2.7 – Cell Respiration
2.7.1 – Define cell respiration
The controlled release of energy from organic compounds in cells in the form of ATP
2.7.2 – State that, in cell respiration, glucose in the cytoplasm is broken down by glycolysis into pyruvate, with a small yield of ATP
Cells transfer energy by breaking down nutrients, mainly carbohydrates like glucose, through the process of cell respiration. Plants synthesis these nutrients using sunlight in photosynthesis, while heterotrophs will digest them from their food.
Cell respiration takes place in a number of steps, which helps to control the release of energy. There are multiple enzymes which catalyze these reactions, and the energy is able to be trapped in the molecule ATP (adenosine triphosphate). It is a nucleotide that carries three phosphate groups. The nature of the molecule means that it is soluble and small enough to pass through the cell membrane quite easily. It contains a lot of chemical energy within its structure.
The first step is glycolysis, which takes place in the cytoplasm of the cell. Once broken down, the glucose molecule (which has six carbon atoms) forms two pyruvate molecules (each with three carbon atoms). A small yield of two molecules of ATP are produced.
2.7.3 – Explain that, during anaerobic cell respiration, pyruvate can be converted in the cytoplasm into lactate, or ethanol and carbon dioxide, with no further yield of ATP
Anaerobic respiration is also called fermentation (a process which is important in making wine, beer, bread, etc). It occurs when there is no oxygen available.
Some organisms will still respire anaerobically, even when there is oxygen available. The most important one is yeast, which has applications mentioned above.
𝒈𝒍𝒖𝒄𝒐𝒔𝒆 → 𝒆𝒕𝒉𝒂𝒏𝒐𝒍+𝒄𝒂𝒓𝒃𝒐𝒏 𝒅𝒊𝒐𝒙𝒊𝒅𝒆+𝑨𝑻𝑷
When a vertebrate uses anaerobic respiration, it instead produces lactic acid (which then ionises in the cell to form lactate). This will mainly take place in the muscle fibres during high demand for energy.
𝒈𝒍𝒖𝒄𝒐𝒔𝒆 → 𝒍𝒂𝒄𝒕𝒂𝒕𝒆+𝑨𝑻𝑷
The pyruvate remains in the cytoplasm and gets further broken down into lactate.
Anaerobic respiration can be seen as wasteful because only two molecules of ATP are produced, making far less energy available to the cell after aerobic respiration.
2.7.4 – Explain that, during aerobic cell respiration, pyruvate can be broken down in the mitochondrion into carbon dioxide and water with a large yield of ATP
Aerobic respiration is summed up in the equation:
𝒈𝒍𝒖𝒄𝒐𝒔𝒆+𝒐𝒙𝒚𝒈𝒆𝒏 → 𝒄𝒂𝒓𝒃𝒐𝒏 𝒅𝒊𝒐𝒙𝒊𝒅𝒆+ 𝒘𝒂𝒕𝒆𝒓+𝑨𝑻𝑷
In the first stage of respiration, glycolysis, two molecules of ATP are formed from two molecules of ADP. The ATP is then used in other reactions for muscle movement, condensation, and movement across the membrane. It may also react with water through hydrolysis. During glycolysis, a glucose molecule in broken up into pyruvic acid, which then becomes pyruvate ions. This takes place in the cytoplasm.
The pyruvate is oxidised by the removal of hydrogen atoms by hydrogen acceptors, and the addition of oxygen to the carbon atoms to form carbon dioxide. The reduced hydrogen acceptor molecules then react with oxygen to form water. ATP is produced in the process.