What you need to know
- 8.8Rods detect light using rhodopsinThe light-sensitive pigment in rod cells, made of the protein opsin and retinal., opsin, retinalThe light-absorbing part of rhodopsin. Light changes it from the cis to the trans form., sodium ions, cation channels and hyperpolarisationWhen the membrane potential becomes more negative than the resting potential., leading to action potentialA rapid reversal of the potential difference across a neurone membrane, caused by Na⁺ ions flowing in, then K⁺ ions flowing out. in optic neurones.
Cones are not required. The rod hyperpolarises. It does not fire the action potential. Do not import the axon sodium spike into the rod.
By the end, you should be able to…
- Describe rhodopsin as opsin plus retinal.
- Explain why light closes sodium channels and hyperpolarises the rod.
- State where the action potentials are produced.
Build your understanding
Rhodopsin is the light-sensitive pigment
Rods are photoreceptors in the retina. They work in dim light. Their pigment is rhodopsinThe light-sensitive pigment in rod cells, made of the protein opsin and retinal., made of the protein opsin and a molecule called retinalThe light-absorbing part of rhodopsin. Light changes it from the cis to the trans form.. In the dark, retinal is in the cis form and is bound to opsin. Many rods share connections toward one optic neurone, which is why rods detect dim light but do not give a sharp picture. Acuity is not the specification focus. The pigment chemistry is.
The National Eye Institute describes photoreceptors turning light into electrical signals. The specification tells you the electrical direction in a rod, which that overview does not. Learn the direction here.
Dark current, then a closing of channels
In the dark, cation channels in the rod membrane are open. Sodium ions enter. The rod is relatively depolarised and releases a transmitter onto the next cell. This resting inflow is sometimes called the dark currentThe steady inflow of Na⁺ ions into a rod cell in the dark, which keeps the rod depolarised.. It is the opposite of the resting axon, which is waiting to open sodium channels.
When light is absorbed, retinal changes from cis to trans and opsin changes shape. Rhodopsin is bleached. A short cascade, which you do not need to name enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. by enzyme, closes the sodium channels. Sodium entry falls. The inside of the rod becomes more negative. That is hyperpolarisationWhen the membrane potential becomes more negative than the resting potential.. The rod then releases less transmitter.
The spike is one cell later
The change in transmitter alters the bipolar cell, and ganglion cells produce action potentialA rapid reversal of the potential difference across a neurone membrane, caused by Na⁺ ions flowing in, then K⁺ ions flowing out.. Those impulses travel in the optic nerve. The specification line is exact: hyperpolarisation of the rod leads to action potentials in the optic neurones. If you write that the rod depolarises and fires a spike, you have used the axon lesson in the wrong cell.
In the dark the pigment is rebuilt and the channels can open again. Bright light bleaches many rods, which is why vision is poor for a moment when you walk into a dark room. That recovery is useful context. It is not a second pathway to memorise.
Follow the process
In the dark: cation channels are open → sodium enters → the rod is relatively depolarised → it releases transmitter. In the light: retinal changes from cis to trans and opsin changes shape → rhodopsin is bleached → sodium channels close → sodium entry falls → the rod hyperpolarises → less transmitter is released → bipolar cells change, and ganglion cells fire action potentials in the optic nerve. The rod itself does not fire that spike. This is the opposite of an axon, where a stimulus opens sodium channels.
NOT: the rod does not fire the action potential, and this point is not about cones.
- Rhodopsin is opsin plus retinal
- Light closes sodium channels
- The rod hyperpolarises
- The spike is in the optic neurone
Two traps this diagram blocks
- Do not say the rod fires an impulse down the optic nerve.
- Do not describe this as the same sodium entry as an axonal action potential.
From the diagram alone
Question
Explain how light changes a rod, and state where the action potentials occur. [4]
Show a model answer
Light changes retinal in rhodopsin, sodium channels close and the rod hyperpolarises, so less transmitter is released. Action potentials are then produced in the optic neurone, not in the rod.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: a rod hyperpolarises in the light. Y: light closes its sodium channels, so the dark currentThe steady inflow of Na⁺ ions into a rod cell in the dark, which keeps the rod depolarised. stops. Z: less transmitter is released, and the next cells produce the action potentialA rapid reversal of the potential difference across a neurone membrane, caused by Na⁺ ions flowing in, then K⁺ ions flowing out. in the optic nerve. Incomplete: “The rod fires an impulse down the optic nerve.” Improved: “Light bleaches rhodopsinThe light-sensitive pigment in rod cells, made of the protein opsin and retinal., sodium channels close, and the rod becomes more negative. The spike is in the optic neurone, not in the rod.”
Light closes sodium channels
Opposite of the axon spike. hyperpolarisationWhen the membrane potential becomes more negative than the resting potential., less transmitter.
Name both parts of the pigment
Rhodopsin is opsin plus retinalThe light-absorbing part of rhodopsin. Light changes it from the cis to the trans form.. Light changes retinal’s shape.
Avoid this misconception
The claim: Light opens sodium channels in the rod and the rod fires an action potential down the optic nerve.
The correction: Light causes sodium channels in the rod to close. The rod hyperpolarises and releases less transmitter. Action potentials are produced in the optic neurones.
Why does a rod go more negative when light hits it?
In the dark, sodium ions flow into a rod all the time. Light changes the shape of retinal in rhodopsin, the sodium channels close, and the rod hyperpolarises. The action potentials come one cell later, in the optic neurone.
What the model shows
One rod, its pigment and channels, and the optic neurone it signals to, in the dark, in dim and bright light, and recovering in the dark.
What to look for
Which way the rod’s voltage moves, what happens to transmitter release, and where the spikes appear.
What it tells us
Light closes Na⁺ channels and hyperpolarises the rod. Bleached rhodopsin must be rebuilt in the dark. The rod does not fire action potentials.
Be careful what you claim
Illustrative values; recovery sped up.
Try it in the model: press Play, then switch to Dim light.
How a rod detects light
Follow one rod in the dark, in dim and bright light and while it recovers in the dark: rhodopsin, the Na⁺ channels, the rod’s voltage and the action potentials in the optic neurone. Values are illustrative and recovery is sped up.
What you see
- A rod: the outer segment with discs of rhodopsin (purple when ready, pale when bleached), Na⁺ channels in its membrane, and the synaptic terminal releasing transmitter onto a bipolar cell and then an optic (ganglion) neurone.
- A close-up of one rhodopsin molecule with retinal in its cis or trans form, and a graph of the rod’s membrane potential with the optic neurone’s action potentials below it.
How to use it
- Choose Dark, Dim or Bright light, press Flash a dim light for a short test flash, and turn on the drug that holds the Na⁺ channels open. Play runs the clock.
What it shows
- Rhodopsin is opsin plus retinal; light changes retinal from cis to trans and bleaches the pigment (8.8).
- Light closes cation channels, so the rod hyperpolarises and releases less transmitter, leading to action potentials in the optic neurones (8.8).
Limits of the model
- Values are illustrative and rebuilding rhodopsin is sped up about tenfold. The cascade between rhodopsin and the channels is not named, as the specification does not require it.
- The bipolar and ganglion cells are simplified to one pathway in which less transmitter means more firing. Cones are not included.
Guided tour · step 1 of 9
- In Rod, just below these steps, keep Light on Dark (options: Dark, Dim light, Bright light).
- Press Play and watch the simulator for a few seconds.
Cation channels in the rod membrane are open, so Na⁺ ions flow in (the dark current). The rod sits relatively depolarised at about −40 mV and releases transmitter all the time.
Rod
Questions
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
What are the two components of rhodopsin?
Show suggested answer
The protein opsin and retinal.
Practice question
Retrieval and explanation · suggested answer
What happens to sodium channels in a rod when light is absorbed?
Show suggested answer
They close, so less sodium enters and the rod hyperpolarises.
Practice question
Retrieval and explanation · suggested answer
Where are the action potentials of this pathway produced?
Show suggested answer
In the optic neurones, not in the rod itself.
Practice question
Retrieval and explanation · suggested answer
Why do rods give poor detail even though they are sensitive?
Show suggested answer
Many rods converge onto the same pathway, so the brain cannot tell which rod absorbed the light.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A drug holds rod sodium channels open even after rhodopsin has bleached. What happens to the rod’s membrane potential and to transmitter release when the light comes on? Assume the optic neurones are otherwise normal.
Show a hint
The light signal is the closing, not the pigment name alone.
Explore the reasoning
If the channels cannot close, sodium keeps entering, the rod does not hyperpolarise, and transmitter release does not fall. The optic neurones therefore do not receive the change that would make them alter their firing. Bleaching of the pigment is not enough if the channel step is blocked. This is a supplied drug effect, not a medicine to know.
Recall this lesson
Lesson recall questions
Question 1 of 5: How does hyperpolarisation of a rod lead to action potentials in optic neurones?
Rod cells and rhodopsin
Practice question
Retrieval and explanation · suggested answer
How does hyperpolarisation of a rod lead to action potentials in optic neurones?
Lesson: 8A 5: Sensory Systems and the Detection of Light
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Photoreceptors start the electrical story
8A · Sensory Systems and the Detection of LightThe National Eye Institute says the retina is a light-sensitive layer and that photoreceptors turn light into electrical signals. It does not mention rhodopsin, retinal, or hyperpolarisation. Those…Specification 8.8Read & use the scienceClose article
Why this matters
The National Eye Institute says the retina is a light-sensitive layer and that photoreceptors turn light into electrical signals. It does not mention rhodopsin, retinal, or hyperpolarisation. Those names are the specification. Use the page only to remember that the eye’s job is a signal, then write the rod mechanism properly. 8.8: rhodopsin, closing of sodium channels, hyperpolarisation, then action potentials in optic neurones. A behaviour or symptom is the output of a pathway, not a direct recording of ion movement in one axon or synapse.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.8 Rod cells, rhodopsin and visual signalling.
Use the science
Why is an answer that only says “the retina detects light” short of 8.8?
Explore a possible response
It misses opsin and retinal, the closing of sodium channels, hyperpolarisation of the rod, and the fact that the spikes are in the optic neurones.
Explore the source
- National Eye Institute: how the eyes workUS National Eye Institute
Dark adaptation is more than pupil size
8A · Sensory Systems and the Detection of LightNEI researchers studied how rod photoreceptors in mice adjust their sensitivity when light conditions change. Rods are especially useful in dim light, and the study followed movement of…Specification 8.8Read & use the scienceClose article
Why this matters
NEI researchers studied how rod photoreceptors in mice adjust their sensitivity when light conditions change. Rods are especially useful in dim light, and the study followed movement of a signalling protein called transducin inside those cells. This is beyond the required rhodopsin–retinal–channel sequence, but it reinforces that vision changes within the retina as well as at the pupil. A dilated pupil lets more light reach the retina; it does not itself restore the molecular state of a bleached pigment or explain every minute of dark adaptation. The mouse result is a research clue, not a direct measurement of one student’s night vision.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 5, pp. 35–38. The application extends beyond required recall.
- 8.8 Rod cells, rhodopsin and visual signalling.
Use the science
Why could two eyes with equally wide pupils still differ in sensitivity to dim light?
Explore a possible response
Pupil diameter controls light entry, but the state and number of rod photoreceptors and their signalling machinery determine how that light is converted to neural signals.
Explore the source
- How eyes adapt to the darkUS National Eye Institute