1. Ch. 4 Part 2 Cell Signaling & Intro to Cell Transport

2. Cell Signaling How living organisms control and coordinate their bodies Based on getting a message from one place to another place – Outside the cell to inside the cell – Inside the cell to outside the cell Two Major mechanisms of signaling – Secretion of Chemical Signals – Direct cell-cell contact 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

4. 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

5. 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

6. How Receptors Alter the Activity of a Cell 1.Activating the release of second messengers 2.Opening an ion channel – Changes membrane potential Ex. Nicotine-accepting acetylcholine receptors 3.Acting directly as a membrane-bound enzyme – Ex. Insulin receptor 4.Acting as an intracellular receptor when the initial signal passes straight through the cell surface membrane – Ex. Oestrogen receptor Found in nucleus and directly controls gene expression when combined with hydrophobic oestrogen hormone

7. 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)

8. Receptor G Protein receives the signal molecule Changes shape to activated alpha G-protein inside the cell membrane “switch” second messenger 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

9. 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

10. 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

11. How Second messengers Work

12. Possible Signaling Pathways in Cells

15. Cells are always trying to Maintain Equilibrium by… 3 ways…  PASSIVE Transport  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  ACTIVE Transport  REQUIRES Energy  Goes Against Conc. Gradient  ACTIVE TRANSPORT  Involves “transporter” membrane protein and ENERGY (ATP)  Different Types  Gated channels  Bulk Transport

16. Terms to Remember!  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

18. 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.

20. 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

21. Diffusion  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

22. What happens when we reach equilibrium?  Particles continue moving across membrane but in both directions! ***No more changes in concentration

23. What diffuses across the membrane???  Small, uncharged (non-polar) molecules  Examples:  Carbon dioxide  Oxygen

24. 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  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

25.  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

27. 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

28. 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

29. 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

31. 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