Chloroplast Function and Cellular Respiration Processes
Chloroplasts and Photosynthesis
Chloroplasts are the sites where photosynthesis occurs. These organelles are generally ovoid or discoidal in shape. They are composed of an outer membrane and an inner membrane; the latter is folded to form structures similar to columns called grana. Each granum is composed of a series of disc-shaped compartments known as thylakoids. Thylakoid membranes contain chlorophyll, the photosynthetic pigment responsible for photosynthesis. These structures are immersed in a liquid called stroma, which contains, in addition to the grana, DNA, ribosomes, and polyribosomes.
Obtaining Energy in Living Organisms
Regardless of how living things get their food—whether incorporated from the outside, like animals, or made by themselves, like green plants—one purpose of food is to make and release energy. While energy can be obtained from any ingested substance, carbohydrates (especially glucose) are the primary source. Food contains a large amount of chemical energy stored in the bonds that hold atoms together to form their molecules. Obtaining energy from glucose is achieved principally through cellular respiration.
The first stage of energy production is the oxidation of the glucose molecule through a process called glycolysis. From that point, the products of glycolysis can follow two pathways:
- Aerobic pathway: Involves molecular oxygen (O2) and comprises the Krebs cycle and the respiratory chain.
- Anaerobic pathway: Does not involve molecular oxygen and is called fermentation.
The Process of Glycolysis
In the cytoplasm, a glucose molecule is broken down, releasing part of the stored energy and transforming it into two pyruvic acid molecules.
The Krebs Cycle
The molecules of pyruvic acid produced during glycolysis enter the mitochondria, where they initiate a series of chemical reactions that release more energy. This process ends with the formation of carbon dioxide and hydrogen. The first is removed from the cell, and the second remains in the mitochondria to continue with the last part of the process.
The Respiratory Chain
Hydrogen is transferred through a series of chemicals that are composed with it, but they quickly give way to the next substance in the chain, releasing a lot of energy during this passage. The last substance combines with oxygen; the hydrogen incorporated through respiration leads to the formation of water molecules, which are the end product of this stage of the process. While energy is released in each of the stages of respiration, the greatest amount is produced during the respiratory chain, which only takes place if oxygen is present in the cell.