The DNA in the rod cells of your eyes contains all information needed to produce a complete baby. The DNA in an ear cell could theoretically construct a foot. We do not do these nonsensical things because in each of us large portion of the DNA template are blocked out. My DNA makes rod cells, nothing else. Each second, millions of cells die and millions of cells born, by the process of old cells pulling apart, each to make two new ones, exact duplicates. Fat cells, large storage bins, reproduce slowly but skin cells reproduce every 10 hours. One notable exception to this constant replacement is the brain. The moment you was born he had his lifetime maximum number of brain cells. Worn-out, damaged ones keep dying; they are never replaced. Yet your initial surplus was so great that you scarcely notices the loss.
My DNA can be compared to an architect whose job is to draw up the grand design for living but it hands the work of building over to contractors - RNA or ribonucleic acid. In the form of molecules, all information is printed on the interlocking twin spirals of DNA. Messenger RNA snuggles up to DNA spirals and gets a blueprint of what is wanted. It then passes the word along to another form of RNA, transfer RNA. And the latter starts to work according to instructions - most likely building one of the hundreds of proteins in your body. It takes the 20-odd amino acids that proteins are made of, and strings them together like beads in a specified pattern. The result may be a pulsating muscle cell for your heart, a contractile leg muscle that permits to walk or whatever the DNA ordered.
To generate electricity, rod cells in your eyes have some thousand power stations that burn sugar and produce electricity. In this chemical process they synthesize a substance called adenosine triphosphate (ATP) for short. It is the universal power source for every living thing, from rhubarb to clams to man. When there is need for energy - to make the heart beat, to expand the chest in breathing, to blink an eyelid - ATP breaks down into simpler substance, releasing power as it dose. As long as you lives, there will be this call for energy and ATP. Even in deepest sleep there is a torrent of activity - cellular furnaces burning to keep the body warm, brain cells discharging electricity to make dreams, heart cells pulsing to keep blood flowing. The breakdown and building up of ATP is constant.
We participate in everything you do. You lifts a suitcase and thinks your arm is doing the job. Actually, it’s invisible muscle cells, contracting. You shaves your face : nerve and muscle cells perform the entire operation. For that matter, the facial hair you chops off were produced by other cells. My task as a rod cell in the eye is to catch faint light - say the twinkle of a star - simplify it and change it into an electrical signal which I then send to your brain. If enough signals arrive, you see the star.
Since each of us 250 million rod cells in your eyes contains 30 million molecules of light catching pigment, we naturally use a lot of electricity. To generate it, I have some thousand sausage-shaped power stations that burn sugar, produce electricity and leave water and carbon dioxide behind. In this complex chemical process they synthesize a substance called adenosine triphosphate (ATP) for short.
Tissues are made of cells that are specialized for particular functions in the body. Cells are the basic living structure of the human body but when they are organized into tissues their activity becomes more specialized. The human body has four primary types of tissue : connective tissues, epithelial tissues, nerve tissues and muscular tissues.
Connective tissues are all over the body. They bind structures together. They also provide support and protection. Connective tissue includes tendons, ligaments, bone and cartilage. Other types of this tissue store fat, produce blood cells and destroy bacteria.
Epithelial tissues cover the whole body inside and out. They form the skin, and the inner lining of the body cavities such as the stomach and the intestines. Epithelial tissue can reproduce quickly in case of damage or injury. That’s why your skin heals so quickly when you get a cut.
Nerve tissues are sensitive to changes. They receive and transmit impulses to various parts of the body. They are located in the brain, spinal cord and the nerves.
Muscle tissues are probably the ones you are most familiar with. Their function is to contract and cause body parts to move. There are three types of muscle tissue skeletal muscle, smooth muscle, and cardiac muscle.