JAYS QUIZPAD: Vision, Vitamin A Deficiency and Xerophthalmia

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Questions and Answers

Xerophthalmia literally means "dry ______"

eye

The earliest symptom of vitamin A deficiency is ______ blindness.

night

Bitot’s Spots are raised, silvery white triangular patches on the ______ conjunctiva.

bulbar

Corneal xerosis begins with punctate ______ in the lower nasal quadrant.

keratopathy Signup and view all the answers

Prevention of vitamin A deficiency includes food ______, breastfeeding, and supplementation.

fortification Signup and view all the answers

Vitamins are potent organic compounds required in the diet in small amounts for optimum growth and health of the ______.

organism Signup and view all the answers

Most vitamins act as ______ that help in biochemical reactions within the body.

co-enzymes Signup and view all the answers

Vitamin A is recognized as the first fat soluble ______.

vitamin Signup and view all the answers

Retinol, retinal, retinoic acid, and retinyl esters are categories of ______.

retinoids Signup and view all the answers

Beta-carotene is a predominant ______, serving as a precursor to active vitamin A.

carotenoid Signup and view all the answers

Animal products like liver and fish are rich sources of ______ vitamin A.

preformed Signup and view all the answers

The body converts provitamin A predominantly found in ______ into active vitamin A.

plants Signup and view all the answers

Vitamin A aldehyde is known as ______, which is obtained by the oxidation of retinol.

retinal Signup and view all the answers

Vitamin A is necessary for maintenance of normal _______.

epithelium Signup and view all the answers

Rhodopsin is used in _______ vision.

night Signup and view all the answers

The _______ stores most of the retinol in the body.

liver Signup and view all the answers

Vitamin A supports the development of _______ and white blood cells.

lymphocytes Signup and view all the answers

The _______ pigment epithelial cells have specific receptors for the retinol-protein complex.

retinal Signup and view all the answers

Vitamin A concentration in the tear fluid is approximately _______ µmol/litre.

0.1 Signup and view all the answers

Carotenoids act as _______ by oxidizing free radicals.

anti-oxidants Signup and view all the answers

During fetal development, retinoic acid aids in the development of _______.

lungs Signup and view all the answers

Retinol enters into the outer segments of ______

photoreceptors Signup and view all the answers

Retinene combines with the protein ______ to form rhodopsin

opsin Signup and view all the answers

Light absorbed by rhodopsin converts its 11-cis retinal into ______ retinal

all-trans Signup and view all the answers

All-trans retinal may be further reduced to ______ by alcohol dehydrogenase

retinol Signup and view all the answers

11-cis retinal in outer segments of photoreceptors reunites with ______ to form rhodopsin

opsin Signup and view all the answers

Under constant light stimulation, the equilibrium between photodecomposition and ______ of visual pigments is known as the visual cycle

regeneration Signup and view all the answers

Vitamin A is essential for the proper function of ______ in the retina

photoreceptors Signup and view all the answers

1 IU of Vitamin A equals ______ µg of retinol

0.3 Signup and view all the answers

Study Notes

Biochemistry of Vision

Vitamins are potent organic compounds needed in small amounts for optimal growth and health.
Vitamins aren't used for energy; they help utilize other nutrients like carbs, proteins, and fats.
Most vitamins are not made in the body; they must be obtained through diet.
Many vitamins act as co-enzymes.

Classification of Vitamins

Vitamins are classified into two main groups:
- Fat-soluble vitamins: Vitamin A, D, E, and K
- Water-soluble vitamins: Vitamin C and the B-complex vitamins. The B-complex vitamins include subgroups for energy-releasing (B1, B2, B3, B6, B7, and pantothenic acid) and hematopoietic (folic acid and B12) functions.

Vitamin A

Vitamin A is a broad term for similar chemical compounds.
It is the first recognized fat-soluble vitamin.
Two main forms of vitamin A:
- Preformed vitamin A: Retinoids (found in animal products). Four forms of retinoids are retinol, retinal, retinoic acid, and retinyl esters.
- Provitamin A: Carotenoids (found in plants). Beta-carotene is the most important.

Preformed Vitamin A: Retinoids

Active (usable) form of vitamin A.
Four categories of retinoids:
- Retinol
- Retinal
- Retinoic acid
- Retinyl esters
All retinoids are absorbed by the body as retinol.
Sources: animal products like liver, fish, fish oils, milk, eggs. Liver is the richest source.

Structure of Vitamin A

Unsaturated organic compounds.
All forms of Vitamin A have a beta-ionone ring with an attached isoprenoid chain, called a retinyl group.

Retinol

Vitamin A alcohol.
Pure alcohol form is unstable.
Present in animal tissues as a retinyl ester with a long chain fatty acid.

Retinal

Vitamin A aldehyde.
Obtained by oxidizing retinol.
Previously known as retinene.
Retinal and retinol are interconvertible (can change from one form to the other).

Retinoic Acid

Vitamin A acid.
Product of oxidizing retinal.
Cannot be converted back to retinal or retinol.

Provitamin A: Carotenoids

Precursors of vitamin A.
Predominantly beta-carotene.
The body must convert them into active vitamin A after consumption.
Sources: plant products like carrots, leafy greens, papaya, mango, and brinjal.

Types of Carotenoids

Alpha-carotene yields 1 molecule of vitamin A.
Beta-carotene yields 2 molecules of vitamin A.
Gamma-carotene yields 1 molecule of vitamin A.

Vitamin A Sources

Vitamin A comes from animal sources (eggs, meat, and dairy products).
Beta-carotene, a precursor to vitamin A, comes from green, leafy vegetables and intensely colored fruits and vegetables.

Biochemical Functions of Vitamin A

Essential for vision in dim light.
Necessary for maintaining normal epithelium. Goblet cells synthesize mucus to help with antimicrobial defense.
Required for embryonic development (e.g. lungs, heart, eyes, and ears). Regulates expression of growth hormone gene.
Acts as an antioxidant. Carotenoids neutralize free radicals, potentially preventing cancer and other diseases.
Crucial for immune function – ensures proper functioning of mucosal cells, membranes, and epithelial layers (body's first line of defense). Helps lymphocytes and white blood cell development.
Crucial for eye health (formation of rhodopsin for night vision and maintenance of healthy cornea and conjunctiva).

Role of Vitamin A in the Eye

Crucial for forming rhodopsin (used in night vision).
Maintains healthy cornea and conjunctival cells.

Sections for Detailed Study of Vitamin A

Vitamin A Absorption and Storage
Transport from Liver to Eye
Synthesis of Visual Pigments
Light-Induced Changes in Visual Pigments

Vitamin A Absorption and Storage

Dietary vitamin A (carotenoids and retinol) is absorbed.
In the intestines, vitamin A is re-esterified.
Enters bloodstream via intestinal lymphatics.
Most retinol goes to the liver (90% stored).
In liver, retinol binds to retinol-binding protein (RBP), creating a stable form.

Transport from Liver to Eye

Retinol-protein complex enters bloodstream.
Binds to specific receptors on basal surfaces of retinal pigment epithelial (RPE) cells.
Only retinol enters the RPE cells; RBP stays outside.

Synthesis of Visual Pigments

Retinol remains unchanged in RPE cells.
Retinol enters photoreceptor outer segments.
Retinol oxidizes into retinene (11-cis retinal).
Retinene combines with opsin to form rhodopsin.
NAD oxidative system in RPE supports rhodopsin formation.

Light-Induced Changes in Visual Pigments

Light falls on the retina, absorbed by photoreceptors.
Photochemical changes in outer segments initiate electrical changes.
Light-induced changes in rods involve the following:
- Rhodopsin bleaching
- Rhodopsin regeneration
- Visual cycle

Rhodopsin Bleaching and Regeneration

Rhodopsin is a combination of opsin (protein) and retinene (vitamin A aldehyde).
Light absorption changes 11-cis retinal to all-trans retinal in rhodopsin.
This separation (photodecomposition) bleaches rhodopsin.
Rhodopsin regeneration involves:
- All-trans retinal entering the chromophore pool in photoreceptors and RPE cells.
- Conversion of all-trans retinal to 11-cis retinal by the enzyme retinal isomerase.
- 11-cis retinal rejoining opsin to reform rhodopsin.

Visual Cycle

Constant light stimulation: Photoreceptor bleaching = photoreceptor regeneration.
Equilibrium between photodecomposition and regeneration of visual pigments forms the visual cycle.

Phototransduction

Process of converting light energy to electrical signals.
Occurs in photoreceptors.

Photoreceptor Membrane Potential

Photoreceptors (rods and cones) are slightly depolarized relative to a typical neuron.
Instead of a -70mV resting potential, photoreceptors have a -50mV potential.

In Dark

Inner segment of photoreceptors pump Na+ out.
Negative potential inside entire cell.
Na+ channels in photoreceptor outer segments are open due to cGMP.
Na+ from extracellular fluid flows into the outer segment, producing a dark current that creates a slight depolarization.

In Light

Light striking photoreceptors reduces cGMP.
Some Na+ channels close.
Results in photoreceptor hyperpolarization.
Increased negativity of membrane potential rather than decreased negativity (depolarization).

Under Dark Conditions

Photoreceptors are depolarized.
Continuously release neurotransmitter glutamate.

Under Light Stimulation

Photoreceptors are hyperpolarized.
Reduction in glutamate release.

Limited Sodium Channels

Number of sodium channels in rod outer segments limits the magnitude of rod hyperpolarization.
Only about 10 percent of a rod's rhodopsin bleaching affects a critical number of Na+ channels needed to prevent further hyperpolarization.

Activation Cascade

Incident light causes retinene transformation to the all-trans form.
Opsin configuration changes.
Transducin (G protein) is activated.
Alpha subunit separates from transducin (Ga).
Ga activates cGMP PDE.
cGMP converts to 5'-GMP.
Closure of leaky Na+ channels
Hyperpolarization to -70mV
"Switching off"

Decrease in Intracellular Ca2+, Decrease Glutamate Release, and Neural Pathways

Light decreases intracellular Ca2+.
Decreases glutamate release by photoreceptors.
Neural pathways transmit the electrical signal.

Vitamin A Deficiency

Causes include dietary deficiency, poor intestinal absorption, insufficient vitamin A stored in the liver, and other diseases.
Conditions like night blindness (early symptom) and xerophthalmia (general term for impaired vitamin A metabolism) may arise. Xerophthalmia can extend to cornea destruction.

Xerophthalmia

General term for eye manifestations resulting from impaired vitamin A metabolism, from night blindness to corneal destruction.
Xeros - dry; ophthalmia - eye.
Typically synonymous with vitamin A deficiency.

Prevalence of Vitamin A Deficiency

Leading preventable cause of childhood blindness worldwide.
Estimated that 30% of the world's blindness is related to vitamin A deficiency.

Night Blindness

Earliest symptom of vitamin A deficiency.
Difficulty seeing in dusky or dark environments.

Prevention of Vitamin A Deficiency

Breastfeeding.
Vitamin A supplementation.
Food fortification.
Promoting vitamin A-rich diets.

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Description

Test your knowledge on vitamin A deficiency and related terms with this quiz. Explore symptoms like xerophthalmia and the importance of vitamin A in overall health. This quiz covers essential concepts about vitamin A and its prevention.