Respiration in the cells

Plant cells are alive, of course, so they use up food for energy. The green plants can provide you with some of the energy you need to live. First you must get this energy to your cells where it can be used. But what happens to food inside the cell? It is broken down, or changed.Respiration– the step in metabolism in which food is broken down - takes place in the mitochondria.

In respiration, food is broken down to give off energy. But the food materials are not broken down all at once. Instead, they are broken down in a series of small steps. Energy is given off with each step (Chart 2.1).

Some of the food materials used by the cell are carbohydrates.Carbohydrates, such as sugars and starches, made up of large molecules, must be broken down into smaller molecules in the mitochondria.

Breaking down molecules means dividing them into smaller, simpler molecules. The smaller, simpler molecules are molecules of materials that are different from the original materials. Energy is given off each time the molecules are divided. Finally, the remaining molecules cannot be divided any more.

In general, the cells in living things break down food molecules to form molecules of water and molecules of a gas calledcarbondioxide. The energy given off in breaking down food is used for the life function of the cells.

Respiration occurs during both light and dark hours. On a warm sunny day, plants produce much more food and oxygen through photosynthesis than they can use. At night, plants use up some of this excess food for respiration.

Due to the types of respiration, all the organisms are divided in for:

Aerobic (air + bios – life): requiring free oxygen for respiration.

Anaerobic: applied to the cells (mostly bacterial) that can live without free oxygen; obligateanaerobescannot live in the presence of oxygen; facultativeanaerobescan live with or without oxygen.

Chart 2.1

Comparison of photosynthesis and respiration.

Glycolysis

The first step in cellular respiration is a process of glycolysis. In glycolysis, a glucose molecule is broken in half to form two three- carbon molecules of the substance called pyruvicacid.

Glycolysis takes place in the cytoplasm of the cell as well as in the mitochondria. The process involves a series of 9 enzyme-controlled reactions. Two reactions early in the process are endergonic; each requires the input of one molecule of ATP. The later reactions release enough energy to combine 4 molecules of ATP. Thus, glycolysis produces an overall gain of two molecules of ATP for each molecule of glucose.

In addition to producing two three- carbon molecules of pyruvic acid and two ATPs, glycolysis also releases 4 hydrogen atoms. These hydrogens then combine with a hydrogen acceptor, as in the light reactions of photosynthesis. The following equation (2.2) summarizes the overall reaction:

C₆H₁₂O₆ è 2C₂H₃OCOOH + 4H (2.2)

glucose pyruvic acid

Glycolysis is an anaerobic process—that is, no oxygen is required for the process to take place. Glycolysis is followed by one of two processes. If oxygen is present in the cell, the pyruvic acid is broken down further through the process of aerobicrespiration. This process results in an additional gain of ATP molecules. Without oxygen, the pyruvic acid cannot be used to release more energy. However, the hydrogen acceptor molecules must release hydrogen atoms, othervise glycolysis could not continue. Hydrogen atoms are removed through fermentation, an anaerobic process that breaks down pyruvic acid into ethyl alcohol or lactic acid. Together, the processes of glycolysis and fermentation make up anaerobic respiration.

Fermentation

Fermentation occurs in some of the less complex organisms, such as some bacteria and yeasts. Most microorganisms convert pyruvic acid to ethyl alcohol, which some animal cells and other microorganisms convert into lactic acid.

Alcoholicfermentation combines the hydrogen and the pyruvic acid formed during glycolysis to produce ethyl alcohol.

Carbon dioxide is also given off as a byproduct. The general equation (2.3) of the process can be written:

2C₂H₃OCOOH + 4H è 2C₂H₅OH + 2CO₂ (2.3)

pyruvic acid ethyl alcohol

Most of the energy originally stored in the glucose remains in the bonds of the ethyl alcohol molecule. For this reason, alcohol is a good fuel.

Lacticacidfermentation combines pyruvic acid and hydrogen from glycolysis to form lactic acid. The general equation for this process can be written (4.4):

2C₂H₃OCOOH + 4H è 2CH₃CHOHCOOH (2.4)

pyruvic acid lactic acid

Lactic acid fermentation occurs in animal muscle cells. When oxygen is available, these cells carry on aerobic respiration. During strenuous exercise, oxygen concentration diminishes, and muscle cells are forced to use lactic acid fermentation. This process uses the hydrogen stored by the hydrogen acceptor molecule and makes the acceptor molecule available for reuse in glycolysis to obtain more energy. The accumulation of lactic acid in the cells is one of the causes of muscle soreness. When the oxygen concentration returns to normal, the lactic acid is converted back to pyruvic acid, aerobic respiration begins, and the soreness fades.