What Is Bioelectricity?

Bioelectricity refers to electrical phenomena produced by living cells and organisms.

Your nervous system, muscles, heart, and many other tissues depend on electrical signals.

These signals allow organisms to:

• detect changes in their surroundings
• transmit information
• control muscles
• coordinate organs
• maintain internal conditions
• respond rapidly to stimuli

Bioelectricity is closely connected to electrochemistry because it depends on the movement and separation of:

charged ions.

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Electricity in Living Organisms

Electricity involves the movement or separation of:

electric charge.

In a metal wire, electrical charge is mainly carried by:

electrons.

Inside biological fluids, electrical charge is mainly carried by:

ions.

Important biological ions include:

• sodium ions, Na⁺
• potassium ions, K⁺
• calcium ions, Ca²⁺
• chloride ions, Cl⁻
• hydrogen ions, H⁺

The movement of these charged particles produces many of the electrical effects found in:

living systems.

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Biological Systems Are Electrochemical

Bioelectricity combines two ideas:

electrical forces

and:

chemical concentration differences.

Ions are affected by both.

An ion may move because:

• it is attracted or repelled by electrical charge
• it moves down a concentration gradient

Together these influences create an:

electrochemical gradient.

This is one of the most important links between electrochemistry and biology.

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The Cell Membrane

Every cell is surrounded by a:

cell membrane.

The membrane separates the inside of the cell from the surrounding extracellular fluid.

Importantly, the membrane does not allow every substance to cross equally easily.

It is:

selectively permeable.

This allows cells to maintain different ion concentrations on opposite sides of the membrane.

Unequal Ion Distribution

Living cells maintain different concentrations of ions inside and outside their membranes.

For a typical animal cell:

Na⁺ concentration is higher outside the cell.

K⁺ concentration is higher inside the cell.

Other ions also contribute to the overall charge distribution.

Because ions carry electrical charge, separating them across a membrane creates an electrical:

potential difference.

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Membrane Potential

The difference in electrical potential between the inside and outside of a cell is called the:

membrane potential.

Voltage is another name for:

electrical potential difference.

Therefore a cell membrane behaves in some ways like a tiny electrical system with different electrical conditions on its two sides.

The membrane potential is measured in:

millivolts (mV).

One millivolt is:

0.001 V.

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Resting Membrane Potential

When an excitable cell such as a neuron is not actively sending a signal, it maintains a:

resting membrane potential.

For many neurons, the inside of the cell is approximately:

−70 mV

relative to the outside, although the exact value varies among cells.

The negative sign means that the inside is electrically more negative than the:

outside.

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How Is the Resting Potential Created?

The resting membrane potential develops because of several interacting factors:

• unequal ion concentrations
• selective membrane permeability
• ion channels
• active transport
• negatively charged molecules inside the cell

The membrane is especially permeable to K⁺ through certain:

leak channels.

As K⁺ moves, charge becomes separated across the membrane.

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Ion Channels

Ion channels are proteins that allow particular ions to cross the cell membrane.

Different channels may be selective for:

• Na⁺
• K⁺
• Ca²⁺
• Cl⁻

Some channels remain open much of the time.

Others open or close in response to:

• voltage changes
• chemical signals
• mechanical forces

Ion channels allow cells to control the movement of:

electrical charge.

The Sodium-Potassium Pump

Cells also use an important membrane protein called the:

sodium-potassium pump.

The pump uses energy from ATP to transport ions against their concentration gradients.

For each cycle, it typically moves:

3 Na⁺ out of the cell

and:

2 K⁺ into the cell.

This helps maintain the Na⁺ and K⁺ concentration gradients needed for:

electrical signaling.

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Active Transport Requires Energy

Moving ions against their concentration gradients requires:

energy.

The sodium-potassium pump obtains this energy from:

ATP.

Therefore maintaining the ion gradients needed for bioelectricity ultimately depends on the cell's:

metabolism.

Cells continuously use energy to maintain the conditions required for electrical activity.

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An Electrochemical Battery?

A useful analogy is to think of a cell membrane as part of a tiny:

electrochemical energy-storage system.

There are different concentrations of ions on opposite sides of the membrane.

The membrane controls when those ions can:

move.

When ion channels open, ions move according to their electrochemical gradients.

However, a living membrane is much more complex and actively regulated than an ordinary:

battery.

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Neurons

A neuron is a specialized cell that transmits information through the nervous system.

Neurons communicate using:

electrical and chemical signals.

Major parts of a neuron include:

• dendrites
• cell body
• axon
• axon terminals

Electrical signals can travel along the:

axon.

Action Potentials

A rapid electrical signal traveling along a neuron is called an:

action potential.

An action potential involves a temporary change in the:

membrane potential.

This occurs because specific ion channels open and close in a carefully controlled sequence.

The major ions involved are:

Na⁺ and K⁺.

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Starting an Action Potential

A stimulus can change the membrane potential.

If the change reaches a sufficient:

threshold,

voltage-gated sodium channels open.

Na⁺ ions move rapidly:

into the neuron.

This makes the inside of the membrane less negative and eventually positive relative to the outside.

This process is called:

depolarization.

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Depolarization

During depolarization:

Na⁺ channels open

and:

Na⁺ moves into the cell.

Why?

Na⁺ is influenced by its:

electrochemical gradient.

Its concentration is higher outside the cell, and the electrical conditions also favor inward movement during the early part of the action potential.

The membrane potential rapidly:

increases.

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Repolarization

After a short time, sodium channels become inactivated and voltage-gated potassium channels open.

K⁺ moves:

out of the cell.

This causes the membrane potential to become negative again.

This stage is called:

repolarization.

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Hyperpolarization

Potassium channels may remain open slightly longer than required to return directly to the resting potential.

The membrane can temporarily become more negative than its normal resting value.

This is called:

hyperpolarization.

The membrane then returns toward its normal resting state.

The Action Potential Sequence

The basic sequence is:

Resting state

↓

Threshold reached

↓

Na⁺ channels open

↓

Na⁺ enters

↓

Depolarization

↓

Na⁺ channels inactivate and K⁺ channels open

↓

K⁺ leaves

↓

Repolarization

↓

Brief hyperpolarization

↓

Return toward resting conditions

This sequence allows neurons to transmit information rapidly over relatively long distances within the:

body.

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Does Electricity Travel Along a Neuron Like a Wire?

Not exactly.

In a metal wire, electrons move through the:

metal.

In neurons, electrical signaling depends mainly on movements of ions across the cell membrane and electrical effects spreading along the:

membrane.

An action potential is repeatedly regenerated as it travels.

Therefore a nerve impulse is an:

electrochemical signal.

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Propagation Along the Axon

When one region of an axon depolarizes, it affects the electrical conditions in the:

adjacent region.

This can cause nearby voltage-gated channels to open.

A new action potential is generated there.

The process repeats along the:

axon.

This allows the electrical signal to propagate without simply fading away.

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Myelin

Many neurons are surrounded by an insulating material called:

myelin.

Myelin reduces ion movement across most of the covered membrane.

Action potentials are regenerated mainly at gaps called:

nodes of Ranvier.

The signal effectively jumps from node to node.

This is called:

saltatory conduction.

Communication Between Neurons

Neurons usually do not physically touch each other.

The small junction between neurons is called a:

synapse.

When an action potential reaches an axon terminal, it can trigger Ca²⁺ channels to open.

Calcium ions enter the terminal and help trigger the release of:

neurotransmitters.

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Chemical and Electrical Communication

At many synapses:

electrical signal → chemical signal → electrical response

The neurotransmitter crosses the synaptic gap and binds to receptors on the next cell.

This can cause ion channels to:

open or close.

The resulting ion movement changes the membrane potential of the next cell.

Bioelectricity and Muscles

Muscle cells also use changes in membrane potential.

A motor neuron sends a signal to a muscle.

This triggers electrical activity in the muscle cell membrane.

The electrical signal helps cause the release of:

Ca²⁺ ions inside the muscle cell.

Calcium then helps initiate the interactions between proteins that produce:

muscle contraction.

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Your Heart Is Electrical

The heart depends on coordinated electrical activity.

Specialized cells generate and conduct electrical signals that cause the heart muscle to contract in an organized pattern.

The natural pacemaker of the heart is the:

sinoatrial (SA) node.

Electrical signals spread through the heart and coordinate contraction of the:

atria and ventricles.

The Electrocardiogram

Electrical activity in the heart produces voltage changes that can be detected at the skin.

An:

electrocardiogram (ECG or EKG)

records these electrical changes.

An ECG does not directly measure the force of the heartbeat.

It records patterns associated with the heart's:

electrical activity.

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The Brain Is Electrical Too

The brain contains billions of neurons whose activity depends on:

electrochemical signaling.

The combined electrical activity of populations of brain cells can produce voltage changes detectable at the scalp.

An:

electroencephalogram (EEG)

records patterns of this electrical activity.

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Sensory Systems

Bioelectricity is also essential for our senses.

Sensory cells convert environmental stimuli into electrical signals.

Examples include:

light → electrical signals in the retina

sound vibrations → electrical signals in the inner ear

pressure → electrical signals in touch receptors

chemicals → electrical signals in taste and smell systems

This conversion of a stimulus into a biological signal is called:

sensory transduction.

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Electric Fish

Some organisms produce much stronger electrical effects than humans.

Electric eels and certain other fish contain specialized cells called:

electrocytes.

Electrocytes generate electrical potential differences using ion gradients across their membranes.

Large numbers of these cells can work together to produce a substantial:

electric discharge.

Electrocytes

Electrocytes are modified cells specialized for producing electrical potentials.

One electrocyte produces only a relatively small voltage.

However, many electrocytes can be arranged so that their voltages:

add together.

This is conceptually similar to connecting electrical cells:

in series.

The resulting electric organ can produce much larger potential differences.

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Why Do Electric Fish Produce Electricity?

Different electric fish use electrical signals for different purposes.

These can include:

• sensing their surroundings
• communication
• navigation
• defense
• capturing prey

This provides a dramatic example of how evolution has adapted biological ion movement for specialized:

functions.

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Electroreception

Some animals can detect weak electric fields.

This ability is called:

electroreception.

It occurs in several groups of aquatic animals.

Electroreception can help animals:

• locate prey
• navigate
• detect nearby organisms
• communicate

Sharks and Electroreception

Sharks possess specialized sensory structures called:

ampullae of Lorenzini.

These organs can detect weak electric fields in the surrounding water.

Because muscle and nerve activity in animals produces electrical signals, electroreception can help a shark detect:

nearby organisms.

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Plants Also Use Electrical Signals

Bioelectricity is not limited to animals.

Plants maintain ion gradients across cell membranes and can produce changes in membrane potential.

Electrical signals contribute to responses involving:

• injury
• environmental stimuli
• movement
• communication between plant tissues

Some plants show particularly noticeable electrical responses.

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The Venus Flytrap

The Venus flytrap provides a striking example.

Touching sensitive trigger hairs can initiate electrical signals.

If appropriate stimulation occurs within a short period, the trap can:

close rapidly.

Ion movement and electrical signaling therefore contribute to a plant response that is easily:

observed.

Bioelectricity and Electrochemistry

The same fundamental principles studied in electrochemistry also appear in biological systems.

Both involve:

• charged particles
• potential differences
• movement of charge
• concentration gradients
• selective pathways for charge movement
• energy transformations

However, biological systems primarily move:

ions through membranes

rather than electrons through metallic wires.

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Concentration Gradients as Stored Energy

Creating an ion concentration gradient requires:

energy.

Once established, the gradient stores potential energy.

When channels open, ions can move down their:

electrochemical gradients.

Cells use this stored energy for:

• electrical signaling
• transport
• muscle activity
• ATP production
• maintaining cellular conditions

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Electrical Gradient vs Concentration Gradient

Consider a positive ion such as K⁺.

A concentration gradient tends to move K⁺ from an area of high concentration toward an area of:

lower concentration.

An electrical gradient may attract K⁺ toward a negatively charged region.

Sometimes these forces act in the:

same direction.

Sometimes they oppose each other.

Their combined effect is the:

electrochemical gradient.

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Electrochemical Equilibrium

Eventually, the chemical tendency for an ion to diffuse can be balanced by the electrical force acting on that ion.

At this point there may be no net movement of that particular ion.

The voltage associated with this balance is called its:

equilibrium potential.

Different ions have different equilibrium potentials because their concentration gradients are:

different.

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Membranes and Batteries: A Useful Comparison

A biological membrane and an electrochemical cell have some useful similarities.

The comparison is useful, but biological membranes are highly dynamic and actively:

regulated.

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Measuring Bioelectricity

Bioelectric signals can be measured using:

electrodes.

Electrodes detect differences in electrical potential.

Examples include:

• ECG — heart activity
• EEG — brain activity
• EMG — muscle activity

An electromyogram (EMG) measures electrical activity associated with:

muscles.

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Why Electrolytes Matter in the Body

In biology, the term electrolyte refers to substances that produce ions when dissolved in body fluids.

Important electrolytes include ions such as:

• Na⁺
• K⁺
• Ca²⁺
• Cl⁻

These ions are essential for:

• nerve signaling
• muscle contraction
• membrane potentials
• fluid balance
• many cellular processes

The word electrolyte therefore has a direct connection to:

electrochemistry.

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Worked Example 1

A neuron has a high Na⁺ concentration outside and a lower Na⁺ concentration inside.

A sodium channel opens.

What tends to happen?

Na⁺ tends to move:

into the neuron.

This movement contributes to:

depolarization.

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Worked Example 2

During an action potential, voltage-gated potassium channels open.

What happens?

K⁺ moves primarily:

out of the neuron.

This helps return the membrane potential toward a negative value.

This process contributes to:

repolarization.

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Worked Example 3

What does the sodium-potassium pump do?

It uses ATP to move:

3 Na⁺ out

and:

2 K⁺ in

during each cycle.

This helps maintain the ion concentration gradients required for:

membrane potentials.

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Worked Example 4

Why is a membrane potential measured in volts?

A membrane potential represents a difference in:

electrical potential

between two locations.

Voltage is the measurement of electrical potential difference.

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Worked Example 5

Why is an action potential described as electrochemical?

Its behavior depends on both:

electrical forces

and:

chemical concentration gradients.

Charged ions move across the membrane according to their electrochemical gradients.

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Worked Example 6

How is a neuron different from a copper wire?

A copper wire primarily carries electrical charge through the movement of:

electrons.

A neuron generates electrical signals primarily through controlled movement of:

ions across its membrane.

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Worked Example 7

Why does the sodium-potassium pump require ATP?

It moves ions against their:

concentration gradients.

This is active transport and therefore requires an input of:

energy.

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Worked Example 8

How can an electric eel generate a much larger voltage than a single biological cell?

Its electric organs contain many:

electrocytes.

Their electrical potentials can combine, producing a much larger overall:

potential difference.

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Worked Example 9

Why can an ECG detect heart activity from the skin?

Coordinated electrical activity in the heart produces changing electrical potentials that spread through body tissues.

Electrodes placed on the skin can detect these:

potential differences.

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Worked Example 10

How does bioelectricity demonstrate electrochemistry?

Biological systems use:

ions, concentration gradients, potential differences, membranes, and controlled charge movement.

These are fundamental electrochemical concepts operating inside:

living organisms.

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Common Mistake: Nerves Carry Electrons Like Wires

Nerve signals are not streams of electrons moving along the axon like electrons through a copper wire.

Neuronal signaling primarily depends on:

ions moving across cell membranes.

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Common Mistake: The Sodium-Potassium Pump Creates Each Action Potential

The sodium-potassium pump is essential for maintaining long-term ion gradients.

However, the rapid changes during an individual action potential are mainly produced by ions moving through:

voltage-gated channels.

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Common Mistake: The Inside of a Neuron Is Always Negative

At rest, the inside is normally negative relative to the outside.

During an action potential, the membrane potential can temporarily become:

positive.

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Common Mistake: Only Animals Use Bioelectricity

Plants, fungi, microorganisms, and animals all use ion gradients and membrane potentials.

Bioelectric phenomena are therefore widespread throughout:

living systems.

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Common Mistake: Electricity and Chemistry Are Separate in Biology

In biological systems, they are closely connected.

The distribution of chemical ions creates electrical potentials, while electrical forces affect how those ions:

move.

That is why the term:

electrochemical gradient

is so important.

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Check Your Understanding

1. Define bioelectricity.
2. What particles mainly carry electrical charge in biological fluids?
3. Name four important biological ions.
4. What is an electrochemical gradient?
5. Why is the cell membrane important for bioelectricity?
6. What does selectively permeable mean?
7. Which ion is generally more concentrated outside a typical neuron: Na⁺ or K⁺?
8. Which is generally more concentrated inside?
9. Define membrane potential.
10. What unit is commonly used for membrane potential?
11. What is a resting membrane potential?
12. What is an approximate resting potential for many neurons?
13. What is an ion channel?
14. What does the sodium-potassium pump do?
15. How many Na⁺ ions are transported during one pump cycle?
16. How many K⁺ ions are transported?
17. Why does the sodium-potassium pump require ATP?
18. Define an action potential.
19. What causes depolarization during a typical neuronal action potential?
20. Which ion moves into the neuron during depolarization?
21. What causes repolarization?
22. Which ion moves out of the neuron during repolarization?
23. What is hyperpolarization?
24. What is threshold?
25. How does an action potential propagate along an axon?
26. What is myelin?
27. What are nodes of Ranvier?
28. Explain saltatory conduction.
29. What is a synapse?
30. What role does Ca²⁺ play at many chemical synapses?
31. How are electrical and chemical signals connected at a synapse?
32. Explain how bioelectricity contributes to muscle contraction.
33. What is the SA node?
34. What does an ECG measure?
35. What does an EEG measure?
36. What does an EMG measure?
37. What is sensory transduction?
38. What are electrocytes?
39. How can electric fish generate relatively large voltages?
40. Give three uses of electrical signals by electric fish.
41. Define electroreception.
42. Give an example of an animal that uses electroreception.
43. Give an example of electrical signaling in a plant.
44. Explain how a concentration gradient can store potential energy.
45. Distinguish between an electrical gradient and a concentration gradient.
46. Explain why biological electrical signals are described as electrochemical.
47. Compare electrical conduction in a metal wire with signaling in a neuron.
48. Compare an electrochemical cell with a biological membrane.
49. Explain why maintaining ion gradients requires cellular energy.
50. Explain how the principles of electrochemistry help us understand nerve signals, muscle activity, and other biological processes.

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Key Terms

Bioelectricity: Electrical phenomena produced by living cells and organisms.

Ion: Electrically charged atom or group of atoms.

Membrane potential: Electrical potential difference across a cell membrane.

Resting membrane potential: Membrane potential of an excitable cell when it is not actively producing an electrical signal.

Electrochemical gradient: Combined effect of concentration and electrical gradients on an ion.

Ion channel: Membrane protein that allows particular ions to cross a cell membrane.

Sodium-potassium pump: Membrane protein that uses ATP to transport Na⁺ out of cells and K⁺ into cells.

Action potential: Rapid, temporary change in membrane potential used for electrical signaling.

Depolarization: Change in membrane potential that makes the inside of a cell less negative or more positive.

Repolarization: Return of membrane potential toward its resting negative value following depolarization.

Hyperpolarization: Temporary change making the membrane potential more negative than its resting level.

Synapse: Junction where a neuron communicates with another cell.

Electrocyte: Specialized cell used by electric fish to generate electrical potentials.

Electroreception: Ability to detect electric fields.

ECG: Recording of electrical activity associated with the heart.

EEG: Recording of electrical activity associated with the brain.

EMG: Recording of electrical activity associated with muscles.

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Key Takeaways

• Bioelectricity depends primarily on the movement and separation of ions.
• Important biological ions include Na⁺, K⁺, Ca²⁺, and Cl⁻.
• Cell membranes maintain different ion concentrations inside and outside cells.
• Separation of charge across a membrane creates a membrane potential.
• The sodium-potassium pump helps maintain Na⁺ and K⁺ concentration gradients using energy from ATP.
• Ion channels allow controlled movement of ions across membranes.
• Neurons transmit information using rapid changes in membrane potential called action potentials.
• Na⁺ movement is important in depolarization, while K⁺ movement is important in repolarization.
• Nervous-system electricity is not simply electrons flowing through neurons like a metal wire.
• Muscle contraction and heartbeat coordination depend on electrical signaling.
• ECGs, EEGs, and EMGs can detect electrical activity produced by living tissues.
• Electric fish can produce strong electrical discharges using specialized electrocytes.
• Some animals use electroreception to detect weak electric fields.
• Plants also use electrical signals to coordinate responses.
• Bioelectricity demonstrates a direct connection between chemistry, electricity, and biology.
• The concept of an electrochemical gradient is central to understanding how living cells use electrical energy.