The Rod Cell Separating Darkness From Light Biology Essay

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The cells of the human body are diverse and amazing. One cell type that is considered very important is the rod cell found in the human eye. Through unique design and specialized functions, the rod cell gives humans the ability to see, especially in low light conditions. "In each eye, there are approximately 125 million rods" (Parker 20). The retina is connected to the optic nerve, and consists of several layers, one of which contains the cells that are sensitive to light. Rod cells are approximately one hundred times more sensitive to light than cone cells, however. These rods are spread over most of the retina, which is the innermost coat of the posterior part of the eyeball that receives the image produced by the eye's lens.

"Rods are responsible for monochromatic vision in dim light" (Alberts et. al. 753). To compare the two types of light-sensitive cells, rods handle vision in low light and cones handle color vision in detail. When light hits these two types of cells, a series of complex chemical reactions occurs: the chemical rhodospin is formed, which causes electrical impulses to be created in the optic nerve. (Bianco par 1). This gives humans the ability to see in lower light levels. Of course, there are variations within this spectrum. If some humans' rod cells are damaged or may become damaged, the cone cells would most likely become damaged as well. Partial blindness will most likely occur. It is hard to understand how rod cells detect light and dark without knowing their structure first ("The Eye" par 3).

"Rod cells have an elongated structure that consists of four distinct regions: an outer segment, an inner segment, the cell body, and the synaptic region" ("rod" par 2). Phototransduction apparatus or "photosensitive discs" are contained within the outer segment of the rod cell. They are closely packed membrane discs and each one is formed by a closed sac of membrane in which photosensitive rhodopsin molecules that are embedded. "Both outer and inner segments are physically isolated from the cell bodies of the rod and cone cells by the outer limiting 'membrane' and the actual cell body is located in the outer nuclear layer" ("Rods and Cones" par 5). The disks of the rod cell are completely separated from the outer plasma membrane. The membrane surrounding the outer segment contains cyclic-GMP-gated Na channels. These Na channels (or sodium channels) are kept open in the dark by cyclic GMP molecules bound to the channels. Paradoxically, light causes a hyperpolarization (which inhibits synaptic signaling) rather than a depolarization of the plasma membrane (which could stimulate synaptic signaling), because the activation by light of rhodospin molecules in the disk membrane leads to the closure of the Na channels in the surrounding plasma membrane (Alberts et. al. 753).

The exact was sight is made possible is as possible. "When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin. Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal. This first reaction takes only a few trillionths of a second. The 11-cis-retinal is an angulated molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable. Rhodopsin breaks down into several intermediate compounds, but eventually (in less than a second) forms metarhodopsin II (activated rhodopsin). This chemical causes electrical impulses that are transmitted to the brain and interpreted as light" (Bianco par 3). When light shines on this pigment, it is broken into the two proteins: retinal and opsin in a process called bleaching, this stimulates an action potential that is detected in the brain. However, the rhodopsin is very sensitive to light, and is therefore best in dim conditions. In brighter conditions it is broken down faster than it is reformed. This is why, in dim conditions we will see mainly in black and white.

Rhodopsin is a vital photoreceptor molecule that allows light to be absorbed in the eye. The molecule is derived from vitamin A (Bianco par 3). Also, this molecule is suspended in a protein framework. When a rhodopsin molecule is struck by a photon of light, it instantly twists into a new shape, which then causes a nerve impulse to emit a signal from the activating of a switch. The inner segment contains organelles and the cell's nucleus. The cell body provides protection and shape of the cell. The synaptic region is the site where the rod cell relays its information to intermediate neurons in the retina. These neurons connect with ganglion neurons whose axons form approximately one million fibers of the optic nerve. Although the cell structure of the rod enables one to see how its purpose is done, there are many different functions of it that haven't been discussed and hopefully will reveal everything else clearly on why these organelles and pieces are within the rod cell.

Each type of organelle has a specific role in its particular cell. The unique organelles of the rod cell are cilia, photoreceptor discs, smooth and rough ER, the Golgi Complex, centriole, and an extent amount of mitochondria. Cilia are the organelles that connect the cell body of the rod cell to the rear of the eye. They are a vital part of the rod cell. Photoreceptor discs are photosensitive organelles that are shaped as "discs" in the retina. Photoreceptors are what allow rod cells to sense light from dark. The smooth ER synthesizes lipids, steroids, and fatty acids. It also stores calcium, detoxifies the cell, and is important to carbohydrate metabolism. Rough ER's main function is to transport and manufacture proteins. It controls the quality of proteins and can mollify and alter them as well. The main function of a Golgi Complex is to sort and process ribosomal proteins. Centrioles are important to the cell because they help cells to divide. They also help the cell through mitosis (Rod Cells Organelles in the Rod Cell).

Finally, rod cells, like all cells, consist of mitochondria, but the mitochondria in rod cells are packed much more densely than others. Mitochondria are considered the "power plants" of the cell, they create ATP, which is the main source of energy within a cell. Cytoplasm is what holds these organelles together. It is the fluid medium for most cells. Without it, nothing would stay intact or together. The most important part of every cell, however, is the nucleus. The nucleus is known as the "master control center" for a cell. It gives cells their genetic code and is normally located in the middle of the cell, through it is located near the bottom of the rod cell. Rod cells have generic and unique organelles, but one of the most important pigments of the cell is rhodopsin (Rod Cells Organelles in the Rod Cell)

The incredible design and functions of the rod cell is truly made by the Creator Himself. The design is amazing in how one can see in the dark through these cells-oppose to some animals that can barely see or sometimes cannot see at all in the dark and have to go by other senses. Rhodopsin allows us to see, and it is found inside the outer segment of the cell, this is an important concept to grasp. Without rhodospin, there wouldn't be a purpose for a rod cell. If there is damage to the rod or cone cells, it could lead to partial or complete blindness, in which is called night-blindness. Finally, the different functions of the rod cell make are normally related or supported by rhodospin, which is another reason why it is such an important molecule. The rod cell is designed incredibly by God the Creator who allows humanity to see in the dark with this complex cell. It is what separates darkness from light in our eyes.