The Cellular Level of Organization

The Cellular Level of Organization

Overview

  • Focus on cellular structure and function only.

Introduction to Cells

  • Cells: Smallest units of life.

  • Over 250 different cell types in the human body.

  • Size range: 2 micrometers to > 1 meter.

  • Cell shape correlates with function.

Components of Cells

  • Plasma Membrane: Outer boundary, selectively permeable.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: Controls cellular activities.

Phospholipid Structure

  • Consists of:

    • Polar phosphate "head" (hydrophilic).

    • Non-polar lipid "tail" (hydrophobic).

Phospholipid Bilayer

  • Comprised of two sheets of phospholipids arranged tail to tail.

  • Hydrophobic tails face inward, while polar heads face extracellular fluid and cytoplasm.

  • Self-assembling; seals when torn.

Cell Membrane

  • Structure: Phospholipid bilayer with various molecular components.

Functions of the Plasma Membrane

  • Physical Barrier: Separates cytoplasm from extracellular fluid.

  • Selective Permeability: Regulates substance entry/exit.

  • Communication: Membrane proteins relay messages via signal transduction.

  • Cell Recognition: Surface carbohydrates identify cells.

Membrane Proteins

  • Functions:

    • Transport: Forms hydrophilic channels for specific solutes.

    • Receptors: Bind chemical messengers, triggering responses.

    • Enzymatic Activity: Catalyze reactions at membrane surface.

    • Cell-Cell Recognition: Glycoproteins serve as identification tags.

    • Attachment: Connect membrane proteins to the cytoskeleton and extracellular matrix.

    • Cell-to-Cell Joining: Intercellular junctions facilitate attachment and communication.

Types of Cell Junctions

  • Tight Junctions: Impermeable seals preventing molecule movement between cells.

  • Desmosomes: Anchoring junctions linking adjacent cells for support.

  • Gap Junctions: Allow small molecules and ions to pass between cells.

Passive Membrane Transport

  • Diffusion: Movement of molecules from high to low concentration.

    • Simple Diffusion: Small, non-polar substances (e.g., O2, CO2).

    • Facilitated Diffusion: Requires protein assistance (channel/carrier proteins).

    • Osmosis: Diffusion of water through a semipermeable membrane.

Tonicity of Solutions

  • Hypertonic: Higher solute concentration.

  • Isotonic: Equal solute concentrations.

  • Hypotonic: Lower solute concentration.

Active Membrane Transport

  • Requires ATP.

    • Primary Active Transport: E.g., Sodium-Potassium Pump, moving ions against gradients.

    • Secondary Active Transport: Utilizes gradients set up by primary transport.

Vesicular Transport

  • Endocytosis: Cell engulfs materials (phagocytosis, pinocytosis, receptor-mediated).

  • Exocytosis: Vesicles release materials outside the cell.

Membrane Potential

  • RMP is established by the balance of ion movement through channels.

  • Depends heavily on K+ ions inside cells.

Cellular Organelles

Endoplasmic Reticulum (ER)

  • Rough ER: Ribosome-studded; site of protein synthesis.

  • Smooth ER: Synthesis of lipids and detoxification processes.

Golgi Apparatus

  • Modifies and packages proteins from ER, prepares for exocytosis.

Mitochondrion

  • ATP production through lipid bilayer membranes.

Peroxisome

  • Detoxifies harmful substances; abundant in liver/kidney.

Cytoskeleton

  • Components: Microtubules, microfilaments, intermediate filaments; aids in cell shape and movement.

Nucleus

  • Control center; houses genetic material defining cell function.