Alaa' Alajrami. Hussam Twaissi. Mohammad khatatbeh

Transcription

1 6 Alaa' Alajrami Hussam Twaissi Mohammad khatatbeh

2 Keep smiling stay beautiful,,, We will continue talking about cell membrane and what cause the potential across it. * Proton depended carrier: (secondary active transport) In this case hydrogen ions diffuse from high concentration to low concentration (from inside the cell to outside the cell), at the same time another ions transport from low concentration to high concentration. G protein: G proteins are not enzymes, they are linking tow structures together. G protein is a complex protein formed of three subunit (α, β, γ ), one of these subunit binds to a receptor causing activating of these three subunit usually (G- α), now the (G- α) can bind to channels once it binds to channel it changes the activity of that channel ( if it s closed now opens ) ** There are four main families of G proteins [Gi/Go, Gq, G12 ] and the[gs] which its function is to activate the enzyme adenylyl cyclase. **you can know more about G-proteins by visiting this link: Types of protein channels Chemical gated channels: The channels that have been activated by receptors for G-proteins. Voltage gated channels: The changes in voltage across membrane can activate these channels. *In chemical gated channels we need chemical substance to bind to its receptors to activate G-protein. 1 P a g e

3 *some receptors change the activity of enzymes. Example: CAMP (cyclic AMP) can be increased by an enzyme which called adenylyl cyclase, CAMP can change the activity of certain proteins inside the cell, for example the CAMP dependent kinase that means it depends on the level of activated CAMP that we can have them in the cell ( once we have increased the activated CAMP parallel we are getting phosphorylation of some proteins and by that we are changing the activity of that proteins). *The chemical reaction that activates kinase is called phosphorylation. Phosphorylation changes the activity of proteins : *Some proteins are active and when they are phosphorylated it become inactive. *Some proteins are inactive and when they are phosphorylate it become active. 2 P a g e

4 The relation between signal transduction mechanisms with transport: *we have sodium channels potassium channels can be activated by increasing CAMP concentration. we will take it in more details later on. *we can have highly amplification by activating an enzyme. Example: when you have activated one enzyme it produces 3 CAMP (for example) and this activate 3 Kinase each kinase starts to phosphorylate other proteins each phosphorylate many proteins. The final result that millions of proteins are phosphorylated by activating one receptor only this is how we can get huge response and having low concentration of hormones and enzymes. Also, we can get different of response according to different types of receptors over the cell. For example: 1. Adenylyl cyclase taking ATP and converting it to CAMP(as second messenger) 2. Guanine cyclase converts GTP to CGTP, which acting over other types of enzymes differ from those which depend on CAMP. **There is a summarization for all previous lectures on doctor s slides. Electro chemical equilibrium: Plasma membrane separating two compartment one inside cell and other outside cell We have very high concentration of [K+] ions inside the cell but very low concentration of [K+] ions outside the cell. We have very high concentration of [Na+] ions outside the cell and very low concentration of [Na+] ions inside the cell. 3 P a g e

5 Assuming : 1. We have a permeable membrane only for [K+] from high to low the movement of [K+] will results: Generation of potential ( positive outside and negative inside ). Any ion of [K+] try to move will face this potential and doesn t move. **In this case we will reach equilibrium but not chemical equilibrium ( that means we will still have high [K+] inside and low [K+] outside) and the potential created will prevent more [K+] to move. 2. We have a permeable membrane only for [Na+] from high to low the movement of [Na+] will results: Generation of potential (negative outside and positive inside). Any ion of [Na+] try to move will face this potential and doesn t move. **In this case we will reach equilibrium but not chemical equilibrium (that means we will still have high [Na+] outside and low [Na+] inside) and the potential created will prevent more [Na+] to move. *This equilibrium is called the equilibrium potential for ions (the Nernst potential ). *We can measures this potential by placing two microelectrode one outside and the other inside the cell then we measured the potential by voltmeter. *We can also calculate this potential by an equation called Nernst equation for any ion at the normal body temperature: *E ion = electromotive force. *Z= electrical charge of the ion. 4 P a g e

6 ** The sign of the potential depends on the all over charge inside the cell, so it becomes positive if the ion diffuses inside is positive and becomes negative if the ion diffuses outside is positive. ** The sign of the potential is positive if the ions are diffusing from inside to outside is a negative ion and the sign is negative if the ion is positive. **The potential determined by the ratio of the concentration of that specific ion on the two side of the membrane, the greater this ratio the greater the tendency for the ion to diffuse in one direction and the greater the Nernst potential required to prevent additional net diffusion. **** the Nernst equation can calculate potential only if the membrane is permeable for one ion and we don t have this situation on our cell so there is another equation called Goldman equation and it is used to calculate potential when the membrane is permeable to several different ions. *** This equation is the Nernst equation but in another form and we can get that form by replacing R, T, F and converting log to lin 5 P a g e

7 By calculating: Contribution of the potassium diffusion potential : we make assumption that the only movement of ions through the membrane is diffusion of [K+], because of th high ratio of [K+] inside to outside the Nernst potential corresponding to this ratio is -94 millivolts. Contribution of the sodium diffusion potential : we make assumption that the only movement of ions through the membrane is diffusion of [Na+] the Nernst potential is +61 millivolts. But the permeability of the membrane to [K+] is about 100 timeas greater as its for [Na+]. Using this value in goldman equation gives a potential inside the membrane of -86 millivolts. Contribution of the Na+,K+ pump : the continuously acting electrogenic Na+, K+ pump adding -4 millivolte so giving a net membrane potential of -90 millivolte. *This equilibrium is called electro chemical equilibrium not just chemical so we will still have concentration gradient but to maintain potential across the membrane constant no more transport of ions across plasma membrane occur. *No net movement doesn t mean that the ions are not moving but it means that the same number of ions move outside and the same number move inside so it means no net change is occurred. Not all angels have wings, some have lab coats. 6 P a g e