All living things require energy to survive and carry out important functions. Humans get energy by eating food, while plants use the energy from the sun to make their own food. But how does this energy reach every cell in our body or in a plant? That's where mitochondria and chloroplasts come in. These tiny structures are known as the "powerhouses" of the cell because they generate energy. They are different from other parts of the cell because they have their own DNA and ribosomes, which suggests they have a unique origin. Mitochondria and chloroplasts are essential for life and play a crucial role in how organisms function.
Cells get energy from their environment, usually in the form of chemical energy from food molecules (like glucose) or solar energy. They then need to convert this energy into useful forms for everyday tasks. Mitochondria and chloroplasts are the organelles that transform the energy for cellular use, although they do this in different ways, as we will discuss.
In most eukaryotic cells, which include plant, animal, fungi, and protist cells, there are hundreds of mitochondria. These organelles can be shaped like ovals or ellipses and have two membranes with an intermembrane space between them (as shown in Figure 1). The outer membrane surrounds the entire mitochondrion and separates it from the cytoplasm. The inner membrane has many folds called cristae, which enclose the interior space known as the matrix. The matrix contains the mitochondrion's own DNA and ribosomes. This double-membrane organelle performs cellular respiration, which breaks down organic molecules with the help of oxygen and produces ATP, an essential molecule for eukaryotic cells.
Mitochondria transfer energy from glucose or lipids into ATP (adenosine triphosphate, the main short-term energetic molecule of cells) through cellular respiration. Different chemical reactions of cellular respiration occur in the matrix and in the cristae. For cellular respiration (in a simplified description), mitochondria use glucose molecules and oxygen to produce ATP and, as by-products, carbon dioxide and water. Carbon dioxide is a waste product in eukaryotes; that is why we exhale it through breathing. The number of mitochondria a cell has depends on the cell’s function and the energy it requires. As expected, cells from tissues that have a high energy demand (like muscles or cardiac tissue that contracts a lot) have abundant (thousands) mitochondria.
Chloroplasts are organelles found exclusively in the cells of plants and algae (photosynthetic protists). They are responsible for performing photosynthesis, which converts energy from sunlight into ATP that is used to synthesize glucose. Chloroplasts belong to a group of organelles known as plastids that produce and store material in plants and algae.
Chloroplasts are lens-shaped and, like mitochondria, have a double membrane and an intermembrane space. The inner membrane encloses the thylakoid membrane that forms numerous interconnected membranous discs called thylakoids. The thylakoids are arranged in stacks called grana, which are surrounded by a fluid called the stroma. The stroma contains the chloroplast's own DNA and ribosomes.
Chlorophyll is the primary pigment found in the thylakoid membrane and is responsible for absorbing solar energy. In photosynthesis, chloroplasts convert energy from the sun into ATP, which is used, along with carbon dioxide and water, to produce carbohydrates (mostly glucose), oxygen, and water. ATP molecules are too unstable and must be used immediately, so macromolecules are used to store and transport this energy throughout the plant.
Chloroplasts are widely distributed in plants, but they are most common and abundant in the cells of leaves and other green organs where photosynthesis primarily occurs. Organs that do not receive sunlight, such as roots, do not contain chloroplasts. Some bacteria, such as cyanobacteria, also perform photosynthesis, but they do not have chloroplasts. Instead, their inner membrane contains chlorophyll molecules.
Both mitochondria and chloroplasts share similarities related to their function of transforming energy from one form to another. They both increase their surface area through membrane folds or interconnected sacs, optimize the use of the interior space, and provide compartments for different reactions needed for cellular respiration and photosynthesis. Additionally, both synthesize ATP through chemiosmosis, transport protons across membranes, and have a double membrane.
Moreover, both organelles have their own DNA and ribosomes that synthesize a small number of proteins, but most proteins for their membranes are directed by the cell nucleus and synthesized by free ribosomes in the cytoplasm. Finally, both mitochondria and chloroplasts reproduce independently of the cell cycle. These similarities suggest that these organelles may have evolved from prokaryotic cells that were engulfed by eukaryotic cells and became endosymbionts.
In summary, while both organelles are involved in energy production, mitochondria and chloroplasts have different functions and are present in different types of organisms. Mitochondria break down carbohydrates to produce ATP through cellular respiration, while chloroplasts produce ATP from solar energy and store it in carbohydrates through photosynthesis. Mitochondria are present in most eukaryotic cells, while chloroplasts are only found in plants and algae, which are autotrophic organisms that produce their own food. These differences explain the distinct metabolic reactions performed by each organelle.
The endosymbiosis hypothesis suggests that mitochondria and chloroplasts originated from free-living bacteria that were engulfed by ancestral archaea organisms, which eventually evolved into eukaryotic cells. This process allowed for the development of a symbiotic relationship between the host cells and the bacterial endosymbionts, with both providing benefits to each other.
The similarities between mitochondria and chloroplasts with bacteria, such as having their own circular DNA molecule, lack of association with histones, homologous inner membranes, and similar reproduction methods, provide evidence for the endosymbiosis hypothesis. Furthermore, the transfer of most endosymbiont genes to the host cell nucleus explains why these organelles have their own DNA and ribosomes, but also why they cannot survive without the host cell. The acquisition of chloroplasts in photosynthetic eukaryotes through a secondary endosymbiosis event, where a cyanobacterium was engulfed by a heterotrophic eukaryote containing the mitochondrial precursor, provides further support for the endosymbiosis hypothesis. Overall, the endosymbiosis hypothesis is the most widely accepted explanation for the origin of mitochondria and chloroplasts, and the similarities between these organelles and bacteria provide strong evidence to support it.
These organelles are essential for the functioning of eukaryotic cells and have played a significant role in the evolution of life on Earth. The ability to produce and utilize energy from macromolecules and sunlight has allowed organisms to become more complex and diverse, leading to the development of multicellular organisms and the emergence of complex ecosystems. The endosymbiosis hypothesis provides a compelling explanation for how these organelles originated and highlights the importance of symbiotic relationships in the evolution of life. Overall, the understanding of mitochondria and chloroplasts provides critical insights into the fundamental processes of life and the mechanisms that drive evolution.
What is the function of mitochondria and chloroplasts?
The function of mitochondria and chloroplasts is to transform the energy from macromolecules (like glucose), or from the sun, respectively, to a useful form for the cell.
What do chloroplasts and mitochondria have in common?
Common features that chloroplasts and mitochondria have: a double membrane, their interior is compartmentalized, they have their own DNA and ribosomes, they reproduce independently of the cell cycle, they synthesize ATP.
What is the difference between mitochondria and chloroplasts?
The inner membrane in mitochondria have folds called cristae, the inner membrane in chloroplasts encloses another membrane that forms thylakoids; mitochondria perform cellular respiration while chloroplasts perform photosynthesis; mitochondria are present in most eukaryotic cells (from animals, plants, fungi, and protists), while only plants and algae have chloroplasts.
Why do plants need mitochondria?
Plants need mitochondria to break down the macromolecules (mostly carbohydrates) produced by photosynthesis that contains the energy that their cells use.
Why do mitochondria and chloroplasts have their own DNA?
Mitochondria and chloroplasts have their own DNA and ribosomes because they probably evolved from ancestral bacteria that were engulfed by the ancestor of eukaryote organisms. This process is known as endosymbiosis.
