Exam 2 Review
Fluidity of Membranes Dependent on Composition
• Conditions that influence fluidity of membranes, and thus permeability
• Temperature
• Hydrocarbon tails (14 to 24 carbon atoms)
• Shorter tails: reduce hydrocarbon interactions and increase fluidity
• Saturation (double bonds)
• Highly unsaturated hydrobcarbon tails more fluid
Why is Membrane Fluidity Important for Cells?
• Lipid Profiles
• HDL versus LDL
• Statins
• trans versus cis fatty acids
• DHA, omega-3 fatty acids
Membranes are transported by a
process of vesicle budding and fusing.
Here, a vesicle is shown budding from
the Golgi apparatus and fusing with
the plasma membrane. Note that the
orientations of both the membrane
lipids and proteins are preserved
during the process: the original
cytosolic surface of the lipid bilayer
(green) remains facing the cytosol, and
the noncytosolic surface (red)
continues to face away from the
cytosol, toward the lumen of the Golgi
or transport vesicle—or toward the
extracellular compartment. Similarly,
the glycoprotein shown here remains
in the same orientation, with its
attached sugar facing the noncytosolic
side.
Protein Sorting
• Necessary for cellular activity
• Proteins need to be sorted to correct
organelles
• Cellular replication
• Function/secretion
• Proteins delivered from cytosol to
nucleus
• Proteins delivered from cytosol to
mitochondria, chloroplasts,
peroxisomes
• Proteins delivered via ER
• Golgi, lysosomes, endosomes, inner
nuclear membrane
Protein Sorting: Three Mechanisms
• Fate of proteins depends on amino acid sequence
• Contains a sorting signal that directs proteins to organelles
• How do hydrophilic proteins cross hydrophobic membranes?
• Transport through pores (nuclear pore complex). Act as selective gates
• Protein translocators move across ER, mitochondria, and chloroplasts
•
Protein must unfold
• Transport vesicles move proteins to endomembrane system
Protein Sorting: Mitochondria and Chloroplasts
• Make some of their own proteins, but majority through genome-encoded mechanism
• N-terminal signal sequence targets to protein translocator
• Protein is unfolded as it is transported and signal sequence is removed
• Specialized proteins have additional signal sequences exposed prior to cleavage that
target to membrane
Protein Sorting: Endoplasmic Reticulum
• Proteins destined for organelles, cell surface, or secreted enter ER then transported via vesicles
• ER signal sequence targets proteins to ER and aids in translocation
•
•
Water soluble translocated into ER lumen
Transmembrane proteins become embedded in the ER membrane
Vesicular Transport: Communication
• Transport outward from Golgi to plasma membrane, and inward to lysosomes
•
Modification to proteins, carbohydrates and lipids during route
Figure 15-20 Essential Cell Biology (© Garland Science 2010)
Figure 15-22 Essential Cell Biology (© Garland Science 2010)
The Endocytic Pathway: Receptor Mediated Endocytosis
• Endosomes have maturation process (early to late endosomes)
• Interior compartments Is kept acididc by an ATP-driven H+ pump (also exploited by pathogens)
• Acidicity helps to sort products, as it disrupts receptor: substrate binding
Transport Across the Membrane
Ch. 12
Figure 12-4 Essential Cell Biology (© Garland Science 2010)
Passive Transport
• Electrochemical gradient
• Most cells have a voltage across them, or membrane potential
•
Inside is negative relative to the outside due to anions
• Drives the movement of ions due to concentration gradient and charge
• Na-K movement
Sodium-Potassium Pump
• Electrochemical gradient
• Most cells have a voltage across them, or membrane potential
• Inside is negative relative to the outside due to anions
• Drives the movement of ions due to concentration gradient and charge
• Na-K pump
• Na+ higher concentration outside of the cell
• K+ higher concertation inside the cell
Na+: movement when possible
K+ : little movement
Gated Channels
• Many types of control of ion channels
• Voltage gated channels
• Ligand gated channels
• Mechanical/stress-gated channels
Voltage-Gated: Action Potential in Neurons
Voltage-Gated: Action Potential in Neurons
• Traveling wave of electrical excitation known as cation potential. Signal does not weaken along the
length of your neuron.
• Voltage gated Na+ channels open due to depolarization of membrane
• Membrane potential shifts to less negative value, towards 0.
•
Hyperpolarization inhibits action
potentials by increasing stimulus
required to move the membrane
potential to the threshold
•
Mediated by efflux of K+ (or influx of
Cl-
Synaptic Conversion to Chemical Signal
Figure 16-7 Essential Cell Biology (© Garland Science 2010)
Cell Signaling: Intracellular Signaling Pathways
Activation of Intracellular signaling
pathways
1. relay the signal (spread)
2. amplify the signal, causing larger
response
3. receive multiple signals and
integrate them
4. distribute signal to multiple
pathways, evoking a complex
response
Cell Signaling: Intracellular Signaling Pathways
• Key signaling proteins act as molecular switches (signal changes protein from inactive to
active state)
• GTP-binding proteins signal based on binding of GTP or GDP
• Self GTP-hydrolyzing shuts off signal
• G-protein coupled receptors common mechanism of signaling
Ion Channel Coupled Receptors
• Function in a direct and simple
way
• Responsible for rapid
transmission of signals across
nervous system synapses
• Transduce chemical
neurotransmitter signal into
an electrical signal (change in
voltage) alters conformation
of ion channels, allowing
specific ions (Na+, K+, Cl- or
Ca+2)
G-Protein Coupled Receptors
• GPCR binds to signal molecule,
causing a conformation change that
activates G protein in cytoplasm
• Composed of three protein
subunits (α, β, and γ)
• α bound to GDP in unstimulated
state, and exchanges for higher
affinity GTP when activated
• Has intrinsic GTPase activity
and regulates time of
activation
• β γ subunit interacts with
target proteins in membrane
Enzyme Coupled Receptors
• Enzyme-coupled receptors are transmembrane proteins that associate with an
enzyme itself
• Important for cell survival, growth factors
• Receptor tyrosine kinases (RTKs) major class of receptors
• Most activate monomeric GTPase Ras (GTP-binding protein
• Ras initiates phosphorylation cascade through MAP kinases (mitogen activated
protein kinase)
• About 30% of all cancers contain mutations in Ras gene (oncogenes)
Figure 16-32 Essential Cell Biology (© Garland Science 2010)
• Major themes
• Dynamics at the cell membrane (composition, transport)
• Cycling of HDL/LDL and how cholesterol effects membrane activates
• MUSCLE CONTRACTION
• NEURON SIGNALING THROUGH ACTION POTENTIAL AND VOLTAGE GATED ION
CHANNELS
• Recognize differences is cell communication (Compare/contrast and
know general mechanism)
• Ion channel coupled
• G protein coupled
• Enzyme coupled
• RTKs, Ras, and MAP Kinase
• PI3-Kinase, Akt
• Recognize differences between the protein movement/synthesis
between different organelles
• Protein sorting mechanism and purpose
• Clathrin coated vesicles and use of SNAREs
• Endocytosis
• Receptor mediated, endosome maturation process
• Phagocytosis/phagolysosome formation