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BIO181_Chp3_S25

Chapter 3: The Cell

  • The unit of life

Different Cell Types

  • Red blood cell

  • Nerve cell

  • Skeletal muscle cell

  • Epithelial cell

Basic Components of a Generalized Cell

  • Plasma membrane: Outer boundary of the cell

  • Cytoplasm: Contains organelles and cytosol

    • Cytosol: Fluid portion

    • Organelles: Specialized structures within the cell

    • Cytoskeleton: Framework that provides structure

  • Nucleus: Contains genetic material

The Cell (Plasma) Membrane - a Phospholipid Bilayer

  • Composed of phospholipids with hydrophilic heads and hydrophobic tails

  • Separates intracellular fluid (cytosol) from extracellular fluid (ECF)

Phospholipid Structure

  • Polar (hydrophilic) head

  • Nonpolar (hydrophobic) tails

    • A schematic structure shows how phospholipids arrange themselves in water

Formation of Phospholipid Bilayers

  • In water, phospholipids spontaneously form bilayers

    • Polar heads face water, and nonpolar tails face each other

  • This arrangement is crucial for cell membrane structure

The Fluid Mosaic Model of the Plasma Membrane

  • Describes the cell membrane as a mosaic of components:

    • Integral proteins (transmembrane proteins)

    • Peripheral proteins

    • Cholesterol

    • Glycoproteins and glycolipids

  • Provides fluidity and flexibility to the membrane

Functions of Membrane Proteins

  • Channels: Allow substances to pass through the membrane

  • Carriers: Bind and transport substances into or out of the cell

  • Receptors: Bind to ligands to initiate cellular responses

  • Enzymes: Catalyze biochemical reactions

  • Structural support: Connect with other proteins for cell support

  • Linker proteins: Connect adjacent cells in tissues

Diffusion and Equilibrium

  • Concentration gradient: Difference in concentration across a membrane

  • Equilibrium: Achieved when molecules are evenly dispersed

  • Example:

    1. Two glucose solutions separated by a divider

    2. Removal of the divider leads to diffusion until equilibrium is reached

Passive Transport: Simple and Facilitated Diffusion

  • Simple diffusion: Nonpolar molecules moving through the phospholipid bilayer

  • Facilitated diffusion: Uses channel or carrier proteins to transport polar/ionic molecules across the membrane

Passive Transport: Osmosis

  • Movement of water across a selectively permeable membrane

  • Water moves to balance solute concentrations on both sides of the membrane

Tonicity: Isotonic, Hypertonic, and Hypotonic Solutions

  • Isotonic: Equal solute concentration inside and outside the cell (normal state)

  • Hypertonic: Higher solute concentration outside the cell (causes crenation)

  • Hypotonic: Lower solute concentration outside the cell (can cause lysis)

Active Transport: Protein Pumps

  • Na+/K+ pump: Transports sodium out and potassium into the cell against their concentration gradients using ATP

Active Transport: Vesicle-Mediated Endocytosis

  • Phagocytosis (cell eating): Engulfing of large particles like bacteria

    1. Particle binds to receptor

    2. Pseudopods surround the particle, forming a phagosome

    3. Phagosome fuses with lysosome for digestion

  • Pinocytosis (cell drinking): Engulfing of small particles or liquids

Active Transport: Vesicle-Mediated Exocytosis

  • Process of expelling materials from the cell via vesicles that fuse with the plasma membrane

Types of Transport Across the Cell Membrane

  • Passive transport: Moves substances down their concentration gradients without energy

    • Simple diffusion: Molecules cross without assistance

    • Facilitated diffusion: Molecules move with the help of transport proteins

    • Osmosis: Movement of water

  • Active transport: Moves substances against their concentration gradients using energy

    • Primary active transport: ATP-driven pumps

    • Secondary active transport: Uses gradients created by primary transport

The Cell and its Organelles

  • Mitochondrion: ATP synthesis

  • Lysosome: Digestion of macromolecules

  • Rough Endoplasmic Reticulum (RER): Protein synthesis and modification

  • Smooth Endoplasmic Reticulum (SER): Lipid synthesis and detoxification

  • Golgi Apparatus: Modifies, sorts, and packages proteins

The Nucleus

  • Contains nucleolus, chromatin, and is surrounded by a nuclear envelope with pores

Chromatin and Chromosomes

  • Chromatin: DNA and proteins (histones); condenses to form chromosomes during cell division

Cell Cycle

  1. Interphase (G1, S, G2 phases)

    • G1: Cell growth

    • S: DNA replication

    • G2: Prepare for division

  2. M phase: Cell division (Mitosis and Cytokinesis)

Central Dogma of Molecular Biology

  • Genetic information flow:

    • DNA -> mRNA -> Protein

Transcription and Translation Overview

  • Transcription: DNA is transcribed into mRNA

  • Translation: mRNA is translated into a polypeptide at the ribosome

Genetic Code

  • Each sequence of three nucleotides (codon) specifies an amino acid

Protein Synthesis Process

  1. Transcription: DNA to pre-mRNA, introns removed to form mature mRNA

  2. Translation: mRNA guides the assembly of amino acids into polypeptides using tRNA

  3. Posttranslational modification: Polypeptide folding and modification into functional proteins in the cytosol or ER.