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Response to Stimuli

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Biology

All biological systems, from single-cell bacteria to multicellular animals, constantly exchange matter and energy with the outside environment. When animals eat or plants take in sunlight, they interact with their environment, exchanging matter and energy. These interactions ensure the survival of the organism but also cause changes within its system. To survive, all living systems must maintain homeostasis, meaning a constant internal environment for optimal functioning despite these interactions. Therefore, all living beings must be able to regulate their responses to stimuli, which are changes in the internal or external environment.

Homeostasis mechanisms

Homeostasis is defined as the regulation and maintenance of a constant internal environment. There are two ways in which we regulate this internal environment when met with changes:

  • Negative feedback loops
  • Positive feedback loops

Negative feedback loop

The homeostatic balance is maintained through control mechanisms that regulate all organisms. Most of these mechanisms are called negative feedback control loops because they counter any change and restore the previous balance through corrective actions.

A change is detected when any factor relevant to homeostatic balance rises above or falls below a certain optimal value. Once the change is detected and assessed, a response is triggered to decrease or increase the factor following the change. These corrections maintain stability in various factors such as the internal temperature (thermoregulation) of biological systems even when the external temperature varies uncontrollably. Figure 1 exemplifies a generic negative loop mechanism.

Response to stimuli example StudySmarterFigure 1. The negative Feedback loop, João Aguiar, StudySmarter Originals

Positive feedback loop

There are also less common homeostatic control mechanisms required for organism survival called positive feedback loops.

Positive loops enhance the change once detected by stimuli instead of correcting it. These systems are far less common because they lead to cascades of repeating events that enhance the stimuli. This type of positive loop is unstable by nature but can also be very important for homeostasis.

Blood coagulation happens through a positive feedback loop that makes sure we don’t bleed out when we cut ourselves, and our blood vessels’ walls open.

Blood clotting is a process that requires platelet aggregation. Once the stimulus is received that the vessel is opened to the outside, platelet aggregation is triggered. Platelets themselves promote their accumulation at the injury site by releasing chemicals and plugging the haemorrhage.

From stimuli to a response

A stimulus is any detectable change in an environmental or physiological factor. These factors can include any condition necessary to the organism and its cell’s optimal function and survival, like temperature or cell pH. Receptors detect stimuli.

Receptors are proteins complementary to a specific molecule or a type of stimulus. They are found intracellularly and extracellularly and can initiate downstream reactions once stimulated.

Coordination and response

Any variation from an optimal level stimulates the receptors. This triggers an adaptive response by effector organs that corrects the imbalance. These responses can be complicated, especially in multicellular organisms like animals and plants, where receptors receive stimuli in one part of the body and effectors generate a response in another part of the body.

Effector organs produce a response to the stimulus. This includes skeletal muscle and glands.

This requires coordination systems to connect the receptors to the effectors through signals and control centres. This happens through the nervous and endocrine systems via electric or chemical/hormonal signalling in animals. In plants, these reactions happen through chemical systems like plant hormones.

External stimuli

External stimuli are changes in environmental factors, meaning external conditions to the organism that can affect its function. This includes environmental temperature or survival threats like environmental dangers.

Our five senses perceive external stimuli, including touch, vision, sound, smell, and taste. These include various types of receptors that can detect environmental changes.

For example, when you cross a road, you look both ways to assess oncoming traffic that may pose a threat to you. The visual and sound stimuli are received through receptors in our eyes and ears, assessed in our control centre in the nervous system, and a decision to either wait or walk is made and executed by effector organs (muscle cells) if you choose to walk.

Internal stimuli

Internal stimuli result from variations in physiological factors detected by internal receptors. These factors include systemic blood pressure and blood water content.

Blood pressure needs to be maintained at a relatively constant level to ensure proper blood flow and oxygen supply reaches the organ systems and their cells. Changes in arterial blood pressure are detected by baroreceptors, a type of pressure sensor present in blood vessels like the aorta. When blood pressure is too high or too low, baroreceptors receive stimuli that trigger a necessary corrective response by effector organs.

If blood pressure is too high, the following responses are triggered to increase it:

  • Kidneys retain less water
  • Vasodilation
  • Heart rate decreases

If blood pressure is too low, the following occurs:

  • Kidneys retain more water
  • Vasoconstriction
  • Heart rate increases

Stimuli and response examples

Let’s review two classic examples of control mechanisms and responses to different stimuli: pain and temperature.

Response to pain stimuli

Pain stimuli are external stimuli most often associated with our sense of touch. This stimulus is received by nociceptors (“pain receptors”) on our skin that signal towards possible threats. This process can elicit a significant behavioural response once the central nervous system determines if the threat presents a true danger to survival or physical integrity.

If significant enough, the control centre may even elicit an automatic response to mitigate the danger through a fast process called a reflex arc. A reflex arc mechanism causes a reflex action, an unconscious effectuated response that happens even before responding voluntarily to the threat. One common example is touching something sharp. Nociceptors on your hand detect the threat and transmit the signal through electric impulses to our brain, which creates the sensation of pain and triggers a fast, involuntary response to move away from the threat, thus stopping its harm.

Response to temperature

Temperature is perhaps one of the most important factors to be maintained in any organism in homeostatic balance. A stable temperature is vital to allow all chemical reactions that sustain life.

Thermoregulation consists of all the behavioural and physiological regulatory mechanisms required to maintain the internal temperature constant despite variations in external temperature. If such responses didn’t exist, then our body temperature would automatically vary and match the outside environment, which is incompatible with life.

Organisms have different strategies to respond to heat change. All mammals, including humans, can generate heat and have negative feedback loops that allow heat retention in cold environments and heat loss in hot environments. This heat balance allows us to maintain a constant temperature independently.

Most other animals rely on the environment for heat sources and are therefore more vulnerable to its variation. They must rely on behavioural responses to maintain the internal temperature constant in these cases. This includes seeking shelter at night when it’s cold or the shade during the day when it’s hot to balance heat exchange.

In humans, thermoregulation is coordinated by the hypothalamus in the brain. This is our internal 'thermostat' and measures our core temperature, which needs to be around 37°C. Receptors spread across the skin and within the body monitor internal and external temperature variation and relay the information to the hypothalamus, which acts upon any change with corrective actions once a stimulus is created.

If the core temperature decreases as a result, for example, of a cold environment, the hypothalamus will try to increase heat production and retention in our body through:

  • Vasoconstriction – reduction of blood vessels’ lumen diameter near the skin, decreasing blood flow and heat loss
  • Shivering – involuntary muscle contractions that generate extra heat
  • Raising body hairs – muscle contraction near the skin to trap air and heat

Meanwhile, if the core temperature increases as a result, for example, of a hot environment, the hypothalamus will try to increase heat loss in our body through:

  • Vasodilation – an increase of blood vessels’ lumen diameter near the skin increasing blood flow and heat loss
  • Sweat production – increased evaporation from the skin in the form of sweat helps to increase heat loss.

Response to Stimuli - Key takeaways

  • Every living being needs to maintain homeostasis to survive, which means maintaining an internal constant environment despite internal and external/environmental pressures.
  • When a relevant parameter to homeostasis changes past a certain point, a stimulus is triggered by receptor cells.
  • Coordination systems like the nervous and endocrine systems act upon stimuli by determining if a response is required to maintain balance. Control centres instruct organ effectors to carry out an adequate response when a response is necessary.
  • Response to stimuli is most often corrective action, which is a response that annuls the cause that triggered the stimuli in the first place. This type of mechanism is known as a negative feedback loop. When a response amplifies the initiating stimuli, the mechanism is known as a positive feedback loop.
  • There are different types of stimuli: internal stimuli are detected by internal receptors, while external stimuli are detected by external receptors divided into our five senses: touch, vision, sound, smell, and taste.

Response to Stimuli

Response to stimuli is any action made by a biological system after a variation in its homeostatic balance is detected through stimuli. Responses are often corrective actions that counteract change restoring balance in the case of the homeostatic negative feedback loops. In the less common positive loops however a response can heighten the imbalance creating a cascade of repeating events.

Thermoregulation is an example of a response to stimuli mechanism. In a cold environment (low-temperature stimuli), blood vessels in humans constrict (vasoconstriction) to increase heat retention, while in a hot environment (high-temperature stimuli), blood vessels dilate (vasodilation) to increase heat loss.

Cells respond to stimuli using organ effectors. Signalling from control centres in the nervous and endocrine system direct effectors such as cells in muscles or blood vessels to respond to the stimuli and ensure homeostasis. Different stimuli are detected by different receptors spread across the organism.

All organisms with appropriate receptors can respond to stimuli. There are different types of receptors for different types of stimuli. Photoreceptors detect light, nociceptors signal potential threats through pain. Whether an organism is able to respond to particular stimuli depends on if it has the appropriate receptors.

The 5 types of external stimuli are often divided into our senses: touch (pressure/movement), vision (light), hearing (sound), smell (chemical), and taste (chemical).

Final Response to Stimuli Quiz

Question

What is the autonomic nervous system part of?

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Answer

Peripheral nervous system.

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Question

What is the autonomic nervous system?

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Answer

The autonomic nervous system is a neural pathway in the peripheral nervous system.

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What is the function of the autonomic nervous system?


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Answer

The autonomic nervous system regulates the automatic responses to stimuli.

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What are the three divisions of the autonomic nervous system?


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Answer

The three divisions of the autonomic nervous system are sympathetic, parasympathetic and enteric.

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Question

The somatic nervous system has thick long neurones with thick myelination. True or False?


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Answer

True.

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Ganglions are mostly found in the autonomic nervous system. True or False? 


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Answer

True.

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The parasympathetic is faster than the sympathetic response. True or False?


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Answer

False.

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The parasympathetic system and sympathetic nervous system are atagonistic. True or False?


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Answer

False - wrong spelling of "antagonistic".

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The parasympathetic nervous system helps the body mobilise resources to deal with danger quickly and effectively. True or False?


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Answer

False.

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The heart rate wouldn't be steady without the input from the sympathetic nervous system. True or False?


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Answer

False.

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What's the function of the sinoatrial node?


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Answer

The sinoatrial node is the pacemaker that controls the rhythmic contractions of the heart muscle.

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The sympathetic nervous system decreases the heart rate. True or False?


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Answer

False.

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Question

What is the part of the brain stem called that controls the influence of the autonomic nervous system on the heart rate?


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Answer

The part of the brain stem that controls the influence of the autonomic nervous system on the heart rate is called the medulla oblongata.

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Chemoreceptors tell the medulla oblongata when blood pressure is too low or too high. True or False?


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Answer

False.

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Baroreceptors can signal to the medulla to use both parasympathetic and sympathetic nervous pathways to increase or decrease the heart rate. True or False? 


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Answer

True.

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Question

In what layer of the skin do we find Pacinian corpuscles? 


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Answer

In a layer called the hypodermis, which is situated deep in the skin below the dermis.

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Where are Pacinian corpuscles most abundant in the skin?


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The fingers, soles of the feet, and the external genitalia

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Aside from the skin, where else are Pacinian corpuscles usually found, and what is the function of them here?


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They are commonly found in joints, ligaments, and tendons. They are useful here as they allow the organism to know which joints are changing direction.

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Why do we say that the structure of a Pacinian corpuscle resembles that of an onion?


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Answer

Similar to the layered structure of an onion, the Pacinian corpuscle consists of layers of connective tissue separated by a gel.

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What type of neurone is in the centre of a Pacinian corpuscle?

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Answer

Sensory neurone

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What is meant by the term ‘stretch-mediated sodium channel’?


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This means that the permeability of the channel to sodium changes when the channel is deformed, for example, by stretching.

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 What is meant by the term ‘resting state’?


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This is the normal state of the Pacinian corpuscle, where the membrane is not deformed as there is no mechanical pressure applied.

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Why does the Pacinian corpuscle not generate an action potential when it is in its resting state?


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During this state, the stretch-mediated sodium channels of the connective tissue are too narrow, so sodium ions are unable to pass along them. This means that the membrane has its resting potential and is not depolarised. Therefore, the membrane cannot reach the generator potential in order to trigger an action potential (nerve impulse).

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When pressure is applied to the Pacinian corpuscle, what causes the sodium ions to diffuse into the neurone?


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The membrane will become stretched as it is deformed. This causes the sodium channels to widen as they are stretch-mediated. So, sodium ions will be able to diffuse into the neurone.

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How does the influx of sodium ions into the membrane lead to the production of an action potential? 


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Answer

The depolarisation continues until a threshold is reached, which triggers a generator potential to be produced. This generator potential will then create an action potential.

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Question

What does it mean when we say receptors are specific?    

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This means that each receptor is specialised to only respond to one certain type of stimulus. For example, the thermoreceptors of the skin will respond to temperature changes and nothing else.

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What receptors are important in helping us with our sense of touch?

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Answer

Pacinian corpuscles allow us to distinguish between the level of pressure we feel when we touch something, while thermoreceptors allow us to distinguish between the hot and cold air touching our skin, so we can feel temperature changes.

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State some examples of receptors. 


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Mechanoreceptors, chemoreceptors, photoreceptors

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What is a receptor?

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A receptor is a cell or a group of cells that receives information from stimuli.

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What does transducer mean?

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Something that converts energy from one form to another.

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What do sensory reception and sensory perception mean?


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Sensory reception is when receptors identify changes in the internal or external environment. Sensory perception is when the brain receives this information and makes sense of it.

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Why is the function of receptors so important?


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They facilitate communication between the brain and different parts of the body, which helps us to adjust to external and internal environmental conditions.

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What is meant by mechanical pressure?

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Answer

The pressure caused by physical force. An example of this would be the pressure of your shoe against the sole of your foot when walking. 

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In a Pacinian corpuscle, what are lamellae?

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Answer

Layers of connective tissue separated by a gel.

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What properties of sodium ions allows them to depolarise membranes?

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Answer

 They are positively charged ions

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Question

What causes the refractory period in a Pacinian corpuscle?

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Answer

Directly after the creation of the action potential, the sodium channels do not open in response to a new signal - they are inactivated. This is what causes the refractory period.

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What happens to the nerve during the refractory period?

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The nerve cannot fire another action potential for a temporary amount of time, normally around 1 millisecond

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Select the answer that is NOT true about the structure of Pacinian corpuscles

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Answer

We can find stretch-mediated calcium channels in a Pacinian Corpuscle 

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What are photoreceptors?

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Answer

The cells in the retina that respond to light stimuli.

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What are the two types of photoreceptors?

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Answer

Rod cells and cone cells

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Where is the retina located?

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The retina is a thin layer of tissue at the back of the eye.

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Where are cone cells located in the eye?

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They are packed together in the fovea region of the eye.

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Where are rod cells located in the eye?

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They are found mainly in the peripheral parts of the retina.

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Why do we have a blind spot?

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The point where the optic nerve connects to the retina has no photoreceptors. This part of the eye is called the blind spot, as we cannot see anything here.

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What is the function of the iris?

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Opens and closes the pupil, controlling the amount of light that enters the eye.

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What does the light do to pigments that causes them to undergo a chemical change?

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Light bleaches the pigments.

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Where does the nerve impulse released by the photoreceptors go?

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Answer

It first goes through the bipolar neurone, followed by the optic nerve, and then to brain.

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What is a bipolar neurone?

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Answer

A neurone with two extensions attached to its cell body. One extension acts as an axon and one acts as a dendrite.

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Which type of photoreceptor is more sensitive and why?

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Answer

Rod cells because many rods are joined to a singular neurone, so summation can occur.

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Question

What is the pigment in rod cells called?

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Answer

Rhodopsin

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