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Neuron and membrane protein channels and pumps

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?

2. What are two types of excitable cells?

3. Where are the electrodes placed when measuring the resting membrane potential of an excitable cell?

4. What does a 0 mV membrane potential indicate?

5. What does a – mV membrane potential indicate?

6. What does a + mV membrane potential indicate?

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.

Phospholipids in water
Phospholipid bilayer
lipid-bilayer-formation-3
lipid-bilayer-formation-4

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 SubstancesRestricted Substances
Small uncharged, nonpolar moleculesSmall polar moleculesIons (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.

Animation showing the permeability of the lipid bilayer to small uncharged nonpolar molecules and small polar molecules
Animation showing the permeability of the lipid bilayer to ions and large polar molecules

Review Questions

1. What molecules form the framework of a cell (plasma) membrane?

2. What are the chemical components of these molecules?

3. What term best describes the arrangement of these molecules?

4. Why do the heads of these molecules face the surrounding watery environment?

5 . Why do the tails of these molecules face each other?

6. What types of substances can freely pass through the membrane framework?

7. What types of substances can not freely pass through the membrane framework?

8. What are the names of the watery compartments formed by the semipermeable membrane?

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.

Channel proteins and carrier proteins embedded in the cell membrane

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?

2. What force causes substances to cross the membrane through channel proteins?

3. What is a carrier protein?

4. What carrier protein is most relevant to the resting membrane potential?

5. In what ratio and direction does this protein move ions?

6. Why is the ion exchange process considered active, and what molecule is involved?

7. How does the pump protein (ATPase) affect ion concentration gradients?

8. How do the ion concentration gradients affect the resting membrane potential?

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References

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