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Vertebrate visual opsin

Vertebrate visual opsin is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Vertebrate visual opsin rather than just read about it. In short: Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells.

Vertebrate visual opsin — main illustration
Vertebrate visual opsin — illustration

Key takeaways

  • Vertebrate visual opsin belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Vertebrate visual opsin to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Vertebrate visual opsin from memory before moving on to harder problems.

Reference excerpt

Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells. They are often abbreviated to opsin, as they were the first opsins discovered and are still the most widely studied opsins.

Opsins

Opsin refers strictly to the apoprotein (without bound retinal). When an opsin binds retinal to form a holoprotein, it is referred to as Retinylidene protein. However, the distinction is often ignored, and opsin may refer loosely to both (regardless of whether retinal is bound). Opsins are G-protein-coupled receptors (GPCRs) and must bind retinal ⁠— typically 11-cis-retinal ⁠— in order to be photosensitive, since the retinal acts as the chromophore. When the Retinylidene protein absorbs a photon, the retinal isomerizes and is released by the opsin. The process that follows the isomerization and renewal of retinal is known as the visual cycle. Free 11-cis-retinal is photosensitive and carries its own spectral sensitivity of 380 nm. However, to trigger the phototransduction cascade, the process that underlies the visual signal, the retinal must be bound to an opsin when it is isomerized. The retinylidene protein has a spectral sensitivity that differs from that of free retinal and depends on the opsin sequence. While opsins can only bind retinal, there are two forms of retinal that can act as the chromophore for vertebrate visual opsins:

Retinal 1 (11-cis-Retinal) - the common form present in most opsins Retinal 2 (11-cis-3,4-Dehydroretinal) - a rarer form that is relatively red-shifted compared to retinal 1. Animals living on land and marine fish form their visual pigments exclusively with retinal 1. However, many freshwater fish and amphibians can also form visual pigments with retinal 2, depending on the activation of the enzyme retinal-3,4-desaturase (GO:0061899). Many of these species can switch between these chromophores during their life cycle, to adapt to a changing habitat.

Function

Isomerization of 11-cis-retinal into all-trans-retinal by light induces a conformational change in the protein that activates the phototransduction pathway.

Subclasses There are two classes of vertebrate visual opsin, differentiated by whether they are expressed in rod or cone photoreceptors.

Cone opsins Opsins expressed in cone cells are called cone opsins. The cone opsins are called photopsins when unbound to retinal and iodopsins when bound to retinal. Cone opsins mediate photopic vision (daylight). Cone opsins are further subdivided according to the spectral sensitivity of their iodopsin, namely the wavelength at which the highest light absorption is observed (λmax).

Rod opsins

Opsins expressed in rod cells are called rod opsins. The rod opsins are called scotopsins when unbound to retinal and rhodopsins or porphyropsins when bound to retinal (1 and 2, respectively). Rod opsins mediate scotopic vision (dim light). Compared to cone opsins, the spectral sensitivity of rhodopsin is quite stable, not deviating far from 500 nm in any vertebrate.

Evolution

Extant vertebrates typically have four cone opsin classes (LWS, SWS1, SWS2, and Rh2) as well as one rod opsin class (rhodopsin, Rh1), all of which were inherited from early vertebrate ancestors. These five classes of vertebrate visual opsins emerged through a series of gene duplications beginning with LWS and ending with Rh1, according to the cladogram to the right; this serves as an example of neofunctionalization. Each class has since evolved into numerous variants. Evolutionary relationships, deduced using the amino acid sequence of the opsins, are frequently used to categorize cone opsins into their respective class. Mammals lost Rh2 and SWS2 classes during the nocturnal bottleneck. Primate ancestors later developed two LWS opsins (LWS and MWS), leaving humans with 4 visual opsins in 3 classes.

History George Wald received the 1967 Nobel Prize in Physiology or Medicine for his experiments in the 1950s that showed the difference in absorbance by these photopsins (see image).

See also Color blindness Melanopsin Retinylidene protein Rhodopsin Visual cycle Visual phototransduction

References

Illustrations

Vertebrate visual opsin: Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The retinal chromophore is shown in red.
Three-dimensional structure of bovine rhodopsin. The seven transmembrane domains are shown in varying colors. The retinal chromophore is shown in red.
Vertebrate visual opsin: Normalised absorption spectra of the three human photopsins and of human rhodopsin (dashed). Drawn after Bowmaker and Dartnall (1980).[5] (Absorption curves do not directly reflect sensitivity spectra.)[6]
Normalised absorption spectra of the three human photopsins and of human rhodopsin (dashed). Drawn after Bowmaker and Dartnall (1980).[5] (Absorption curves do not directly reflect sensitivity spectra.)[6]

Worked examples

Example 1 — a first encounter with Vertebrate visual opsin

Start with the simplest possible case. Write down what Vertebrate visual opsin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Vertebrate visual opsin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Vertebrate visual opsin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Vertebrate visual opsin

In research
Vertebrate visual opsin appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Vertebrate visual opsin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Vertebrate visual opsin is common in secondary-school and first-year university syllabi. It links to neighbouring topics G protein-coupled receptors, Vision, so understanding it makes those chapters shorter.
In everyday life
Look for Vertebrate visual opsin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Vertebrate visual opsin in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Vertebrate visual opsin means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Vertebrate visual opsin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Vertebrate visual opsin in simple terms?

Vertebrate visual opsins are a subclass of ciliary opsins and mediate vision in vertebrates. They include the opsins in human rod and cone cells.

Why does Vertebrate visual opsin matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Vertebrate visual opsin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Vertebrate visual opsin.

Tags

  • G protein-coupled receptors
  • Vision

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