4.4 Metabolic Organelles
KEY CONCEPTS
By the end of this section, you will be able to do the following:
- Recognize the structure and describe the function of mitochondria, chloroplasts, and peroxisomes
- Evaluate the role of mitochondria, chloroplasts, and peroxisomes in cellular metabolism
- Give examples of how the abundance or structure of mitochondria, chloroplasts, and peroxisomes varies among different cell types
All cellular life requires energy. To get this energy, cells must undergo a series of efficient and controlled chemical reactions. The combination of all of these reactions is referred to as metabolism (from the Greek metabole, change). In this section, we will focus on three important organelles in which important metabolic reactions occur.
Mitochondria
Mitochondria (singular: mitochondrion) are oval-shaped, double membrane organelles (Figure 4.14) that have their own ribosomes and DNA. The ribosomes are smaller (70S) than those found in the cytosol and on the rough ER (80S), and the DNA is circular (rather than linear like the . These organelles are typically approximately 1 μm long. Each membrane (inner and outer) is a phospholipid bilayer embedded with proteins. The space between these membranes is called the intermembrane space. The inner layer has folds called cristae. We call the area surrounded by the folds the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration, a process used to generate lots of ATP for the cell.
Mitochondria are responsible for generating large amount of adenosine triphosphate (ATP). ATP functions as the cell’s short-term stored energy. Cellular respiration is the process of making large amounts of ATP using the chemical energy in glucose and other nutrients (Figure 4.14). In mitochondria, this process consumes oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a by-product. To facilitate this multistep process, mitochondria are equipped with many enzymes within the matrix and inner membrane which facilitate this multistep process. As the ATP is generated, it needs to be distributed throughout the cell. However, since ATP is a charged molecule, it cannot move freely across the semipermeable plasma membranes of the mitochondrion. Thus, transport proteins in the inner and outer mitochondrial membranes enable a hydrophilic passageway for ATP out of the mitochondrion and throughout the cell where it can then be used for endergonic reactions. It should be noted that the reaction for cellular respiration is the inverse of photosynthesis (Figure 4.14).
Mitochondrial abundance increases in cells performing processes that require large amounts of ATP (e.g., energy-demanding processes). An example of this is human muscle cells, which have a very high concentration of mitochondria. Your muscle cells need considerable energy to keep your body mo