Protein Synthesis, The Cell, and Cytoplasmic Organelles: Core Concepts in Anatomy & Physiology

Protein Synthesis: DNA → RNA → Protein

Overview of Protein Synthesis

Protein synthesis is the process by which cells generate proteins from genetic information. It involves two main stages: transcription (DNA to RNA) and translation (RNA to protein). This process is fundamental to gene expression and cellular function.

• Transcription: DNA is copied into messenger RNA (mRNA) by the enzyme RNA polymerase.

• Translation: Ribosomes read mRNA and synthesize a polypeptide chain (protein).

Segments of DNA and Amino Acid Sequence

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid.

• Anticodon: A sequence of three nucleotides in tRNA complementary to the mRNA codon.

• Each amino acid is represented by one or more codons; e.g., UUU and UUC both code for phenylalanine.

• Start codon: AUG (codes for methionine) signals the beginning of translation.

• Stop codons: UAA, UAG, UGA signal the end of translation.

Transcription Process

• Transcription factors bind to DNA promoter regions.

• RNA polymerase unwinds DNA and synthesizes pre-mRNA.

• Pre-mRNA is processed: introns are removed, exons are spliced together.

• Final mRNA exits the nucleus to enter the cytoplasm for translation.

Translation Process

• Ribosomes bind mRNA and facilitate the assembly of amino acids into a polypeptide chain.

• tRNA molecules bring specific amino acids to the ribosome, matching their anticodon to the mRNA codon.

• Peptide bonds form between amino acids, elongating the protein.

• Translation ends at a stop codon; the completed protein is released.

Post-Translational Modification

• Proteins may be folded, cleaved, or chemically modified after synthesis.

• Some proteins are inserted into membranes or secreted from the cell.

Example:

The mRNA codon UCC specifies the amino acid serine. The tRNA anticodon AGG pairs with UCC, bringing serine to the growing polypeptide chain.

The Cell: Structure and Function

Major Components of the Cell

Cells are the basic units of life, consisting of three main compartments: plasma membrane, cytoplasm, and nucleus.

• Plasma Membrane: Composed of a phospholipid bilayer with hydrophilic heads and hydrophobic tails. Functions include separation of environments, selective permeability, communication, and cell identification.

• Cytoplasm: Contains mostly water, dissolved solutes, organelles, and cytoskeletal elements.

• Nucleus: Surrounded by a double membrane (nuclear envelope), contains DNA and controls cell activities.

Plasma Membrane Structure and Function

• Fluid Mosaic Model: Describes the membrane as a dynamic structure with proteins and lipids that move laterally.

• Functions: Receptors, enzymes, transport, structural support, and cell linking.

• Selective Permeability: Allows certain substances to pass while restricting others.

Transport Across the Plasma Membrane

• Diffusion: Movement of molecules from high to low concentration. Driven by kinetic energy.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Facilitated Diffusion: Movement of polar solutes via membrane proteins.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (ATP).

Key Equations:

• Osmotic Pressure: $\Pi = iMRT$

• Membrane Potential: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$

Types of Transport

• Primary Active Transport: Direct use of ATP (e.g., Na+/K+ pump).

• Secondary Active Transport: Indirect use of ATP via coupled transport (e.g., glucose transport with Na+).

• Symport: Two substances move in the same direction.

• Antiport: Two substances move in opposite directions.

Cytoplasmic Organelles

Membrane-Bound Organelles

• Mitochondria: "Power plants" of the cell; produce ATP via oxidative phosphorylation. Double membrane structure with inner folds (cristae).

• Endoplasmic Reticulum (ER): Large folded membrane continuous with the nuclear envelope. Smooth ER synthesizes lipids and detoxifies; Rough ER has ribosomes and synthesizes proteins.

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

• Lysosomes: Contain hydrolytic enzymes for digestion of cellular debris and pathogens.

• Peroxisomes: Break down fatty acids and synthesize certain phospholipids; produce hydrogen peroxide.

Non-Membrane Bound Organelles

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins. Can be free in cytosol or bound to ER.

• Centrosomes and Centrioles: Organize microtubules during cell division.

Cytoskeleton

• Actin Filaments: Support plasma membrane and cell movement.

• Intermediate Filaments: Provide structural stability (e.g., keratin).

• Microtubules: Form the mitotic spindle and provide tracks for organelle movement.

Vesicular Transport

• Endocytosis: Uptake of large molecules or particles into the cell.

• Exocytosis: Release of large molecules from the cell.

• Pinocytosis: "Cell drinking"; uptake of fluids and solutes.

• Phagocytosis: "Cell eating"; uptake of large particles (e.g., bacteria).

Summary Table: Major Cell Organelles and Functions

Organelle	Structure	Main Function
Mitochondria	Double membrane, inner folds (cristae)	ATP production via aerobic respiration
Endoplasmic Reticulum (ER)	Large folded membrane, rough (ribosomes) & smooth	Protein and lipid synthesis, detoxification
Golgi Apparatus	Stacked flattened sacs	Modification, sorting, and packaging of proteins/lipids
Lysosomes	Membrane-bound vesicles with enzymes	Digestion of cellular debris and pathogens
Peroxisomes	Small vesicles with oxidative enzymes	Fatty acid breakdown, peroxide production
Ribosomes	Large and small subunits, free or bound	Protein synthesis
Centrosome/Centrioles	Microtubule organizing center	Cell division, spindle formation

Additional info:

• Some details (e.g., specific diseases like Tay Sachs) were inferred for completeness.

• Equations for osmotic pressure and membrane potential added for academic context.