Physical Science 8th grade (1).pdf

Cellular respiration on the molecular level cellular

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Cellular respiration On the molecular level, cellular respiration breaks down glucose into water and carbon dioxide again, extracting energy in the process. The reactions of respiration proceed in many steps, but the end result is that glucose and oxygen are used up and carbon dioxide and water are produced. Respiration is almost the reverse of photosynthesis, releasing energy that originally came from the sun. The ATP cycle Each cell converts the energy in glucose into chemical energy stored in molecules of ATP. A cycle between ADP and ATP is the energy source of cells. In a series of complex reactions that also require oxygen, one molecule of glucose is used to convert a maximum of 36 to 38 molecules of ADP to ATP. The ATP molecule is like a battery that distributes energy to where it is needed. Cells use the energy by converting the ATP back into ADP and the cycle starts over. Phosphorus is a critical part of the ADP-ATP cycle and one reason this element is an important nutrient.
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227 11.1 T HE C HEMISTRY OF C ARBON C HAPTER 11: T HE C HEMISTRY OF L IVING S YSTEMS The importance of water Why water is necessary Liquid water is essential to life as we know it. The human body is typically between 60 and 65 percent water by weight. Most of the chemical reactions that sustain life only work in solution . Therefore, when scientists look for life on other planets, the first thing they look for is water. We believe Mars either had or has water on its surface or beneath its surface. That raises the tantalizing possibility that life may exist there. There are three important characteristics of water that make it essential for life. Water is a good solvent Water is a good solvent. In order to have a chemical reaction, molecules must be able to move around and contact each other. In a solid, this is just not possible. However, in a solution, molecules can move relatively large distances carrying energy and nutrients throughout a cell. Water also allows transport through the body on a larger scale. For example, oxygen is required by cells throughout the body, but it comes into the body in a centralized place: your lungs. Red blood cells absorb oxygen in the lungs and are carried throughout the body so they can distribute the oxygen. Liquid over a wide temperature range Water exists as a liquid over a large range of temperatures. In fact, virtually all living organisms on Earth are most active between the freezing and boiling point of water. The wide range over which water remains liquid allows most of Earth to be habitable most of the time. Very few biological processes can proceed when completely frozen because molecules: (a) cannot reach each other, and (b) have less thermal energy for activating reactions. High specific heat Water has a high specific heat — one of the highest of any substance known. Water’s high specific heat means it takes a lot of energy to raise the temperature a small amount. This property of water helps living organisms maintain a stable body temperature even though outside temperatures may fluctuate a great deal.
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