Biology Organelles and Cell Membrane

Organelles

• Two types of organelles: membranous and non-membranous
• Membranous Organelles: enclosed by a phospholipid membrane
  • Examples: mitochondria, nucleus, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes
• Non-Membranous Organelles: not enclosed by a membrane, components are in direct contact with the cytosol
  • Examples: cytoskeleton, centrioles, microvilli, cilia, flagella, and ribosomes

Cell Membrane

• Forms a barrier between the cell and its environment
• Consists of a phospholipid bilayer with scattered proteins
  • Phospholipid Bilayer:
    • Thin and delicate (6-10 nm)
    • Hydrophilic heads face outward, hydrophobic tails face inward
    • Creates a barrier between the cytoplasm and extracellular fluid
  • Proteins:
    • Integral Proteins: embedded in the membrane, pass all the way through (transmembrane proteins), can form channels or pores
    • Peripheral Proteins: bound to the inner or outer surface of the membrane, easily separated
  • Functional Classes of Membrane Proteins:
    • Anchoring Proteins: connect the plasma membrane to other structures
    • Recognition Proteins: recognized by cells of the immune system
    • Receptor Proteins: bind to specific extracellular molecules (ligands)
• Diffusion: movement of molecules from an area of high concentration to low concentration
  • Factors Influencing Diffusion Rate: distance, molecule size, temperature, gradient size, electrical forces
  • Facilitated Diffusion: transport of large molecules with the help of carrier proteins (no ATP required)
  • Osmosis: diffusion of water across a selectively permeable membrane from an area of high water concentration to low concentration
• Osmotic Pressure: force with which pure water moves into a solution due to solute concentration
• Osmolarity: solute concentration of a solution
• Tonicity: effect of a solution on a cell
  • Isotonic: equal solute concentration inside and outside the cell (no net movement)
  • Hypertonic: higher solute concentration outside the cell (water moves out, cell shrinks)
  • Hypotonic: lower solute concentration outside the cell (water moves in, cell swells)
• Filtration: bulk flow of substances driven by a pressure gradient (no ATP required)
• Active Processes: require energy (ATP)

Mitochondria

• Powerhouses of the cell
• Responsible for ATP synthesis
• Double-layered membrane
  • Cristae: folded inner membrane, increases surface area for ATP production
  • Matrix: liquid enclosed by the inner membrane
• Aerobic Respiration: multi-step process for ATP production
  • Glycolysis: glucose breakdown into pyruvate (occurs in cytoplasm, produces 2 ATP)
  • Citric Acid Cycle: pyruvate breakdown into carbon dioxide, hydrogen, and 2 ATP (occurs in mitochondrial matrix)
  • Electron Transport System: hydrogen atoms used to create 32 ATP, oxygen combines with hydrogen to form water

Cytoskeleton

• Provides internal framework, strength, and flexibility
• Composed of microfilaments, intermediate filaments, and microtubules
  • Microfilaments: smallest, primarily composed of actin, found in the cell periphery
  • Intermediate Filaments: strong and durable, various protein types, present throughout the cytoplasm
  • Microtubules: largest, extend from the centrosome, involved in cell division

Centrioles

• Cylindrical structures composed of microtubules arranged in nine triplets
• Located at the centrosome
• Involved in cell division, forming spindle fibers that move chromosomes to the poles

Cilia

• Long, slender extensions for propelling fluids or solids
• Microtubules arranged in nine doublets around a central pair

Microvilli

• Finger-shaped extensions of the plasma membrane
• Core of microfilaments for stiffness
• Increase surface area for nutrient absorption

Ribosomes

• Responsible for protein synthesis
• Consist of two subunits: small and large
• Attached to the endoplasmic reticulum

Nucleus

• Control center of the cell
• Contains genetic information (DNA)
• Directs protein synthesis
• Usually the largest and most prominent structure in the cell
• Nuclear Envelope: surrounds the nucleus, double membrane with pores
  • Nuclear Pores: allow communication between the nucleus and cytoplasm

DNA Replication

• Process of duplicating DNA
• Occurs during the S-phase of the cell cycle
• Steps:
  • Helicase: unwinds and unzips the DNA
  • DNA Polymerase: adds complementary nucleotides to create two daughter strands
  • Lagging Strand: synthesized discontinuously, forming Okazaki fragments that are glued together by Ligase
  • Leading Strand: synthesized continuously
  • Semi-Conservative Replication: each daughter strand has one original and one new strand
• Sister Chromatids: two identical strands of DNA connected at the centromere
  • Kinetochore: raised region on the centromere where spindle fibers attach

Cell Cycle

• Cycle of growth and division
• Interphase:
  • G1: cell growth, normal functions, organelle duplication
  • S: DNA replication
  • G2: preparation for division
  • G0: a resting state where cells permanently stop dividing
• M Phase: mitosis and cytokinesis
• Mitosis: identical nuclear division in somatic cells (body cells), for repair and replacement
  • Prophase: nuclear membrane breaks down, chromosomes condense, spindle fibers form
  • Metaphase: chromosomes line up at the equator of the cell, attached to spindle fibers at the kinetochore
  • Anaphase: sister chromatids separate and move to opposite poles
  • Telophase: nuclear membrane reforms, chromosomes decondense, cytokinesis begins
• Meiosis: nuclear division in sex cells (gametes), resulting in genetically diverse daughter cells

Protein Synthesis

• Steps:
  • Transcription: DNA is copied into mRNA
  • Translation: mRNA is translated into a polypeptide chain by ribosomes
    • mRNA: messenger RNA, carries the genetic code from DNA to ribosomes
    • tRNA: transfer RNA, carries amino acids to the ribosomes
    • Ribosome: reads mRNA and assembles amino acids into a polypeptide
    • Codon: three-nucleotide sequence on mRNA that specifies an amino acid
    • Anticodon: three-nucleotide sequence on tRNA that complements a codon
    • Elongation: process of adding amino acids to the polypeptide chain
    • Translocation: shift of tRNA from one site on the ribosome to another
    • Chain Termination: stop codon signals the ending of polypeptide synthesis
• Polypeptide Folding: polypeptide chain folds into a specific three-dimensional shape to become a functional protein
• Protein Structure: primary, secondary, tertiary, and quaternary

Test your knowledge on the various types of organelles, including membranous and non-membranous structures. This quiz also covers the essential functions and composition of the cell membrane, highlighting its importance in cellular processes. Brush up on your biology concepts!