Resting Membrane Potential Simulation
– Section 1 –
RMPs and Cell Membranes
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Introduction to Resting Membrane Potentials
What is a Resting Membrane Potential?
A resting membrane potential (RMP) is the electrical potential across the cell (plasma) membrane of an excitable cell, such as a neuron or muscle cell, when it is not responding to stimuli.
How Membrane Potentials Are Measured
The resting membrane potential can be measured by placing a reference electrode outside the cell membrane and a recording electrode inside the membrane.
The charge difference between the two electrodes, the membrane potential, is measured in millivolts (mV) using a voltmeter.
When the charges on the inside and outside of the cell membrane differ, the measured membrane potential will be greater or less than 0 mV.
Typically, the resting membrane potential of excitable cells is around -70 mV (ranging from -60 mV to 90 mV), indicating that the inside of the cell membrane is relatively negative compared to its outside. This charge imbalance results from an uneven distribution of ions across the cell membrane, mainly involving potassium (K+), sodium (Na+), and chloride (Cl–).
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Review Questions
1. What is an excitable cell?
Excitable cells produce electrochemical impulses in response to stimuli.
2. What are two types of excitable cells?
Nerve cells (neurons) and muscle cells are types of excitable cells.
3. Where are the electrodes placed when measuring the resting membrane potential of an excitable cell?
A reference electrode is placed outside the cell membrane, and a recording electrode is placed inside the membrane.
4. What does a 0 mV membrane potential indicate?
No charge difference can be detected between the inside and outside of the cell membrane.
5. What does a – mV membrane potential indicate?
It indicates that a charge difference exists between the two sides of the membrane, with the inside being relatively negative to the outside.
6. What does a + mV membrane potential indicate?
The inside of the cell membrane is negatively charged compared to the outside.
Review of Cell Membranes
Basic Components
All cells in the body are surrounded by a cell (plasma) membrane primarily composed of lipids and proteins.
The cell membrane’s basic framework consists of a bilayer of phospholipid molecules.
Interspersed among the phospholipids are cholesterol molecules, which help make the membrane more fluid.
Several types of proteins are also associated with the cell membrane.
They are either intrinsic (embedded) or extrinsic (attached) to the membrane and assist with membrane transport, cell communication, and cell recognition.
Many membrane lipids and proteins have attached carbohydrates, which play a role in cell recognition.
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Phospholipid Bilayer
Components
A phospholipid molecule consists of a polar head and two neutral tails.
The head of a phosphate contains a negatively charged phosphate group (PO₄³⁻), which makes it both polar and hydrophilic. Therefore, this portion of the molecule is attracted to the polar water molecules surrounding the membrane.
Each phospholipid tail consists of a hydrocarbon chain called a fatty acid (CH₃(CH₂)nCOOH), a type of lipid. The fatty acid chains are hydrophobic (repelled by water) and attracted to each other.
Phospholipid molecules orient into a bilayer because their polar heads are drawn to the surrounding water molecules, and their tails are drawn to each other.
Functions
The phospholipid bilayer acts as a semipermeable barrier that allows certain substances to pass through the cell membrane while preventing others from doing so.
Small nonpolar molecules, like oxygen and other gases, and small polar molecules, like water, can diffuse through the lipid barrier.
However, ions such as K+ and large polar molecules like glucose cannot pass through this barrier.
| Nonrestricted Substances | Restricted Substances | ||
| Small uncharged, nonpolar molecules | Small polar molecules | Ions (cations and anions) | Large polar molecules |
| Oxygen (O2) | Water (H2O) | Sodium (Na+) | Amino acids |
| Carbon dioxide (CO2) | Ethanol (C2H5OH) | Potassium (K+) | Glucose (other sugars) |
| Nitrogen (N2) | Glycerol (C3H8O3) | Chloride (Cl–) | Nucleosides |
As a result, the lipid bilayer separates the cell cytoplasm from the surrounding environment, creating extracellular (ECF) and intracellular fluid (ICF) compartments.
Review Questions
1. What molecules form the framework of a cell (plasma) membrane?
Phospholipids.
2. What are the chemical components of these molecules?
Each phospholipid has a polar head and two neutral fatty acid (lipid) tails.
3. What term best describes the arrangement of these molecules?
They are arranged in a bilayer.
4. Why do the heads of these molecules face the surrounding watery environment?
The polar heads of the phospholipids are attracted to the water molecules in the surrounding environment.
5 . Why do the tails of these molecules face each other?
The neutral fatty acid tails are attracted to each other while being repelled by the surrounding water molecules.
6. What types of substances can freely pass through the membrane framework?
Small nonpolar molecules, such as O2, and small polar molecules, such as H2O.
7. What types of substances can not freely pass through the membrane framework?
Ions, such as K+ or Na+, and large polar molecules, such as glucose and amino acids.
8. What are the names of the watery compartments formed by the semipermeable membrane?
The watery environment outside the cell membrane is called extracellular fluid, and the environment inside the membrane is called intracellular fluid.
Integral Proteins
Integral proteins, called membrane transport proteins, play a significant role in the proper function of excitable cells. They span the width of the cell membrane and function as gatekeepers, allowing only particular ions in and out of the cell. There are two main types of membrane transport proteins: channel and carrier proteins.
Channel Proteins
Channel proteins allow ions to move by diffusion through the cell membrane along their concentration gradients.
Some channels, called leak channels, are continuously open, while others are gated and open only when stimulated.
Voltage-gated channels open in response to changes in membrane potential, and ligand-gated channels open when chemicals bind.
Other types of channels can respond to stimuli such as mechanical pressure and changes in temperature.
Carrier Proteins
Carrier proteins differ from channel proteins because they are only open to one side of the membrane at a time. They have binding sites that attach only to particular substances. Once the carrier protein has bonded with a specific substance, the protein alters its shape to transport the substance from one side of the membrane to the other.
The most relevant carrier protein to the resting membrane potential is the sodium-potassium pump or Na+/K+ ATPase. Each pump protein moves 3 Na+ ions out of the cell while simultaneously moving 2 K+ ions into the cell. The sodium-potassium pump moves the ions by changing conformation (shape) using the energy stored in a molecule of ATP (adenosine triphosphate).
While facing the cell’s interior (cytoplasmic side), the pump protein has a high affinity for Na+ and binds with three Na+ ions and an ATP molecule. The pump protein then acts as an enzyme and hydrolyzes the ATP to ADP + P, allowing a low-energy phosphate group to bond.
The pump protein then changes conformation to face the cell’s exterior, where its affinity for Na+ decreases and its affinity for K+ increases. As a result, the three Na+ ions detach and enter the extracellular fluid, and two K+ ions attach.
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The attachment of the K+ ions causes the phosphate to detach, and the pump protein changes conformation to face the cell’s interior again. The repositioning of the pump protein alters its ion affinities, causing it to release the two K+ ions and bind three Na+ ions, which allows the active transport process to start again.
The sodium-potassium pump functions continuously to maintain a higher concentration of Na+ ions outside the cell membrane and a higher concentration of K+ ions inside. The membrane’s open channels permit these ions to move down their respective gradients, contributing to a separation of charges across the membrane that produces the resting membrane potential.
Review Questions
1. What are the two general types of channel proteins?
Leak (or continuously open) channels and gated channels.
2. What force causes substances to cross the membrane through channel proteins?
Substances move by diffusion down their respective concentration gradients.
3. What is a carrier protein?
Carrier proteins are integral proteins that bind to substances and move them across the membrane by changing their shape.
4. What carrier protein is most relevant to the resting membrane potential?
The sodium-potassium pump or Na+/K+ ATPase is the most relevant carrier protein.
5. In what ratio and direction does this protein move ions?
The pump moves 3 Na+ out of the cell and 2 K+ ions into the cell.
6. Why is the ion exchange process considered active, and what molecule is involved?
The pumping process is active because it requires the energy stored in ATP or adenosine triphosphate.
7. How does the pump protein (ATPase) affect ion concentration gradients?
It creates a high concentration gradient of Na+ ions outside the cell and a high concentration of K+ ions inside the cell.
8. How do the ion concentration gradients affect the resting membrane potential?
When these ions move down their gradients through open channels, a separation of charges develops across the membrane, producing the resting membrane potential.
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