Chapter 11: Cell Communication

Introduction to Cell Communication

• Fight-or-Flight Response: Example of impala fleeing from a cheetah demonstrates physiological functions driven by hormones from adrenal glands.

  • Rapid breathing, increased heart rate, and active leg muscles are all part of this response.

  • The process involves signal reception when an epinephrine molecule binds to a receptor on muscle cells, activating a pathway that fuels the muscles for escape.

Key Concepts in Cell Signaling

• Stages of Cell Signaling:

  • Signal Reception: Binding of a signaling molecule (ligand) to a receptor.

  • Signal Transduction: Series of molecular interactions relay the signal inside the cell.

  • Cellular Response: Activation of a cellular activity, often involving gene regulation or enzymatic action.

Types of Cell Signaling

• External Signals: Converted into cellular responses, important for communication within multicellular organisms.

• Quorum Sensing: Bacteria communicate to measure cell density and coordinate behaviors, such as forming biofilms or producing toxins.

• Cell Types in Communication:

  • Yeast (e.g., Saccharomyces cerevisiae) uses chemical signaling during reproduction via mating factors.

Mechanisms of Cell Signaling

1. External Signal Reception

• Binding Anticipation: When a signaling molecule binds to its specific receptor, it causes a conformational change initiating intracellular signaling.

2. Signal Transduction Pathways

• Relay Molecules: Proteins that transmit the signal inside the cell, often involving phosphorylation cascades.

• Protein Kinases: Enzymes that add phosphate groups to other proteins, modifying their functions.

• Second Messengers: Small molecules (e.g., cAMP, Ca²⁺) that rapidly disseminate signaling within the cytosol after a signaling event.

3. Cellular Response Activation

• Gene Expression: Signals can activate transcription factors leading to gene expression changes and regulation of cellular activities.

• Examples:

  • Epinephrine activates glycogen breakdown for energy release.

  • Other signaling pathways can regulate processes such as apoptosis.

Specific Types of Cell-Surface Receptors

1. G Protein-Coupled Receptors (GPCRs)

• Structure: Span the cell membrane and interact with G proteins.

• Mechanism: When activated by a ligand, G proteins trigger various signal transduction pathways.

2. Receptor Tyrosine Kinases (RTKs)

• Function: Enzymatic receptors that add phosphate groups to tyrosine residues in proteins, initiating multiple signaling pathways simultaneously.

3. Ion Channel Receptors

• Action: Open or close in response to ligand binding, allowing ions to flow across membranes, crucial for quick cell signaling in nerve cells.

Long-Distance Signaling: Hormonal Communication

• Endocrine Signaling: Hormones, such as adrenaline, are secreted into the bloodstream, affecting target cells throughout the body.

• Effects of Hormones: Depending on the receptors present, the same hormone can trigger various responses in different tissues (e.g., epinephrine impacting muscle versus liver cells).

Cell Signaling Pathway Integration and Regulation

• Cellular Responses to Signals: Involve intricate pathways where multiple signals can still lead to a cohesive cellular outcome.

• Regulating Responses:

  • Efficient signal propagation requires components such as scaffolding proteins, which enhance interactions among signaling molecules and maintain a coordinated response.

  • Termination mechanisms ensure pathways do not remain perpetually active, allowing for new signals to be processed effectively.

Integration of Signals: Apoptosis

• Apoptosis and Signaling Pathways: Apoptotic pathways can integrate multiple signals, emphasizing the importance of regulated cell death in development and maintenance of health.

Conclusion on Cell Communication

• The mechanisms of cell signaling are vital for responding to internal and external cues, demonstrating how cells coordinate their activities for survival and functionality in multicellular organisms.