Cell Structure and Function: Ch 4

Cellular Membranes and the Phospholipid Bilayer

Structure and Function of Cellular Membranes

The cell membrane is a fundamental structure in all cells, primarily composed of a phospholipid bilayer. This bilayer forms a selective barrier, regulating the movement of substances into and out of the cell.

• Phospholipid Bilayer: Consists of two layers of phospholipids with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Function: Maintains cellular integrity, enables communication, and controls transport.

• Embedded Proteins: Integral and peripheral proteins assist in transport, signaling, and structural support.

Cellular Organelles and Their Functions

Major Organelles in Eukaryotic Cells

Organelles are specialized structures within cells that perform distinct functions necessary for cellular life.

• Ribosomes: Sites of protein synthesis, translating RNA into amino acid sequences. Two types:

  • Bound Ribosomes: Attached to the Rough Endoplasmic Reticulum (ER); synthesize proteins for export or membrane insertion.

  • Free Ribosomes: Float in the cytosol; synthesize proteins for use within the cell.

• Vacuoles: Large, membrane-bound sacs for storage; more prominent in plant cells and often larger and longer-lasting than vesicles.

• Vesicles: Small, membrane-bound sacs primarily used for transport; designed to merge with other membranes to deliver contents.

• Lysosomes: Contain digestive enzymes to break down cellular waste, old organelles, and ingested materials (e.g., bacteria, viruses). The term "lyse" means to break apart.

• Peroxisomes: Involved in detoxification; manage hydrogen peroxide chemistry to safely break down harmful substances.

• Mitochondria: Known as the "powerhouse of the cell"; site of ATP generation through cellular respiration.

• Chloroplasts: Found in plant cells; site of photosynthesis, converting sunlight into glucose.

• Nucleus: Stores genetic material (DNA); controls cellular activities.

• Nucleolus: Located within the nucleus; responsible for manufacturing ribosomes.

• Endomembrane System: Includes the ER, Golgi apparatus, vesicles, and lysosomes; coordinates synthesis, modification, and transport of cellular products.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins and acts as a membrane factory.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis, toxin processing, carbohydrate metabolism, and calcium ion storage.

• Golgi Apparatus: Modifies, sorts, and packages proteins for secretion or delivery to other organelles.

• Cytoplasm: The cellular medium in which organelles are suspended.

• Cytoskeleton: Provides structural support and enables movement within and outside the cell.

Gene Expression and Protein Targeting

Pathways of Protein Synthesis and Export

Proteins are synthesized according to genetic instructions and may be destined for export or for use within the cell. The pathway depends on the protein's final destination.

• Exported Proteins: Proteins that are secreted from the cell follow this pathway:

  1. DNA is transcribed into mRNA.

  2. mRNA is translated by bound ribosomes in the Rough ER.

  3. Protein enters the ER lumen and is packaged into a transport vesicle.

  4. Vesicle moves to the Golgi apparatus for modification and sorting.

  5. Protein is repackaged into a transport vesicle.

  6. Vesicle fuses with the cell membrane, releasing the protein outside the cell.

• Proteins for Cytosolic Use: Proteins that function within the cytosol follow this pathway:

  1. DNA is transcribed into mRNA.

  2. mRNA is translated by free ribosomes in the cytosol.

  3. Protein remains in the cytosol for cellular functions.

Endosymbiotic Theory: Origin of Mitochondria and Chloroplasts

Evidence Supporting the Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly independent prokaryotic organisms that were engulfed by ancestral eukaryotic cells.

• Double Membranes: Both organelles have double membranes, consistent with engulfment by a host cell.

• Own DNA: Mitochondria and chloroplasts contain their own circular DNA, similar to bacterial genomes.

• Independent Reproduction: These organelles can grow and reproduce independently within the cell by binary fission.

• Ribosomes: Their ribosomes resemble those of bacteria, not eukaryotes.

• Genetic Similarity: DNA sequences are closely related to certain bacteria (e.g., mitochondria to proteobacteria, chloroplasts to cyanobacteria).

Summary Table: Key Organelles and Their Functions

Organelle	Main Function	Additional Notes
Ribosome	Protein synthesis	Bound (Rough ER) vs. Free (cytosol)
Vacuole	Storage	Larger in plant cells
Vesicle	Transport	Merges with membranes
Lysosome	Digestion	Contains enzymes
Peroxisome	Detoxification	Manages hydrogen peroxide
Mitochondria	ATP generation	Cellular respiration
Chloroplast	Photosynthesis	Glucose from sunlight
Nucleus	DNA storage	Controls cell activities
Nucleolus	Ribosome production	Inside nucleus
Rough ER	Protein synthesis	Membrane factory
Smooth ER	Lipid synthesis	Toxin processing, Ca2+ storage
Golgi Apparatus	Protein modification	Shipping and handling
Cytoskeleton	Structure & movement	Internal and external

Key Equations

• Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• ATP Generation (Cellular Respiration):

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

• Photosynthesis (Chloroplasts):

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Additional info: Some details about the endosymbiotic theory and organelle functions were expanded for clarity and completeness.