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Unit 3: Cells and Cellular Communication
Chapters 6-7,11
“I’m talking to you!...You’re so…so…so thick-membraned sometimes.”
CHAPTER 8
Cell Membrane Structure and Function
Cell Memranes are… Selectively permeable Fluid
Discovering Membrane Sructure
Made of lipids and proteins
Evidence: 1895 substances that dissolve in lipids
enter cells fastest (Overton) 1917 man-made membrane of
phospholipids created (Langmuir) 1925 proposal that memranes are
phospholipid bilayers (Gorter + Grendel)
Memorizing this timeline is unnecessary- Important concept is that each researcher worked from previous ideas, up to the currently accepted theory.
Discovering Membrane Structure
1935 theory that proteins surround both sides of the phospholipid bilayer (Davson +Danielli)
1950’s + 60’s Questions about this model because membrane proteins have large hydrophobic sections, not all membranes are the same
1972 fluid mosaic model proposed- and still in use in 2008 (Singer + Nicolson)
Fluid Mosaic Model of Membranes
Insert pic of model
Chemistry: phospholipids
Insert pic Amphipathic molecules = has a hydrophobic region and a hydrophilic region
Fluidity within the membrane
Movement of lipids
2 µm/s
Same size as bacterial cell
Movement of Phospholipids is controlled by
Cytoskeleton- some are immobile because of it
Temp- as temp ↓ movement ↓ until it solidifies
Cholesterol- at 37C it limits fluidity, but it allows the membrane to be fluid at lower than normal temp b/c it blocks the packing of lipids
Cells- can change lipid composition to adjust to temp
Cholesterol in membranes
Insert pic of cholesterol blocking movement
Mosaic= huge variety of molecules
50+ types of proteins in membranes of RBC’s!
Integral proteins- cross into the hydrophobic core
Transmembrane proteins- span entire memb
Peripheral proteins- stuck to the surface
A Transmembrane Protein with 7 passes through the membrane
Inside ≠ Outside of Membrane
Membrane showing all types labeled
Carbs in the Membrane
cell-cell recognition “Name Tags” Branched oligosaccharides (less
than 15 monosaccharides) Uses:
Correct placement of cells as embryo develops
Immune system defenses
Glycolipids and Glycoproteins
Insert image of glycolipid and label parts
Structure + Function = moving things across the cell membrane
IN Sugar Amino acids water Oxygen Na+ K+ Ca2+ Cl-
OUT Carbon Dioxide Water Other waste
products Na+ K+ Ca2+ Cl-
How to Cross a Membrane
Hydrophobics: Dissolve in the lipid bilayer and pass
through Hydrocarbons Carbon dioxide Oxygen
What if you are hydrophilic??
Transport proteins
Let water, ions, and polar molecules through
“toll gate” Some are just
tunnels, others carry a molecule through
Passive Transport
Diffusion – [high] [low]
Concentration gradient- the natural direction of flow (different for each molecule)
Passive Transport for Life: Cellular Respiration
Oxygen diffuses INTO a cell as long as C.R. is happening
Water diffuses in/out of cells freely through transport proteins
No energy needed!
OSMOSIS
DIFFUSION OF WATER Pic of hypo, hyper, isotonic solns
What type of solution is this cell sitting in?
Osmoregulation
Control of water balance in organisms without cell walls
Paramecium live in ponds that are hypotonic compared to their cell.
Adaptation- Contractile Vacuole collects water and forces it out.
Pic Of contractile vacuole in para.
Water Balance With Cell Walls
When surrounded by incoming rain water, it is in a hypotonic solution.
Turgid- cell membrane pushed up against cell wall- normal for plants
Flaccid- cell membrane pulls in from cell wall- plant wilts
Plasmolysis- water leaves cell, cell shrivels, fatal to cell
Facilitated Diffusion
Passive transport Hydrophilic molec. diffuse via
transport proteins
Transport Proteins are Like Enzymes
Specific match to molecules they transport
Some have specific active sites Rate of transport slows down when
it becomes saturated Inhibited by “imitation” molecules Catalyze movement of molecules
instead of reactions
Channel Protein
Example: Aquaporins
Gated Channels
Example: Neurotransmitter arrives at a nerve cell, causes Na+ to enter the cell
Some proteins have “induced fit” like an enzyme
Triggered by binding/release of the molecule to be moved
How can transport proteins affect your health?
Cystinuria Body is unable to make memb.
Proteins that transport cystine and other A.A. out of the kidney cells
Affect: chronic kidney stones
Active Transport
Movement AGAINST the concentration gradient
Requires Energy! ATP is the key Allows cells to have a different
internal environment than surroundings More K+ and less Na+ inside than
outside
Na-K Pump
Well studied example of active transport
Exchanges Na+ for K+ across plasma membrane of animal cells
Membrane Potential
Electrical voltage formed when there is an unequal distribution of ions across a membrane
Inside is negative compared to outside
-50mV to -200mV
Very important concept that will be used in examples all year long.
Electrochemical Gradient
Combination of concentration gradient and electrical force
Inside a cell more negative than outside THEREFORE
Passive transport will move cations IN and anions OUT
How this works in a Neuron
Resting nerve cell
Stimulated by trigger
Gated channels open
Na+ enters following electrochemical gradient
Electrogenic Pumps
Na-K pump does not give equal trade of ions
3Na+ OUT and 2K+ IN Result: storing energy every time
the pump cycles for other jobs
ANIMALS use Na-K pumps PLANTS, BACTERIA, FUNGI use
Proton pumps
Proton Pump is the main electrogenic pump in plants, fungi, and bacteria
Contransport
A single ATP pump works, and then diffusion of the same substance assists in moving a molecule AGAINST the concentration gradient
Moving Very Large Molecules that don’t fit through transport proteins
OUT exocytosis Insulin created in pancreatic cells is secreted
into the blood Carbs finished in Golgi Apparatus moved
outside plant cells to make a new cell wall
IN endocytosis
Process: Vesicles containing molecules fuse with the membrane and release contents
Ligand- extracellular substances that bind to receptors
Why should you care?
Receptor Mediated Endocytosis Cholesterol carried in blood in Low
Density Lipoproteins (LDL’s) Receptors on cell membranes bind
to LDL’s Vesicle brings cholesterol into cell If LDL receptor proteins are
defective cholesterol accumulates in blood- atherosclerosis…VERY BAD
CHAPTER 7
A TOUR OF THE CELL
We will do this very quickly as 95% is review!
If you need extra time see me ASAP! I am happy to help you catch up!
Differential Centrifugation
Prokaryotes= small + simple
E.coli- many different variations exist
Cell Membrane- FLASHBACK!!
Animal Cells
Plant Cell
Prokaryotic Cells vs. Eukaryotic Cells
Peroxisomes
FUNCTION IN THE CELL:
Centrosomes
FUNCTION IN THE CELL:
Microvilli
FUNCTION IN THE CELL:
PLANT CELL VS. ANIMAL CELL
Cell Motility: animals, protists + plants
CHAPTER 11
CELL COMMUNICATION
This is likely to be 100% new information for you. Be patient and focused, and you will see that it is not as scary as you may think.
Sex in Saccharomyces cerevisiae
Signal transduction pathway
Process of a signal converted to cellular response
yeast + animal cells similar processPlant + bacterial cells similar process
THEREFORE…APPROXIMATELY WHEN DID THIS EVOLVE???
Communication between starving bacteria
Messages over short distances
Local regulators Paracrine signaling
Growth factors sent out to all nearby cells Benefits of this method?
Neurotransmitters Electrical signal secretion of
neurotransmitter into synapse message to one immediate neighbor only
Benefits of this method?
Long Distance Communication: Hormones
Endocrine signaling Travel in blood in animals Travel in by diffusion, or through
cells in plants Less known about these mechanisms
Ethylene- causes fruit to ripen is a gas C2H4
Direct Contact Communication
Pass signal molecules directly from cytoplasm cell 1 cytoplasm cell 2
Receptor molecules on cell membranes
3 general steps to cell signaling
11.5
1971 Nobel Prize Work: Earl W. Sutherland
Hormone epinephrine stimulates depolymerization of glycogen in liver and skeletal muscles cells
Depolymerization relases glucose-1-phosphate
Cell converts it to glucose-6-phosphate
Cell uses it for glycolysis (make energy)
Conclusion 1
1 effect of epinephrine secreted from adrenal gland in times of stress is to quickly mobilize fuel for cells
Mechanism Shown by Sutherland
Epinephrine activates enzyme glycogen phosphorylase that starts depolymerization of glycogen
Experiment Epinephrine + enzyme + substrate
= no reaction Only worked in living cells
Conclusion 2
Epinephrine does not directly work with the enzyme
Plasma membrane must be involved in the process
Step 1 Reception
Pic 11.6
Induced fit of Membrane Receptor Proteins
Each cell has ID tags so the message doesn’t get to the wrong cell.
Ligand- a small molecule that specifically binds to a larger molecule
The induced fit is the trigger to cause another molecular interaction
Signal molecues do not need to enter the cell to start the chain reaction of events!!
Epinephrine is HUGE, it will never get into the cell!
Review of Friday’s main idea
Video clip 11-05 on CD
G-protein Linked Receptors
Require the help of a G protein Yeast mating factor Epinephrine Many neurotransmitters and hormones
G proteins are… On/off swtiches GDP bound = off GTP bound =on
Diagram of a G Protein linked receptor
G Protein act can as GTPase
Enzyme that hydrolizes GTP GDP
Function= shut down reaction when the extracellular signal is gone
WHY do we care????
Studies of some G proteins show that if a mouse is conceived and lacks a certain G- protein, blood vessels form incorrectly, mouse is never born.
Human vision and smell depend on G proteins
Cholera bacteria form toxins that interfere with G proteins
FYI from the CDC
What is cholera? Cholera is an acute, diarrheal illness caused by
infection of the intestine with the bacterium Vibrio cholerae. The infection is often mild or without symptoms, but sometimes it can be severe. Approximately one in 20 infected persons has severe disease characterized by profuse watery diarrhea, vomiting, and leg cramps. In these persons, rapid loss of body fluids leads to dehydration and shock. Without treatment, death can occur within hours.
Tyrosine-Kinase receptors
Used for growth factors
Cytoplasmic side= enzyme tyrosine kinase. Catalyzes transfer of phosphate group from
ATP to tyrosine (AA) on a substrate protein Extra cellular side = tyrosine kinase
receptor Attach phosphates to tyrosines on proteins
Tyrosine Kinase Receptor model
2 steps to the T-K Receptor reaction
1. ligan binding causes 2 receptor polypeptides to aggregate (dimer)
2. activates the T-K parts of polypeptides , which adds phosphates to the tyrosine on the tail of the 2nd polypeptide
AP exam word: phosphorylation
T-K receptors vs. G protein receptors
T-K receptors can trigger many different reaction pathways
G protein receptors are more specific
Some cancers form when the T-K receptors aggregate w/o the ligand
Ligand gated ion channels
Protein pores in membrane that open/close in response to signals from ions (Ca+, Na+)
Important for the nervous system cell to cell communication
When a specific ligand binds to the protein, the channel opens for ions to pass through.
Intracellular receptors
Some receptors are inside the cell
Steroiods, ex testosterone works this way.
Activated testosterone receptor is a transcription factor to regulate specific genes.
Steroid hormone goes through membrane
Hormone binds to receptor
Hormone+ receptor enter nucleus binds to specific genes
Starts transcription of the gene
Translation of mRNA completed
Signal Transduction Pathway
Once the chain reaction has started there are multiple intermediate steps
Can amplify the message being sent
Better regulation of chemical processes
Relay molecules are usually proteins
Phosphorylation is often the way that a message is passed from one molecule to the next
(Add a phosphate group from ATP to the next molecule in line)
Enzymes that do this are protein kinases
THIS IS A HYPOTHETICAL MECHANISM!!!! NOT A REAL ONE!!
Importance of Protein Kinases
1% of your genes are for protein kinases
1 cell can have hundreds of different protein kinases
Abnormal activity of those regulating cell division contributes to cancer
Turning OFF a signal transduction pathway
Protein phosphatease- enzyme that remove phosphate groups from proteins
Second Messengers
Ions, or water soluble molecules that help carry on the chain reaction
Move by diffusion through the cell Used in G protein linked receptors
and T-K receptors
Ca2+ and cAMP most common
cyclic AMP
Links back to epinephrine
Sutherland observed that the binding of epinephrine caused and increase in the production of cAMP.
Converted back to inactive AMP almost immediately after the epinephrine leaves the receptor
Application: Cholera Infections
Vibrio cholerae enters body in water Bacteria colonize in small intestine Bacteria form a toxin, that modifies a G
protein that regulates salt and water secretion
GTP never made into GDP Adenylyl cyclase never stops making
cAMP Intestinal cells continue to excrete water
and salt diarrhea
Ca2+ as second messenger
Always in cells, BUT there is more than 10,000X more in the blood and extracellular fluid
Protein pumps move it out of the cell or into the ER
A signal molecule tells the cell to release Ca2+ from the ER
Ca2+ and inositol triphosphate signal pathway
Regulation of cell processes
Pathways may regulate Activity of enzymes Synthesis of enzymes Transcription of DNA
Transcription factors can regulate the on/off of several genes
Signal amplification
Epinephrine sends different signals to different cells
Liver cells- break down glycogen Heart cells- contraction, rapid
heartbeat
The combination of proteins in each type of cell determine the message
Same signal molecule, different messages
Scaffolding proteins
Large proteins that “hold” other proteins in the correct order so they are ready for a chain reaction.
In the brain, there are permanent scaffolding proteins that keep synapse proteins in place
Signal molecules are only present for a short time
BIG problems if the molecule stays too long!
Go back to cholera example!
THE END!!!
Practice Essay Question
Cell to cell communication is vital for a multicellular organism.
A. Discuss the ways that cells communicate locally and over long distances.
B. Describe the importance of multistep pathways using one of the following: Tyrosine kinase receptors G protein linked receptors