dilute urine).

  3. Uniport Transporters (Protéines Porteuses):

     • Carrier proteins that undergo a conformational change upon binding a single solute.

     • Transport Cycle: Solute binds weakly to a specific binding site $\rightarrow$ Conformational change $\rightarrow$ Solute translocation $\rightarrow$ Dissociation and release on opposite side $\rightarrow$ Return to initial conformation.

     • Glucose Transporters (GLUT Family): 14 identified isoforms divided into 3 structural classes:

       • GLUT1: Ubiquitous (highest density in erythrocytes and brain); high glucose affinity (low $\text{K}_m$).

       • GLUT2: Expressed in hepatocytes, enterocytes, renal proximal tubule cells, and pancreatic $\beta$-cells; low glucose affinity (high $\text{K}_m$).

       • GLUT3: Predominant in neurons/brain.

       • GLUT5: Specific fructose transporter.

  4. Porins:

     • Pore-forming transmembrane proteins allowing passive diffusion of small hydrophilic molecules.

     • Found in the outer membrane of Gram-negative bacteria, mitochondria, and chloroplasts.

• Comparison of Passive Transport Families:

  • Channels (Canaux): Activation event = Opening (ouverture), Transferred solutes per event = $6 \times 10^4$, Transfer rate = $2 \times 10^9\,\text{molecules/sec}$, Example = $\text{K}^+$ channels.

  • Uniport Transporters: Activation event = Conformational change, Transferred solutes per event = $1\text{--}5$, Transfer rate = $10^6\text{--}10^8\,\text{molecules/sec}$, Example = GLUT1.

  • Porins: Activation event = None (aucun), Transfer rate = $200\text{--}50\,000\,\text{molecules/sec}$, Example = Mitochondrial porins.

Transmembrane Transport: Active Transport Mechanisms

• General Features:

  • Moves ions or small molecules against their concentration or electrochemical gradient.

  • Requires metabolic energy expenditure (ATP hydrolysis).

  • Mediated by specific transmembrane ATPases/permeases undergoing deep conformational alterations.

• Primary Active Transport (Pumps):

  1. $\text{Na}^+/\text{K}^+$ ATPase Pump:

     • Stoichiometry: Hydrolyzes $1\,\text{ATP}$ to extrude $3\,\text{Na}^+$ ions out of the cell and import $2\,\text{K}^+$ ions into the cell.

     • Electrogenic Nature: Net loss of one positive charge per cycle, contributing to the negative resting membrane potential.

     • Kinetics: Hydrolyzes up to $100\,\text{ATP molecules/sec}$. Consumes $1/3$ of total cellular ATP energy ($>2/3$ in excitable nerve cells).

     • Physiological Roles: Maintains intracellular $\text{Na}^+$ and $\text{K}^+$ concentration gradients, osmotic balance, and resting potential.

  2. $\text{Ca}^{2+}/\text{H}^+$ ATPase Pump:

     • Maintains cytosolic free $\text{Ca}^{2+}$ at extremely low levels ($0.0001\,\text{mM}$) relative to extracellular levels ($1\text{--}2\,\text{mM}$).

  3. $\text{H}^+/\text{K}^+$ ATPase Pump (Proton Pump):

     • Expressed in colon, renal, and gastric mucosal parietal cells.

     • Generates extreme gastric luminal acidity (pH = $1$).

     • Located in apical membranes of parietal cells. In non-secreting states, stored in cytoplasmic vesicles; upon activation, vesicles fuse with the apical membrane.

     • Electroneutral exchange: Hydrolyzes $1\,\text{ATP}$ to pump $1\,\text{H}^+$ into the stomach lumen in exchange for $1\,\text{K}^+$.

     • Stimulated by meal intake via histamine, gastrin, and acetylcholine binding, which activate protein kinases.

• Secondary Active Transport (Cotransporters & Exchangers):

  • Does not directly hydrolyze ATP; utilizes the potential energy stored in an ion gradient (typically the steep $\text{Na}^+$ gradient created by the primary $\text{Na}^+/\text{K}^+$ ATPase pump).

  • Symport (Cotransport): Both solutes move in the same direction.

    • $\text{Na}^+/\text{Glucose}$ Symport (SGLT): Drives glucose entry against its concentration gradient coupled to downhill $\text{Na}^+$ entry.

    • Other Symports: $\text{Na}^+/\text{phosphate}$ (NaPi IIa/b), $\text{Na}^+/\text{Iodide}$ (NIS), $\text{Na}^+/\text{K}^+/2\text{Cl}^-$ (NKCC), $\text{Na}^+/\text{Cl}^-$ (NCC), $\text{K}^+/\text{Cl}^-$ (KCC).

  • Antiport (Exchange): Solutes move in opposite directions.

    • $\text{Na}^+/\text{Ca}^{2+}$ Exchanger (NCX): Imports $3\,\text{Na}^+$ down their gradient to export $1\,\text{Ca}^{2+}$ against its gradient.

    • $\text{Na}^+/\text{H}^+$ Exchanger (NHE): Imports $1\,\text{Na}^+$ in exchange for exporting $1\,\text{H}^+$ to reduce cytosolic acidity and maintain intracellular pH, often functioning alongside the $\text{Cl}^-/\text{HCO}_3^-$ anion exchanger.

Integrated Physiological Transport Examples

• Intestinal Epithelial Glucose Absorption:

  1. Apical Transport: Intestinal luminal glucose is actively transported into enterocytes via apical $\text{Na}^+/\text{glucose}$ secondary active symporters (SGLT1).

  2. Basolateral Transport: Accumulated intracellular glucose exits across the basolateral membrane into the bloodstream via passive facilitated diffusion through GLUT2.

• Renal Tubular Glucose Reabsorption:

  • Glomerular Filtration: Glucose is freely filtered at the glomerulus into the nephron.

  • Proximal Tubule Segments S1 and S2: Reabsorb $90\%$ of filtered glucose via apical secondary active SGLT2 cotransporters ($1\,\text{Na}^+:1\,\text{glucose}$), followed by basolateral exit via GLUT2.

  • Proximal Tubule Segment S3: Reabsorbs the remaining $10\%$ of glucose via apical SGLT1 cotransporters ($2\,\text{Na}^+:1\,\text{glucose}$), followed by basolateral exit via GLUT1.

  • Physiological Threshold & Glucosuria: Reabsorption is saturable. Normal transport capacity accommodates filtered glucose up to a blood glucose concentration of $11\,\text{mmol/L}$. Exceeding this renal threshold results in glucosuria (glucose excretion in urine).

Vesicular Transport: Endocytosis and Exocytosis

• Endocytosis:

  • Active vesicular transport mechanism internalizing bulk macromolecules, fluid, or particles from the extracellular environment into membrane-bound intracellular vesicles.

  • Requires metabolic energy (ATP) and dynamic plasma membrane structural remodeling.

  • Facilitates membrane component recycling, cell nutrition, immune defense, and cellular homeostasis.

• Exocytosis:

  • Active vesicular mechanism fusing intracellular secretory vesicles with the plasma membrane to discharge contents into the extracellular space.

  • Requires metabolic energy expenditure.

  • Functions:

    • Cellular waste elimination.

    • Secretion of regulatory signaling molecules (neurotransmitters, peptide hormones).

    • Delivery of transmembrane receptors, extracellular matrix components, and cell wall constituents to the cell exterior.