Cell Communication
Overview: The Cellular Internet
Cell-to-cell communication is absolutely essential for multicellular organisms
Nerve cells must communicate pain signals to muscle cells (stimulus) in order for muscle cells to initiate a response to pain
Biologists have discovered some universal mechanisms of cellular regulation
External Signals
Signal Transduction Pathway
Yeast cells identify their mates by cell signaling (early evidence of signaling) factor
Receptor
Exchange of mating factors. Each cell type secretes a mating factor that binds to receptors on the other cell type.
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Mating. Binding of the factors to receptors induces changes in the cells that lead to their fusion. New a/ cell. The nucleus of the fused cell includes all the genes from the a and a cells.
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3
factorYeast cell,mating type a
Yeast cell,mating type
a/
a
a
Hello tiger, go back to the previous slide to answer # 2 (part 2) question!
Signal Transduction Pathways
Convert signals on a cell’s surface into cellular responses
Are similar in microbes and mammals, suggesting an early origin
Cells in a multicellular organism (tissues, organs, systems) communicate via chemical messengers
A hormone is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism
All multicellular organisms produce hormones
Plant hormones are also called phytohormones
Hormones in animals are often transported in the blood
Local and Long-Distance Signaling
Animal and plant cellsHave cell junctions that directly connect the cytoplasm of adjacent cells
Plasma membranes
Plasmodesmatabetween plant cells
Gap junctionsbetween animal cells
Figure 11.3(a) Cell junctions. Both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes.
Figure 11.3(b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces.
In local signaling, animal cellsMay communicate via direct contact
In other cases, animal cellsCommunicate using local regulators
(a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid.
(b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell.
Local regulator diffuses through extracellular fluid
Target cell
Secretoryvesicle
Electrical signalalong nerve celltriggers release ofneurotransmitter
Neurotransmitter diffuses across
synapse
Target cellis stimulated
Local signaling
In long-distance signalingBoth plants and animals use hormones (e.g. Insulin)
Hormone travelsin bloodstreamto target cells
(c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells.
Long-distance signaling
Bloodvessel
Targetcell
Endocrine cell
Figure 11.4 C
Earl W. SutherlandDiscovered how the hormone epinephrine acts on cells
The Three Stages of Cell Signaling
Sutherland’s Three Steps
Sutherland suggested that cells receiving signals went through three processesReceptionTransductionResponse
EXTRACELLULARFLUID
Receptor
Signal molecule
Relay molecules in a signal transduction pathway
Plasma membraneCYTOPLASM
Activationof cellularresponse
Figure 11.5
Overview of cell signaling
Reception1 Transduction2 Response3
Step One - Reception
Reception occurs when a signal molecule binds to a receptor protein, causing it to change shape
Receptor protein is on the cell surface
The binding between signal molecule (ligand) and receptor is highly specific
A conformational change in a receptor is often the initial transduction of the signal
Step Two - TransductionThe binding of the signal molecule
alters the receptor protein in some way
The signal usually starts a cascade of reactions known as a signal transduction pathway
Multistep pathways can amplify a signal
Step Three - ResponseCell signaling leads to regulation of cytoplasmic activities or transcription
Signaling pathways regulate a variety of cellular activities
Hormone(testosterone)
EXTRACELLULARFLUID
Receptorprotein
DNA
mRNA
NUCLEUS
CYTOPLASM
Plasmamembrane
Hormone-receptorcomplex
New protein
Figure 11.6
Example of PathwaySteroid hormones bind to intracellular
receptors1 The steroid
hormone testosterone passes through the plasma membrane.
The bound proteinstimulates thetranscription ofthe gene into mRNA.
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The mRNA istranslated into aspecific protein.
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Testosterone bindsto a receptor proteinin the cytoplasm,activating it.
2
The hormone-receptor complexenters the nucleusand binds to specific genes.
3
Other pathways regulate genes by activating transcription factors that turn genes on or off
Reception
Transduction
Response
mRNANUCLEUS
Gene
P
Activetranscriptionfactor
Inactivetranscriptionfactor
DNA
Phosphorylationcascade
CYTOPLASM
Receptor
Growth factor
Figure 11.14
Signal response is terminated quickly by the reversal of ligand binding
Termination of the Signal
There are three main types of membrane receptors:G-protein-linkedTyrosine kinasesIon channel
Receptors in the Plasma Membrane
G-protein-linked receptors
G-protein-linkedReceptor
Plasma Membrane
EnzymeG-protein(inactive)CYTOPLASM
Cellular response
Activatedenzyme
ActivatedReceptor
Signal molecule Inactivateenzyme
Segment thatinteracts withG proteins
GDP
GDP
GTP
GTP
P i
Signal-binding site
Figure 11.7
GDP
Receptor tyrosine kinases
Signalmolecule
Signal-binding site
CYTOPLASM
Tyrosines
Signal moleculeHelix in the
Membrane
Tyr
Tyr
Tyr
Tyr
Tyr
TyrTyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
TyrTyr
Tyr
Tyr
Tyr Tyr
Tyr
TyrTyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
DimerReceptor tyrosinekinase proteins(inactive monomers)
PP
PP
P
P Tyr
TyrTyr
Tyr
Tyr
TyrP
P
P
P
P
PCellularresponse 1
Inactiverelay proteins
Activatedrelay proteins
Cellularresponse 2
Activated tyrosine-kinase regions(unphosphorylateddimer)
Fully activated receptortyrosine-kinase(phosphorylateddimer)
6 ATP 6 ADP
Figure 11.7
Ion channel receptors
Cellularresponse
Gate open
Gate close
Ligand-gatedion channel receptor
Plasma Membrane
Signalmolecule(ligand)
Figure 11.7
Gate closed Ions
Organisms detect changes in their environment and respond to these changes in a variety of ways.
These changes may occur at the cellular or organism level
Feedback Mechanism
These have evolved in living things as a mechanism by which they maintain homeostasis or dynamic equilibrium. It occurs when the level of one substance influences the level of another substance or activity of another organ.
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Feedback Mechanism
An example of a feedback mechanism in humans would be the increase in heart rate and respiratory rate which occurs in response to increased exercise or other increased muscle cell activity.
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examples of feedback
mechanisms
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examples of feedback
mechanisms
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The pancreas is an endocrine gland which produces hormones which
regulate blood glucose (sugar) levelsAn increase in blood sugar level
triggers the release of the hormone insulin by the pancreas
the hormone insulin lowers blood sugar level restoring the body to its original
blood glucose level in two major ways:it increases the ability of body cells to
take in glucose from the bloodit converts blood glucose to the
compound glycogen -- this compound is also called animal starch and is
stored in our liver and muscles
Maintenance of Water : plants need to regulate water loss and carbon dioxide intake for photosynthesis and other life activitieswhen plants do not keep enough water in their cells, they wilt and die.Stomate: a microscopic hole in a plant leaf which allows gases to enter and leave and water vapor to leave as well. Stomata is the plural of stomate.Guard cells: open and close the stomate.the ability of the guard cell to close during periods of limited water availability for the plant allows the plant to maintain water homeostasis 32
Negative feedback occurs when the rate of the process decreases as the concentration of the product increases. It controls the rate of a process to avoid accumulation of a product.
Positive feedback occurs when the rate of a process increases as the concentration of the product increases. The rate of a process will continuously accelerate under positive feedback as long as substrate is available and the product is not consumed by some other process.
Positive and Negative Feedback
video
The central nervous system can directly release hormones, or it can signal tissues throughout the body to release hormones to provide rapid, short term communication between different body regions.
Hormones can stimulate nervous activity and the release of hormones that can stimulate the parasympathetic nervous system without any input from the brain. They act more slowly but generally have a longer effect.
Hormonal Communication
video
Timing and coordination of physiological events are regulated by multiple mechanisms.
What are circadian rhythms?They are physical, mental and behavioral
changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment.
They are found in most living things, including animals, plants and many tiny microbes.
They are produced by natural factors within the body, but they are also affected by signals from the environment. Light is the main cue influencing circadian rhythms, turning on or turning off genes that control an organism’s internal clocks.
How do circadian rhythms affect body function and health?
They can influence sleep-wake cycles, hormone release, body temperature and other important bodily functions. They have been linked to various sleep disorders, such as insomnia. Abnormal circadian rhythms have also been associated with obesity, diabetes, depression, bipolar disorder and seasonal affective disorder.
How are circadian rhythms related to jet lag?
Jet lag occurs when travelers suffer from disrupted circadian rhythms. When you pass through different time zones, your body’s clock will be different from your wristwatch. For example, if you fly in an airplane from California to New York, you “lose” 3 hours of time. So when you wake up at 7:00 a.m., your body still thinks it’s 4:00 a.m., making you feel groggy and disoriented. Your body’s clock will eventually reset itself, but this often takes a few days.
Circadian clocks in plantsare endogenous timekeepers that keep
plant responses synchronized with the environment. They must continue to run:
in absence of external inputs must be about 24 hours in durationcan be reset or entrainedcan compensate for temperature
differences
In plants, physiological events involve interactions between environmental stimuli and internal molecular signals.
Plants and LightPlants have three basic responses or reactions to light. They are:
photosynthesis Phototropism photoperiodism
Plants and LightPhotosynthesis is the process on which all
life on earth depends. Radiant energy from the sun is converted
into chemical energy. The energy is stored in chemical bonds in
sugars like glucose and fructose.
Plants and LightPhototropism is the plant's movement in
response to light. All of us have seen the houseplant that leans toward the window. That is phototropism.
Growth hormones are produced which cause the stem cells on the side away from the light to multiply causing the stem to tilt.
The leaves are then closer to the light source and aligned to intercept the most light.
Plants and LightPhotoperiodism is the plant's reaction to
dark, and it is controlled by the phytochrome pigment in the leaves.
The pigment shifts between two forms based on whether it receives more red or far red light.
The reaction controls several different plant reactions including seed germination, stem elongation, dormancy, and blooming in day length sensitive plants.
Plant Hormones: Auxin: causes stem elongation and growth
- formation of adventitious and lateral roots, inhibits leaf loss, promotes cell division (with cytokinins), increases ethylene production, enforces dormancy of lateral buds produced by shoot apical meristems and other immature parts
Plant Hormones: Cytokinins: stimulate cell division (with
auxin), promote chloroplast development, delay leaf aging, promote formation of buds, inhibit formation of lateral roots produced by root apical meristems and immature fruits
Plant HormonesGibberellins: promote stem elongation,
stimulate enzyme production in germinating seeds produced by roots and shoot tips, young leaves, seeds
Ethylene: controls shedding of leaves, flowers, fruits, promotes fruit ripening produced by apical meristems, leaf nodes, aging flowers, ripening fruit