Design in Nature Demands a Designer, Let's Take a Look at the Chemistry of Sight
by Robert Harsh
Light enters the eye through the cornea and the lens, which then bends the light to focus on the retina. The lens and cornea have no blood vessels but are nourished by aqueous humor that is contained in a special chamber in the front of the eye. It is absolutely clear so as to limit the refraction of light. Special ion pumps at the cellular level keep cornea cells dehydrated when compared with other cells. This further promotes clarity.
Your cornea is seven layered and the outside is very sensitive to pressure. This, of course, allows the split-second blink to be accomplished that may save the eye from being damaged. Your cornea is made of specially arranged collagen fibrils that permit 99% of the light to pass through without scattering.
The iris and the pupil control the amount of light that goes to the retina while the lens fine-tunes the focus.
Our lens is constructed by layering of very elongated cells that synthesize and then become filled with the protein, crystallins. Eventually synthesis stops, the cell dies and functions as a fiber that helps build one of nature's most perfectly engineered structures. The lens is flexible and is attached to ciliary muscles which change lens' shape.
Light travels through the near-perfectly-clear Vitreous Humor to the retina. Vitreous humor is a special liquid that inflates the eyeball and gives it the proper shape. It provides exactly the right pressure to protect the retina. This fluid even gets filtered and recycled which prevents loss of vision due to internal bleeding.
Light is captured by photoreceptor cells (rods and cones) of retina. Each eye has 6.5 million light sensitive cones and 125 million rods, each wired to the central NERCO's system and each sensitive to one photon. A computer screen has 1/2 million light sensitive spots (pixels), each sending messages to the computer.
The size of one photoreceptor cell is two microns. Now get this. The optical resolution of the human eye is, you guessed it, two microns! Is that a coincidence or what?!
The message of light is collected by the retina and is transformed into a chemical message by neuroreceptors found in the retina. The choroid is located behind the retina and is black to prevent reflected light from blurring the image.
Let's explore the chemical sequence in the retina involved in reception and transmission of the visual image. The following takes place within the membrane-lined disks located inside the visual pigment cells know as rods and cones;
1. Rhodopsin absorbs light. Rhodopsin consists of a protein called Retinal and the membrane that surrounds it called Opsin.
2. The absorption of light causes the Retinal component to change its shape from the cis-retinal form that has an angle in its shape to the trans-retinal, which is a straight molecule.
3. This produces a change in shape of the Opsin component of the Rhodopsin. So it is now called Metarhodopsin.
The following events involve the whole rod cell:
1. The shape change triggers the signal-transduction pathway. The altered shape of Rhodopsin activates the G protein (of the signal-transduction pathway) called Transducin. This G protein is located on the membrane of the discs. Inactivated Transducin is bound to a small molecule known as GDP. When Transducin is activated by the change in shape of Rhodopsin, the GDP is chemically changed into GTP.
2. Activated Transducin in turn activates an enzyme known as Cyclic GMP Phosphodiesterase. This enzyme digests c-GMP (Cyclic GMP), causing c-GMP to become detached from Na+ channels which normally have c-GMP channels on the rod's cell membrane. The Na+ channels normally have c-GMP attached to them, and they allow Na+ and Ca+ to pass through them.
3. This makes the cell non-polarized. When c-GMPs are digested, they detach and the Na+ channels close. The major consequence is that Ca+ ions do not come in through the cell's membrane, resulting in an abundance of positive ions outside the cell as compared to inside. The cell becomes hyper polarized (really polarized, negative on the inside and positive on the outside).
4. This hyper polarization causes the rod cells to stop releasing Inhibitors Neurotransmitter in the synapse with other neurons. The stopping of the release of inhibitory neurotransmitter is the message to the rest of the nervous system that light has stimulated that particular rod cell .
In order to use a photoreceptor cell again, the transretinal must be converted back to cis-retinal. This is done by the Retinal Pigment Epithelium (RPE). Cones and rods must be in physical contact with RPF. RPF digests and replaces photoreceptor cell membrane. RPE collects used retinal and uses Vitamin A to convert trans-retinal back to cis-retinal.
Each of these chemicals is a protein and is coded for by our DNA.
Does it really make sense that pure chance events caused the DNA for these chemicals to come together in just the right sequences by pure chance?
Photoreceptors and the RPE require a huge amount of energy. The per gram of tissue energy expenditure is:
--50% greater than kidney
--300% greater than cerebral cortex
--600% greater than cardiac muscle
What does a tissue that has a high metabolic rate need?
Abundant Oxygen and Glucose
How does large amounts of anything get to a cell?
Approximately 80% of blood supply to the eye travels through very large choriocapillaries. Be comparison, only 5% flows through the retinal artery that supplies the retinal layer in front of the photoreceptors. Choriocapillaries literally bathe the photoreceptors in blood.
The famous evolutionist, Richard Dawkins, in his book The Blind Watchmaker, ridicules the engineering of the retina. He laughed at the very idea of placing the neurons in front of the photoreceptor cells. "The principle of the thing would offend any tidy-minded engineer. (Dawkins, 1986)."
Dawkins was dead wrong again!
If the neurons were located behind the photoreceptors and the choriocapillaries were in front, they would absorb all of the light! Blood readily absorbs light.
The optic nerves from each eye travel to both left and right sides of the brain. The brain interprets the various messages sent to it by: form/color/motion/depth
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberrations, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.
Who made this valid observation?
The wisest man who ever lived, Solomon, agreed with Darwin when he wrote: "Ears that hear and eyes that see the Lord made them both ( Proverbs 20:12 )."
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