Cell Biology and Homeostasis II: Comprehensive Academic Study Notes: Homeostasis II - Homeostasis II: Cellular Homeostasis II

Morphology and Functions of the Plasma Membrane

The plasma membrane functions as a dynamic interface that controls exchanges between the cell and its environment. Beyond its role as a protective envelope, it serves as a site for biochemical activities, molecular transport, specific recognition, and cellular signaling. Under Transmission Electron Microscopy (MET), the membrane appears as a 7.5 nm thick trilaminar structure consisting of two dark outer layers flanking a clear central layer. This organization is described by the fluid mosaic model, where lipids and proteins move laterally within the membrane plane rather than remaining static.

Chemically, the membrane is composed of lipids (40% by mass), proteins (50% by mass), and carbohydrates (10% by mass). While proteins account for half the mass, they are much larger than lipids; the molecular ratio is approximately one protein for every 50 lipids. The lipid component is organized into a bilayer with a hydrophobic core that renders the membrane relatively impermeable to most substances. The protein component includes transmembrane proteins, anchored proteins, and extrinsic peripheral proteins located on either side of the bilayer. Carbohydrates, in the form of oligosaccharides or glycans, extend from the external face to facilitate recognition mechanisms.

Specialized Membrane Domains and Ciliary Structures

Polarized cells, such as epithelial or nerve cells, exhibit specialized membrane domains with distinct morphologies and functions. For example, epithelial cells are defined by a basal pole and an apical pole, often facing an organ lumen. The respiratory mucosa is characterized by ciliated epithelial cells. These cells possess cilia that beat synchronously to evacuate inhaled dust and particles from the upper airways toward the trachea. This synchronous movement is essential for mobilizing the bronchial environment.

Flagella are elongated ciliary structures that provide mobility to isolated cells, such as spermatozoa or unicellular organisms like Chlamydomonas. In contrast, stereocilia are specialized membrane domains found in the Organ of Corti within the cochlea of the inner ear. Each ear contains approximately 3500 sensory cells equipped with stereocilia arranged in increasing sizes forming a V-shape. When sound vibrations move the endolymph and the basilar membrane, the stereocilia bend. They are connected by small spring-like structures that stretch during movement, triggering the opening of ion channels. This converts mechanical vibrations into electrical signals, which are then transmitted via the auditory nerve to the brain to generate conscious sound perception.

Similar sensory principles apply to the vestibular system of the inner ear, which controls balance. Here, hair cells are covered by a gelatinous substance containing calcium carbonate crystals called otoliths. Head movements cause the endolymph to shift, bending the cilia through the inertia of the otolithic mass. This process informs the brain about spatial positioning. If otoliths become detached and block a semi-circular canal, it can result in Benign Paroxysmal Positional Vertigo (BPPV), which is typically treated by physical maneuvers to clear the canal.

Microvilli and Intestinal Absorption

The digestive system utilizes microvilli to maximize surface area for nutrient absorption. The small intestine, which measures 5 to 7 meters in length, contains numerous folds called villi. At the cellular level, enterocytes possess apical membrane projections known as microvilli, creating a total absorptive surface of over $30\,m^2$. Enterocytes are highly polarized cells; their apical membrane faces the intestinal lumen for nutrient import, while the basal membrane exports nutrients toward the bloodstream. This directed transport depends on specific transporters, such as the SGLT sodium-glucose symport at the apical side and GLUT permeases at the basal side.

Intercellular Junctions and Cellular Adhesion

Cellular junctions are critical for maintaining tissue structure and establishing barriers. Tight junctions (occlusive or waterproof junctions) are located at the most apical part of epithelial cells. They involve close interactions between proteins such as Jam, Occludin, and Claudine, creating lines of closure that prevent the free diffusion of membrane components between apical and basolateral domains. This ensures the sealing of the epithelium, forcing nutrients to pass through cells rather than between them.

Desmosomes act as anchoring junctions, functioning like rivets to provide mechanical attachment between cells or their supports. Deficiencies in these anchoring systems are associated with pathologies; for instance, Pemphigus and certain forms of Lupus are autoimmune diseases caused by antibodies targeting these junctions, resulting in skin detachment. In cancer, the loss of contact inhibition and the inability to maintain junctions are characteristics of tumor cells capable of metastasis.

Cellular adhesion can also be transient, as seen in the extravasation (or diapedesis) of white blood cells. While red blood cells travel passively with blood flow, leukocytes interact with endothelial cells via membrane proteins and carbohydrates. In the presence of injury or infection signals, leukocytes adhere more strongly to the endothelium, roll along the surface, and eventually deform to squeeze between endothelial cells to reach the underlying tissue.

Transmembrane Transport and Osmosis

Molecular transport across the plasma membrane is governed by chemical properties and gradients. Simple diffusion is limited to small hydrophobic molecules, such as gases or lipophilic substances. Water, although a small molecule, diffuses slowly on its own and requires specialized channels called aquaporins for rapid transit. The movement of water is dictated by osmosis, moving from low solute concentration to high solute concentration.

In the context of human red blood cells (hematites), the plasma is normally an isotonic medium, often mimicked by physiological serum ($0.9\,g$ of $NaCl$ per $100\,ml$ of water). In an isotonic environment, water entry and exit are balanced. In a hypertonic environment (high salt), water leaves the cell, causing it to shrink or plasmolyze. In a hypotonic environment (low salt), water enters the cell, causing it to swell (turgidity) and potentially burst (hemolysis). This illustrates the necessity of maintaining osmotic balance for cellular homeostasis.

Active and Passive Transport Systems

Passive transport, such as facilitated diffusion via GLUT permeases, moves molecules down their concentration gradient without energy expenditure. For example, glucose concentration is typically higher in the blood than inside most cells, where it is consumed for metabolism. However, living cells must often move substances against their electrochemical gradients using active transport, which requires energy. This energy is derived from ATP hydrolysis, light, or the coupling of two transports.

The Sodium-Potassium Pump ($Na^+/K^+$-ATPase) is a universal active transport system. It hydrolyzes ATP to export $3\,Na^+$ ions and import $2\,K^+$ ions against their respective gradients. Because it moves more positive charges out than in, the pump is electrogenic. This maintainence of high extracellular $Na^+$ ($150\,mM$ vs $15\,mM$ intracellular) and high intracellular $K^+$ is vital for many biological processes.

In enterocytes, the import of glucose from the intestinal lumen occurs against its concentration gradient. To achieve this, the cell uses a sodium-glucose symport (SGLT) which harnesses the energy of the $Na^+$ gradient (generated by the $Na^+/K^+$ pump) to pull glucose into the cell. Once concentrated inside the cell, glucose is then exported to the blood via a GLUT permease through passive facilitated diffusion.

Ion Channels and Cell Signaling

Ion channels are transmembrane proteins that allow the rapid, selective passage of ions ($Na^+$, $K^+$, $Ca^{2+}$, $Cl^-$) based on electrochemical gradients. These channels can be voltage-gated (sensitive to electrical charge distribution) or ligand-gated (sensitive to the binding of signaling molecules). In the nervous system, signal propagation relies on the action potential, which involves the sequential opening of voltage-gated channels along the axon.

The Nicotinic Acetylcholine Receptor is a prime example of a ligand-gated ion channel located at neuromuscular junctions. When acetylcholine is released into the synaptic cleft, it binds to the receptor, causing a sudden influx of $Na^+$ that depolarizes the membrane. This depolarization triggers muscle contraction. Nicotine acts as an agonist for these receptors, mimicking the signal. Over-stimulation by nicotine can lead to an increase in the number of receptors, contributing to tobacco dependence. Antagonists, such as curare, bind to the receptor without opening the channel, causing muscle relaxation or paralysis, which is utilized in certain surgical procedures.

Signal Transduction and the MAP Kinase Pathway

Cellular communication involves the reception of a signal (ligand) by a membrane receptor and the subsequent transduction of that information into a cellular response. Receptors coupled to enzymatic activity, such as Receptor Tyrosine Kinases (RTK), undergo dimerization and phosphorylation upon ligand binding. This activates a cascade of internal signaling proteins.

The MAP Kinase (Mitogen-activated protein) pathway is a critical signaling route for cell proliferation. The process begins with a growth factor binding to an RTK, which activates the small protein RAS. RAS then triggers a cascade: MAP-Kinase-Kinase-Kinase phosphorylates MAP-Kinase-Kinase, which in turn phosphorylates MAP-Kinase. This terminal kinase enters the nucleus to modify gene expression or activate proteins that drive the cell into mitosis. Dysregulation of this pathway is a hallmark of cancer. For instance, the Ras gene is an oncogene; a mutation keeping the RAS protein permanently active leads to uncontrolled cell division. Biological therapies like Herceptin target these pathways to inhibit tumor growth.

G-Protein Coupled Receptors (RCPG)

G-Protein Coupled Receptors (GPCR) represent a large family of receptors characterized by seven transmembrane domains ($7\,TM$). They mediate responses to odors, tastes, hormones, and photons (via opsins). Upon ligand binding, these receptors activate an anchored G-protein, which then interacts with either an ion channel or an enzyme. Acetylcholine also acts on muscarinic receptors, which are GPCRs. Depending on the specific G-protein and target cell, the response varies; for example, muscarinic activation causes saliva secretion in glandular cells but relaxation in cardiac muscle cells. Antagonists of these receptors, like atropine found in Belladonna, can be toxic, potentially causing respiratory or cardiac arrest.

Intracellular Compartmentation and the Nucleus

Eukaryotic cells are distinguished from prokaryotes by their extreme compartmentation. While prokaryotes are small ($\approx 1\,\mu m$), eukaryotes are roughly 10,000 times larger in volume. Compartmentation allows the cell to concentrate enzymes and enzymes for specific reactions in localized spaces, optimizing biochemical efficiency. Each organelle, from the nucleus to the mitochondria, serves a specialized physiological function.

The nucleus is the repository for genetic material, delimited by a nuclear envelope with nuclear pores that regulate nucleocytoplasmic exchange. Inside, DNA is organized into 23 pairs of chromosomes ($46$ total) associated with histone proteins to form chromatin. The nucleus is the site of DNA replication, maintenance, and repair, as well as transcription (converting DNA to RNA). The nucleolus is a specialized region within the nucleus dedicated to producing ribosomal subunits.

The Cytoskeleton and Cytoplasm

The cytoplasm (or cytosol) is a complex, viscous colloidal phase containing water, ions, gases, proteins, and metabolites. It is the site of glycolysis, which produces $2\,ATP$ and $2\,NADH + 2H^+$ molecules per glucose molecule. The structural organization of the cell is provided by the cytoskeleton, which consists of three types of filaments:

1. Microfilaments (Actine): Flexible two-strand helical polymers concentrated under the plasma membrane for structural support.
2. Microtubules (Tubuline): Rigid hollow cylinders that act as "rails" for vesicular transport and are essential for chromosome separation during mitosis and the structure of cilia and flagella.
3. Intermediate Filaments: Rope-like structures (including lamins in the nucleus) that provide mechanical resistance to cells and tissues.

Mitochondria and Bioenergetics

Mitochondria are double-membrane organelles responsible for converting energy from nutrients into ATP via oxidative phosphorylation. The outer membrane is smooth, while the inner membrane forms numerous folds called cristae to increase surface area. The matrix contains circular DNA and enzymes for the Krebs cycle. Carbon molecules from catabolism generate Acetyl-coA, which feeds into the Krebs cycle, producing reduced coenzymes. These are oxidized by the respiratory chain, using $O_2$ as the final electron acceptor, to produce a proton gradient that drives ATP synthesis. Mitochondria are dynamic organelles that constantly undergo fission and fusion to form interconnected networks.

The Endoplasmic Reticulum (RE) and Golgi Apparatus

The Endoplasmic Reticulum is a network of cisternae and tubes extending from the nuclear envelope. It exists in two forms: Smooth ER (REL) and Rough ER (RER or REG). The REL is the primary site for lipid synthesis and serves as a storage reserve for Calcium ($Ca^{2+}$). The RER is studded with ribosomes for protein synthesis. Proteins synthesized here are either destined for the membrane, secretion, or lysosomes. The RE is biogenic, meaning it produces the components for its own membrane and others.

During synthesis, proteins are translocated into the RE and may undergo N-glycosylation, where an oligosaccharide is attached to an asparagine residue. These proteins then travel via centrifugal transport vesicles to the Golgi apparatus. The Golgi is an oriented organelle (Cis face toward the RE, Trans face toward the membrane) that functions as a sorting station. It modifies protein sugars and tags lysosomal enzymes with Mannose 6-Phosphate (M6P) to ensure they are directed to the correct destination.

Vesicular Trafficking and Secretion

Cells maintain a balance between centrifugal flux (from the center to the periphery) and centripetal flux (from the periphery to the center). Secretion occurs through two pathways:

1. Constitutive Secretion: A default, continuous pathway used for membrane renewal.
2. Controlled Secretion: Found in specialized cells, where substances are sequestered in secretory granules and released only upon a signal. Examples include the release of histamine by mastocytes during allergic reactions and the release of insulin by pancreatic beta cells in response to elevated blood glucose and intracellular calcium levels.

Exocytosis can also be used in defense; Natural Killer (NK) and Cytotoxic T Lymphocytes (CTL) engage in a "kiss of death" where they release perforins via exocytosis to puncture the membrane of abnormal or infected target cells, leading to their implosion.

Lysosomes and Endocytosis

Lysosomes are acidic compartments (pH 5) containing roughly 40 different types of acid hydrolases for intracellular digestion. They receive enzymes from the Golgi and materials to be digested through endocytosis, phagocytosis, or autophagy (the digestion of damaged internal organelles like mitochondria). Endocytosis captures extracellular substances via three main processes:

1. Pinocytosis: Taking in small volumes of fluid ($< 250\,nm$).
2. Receptor-mediated Endocytosis: Highly specific capture of molecules like LDL-cholesterol.
3. Phagocytosis: Engulfing large particles ($> 250\,nm$).

In cholesterol capture, LDL particles bind to surface receptors and are internalized in clathrin-coated vesicles. Once in the endosome, the acidic pH causes the LDL to dissociate from its receptor. The receptor is recycled to the surface, while the LDL is sent to the lysosome for hydrolysis into cholesterol.

Lysosomal Pathologies and Transcytosis

Failure in lysosomal function leads to "storage diseases." Tay-Sachs disease is a neurodegenerative lipidosis caused by a lack of hexosaminidase, leading to the toxic accumulation of gangliosides in the brain. Gaucher disease results from a glucocerebrosidase deficiency. Modern treatments for Gaucher involve "enzyme rescue therapy," where functional enzymes tagged with M6P are administered intravenously and taken up by cells through endocytosis.

Mechanical damage to lysosomes also causes illness. Gout is caused by the accumulation of sharp sodium urate crystals in joints. Phagocytic cells attempt to ingest these crystals, but the crystals puncture the lysosomal and cellular membranes, releasing digestive enzymes into the tissue and causing intense inflammation and necrosis. Similarly, Silicosis and Asbestosis are chronic lung inflammations caused by inhaling silica or asbestos particles that cannot be digested by alveolar macrophages, eventually leading to irreversible pulmonary fibrosis.

Transcytosis is a specialized process where substances are transported across an entire epithelial barrier without being degraded. This occurs in the mammary gland to transfer maternal antibodies into milk and in the intestine of newborns to absorb those same antibodies. This provides the infant with passive immune protection while their own immune system matures. For instance, in the mammary gland, antibodies are endocytosed at the basal pole and exocytosed intact at the apical pole.