🐇Honors Biology Unit 3 – Cell Structure and Function

Key Concepts and Terminology

• Cells are the fundamental unit of life, serving as the building blocks for all living organisms
• Organelles specialized structures within cells that perform specific functions (mitochondria, endoplasmic reticulum, Golgi apparatus)
• Prokaryotic cells lack membrane-bound organelles and a true nucleus (bacteria, archaea)
• Eukaryotic cells contain membrane-bound organelles and a true nucleus (plants, animals, fungi, protists)
• Cell membrane a selectively permeable barrier that controls the movement of substances in and out of the cell
• Diffusion passive movement of molecules from an area of high concentration to an area of low concentration
• Osmosis diffusion of water across a selectively permeable membrane
• Active transport movement of molecules against a concentration gradient, requiring energy input
• ATP (adenosine triphosphate) primary energy currency of the cell, used to power various cellular processes
• Cell signaling communication between cells through chemical messengers (hormones, neurotransmitters)

Cell Theory and Historical Context

• Cell theory states that all living organisms are composed of cells, cells are the basic unit of life, and all cells arise from pre-existing cells
• Robert Hooke first observed and described cells in 1665 using a primitive microscope to examine thin slices of cork
• Antonie van Leeuwenhoek made significant improvements to the microscope in the late 17th century, allowing him to observe single-celled organisms (protozoa, bacteria)
• Matthias Schleiden and Theodor Schwann independently concluded in 1838-1839 that cells are the fundamental units of both plant and animal tissues
• Rudolf Virchow proposed in 1855 that all cells arise from pre-existing cells, completing the cell theory
• Advancements in microscopy and cell staining techniques in the late 19th and early 20th centuries led to the discovery of various organelles and their functions

Types of Cells: Prokaryotes vs. Eukaryotes

• Prokaryotic cells are typically smaller and simpler than eukaryotic cells, lacking membrane-bound organelles and a true nucleus
  • Examples of prokaryotes include bacteria and archaea
  • Genetic material in prokaryotes is a single circular DNA molecule located in the nucleoid region
• Eukaryotic cells are larger and more complex, containing membrane-bound organelles and a true nucleus
  • Examples of eukaryotes include plants, animals, fungi, and protists
  • Genetic material in eukaryotes is linear DNA organized into chromosomes within the nucleus
• Cell walls are present in most prokaryotes, plants, fungi, and some protists, providing structural support and protection
  • Bacterial cell walls contain peptidoglycan, while plant cell walls are composed primarily of cellulose
• Eukaryotic cells have a cytoskeleton, a network of protein filaments (microfilaments, intermediate filaments, microtubules) that provides structure, support, and facilitates intracellular transport

Cell Membrane Structure and Function

• Cell membrane is a selectively permeable phospholipid bilayer with embedded proteins that separates the cell's interior from the external environment
• 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 (ion channels, receptors)
  • Peripheral proteins are loosely attached to the surface of the membrane (enzymes, cytoskeletal proteins)
• Fluid mosaic model describes the cell membrane as a dynamic, fluid structure with proteins and lipids that can move laterally within the plane of the membrane
• Selectively permeable nature of the cell membrane allows it to control the passage of substances in and out of the cell
  • Small, nonpolar molecules (oxygen, carbon dioxide) can easily diffuse across the membrane
  • Larger, polar molecules (glucose, amino acids) require specific transport proteins to cross the membrane

Major Organelles and Their Roles

• Nucleus contains the cell's genetic material (DNA) and controls cellular activities
  • Nuclear envelope a double membrane that separates the nucleus from the cytoplasm
  • Nucleolus a dense region within the nucleus where ribosomal RNA is synthesized
• Endoplasmic reticulum (ER) a network of membranous channels involved in protein and lipid synthesis, transport, and modification
  • Rough ER studded with ribosomes, site of protein synthesis
  • Smooth ER lacks ribosomes, involved in lipid synthesis and detoxification
• Golgi apparatus a stack of flattened membranous sacs that modifies, packages, and sorts proteins and lipids for transport to various destinations
• Mitochondria rod-shaped organelles that generate ATP through cellular respiration
  • Mitochondrial DNA allows mitochondria to self-replicate
• Lysosomes membrane-bound vesicles containing digestive enzymes that break down cellular waste, debris, and foreign particles
• Ribosomes small organelles composed of RNA and protein, site of protein synthesis
  • Free ribosomes synthesize proteins for use within the cell
  • Bound ribosomes attached to the rough ER, synthesize proteins for export or insertion into the cell membrane
• Cytoskeleton a network of protein filaments that provides structure, support, and facilitates intracellular transport
  • Microfilaments thin, flexible filaments composed of actin, involved in cell movement and division
  • Intermediate filaments medium-sized filaments that provide mechanical strength and resistance to stress
  • Microtubules hollow, cylindrical filaments composed of tubulin, involved in cell division, organelle transport, and cilia/flagella formation

Cell Transport Mechanisms

• Passive transport movement of substances across the cell membrane without the input of energy
  • Simple diffusion movement of small, nonpolar molecules (oxygen, carbon dioxide) down their concentration gradient
  • Facilitated diffusion movement of larger, polar molecules (glucose, amino acids) through specific transport proteins
  • Osmosis diffusion of water across a selectively permeable membrane from an area of high water potential to an area of low water potential
• Active transport movement of substances across the cell membrane against their concentration gradient, requiring energy input (usually ATP)
  • Primary active transport directly uses ATP to power the movement of molecules (sodium-potassium pump)
  • Secondary active transport uses the concentration gradient of one molecule to power the transport of another (sodium-glucose cotransport)
• Endocytosis a process by which cells take in materials from the external environment by engulfing them with the cell membrane
  • Phagocytosis cell engulfs large particles or other cells (immune cells engulfing bacteria)
  • Pinocytosis cell takes in small dissolved molecules or fluids (nutrient uptake)
  • Receptor-mediated endocytosis specific molecules bind to receptors on the cell surface, triggering invagination and uptake (cholesterol uptake)
• Exocytosis a process by which cells release materials to the external environment by fusing vesicles with the cell membrane (neurotransmitter release, secretion of hormones)

Energy Production in Cells

• Cellular respiration a series of metabolic reactions that break down glucose to produce ATP
  • Glycolysis breaks down glucose into two pyruvate molecules, producing a small amount of ATP and NADH (occurs in the cytoplasm)
  • Citric acid cycle (Krebs cycle) oxidizes acetyl-CoA derived from pyruvate, producing CO2, NADH, and FADH2 (occurs in the mitochondrial matrix)
  • Electron transport chain a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, creating a proton gradient that powers ATP synthesis (oxidative phosphorylation)
• Photosynthesis a process by which plants and other autotrophs convert light energy into chemical energy (glucose)
  • Light-dependent reactions occur in the thylakoid membranes of chloroplasts, converting light energy into ATP and NADPH
  • Calvin cycle (light-independent reactions) uses ATP and NADPH from the light-dependent reactions to fix CO2 into glucose (occurs in the stroma of chloroplasts)
• Fermentation an anaerobic process that allows cells to produce ATP in the absence of oxygen
  • Lactic acid fermentation pyruvate is reduced to lactic acid, regenerating NAD+ for glycolysis (occurs in muscle cells during intense exercise)
  • Alcoholic fermentation pyruvate is converted to ethanol and CO2, regenerating NAD+ for glycolysis (occurs in yeast during beer and wine production)

Cell Communication and Signaling

• Cell signaling allows cells to respond to their environment and coordinate their activities
• Signaling molecules (ligands) bind to specific receptors on the target cell's surface or interior
  • Hydrophobic signaling molecules (steroid hormones) can diffuse directly through the cell membrane and bind to intracellular receptors
  • Hydrophilic signaling molecules (peptide hormones, neurotransmitters) bind to cell surface receptors
• Signal transduction the process by which a signal is transmitted from the receptor to the cell's interior, often involving a series of molecular interactions and cascades
  • G protein-coupled receptors (GPCRs) a large family of cell surface receptors that activate intracellular signaling pathways via G proteins (adenylyl cyclase pathway, phospholipase C pathway)
  • Receptor tyrosine kinases (RTKs) cell surface receptors that dimerize upon ligand binding, activating their intrinsic tyrosine kinase activity and initiating intracellular signaling cascades (MAP kinase pathway, PI3 kinase pathway)
• Second messengers small molecules that relay signals from receptors to target molecules within the cell (cyclic AMP, calcium ions, inositol triphosphate)
• Cell responses to signaling can include changes in gene expression, protein activity, or cellular processes (cell division, differentiation, apoptosis)

Practical Applications and Research

• Stem cell research holds promise for regenerative medicine and tissue engineering
  • Embryonic stem cells pluripotent cells derived from early-stage embryos, can differentiate into any cell type
  • Adult stem cells multipotent cells found in various tissues, can differentiate into a limited number of cell types
  • Induced pluripotent stem cells (iPSCs) adult cells reprogrammed to a pluripotent state, avoiding ethical concerns associated with embryonic stem cells
• Cancer research focuses on understanding the molecular basis of cancer and developing targeted therapies
  • Cancer cells exhibit uncontrolled growth, evade apoptosis, and can metastasize to other tissues
  • Oncogenes genes that, when mutated or overexpressed, can promote cancer development (Ras, Myc)
  • Tumor suppressor genes genes that normally regulate cell growth and division, but when mutated or lost, can contribute to cancer development (p53, BRCA1)
• Drug discovery and development relies on understanding cellular processes and identifying molecular targets
  • High-throughput screening allows researchers to test large numbers of compounds for desired effects on cells or specific molecular targets
  • Cell culture techniques enable the study of drug effects on specific cell types or tissues
  • Personalized medicine aims to tailor treatments based on an individual's genetic profile and cellular characteristics
• Synthetic biology involves the design and construction of novel biological systems or organisms
  • Genetically engineered cells can be used to produce pharmaceuticals, biofuels, or other valuable compounds
  • Synthetic gene circuits can be designed to perform specific functions (biosensors, logic gates)
  • Genome editing technologies (CRISPR-Cas9) allow precise modification of cellular genomes for research and therapeutic purposes