AP Biology, Chapter 11
Cell Communication

Summary
Introduction

AN OVERVIEW OF CELL SINGALING
	Intro
	Cell signaling evolved early in the history of life
		1. Describe the basic signal-transduction pathway of yeast. Explain why we believe these 
		pathways in yeast, mammals, and plants evolved before the first multicellular organism 
		appeared on Earth.
			a. Yeast mating 
				i. Two haploid mating types, a and 
				ii. Type a cells secrete a-factor;  cells, -factor
 				iii. Each type has a receptor for the opposite type's factor
				iv. Responses: grow toward one another, form diploid
			b. Homology of molecular process of signal transduction
	Communicating cells may be close together or far apart
		2. Categorize chemical signals in terms of the proximity of the communicating cells.
			a. Local signaling with local regulators
				i. Growth factors stimulate cell division in paracrine signaling
				ii. Neurotransmitters depolarize or hyperpolarize cells
			b. Long distance with hormones
				i. In animals, transported by blood = endocrine signaling
				ii. In plants, transported by vessels or through cells
			c. Chemically, may be gas, lipid, protein, ?
	The three stages of cell signaling are reception, transduction, and response
		3. Describe the three main stages of cell signaling.
			a. Example
				i. Epinephrine in the blood mobilizes glucose in the liver
				ii. Glycogen + Pi  glucose-1-phosphate 
					.  glucose-6-phosphate  glycolysis
				 	. or  glucose + Pi  into the blood
				iii. Epinephrine acts from outside by activating cytoplasmic glycogen 
				phosphorylase
			b. Three stages
				i. Chemical signal binds to a specific protein receptor usually on cell 
				surface
				ii. Receptor changes and initiates transduction or change in form of the 
				signal in a signal-transduction pathway
				iii. Transduced signal triggers cellular response

SIGNAL RECEPTION AND THE INITIATION OF TRANSDUCTION
	Intro
	A signal molecule binds to a receptor protein, causing the protein to change shape
		4. Describe the nature of a ligand-receptor interaction and state how such interactions 
		initiate a signal-transduction system.
			a. Chemical signal or ligand and receptor have complementary shapes
			b. Generally causes the receptor to change shape
	Most signal receptors are plasma-membrane proteins
		5. Compare and contrast G-protein-linked receptors, tyrosine kinase receptors, and 
		ligand-gated ion channels.
			a. Cell surface receptors
				i. G-protein-linked receptors
					. Examples: yeast mating, epinephrine, some neurotransmitters
					. Receptor proteins span the cell membrane with -helices
					. G-protein is attached just inside the membrane
					. Receptor activates G-protein linked to GDP
					. GTP replaces GDP and the G-protein activates an enzyme
					. G-protein cleaves GTP and shuts off
				ii. Tyrosine kinase receptors
					. Example: growth factors and insulin
					. When ligand binds tyrosine kinase monomers aggregated
					. Aggregated receptors act as a kinase
					. Phosphorylated tyrosines activate many cytoplasmic proteins
					. Abnormal tyrosine kinase receptors may cause cancer
				iii. Ligand-gated ion channels
					. Example: acetylcholine receptor on skeletal muscle cells
					. Ligand binding opens or closes and ion channel
			b. Intracellular receptors
				i. Small or lipid chemical signals may penetrate: steroids, NO, thyroid 
				hormones
				ii. Example: testosterone
					i. Binds to and activates receptor proteins in cytoplasm
					ii. Activated protein turns on genes in the nucleus

SIGNAL TRANSDUCTION PATHWAYS
	Intro
	Pathways relay signals from receptors to cellular responses
		6. Describe several advantages of using a multistep pathway in the transduction stage of 
		cell signaling.
			a. Signal amplification allows the signal to grow
			b. More opportunities for coordination and regulation
	Protein phosphorylation, a common mode of regulation in cells, is a major mechanism 
	of signal transduction
		7. Explain what is usually passed along in a signal-transduction pathway.
			a. Signal-transduction pathways are chains of protein activation
			b. Each step involves specific protein interaction
			c. Information is passed along
		8. Describe how phosphorylation propagates signal information.
			a. Most protein kinases phosphorylate serine or threonine on other proteins
			b. Addition of phosphate changes conformation
			c. Often the change is to an active form
			d. In a pathway that usually activates a kinase to phosphorylate the next kinase
			e. Protein phosphateases remove phosphate and deactivate kinases
			f. 1% of our genes are for kinases
	Certain small molecules and ions are key components of signaling pathways (second 
	messengers)
		9. Describe how cyclic AMP is formed and how it propagates signal information.
			a. Non-protein, small, water-soluble pathway components are second 
			messengers
			b. cAMP formation in epinephrine stimulation of hepatocytes
				i. Epinephrine binds to a receptor
				ii. Receptor activates G protein
				iii. G protein activates adenylyl cyclase
				iv. Adenylyl cyclase converts ATP  cAMP
			c. cAMP binds to and activates protein kinase A
		10. Describe how the cytoplasmic concentration of Ca2+ can be altered and how this 
		increased pool of Ca2+ is involved with signal transduction.
			a. Cytoplasmic Ca2+ is kept low by active transport into ER and out of the cell
			b. Opening of gated Ca2+ channels can quickly change the cytoplasmic 
			concentration
				i. Phospholipase C can be activated by G protein or tyrosine kinase
				ii. Phospholipase C cleaves inositol triphosphate (IP3) from a 
				phospholipid
				iii. IP3 opens the gate on the Ca2+ channel
			c. Ca2+ released may activate kinases directly or by binding to calmodulin

CELLULAR RESPONSES TO SIGNALS
	Intro
	In response to a signal, a cell may regulate activities in the cytoplasm or transcription 
	in the nucleus
		11. Describe how signal information is transduced into cellular responses in the 
		cytoplasm and in the nucleus.
			a. Cytoplasmic responses
				i. Rearrange cytoskeleton
				ii. Open or close ion channel
				iii. Turn on or off an enzyme in metabolism
				iv. Proceed to the next task in the cell cycle
			b. Nuclear responses: activate or inactivate a transcription factor
	Elaborate pathways amplify and specify the cell's response to signals
		12. Describe how signal amplification is accomplished in target cells.
			a. The first activated protein turns on several more
			b. Those activate several more
			c. The response grows exponentially
		13. Describe how target cells discriminate among signals and how the same signal can 
		elicit multiple cellular responses.
			a. Cell-type specific receptors
			b. Cell-type specific relay proteins
			c. Cell-type specific cellular response systems
		14. Explain how scaffolding proteins help to increase the efficiency of signal transduction. 
			a. Can be branch points in relay pathways and response systems
			b. Acts a concentrated informational resource