Organelles and Cell Specialization

Phospholipid Bilayer – Structure and Function

    Key Components

  • Phospholipid bilayer: Forms spontaneously from amphipathic phospholipids
  • Amphipathic: Molecules with both hydrophilic heads and hydrophobic tails

    Function of Amphipathic Nature

  • Allows bilayer formation: hydrophilic heads face water; hydrophobic tails form inner barrier
  • Creates a selective barrier for transport

    Functions of the Lipid Bilayer

  • Semi-permeable: Controls what enters/leaves the cell
  • Selectively permeable:
    • Small, nonpolar molecules pass freely
    • Polar/large molecules need assistance

    Importance in Eukaryotic Cells

  • Maintains compartmentalization (organelles)

    Fluidity of the Bilayer

  • Lipids move laterally → membrane adapts, repairs, and divides
  • Cholesterol:
    • Reduces fluidity in heat
    • Prevents stiffness in cold
    • Stabilizes the membrane

    Barrier Effectiveness

  • Hydrophobic core repels water, ions, and large polar molecules

    Substances That Cannot Cross Easily

  • Ions (e.g. Na⁺, Cl⁻)
  • Polar molecules (e.g. glucose)
  • Large macromolecules (e.g. proteins)

Membrane Proteins and Carbohydrates

    Glycoproteins and Glycolipids

  • Located on extracellular side
  • Roles:
    • Cell recognition
    • Signaling
    • Adhesion (e.g. sperm binding, immune response)
  • Organ transplants: Crucial for identifying compatible tissue

    Membrane Proteins (Functions - JET RAT)

  • Junctions – Connect adjacent cells
  • Enzymes – Catalyze reactions
  • Transport – Move substances across membrane
  • Recognition – Cell identity markers
  • Anchorage – Attach to cytoskeleton
  • Transduction – Receive signals (e.g., hormones)

    Types of Membrane Proteins

  • Integral proteins:
    • Span the bilayer
    • Hydrophobic regions interact with membrane core
  • Peripheral proteins:
    • Loosely attached to surface or integral proteins
    • Temporary and dynamic

Fluid Mosaic Model

Explains the membrane’s flexible, dynamic nature.


    5 Key Components

  • 1. Phospholipid bilayer
  • 2. Integral proteins
  • 3. Peripheral proteins
  • 4. Glycoproteins
  • 5. Cholesterol

Passive Transport

    1. Simple Diffusion

  • Movement: High → Low concentration
  • No energy (ATP) needed
  • Features:
    • Selectively permeable membrane
    • Concentration-gradient driven
  • Example: Oxygen diffusing into a cell

    2. Facilitated Diffusion

  • Uses channel or carrier proteins
  • Still passive – no ATP needed
  • Moves particles down the concentration gradient
  • Channel proteins:
    • Form specific pores for ions/molecules
    • Faster than simple diffusion
  • Carrier proteins:
    • Bind particles → change shape → release particle inside cell

    3. Comparison

    Simple Diffusion Facilitated Diffusion
    Uses proteins? No Yes (channel or carrier)
    Energy required? No No
    Particle type Small and nonpolar Large or polar

Osmosis

    Definition

  • Movement of water from low solute → high solute across a semi-permeable membrane

    Supports Plant Turgor Pressure:

  • Water enters vacuole
  • Exerts pressure on cell wall
  • Keeps plant rigid and upright

    Osmosis and Solutions

  • Hypotonic: High water, low solutes outside → water enters cell (cell swells)
  • Isotonic: Equal water and solute concentration inside and outside → no net movement
  • Hypertonic: Low water, high solutes outside → water exits cell (cell shrinks)

    Importance of Osmosis

  • Maintains turgor pressure in plants
  • Regulates cell volume
  • Essential for homeostasis in tissues and organs

Active Transport

    Movement of Molecules

  • Movement of molecules against concentration gradient
  • Requires ATP
  • Uses pump proteins

    Importance

  • Maintains concentration gradients
  • Enables nutrient absorption, waste removal, and nerve signaling

    Mechanism

  • Molecule binds to the pump protein
  • ATP causes a shape change
  • Molecule is released on the opposite side of the membrane
  • Protein resets without more energy

    Pump vs. Channel Proteins

    Feature Pump Proteins Channel Proteins
    Energy required Yes (ATP) No
    Direction One-way Both directions
    Gradient movement Against gradient With gradient

Organelles

    Importance

  • Compartmentalization increases efficiency and specialization

    Key Organelles and Functions

  • Nucleus: DNA storage and control center
  • Ribosomes: Protein synthesis
  • Golgi Apparatus: Packages and ships materials
  • Lysosomes: Waste breakdown
  • Chloroplasts: Photosynthesis (in plants)

    Organelle Criteria

  • Must be membrane-bound
  • Must participate in metabolism
  • Note: the cytoskeleton, cell wall, and cytoplasm are not organelles

Cell Size Limitations

    2 Main Factors

  • Metabolism rate → Depends on volume
  • Exchange rate → Depends on surface area

    As the Cell Grows:

  • Surface area to volume ratio decreases
  • Less efficient exchange
  • Large cells divide to survive

    Why Multicellularity Helps

  • More small cells = higher total surface area to volume ratio
  • Allows for specialization and the division of labor

Cell Differentiation

    Definition

  • Process where unspecialized cells develop into specialized cells

    Genetic Control

  • All cells retain the same genome
  • Gene expression determines specialization

    Form and Function

  • Structure supports function
  • Example: RBCs have no nucleus + biconcave shape → max oxygen transport

Stem Cells

    Definition

  • Undifferentiated cells that:
    • Can divide endlessly
    • Can differentiate into other cell types

    Key Properties

  • Self-renewal: Reproduce indefinitely
  • Potency: Ability to become different cell types

    Types of Stem Cells

    Type Potential
    Totipotent All cell types (including embryo + placenta)
    Pluripotent Any cell from 3 germ layers
    Multipotent Limited related types (e.g. blood, skin)

    Roles in Adults

  • Replace damaged/worn-out cells (e.g. skin, blood)

Embryonic vs Adult Stem Cells

    Embryonic Stem Cells

  • Totipotent and pluripotent
  • Differentiation triggered by morphogen gradients (signaling molecules)

    Adult Stem Cells

  • Only multipotent
  • Found in bone marrow, hair follicles, etc.