Organelles and Cell Size Study Guide

Essential Organelles and Functions

• Ribosomes

  • Structure: Non-membrane-bound complexes composed of ribosomal RNA (rRNA) and proteins, organized into large and small subunits.
  • Function: Sites of protein synthesis (translation).
  • Classification:
  • Free Ribosomes: Suspended in the cytosol; synthesize water-soluble proteins that function within the cytoplasm.
  • Bound Ribosomes: Attached to the rough endoplasmic reticulum; synthesize membrane-bound proteins, secretory proteins, and organelle-specific proteins (e.g., lysosomal enzymes).

• Rough Endoplasmic Reticulum (RER)

  • Structure: An extensive network of interconnected membranous sacs (cisternae) studded with ribosomes on the cytosolic surface.
  • Function: Folds nascent polypeptide chains into functional conformations, carries out initial protein modifications (such as N-linked glycosylation), and packages proteins into transport vesicles destined for the Golgi apparatus.

• Smooth Endoplasmic Reticulum (SER)

  • Structure: Tubular membranous network continuous with the RER, lacking attached ribosomes.
  • Function: Synthesizes lipids (phospholipids, steroid hormones, and oils), metabolizes carbohydrates, detoxifies drugs and chemical poisons (prominent in hepatocytes), and sequesters calcium ions ($\text{Ca}^{2+}$) crucial for cellular signaling and muscle contraction.

• Golgi Apparatus

  • Structure: Series of flattened, membrane-bound discs (cisternae) exhibiting distinct polarity: the receiving cis face (oriented toward the ER) and the shipping trans face (oriented toward the plasma membrane).
  • Function: Modifies, sorts, and packages ER products. Modifies carbohydrate chains on glycoproteins, manufactures specific polysaccharides, attaches molecular identification tags (e.g., phosphate groups), and buds transport vesicles directed to their targeted destinations.

• Mitochondria

  • Structure: Double-membrane organelle; consists of a smooth outer membrane, a highly convoluted inner membrane folded into cristae (maximizing surface area), an intermembrane space, and an internal mitochondrial matrix containing circular DNA and autonomous ribosomes.
  • Function: Primary site of cellular respiration; oxidizes fuel molecules to generate ATP via the Krebs cycle and oxidative phosphorylation.

• Chloroplasts

  • Structure: Double-membrane photosynthetic organelle containing an internal fluid called the stroma, autonomous circular DNA, ribosomes, and flattened thylakoid sacs stacked into structures called grana.
  • Function: Converts solar energy into chemical bond energy (glucose) via light reactions across the thylakoid membrane and the Calvin cycle in the stroma.

Cytoskeleton Components

• Microtubules

  • Structure: Hollow cylindrical tubes ($25\,\text{nm}$ diameter) formed from polymerized dimers of $\alpha$-tubulin and $\beta$-tubulin.
  • Function: Resist compressive forces to preserve cell shape, serve as structural tracks along which motor proteins (kinesin and dynein) transport vesicles, separate chromosomes during cell division by forming the mitotic spindle, and form the internal core of motile cilia and flagella ($9+2$ arrangement).

• Microfilaments (Actin Filaments)

  • Structure: Two intertwined strands of globular actin polymers ($7\,\text{nm}$ diameter).
  • Function: Bear tension (pulling forces) within the cell, support the cell cortex beneath the plasma membrane, facilitate muscle contraction via interactions with myosin, power amoeboid movement (pseudopodia), drive cytoplasmic streaming, and form the contractile ring during animal cell cytokinesis.

• Intermediate Filaments

  • Structure: Tough, fibrous proteins (e.g., keratins) supercoiled into durable cables ($8\,\text{nm}$ to $12\,\text{nm}$ diameter).
  • Function: Reinforce cell shape under mechanical stress, anchor the nucleus and other stable organelles in place, and form the nuclear lamina that lines the interior of the nuclear envelope; far more permanent than dynamic microtubules and microfilaments.

The Endomembrane Pathway

The sequential pathway governing the production, modification, and transport of secretory and membrane proteins occurs in six key stages:

1. Synthesis: Ribosomes bind to the rough ER; nascent polypeptides enter the RER lumen through translocons, fold into their tertiary shapes, and undergo initial glycosylation.
2. Vesicle Packaging: Processed proteins accumulate in transitional ER regions and bud off into membrane-bound transport vesicles.
3. Cis-Golgi Fusion: Transport vesicles travel along microtubule tracks and fuse with the cis face of the Golgi apparatus, releasing their luminal contents.
4. Processing and Sorting: Molecules move through the medial cisternae toward the trans face, undergoing successive carbohydrate remodeling and phosphorylation tags indicating their destination.
5. Trans-Golgi Budding: Proteins are segregated and packaged into distinct secretory or transport vesicles at the trans face.
6. Target Delivery:
   • Secretory vesicles fuse with the plasma membrane to release proteins externally via exocytosis.
   • Integral membrane proteins fuse with the plasma membrane to become part of the cell surface.
   • Lysosomal vesicles fuse with or develop into mature lysosomes containing active hydrolytic enzymes.

Surface Area-to-Volume Ratio ($\text{SA:V}$ Ratio)

• Biological Significance

  • All exchange of nutrients, dissolved gases, and metabolic wastes occurs across the plasma membrane, while metabolic consumption and waste generation occur within the volume.
  • As cell dimensions increase, volume ($V$) increases proportionally to the cube of the radius ($r^3$) or side length ($s^3$), while surface area ($\text{SA}$) only increases proportionally to the square ($r^2$ or $s^2$).
  • High $\text{SA:V}$ Ratio (Small Cells): The membrane has ample surface area to support the cell's metabolic intake and waste expulsion via diffusion, maintaining cellular efficiency.
  • Low $\text{SA:V}$ Ratio (Large Cells): The interior volume's metabolic demands surpass the diffusion capacity of the membrane surface; limits maximum cell size and necessitates cell division, membrane folding (e.g., microvilli), or flattened cell shapes.

• Calculation Methods for a Cube

  • Surface Area:
    $\text{SA} = 6s^2$
  • Volume:
    $V = s^3$
  • Ratio:
    $\frac{\text{SA}}{V} = \frac{6s^2}{s^3} = \frac{6}{s}$

• Calculation Methods for a Sphere

  • Surface Area:
    $\text{SA} = 4\pi r^2$
  • Volume:
    $V = \frac{4}{3}\pi r^3$
  • Ratio:
    $\frac{\text{SA}}{V} = \frac{4\pi r^2}{\frac{4}{3}\pi r^3} = \frac{3}{r}$