Ch.3&4 Voice

Overview of Cell Structure and Function

  • Covers Chapters 3 and 4, focusing on organelles and cells

Learning Outcomes

  • Understanding the components and functions of the plasma membrane, various types of transport mechanisms, and implications for cellular metabolism and physiology

Cell Boundary and Fluid

Plasma Membrane

  • The outer boundary of the cell, visually represented as a dark line in diagrams
  • Outside the plasma membrane is the extracellular fluid (ECF)
  • Inside the cell is called intracellular fluid
  • Importance of Ion Distribution:
    • The distribution of ions creates a membrane charge, crucial for processes like nerve impulses

Composition of Plasma Membrane

  • Predominantly composed of phospholipids (75% of the membrane)
    • Structure of Phospholipids:
      • Hydrophilic heads facing outward
      • Hydrophobic tails facing inward
    • Fluidity:
      • Remains liquid at room temperature, allowing for flexibility
  • Cholesterol (a rigid molecule):
    • Provides structural support to the membrane
    • Remains solid at room temperature
  • Glycolipids:
    • Extends into ECF, important for cell recognition processes

Proteins in Plasma Membrane

Types of Membrane Proteins

  • Transmembrane Proteins:
    • Span the entire plasma membrane, implicating both hydrophilic regions (outer and inner) and a hydrophobic core
  • Peripheral Proteins:
    • Only hydrophilic; located on either inside or outside surfaces of the membrane

Functions of Membrane Proteins

  • Receptor Proteins:
    • Bind chemical messengers (ligands), starting events such as chemical reactions or cellular responses.
    • Messages have temporary bindings, eventually detaching to prevent continuous stimulation
  • Enzymes:
    • Break down chemical messengers to terminate their activity after signaling
  • Channel Proteins:
    • Enable selective transport into and out of the cell, particularly for larger or charged particles
  • Types of Channel Proteins:
    • Leak Channels: Always open, allowing free passage for small solutes (e.g., water, ions)
    • Ligand Gated Channels: Activated by ligands binding; opens channels for solutes
    • Voltage Gated Channels: Open based on changes in membrane voltage
    • Mechanically Gated Channels: Open due to mechanical pressure or stretching
  • Carrier Proteins:
    • Utilize ATP to transport molecules across the membrane; distinct from channel proteins due to energy requirements
  • Cell Identity Markers:
    • Glycolipids/proteins acting like name tags for immune recognition
  • Cell Adhesion Molecules:
    • Involved in the physical linking of cells and other structures

Cellular Extensions

Microvilli

  • Short, densely packed extensions
  • Compare to carpet or brush bristles
  • Function: Increase surface area, enhancing absorption in organs (e.g., intestines)

Cilia

  • Longer, slender hair-like structures
  • Found in respiratory tracts, functioning to move mucus and keep surfaces moist
  • Ciliary Motion:
    • Power stroke (forward thrust) followed by a recovery stroke

Flagellum and Pseudopods

  • Flagellum:
    • Single whip-like tail seen in sperm cells
  • Pseudopods:
    • Extensions of the cell membrane, used by some cells (e.g., immune cells) to engulf pathogens

Membrane Transport Mechanisms

Types of Transport

  • Filtration:
    • Movement through capillary walls due to blood pressure creating a filtration force
  • Simple Diffusion:
    • Passive process; solute moves from high to low concentration without energy
    • Factors affecting diffusion:
    • Temperature: Higher temperatures increase diffusion rates
    • Molecular Weight: Larger mols diffuse slower
    • Concentration Gradient: Steeper gradients increase rates
    • Surface Area: More area increases rates
    • Permeability: Selective permeability affects ease of diffusion
  • Facilitated Diffusion:
    • Passive transport utilizing proteins (either channel or carrier)
    • No ATP required
  • Osmosis:
    • Passive movement of water; adjusts concentration levels in solutions
    • Osmotic Pressure: Can impact water movement across semipermeable membranes
  • Tonicity Definitions:
    • Hypotonic Solution: Lower solute concentration outside; causes cells to swell and potentially burst (lysis)
    • Hypertonic Solution: Higher solute concentration outside; causes cells to shrivel (crenation)
    • Isotonic Solution: Equal solute concentrations; no net water movement

Active Transport Mechanisms

Overview

  • Requires ATP and involves energy to pump solutes against their concentration gradient
  • Sodium-Potassium Pump:
    • Transports 3 sodium ions out and 2 potassium ions into the cell, essential for maintaining resting membrane potential and generating heat

Vesicular Transport

Endocytosis Types
  • Phagocytosis (Cell eating): Invaginating membrane to engulf large particles
  • Pinocytosis (Cell drinking): Invaginating membrane to take in liquids
  • Receptor Mediated Endocytosis: Uses specific receptors for targeted uptake

Exocytosis

  • Reverse of endocytosis; proteins and other molecules packaged in vesicles are exported out of the cell

Summary of Transport Mechanisms

Passive Transport

  • Includes filtration, simple diffusion, facilitated diffusion, and osmosis; does not use ATP

Active Transport

  • Involves energy expenditure (ATP) for processes like active transport and vesicular transport (endocytosis, exocytosis)

Conclusion

  • Understanding the structure and function of the plasma membrane and cellular mechanisms is crucial for grasping biological processes and the maintenance of homeostasis in living organisms.