Animal Physiology Unit 2 ReviewCell Physiology and Cellular Signaling

Key Concepts and Terminology

• Cell physiology studies the functions and activities of cells, including their structure, metabolism, and interactions with the environment
• Cellular signaling refers to the communication between cells through chemical or electrical signals, allowing them to coordinate their activities and respond to stimuli
• Homeostasis maintains a stable internal environment within cells and organisms, despite changes in the external environment
• Membrane transport mechanisms facilitate the movement of molecules across cell membranes, including passive diffusion, facilitated diffusion, and active transport
• Cellular energetics involves the processes by which cells obtain and utilize energy, such as cellular respiration and photosynthesis
• Intracellular signaling pathways transmit signals within cells, often through the activation of protein kinases and second messengers (cyclic AMP, calcium ions)
• Intercellular communication allows cells to exchange information and coordinate their activities through various means, such as gap junctions, hormones, and neurotransmitters
• Cellular responses to environmental stimuli enable cells to adapt to changes in their surroundings, such as temperature, pH, and nutrient availability

Cell Structure and Function

• Cells are the basic functional units of living organisms, consisting of specialized structures called organelles that perform specific functions
• The plasma membrane serves as a selective barrier, regulating the movement of molecules in and out of the cell
• The nucleus contains the cell's genetic material (DNA) and controls cellular activities through gene expression
• Mitochondria generate ATP through cellular respiration, providing energy for various cellular processes
• The endoplasmic reticulum (ER) plays a role in protein and lipid synthesis, as well as calcium storage
  • Rough ER is studded with ribosomes 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 within or outside the cell
• Lysosomes contain digestive enzymes that break down cellular waste, foreign particles, and damaged organelles
• Cytoskeleton provides structural support and enables cell movement, consisting of microfilaments, intermediate filaments, and microtubules

Membrane Transport Mechanisms

• Passive diffusion allows molecules to move down their concentration gradient without requiring energy input (oxygen, carbon dioxide)
• Facilitated diffusion uses carrier proteins or channels to transport molecules down their concentration gradient, also without energy input (glucose, amino acids)
• Active transport moves molecules against their concentration gradient, requiring energy input in the form of ATP (sodium-potassium pump, calcium pump)
• Endocytosis involves the ingestion of extracellular material by invagination of the plasma membrane, forming vesicles (phagocytosis, pinocytosis)
• Exocytosis releases cellular products or waste by fusion of intracellular vesicles with the plasma membrane (neurotransmitter release, hormone secretion)
• Osmosis is the movement of water across a semipermeable membrane from a region of high water potential to a region of low water potential
  • Hypotonic solutions have a higher water potential than the cell, causing water to enter the cell and leading to cell swelling or lysis
  • Hypertonic solutions have a lower water potential than the cell, causing water to leave the cell and leading to cell shrinkage
  • Isotonic solutions have the same water potential as the cell, resulting in no net movement of water across the membrane

Cellular Energetics and Metabolism

• Cellular respiration is the process by which cells break down organic molecules to generate ATP, occurring in the mitochondria
  • Glycolysis breaks down glucose into pyruvate in the cytoplasm, producing a small amount of ATP and NADH
  • Citric acid cycle (Krebs cycle) oxidizes acetyl-CoA derived from pyruvate, generating NADH, FADH2, and a small amount of ATP
  • Electron transport chain transfers electrons from NADH and FADH2 to oxygen, creating a proton gradient that drives ATP synthesis through chemiosmosis
• Photosynthesis captures light energy to synthesize organic compounds (glucose) from carbon dioxide and water, occurring in chloroplasts of plant cells
• Metabolism encompasses all the biochemical reactions involved in the synthesis (anabolism) and breakdown (catabolism) of molecules within cells
• Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy, without being consumed in the process
  • Enzyme activity can be regulated by factors such as substrate concentration, temperature, pH, and the presence of inhibitors or activators
• Cellular energy storage molecules include carbohydrates (glycogen), lipids (triglycerides), and proteins, which can be mobilized when energy is needed

Intracellular Signaling Pathways

• G protein-coupled receptor (GPCR) signaling involves the activation of G proteins by ligand-bound receptors, leading to the production of second messengers (cyclic AMP, IP3, DAG)
• Receptor tyrosine kinase (RTK) signaling is initiated by the binding of growth factors to RTKs, resulting in receptor dimerization and autophosphorylation, followed by the activation of downstream signaling cascades (MAPK, PI3K/Akt)
• Second messengers amplify and propagate signals within the cell, leading to changes in enzyme activity, gene expression, or ion channel function
  • Cyclic AMP (cAMP) is produced by adenylyl cyclase and activates protein kinase A (PKA), which phosphorylates various target proteins
  • Calcium ions (Ca2+) are released from the endoplasmic reticulum in response to IP3, activating calcium-dependent enzymes and ion channels
• Phosphorylation cascades involve the sequential activation of protein kinases, which phosphorylate and activate downstream targets, ultimately leading to cellular responses
• Transcription factors are proteins that regulate gene expression by binding to specific DNA sequences, either promoting or repressing transcription
  • Activation of transcription factors can occur through phosphorylation, ligand binding, or translocation to the nucleus
• Feedback loops help maintain homeostasis by regulating the activity of signaling pathways, either through positive feedback (amplification) or negative feedback (attenuation)

Intercellular Communication

• Gap junctions are channels that directly connect the cytoplasm of adjacent cells, allowing the exchange of small molecules and ions (electrical coupling in heart muscle cells)
• Hormones are chemical messengers secreted by endocrine glands into the bloodstream, acting on target cells that express specific receptors (insulin, thyroid hormones)
  • Steroid hormones (estrogen, testosterone) diffuse through the plasma membrane and bind to intracellular receptors, regulating gene expression
  • Peptide hormones (insulin, growth hormone) bind to cell surface receptors, initiating intracellular signaling cascades
• Neurotransmitters are chemical messengers released by neurons at synapses, binding to receptors on the postsynaptic cell and modulating its activity (acetylcholine, dopamine, serotonin)
• Paracrine signaling involves the release of local mediators that affect nearby cells, often within the same tissue (prostaglandins, growth factors)
• Juxtacrine signaling requires direct cell-cell contact, with signaling molecules expressed on the surface of one cell interacting with receptors on the adjacent cell (Notch signaling, T cell activation)
• Extracellular vesicles (exosomes, microvesicles) contain signaling molecules and can be released by cells to communicate with distant targets

Cellular Responses to Environmental Stimuli

• Thermosensors detect changes in temperature and initiate appropriate cellular responses, such as the activation of heat shock proteins to protect against protein denaturation
• Mechanosensors respond to mechanical forces, such as stretch, pressure, or shear stress, and are important in various physiological processes (blood pressure regulation, hearing, touch)
• Chemosensors detect changes in the concentration of specific molecules, such as nutrients, toxins, or signaling molecules, and trigger cellular responses (taste receptors, olfactory receptors)
• Osmosensors monitor changes in extracellular osmolarity and regulate the activity of ion channels and transporters to maintain cell volume and prevent osmotic stress
• Redox sensors detect changes in the cellular redox state, which can be influenced by factors such as oxidative stress or hypoxia, and activate antioxidant defense mechanisms or hypoxia-inducible factors
• Cellular stress responses help cells adapt to and survive adverse conditions, such as oxidative stress, DNA damage, or nutrient deprivation
  • The heat shock response involves the upregulation of heat shock proteins (HSPs) that act as molecular chaperones to prevent protein misfolding and aggregation
  • The unfolded protein response (UPR) is activated in response to ER stress, leading to the upregulation of genes involved in protein folding and degradation
  • Autophagy is a cellular process that degrades damaged organelles and proteins, helping cells to recycle nutrients and maintain homeostasis under stress conditions

Practical Applications and Real-World Examples

• Cancer biology heavily relies on understanding cellular signaling pathways, as many cancers arise from mutations in genes that regulate cell growth and division (p53, Ras, PTEN)
  • Targeted cancer therapies aim to specifically inhibit overactive signaling pathways in cancer cells, while minimizing damage to healthy cells (tyrosine kinase inhibitors, monoclonal antibodies)
• Regenerative medicine and tissue engineering utilize knowledge of cell physiology to develop strategies for repairing or replacing damaged tissues and organs (stem cell therapies, biomaterials)
• Drug discovery and development often involve targeting specific cellular processes or signaling pathways to treat diseases (ion channel modulators, enzyme inhibitors, receptor agonists/antagonists)
• Personalized medicine takes into account individual genetic variations that can affect drug metabolism, transport, and response, allowing for tailored treatment plans
• Optogenetics is a technique that uses light-sensitive proteins to control the activity of specific cells or signaling pathways, enabling precise manipulation of cellular functions (neuroscience research, potential therapeutic applications)
• Synthetic biology applies principles of cellular signaling and gene regulation to design and engineer novel biological systems or organisms with desired functions (biosensors, biofuels, synthetic gene circuits)
• Bioremediation employs microorganisms or engineered cells to detoxify or degrade environmental pollutants, relying on their metabolic capabilities and stress responses
• Agricultural biotechnology utilizes knowledge of plant cell physiology to develop crops with improved traits, such as increased yield, nutrient content, or resistance to pests and environmental stresses (drought-tolerant crops, genetically modified organisms)