Tutor profile: Alex P.
Subject: SAT II Biology E/M
If a Molecule of DNA is composed of approximately, 16.2% Adenine(A) and 33.4% Guanine(G), the percentages of Thymine (T) and Cytosine (C) must be approximately, a) 16.3% T and 16.3% C b) 34.1% T and 34.1% C c) 34.1% T and 16.3% C d) 16.3% T and 34.1% C e) 33.4% T and 16.2% C
The Correct answer is (d). 16.3%T and 34.1% C In the DNA molecule, base pairing occurs between Adenine(A) and Thymine(T) & Guanine(G) and Cytosine(C). Adenine and Thymine are held together by 2 hydrogen bonds and Guanine and cytosine are held together by 3 hydrogen bonds. Here in this question ,there is approximately 16.2% of Adenine(A) ,hence the percentage of Thymine(T) would be around the same. Similarly the percentage of Guanine is approximately 33.4% ,which would be almost same for the Cytosine(C).
How is our Heart pump the blood to different organs and receive blood from different parts of the body against gravity? Is the Heart is so strong?
Our Heart is the main organ is our body, which continuously beat ,right from 1st month of Intrauterine life till we die. Heart has several modification within itself to work continuously. It is made up of cardiac smooth muscles which are not under our control and also they exhibit certain properties like automaticity which enables them to beat on its own, which aids in pumping the blood to different organs. Have you ever wondered , how this small organ is able to pump blood against the gravity? The heart muscles are so strong enough to pump blood against the gravity to peripheral parts of the body especially the brain. The Left Ventricle of the heart have thickened muscular wall compared to the Right ventricle of the heart, which aids in pumping the oxygenated blood to different organs. But ,this is not the case ,when we come to the blood reaching the heart. How does the blood flow against the gravity from the peripheries? This is made possible by the use of the another system : Blood Vessels. The veins contains small valves and these valves close when blood starts flow in one direction , so that blood flow is unidirectional and backflow is prevented. in addition to this, the skeletal muscles aids in pumping blood, especially in the lower limb , the calf muscle serve as a calf pump. While walking , these muscles contract and squeeze the veins and force the blood to move forward . Thus as the blood moves forward, the valves and the squeezing action of muscles push the blood against gravity and finally reaches the heart. If there is any malfunction of the valves in the veins resulting in pooling of blood especially in the lower extremities for example as in Varicose veins. If there is any defect in any of the process, resulting in failure of the pump system can cause decreased Blood pressure and can result in syncope and may fall due to low blood supply to the brain as a result of lowered blood pressure. So, If anyone feel like fainting, it may be due to lowered blood pressure and ask him to lie down and drink water to make the Blood pressure even all over the body and thus cause blood flow to the brain, may help him.
How does our eyes adapt to darkness? How long does it take to fully adapt?
Our Eyes are created magnificently. Human Eyes take hours to fully adapt to the darkness and become sensitized in the low light conditions. You might have experienced it. When you enter dark room , you will not be able to visualize anything for few seconds. But several hours into the darkness exposure, the human eyes continue to adapt and become sensitized. There are several factors that contribute to our eyes adapting to darkness. The iris that surrounds the pupil regulate the size of the pupil, thereby control the amount of light entering the eyes. When confronted with low light conditions, the iris expands the pupil as wide as possible, which will let as much as light possible into the eye and sensitivity is enhanced. In the retina , we have 2 specialized cells- Rods and Cones. These cells also help in dark adaptation. Human Eyes have Red, Green,& Blue sensitive cone cells, All other colors are experienced by humans as a mixture of red, green &blue. Also, cones contain Rhodopsin which is one of the light sensitive chemical and is the primary chemical used by cones, when seeing in low light conditions. Since Rhodopsin is highly sensitive to light, these get deactivated ,when we walk around in the normal light. Upon exposure to dark, this rhodopsin pigment is regenerated and become sensitive .Thus improves our night vision. Cone cells take about 10 minutes to adapt to dark. The most important cells are the Rods: these are cells responsible for low light vision. Rod cell also contains a pigment called Rhodopsin, which is very sensitive to the light but ,Rods respond slowly to light . Thus take hours to completely adapt. But, Rods are responsible for greater night vision than cones because: (i) Rod cells are more than cones and (ii) Rods rely more on the rhodopsin pigment cone cell. Thus each single rod cell is 100-1000 times as sensitive as a fully adapted cone cell. In Conclusion, Cone cells adapt in 10 minutes and Our rods completely adapt after several hours and Our pupils dilate in a matter of seconds. remember this, if you experience a delay in adapting your vision when you enter a dark room.
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