💀Anatomy and Physiology I Unit 3 – The Cellular Level of Organization

Cell Structure and Function

• Cells are the basic structural and functional units of living organisms
• Prokaryotic cells lack a membrane-bound nucleus and organelles while eukaryotic cells contain a nucleus and membrane-bound organelles
• The nucleus houses the cell's genetic material (DNA) and directs cellular activities
• Ribosomes synthesize proteins using mRNA as a template
• The endoplasmic reticulum (ER) modifies, packages, and transports proteins and lipids
  • Rough ER studded with ribosomes for protein synthesis
  • Smooth ER lacks ribosomes and involved in lipid synthesis and detoxification
• Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or transport to other parts of the cell
• Mitochondria generate ATP through cellular respiration (powerhouses of the cell)
• Lysosomes contain digestive enzymes that break down cellular waste, damaged organelles, and foreign particles

Cellular Membrane and Transport

• The plasma membrane is a selectively permeable phospholipid bilayer that separates the cell's interior from the external environment
• Phospholipids have hydrophilic heads and hydrophobic tails that spontaneously form a bilayer in aqueous environments
• Membrane proteins embedded in the phospholipid bilayer perform various functions (receptors, channels, transporters, enzymes)
• Simple diffusion is the passive movement of molecules from high to low concentration without requiring energy
• Facilitated diffusion uses carrier proteins to transport molecules down their concentration gradient without energy input
• Active transport moves molecules against their concentration gradient using energy (ATP) and specific transport proteins
  • Primary active transport directly uses ATP (sodium-potassium pump)
  • Secondary active transport relies on electrochemical gradients established by primary active transport (sodium-glucose cotransporter)
• Endocytosis involves the cell engulfing extracellular materials by invaginating the plasma membrane to form vesicles (phagocytosis, pinocytosis, receptor-mediated endocytosis)
• Exocytosis releases cellular materials by fusing vesicles with the plasma membrane and expelling the contents into the extracellular space

Cell Communication

• Cells communicate with each other and respond to their environment through various signaling mechanisms
• Ligands (hormones, neurotransmitters, growth factors) bind to specific receptors on the target cell's surface or interior
• Receptor-ligand binding initiates intracellular signaling cascades that amplify the signal and lead to cellular responses
• G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and activate intracellular signaling pathways via G proteins
• Receptor tyrosine kinases (RTKs) dimerize upon ligand binding and initiate signaling cascades through phosphorylation
• Second messengers (cyclic AMP, calcium, inositol triphosphate) relay and amplify signals within the cell
• Gap junctions allow direct communication between adjacent cells by forming channels that permit the exchange of small molecules and ions
• Cellular responses to signaling include changes in gene expression, protein activity, metabolism, and cell fate (growth, differentiation, apoptosis)

Cell Metabolism and Energy

• Metabolism encompasses all the chemical reactions that occur within a cell to maintain life
• Anabolism involves the synthesis of complex molecules from simpler ones and requires energy input
• Catabolism breaks down complex molecules into simpler ones and releases energy
• Adenosine triphosphate (ATP) is the primary energy currency of the cell
  • ATP consists of adenosine and three phosphate groups
  • Hydrolysis of ATP to ADP or AMP releases energy for cellular processes
• Cellular respiration is the process of breaking down glucose to generate ATP
  • Glycolysis occurs in the cytoplasm and converts glucose into pyruvate, producing a net gain of 2 ATP
  • Pyruvate enters the mitochondria and is oxidized to acetyl-CoA, which enters the Krebs cycle
  • The Krebs cycle generates high-energy electrons (NADH and FADH2) and produces 2 ATP
  • Oxidative phosphorylation involves the electron transport chain and chemiosmosis to generate a large amount of ATP (up to 34 ATP per glucose molecule)
• Photosynthesis in plant cells captures light energy to synthesize glucose from carbon dioxide and water, releasing oxygen as a byproduct

Cell Division and Reproduction

• Cell division is the process by which cells reproduce to generate new cells
• The cell cycle consists of interphase (G1, S, and G2 phases) and mitosis (M phase)
  • G1 phase: cell growth and preparation for DNA replication
  • S phase: DNA replication occurs, doubling the genetic material
  • G2 phase: cell growth and preparation for mitosis
  • M phase: mitosis (nuclear division) and cytokinesis (cytoplasmic division)
• Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase
  • Prophase: chromatin condenses into chromosomes, nuclear envelope breaks down, and spindle fibers form
  • Metaphase: chromosomes align at the cell's equator
  • Anaphase: sister chromatids separate and move towards opposite poles
  • Telophase: nuclear envelopes reform around the separated chromosomes and cytokinesis begins
• Cytokinesis differs between animal and plant cells
  • Animal cells: cleavage furrow forms and pinches the cell into two
  • Plant cells: cell plate forms and develops into a new cell wall separating the daughter cells
• Meiosis is a specialized form of cell division that produces haploid gametes (eggs and sperm) for sexual reproduction
  • Meiosis I: homologous chromosomes separate, resulting in two haploid daughter cells
  • Meiosis II: sister chromatids separate, resulting in four haploid gametes
• Meiosis introduces genetic variation through independent assortment and crossing over

Cellular Specialization

• Cellular specialization, also known as cell differentiation, is the process by which cells become structurally and functionally distinct to perform specific roles within an organism
• Stem cells are unspecialized cells that can 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 within a specific lineage
• Gene expression patterns determine a cell's fate and specialization
  • Transcription factors regulate gene expression by binding to specific DNA sequences and promoting or repressing transcription
  • Epigenetic modifications (DNA methylation, histone modifications) alter gene expression without changing the DNA sequence
• Specialized cells have unique structures and functions that enable them to perform their roles effectively
  • Neurons have long axons and dendrites for transmitting electrical signals
  • Muscle cells contain contractile proteins (actin and myosin) for generating force
  • Secretory cells have an extensive rough ER and Golgi apparatus for producing and secreting proteins
• Cell-cell interactions and signaling pathways guide cellular specialization during development
• Cellular specialization is essential for the formation of tissues, organs, and organ systems in multicellular organisms

Cellular Homeostasis

• Homeostasis is the maintenance of a stable internal environment despite changes in the external environment
• Cells maintain homeostasis through various regulatory mechanisms and feedback loops
• The plasma membrane plays a crucial role in maintaining cellular homeostasis by controlling the movement of substances in and out of the cell
• Ion pumps and channels regulate the concentration of ions (sodium, potassium, calcium) within the cell
• pH homeostasis is maintained by buffers and the regulation of hydrogen ion concentration
• Osmotic balance is achieved by controlling the movement of water across the plasma membrane through aquaporins and the regulation of solute concentrations
• Thermoregulation in cells involves the expression of heat shock proteins (HSPs) that protect against protein denaturation during heat stress
• Cellular waste and toxins are removed by various mechanisms
  • Lysosomes break down cellular debris and damaged organelles
  • Peroxisomes detoxify harmful substances and break down fatty acids
  • Cells export waste products through exocytosis or specialized transport proteins
• Apoptosis, or programmed cell death, is a regulated process that maintains cellular homeostasis by removing damaged, infected, or unnecessary cells
• Disruption of cellular homeostasis can lead to various pathological conditions, such as metabolic disorders, cancer, and neurodegenerative diseases

Key Cellular Processes in Human Physiology

• Cellular respiration in mitochondria generates ATP to power various physiological processes
  • Muscle contraction requires ATP hydrolysis to drive the sliding of actin and myosin filaments
  • Active transport in neurons maintains the resting membrane potential and enables the propagation of action potentials
  • Synthesis of complex molecules (proteins, lipids, carbohydrates) demands ATP for anabolic reactions
• Cellular signaling is crucial for the coordination and regulation of physiological functions
  • Hormones (insulin, glucagon) regulate blood glucose levels by signaling to liver, muscle, and fat cells
  • Neurotransmitters (acetylcholine, dopamine, serotonin) mediate communication between neurons and target cells
  • Growth factors (epidermal growth factor, platelet-derived growth factor) stimulate cell proliferation and differentiation during development and tissue repair
• Secretion of proteins, hormones, and other signaling molecules by specialized cells is essential for various physiological processes
  • Pancreatic beta cells secrete insulin to regulate blood glucose levels
  • Thyroid follicular cells secrete thyroid hormones (T3 and T4) to regulate metabolism
  • Goblet cells in the respiratory and digestive tracts secrete mucus for lubrication and protection
• Cellular transport mechanisms are critical for the absorption, distribution, and elimination of nutrients, gases, and waste products
  • Intestinal epithelial cells absorb nutrients through various transport proteins (glucose transporters, amino acid transporters)
  • Alveolar cells in the lungs facilitate gas exchange between the blood and the atmosphere
  • Kidney tubule cells reabsorb essential nutrients and ions while secreting waste products into the urine
• Cell division and differentiation are fundamental for growth, development, and tissue repair
  • Hematopoietic stem cells in the bone marrow give rise to all blood cell types (erythrocytes, leukocytes, platelets)
  • Skin stem cells in the basal layer of the epidermis continuously divide to replace dead skin cells
  • Satellite cells in skeletal muscle proliferate and differentiate to repair damaged muscle fibers