M

Chapter 1-6: Cellular Transport, Endocytosis, and Organelles

Active Transport and Membrane Pumps

  • All active transport types move substances against their concentration gradients using energy (ATP).
  • Primary amino acid: ATP hydrolysis powers pumps by releasing a phosphate group, causing a conformational change that moves substrates.
  • Sodium–potassium pump (Na⁺/K⁺-ATPase) is the most common and well-understood pump in the cell membrane.
  • Key mechanism details:
    • ATP is hydrolyzed to ADP and Pi; the released energy drives pump conformational changes.
    • The pump moves Na⁺ out of the cell and K⁺ into the cell against their gradients.
    • Stoichiometry (classic textbook and widely cited): 3\,\mathrm{Na}^+\,\text{out},\quad 2\,\mathrm{K}^+\,\text{in per ATP}
  • Other primary active pumps exist, including:
    • Calcium pumps (Ca²⁺-ATPases)
    • Proton (hydrogen) pumps (H⁺-ATPases)
    • Note: Proton pump inhibitors reduce free H⁺, increasing H⁺ concentration in some compartments.
  • Resting membrane potential maintenance requires both pumps and leakage channels.
  • Leakage channels (always open) allow Na⁺ to diffuse into the cell and K⁺ to diffuse out, which, if unopposed, would dissipate the resting potential.
  • The Na⁺/K⁺-ATPase helps maintain the electrochemical gradient at rest when leakage channels allow ion fluxes.
  • Vesicle formation and endocytosis example:
    • When the cell uptakes material by endocytosis, the plasma membrane indents to form a vesicle that traps extracellular material.
    • The vesicle can fuse with a lysosome to digest contents; the membrane of the vesicle becomes part of the vesicular system and is recycled.

Vesicular Transport and Endocytosis

  • Endocytosis overview: uptake of material via vesicle formation from the plasma membrane; vesicles transport cargo through the cytoplasm and can fuse with lysosomes for degradation or with other organelles for processing.
  • Phagocytosis (cellular eating):
    • Performed primarily by phagocytes (e.g., macrophages, certain immune cells).
    • The ingested material forms a phagosome, which fuses with lysosomes to digest contents.
    • In immune responses, phagosomes digest pathogens and debris; residual material is processed and remnants are expelled or reused.
  • Pinocytosis (cellular drinking), aka panocytosis: a non-specific sampling of extracellular fluid and solutes via vesicle formation; used extensively in the small intestine for absorption.
    • Pinocytosis is nonspecific and involves the formation of small vesicles from the plasma membrane.
  • Receptor-mediated endocytosis: a specific form of endocytosis that relies on receptor binding to trigger endocytosis.
    • Involves clathrin-coated pits (plasma membrane pits with a clathrin coat).
    • Caveolae: smaller pits shaped by caveolin; a distinct coat (not clathrin) that can mediate endocytosis.
    • Toxins exploit receptor-mediated endocytosis (e.g., toxins entering cells via receptor binding to pits).
    • Nutrient uptake examples: folic acid is absorbed via receptor-mediated endocytosis.
    • Terminology nuance: clathrin-coated pits are one type; caveolae are another type with different protein coats.
  • Endocytosis plus transcytosis: moving cargo across the cell via vesicular trafficking.
    • Transcytosis can mean moving a vesicle across the cell to the opposite plasma membrane for secretion or release on the other side.
    • In some contexts, transcytosis is used to describe endocytosis followed by exocytosis across a cell (e.g., endothelial transport across barriers).
  • Exocytosis and secretory pathways:
    • Secretory vesicles contain synthesized products that are released outside the cell.
    • Exocytosis involves fusion of vesicles with the plasma membrane, releasing contents extracellularly; the plasma membrane is preserved and recycled.
    • The secretory pathway involves visualizing vesicle maturation from the rough endoplasmic reticulum (RER) to the Golgi apparatus and then to secretory vesicles.
  • SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) regulate vesicle fusion with target membranes; mentioned as a topic for later discussion.
  • Vesicle trafficking and motor transport within cells:
    • Vesicles are transported along cytoskeletal tracks (microtubules) by motor proteins.
    • A vesicle can be moved by motor proteins that resemble “plow shoes” and carry cargo along microtubules; a vesicle can be very large, roughly ~500 times the mass of the motor protein, illustrating the energetic and mechanical load.
    • Once delivered to the correct membrane, vesicles fuse to deliver their cargo.
  • Upregulation and downregulation of membrane proteins:
    • Upregulation increases the expression of proteins (e.g., receptors, channels) at the plasma membrane.
    • Downregulation reduces their expression.
    • Relevance to pharmacology and addiction: many medications exploit receptor/channel regulation; chronic exposure can lead to downregulation or upregulation altering drug effectiveness.
  • Receptor localization and signaling consequences:
    • Cells possess thousands of cell adhesion molecules in the plasma membrane that mediate contact signaling and cell–cell interactions.
    • Adhesion molecules help anchor cells to extracellular matrices or guide movement, signaling need for cell processes like division or apoptosis.
  • Chemical signaling vs second messenger signaling:
    • Chemical-gated signaling: a ligand binds a receptor that directly gates ion channels, allowing immediate ion flow into the cell.
    • Second messenger signaling: ligand binds a receptor that activates intracellular signaling cascades (second messengers) that trigger responses inside the cell without immediate ion entry.
    • Classic second messengers include G-proteins and cyclic AMP (cAMP).
    • Simple analogy: chemical gating is like a car door opening to allow immediate entry; second messenger signaling is like issuing internal instructions that start events inside the cell.
  • Note on terminology: SNARes (SNARE proteins) and related trafficking machinery are introduced conceptually here and will be discussed later in class.

Cytoplasm, Organelles, and Intracellular Environment

  • Cytoplasm definition: the intracellular fluid matrix that includes cytosol, organelles, and inclusions.
  • Cytosol: gel-like aqueous solution, mostly water but containing solutes (protein, metabolites, salts, sugars, etc.).
  • Inclusions (insoluble materials within the cytosol):
    • Glycogen granules (rapid energy source, e.g., skeletal muscle)
    • Pigments (e.g., melanin)
    • Lipids
    • Crystals (e.g., otoliths in the inner ear for balance sensing)
  • Organelles: membrane-bound structures that perform specialized functions to support cell life.
  • Mitochondrial dynamics:
    • Damaged mitochondria can be degraded or fused with other mitochondria to form a healthy organelle (mitochondrial fusion) as a quality-control mechanism.
    • Mitochondrial dysfunction and ion disturbances can contribute to neuropathology, including brain injury and concussion.
  • Concussion, excitotoxicity, and ionic disruption:
    • Injury can stretch/rest potential, causing dysregulated sodium and calcium flux.
    • Excess intracellular Ca²⁺ (calcium overload) is toxic (excitotoxicity).
    • The Na⁺/K⁺ pump and other ion pumps work overtime to restore homeostasis, which can prolong recovery after brain injury or concussion.
  • Endoplasmic reticulum (ER): two forms and their roles
    • Rough ER (RER): studded with ribosomes; site of protein synthesis, particularly for proteins that will be secreted or inserted into membranes.
    • Proteins synthesized on ribosomes enter the RER lumen, are processed, and are packaged into transport vesicles.
    • The vesicles ferry proteins from the RER to the Golgi apparatus for further processing.
    • A key concept: proteins synthesized in the RER are delivered to target destinations via vesicles and subsequent fusion with target membranes.
  • Rough ER to Golgi to plasma membrane:
    • Vesicles exit the rough ER and fuse with the Golgi apparatus (a post-translational processing and sorting center).
    • The Golgi sorts proteins into three main pathways:
    • Pathway A: Secretory vesicles destined to exocytose contents outside the cell.
    • Pathway B: Proteins inserted into the plasma membrane (e.g., ion channels, transporters).
    • Pathway C: Proteins stored or held in reserve within the Golgi/secretory pathway for later use.
  • Smooth ER and specialized forms:
    • Smooth ER is involved in lipid metabolism and fat storage.
    • In skeletal muscle, the smooth ER is specialized as the sarcoplasmic reticulum (SR), which stores and releases calcium for muscle contraction.
  • Lysosomes and cellular recycling:
    • Lysosomes contain digestive enzymes for breaking down cellular waste, damaged organelles, pathogens, and other materials.
    • Damaged organelles can be degraded by lysosomal pathways as part of cellular maintenance and homeostasis.
    • In bone tissue, lysosomes participate in bone remodeling by degrading bone matrix to release calcium into the blood; this process is regulated and occurs continuously.
    • Age- and hormonal-status factors (e.g., menopause) can influence bone remodeling and increase osteoporosis risk due to decreased estrogen protection and increased osteoclast activity.
  • Golgi apparatus and organelle maintenance:
    • After processing, proteins are directed to secretory vesicles, plasma membrane insertion sites, or storage compartments.
    • Lysosomes can receive damaged organelles for breakdown; this maintains cellular homeostasis.

Plasma Membrane Specializations and Surface Structures

  • Cell adhesion molecules (CAMs):
    • Thousands of CAMs in the plasma membrane mediate cell–cell and cell–matrix interactions.
    • Functions include anchoring cells to extracellular matrix, enabling immune cell sampling, and influencing signaling cascades.
  • Cilia, Flagella, and Microvilli: structural adaptations for function
    • Cilia: short, hair-like projections that move substances across the cell surface (e.g., mucus transport in the trachea).
    • Flagella: longer projections that move the entire cell (e.g., sperm mobility).
    • Microvilli: small, finger-like projections that increase surface area for absorption (especially in the small intestine).
  • Tracheal cilia and mucus clearance:
    • Cilia move mucus toward the esophagus and stomach for digestion; mucus traps debris from inspired air.
    • Smoking and pollutants can damage cilia, reducing clearance and increasing respiratory infections.
  • Absorptive surface area enhancement by microvilli:
    • Microvilli dramatically increase the surface area of absorptive cells, enhancing nutrient uptake in the small intestine.

Nervous System Context and Clinical Relevance

  • Neuronal protein trafficking and membrane insertion:
    • Neurons synthesize proteins (e.g., Na⁺/K⁺ pumps) in the nucleus and rough ER, package them into vesicles, and transport them down axons to the plasma membrane for insertion, increasing or decreasing membrane protein numbers as needed (upregulation/downregulation).
    • Vesicle trafficking within neurons relies on motor proteins and cytoskeletal tracks; this trafficking is essential for maintaining neuronal signaling and excitability.
  • Resting potential disruption and recovery time:
    • After brain injury, disturbances in resting membrane potential can cause sustained misregulation of ion flux and neurotransmitter release, contributing to prolonged symptoms.
  • Chemical signaling vs direct ion flow:
    • A chemical ligand can activate a receptor that opens ion channels directly (chemical-gated entry).
    • Alternatively, receptor activation can trigger intracellular signaling cascades (second messengers) that alter cell function without immediate ion flux, such as G-protein signaling leading to cAMP production.
  • Second messenger systems and receptors:
    • G proteins and cyclic AMP (cAMP) are key second messengers in many signaling pathways.
    • Distinguish: direct gating of channels by ligands versus intracellular signaling cascades initiated by receptor activation.

Practical Concepts and Real-World Relevance

  • Addiction, medications, and receptor regulation:
    • Receptor upregulation/downregulation affects drug efficacy and tolerance.
    • Understanding these processes helps explain why certain medications lose effectiveness over time or require dosage adjustments.
  • Phagocytosis in immunity:
    • Phagocytosis by macrophages and other phagocytes is central to innate immunity and pathogen clearance.
    • Phagosomes merge with lysosomes to digest pathogens; degradation products are recycled or presented to other immune cells.
  • Pinocytosis in nutrient absorption:
    • Pinocytosis contributes to nonspecific uptake of extracellular fluids and nutrients, particularly in the gut.
  • Receptor-mediated endocytosis in toxin uptake and nutrient transport:
    • Toxins exploit receptor-mediated endocytosis to enter cells; understanding this process informs how toxins gain intracellular access and potential interventions.

Key Takeaways and Interconnections

  • Active transport, including the Na⁺/K⁺-ATPase, is essential for maintaining the electrochemical gradients that underpin resting membrane potential and cellular excitability.

  • Leakage channels provide a background ion flux that must be balanced by the Na⁺/K⁺-ATPase to maintain steady-state conditions.

  • Vesicular transport (endocytosis, exocytosis, transcytosis) enables cells to uptake, process, secrete, and transport materials while preserving membrane integrity.

  • Endocytosis variants (phagocytosis, pinocytosis, receptor-mediated) differ in specificity and mechanism; clathrin-coated pits and caveolae are two major endocytic pathways with distinct roles.

  • The Golgi apparatus and ER coordinate protein synthesis, processing, and trafficking, including three main destinations: secretion, membrane insertion, and storage.

  • Lysosomes and mitochondria contribute to cellular maintenance and energy balance; their dysfunction or stress (e.g., excitotoxicity from Ca²⁺ overload) can underlie injury and disease.

  • Cell surface structures (CAMs, microvilli, cilia, flagella) and signaling pathways (G proteins, cAMP) integrate external cues with cellular responses.

  • In the nervous system, vesicular trafficking and membrane protein regulation directly influence neuronal signaling, plasticity, and responses to injury; changes in these processes have broad clinical implications.

  • Biochemical placeholders (e.g., the Na⁺/K⁺-ATPase stoichiometry, adenylyl cyclase producing cAMP, and microtubule-based vesicle transport) are central to understanding how cells maintain homeostasis and respond to stress.

  • Equations highlighted:

    • Na⁺/K⁺-ATPase stoichiometry: 3\,\mathrm{Na}^+\,\text{out},\quad 2\,\mathrm{K}^+\,\text{in per ATP}
    • cAMP signaling: \text{ATP} \xrightarrow{\text{adenylyl cyclase}} \text{cAMP} + \text{PPi}

  • Important terms to review for exams:

    • Primary active transport, Na⁺/K⁺-ATPase, leakage channels, electrochemical gradient, resting membrane potential
    • Endocytosis: phagocytosis, pinocytosis, receptor-mediated endocytosis; clathrin-coated pits; caveolae
    • Exocytosis, SNAREs, vesicular trafficking, transcytosis
    • Upregulation vs downregulation; receptor dynamics; addiction pharmacology implications
    • Rough ER, ribosomes, Golgi apparatus, secretory pathways (A, B, C)
    • Smooth ER and sarcoplasmic reticulum; lipid metabolism; calcium storage in muscle
    • Lysosomes, mitochondria, mitophagy; calcium toxicity and excitotoxicity
    • CAMs, cilia, microvilli, flagella; mucus clearance; nutrient absorption
    • G proteins, cyclic AMP, second messengers; direct gating vs second messenger cascades