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Chapter 3

BIOL 1520 Chapter Notes - Chapter 3: Autocrine Signalling, Paracrine Signalling, Plasmodesma

Course Code
BIOL 1520

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Principle of Chemical Signaling and Communication by
Communication between and within cells
Cellular communication: communication between cells is called intercellular signaling, and
communication within a cell is called intracellular signaling
Ligands chemical signals between cells
Receptor a protein in or on the target cell
Specificity in cell signaling occurs in a couple different ways:
1. Ligands and receptors are highly specific; a specific ligand will have a specific receptor that
typically binds only that ligand
2. Not all cells have receptors for each ligand, so that only cells that have the receptor are capable
of detecting and responding to the signal
Categories of Cell Signaling
direct, autocrine, paracrine, endocrine, and pheromone
Direct signaling involves communication between cells that are in direct contact with each other. This
communication is often mediated by gap junctions in animal cells and plasmodesmata in plant cells
Autocrine occurs when a ligand acts on the same cell that releases it
Paracrine - occurs when a ligand diffuses in a small area and only acts on neighboring cells
Endocrine hooes, hih ae aied thoughout the ogais’s od ia the asula sste to act
on cells that may be very far away from the cells which released the ligand. Neuroendocrine signals are a
specialized subcategory of endocrine signals, and are released by neurons but that travel via the vascular
system to act on cells that are far away.
Pheromones released into the environment to act on the cells in a different individual
Functions and Types of Hormones
A molecule is a hormone if it:
Is secreted from a cell or gland into the vascular system
Acts on distant cells in other locations in the body
Causes large effects even with only small amounts of the molecule
Causes a response only in specific target cells
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Causes a characteristic response
Is part of a feedback loop
Hormones regulate many different functions, including homeostasis, development, reproduction, and
Four general structural classes of hormones:
Peptide hormones are small proteins
Amino acid-derived hormones are modified amino acids
Steroid hormones are small organic compounds with characteristic carbon ring structures
Gas hormones are gases capable of acting like ligands
Peptide and amino acid-deied hooes ted to e hdophili ad thus aot oss the ell’s
hydrophobic plasma membrane. Thus these hooes id to eeptos o the ell’s sufae.
In contrast, steroid and gas hormones are both able to cross the plasma membrane because they are small
and nonpolar; their receptors are located within the cell.
Hormone Signaling Pathways
Internal receptors:
These are found in the cytoplasm of the cell and respond to hydrophobic ligand molecules that
are able to travel across the plasma membrane
The hormone binds to its receptor which causes the receptor to change shape, allowing the
receptor-hormone complex to enter the nucleus and cause changes in gene expression.
Hormone binding exposes regions of the receptor that have DNA-binding activity, meaning they
can attach to specific sequences of DNA
These sequences are found next to certain genes in the DNA of the cell, and when the receptor
binds next to these genes, it alters their levels of transcription
Cell-surface receptors/transmembrane receptors:
Membrane-anchored proteins that bind to external ligand molecules
Necessary for ligands that cannot cross the plasma membrane
Spans and plasma membrane and performs signal transduction, or the process of converting an
extracellular signal into an intercellular signal
The receptor does not alter gene expression directly, but must activate chemical/protein
esseges to ela the sigal ito the ell
A chain of events: a signaling pathway or signaling cascade
Pathways activated by cell surface receptors include synthesis of second messengers such as
calcium or cyclic AMP which propagate throughout the cell to spread the signal, or initiation of a
phosphorylation cascade where a series of proteins are activated by having a phosphate group
added to them, which changes their activity.
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Signal amplification the sigal fo the hooe is aplified o agified ia oe of seeal
a) For hormones that bind to cell-surface receptors, the amplification can occur via second
messengers, where thousands of molecules are produced or released in response to the hormone
b) For hormones that bind to intracellular receptors, the amplification can occur during the process
of both transcription, where hundreds of copies of mRNA are synthesized from a single gene, and
during translation, where hundreds of copies of each protein are synthesized from a single mRNA
Responding to the Signal
There are many different types of cellular responses to a hormone, including:
o Changes in gene expression
o Changes in cell metabolism
o Cell growth and division
Positive feedback when the hormone amplifies the signal and increases the response
Negative feedback when the hormone decreases the signal and the response
There can be differences between different cell types and in different conditions for a number of reasons:
The same ligand can cause different responses in different cell types due to differences in protein
expression in the different cells, where the same signal activates different signaling pathways,
leading to a different response in each cell type
The same ligand can cause different responses in different cell types due to different receptors in
the two different cell types, which then activate different signaling pathways, leading to different
responses in each cell type
Often multiple signaling pathways interact with each other because the same signaling proteins
are involved in each pathway. As a result, if two different signals come in at the same time, the
outcome can be different than if they came in separately. The interaction between different
signaling pathways is called signaling crosstalk.
Hormone-response elements sites o a ell’s DNA that epeiee additioal tasiptio i the
presence of steroids
Signaling in Single-Celled Organisms
Quorum sensing a phenomenon where individual bacteria monitor the density of their population; once
the population reaches a specific density, the bacteria collectively change their gene expression and
behavior at the same time
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