Chapter 3: The Cell

Diffusion and Membrane Transport

Diffusion is a fundamental process by which molecules move in and out of cells, essential for maintaining cellular homeostasis. Understanding diffusion and related transport mechanisms is crucial for grasping how cells exchange substances with their environment.

• Definition of Diffusion: Diffusion is the movement of solute molecules from an area of higher concentration to an area of lower concentration, driven by the concentration gradient.

• Facilitated Diffusion: In facilitated diffusion, substances move across the cell membrane with the help of membrane proteins, such as channel or carrier proteins. This process does not require energy and is used for molecules that cannot freely cross the lipid bilayer.

• Example: Glucose and ions often use facilitated diffusion to enter or exit cells via specific membrane proteins.

• Key Terms:

  • Solute: The substance dissolved in a solution.

  • Solvent: The liquid in which the solute is dissolved, typically water in biological systems.

  • Concentration Gradient: The difference in concentration of a substance between two regions.

Additional info: Osmosis is a related process involving the movement of solvent (water) across a semipermeable membrane from an area of lower solute concentration to higher solute concentration.

Protein Synthesis and Gene Expression

Protein synthesis is the process by which cells build proteins, following instructions encoded in DNA. This process involves two main stages: transcription and translation, and is central to gene expression.

• DNA and Protein Synthesis: DNA contains the genetic information required to make all proteins in the body. This information is stored as sequences of nucleotides that specify the amino acid sequence of proteins.

• Transcription: During transcription, a segment of DNA unwinds and serves as a template for the synthesis of messenger RNA (mRNA). RNA polymerase is the enzyme responsible for building the complementary mRNA strand.

• Translation: The mRNA leaves the nucleus and enters the cytoplasm, where ribosomes read its sequence and synthesize a polypeptide by joining amino acids in the correct order. Transfer RNA (tRNA) brings amino acids to the ribosome, matching its anticodon to the mRNA codon.

• Gene Expression: Gene expression refers to the process by which the information in a gene is used to produce a functional product, such as a protein. This determines traits and cellular functions.

• Example: The melanocortin-1 receptor gene codes for a membrane protein in skin cells. Variants of this gene can result in different responses to ultraviolet radiation, leading to tanning or freckling.

Additional info: After translation, proteins may undergo posttranslational modifications, such as folding or chemical changes, to become fully functional.

Steps of Protein Synthesis

The synthesis of proteins from DNA involves several coordinated steps, ensuring accurate transfer of genetic information and assembly of amino acids.

1. Transcription:

   • DNA unwinds in the nucleus.

   • RNA polymerase binds to the promoter region and synthesizes a complementary mRNA strand.

   • mRNA is processed (splicing, addition of cap and tail) and exits the nucleus.

2. Translation:

   • mRNA attaches to a ribosome in the cytoplasm.

   • Ribosome reads mRNA codons; tRNA brings corresponding amino acids.

   • Peptidyl transferase forms peptide bonds between amino acids.

   • Process continues until a stop codon is reached, releasing the completed polypeptide.

3. Posttranslational Modification:

   • Polypeptide may be folded, cleaved, or chemically modified to become a functional protein.

Key Molecules in Protein Synthesis

• Messenger RNA (mRNA): Carries the genetic code from DNA to ribosomes.

• Ribosomal RNA (rRNA): Structural and enzymatic component of ribosomes.

• Transfer RNA (tRNA): Delivers amino acids to the ribosome, matching anticodon to mRNA codon.

Amino Acids and Their Abbreviations

Amino acids are the building blocks of proteins. Each has a standard abbreviation used in genetic and biochemical contexts.

Relevant Equations

• General Diffusion Equation:

• Central Dogma of Molecular Biology:

Summary Table: Diffusion vs. Facilitated Diffusion

Additional info: Meiosis is referenced but not covered in detail; it is a process of cell division that produces gametes and will be discussed in a later section.