AP Biology Unit 2: Cell Structure, Transport, and Compartmentalization Notes

Subcellular Components and Organelle Function

• Ribosomes:
  • Composed of ribosomal RNA (rRNA) and proteins.
  • Non-membrane-bound subcellular structures present in cells across all forms of life, reflecting universal common ancestry.
  • Synthesize proteins by translating messenger RNA (mRNA) sequences.
• Endomembrane System:
  • Consists of a coordinated group of membrane-bound organelles and subcellular components: the nuclear envelope, endoplasmic reticulum (ER), Golgi complex, lysosomes, vacuoles, transport vesicles, and the plasma membrane.
  • Functions collectively to modify, package, and transport proteins, lipids, and polysaccharides intercellularly.
• Endoplasmic Reticulum (ER):
  • Provides mechanical support to help cells maintain structural shape and facilitates intracellular transport.
  • Rough ER:
    • Studded with membrane-bound ribosomes on its surface.
    • Enables compartmentalization of the cell and directly carries out protein synthesis.
  • Smooth ER:
    • Lacks ribosomes.
    • Functions in lipid synthesis and cellular detoxification.
• Golgi Complex:
  • A membrane-bound organelle consisting of a series of flattened membrane sacs (cisternae).
  • Correctly folds and chemically modifies newly synthesized cellular products.
  • Packages proteins into vesicles for intracellular trafficking or secretion.
• Mitochondria:
  • Possess a double-membrane structure that creates distinct compartments for metabolic reactions in aerobic cellular respiration.
  • The outer membrane is smooth, whereas the inner membrane is highly convoluted and folded into cristae.
  • The inner membrane folds increase surface area, enabling highly efficient synthesis of ATP.
• Lysosomes:
  • Membrane-enclosed sacs containing hydrolytic enzymes used to digest macromolecules, cellular debris, and foreign material.
  • Play a critical role in programmed cell death (apoptosis).
• Vacuoles:
  • Membrane-bound storage sacs with specialized roles across different cell types.
  • Plant Cells: Contain a large central vacuole that maintains turgor pressure and structural rigidity through nutrient and water storage.
  • Animal Cells: Contain smaller, more numerous vacuoles used for storing cellular materials and endocytosed contents.
• Chloroplasts:
  • Specialized double-membrane organelles found in plants and photosynthetic algae.
  • Serve as the site of photosynthesis, capturing light energy to synthesize organic molecules.

Cell Size Mechanics and Environmental Exchange

• Surface Area-to-Volume Ratio ($SA/V$):
  • Affects the ability of a biological system to acquire necessary resources, eliminate waste products, absorb or dissipate thermal energy, and exchange chemicals with the environment.
  • The plasma membrane surface area must be sufficiently large to support adequate material exchange.
• Cell Size Constraints:
  • Smaller cells have a higher $SA/V$ ratio, allowing for a more efficient exchange of materials across the plasma membrane.
  • As a cell increases in volume, its relative surface area decreases while internal resource demand increases.
  • To overcome volume-related exchange constraints, cells develop complex structural features, such as membrane folds, to expand surface area.
• Organismal Scale and Heat Exchange:
  • As organisms increase in size, overall $SA/V$ ratio decreases, directly impacting thermal exchange rates with the environment.
  • Smaller body masses exchange proportionally more heat with the surrounding environment than larger masses.
  • As body mass increases, both $SA/V$ ratio and the rate of relative heat exchange decrease.
• Metabolic Rate Relationships:
  • An inverse relationship exists between metabolic rate per unit body mass and total body size in multicellular organisms.
  • Smaller organisms exhibit higher metabolic rates per unit body mass compared to larger organisms.
• Specialized Structures Optimizing Surface Area:
  • Root hair cells
  • Guard cells
  • Gut epithelial cells
  • Cilia
  • Stomata

• Mathematical Formulas for Surface Area and Volume:
  • Surface Area of a Rectangular Solid:         $SA = 2lh + 2lw + 2wh$
  • Surface Area of a Cylinder:         SA = 2\r\n\r\n\r\n\pi rh + 2\pi r^2
  • Surface Area of a Sphere:         $SA = 4\pi r^2$
  • Surface Area of a Cube:         $SA = 6s^2$
  • Volume of a Sphere:         $V = \frac{4}{3}\pi r^3$
  • Volume of a Cube:         $V = s^3$
  • Volume of a Rectangular Solid:         $V = lwh$
  • Volume of a Cylinder:         $V = \pi r^2 h$
  • Variable Definitions:
    • $r$ = radius
    • $l$ = length
    • $h$ = height
    • $w$ = width
    • $s$ = length of one side of a cube

Plasma Membrane Structure and the Fluid Mosaic Model

• Phospholipid Bilayer Architecture:
  • Phospholipids are amphipathic molecules containing both hydrophilic and hydrophobic regions.
  • Polar, hydrophilic phosphate head groups face outward toward the aqueous external environment and internal cytosol.
  • Nonpolar, hydrophobic fatty acid tail groups orient inward toward each other, forming a water-repelling core.
• Embedded Membrane Proteins:
  • Proteins embedded in the bilayer contain hydrophilic regions (with charged and polar amino acid side groups) and hydrophobic regions (with nonpolar amino acid side groups).
  • Hydrophilic protein domains reside within the interior of channel pores or are exposed to external/internal aqueous environments.
  • Hydrophobic protein domains interact directly with the nonpolar fatty acid tails inside the membrane interior.
• Fluid Mosaic Model:
  • Describes the cell membrane as a dynamic matrix composed of a phospholipid fluid framework embedded with diverse proteins, steroids (such as cholesterol in vertebrate animals), glycoproteins, and glycolipids.
  • These components move laterally within the plane of the membrane.

Biological Membrane Permeability and Cell Wall Functions

• Selective Permeability:
  • The hydrophobic interior of the phospholipid bilayer establishes selective permeability, separating internal cellular conditions from the external environment.
  • Small, nonpolar molecules—including $\text{N}_2$, $\text{O}_2$, and $\text{CO}_2$—freely diffuse across the membrane without assistance.
  • Small, polar, uncharged molecules—such as $\text{H}_2\text{O}$ and $\text{NH}_3$—pass across the membrane independently in small quantities.
  • Hydrophilic substances, including large polar molecules and charged ions, are blocked by the nonpolar hydrocarbon tails and require specialized channel or transport proteins to cross.
• Cell Wall Functions:
  • Present in Bacteria, Archaea, Fungi, and Plants.
  • Acts as a structural boundary, offers protection against osmotic lysis, and provides a permeability barrier to specific environmental substances.

Mechanisms of Membrane Transport and Vesicular Transport

• Concentration Gradients:
  • Selective permeability enables cells to maintain concentration gradients of solutes across the plasma membrane.
• Passive Transport:
  • Net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient.
  • Occurs spontaneously without direct metabolic energy expenditure.
• Active Transport:
  • Moves molecules across a membrane against concentration gradients (from low concentration to high concentration).
  • Requires direct input of metabolic energy (such as ATP).
• Bulk Vesicular Transport:
  • Requires metabolic energy to move large particles or bulk quantities of materials across the plasma membrane.
  • Endocytosis:
    • The plasma membrane invaginates around extracellular material, pinching off to form new internal vesicles that engulf external molecules and particulates.
  • Exocytosis:
    • Internal membrane-bound vesicles fuse with the plasma membrane, releasing their contents into the extracellular space.

Facilitated Diffusion and Ion Transport

• Facilitated Diffusion Mechanics:
  • Passive transport mechanism that enables large polar molecules and charged ions to move down their concentration gradient without energy expenditure.
  • Utilizes transmembrane channel proteins or transport carrier proteins.
• Ion Transport and Membrane Polarization:
  • Charged ions, such as $\text{Na}^+$ (sodium) and $\text{K}^+$ (potassium), cannot cross the hydrophobic core and require specific channel proteins.
  • The movement of charged ions across the membrane can polarize the membrane, establishing an electrical potential.
• Aquaporins:
  • Specialized channel proteins that facilitate the rapid transport of large quantities of water molecules across biological membranes.

Tonicity, Water Potential, and Osmoregulation

• Environmental Tonicity:
  • Hypotonic: Lower solute concentration relative to the cell cytosol (causes net water influx).
  • Hypertonic: Higher solute concentration relative to the cell cytosol (causes net water efflux).
  • Isotonic: Equal solute concentration relative to the cell cytosol (no net water movement).
• Osmotic Dynamics:
  • Water moves via osmosis from hypotonic regions (low solute concentration / low osmolarity) to hypertonic regions (high solute concentration / high osmolarity).
  • Osmotic flow occurs from regions of high water potential to regions of low water potential.
• Water Potential Formulas:
  • Overall Water Potential:         $\Psi = \Psi_s + \Psi_p$
    • $\Psi$ = total water potential
    • $\Psi_p$ = pressure potential
    • $\Psi_s$ = solute potential
  • Solute Potential of a Solution:         $\Psi_s = -iCRT$
    • $i$ = ionization constant
    • $C$ = molar concentration
    • $R$ = pressure constant ($0.0831\,\text{L}\cdot\text{bars}\cdot\text{mol}^{-1}\cdot\text{K}^{-1}$)
    • $T$ = absolute temperature in Kelvin ($^\circ\text{C} + 273$)
• Osmoregulation:
  • Maintains internal water balance and controls solute concentration within acceptable physiological limits.
  • Essential for cellular growth, volume control, and metabolic homeostasis.

Active Transport Mechanisms and Electrochemical Gradients

• Energy Requirements:
  • Active transport relies on metabolic energy (ATP hydrolysis) to drive molecules or ions against concentration gradients.
  • Establishes and maintains steep electrochemical gradients across cellular membranes.
• Membrane Transport Proteins:
  • Transmembrane pump proteins are required for active transport processes.
• $\text{Na}^+/\text{K}^+$ Pump and ATPase:
  • The sodium-potassium pump ($\text{Na}^+/\text{K}^+$ ATPase) actively exports $\text{Na}^+$ ions while importing $\text{K}^+$ ions.
  • Directly maintains membrane potential, cellular volume, and electrochemical gradients required for secondary active transport and nerve impulse transmission.

Intracellular Compartmentalization

• Functional Partitioning:
  • Membranes and membrane-bound organelles in eukaryotic cells compartmentalize intracellular metabolic processes and specific enzymatic reactions.
• Reaction Efficiency:
  • Internal membrane partitioning minimizes competing metabolic interactions.
  • Increases localized surface area available for enzymatic complexes and reaction pathways to proceed efficiently.

Evolutionary Origins of Cell Compartmentalization

• Structural Differences Between Kingdoms:
  • Prokaryotes: Lack internal membrane-bound organelles, but maintain internal specialized regions to organize specific processes.
  • Eukaryotes: Possess extensive internal membrane systems that partition the cytoplasm into functionally specialized organelles.
• Endosymbiotic Theory:
  • Proposes that membrane-bound eukaryotic organelles, specifically mitochondria and chloroplasts, evolved from once free-living prokaryotic cells engulfed by an ancestral nucleated cell.
  • Evidence Supporting Endosymbiosis:
    • Double Membrane Systems: Both mitochondria and chloroplasts possess double membranes, consistent with an engulfment mechanism.
    • Physical Size: Both organelles are similar in size to typical bacteria.
    • Genome and Protein Machinery: Both organelles contain their own circular loop DNA and distinct prokaryote-like ribosomes (though most organelle proteins are encoded by nuclear genes).
    • Replication: Mitochondria and chloroplasts reproduce independently within host cells via binary-fission-like self-replication.