Biology: Life processes

Every living cell, from bacteria to human cells, requires a continuous supply of energy to carry out essential life processes. This energy is obtained through a chemical process called respiration. Respiration is the controlled release of energy from food substances, primarily glucose, within cells. It is crucial for activities such as muscle contraction (movement), active transport (moving substances against a concentration gradient), growth and repair of tissues, nerve impulse transmission, and maintaining a constant body temperature. Without respiration, cells cannot function, and the organism cannot survive. Respiration is broadly categorised into two types: aerobic respiration, which requires oxygen, and anaerobic respiration, which occurs in the absence of oxygen.

Aerobic respiration is the most efficient form of respiration, occurring when there is a sufficient supply of oxygen. It involves the complete breakdown of glucose into carbon dioxide and water, releasing a large amount of energy. This process begins in the cytoplasm of the cell, but the majority of energy release occurs in specialised organelles called mitochondria. The energy released is captured in the form of adenosine triphosphate (ATP) molecules, which act as the immediate energy currency for cellular activities. The reactants for aerobic respiration are glucose (from digested food) and oxygen (from breathing), and the products are carbon dioxide, water, and a significant amount of energy.

Anaerobic respiration occurs when oxygen is absent or in short supply. This process is less efficient than aerobic respiration because glucose is only partially broken down, resulting in a much smaller release of energy. In animal cells, such as muscle cells during intense exercise, glucose is converted into lactic acid and a small amount of energy. The accumulation of lactic acid causes muscle fatigue and pain. After exercise, extra oxygen is needed to break down the accumulated lactic acid; this is known as oxygen debt. In plant cells and yeast (a type of fungus), anaerobic respiration is called fermentation. Glucose is broken down into ethanol, carbon dioxide, and a small amount of energy. This process is economically important in brewing (ethanol) and baking (carbon dioxide).

Respiration is an exothermic reaction, meaning it releases energy into its surroundings, often in the form of heat. This heat energy is crucial for warm-blooded animals to maintain a constant body temperature. We can demonstrate the release of heat during respiration through a simple experiment using germinating seeds. Germinating seeds are highly metabolically active and respire rapidly. By comparing the temperature of respiring seeds to non-respiring (boiled) seeds, the heat released can be observed. The experiment typically involves setting up two thermos flasks: one with living, germinating seeds and another with an equal mass of boiled (dead) seeds (as a control). A thermometer is placed in each flask, and the flasks are sealed to prevent heat loss to the surroundings. Over a period of hours or days, the flask containing germinating seeds will show a noticeable rise in temperature, while the control flask will show little to no change, demonstrating that respiration releases heat.

It is a common misconception to confuse breathing with respiration, but they are distinct processes. Breathing, also known as ventilation, is a physical, mechanical process involving the movement of air into (inhalation) and out of (exhalation) the lungs. Its primary purpose is gas exchange: to bring oxygen into the body and remove carbon dioxide. This gas exchange occurs in the alveoli (air sacs) of the lungs, where oxygen diffuses from the inspired air into the bloodstream, and carbon dioxide diffuses from the bloodstream into the expired air. Respiration, on the other hand, is a chemical process that occurs inside every living cell. It is the breakdown of glucose to release energy. While breathing supplies the oxygen needed for aerobic respiration and removes the carbon dioxide produced, it is not the same as the energy-releasing chemical reactions of respiration itself. The composition of inspired air is approximately 21% oxygen, 0.04% carbon dioxide, and 78% nitrogen, while expired air contains about 16% oxygen, 4% carbon dioxide, and 78% nitrogen, reflecting the gas exchange that has occurred.