Plants require energy in the form of ATP and they produce it during the day through photosynthesis and at night through cellular respiration. The two organelles known as mitochondria are crucial to this process.
Mitochondria are considered the powerhouse of a cell and they bidirectionally communicate with other organelles on a moment to moment basis. This communication becomes faulty during cellular stress and results in mitochondrial associated disorders like Alzheimer’s disease.
Photosynthesis
In photosynthesis, the Sun’s energy causes a chemical reaction that changes water and carbon dioxide molecules into sugar and oxygen gas. The sugar molecules provide fuel for the plant. The oxygen can be used by the plant itself, or it can be released into the air to help other organisms, such as animals, survive.
The process of photosynthesis is carried out in organelles called chloroplasts, which are found only in plants and some algae. Chloroplasts are disc-shaped and have outer and inner membranes with a space in between. The center of the chloroplast contains membrane stacks called thylakoids, which are packed together in interconnected “grana.”
Light reactions take place inside the thylakoids in the presence of protein-containing pigments called chlorophyll. The proteins in the thylakoids absorb sunlight and convert its energy into adenosine triphosphate (ATP). In this way, ATP provides the cell with a steady supply of chemical energy that can be used to do work. The ATP is also used to fix carbon dioxide, which is a waste product of the plant’s normal respiration.
Dark reactions take place outside the thylakoids in the stroma of the chloroplast. The stroma is the aqueous fluid surrounding the stacks of thylakoids. In these reactions, the electrons from the ATP are transferred to oxidative enzymes in the chloroplast and other cellular oxidative pathways that transfer CO2 back into carbohydrates, which are then used for energy.
These reactions require a large amount of energy, and the energy comes from a molecule called NADH, which is produced in the mitochondria of the plant’s cells. The most important of these oxidative reactions is the Calvin cycle. The cyclical reaction involves the oxidation and reduction of six molecules of acetyl CoA to four molecules of glycine, and then two molecules of glycine to one serine. The cyclic reaction requires a large number of molecular oxidoreductases, including complexes like cytochrome c reductase and Phosphoglycerate dehydrogenase.
Mitochondria are often called the powerhouses of the cell. They make a steady supply of ATP, the cell’s main energy-carrying molecule. ATP is created in a process called cellular respiration, and many steps of that process happen in the mitochondria. Mitochondria are suspended in the jelly-like cytosol of the cell, and have an outer membrane and an inner one with many inward protrusions called cristae that increase surface area. The inner membrane contains mitochondrial DNA and ribosomes. Evidence suggests that mitochondria and chloroplasts evolved in a relationship known as endosymbiosis. This is a type of symbiosis in which two different organisms, each with unique genetic and metabolic characteristics, live in a close, mutually beneficial relationship.
Cell Respiration
During cellular respiration, mitochondria take the energy-rich sugars that have been created in photosynthesis and break them down into easier to use molecules like ATP. This is the process that gives cells their basic energy, and it happens in all eukaryotic organisms – plants and animals alike. The ATP produced by this process is used to power the cell’s functions, including the movement of its components and the generation of nerve impulses.
Unlike animal cells, which obtain their energy from the food that they consume, plants get their energy from sunlight via photosynthesis, so plants need mitochondria to help them produce this energy. While cellular respiration does produce some energy, most of the 38 ATPs produced are used for other processes, such as the conversion of glucose to fatty acids and proteins and other organic molecules. Only a small amount of the energy from this process is used to generate heat, which helps keep the cell warm.
The ATP production happens in the matrix space of the mitochondria, a compartment surrounded by an inner membrane. This matrix contains the enzyme complexes of the electron transport chain and oxidative phosphorylation, as well as other proteins and coenzymes. The enzymes of this complex convert two molecules of adenosine triphosphate (ATP) into the energy-rich compound adenosine diphosphate (ADP). This is accomplished through a series of reactions that also involve the Krebs cycle and other coenzymes such as NAD+, FADH2.
Mitochondria are sometimes referred to as the “powerhouses” of the cell, because they are known for providing much of the basic energy needed by the cell. The rest of the cellular energy comes from lipids, which are stored in lipid bilayers and phospholipid membranes, and from sugars.
The overall function of mitochondria is regulated at various levels, and the relationship between this organelle and other parts of the cell has been studied for decades. One important discovery is that mitochondria communicate with the cytoplasm through specific transporters, which allow metabolites to move back and forth between these two spaces. This type of signaling is important because it allows a plant to respond to changing conditions in the environment.
Coenzyme Production
During the day, photosynthesis is a very energy-intensive process. It requires a lot of oxygen, and so plants use cellular respiration to produce adenosine triphosphate (ATP), the cell’s main energy carrier. Once again, mitochondria act as the site for this process, capturing and transporting energy from the molecular level to the cytosol.
In addition to acting as the powerhouse of the cell, mitochondria also perform a number of other important tasks. They produce coenzymes that are required for a number of essential cellular functions, including DNA production and protein synthesis. They also play a role in cell metabolism, oxidative stress responses and apoptosis.
The majority of the proteins found in mitochondria are encoded by nuclear genes. Most of them are assembled into multi-subunit complexes within the cytoplasm, and then imported into mitochondria by mitochondrial transfer RNA (mtRNA). The remaining proteins that are encoded in mitochondria are involved in energy metabolism, protein synthesis and apoptosis.
Mitochondria are suspended in the jelly-like cytoplasm of cells, and are oval-shaped with two membranes, an outer one that surrounds the organelle and an inner one that has many inward protrusions called cristae that increase surface area. The space between the membranes contains a matrix that contains mitochondrial DNA and ribosomes.
Over the past decade, a large body of research has been performed to understand how mitochondria are formed and how they function. It is becoming increasingly clear that mitochondrial biogenesis occurs via a multi-faceted, cell-specific mechanism. This is particularly true for plant mitochondria, which can vary in size, shape and protein content across tissue types and over development.
It is believed that a series of signals are received from the cytoplasm that control mitochondrial biogenesis in order to meet the energy demands of the plant. These signals are transmitted from a small group of nuclear genes that are known as the “mitochondrial ribosomal RNA” genes.
During stress, a variety of factors have been shown to regulate mitochondrial proteins and genes via transcriptional regulation. Specifically, the transcription factor AP2/ERF along with several bZIP and trihelix-domain proteins have been shown to regulate the expression of mitochondrial proteins (Gonzalez et al., 2009). It has also been demonstrated that dynamins and related proteins such as DRP3A and DRP3B are dual targeted to both mitochondria and peroxisomes, and their loss of function correlates with an alteration in both mitochondrial and peroxisomal fission and morphology.
Mitochondrial Biogenesis
The mitochondria in a cell are formed through a process known as biogenesis. Once formed, the mitochondrial DNA is replicated and translated to make proteins. This protein production is regulated by a complex system called mitochondrial transcription. Recent studies on mitochondrial biogenesis have revealed that the nuclear genes encoding for mitochondrial proteins can be triggered to activate in response to certain environmental conditions. This is known as retrograde signaling, and the signals are relayed back to the mitochondrial genome. Mitochondria also contain specific cofactors and transcription factors that regulate their own gene expression.
The number of mitochondria in a cell can vary from one tissue to another, and it can increase or decrease as the cell adapts to different stress conditions and metabolic demands. For example, the number of mitochondria in skeletal muscle cells increases with exercise training, and this is associated with improved ATP-producing capacity. This increased synthesis of mitochondrial proteins is mediated by the transcription factor NFE2L2, which is a protein that is normally retained in the cytoplasm by interacting with the kelch ECH-associated protein 1 (KEAP1).
It has been shown that mitochondrial protein composition and abundance can vary between cells within the same organ, or even the same species. This is because the morphology and dynamics of mitochondria vary in a cell, with fusion and fission being important events. The exact mechanisms behind these variations have yet to be fully elucidated, but they may involve changes in mitochondrial shape or the movement of proteins between the mitochondrial matrix and the outer membrane.
Interestingly, it has been shown that the endoplasmic reticulum and mitochondria have a bidirectional communication network that is used to promote physiological processes such as lipid metabolism, ATP production and calcium signaling. Defects in this communication can lead to mitochondrial dysfunction, which is associated with a variety of severe human pathologies including type-2 diabetes and cardiovascular disease.
The structure of mitochondria is fascinating, as it consists of an outer membrane and an inner membrane with many inward protrusions called cristae that increase surface area. The space in between these membranes contains a matrix that contains mitochondrial DNA and ribosomes.