Bio Lecture 01/28

Overview of Gradients in Cells

• Definition of Gradient: A gradient refers to a difference in concentration across a membrane, which is essential for various cellular processes.

• Types of Gradients:

  • Chemical Gradient: Refers to differences in solute concentrations (e.g., ion distribution inside vs. outside the cell).

  • Electrical Gradient: Involves differences in charge across a membrane (explored in greater detail in subsequent courses).

  • Electrochemical Gradient: Combines both chemical and electrical gradients.

Cell B Analysis

• Assessment of Gradients: Observations suggest that cell B has similar concentrations of stars inside and outside, indicating no significant gradient.

• Movement Across Membrane: Facilitation of movement through newly formed channels indicates a process called facilitated diffusion.

Facilitated Diffusion

• Process: Molecules move down their concentration gradient through proteins in the membrane, called channel proteins or carrier proteins.

• Carrier Proteins vs. Channel Proteins:

  • Carrier proteins undergo conformational changes to transport molecules.

  • Channel proteins act as pores facilitating passive transport without changing shape.

• Energy Requirement: Neither carrier nor channel proteins use energy since they transport molecules down their concentration gradients.

Active Transport vs. Facilitated Diffusion

• Active Transport: Requires energy to move substances against their concentration gradient (e.g., sodium-potassium pump). This process often involves carrier proteins.

• Distinctions: Understanding the difference between active transport and passive transport, like facilitated diffusion, is critical for grasping cellular transport mechanisms.

Importance of Signal Sequences in Protein Targeting

• Protein Synthesis Location: In eukaryotic cells, proteins are synthesized in the cytoplasm, specifically by ribosomes.

• Signal Sequences: Proteins that are destined to go to specific locations (like the ER or nucleus) contain signal sequences, which serve as "zip codes" regarding their destination.

• Default Pathway: Proteins without a signal sequence remain in the cytoplasm, while those with a signal are sorted to their respective locations.

• Signal Recognition Particle (SRP): Binds to the nascent signal sequence, halting translation until it interacts with the SRP receptor on the ER membrane. Once bound, translation resumes, facilitating entry into the ER.

Pathway and Mechanisms of Membrane Proteins

• Translation Process:

  1. Ribosome Translation: Ribosome starts translating mRNA, producing the protein.

  2. Signal Sequence Exposure: The signal sequence emerges as translation proceeds and binds to the SRP.

  3. Binding to ER: SRP binds to its receptor on the ER membrane, stopping translation.

  4. Protein Entering ER: Once trafficking begins, translation resumes as the rest of the protein enters the ER through a translocon.

  5. Cleavage of Signal Sequence: The signal peptide is often cleaved off during or after translation.

Protein Targeting and Structural Function

• Different Targeting Signals: Each protein's signal sequence varies depending on its destination (e.g., ER, nucleus).

• Pulse-Chase Experiment Overview:

  • Purpose: To track the location of synthesized proteins over time.

  • Pulse Phase: Introduce a tagged amino acid to mark proteins during synthesis.

  • Chase Phase: Flush the system with unlabeled amino acids to track where labeled proteins go without interference.

• Expected Outcomes: Helps demonstrate sequential transport: from rough ER, to Golgi, to secretory vesicles.

Final Thoughts on Protein Targeting

• Understanding Trafficking: Comprehension of protein synthesis, targeting sequences, and the pulse-chase experiment is pivotal for understanding cell biology and membrane dynamics.