Cell Communication

Introduction to Cell Communication

Cell communication is essential for coordinating cellular activities and responses to environmental signals. Cells use a variety of signaling mechanisms to transmit information, which can result in changes in gene expression, metabolism, or movement.

Section 11.1: External Signals and Cellular Responses

Yeast Mating Types and Signal Transduction

• Yeast mating types: Two types, a and α, fuse during sexual reproduction to form a diploid cell.

• Signal transduction pathway: A series of proteins relay a mating signal, triggering fusion. This pathway is conserved in animal cells, making yeast a model for human cell signaling.

Quorum Sensing in Bacteria

• Quorum sensing: Bacteria release signaling molecules (AHL). At high density, AHL accumulates and triggers group behaviors, such as biofilm formation or virulence.

• Application: Bacteria act collectively only when their population is sufficient to impact the environment or host.

Forms of Chemical Signaling

• Autocrine: Cell signals itself.

• Paracrine: Cell signals nearby cells.

• Endocrine: Cell signals distant cells via the bloodstream.

• Signaling across gap junctions: Direct signaling to adjacent cells.

The Endocrine System

The endocrine system produces hormones that regulate body functions. Major glands include the hypothalamus, pituitary, pineal, thyroid, thymus, adrenal, pancreas, ovaries, and testes.

Hormone Types: Fat-Soluble vs. Water-Soluble

• Fat-soluble hormones: Diffuse into cells and bind to internal receptors, often affecting gene expression directly.

• Water-soluble hormones: Bind to cell surface receptors, triggering a cascade of internal reactions (signal transduction).

Examples of Hormonal Regulation

• Pancreas: Produces insulin and glucagon to regulate blood sugar levels.

• Human Growth Hormone (HGH): Stimulates growth and development; imbalances can cause gigantism or dwarfism.

Nervous System Overview

CNS vs. PNS

• Central Nervous System (CNS): Brain and spinal cord; processes information and coordinates responses.

• Peripheral Nervous System (PNS): Somatic nerves (voluntary movement) and autonomic nerves (involuntary functions).

• Autonomic system: Parasympathetic (rest, "the brakes") and sympathetic (fight-or-flight, "the gas").

Parts of the Brain

• Cerebrum: Complex thought and voluntary movement.

• Cerebellum: Coordinates movement using sensory input.

• Brainstem: Controls automatic functions.

• Medulla: Regulates survival functions (heart rate, breathing).

• Thalamus: Relays sensory information.

• Hypothalamus: Regulates hormones and homeostasis.

Phineas Gage: Brain Injury and Personality

• Phineas Gage survived a severe brain injury, leading to personality changes and providing early evidence that brain regions control behavior and personality.

Neuron Structure and Function

Neuron Anatomy

• Cell body (soma): Contains the nucleus and organelles.

• Dendrites: Receive signals from other neurons.

• Axon: Conducts nerve impulses away from the cell body.

• Axon terminal: Transmits signals to other cells.

• Myelin sheath: Insulates axon, speeding up signal transmission.

Neuronal Action Potential

• Resting state: Membrane potential at -70 mV (Na+ outside, K+ inside).

• Depolarization: Stimulus opens Na+ channels; Na+ enters, making inside positive.

• Repolarization: K+ channels open; K+ exits, restoring negative charge.

• Hyperpolarization: Membrane potential dips below resting value.

• Na+/K+ pump: Restores ion balance using ATP: out, in$.

Synapse and Neurotransmitters

• Synapse: The gap between two neurons where neurotransmitters carry the signal.

• Neurotransmitters: Chemicals released by neurons to transmit signals to other cells.

• Drugs: Substances like caffeine, nicotine, alcohol, and THC can mimic or block neurotransmitter action, affecting brain function.

Sensory Systems

The Five Senses

• Smell: Detects odor molecules.

• Sight: Detects visible light.

• Hearing: Detects sound waves.

• Taste: Detects chemicals in food.

• Touch: Detects temperature, pressure, pain, and movement.

• Areas with more sensory nerves (e.g., lips, fingertips) are more sensitive than areas with fewer nerves (e.g., back).

Section 11.2: Reception

Reception: Ligands and Receptors

• Reception: A signaling molecule (ligand) binds to a specific receptor protein, causing it to change shape and initiate a response.

• Ligand: A molecule that specifically binds to a receptor.

Ion-Channel Receptors

• Mechanism: Ligand binds to channel protein, channel opens, ions flow down their gradient, triggering a response.

• Example: Neurotransmitters open ion channels to propagate action potentials.

G-Protein-Linked Receptors (GPCRs)

• Mechanism: Ligand binding activates receptor, which activates a G-protein by exchanging GDP for GTP. The G-protein then activates an enzyme, leading to a cellular response.

• Example: Rhodopsin in the eye is a GPCR activated by light.

Enzyme-Linked Receptors: Receptor Tyrosine Kinases (RTKs)

• Mechanism: Signal molecule binds two receptor monomers, causing dimerization and activation of the catalytic domain. The receptor then triggers a cellular response.

• Example: Growth factor receptors control cell growth and differentiation.

GPCR vs. RTK Signaling

• Both pathways activate kinases and lead to protein phosphorylation, but use different receptors and intermediate steps.

Intracellular Receptors

• Mechanism: Hormone enters the cell, binds to an internal receptor, and the complex activates gene transcription in the nucleus.

• Example: Testosterone regulates gene expression during puberty.

Section 11.3: Transduction

Signal Transduction Pathways

• Transduction: Cascades of molecular interactions relay signals from receptors to target molecules inside the cell.

Phosphorylation Cascade

• Protein kinases: Enzymes that transfer phosphate groups from ATP to proteins, activating them in a sequence.

• The final protein in the cascade initiates the cellular response.

Second Messengers

• Second messengers: Small molecules (e.g., cAMP) that relay and amplify signals inside the cell after receptor activation.

• Example: Epinephrine activates a G-protein, which activates adenylate cyclase to convert ATP to cAMP. cAMP then activates protein kinases, leading to cellular responses such as ion channel opening.

Section 11.4: Cellular Response

Nuclear Response

• Hormones can directly activate gene transcription by binding to nuclear receptors, releasing corepressors, and recruiting coactivators and RNA polymerase.

• Example: Sex hormones trigger gene expression for development.

Signal Amplification

• One ligand or second messenger can activate multiple downstream molecules, amplifying the cellular response and conserving resources.

Controlling Cell Signaling

• Cells integrate signals through multiple pathways, cross-talk, and scaffolding proteins to fine-tune responses.

Caffeine and Signal Transduction

• Caffeine blocks adenosine receptors, preventing the signaling cascade that causes tiredness, allowing target proteins to be dephosphorylated and reducing fatigue signals.

Section 11.5: Apoptosis

Programmed Cell Death

• Apoptosis: Programmed cell death, triggered by signaling pathways, resulting in cell breakdown and removal by macrophages without inflammation.

• Causes: Infection, damage, or developmental processes.

• Significance: Maintains tissue health and prevents immune response to cell contents.