🤾🏻‍♂️Human Physiology Engineering Unit 2 – Cell Physiology

Key Concepts and Terminology

• Cell theory states that all living organisms are composed of one or more cells, the cell is the basic unit of life, and all cells arise from pre-existing cells
• Eukaryotic cells contain membrane-bound organelles (nucleus, mitochondria, endoplasmic reticulum) while prokaryotic cells lack membrane-bound organelles
• Homeostasis maintains a stable internal environment within cells and organisms through various regulatory mechanisms
• Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration driven by a concentration gradient
• Osmosis refers to the movement of water across a semipermeable membrane from a region of high water potential to a region of low water potential
• Active transport requires energy input (ATP) to move molecules against their concentration gradient
• Metabolism encompasses all chemical reactions involved in maintaining the living state of cells and organisms
• Cell signaling involves the transmission of signals between cells through various mechanisms (hormones, neurotransmitters, cell surface receptors)

Cell Structure and Organization

• Plasma membrane is a selectively permeable barrier that separates the interior of the cell from the external environment
  • Composed of a phospholipid bilayer with embedded proteins that regulate transport and cell communication
• Cytoskeleton provides structural support and enables cell movement
  • Consists of microfilaments (actin), intermediate filaments, and microtubules
• Nucleus contains the cell's genetic material (DNA) and controls cellular activities
  • Nuclear envelope is a double membrane that separates the nucleus from the cytoplasm
  • Nucleolus is the site of ribosomal RNA synthesis and ribosome assembly
• Endoplasmic reticulum (ER) is a network of membrane-bound channels involved in protein and lipid synthesis
  • Rough ER has ribosomes attached and is involved in protein synthesis and modification
  • Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification
• Golgi apparatus modifies, packages, and sorts proteins and lipids for transport to their final destinations
• Mitochondria are the powerhouses of the cell, generating ATP through cellular respiration
• Lysosomes contain digestive enzymes that break down cellular waste, damaged organelles, and foreign particles

Membrane Transport Mechanisms

• Passive transport does not require energy input and includes diffusion, facilitated diffusion, and osmosis
  • Diffusion occurs when molecules move down their concentration gradient without the assistance of membrane proteins
  • Facilitated diffusion involves the movement of molecules across the membrane through specific carrier proteins or channels
• Active transport requires energy input (ATP) to move molecules against their concentration gradient
  • Primary active transport directly uses ATP to power the movement of molecules (sodium-potassium pump)
  • Secondary active transport utilizes the electrochemical gradient created by primary active transport to move other molecules (glucose transport in intestinal cells)
• Endocytosis is the process by which cells take in materials from the external environment by engulfing them with the plasma membrane
  • Phagocytosis involves the ingestion of large particles (bacteria) by specialized cells (macrophages)
  • Pinocytosis is the uptake of fluids and dissolved solutes through small vesicles
• Exocytosis is the process by which cells release materials (hormones, neurotransmitters) to the external environment by fusing vesicles with the plasma membrane

Cellular Energy Production

• Cellular respiration is the process by which cells break down organic molecules (glucose) to generate ATP
  • Glycolysis occurs in the cytoplasm and breaks down glucose into pyruvate, producing a small amount of ATP
  • Citric acid cycle (Krebs cycle) takes place in the mitochondrial matrix and oxidizes acetyl-CoA to generate high-energy electrons (NADH, FADH2)
  • Electron transport chain is located in the inner mitochondrial membrane and uses the high-energy electrons to create a proton gradient, driving ATP synthesis through chemiosmosis
• Fermentation is an anaerobic process that allows cells to generate ATP in the absence of oxygen
  • Lactic acid fermentation occurs in muscle cells during intense exercise, converting pyruvate to lactate
  • Alcoholic fermentation is used by yeast to produce ethanol and carbon dioxide, converting pyruvate to ethanol
• Photosynthesis is the process by which plants and other autotrophs convert light energy into chemical energy (glucose)
  • Light-dependent reactions occur in the thylakoid membranes of chloroplasts and use light energy to generate ATP and NADPH
  • Calvin cycle (light-independent reactions) takes place in the stroma of chloroplasts and uses ATP and NADPH to fix carbon dioxide into organic compounds (glucose)

Cell Signaling and Communication

• Cell signaling allows cells to respond to external stimuli and communicate with each other
  • Ligands are signaling molecules (hormones, neurotransmitters, growth factors) that bind to specific receptors on the target cell
  • Receptors are proteins that recognize and bind to specific ligands, initiating a cellular response
• Signal transduction is the process by which a signal is transmitted from the receptor to the cell's interior, triggering a cellular response
  • G protein-coupled receptors (GPCRs) are a large family of receptors that activate intracellular signaling cascades through G proteins (adrenaline, dopamine)
  • Receptor tyrosine kinases (RTKs) are a class of receptors that possess intrinsic enzymatic activity and phosphorylate intracellular proteins (insulin, growth factors)
• Second messengers are small molecules that relay signals from receptors to target molecules within the cell
  • Cyclic AMP (cAMP) is a common second messenger that activates protein kinase A (PKA), which phosphorylates various target proteins
  • Calcium (Ca2+) is another important second messenger that regulates numerous cellular processes (muscle contraction, neurotransmitter release)
• Gap junctions are specialized intercellular connections that allow direct communication between adjacent cells
  • Composed of connexin proteins that form channels, allowing the passage of small molecules and ions

Cellular Homeostasis

• Homeostasis is the maintenance of a stable internal environment within cells and organisms
  • Negative feedback loops are regulatory mechanisms that counteract changes in a system to maintain stability (thermoregulation, blood glucose regulation)
  • Positive feedback loops amplify changes in a system, leading to a new steady state (blood clotting, childbirth)
• Ion homeostasis is the regulation of ion concentrations (sodium, potassium, calcium) within cells and extracellular fluids
  • Sodium-potassium pump maintains the electrochemical gradient across the plasma membrane, essential for nerve and muscle function
  • Calcium pumps and exchangers regulate intracellular calcium levels, crucial for cell signaling and muscle contraction
• pH homeostasis is the maintenance of a stable pH within cells and bodily fluids
  • Buffers are substances that resist changes in pH when acids or bases are added (bicarbonate buffer system in blood)
  • Respiratory system helps regulate blood pH by adjusting the rate of carbon dioxide removal
• Osmotic homeostasis is the regulation of water and solute concentrations within cells and bodily fluids
  • Aquaporins are water channels that facilitate the movement of water across cell membranes in response to osmotic gradients
  • Kidneys play a crucial role in maintaining osmotic balance by regulating the excretion of water and solutes (sodium, potassium)

Specialized Cell Functions

• Neurons are specialized cells that transmit electrical and chemical signals in the nervous system
  • Axons are long, thin extensions that conduct electrical impulses away from the cell body
  • Dendrites are shorter, branched extensions that receive signals from other neurons
  • Synapses are specialized junctions between neurons where neurotransmitters are released to transmit signals
• Muscle cells are specialized for contraction and movement
  • Skeletal muscle cells are multinucleated and contain sarcomeres, the basic contractile units
  • Cardiac muscle cells are branched and interconnected, allowing for coordinated contraction of the heart
  • Smooth muscle cells are spindle-shaped and found in the walls of hollow organs (blood vessels, intestines)
• Epithelial cells form protective barriers and regulate transport across body surfaces
  • Simple epithelium consists of a single layer of cells (squamous, cuboidal, columnar)
  • Stratified epithelium consists of multiple layers of cells (keratinized in skin, non-keratinized in esophagus)
  • Glandular epithelium secretes substances (hormones, enzymes) into ducts or directly into the bloodstream
• Stem cells are unspecialized cells that have the ability to differentiate into various cell types
  • Embryonic stem cells are pluripotent and can give rise to all cell types in the body
  • Adult stem cells are multipotent and can differentiate into a limited number of cell types (hematopoietic stem cells, mesenchymal stem cells)

Engineering Applications in Cell Physiology

• Tissue engineering involves the development of biological substitutes to restore, maintain, or improve tissue function
  • Scaffolds are three-dimensional structures that support cell attachment, proliferation, and differentiation
  • Bioreactors are devices that provide controlled conditions (temperature, pH, oxygen) for cell culture and tissue growth
• Drug delivery systems are designed to target specific cells or tissues and release therapeutic agents in a controlled manner
  • Nanoparticles can be engineered to encapsulate drugs and deliver them to specific sites in the body (tumor cells)
  • Liposomes are spherical vesicles composed of lipid bilayers that can carry both hydrophilic and hydrophobic drugs
• Biosensors are analytical devices that combine a biological component (enzymes, antibodies) with a physicochemical detector to measure specific analytes
  • Glucose biosensors use glucose oxidase to detect blood glucose levels in diabetic patients
  • ELISA (enzyme-linked immunosorbent assay) is a widely used biosensor technique that detects specific antigens or antibodies
• Microfluidic devices are miniaturized systems that manipulate and analyze small volumes of fluids
  • Organ-on-a-chip platforms simulate the functions of human organs by culturing cells in microfluidic chambers
  • Point-of-care diagnostic devices use microfluidics to perform rapid, on-site testing for various diseases (HIV, malaria)
• Synthetic biology aims to design and construct novel biological systems or redesign existing ones for specific purposes
  • Genetic circuits can be engineered to perform logical operations and regulate gene expression (toggle switches, oscillators)
  • Metabolic engineering involves modifying cellular metabolic pathways to produce desired compounds (biofuels, pharmaceuticals)