Chemical Signaling and Communication by Microbes Flashcards
Types of Communication
- Intercellular signaling (between cells)
- Intracellular signaling (within a cell)
Ligands
chemical signals sent between cells; a molecule that binds another specific molecule
- ligand binds to a receptor
- receptor is a protein IN or ON the target cell
- EX: hormones + neurotransmitters
Specificity in Cell Signaling
- ligands and receptors are highly specific in the ways they bind; each receptor typically only can bind to one specific ligand
- not all cells have receptors for each ligand, meaning only cells that have the receptor are able to detect/respond to a particular ligand
Categories of Cell Signaling
- Direct
- Autocrine
- Paracrine
- Pheromones
- Endocrine
Direct Signaling
AKA juxtracrine signaling; involves communication between cells that are in direct contact with each other
- often mediated by gap junctions in animal cells and plasmodesmata in plant cells
Autocrine Signaling
occurs when a ligand acts on the same cell that releases it
Paracrine Signaling
occurs when a ligand diffuses in a small area and only acts on neighboring cells
- NEURAL signals are a specialized subset, diffusing a short distance between adjacent neurons
Endocrine Signaling
HORMONES; carried throughout an organism’s body via the vascular system to act on cells that may be very far away from the cells that released the ligand
- NEUROENDOCRINE signals are a specialized subset, released by neurons to travel the vascular system to act on far away cells
Pheromones
released into the environment to act on the cells in a different individual
Hormone
- regulate homeostasis, development, reproduction, and stress
- function in cell signaling in all multicellular organisms and single-celled organisms
- hormones are defined by shared FUNCTION rather than shared structure
Hormone Classification
- is secreted from a cell or gland into the vascular system (or into the environment, in the case of single-celled organisms)
- acts on distant cells in other locations in the body which occurs because the hormone travels through the circulatory system (or environment)
- causes large effects even with only small amounts of the molecule which occurs due to signal amplification
- causes a response only in specific target cells even though they come into contact with many different cell types which occurs because only those specific target cells possess a receptor that is capable of binding to that particular hormone
- causes a characteristic response (always the same response in a given set of circumstances) which occurs because activation of the receptor by the hormone initiates a specific signaling pathway in response to the hormone
- is part of a feedback loop (either positive or negative)
Hormones: General Structures
- Hydrophilic
- Hydrophobic
Hydrophilic Hormones
PEPTIDE HORMONES (small proteins) + AMINO ACID-DERIVED HORMONES (modified amino acids)
- soluble in water/aqueous solution
- cannot cross the cell’s hydrophobic PM
- the receptor for these hormones must be present on the cell’s surface for the hormone to reach it
Hydrophobic Hormones
STEROID HORMONES (small organic compounds with characteristic carbon ring) + GAS HORMONES (gases capable of acting like ligands)
- typically small, nonpolar molecules
- insoluble in water
- they can easily cross the cell’s hydrophobic PM
- the receptor for these hormones are located within a cell’s cytoplasm/nucleus
Hormone Signaling Pathway Steps
- Signal Reception
- Signal Transduction
- Signal Amplification
Pathway Step 1: General
the first step in hormone signaling is the hormone binding to the receptor
- can occur either inside or on the cell’s surface, depending on the hormone class
- different behavior depending if the ligand is hydrophobic or hydrophilic
Pathway Step 1: Hydrophobic
Nonpolar, hydrophobic ligands can travel the PM to bind to INTERNAL RECEPTORS (intracellular/cytoplasmic receptors) found in the cell’s cytoplasm
- once binded, the receptor changes shape, allowing the receptor-hormone complex to enter the nucleus (if it wasn’t there already)
- the complex is capable of binding to specific sequences of DNA and activating/repressing transcription of certain genes (the receptor requires the hormone to bind to DNA)
Pathway Step 1: Hydrophilic
cannot cross the cell’s PM and must bind to CELL-SURFACE RECEPTORS (transmembrane receptors) on the cell surface
- once the hormone binds, the receptor changes shape and activates chemical/protein “messengers” to relay the signal from outside to inside the cell
- this signal process is called SIGNAL TRANSDUCTION
- as a result, the receptor does not alter gene expression directly, but must activate chemical/protein “messengers” to rely the signal from outside to inside the cell
Signal Transduction
the process of converting a signal from one form to another
- in hormone signaling, it means converting an extracellular signal into an intracellular signal
Step 2: General
this step is necessary ONLY FOR HYDROPHILIC LIGANDS that must bind to extracellular receptors bc they cannot cross the PM
- once a hormone binds to the extracellular portion of the cell-surface receptor, the intracellular portion changes shape, resulting in activation of a chain of events called a SIGNALING PATHWAY/CASCADE
Signaling Pathway/Cascade
events in the cascade occur in a defined series of events; different specific receptors activate many different enzymes; generally, this activated enzyme then activates other proteins to carry the signal into the cell to elicit a response
- pathways activated by cell surface receptors might include either second messengers or a phosphorylation cascade
- the activation results in some type of cellular response, which may include gene expression changes
- ultimately, the activation of the pathway results in some CELLULAR RESPONSE (may include gene expression changes)
Signaling Pathway/Cascade: Second Messengers
synthesis of second messengers (non-protein signaling molecules) such as calcium/cAMP which propagate throughout the cell to spread the signal
Signaling Pathway/Cascade: Phosphorylation
initiation of a phosphorylation cascade, where a series of proteins are activated by having a phosphate group added to them, changing their activity
Step 3: Signal Amplification
one feature of hormones is that a small amount can cause a strong physiological response
- mediated through SIGNAL AMPLIFICATION, where the signal from the hormone is amplified/magnified
- different actions depending if the hormone if hydrophilic or hydrophobic
