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