Cell Communication & Homeostasis

UNIT 4.1 CELL COMMUNICATION & HOMEOSTASIS SIGNAL RECEPTION

Overview of Cell Communication

• Cell signaling is crucial for unicellular and multicellular organism survival.

  • In unicellular organisms, cell signaling dictates responses to environmental changes.

  • In multicellular organisms, it coordinates individual cell activities to support overall functional integrity.

• The similarity in signaling pathways and molecules across all organisms suggests an evolutionary origin in prokaryotes that was later adopted by eukaryotes, providing evidence for evolution.

Signal Transduction Pathways

• Signaling occurs through signal transduction pathways, which convert external signals into cellular responses.

• Cell signaling consists of three main stages:

  • Reception: Detection of signaling molecules (ligands).

  • Transduction: Transformation of the signal into a cellular response.

  • Response: The change in cellular behavior or function.

Signal Reception

• Ligands: Molecules that bind to receptor proteins on target cells, initiating a response.

• Target Cells: Cells equipped with specific receptor proteins that respond to the ligands only.

• Receptor Specificity: Only target cells with the necessary receptors can respond to a specific signal.

Mechanism of Signal Reception

• Signaling begins when a ligand binds to its receptor protein, typically non-covalently and reversibly.

• Ligand-binding domains of receptors interact specifically with chemical messengers, which can include peptides, small chemicals, or proteins.

• The binding is reversible, allowing cells to halt the response once it is no longer needed.

Classifications of Receptors

• Intracellular Receptors: Located within the cytoplasm or nucleus, binding ligands that are small or nonpolar (e.g., steroid hormones).

• Membrane Receptors: Located on the cell surface with large or polar ligands that cannot diffuse through the membrane:

  • G-Protein Linked Receptors

  • Ligand-Gated Ion Channels

Intracellular Receptors

• Activated by small or hydrophobic ligands that cross the membrane (e.g., testosterone, estrogen).

• These receptors can function as transcription factors, regulating gene expression by activating specific genes inside the cell.

Membrane Receptors: Ligand-Gated Ion Channels

• These receptors are closed channel proteins that open upon ligand binding, allowing ions to flow across the membrane, altering ion concentrations.

• Example: Acetylcholine receptors on skeletal muscle cells facilitate Na+ and Ca2+ ion influx, triggering muscle contraction.

Membrane Receptors: G-Protein Linked Receptors

• These receptors rely on G-proteins that mediate cellular signaling following ligand binding.

• G-proteins are activated upon GDP-to-GTP exchange, leading to downstream signaling events triggering specific cellular responses.

• Key trait: All G-protein linked receptors feature 7 transmembrane domains, highlighting an evolutionary commonality.

UNIT 4.1 CELL COMMUNICATION & HOMEOSTASIS SIGNAL TRANSDUCTION & CELLULAR RESPONSE

Signal Transduction

• Ligand binding initiates a signal transduction pathway, amplifying the signal through multiple steps.

• Multistep pathways efficiently regulate cellular responses, often involving phosphorylation cascades and secondary messengers.

Phosphorylation Cascades

• Phosphorylation mechanisms regulate protein activity by adding phosphate groups, making proteins active.

• A phosphorylation cascade involves a series of sequential protein activations, enhancing signal transduction efficacy.

Secondary Messengers

• These are small, hydrophilic molecules that relay signals from receptors to target molecules within the cell:

  • Examples include cyclic AMP (cAMP) and calcium ions (Ca2+).

  • cAMP activates protein kinase A, leading to phosphorylation cascades for amplified responses.

Signal Amplification

• Signal transduction pathways exhibit amplification, where a single ligand can lead to the activation of extensive signaling cascade outputs (e.g., 1 epinephrine molecule may release 100 million glucose molecules).

Signal Specificity

• Cellular response specificity is determined by the ensemble of receptors and relay proteins present, leading to different responses in different cell types to identical signals.

• Examples:

  • Liver cells break down glycogen when exposed to epinephrine while heart muscle cells increase contraction.

Cellular Response

• Cells respond to environmental signals in a multitude of ways:

  • Opening of ion channels altering ion concentration and electrical potential.

  • Changes in gene expression through transcription factor activation.

  • Regulation of enzyme activity impacting cellular metabolism.

Benefits of Signal Transduction Pathways

• They enable responses to signals that cannot cross the plasma membrane.

• Diversity in pathways allows tailored cellular responses.

• Amplification of signals ensures efficient responses, enhancing evolutionary fitness.

Signal Disruption: Anthrax

• Disruptive conditions in signal transduction can lead to harmful effects; for example, anthrax toxins increase cAMP levels, impairing proper cell signaling responses.

Signal Disruption: Diabetes

• Insulin signaling pathways can become defective, leading to impaired glucose uptake as observed in Type 1 and Type 2 diabetes.

Distances of Cell Communication

• Cells communicate through direct contact (juxtacrine) or through signaling molecules over short (autocrine and paracrine) and long distances (endocrine).

Direct Contact Communication

• Juxtacrine signaling requires direct cell contact; examples include cell junctions (plasmodesmata and gap junctions).

• Cell surface markers can serve as ligands initiating signaling between adjacent cells.

Short Distance Communication

• Local regulators target nearby cells; include autocrine signaling (self-targeting) and paracrine signaling (neighboring cell targeting).

• Specialized examples: Morphogens provide positional information during embryonic development, influencing cell differentiation based on concentration gradients.

• Quorum sensing in bacteria allows communication based on population density, affecting gene expression in unison.

Long Distance Communication

• Endocrine signaling utilizes hormones secreted into the bloodstream to relay signals across the body; many hormones are protein-based, with exceptions like steroid hormones.

• Insulin and human growth hormones exemplify key long-distance signaling molecules vital for metabolic regulation and growth, respectively.

HOMEOSTASIS & FEEDBACK MECHANISMS

Homeostasis

• Homeostasis is the capability of organisms to maintain stable internal conditions amidst external shifts.

• Defined as dynamic homeostasis due to constant regulatory interactions.

Negative Feedback Mechanisms

• Negative feedback maintains homeostasis by counterbalancing changes to a set point through a stimulus-response mechanism.

  • Example: Thermoregulation involves blood vessel dilation/constriction and sweat/or shivering responses.

Positive Feedback Mechanisms

• In contrast, positive feedback mechanisms amplify processes essential for completing specific biological functions, moving away from homeostasis temporarily.

  • Examples include childbirth, where oxytocin release promotes uterine contractions, and fruit ripening, where ethylene accelerates surrounding fruit ripening.