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