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All organisms need energy to perform vital processes and stay alive. That is why we need to eat, and organisms like plants gather energy from the sun to produce their food. How does the energy contained in the food we eat or in the sun get to every cell in an organism’s body? Fortunately, organelles called mitochondria and chloroplast do this job. Hence, they are considered the “powerhouses” of the cell. These organelles differ from other cell organelles in many ways, such as having their own DNA and ribosomes, suggesting a remarkably distinct origin.
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.
Most eukaryotic cells (protist, plant, animal, and fungi cells) have hundreds of mitochondria (singular mitochondrion) dispersed in the cytosol. They can be elliptical or oval-shaped and have two bilayered membranes with an intermembrane space between them (Figure 1). The outer membrane surrounds the whole organelle and separates it from the cytoplasm. The inner membrane has numerous inward folds extending into the interior of the mitochondrion. The folds are called cristae and surround the interior space called the matrix. The matrix contains the mitochondrion’s own DNA and ribosomes.
A mitochondrion is a double membrane-bounded organelle that performs cellular respiration (uses oxygen to break down organic molecules and synthesize ATP) in eukaryotic cells.
Figure 1. Left: diagram of a mitochondrion and its components. Source: modified from Margaret Hagen, Public domain, www.flickr.com. Right: microscope image of mitochondria inside a mammalian lung cell. Source: Louisa Howard, Public domain, via Wikimedia Commons.
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 found in the cells of plants and algae (photosynthetic protists) only. They perform photosynthesis, transferring energy from the sunlight into ATP, which 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, they have a double membrane and an intermembrane space (Figure 2). The inner membrane encloses the thylakoid membrane that forms numerous piles of interconnected fluid-filled membranous discs called thylakoids. Each pile of thylakoids is a granum (plural grana), and they are surrounded by a fluid called the stroma. The stroma contains the chloroplast’s own DNA and ribosomes.
Figure 2. Left: diagram of a chloroplast and its components (DNA and ribosomes not shown). Source: Margaret Hagen, Public domain, www.flickr.com. Right: microscope image of plant cells containing numerous oval-shaped chloroplasts. Source: HermannSchachner, CC0, via Wikimedia Commons.
Thylakoids contain several pigments (molecules that absorb visible light at specific waves) incorporated into their membrane. Chlorophyll is more abundant and the main pigment that captures the energy from sunlight. In photosynthesis, chloroplasts transfer energy from the sun into ATP which is used, along with carbon dioxide and water, to produce carbohydrates (mainly glucose), oxygen, and water (simplified description). ATP molecules are too unstable and must be used in the moment. Macromolecules are the best way to store and transport this energy to the rest of the plant.
Chloroplast is a double-membrane organelle found in plants and algae that capture energy from sunlight and uses it to drive the synthesis of organic compounds from carbon dioxide and water (photosynthesis).
Chlorophyll is a green pigment that absorbs solar energy and is located in membranes within the chloroplasts of plants and algae.
Photosynthesis is the conversion of light energy to chemical energy that is stored in carbohydrates or other organic compounds.
In plants, chloroplasts are widely distributed but are more common and abundant in leaves and other green organs’ cells (like stems) where photosynthesis primarily occurs (chlorophyll is green, giving these organs their characteristic color). Organs that do not receive sunlight, like roots, do not have chloroplasts. Some cyanobacteria bacteria also perform photosynthesis, but they do not have chloroplasts. Their inner membrane (they are double-membrane bacteria) contains the chlorophyll molecules.
As both organelles transform energy from one form to another, they have similarities related to this function. Other similarities are more related to the origin of these organelles (like having a double membrane and their own DNA and ribosomes, which we will discuss shortly). Some similarities between these organelles are:
The ultimate purpose of both organelles is to provide cells with the required energy to function. However, they do so in different ways. The differences between these two organelles are:
This diagram (Figure 3) summarizes the similarities and differences between mitochondria and chloroplasts:
Figure 3. Venn diagram summarizing the similarities and differences between a mitochondrion and a chloroplast. Liza, StudySmarter Originals.
As discussed above, mitochondria and chloroplasts have striking differences compared to other cell organelles. How can they have their own DNA and ribosomes? The most accepted hypothesis suggests that eukaryotes originated from an ancestral archaea organism (or an organism closely related to archaea). Evidence suggests that this archaea organism engulfed an ancestral bacterium that was not digested and eventually evolved into the organelle mitochondrion. This process is known as endosymbiosis.
Two separate species with a close association and typically exhibit specific adaptation to each other live in symbiosis (the relationship can be beneficial, neutral, or disadvantageous for one or both species). When one of the organisms lives inside the other, it is called endosymbiosis (endo = within). Endosymbiosis is common in nature, like photosynthetic dinoflagellates (protists) that live inside coral cells—the dinoflagellates exchange products of photosynthesis for inorganic molecules with the coral host. However, mitochondria and chloroplasts would represent an extreme case of endosymbiosis, where most of the endosymbiont genes have been transferred to the host cell nucleus, and neither symbiont can survive without the other anymore.
In photosynthetic eukaryotes, a second event of endosymbiosis is thought to have happened. In this way, a lineage of the heterotrophic eukaryotes containing the mitochondrial precursor acquired an additional endosymbiont (probably a cyanobacterium, which is photosynthetic).
Plenty of morphological, physiological, and molecular evidence supports this hypothesis. When we compare these organelles with bacteria, we find many similarities: a single circular DNA molecule, not associated with histones (proteins); the inner membrane with enzymes and transport system is homologous (similarity due to a shared origin) with the plasma membrane of bacteria; their reproduction is similar to the binary fission of bacteria, and they have similar sizes.
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.
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.
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.
Plants need mitochondria to break down the macromolecules (mostly carbohydrates) produced by photosynthesis that contains the energy that their cells use.
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.
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