The chloroplast, an exclusive cell organelle in plant and algal cells is a critical site for photosynthesis-driven energy production. The etymology of the term “chloroplast” is derived from the Greek words khloros, which means “green,” and plates, which means “formed.”
The organelle is distinguished by its heightened concentration of chlorophyll, the light-harvesting molecule, which contributes to the green coloration of numerous plants and algae. It is believed that similar to mitochondria, chloroplasts have evolved from independent bacterial organisms.
Function of Chloroplasts
Chloroplasts, intricate organelles residing within plant and algal cells, play a pivotal role in the process of photosynthesis by harnessing light energy and converting it into the form of sugar and other organic molecules, which are further utilized as food by the plant or alga.
The photosynthesis mechanism can be broadly divided into two stages: the light-dependent reactions and the light-independent reactions, also known as the Calvin cycle.
During the first stage, light-dependent reactions occur, during which chlorophyll and carotenoids capture sunlight to produce adenosine triphosphate (ATP), a form of energy used by the cell, and nicotinamide adenine dinucleotide phosphate (NADPH), an electron carrier.
The second stage, the light-independent reactions, involves the conversion of inorganic carbon dioxide and the organic molecule, resulting from NADPH, into carbohydrates in a process called CO2 fixation. Carbohydrates and other organic molecules can then be stored and utilized by the cell for energy.
The survival and growth of plants and photosynthetic algae rely heavily on the presence of chloroplasts, which function similarly to solar panels, capturing light energy and converting it into a usable form that powers the cell’s activities.
However, there are a few plants that have lost their chloroplasts over evolutionary time. One such example is the parasitic plant genus Rafflesia, which relies entirely on other plants, specifically Tetrastigma vines, for its nutrients.
Since Rafflesia acquires all its energy by parasitizing other plants, it no longer needs its chloroplasts and has lost the genes essential for chloroplast development. Over a long period, Rafflesia is the only known land plant genus that lacks chloroplasts.
Diagram of Chloroplast
Below is a visual Diagram of the chloroplast, highlighting its various components. These include the outer and inner membranes, the intermembrane space, thylakoid membranes, stroma, and lamellae. By identifying and marking these parts, we can gain a better understanding of the chloroplast’s structure and function.
Structure of Chloroplasts
Chloroplasts, similar to mitochondria, are oval-shaped organelles characterized by a dual membrane system, comprised of an outer membrane and an inner membrane situated just beneath it. Sandwiched between these membranes, lies a slender intermembrane gap, around 10-20 nanometers in width.
The region confined within the inner membrane is referred to as the stroma. Unlike the folded inner membranes of mitochondria, chloroplasts have an untextured surface. In its place, many minute disk-shaped pouches called thylakoids, are present in the stroma.
Thylakoids, found in vascular plants and green algae, are arranged in a stack-like manner, forming a structure known as a granum (plural: grana). Chlorophylls and carotenoids, which are photosynthetic pigments, are found within the thylakoids and are involved in absorbing light energy.
Such pigments are combined with other molecules such as proteins, to give rise to photosystems. There are two types of photosystems: Photosystem I and Photosystem II, and they play distinct roles in various stages of light-dependent reactions.
In the stroma, enzymes assist in the formation of intricate organic compounds that are employed to store energy, such as carbohydrates. Moreover, the stroma harbors its own DNA and ribosomes, which are strikingly similar to those that exist in photosynthetic bacteria.
This has led to the proposal that chloroplasts may have evolved from free-living bacteria, similar to the case of mitochondria, in eukaryotic cells.
Evolution of Chloroplasts
Chloroplasts, which are distinguished from other plastids by their green hue, are organelles situated in plant cells that participate in the synthesis and storage of nourishment. These cell structures are believed to have emerged from endosymbiotic cyanobacteria.
A eukaryotic cell is thought to have enveloped an aerobic prokaryote, which then forged an endosymbiotic alliance with the host eukaryote, ultimately transforming into a chloroplast.
The genome of the cyanobacterium that originated the chloroplast underwent substantial shrinkage during evolution, mainly due to gene loss and the transfer of genes to the nuclear genome.
Enclosed in a double-membrane chloroplast envelope consisting of outer and inner layers with an intermembrane space in between, these organelles are approximately 1-2 μm in thickness and 5-7 μm in diameter. Additionally, the thylakoid membrane system, which comprises a third internal membrane, is heavily folded and arranged into stacks of thylakoids.
Chloroplasts have evolved differently in various organisms. For instance, chlorarachniophyte chloroplasts are bounded by four membranes, with the exception of the region near the cell membrane, where the chloroplast membranes merge into a double membrane.
Their thylakoids are loosely arranged in stacks of three. Furthermore, chlorarachniophytes store a type of polysaccharide known as chrysolaminarin.
Dinoflagellates, another group of protists, features around half of photosynthetic dinophytes. Most dinophyte chloroplasts are secondary red algal-derived chloroplasts. Many other dinophytes have either discarded or substituted their chloroplasts via tertiary endosymbiosis.