Download Ch. 14 Part 5

Ch. 4 Part 2
Cell Signaling & Intro to Cell Transport
Cell Signaling
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How living organisms control and coordinate their bodies
Based on getting a message from one place to another place
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Outside the cell to inside the cell
Inside the cell to outside the cell
Two Major mechanisms of signaling
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Secretion of Chemical Signals
Direct cell-cell contact
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Ex. Embryonic development and lymphocyte attacks
Cell membrane plays big role in cell signaling
– Contains protein “receptors”
Stimulus (signal) receptor  transmission of signal “signal transduction” target (effector) response
Signals
• Signals
– Produced by body
• Hormones and neurotransmitters
– Stimuli
• Light, drugs, pheromones
• Single molecules are usually very small
and diverse
– Hydrophobic, insoluble molecules
– get across membrane no problem
• Steroids
– Ex. oestrogen
– Most signal molecules are water soluble
• need to attach to cell membrane receptors to
relay signal
• Distance travelled by molecules can be
small or short
• Signals can be electrical (nervous system)
or chemical (endocrine system) events
Signal Receptor in Cell Membrane
• Transmembrane proteins
• “Receptor”
– Specific shape
– Only on certain cells
• When signal molecule attaches
to receptor on the outside of
the cell:
– conformational changes occur
to protein that spans the
membrane
– Signal from outside is
“transduced”
– Changes to the shape of the
receptor protein initiates a
“domino” effect
• Transmits the signal to another
protein inside the cell
– G-Proteins (alpha, beta,
gamma)
– G proteins activate another
molecule/protein
How Receptors Alter the Activity of a Cell
1. Activating the release of second messengers
2. Opening an ion channel
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Changes membrane potential
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Ex. Nicotine-accepting acetylcholine receptors
3. Acting directly as a membrane-bound enzyme
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Ex. Insulin receptor
4. Acting as an intracellular receptor when the initial
signal passes straight through the cell surface
membrane
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Ex. Oestrogen receptor
•
Found in nucleus and directly controls gene expression when
combined with hydrophobic oestrogen hormone
G Proteins
• Protein Family
– G Protein Coupled Receptors
(GPCRs)
• Transmembrane protein
– Guanine nucleotide-binding
proteins
• Intracellular proteins
• Alpha, beta, and gamma
• Respond diverse stimuli
– light, compounds, bioactive
lipids, cytokines, hormones, and
neurotransmitters
• Activation puts specific G proteins
into play
– Changes in G-protein structure
alters the activity of enzymatic
signaling pathways
– they bind the guanine
nucleotides GDP and GTP (that is
the “switch” that relays the
signal from outside)
• Receptor G Protein receives the signal molecule
• Changes shape to activated alpha G-protein inside the cell
membrane
• “switch”
• Activated G protein (now with GTP) moves to stimulate another
receptor molecule that releases the “second messenger” molecule
– “amplifies” original signal
– Key feature of cell signaling
Second Messenger Molecule
• Amplifies original signal
• Usually activates enzyme
that activates other
enzymes
– Increases amplification of
original signal
– “signaling cascade”
• Sequence of events
triggered by G protein
• Final enzyme activated
brings about required
change in cell metabolism
• Common Second
messagner is cAMP
– Cyclic Adenosine
Monophosphate
– Made from ATP
– Binds to Kinase Enzymes
Kinases
• Enzyme that modifies other
proteins by chemically adding
phosphate groups to them
(phosphorylation)
• Phosphorylation usually
results in a functional change
of the target protein
(substrate) by:
– changing enzyme activity
– cellular location
– association with other
proteins
How Second messengers Work
Possible Signaling Pathways in
Cells
Cells are always trying to Maintain Equilibrium by…
3 ways…
 PASSIVE Transport
 ACTIVE Transport
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2 types
Requires NO energy
Goes with Conc. Gradient
types:
 SIMPLE DIFFUSION No protein required
 Small, uncharged particles
 FACILITATED DIFFUSION CHANNEL or CARRIER proteins
 Trans-membrane protein channel
 Protein with a specific shape; open-close
mechanism
 Ex. Osmosis and ligand-gated channels
REQUIRES Energy
Goes Against Conc. Gradient
 ACTIVE TRANSPORT
 Involves “transporter”
membrane protein and
ENERGY (ATP)
 Different Types
 Gated channels
 Bulk Transport
Terms to Remember!
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Solute + solvent = solution
Solute thing being mixed in solvent
Solvent larger component doing the dissolving (water)
Concentrated= more solute than solvent (less water, hypertonic)
Dilute= more solvent than solute (more water, hypotonic)
Isotonic = equal concentrations on BOTH sides of the membrane
Tonicity – the ability of a solution to cause a cell to gain or lose
water
 Osmoregulation – the control of water balance e.g. contractile
vacuole in paramecium
Cells in Isotonic
Solution
•If the concentration
of solute (salt) is = on
both sides, there will
be no net movement
of water
•"ISO" means the
same
Cells in Hypertonic Solution
• More solute (salt)
molecules outside the cell,
which causes water to be
sucked out of the cell.
• In plant cells, the central
vacuole loses water and
the cells shrink, causing
plasmolysis resulting in the
plant wilting.
• In animal cells, the cells
also shrink.
• In both cases, the cell may
die.
• This is why it is dangerous
to drink sea water
Cells in Hypotonic Solution
• There are less solute (salt)
molecules outside the cell,
since salt sucks, water will
move into the cell.
• The cell will gain water and
grow larger. In plant cells,
the central vacuoles will fill
and the plant becomes stiff
and rigid, the cell wall keeps
the plant from bursting
• In animal cells, the cell may
be in danger of bursting,
organelles called
CONTRACTILE
VACUOLES will pump water
out of the cell to prevent
this.
Passive Transport
• Requires no energy
• Occurs due to natural concentration gradient
• Molecules move from high concentration to low concentration (DOWN
the gradient)
3 Types
 Diffusion
 Osmosis
 Facilitated Diffusion
Diffusion
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Particles constantly move
Collide randomly
Spread out randomly
Diffusion is moving from area of HIGH conc. to
area of LOW conc.
 This is what we call the CONCENTRATION
GRADIENT
What happens when we
reach equilibrium?
 Particles continue moving across membrane but in
both directions!
***No more changes in concentration
What diffuses across the
membrane???
 Small, uncharged
(non-polar) molecules
 Examples:
 Carbon dioxide
 Oxygen
Factors that influence diffusion
1.
Steepness of concentration gradient
 Greater difference in concentration =
 Greater difference b/t molecules passing in 2 directions =
 Faster net rate of diffusion
2. Temperature
 High temp. = more kinetic energy
 Move across membrane faster
 Faster net rate of diffusion
3.
Surface Area
 Greater SA = more molecules can cross at any given moment
 Faster net diffusion rate
 Folding of surface membranes increases SA
 Ex. Microvilli in digestive tract and kidneys
 Cristae in mitochondria
4.
Nature of molecules or ions
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Large molecules need MORE energy to move compared to small molecules
Large molecules diffuse slowly compared to small molecules
NON-POLAR diffuse more easily than POLAR molecules
Faster net diffusion rate for SMALL, NON-POLAR molecules
 Passive Transport
 Two types
 Passive diffusion of water
 Individual water molecules can diffuse across the
phospholipid bilayer
 Facilitated diffusion of water
 BULK FLOW of water molecules through a
transmembrane protein called an AQUAPORIN
Passive Transport: Facilitated Diffusion
Facilitated Diffusion
•diffusion of a substance
through transport proteins in
cell membrane
•Proteins provide hydrophilic
areas that allow molecules or
ions to pass through
membrane
•Channel proteins
•gated channels
•Carrier proteins
Intrinsic/Transmembrane Proteins
Channel Proteins
 FIXED shape
 Water filled pores in
membrane
 Some are created by
single protein while others
are formed by multiple
proteins
 Allow charged substances
(ions) in/out of cell
 Some are “GATED”
 Portion on the interior of
cell membrane opens/and
closes
Carrier
Proteins
 NO fixed
shape
 Shape
changes
 Binding site
opens at on
end and
closes at the
other end
 Alternates
opening and
closing at
different ends
Intrinsic Proteins
Both Carrier & Channel
Proteins…
 HIGHLY specific for one molecule
 Rates at which diffusion takes place is affected by the
number of channel or carrier proteins
 Rate of diffusion is also affected by whether channel
proteins are opened or closed
Channel Proteins
 Voltage-Gated Channels
 When membrane is at resting potential (inside of
membrane is 60 to 70 mV LOWER than the
outside of the cell…more negative), these
channels are closed
 2 types
1. Na+ voltage gated channel
 Allows Na+ ions IN during production of action potential
2. K+ voltage gated channel
 Allows EXIT of K+ ions from cell during repolarization of cell
membrane