When you think about muscles, you may immediately think about exercise and a defined physique. But no matter what shape you're in, your muscles work all day long to keep you moving and to hold your skeleton upright when you're sitting or standing. To do so, muscle cells must continually contract by producing energy via aerobic cellular respiration.
ATP and Cellular Respiration
When muscle cells contract, or shorten, they do so by drawing on ATP, or adenosine triphosphate, the molecule that is the fundamental energy source for cellular work. While small amounts of ATP can be made without oxygen to supply enough energy for a few seconds of cellular demand, ongoing muscle contraction requires the presence of oxygen. In a sense, the cells "breathe" oxygen carried to them via red blood cells and use it to make ATP in intra-cellular structures called mitochondria.
Mitochondria
Cellular respiration takes place in the mitochondria of the muscle cell. Mitochondria are oval-shaped organelles that are often referred to as the powerhouse of the cell. Mitochondria have an outer and inner membrane and a center filled with water and enzymes called the matrix. Within the matrix of the mitochondria, glucose is combined with oxygen to manufacture ATP. Cells with a high demand for ATP like those of the quadriceps, hamstrings and gluteal muscles used for aerobic activities may each have thousands of mitochondria.
Glucose and Cellular Respiration
Carbohydrates are the primary supplier of glucose for ATP production. Carbohydrates are plant-based foods like fruits, vegetables and grains. Your digestive system breaks down carbohydrate foods to glucose which is then stored in the form of glycogen in muscle and liver cells. When your glycogen stores are full, excess glucose is converted to fat and stored in your adipose tissue. In the absence of ample glycogen, your body will convert fat and protein to glucose for cellular respiration. Each molecule of glucose used during cellular respiration makes 38 ATPs.
Exercise and Respiration
Aerobic exercise that increases the ATP demand of muscles enhances the mechanisms of cellular respiration by increasing capillary access to the cells to allow more oxygen to be delivered, by enhancing your cells' storage capacity for glycogen and by increasing the size and number of ATP-producing mitochondria within the muscle cells. Enhanced cellular respiration means that your heart and lungs do not need to work as hard to meet muscular oxygen demands, enabling you to work at a higher intensity with less effort.
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