Honors Anatomy and Physiology Unit 3 ReviewCellular Structure and Function

Key Concepts and Terminology

• Cells are the fundamental units of life and the building blocks of all living organisms
• Eukaryotic cells contain membrane-bound organelles, while prokaryotic cells lack membrane-enclosed structures
• Cellular respiration is the process by which cells break down glucose to produce ATP (adenosine triphosphate), the primary energy currency of the cell
• Mitochondria are the powerhouses of the cell, responsible for generating most of the cell's supply of ATP through aerobic respiration
• Ribosomes are the sites of protein synthesis, translating mRNA (messenger RNA) into polypeptide chains
• The cytoskeleton provides structural support and enables cell movement, consisting of microfilaments, intermediate filaments, and microtubules
• Endocytosis is the process by which cells take in substances from the extracellular environment, while exocytosis involves the secretion of substances out of the cell
• Apoptosis refers to programmed cell death, a highly regulated process that helps maintain tissue homeostasis and eliminates damaged or infected cells

Cell Structure Overview

• Cells are typically microscopic in size, with diameters ranging from 1 to 100 micrometers (μm)
• The plasma membrane is a selectively permeable barrier that separates the cell's interior from the external environment, controlling the movement of substances in and out of the cell
• The nucleus contains the cell's genetic material (DNA) and serves as the control center for cellular activities
• The cytoplasm is the gel-like substance within the cell, containing organelles, proteins, and other cellular components
• The endoplasmic reticulum (ER) is a network of membranous channels involved in protein and lipid synthesis, as well as calcium storage
  • The rough ER is studded with ribosomes and plays a role in protein synthesis and modification
  • The smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage
• The Golgi apparatus is responsible for modifying, packaging, and sorting proteins and lipids for transport to their final destinations within or outside the cell
• Lysosomes are membrane-bound organelles containing digestive enzymes that break down cellular waste, foreign particles, and damaged organelles

Organelles and Their Functions

• Mitochondria are the sites of cellular respiration, generating ATP through the oxidation of glucose and other organic molecules
  • The inner mitochondrial membrane is highly folded, forming cristae that increase the surface area for energy production
  • The mitochondrial matrix contains enzymes involved in the citric acid cycle (Krebs cycle) and oxidative phosphorylation
• Ribosomes are composed of rRNA (ribosomal RNA) and proteins, and they catalyze the synthesis of polypeptide chains based on the genetic code carried by mRNA
• Peroxisomes are organelles that contain enzymes for detoxification and lipid metabolism, breaking down fatty acids and neutralizing harmful substances like hydrogen peroxide
• The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and helps organize cellular components
  • Microfilaments are composed of actin and are involved in muscle contraction, cell division, and cell motility
  • Intermediate filaments provide mechanical strength and help maintain cell shape (keratin, desmin, vimentin)
  • Microtubules are hollow tubes made of tubulin proteins, involved in cell division, intracellular transport, and the formation of cilia and flagella
• Centrioles are paired organelles located near the nucleus, composed of microtubules arranged in a 9+0 pattern, and play a role in cell division and the formation of cilia and flagella

Cell Membrane and Transport

• The plasma membrane is a fluid mosaic structure composed of a phospholipid bilayer with embedded proteins and carbohydrates
• Phospholipids have hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails, spontaneously forming a bilayer in aqueous environments
• Membrane proteins serve various functions, such as transport, cell signaling, and enzymatic activity
  • Integral proteins are embedded within the phospholipid bilayer, while peripheral proteins are loosely attached to the membrane surface
  • Transmembrane proteins span the entire width of the membrane and often function as channels or receptors
• Passive transport does not require energy input and includes simple diffusion (movement down a concentration gradient) and facilitated diffusion (movement through specific protein channels)
• Active transport requires energy input (usually ATP) to move substances against their concentration gradients, examples include the sodium-potassium pump and the calcium pump
• Endocytosis involves the invagination of the plasma membrane to take in substances, and can be divided into phagocytosis (cell eating) and pinocytosis (cell drinking)
• Exocytosis is the process by which cells release substances (e.g., hormones, neurotransmitters) by fusing secretory vesicles with the plasma membrane

Cell Communication and Signaling

• Cells communicate with each other and respond to their environment through various signaling pathways
• Ligands are signaling molecules that bind to specific receptors on the cell surface or within the cell
  • Examples of ligands include hormones, growth factors, neurotransmitters, and cytokines
  • Receptors can be membrane-bound (e.g., G protein-coupled receptors, receptor tyrosine kinases) or intracellular (e.g., nuclear receptors, cytoplasmic receptors)
• Signal transduction is the process by which a cell converts an extracellular signal into an intracellular response
  • The binding of a ligand to its receptor initiates a cascade of molecular events that ultimately lead to changes in gene expression, protein activity, or cellular behavior
  • Common signaling pathways include the cAMP (cyclic AMP) pathway, the IP3 (inositol triphosphate) pathway, and the MAPK (mitogen-activated protein kinase) pathway
• Gap junctions are specialized intercellular connections that allow direct communication between adjacent cells, enabling the exchange of small molecules and ions
• Paracrine signaling involves the release of signaling molecules that act on nearby cells, while endocrine signaling involves the release of hormones that travel through the bloodstream to target distant cells
• Autocrine signaling occurs when a cell secretes a signaling molecule that binds to receptors on its own surface, regulating its own behavior

Energy Production in Cells

• Cells require a constant supply of energy in the form of ATP to carry out various functions, such as synthesis, transport, and movement
• Cellular respiration is the process by which cells break down organic molecules (primarily glucose) to produce ATP
  • Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm and converting glucose into pyruvate, producing a small amount of ATP
  • The citric acid cycle (Krebs cycle) takes place in the mitochondrial matrix, oxidizing acetyl-CoA derived from pyruvate and generating high-energy electron carriers (NADH and FADH2)
  • Oxidative phosphorylation occurs in the inner mitochondrial membrane, where the electron transport chain uses the energy from NADH and FADH2 to pump protons (H+) into the intermembrane space, creating an electrochemical gradient
    • ATP synthase harnesses the energy of the proton gradient to generate ATP through chemiosmosis
• Fermentation is an anaerobic process that allows cells to produce ATP in the absence of oxygen, examples include lactic acid fermentation in muscle cells and alcohol fermentation in yeast
• Photosynthesis is the process by which plants and other autotrophic organisms convert light energy into chemical energy, producing glucose and oxygen
  • The light-dependent reactions occur in the thylakoid membranes of chloroplasts, where light energy is used to split water and generate ATP and NADPH
  • The light-independent reactions (Calvin cycle) take place in the stroma of chloroplasts, using ATP and NADPH to fix carbon dioxide into organic compounds (glucose)

Cell Division and Reproduction

• Cell division is the process by which a parent cell divides into two or more daughter cells, ensuring growth, repair, and reproduction in multicellular organisms
• The cell cycle is a series of events that lead to cell division, consisting of interphase (G1, S, and G2 phases) and mitosis (M phase)
  • During interphase, the cell grows, replicates its DNA, and prepares for division
  • Mitosis is the division of the nucleus, resulting in two genetically identical daughter nuclei
    • Prophase: chromosomes condense, 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 re-form around the daughter nuclei, and chromosomes decondense
  • Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells
• Meiosis is a specialized form of cell division that produces haploid gametes (eggs and sperm) for sexual reproduction
  • Meiosis involves two rounds of division (meiosis I and II) and results in four genetically distinct daughter cells with half the number of chromosomes as the parent cell
  • Crossing over during prophase I allows for genetic recombination, increasing genetic diversity in offspring
• Apoptosis is programmed cell death, a highly regulated process that helps maintain tissue homeostasis and eliminates damaged or infected cells
  • Apoptosis is triggered by specific signals (e.g., DNA damage, viral infection) and is carried out by a family of proteases called caspases
  • Cells undergoing apoptosis display characteristic morphological changes, such as cell shrinkage, chromatin condensation, and the formation of apoptotic bodies

Specialized Cell Types in Human Anatomy

• Neurons are the primary cells of the nervous system, responsible for transmitting electrical and chemical signals
  • Neurons consist of a cell body (soma), dendrites (which receive signals), and an axon (which transmits signals)
  • Synapses are specialized junctions between neurons where neurotransmitters are released to propagate signals
• Muscle cells (myocytes) are specialized for contraction and movement
  • Skeletal muscle cells are long, cylindrical, and multinucleated, with sarcomeres containing actin and myosin filaments for contraction
  • Cardiac muscle cells are branched and interconnected by intercalated discs, allowing for coordinated contraction of the heart
  • Smooth muscle cells are spindle-shaped and lack sarcomeres, found in the walls of blood vessels and hollow organs
• Epithelial cells form the lining of body cavities, ducts, and surfaces, providing protection, secretion, and absorption
  • Examples include the skin epidermis, the lining of the digestive tract, and the cells of glands
• Connective tissue cells provide structural support and help connect different tissues and organs
  • Fibroblasts secrete collagen and other extracellular matrix components
  • Adipocytes (fat cells) store energy in the form of lipid droplets
  • Osteoblasts and osteoclasts are involved in bone formation and remodeling, respectively
• Blood cells are specialized for various functions within the circulatory system
  • Red blood cells (erythrocytes) contain hemoglobin and transport oxygen to tissues
  • White blood cells (leukocytes) are involved in the immune response, defending against pathogens and foreign substances
    • Examples include neutrophils, lymphocytes, monocytes, eosinophils, and basophils
  • Platelets (thrombocytes) are cell fragments that play a crucial role in blood clotting and wound healing