Biochemistry and Metabolism- Lens

Question 1

What is the lens index of refraction

Answer 1

1.390, which is higher than that of the surrounding media and a result of its high protein content (SOLUBLE PROTEIN CALLED CRYSTALLINS)

Question 2

How much do proteins constitute of the lens weight

Answer 2

33% of the weight of the lens, which is 2–3 times higher than their concentration in other tissues in the body

Question 3

How does the lens generate ATP

Answer 3

glycolysis. Alterations in this metabolic pathway have been implicated in the development of congenital cataract as well as diabetic cataract

Question 4

What type of collagen does the lens capsule comprise of

Answer 4

type IV collagen

Question 5

Which layer of the capsule covering the lens has an epithelial layer

Answer 5

Anterior layer.
Posterior layer does not have an epithelium. It is very thin layer

Question 6

Where do the epithelial cells divide

Answer 6

At the germinative zone anterior to the equator

Question 7

Pathophysiology of posterior subcapsular cataracts

Answer 7

In the adult lens, lens epithelial cell migration posterior to the equator results in the development of posterior subcapsular cataract

Question 8

What is the lens composed of

Answer 8

lens fibres which are long ribbonlike cells

Question 9

Where are the oldest and youngest cells located in the lens

Answer 9

The lens structure can be equated with the growth rings of a tree: the oldest cells are in the center, and the progressively younger layers, or shells, of fiber cells are toward the periphery.

Question 10

What are the 2 types of crystallin protein in the lens

Answer 10

α-crystallin and the β,γ-crystallin family, both of which seem to be present in all vertebrate lenses but have also been demonstrated in other ocular tissues.The second group consists of the taxon- specific crystallins, which are present only in certain species.

Question 11

What is alpha crystallin

Answer 11

a member of the small heat shock protein family. Heat shock proteins are molecular chaperones; they stabilize partially folded proteins and prevent them from aggregating.
Mutations in the α-crystallin gene result in premature cataract development

Question 12

What is beta,gamma Crystallins

Answer 12

β-Crystallins exist as polymers, and γ-crystallins are monomeric. The specific functions of the β,γ-crystallins are unknown.Gamma crystallins are concentrated in the nucleus of the lens.
Given their compact and symmetric structures (which can pack very densely), γ-crystallins tend to be highly con- centrated in aged, hard lenses, which have little to no accommodative ability.

Question 13

What are taxon specific crystallins

Answer 13

Taxon-specific crystallins have not been demonstrated in humans. Reduced nucleotides absorb ultra- violet (UV) light and may protect the retina from UV-induced oxidation.

Question 14

Which 2 proteins form a cytoskeletal structure of the lens

Answer 14

filensin and phakinin

Question 15

Which wavelengths of light does the cornea and aqueous humour protect the eye from

Answer 15

wavelengths below the visible spectrum (mainly UV). Wavelengths of 300 nm or below are blocked by the cornea and by ascorbate (vitamin C), which is present at high levels in the aqueous humor. Wavelengths of 360 nm or below are blocked by the lens

Question 16

Lens fibre packaging

Answer 16

hexagonal
packing of lens fibers

Question 17

Which pump in the lens maintains the ionic balance

Answer 17

Na+,K+-ATPase (also called sodium-potassium pump). Functional Na+,K+-ATPase pumps are found primarily at the anterior surface of the lens, in the epithelium and the outer, immature fibers

Question 18

Which family do gap junctions in the lens originate from

Answer 18

Connexins

Question 19

Composition of electrolytes in the lens

Answer 19

sodium (Na+) levels are low (≈10 mmol/L), and potassium (K+) levels are high (≈120 mmol/L). When normal regulatory mechanisms are abrogated, K+ leaks out of the lens and Na+ floods in, followed by chloride (Cl–). Water then enters in response to the osmotic gradient, causing loss of transparency by disrupting the normally smooth gradient of refractive index, as can occur following traumatic violation of the lens capsule.

Question 20

How does oxygen reach the lens

Answer 20

diffusion from the aqueous humour

Question 21

What happens in glycolysis

Answer 21

2 molecules of ATP are formed per original molecule of glucose. A small proportion of glucose-6-phosphate is metabolized through the pentose phosphate pathway

Question 22

What is the pentose phosphate pathway

Answer 22

This path- way is activated under conditions of oxidative stress because it is responsible for replenishing the supply of nicotinamide adenine dinucleotide phosphate (NADPH) that becomes oxidized through the increased activity of glutathione reductase under such conditions

Question 23

Which enzyme is affected in galactosaemia

Answer 23

deficiency of galactose-1-phosphate uridyltransferase

Question 24

What is the sorbitol pathway

Answer 24

Aldose reductase is the key enzyme for the pathway, and it converts the sugars into the corresponding sugar alcohols. Because aldose reductase has a very high Km (apparent affinity constant) value—that is, low affinity—for glucose under normal conditions little or no activity occurs through this pathway