why do we need energy to survive

All the cells in our body require energy to carry out the required chemical reactions to complete their specific functions. respiration, motion, cognitive. All the cells in our body require energy to carry out the required chemical reactions to complete their specific functions. respiration, motion, cognitive processes, growth, all need energy for reactions to occur. The energy could be to allow a certain compound to be formed, or to produce a chemical gradient across a membrane with which reactions may be triggered or propogated. There are plenty of functions which require energy, which could come under the umbrella term of metabolism, the reactions which are necessary for life. The body digests complex molecules such as proteins, lipids and carbohydrates into more flexible smaller units, such as glucose, amino acids and monoglycerides. These units can be transported in a suitable form to the cells in the body through the circulatory system (as well as waste products being transported away using the lymphatic system) which are later used to build membranes, organelles, chemical compounds, hormones, etc by cells depending on the function of the cell. Remember, everything you do requires a chemical reaction. Everything needs maintenance, so everything will need energy to occur. Secondly, as chemical reactions are going off inside of us all the time, a significant portion of which are exothermic reactions, heat can be generated, which is why we are warmer than our surroundings.

This heat energy is not considered waste energy however, as the heat is required to keep our body at the correct temperature for the correct function of many enzymes and chemical pathways in our bodies. I hope this is at the level of detail you require, I could go into much, much more detail, but I doubt you will require it. As long as you put something along these lines, your work should be fine.
The human body is made up of cells, each one of which needs to be able to provide for its energy needs by taking up nutritional molecules from the bloodstream and chemically burning them as part of cellular metabolism. Glucose is an important nutrient molecule that cells rely upon for energy, both as a component of diet and when stored for later use in the form of the carbohydrate molecule glycogen. Any food that contains carbohydrates serves as a source of glucose. Even if the food itself doesn't contain glucose, your body can break down sugar and starch in the foods you eat into glucose and other simple sugars, and can then convert the other simple sugars into glucose. Thats why carbohydrate-containing foods increase your blood glucose levels -- your body breaks them own into glucose, then absorbs the glucose into your bloodstream.

The function of glucose in the body is an energy molecule. Cells take up glucose absorbed by the digestive tract and either use the glucose or store it for later. While proteins and fat also provide cells with energy, some body cells -- the cells of the brain in particular -- preferentially rely upon glucose, notes Dr. Lauralee Sherwood in her book, "Human Physiology. " Further, glucose breaks down into smaller molecules that provide building blocks for many cellular products. All cells need energy to function. With some cells, the reason for the energy requirement is more apparent than with others -- muscle cells, for instance, require energy in order to shorten and produce movement. However, even cells not involved in producing motion need a regular source of energy, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology. " Cells use energy derived primarily from glucose to produce electrical currents that allow communication, to synthesize hormones and other products, and to grow and divide. Since the primary utility of glucose in the human body is metabolism, the vast majority of ingested glucose is processed through a series of reactions. The first reaction, called glycolysis, splits glucose into two smaller molecules called pyruvate.

The second reaction converts pyruvate to a molecule called acetyl-CoA, which enters the Kreb's cycle. Note Drs. Reginald Garrett and Charles Grisham in their book, "Biochemistry," the Kreb's cycle is one of the most important metabolic processes in a cell, since it ultimately leads to generation of massive amounts of energy from glucose. Unlike protein and fat, glucose presents a distinct advantage to many cells as a source of energy, because it can be metabolized either with or without oxygen, note Garrett and Grisham. While the Kreb's cycle requires oxygen to function, there is another metabolic branch that allows oxygen-independent energy generation from glucose. This process, called fermentation, doesn't generate as much energy as the Kreb's cycle, but it does allow muscles and other cells to continue to function in low-oxygen conditions. The human body stores energy in two ways -- as fat, and as a carbohydrate called glycogen. While most of the energy stored in the body is stored as fat, glucose is so important to normal function that the liver and muscle cells store a certain amount of it for times during which blood glucose levels begin to run low. Note Garrett and Grisham, without glucose, the body can't store glycogen. This leads to fatigue and muscle weakness.

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