Learning Outcomes - Week 2 Flashcards
Be able to describe what is meant by the term potential difference.
Potential difference is simply defined as “the difference in charge between two points”
Understand how potential difference is measured and what its units are.
Potential difference can be readily measured with a voltmeter (see image on right) and its unit is volts (V).
Example:
For example if we take a voltmeter and connect its leads to the terminals of a typical car battery then the display will read 12 V because there is a difference in charge between one end of the battery and the other. On the other hand if we stick it on a AA battery the display will show 1.5 V because there is a smaller difference in charge between the two ends of the AA battery. In physiology the potential differences we measure are usually quite small so we tend to talk about millivolts (mV) rather than V for convenience (1 mV = 0.001 V).
What are ions?
Difference between cations and anions?
Understand the difference between monovalent and divalent cations and anions.
Ions are molecules with an electrical charge due to varying number of electrons and protons.
Cations is a positively charged (Less electrons than protons) and anion is negatively charged (more electrons than protons)
The size of the charge on different ions is referred to as their valency and this is denoted in superscript following the symbol for the atom or molecule. For example chloride has a valency of -1 so would be a monovalent anion and denoted as Cl- whereas calcium has a valency of +2 so is classified as a divalent cation and would be written as Ca2+.
Examples: Valency of +1 = monovalent, valency of 2+ or 2- = divalent, 3+/- or more = multivalent
Ions with different charges are _________ to each other while similarly charged ions _________ each other.
attracted
repel
Be able to explain what a membrane potential is and how you would measure it.
If we take a voltmeter with very fine leads (known as microelectrodes) and leave one outside the cell and carefully place the other into the cell then we record the potential difference between the inside and outside of the cell. We refer to this potential difference as the membrane potential because it is the potential difference across the membrane.The value of the membrane potential varies from cell to cell and as we will see later in semester in some cells it can also change over time.
Understand the difference between a concentration gradient and an electrical gradient.
Concentration gradient results in the flow of solutes down the concentration gradient until concentration equilibrium is reached (still some movement but constantly being balanced)
Electrical gradient refers to the charge across the membrane (membrane potential). As solutes flow down the concentration gradient the electrical gradient begins to shift resulting in an electrical charge difference between ICF and ECF. This will cause the solutes at the lower end of the concentration gradient to begin moving back up the concentration gradient which is actually now DOWN the electrical gradient.
Eventually a point of equilibrium is reached where the concentration gradient is exactly balanced by the electrical gradient and no further net movement of ions occurs and the membrane potential is stable.
When studying the effects of ions flowing across membrane we only need consider what is happening _______ the cell.
inside
Understand that we can predict the polarity of the membrane potential if we know what ____ _________ are open and the relative ____________ of these ions ________ and ________ the cell.
ion channels
concentration
inside
outside
The magnitude of the membrane potential is dependent upon the size of the __________________ of the ion across the __________.
concentration gradient
membrane
What is an equilibrium potential is (1) and why is it useful (2)?
- If the membrane is selectively permeable to only one ion and we know the intracellular and extracellular concentrations of the ion then we should be able to predict the theoretical value of the membrane potential once equilibrium has been reached. This theoretical value is known as the equilibrium potential because it is the potential difference across the membrane that will exist after equilibrium has been achieved
2.
Know the historical origins of the Nernst equation and be able to use it calculate the equilibrium potential for monovalent anions and cations at body temperature.
Write the equation out and label it’s parts
The equilibrium potential for any ion can be conveniently calculated using the Nernst equation which was named after Walther Nernst who was awarded a Nobel prize in 1920 for his contributions to electrochemistry
The equilibrium potential for any ion (Eion) can be calculated using the Nernst equation:
Be able to explain the fundamental assumption that is made when you use the Nernst equation to predict membrane potentials.
This version of the Nernst equation is much more convenient to use providing we remember that it applies only to monovalent cations at 37oC.
Label each arrow
Label each arrow
Nernst Equation Example 1.
In this example the intracellular [K+] = 100 mM and the extracellular [K+] = 5 mM.
Nernst Equation Example 2.
In this example the intracellular [Na+] = 15 mM and the extracellular [Na+] = 105 mM.
Nernst Equation Example 3.
In this example the intracellular [Cl-] = 35 mM and the extracellular [Cl-] = 165 mM.
