Ocular Biochemical Compositions: High yield Notes

The ocular biochemistry, often a daunting and boring topic, is one of the most commonly asked. Understanding this topic is crucial. In this short article, we try to enumerate all those high-yield important points regarding the ocular biochemical composition and try providing logical memory aids where ever possible.

Table of Contents

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Biochemical Composition of Cornea

The cornea’s transparency and refractive power make it vital for vision. To understand its composition:
Water content of cornea is 80%
Dry Weight: The cornea has 20% dry weight.
The cornea maintains a relative state of dehydration to ensure its remains transparent . For more, click here.
Collagen: 70% of the cornea’s dry weight is Type 1 collagen.
Proteoglycans: It contains 10% proteoglycans, with a 60/40 ratio of keratan sulfate to dermatan sulfate.

MEMORY AID
Keratan (sounds like “cornea”). Therefore , the abundant proteoglycan in cornea is keratan sulphate

Scleral Biochemical Compositiom

The sclera is the eye’s tough, opaque outer layer. Here’s how its constituents stacks up:
- Water content of sclera is 70%
- Dry Weight: The sclera is 30-35% dry weight, making it denser than the cornea.
- Collagen: Like the cornea, the sclera is rich in Type 1 collagen.
- Proteoglycans: However, dermatan sulfate dominates here (unlike the cornea).
  
  MEMORY AID
  - Both cornea and sclera have TYPE 1 COLLAGEN
  - Dermatan sulfate as the main proteoglycan in sclera

Lens Biochemical Composition

The lens is unique in its biochemical composition, playing a major role in focusing light onto the retina.
- Glutathione: The lens has 1000x higher levels of reduced glutathione than the aqueous, which helps protect it from oxidative damage.
- Potassium (K+): Potassium levels are 125 mmol/L, about 25x higher than in the plasma or aqueous.
- Chloride (Cl-): High chloride levels also exist in the lens.
- Sodium (Na+): In contrast, sodium levels are low at only 20 mmol/L.
- Calcium (Ca2+): Similarly, calcium levels are kept low, preventing calcification.
- MEMORY AIDS
  1. “Greater GLutathione, potassium and chloride “
  2. “Lower Sodium and Calcium “
- SODIUM _POTTASIUM BALANCE IN LENS :- Sodium-Potassium ATPase pumps, located primarily in the lens epithelium and fiber cells, actively transport Na+ out of the lens and K+ into the lens. This process maintains the high intracellular concentration of K+ (125 mmol/L) and the low intracellular concentration of Na+ (20 mmol/L). The pump prevents osmotic swelling and ensures proper cellular function.
- Calcium (Ca2+) Control:-Calcium-ATPase pumps and calcium-binding proteins in the lens help keep Ca2+ levels low. Elevated calcium levels can lead to lens opacification (cataract formation) by promoting the aggregation of lens proteins. The active regulation of calcium prevents this and ensures the lens remains clear.
- Chloride (Cl-) Regulation: Chloride channels (such as ClC channels) help regulate the flow of Cl- ions across the lens membrane. Usually to balance the potasium inside chloride content inside is more .

Aqueous Humor: Biochemical Composition

The aqueous humor, circulating in the anterior chamber, plays a role in nourishing avascular structures like the lens and cornea.

Volume: The total volume in the anterior chamber is about 250 μL.
Production Rate: It’s produced at 2.5 μL/min or approximately 3.6 mL/day.
Ascorbic Acid: The aqueous humor contains 50x higher ascorbic acid (vitamin C) compared to plasma, which helps protect the eye from UV light.
Lactate: Levels are 2x higher than in plasma.
Electrolytes:
- Chloride (Cl-) is higher than plasma.
- Sodium (Na+) and Potassium (K+) levels are lower than in plasma.
Glucose: The aqueous contains about 80% of the glucose level found in plasma.
Urea: Present in amounts similar to plasma.
Protein: Only trace amounts of protein are found in the aqueous humor.

MEMORY AID AND LOGIC BEHIND
Ascorbate levels :Ascorbate is actively transported by specific transporters located in the non-pigmented epithelium of the ciliary body, which produces the aqueous humor. These transporters, known as sodium-dependent vitamin C transporters (SVCT), are responsible for concentrating ascorbate in the aqueous humor to levels much higher (50x) than in the plasma.The active transport process ensures that the aqueous humor maintains high levels of ascorbate to serve as a potent antioxidant, protecting the eye from oxidative damage caused by UV light and other environmental factors.
In contrast to passive diffusion, this active mechanism enables the eye to accumulate ascorbate against a concentration gradient, ensuring its availability for defense against oxidative stress.
Lactate levels : The levels of lactate are 2x higher than in plasma because the cornea and lens rely predominantly on anaerobic glycolysis (breakdown of glucose without oxygen) to generate energy. This is due to the limited availability of oxygen in these avascular structures. The lactate produced as a byproduct of anaerobic metabolism diffuses into the aqueous humor, increasing its concentration.
Chloride is higher than plasma here as well just like its high in the lens
Sodium (Na+) and Potassium (K+): Both sodium and potassium levels are lower than in plasma to maintain a delicate osmotic balance .Low Na+ and K+ keeps osmotic load low and helps avoid excessive fluid accumulation or dehydration in the cornea and lens.
Glucose concentration in the aqueous humor is about 80% of plasma levels. The eye structures, particularly the cornea and lens, use glucose as their primary source of energy. However, the concentration of glucose is lower than in plasma to minimize the risk of glycation, which could lead to structural changes and protein aggregation, particularly in the lens. This is one of the protective mechanisms against cataract formation.
Urea levels are similar to those in plasma because it is a small molecule that diffuses freely between the plasma and aqueous humor

Vitreous: Biochemical Composition

The vitreous humor, occupying the space behind the lens, shares similar characteristics with the aqueous but has some distinct features.
- Composition:
  - Almost identical to aqueous humor.
  - Contains 1% hyaluronic acid, mainly in the vitreous cortex.
- Cells: Fibroblasts, primarily at the vitreous base, and glycoproteins are present.
- Ascorbic Acid: The vitreous has 10x higher ascorbic acid than plasma, which diffuses passively from the aqueous humor.
MEMORY AID and LOGICAL EXPLANATION
Here in vitreous there is no active transport, ascorbate recahed here through passive transport.
Therefore , here it is only 10x of the plasma( contrary to aqueous where it is 50 X)
To read about anatomy of vitreous, click here

Tear Film: Biochemical Composition

Lastly, understanding the tear film’s structure and function is crucial, as it maintains ocular surface health and clear vision.
- Volume: The tear film has a 6.5 μL volume.
- Thickness: It’s 10 μm thick, with 90% of this being the middle aqueous layer (9 μm), and the lipid and mucin layers making up 0.5–1 μm each.
- Production Rate: It is produced at 1.2 μm/min, replenishing 20% per minute.
- Chemical Properties:
  - Slightly hypotonic compared to plasma.
  - pH: Slightly acidic than plasma.
  - Contains higher K+ and Cl- but lower glucose than plasma.
- Proteins:
  - Lactoferrin and Lysozyme make up 25% of tear proteins and have antimicrobial properties.
  - Lipocalin assists in transport.
  - Immunoglobulins: All are present except IgD, with IgA being the most abundant.
- Lipid Content: Composed of wax esters (30%), sterols (25%), and minimal fatty acids (<2%).
Memory aid
High potassium levels in the tear film help maintain osmotic balance, promoting hydration of the ocular surface and preventing cellular dehydration. They also stabilize the tear film by promoting mucin production (now you know why tears supplements have potassium in them)
Cl- also plays a role in maintaining the acidity of tears (slightly more acidic than plasma), which helps prevent microbial growth on the ocular surface.
High glucose levels in tears could promote bacterial growth, so maintaining a low glucose concentration in tears helps protect against infection.
To know more about tear film anatomy , visit our video on our channel

Final thoughts

That’s all on the biochemical composition of the ocular structures. I hope by the end of this blogpost you will be able to memorize and logically understand all the high yield facts. Thank you and have a nice day!