Campbell Biology in Focus Study Notes
Overview
Lecture presentations by Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University.
Second Edition © 2016 Pearson Education, Inc.
Concept 5.6: The Plasma Membrane and Cell Signaling
The plasma membrane plays a critical role in most cell signaling, acting as a selective barrier and housing key receptor proteins.
In multicellular organisms, cell-to-cell communication is essential for coordinating cellular activities, growth, differentiation, and overall organismal function.
Communication can occur at both local (short distance) via direct contact or local regulators, and long distances through hormones.
Unicellular organisms also rely on communication between cells for various functions like quorum sensing, nutrient foraging, and mating.
Local Signaling
Direct Contact Communication
Eukaryotic cells may communicate via direct contact, allowing for immediate signal transmission between adjacent cells.
Animal and plant cells feature specialized junctions that physically connect adjacent cells' cytoplasms:
Gap junctions (in animal cells): Form direct cytoplasmic channels between adjacent cells, allowing small molecules and ions to pass.
Plasmodesmata (in plant cells): Channels through cell walls that connect the cytoplasm of adjacent plant cells, facilitating the flow of water, solutes, and macromolecules.
This direct connection allows free passage of substances in the cytosol, facilitating rapid and efficient local signaling by enabling signals to move directly from one cell to another.
Local signaling specifically refers to short-distance communication between cells, often within the same tissue or tightly clustered group of cells.
Messenger Molecules
In many instances of local signaling, signaling cells secrete messenger molecules known as local regulators, which diffuse short distances to target cells.
Local regulators typically travel only short distances to exert their effects on nearby cells, not entering the bloodstream for general circulation.
Growth factors represent a significant class of local regulators that stimulate adjacent cells to grow and divide, a process known as paracrine signaling in animals. This is crucial for tissue repair and development.
Synaptic Signaling
A specialized and rapid form of local signaling found exclusively within the nervous system is called synaptic signaling.
An electrical signal (action potential) travels along a nerve cell (neuron) and, upon reaching the synapse, triggers the secretion of chemical messenger molecules called neurotransmitters.
Neurotransmitters diffuse rapidly across the narrow synapse (the junction between the neuron and its target cell) and bind to specific receptor proteins on the target cell, eliciting a rapid and precise response.
Long-Distance Signaling
In both plants and animals, long-distance signaling primarily involves hormones, which are chemical messengers transported over greater distances.
Hormonal signaling in animals, specifically referred to as endocrine signaling, involves specialized endocrine cells releasing hormones directly into the circulatory system. These hormones travel via the bloodstream to reach target cells throughout the body, often at very low concentrations.
Hormones are diverse in their chemical nature, varying significantly in size and shape (e.g., steroids, peptides, amino acid derivatives), which influences their transport and receptor binding specificities.
The Three Stages of Cell Signaling
Earl W. Sutherland, through his pioneering work on hormone detection (e.g., epinephrine's effect on liver cells), identified three key processes fundamental to all cell signaling pathways:
Reception: The target cell's detection of a signaling molecule (ligand) that binds to a receptor protein.
Transduction: The conversion of the signal into a form that can bring about a specific cellular response. This often involves a cascade of biochemical reactions.
Response: The specific cellular activity or change triggered by the transduced signal.
Reception
Reception involves the highly specific binding of a signaling molecule (ligand) to a complementary receptor protein, much like a key fitting into a lock. This specificity ensures that only certain cells respond to certain signals.
Ligand binding typically induces a conformational change (shape change) in the receptor protein, which is the initial step in activating the receptor and initiating the signaling pathway.
Many receptors are membrane proteins, located on the cell surface, for ligands that are hydrophilic and cannot easily cross the plasma membrane. Other receptors are intracellular, for ligands that can pass through the membrane.
Types of Membrane Receptors
G Protein-Coupled Receptors (GPCRs)
These receptors are a large family of cell-surface transmembrane receptors that work with G proteins, which are peripheral membrane proteins.
G proteins function as on/off switches: when GDP (guanosine diphosphate) is bound to the G protein, it is inactive; when GTP (guanosine triphosphate) replaces GDP, the G protein becomes active.
Upon ligand binding, the GPCR changes conformation and activates an associated G protein, which then moves horizontally across the membrane to activate an effector enzyme, initiating a signal transduction pathway.
GPCRs are involved in a wide variety of cellular processes, including vision, smell, and regulation of mood and immune system activity.
Ligand-Gated Ion Channels
These act as membrane receptors that contain a gate, which opens or closes in response to the binding of a specific signaling molecule (ligand).
When a ligand binds to the receptor, the gate opens, allowing specific ions (such as Na^{+}, K^{+}, or Ca^{2+}) to flow across the plasma membrane down their electrochemical gradient.
This rapid influx or efflux of ions changes the membrane potential of the cell (e.g., depolarization or hyperpolarization) or alters the intracellular concentration of specific ions, thereby triggering a rapid cellular response, such as a nerve impulse or muscle contraction.
These channels are crucial in the nervous system for neurotransmission.