Chloroplasts and mitochondria are two cellular structures present in plants and algae (organisms that perform photosynthesis). They harness energy from light to produce sugars like glucose.
Both organisms share a genetic system, reflecting their shared origins as photosynthetic bacteria. Genes from these endosymbionts were transferred to their host genomes through endosymbiont lateral gene transfer.
They have an inner membrane
Chloroplasts and mitochondria are similar in that they both possess an inner membrane. These endocytic organelles float within a cell’s cytosol, performing several essential functions such as photosynthesis (producing carbohydrates from carbon dioxide) and oxidative phosphorylation (combining acetyl CoA oxidation with NADH and FADH2 reduction).
The inner membrane consists of multiple folds that encase layered structures known as cristae. These cristae are connected to the outer membrane via pores. Furthermore, there is a large protein complex within this inner membrane called mitochondrial inner membrane organizing system, which maintains cristae morphology.
Another essential feature of the inner membrane is diphosphatidylglycerol, which plays an integral role in its structure and function. X-ray crystallography studies have demonstrated that diphosphatidylglycerol binds to specific transmembrane proteins.
Mitochondria are ubiquitous in cells and provide the primary energy source for cellular activity. Their inner membrane contains a proton gradient which drives ATP synthesis while serving as an important site for oxygen production and oxidative phosphorylation reactions.
Two major types of molecules enter a cell through its membranes: proteins and nucleotides. Most proteins found inside are encoded by nuclear genes and synthesized on cytosolic ribosomes.
Some proteins, however, cannot be produced within the cell but must pass through both inner and outer membranes to enter a cell. Examples include enzymes involved in the Krebs cycle and fatty acid b-oxidation pathways.
Proteins must pass through a pore containing two molecular chaperones, hsp70 and hsp60, in order to enter the inner membrane. As hsp70 binds and moves the protein into place, it must then dissociate from hsp70 so it can fold properly into its intended shape.
They have a thylakoid membrane
Chloroplasts and mitochondria both possess a thylakoid membrane, an internal system in chloroplasts which conducts light reactions and generates ATP.
The thylakoid membrane is composed of a lipid bilayer surrounded by the stroma, which contains proteins and other components. The stroma serves as the site for protein synthesis and transcription.
Chloroplasts rely on thylakoid membranes for photosynthesis, transporting and storing chlorophyll. These membranes are organized in a network known as the grana, each consisting of 10-20 thylakoids.
Helical fretwork connects these stacks, making it difficult for them to separate and move independently (Paolillo et al., 2005).
However, this does not denote that the thylakoid lipid bilayer isn’t essential for photosynthesis in chloroplasts. Indeed, several studies have demonstrated its necessity (Tsvetkova et al., 1994; Tietz et al., 2015)
These studies reveal that phospholipids are the primary lipid component of the thylakoid bilayer. Phospholipids in this layer play an essential role in transporting phosphate, an essential step for ATP synthesis.
Comparative to mitochondria, which have a higher membrane potential due to charge separation, thylakoid has a much lower potential due to lack of charge gradient.
Additionally, thylakoid membranes have the capacity to adapt their function according to changes in environmental conditions. For instance, Synechococcus sp PCC 7942 exhibits ‘radial asymmetry’ whereby photosynthetic complexes vary lateral distribution within different layers; this adaptation helps the plant adapt to changes in environment.
They have cristae
Both chloroplasts and mitochondria share a common feature: their inner membrane that forms multiple infoldings. Cristae are essential to organelles because they increase surface area and provide more room for chemical reactions.
They also increase the amount of proteins that can live within them, which is essential for ATP production. Furthermore, these protein complexes help create an electrochemical gradient that promotes oxidative phosphorylation and produces ATP.
Furthermore, cristae are filled with molecules that store solar energy as glucose and fatty acids. They play an integral role in photosynthesis, helping convert light into food for cells.
Cristae have long baffled scientists, as their structure is unlike any other membrane structure found in cells and tissues. Early models depicted them as baffle-like folds; however, 3D images of different organisms reveal a very distinct internal architecture.
Cristae are hollow compartments instead of disk-like structures that connect to the outside boundary of a membrane via narrow tubular segments known as crista junctions (Figure 2); these can range in length from several tens of nanometers up to several hundred.
They contain most of the fully assembled protein complexes responsible for electron transport chain, oxidative phosphorylation, beta oxidation and ATP synthase. Furthermore, they include most of cytochrome c – a small soluble electron carrier used to transfer protons across membranes to drive these processes.
Most importantly, the cristae contain dimers necessary for ATP synthesis. These dimers can be found along tightly curved crista ridges or around narrow tubular cristae and organized into long rows. This arrangement is an essential feature of all mitochondria and provides a key mechanism for efficient assembly and utilization of ATP.
They have compartments
Chloroplasts and mitochondria share a similar feature; they both possess compartments which create distinct environments for various reactions within their cells. These compartments enable organelles to carry out essential processes like cellular respiration and photosynthesis – the conversion of sunlight into chemical energy.
The compartments created by mitochondria and chloroplast membrane folds provide for segregated environments for ATP (adenosine triphosphate) synthesis, one of the key roles these organelles perform.
Chloroplasts, found in plants and algae, convert sunlight into carbon dioxide and sugars that the cell uses for energy (ATP).
Mitochondria, found in most eukaryotic cells, are an important source of energy for the body. Furthermore, they store calcium and produce iron compounds.
Organelles possess their own DNA and ribosomes to synthesize proteins. This suggests they likely evolved from ancestral bacteria that were engulfed by eukaryotes and became endosymbionts.
An intriguing fact is that both organelles possess double membranes. These membranes look similar to those found in bacterial cells and suggest these organelles probably originated from prokaryotic cells that were then consumed by eukaryotic organisms.
The outer membrane of mitochondria consists of a simple phospholipid bilayer with protein structures called porins. These porins allow certain molecules, like ions and nutrients, to pass through while the inner membrane has many infoldings called cristae that are studded with proteins for selective passage of specific-sized particles.
They have ATP synthesis
Mitochondria produce ATP (adenosine triphosphate), the primary short-term energetic molecule for cells through cellular respiration. Found in nearly all eukaryotic cells, mitochondria provide energy for many different cellular reactions.
Cellular respiration utilizes glucose and oxygen to convert sugars (glucose or lipids) into ATP, creating carbon dioxide and water as byproducts. The number of mitochondria a cell has depends on its function as well as how much energy it requires.
The production of ATP takes place within a chemical engine called F1-ATPase. It’s attached to Fo protein, soluble within the matrix which serves as an electrostatic channel between intermembrane space and its environment.
Fo and F1 are intricate structures that use protons flowing into the matrix to bind ADP + Pi and release ATP. Their trimeric nature allows them to switch conformational states, altering ADP+P binding, phosphorylation reactions, and ATP release sequentially.
This process is controlled by the chloroplast g subunit and its disulfide bond between b-hairpins inserted before its C-terminal a-helix. The g subunit transports protons from intermembrane space into the matrix through its g-rotor.
A proton gradient across the thylakoid membrane creates a pH gradient of 3.5 units (Fig. 1), which is sufficient to produce a proton driving force of 0.20 V.
Plants benefit from efficient collaboration between chloroplast and mitochondria, increasing photosynthesis, ATP production in the mitochondria, and sucrose synthesis in the cytosol. This increase is largely attributed to higher photosystem I/II ratios, increased LEF capacity, and an uptick in NADH production and consumption within mitochondria.
