4.4 Metabolic Organelles

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 moving. Under normal conditions, our muscle cells use cellular respiration to produce ATP. However, during strenuous exercise, our muscle cells experience a deficit of oxygen so have to switch to using a different metabolic process (lactic acid fermentation) in the cytosol to generate smaller amounts of ATP in the cytosol.

Chloroplasts 

Like mitochondria, chloroplasts have their own DNA and ribosomes, but chloroplasts have an entirely different function. These unique organelles span 1-5 µm wide and 1-10 µm long. Chloroplasts are responsible for carrying out photosynthesis within plant cells and photosynthetic protists. Note that some bacteria perform photosynthesis, but not within an organelle. Like mitochondria, chloroplasts have outer and inner membranes. The inner membrane encloses a space that contains a fluid called stroma, as well as a set of interconnected and stacked fluid-filled membrane sacs we call thylakoids (Figure 4.17). Each thylakoid stack is a granum (plural = grana). The chloroplasts contain a green pigment, chlorophyll, which is found in the thylakoid membranes. Chlorophyll captures the light energy that drives the reactions of photosynthesis. 

Chloroplasts are responsible for harvesting light energy to synthesize organic molecules like sugar. Photosynthesis is the series of reactions that use carbon dioxide, water, and light energy to make glucose and oxygen (Figure 4.14). The ability to carry out photosynthesis is a major difference between plants, animals, and fungi. All eukaryotic cells can perform cellular respiration to break down sugars to generate ATP within mitochondria. However, the source of the sugar is different for heterotrophs and autotrophs. Animals and fungi (heterotrophs) cannot photosynthesize, and so require external sources of food to provide this sugar. However, plants and photosynthetic protists (autotrophs) can produce their own sugar via photosynthesis.

The number of chloroplasts in a cell varies depending on the organism and cell function. On a given plant, a mature leaf cell contains about 20-100 chloroplasts. The root cells on that same plant will have no chloroplasts. This is due to the structure and function of the different plant tissues. Leaves have a large surface area to maximize the amount of light harvesting during photosynthesis. However, root cells do not participate in light-harvesting. Instead, their function is to extend and elongate to access nutrients and water inside the soil. Single-celled algae only contains one or a few chloroplasts. This is because there is a difference in energy requirements: a terrestrial plant will require more energy to maintain growth and survive compared to single-celled organism.

Peroxisomes 

Peroxisomes are small, round organelles enclosed by single membranes (Figure 4.16). They are generally small (0.5 – 1.0 µm in diameter). The defining feature of peroxisomes is the presence of an enzyme called catalase. In animal cells, peroxisomes carry out oxidation reactions that break down fatty acids. The product of the oxidation of fatty acids in peroxisomes is acetyl-CoA, which is then exported to the cytosol and mitochondria, where it can be used in cellular respiration to generate ATP. Oxidation of fatty acids within peroxisomes accounts for 25-50% of fatty acid oxidation within the entire animal cell. Many of these oxidation reactions release hydrogen peroxide, H₂O₂, which would be damaging to cells. However, when these reactions are confined to peroxisomes, catalase safely breaks down the H₂O₂ into oxygen and water. Peroxisomes also detoxify many poisons that may enter the human body.

Peroxisomes can be found in all eukaryotic cells. However, in animal cells, they are more common in liver and kidney cells because of their ability to detoxify harmful molecules (ethanol in the liver and uric acid in the kidneys, for example).  Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars. Plant cells contain many different types of peroxisomes that play a role in metabolism, pathogen defense, and stress response, to mention a few.