🐇Honors Biology Unit 4 – Cell Membrane and Transport Mechanisms

Membrane Structure and Composition

• Composed of a phospholipid bilayer with hydrophilic heads facing outward and hydrophobic tails facing inward
• Phospholipids are amphipathic molecules containing a hydrophilic head and two hydrophobic fatty acid tails
• Integral proteins are embedded within the membrane and can span the entire bilayer (transmembrane proteins)
• Peripheral proteins are attached to the surface of the membrane through interactions with integral proteins or lipids
• Cholesterol is a steroid molecule that helps regulate membrane fluidity and permeability
  • Cholesterol inserts between phospholipids and stabilizes the membrane
  • Higher cholesterol content reduces membrane fluidity and permeability
• Glycolipids and glycoproteins are lipids and proteins with attached carbohydrate chains that participate in cell recognition and adhesion
• The fluid mosaic model describes the membrane as a dynamic structure with components that can move laterally within the plane of the membrane

Membrane Functions and Properties

• Acts as a selective barrier controlling the passage of molecules in and out of the cell
• Maintains cellular homeostasis by regulating the concentration of ions and molecules within the cell
• Participates in cell signaling through the action of receptor proteins that bind to specific ligands
• Provides a platform for enzymatic reactions and energy production (ATP synthesis in mitochondria and chloroplasts)
• Facilitates cell-to-cell communication and recognition through surface markers (glycoproteins and glycolipids)
• Allows for cell movement and changes in shape through the action of cytoskeletal elements attached to the membrane
• Exhibits fluidity, allowing components to move laterally within the plane of the membrane
  • Fluidity is influenced by temperature, lipid composition, and the presence of cholesterol
• Demonstrates selective permeability, allowing some molecules to pass through while restricting others based on size, charge, and polarity

Passive Transport Mechanisms

• Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration driven by a concentration gradient
  • Occurs without the input of energy and does not require membrane proteins
  • Rate of diffusion is influenced by the concentration gradient, membrane permeability, and temperature
• Osmosis is the diffusion of water across a selectively permeable membrane from a region of high water potential to a region of low water potential
  • Water moves to equalize solute concentrations on both sides of the membrane
  • Tonicity describes the relative solute concentrations of two solutions separated by a selectively permeable membrane (isotonic, hypotonic, or hypertonic)
• Facilitated diffusion is the passive movement of molecules across the membrane with the help of transport proteins
  • Carrier proteins bind to specific molecules and undergo conformational changes to transport them across the membrane
  • Channel proteins form hydrophilic pores that allow specific ions or small molecules to pass through
• Ion channels are selective for specific ions (sodium, potassium, calcium, chloride) and can be gated by voltage, ligands, or mechanical stimuli
• Aquaporins are channel proteins that facilitate the rapid movement of water molecules across the membrane

Active Transport Mechanisms

• Requires the input of energy (usually ATP) to move molecules against their concentration gradient
• Primary active transport directly uses ATP to power the transport of molecules across the membrane
  • Sodium-potassium pump (Na+/K+ ATPase) maintains the electrochemical gradient by pumping sodium ions out and potassium ions into the cell
  • Calcium pump (Ca2+ ATPase) removes calcium ions from the cytoplasm to maintain low intracellular calcium levels
• Secondary active transport utilizes the electrochemical gradient generated by primary active transport to move molecules against their concentration gradient
  • Symporters transport two molecules in the same direction across the membrane (glucose and sodium in the intestinal epithelium)
  • Antiporters transport two molecules in opposite directions across the membrane (sodium-calcium exchanger in cardiac muscle cells)
• ABC transporters are a family of proteins that use ATP hydrolysis to transport various molecules (lipids, drugs, toxins) across the membrane
• Endocytosis is the process by which cells take in materials from the extracellular environment by invaginating the plasma membrane to form vesicles
  • Phagocytosis involves the engulfment of large particles (bacteria, cell debris) by specialized cells (macrophages, neutrophils)
  • Pinocytosis is the non-specific uptake of fluids and small dissolved molecules by small vesicles

Membrane Potential and Electrical Signaling

• Membrane potential is the difference in electrical charge between the interior and exterior of the cell
  • Resting membrane potential is typically around -70 mV (inside negative relative to outside) in animal cells
  • Maintained by the unequal distribution of ions (primarily sodium and potassium) across the membrane and the selective permeability of ion channels
• Action potentials are rapid, transient changes in membrane potential that propagate along the membrane of excitable cells (neurons and muscle cells)
  • Triggered by depolarization of the membrane to a threshold value
  • Involves the sequential opening and closing of voltage-gated sodium and potassium channels
  • Divided into phases: resting, rising (depolarization), falling (repolarization), and hyperpolarization
• Synaptic transmission is the process by which an action potential in one neuron leads to the release of neurotransmitters that bind to receptors on the postsynaptic cell
  • Electrical synapses allow direct transmission of electrical signals through gap junctions
  • Chemical synapses involve the release of neurotransmitters from synaptic vesicles into the synaptic cleft
• Neurotransmitters are chemical messengers that transmit signals between neurons or from neurons to other cells (muscle cells, gland cells)
  • Bind to specific receptors on the postsynaptic membrane to elicit a response (excitation or inhibition)
  • Examples include acetylcholine, dopamine, serotonin, GABA, and glutamate

Vesicular Transport and Exo/Endocytosis

• Vesicles are small, membrane-bound sacs that transport molecules within the cell or between the cell and its environment
• Exocytosis is the process by which vesicles fuse with the plasma membrane to release their contents into the extracellular space
  • Constitutive exocytosis occurs continuously and is involved in the secretion of extracellular matrix components and membrane proteins
  • Regulated exocytosis is triggered by a specific signal (calcium influx) and is involved in the release of neurotransmitters, hormones, and digestive enzymes
• Endocytosis is the process by which cells take in materials from the extracellular environment by invaginating the plasma membrane to form vesicles
  • Receptor-mediated endocytosis involves the specific uptake of molecules (low-density lipoprotein, transferrin) that bind to receptors on the cell surface
  • Clathrin-mediated endocytosis is a type of receptor-mediated endocytosis that involves the formation of clathrin-coated pits and vesicles
• Vesicle formation and trafficking are regulated by a variety of proteins, including coat proteins (clathrin, COPI, COPII), Rab GTPases, and SNARE proteins
• Lysosomes are membrane-bound organelles that contain digestive enzymes and are involved in the breakdown of molecules taken in by endocytosis or autophagy

Membrane Disorders and Diseases

• Cystic fibrosis is caused by mutations in the CFTR gene, which encodes a chloride channel, leading to the accumulation of thick, sticky mucus in the lungs and digestive system
• Duchenne muscular dystrophy is caused by mutations in the dystrophin gene, which encodes a protein that links the cytoskeleton to the extracellular matrix, leading to progressive muscle weakness and degeneration
• Familial hypercholesterolemia is caused by mutations in the LDL receptor gene, leading to high levels of LDL cholesterol in the blood and an increased risk of cardiovascular disease
• Tay-Sachs disease is caused by mutations in the HEXA gene, which encodes an enzyme involved in the breakdown of GM2 ganglioside, leading to the accumulation of this lipid in the brain and progressive neurodegeneration
• Multidrug resistance in cancer cells is often associated with the overexpression of ABC transporters (P-glycoprotein), which pump chemotherapeutic drugs out of the cell, reducing their effectiveness
• Neurological disorders, such as Alzheimer's disease, Parkinson's disease, and epilepsy, involve disruptions in membrane potential, ion channel function, and neurotransmitter signaling
• Channelopathies are a group of disorders caused by mutations in genes encoding ion channels, leading to altered membrane excitability and function (Long QT syndrome, epilepsy, migraine)

Applications in Biotechnology and Medicine

• Liposomes are artificial vesicles composed of phospholipid bilayers that can be used to deliver drugs, vaccines, or genetic material to specific targets in the body
• Membrane proteins, such as receptors and ion channels, are important drug targets for the treatment of various diseases (G protein-coupled receptors, ion channel modulators)
• Patch-clamp technique allows for the study of ion channel function and the screening of drugs that modulate channel activity
• Membrane-based biosensors use immobilized enzymes, antibodies, or receptors to detect specific molecules (glucose, toxins, pathogens) in biological samples
• Cell-based therapies, such as chimeric antigen receptor (CAR) T-cell therapy, involve the modification of cell surface receptors to target and eliminate cancer cells
• Membrane filtration techniques, such as dialysis and ultrafiltration, are used in the purification of proteins, antibodies, and other biomolecules
• Membrane-based separation processes, such as reverse osmosis and nanofiltration, are used in water purification, desalination, and the concentration of biological products
• Membrane-based fuel cells convert chemical energy into electrical energy through the controlled movement of ions across a membrane, with potential applications in sustainable energy production