Chloroplasts are membrane bound, spheroid, filamentous, saucer-shaped, discoid or ovoid shaped cell organelles, rich in chlorophyll, found in green plants and responsible for absorbing sunlight during photosynthetic process.
They are found in the cells of green plants and certain algae, but not in animal or bacterial cells, as they are unique to photosynthetic cells. These organelles are rich in chlorophyll, a molecule that absorbs light energy and gives plants and many algae their green color.
All green plants participate in photosynthesis, a process that transforms solar energy into sugars, with oxygen as a byproduct. This oxygen is essential for animals to breathe.
The number of chloroplasts varies by cell type, ranging from 1 to 100, such as in unicellular algae and plants like Arabidopsis and wheat. On average, a plant cell contains about 50 in number.
Features of Chloroplast
- Chloroplasts are a type of plastid distinguished by their membranes and high chlorophyll content. Other plastid types include leucoplasts and chromoplasts, which have little to no chlorophyll and do not perform photosynthesis.
- Chloroplast behavior is significantly affected by environmental factors like light intensity and color. Within the cytoplasm, They are usually evenly distributed, though in some cells, they cluster around the nucleus or just beneath the plasma membrane.
- They are highly dynamic, moving and circulating within plant cells, and sometimes divide by pinching in two to replicate.
- They are responsible for photosynthesis, where chlorophyll, the photosynthetic pigment, absorbs sunlight.
- This pigment captures sunlight energy (photons) and transforms it into stored energy in the form of ATP and NADPH, while splitting water molecules to release oxygen in autotrophic plant and algal cells.
- The ATP and NADPH are then used in the Calvin cycle to produce organic molecules from CO₂.
- In addition to photosynthesis, it perform other essential functions such as synthesizing amino acids, fatty acids, and contributing to the plant’s immune response.
Morphology of Chloroplast
- In land plants, they are generally lens-shaped, measuring around 3–10 μm in diameter and 1–3 μm in thickness.
- The size of chloroplasts varies by species but remains consistent within a specific cell type.
- These vesicular structures have a colorless center and are located in the mesophyll cells, where photosynthesis takes place.
- A typical leaf has two types of mesophyll cells: palisade parenchyma, which is the main site of photosynthesis, and spongy parenchyma.
- They are found in these mesophyll cells and in the bundle sheath cells.
- Chloroplast shapes differ across plant species; they can be spheroid, filamentous, saucer-shaped, discoid, or ovoid. For example, they appear spherical or ovoid in maize plants.
- Some of them are club-shaped, with a thin middle and chlorophyll-rich ends.
- In certain algae, a single, large chloroplast may form a network, a spiral band, or a stellate plate, as seen in Spirogyra (water silk), where it appears as spiral coils.
Ultra Structure of Chloroplast
- Chloroplasts have a structure enclosed by a double membrane and consist of three main membrane systems: the outer membrane, the inner membrane, and the thylakoid system.
- In some cases, they are formed through secondary endosymbiosis may have extra membranes surrounding these three layers.
- Within the chloroplast, a semi-gel-like fluid called the stroma occupies most of the internal volume.
- Floating within the stroma is the thylakoid system, which includes both stacked grana and stroma thylakoids.
- Inside chloroplasts, there are two main regions: the grana and the stroma.
- Grana consist of stacks of disc-shaped thylakoids, which contain chlorophyll and serve as the chloroplast’s functional units.
- The stroma, similar to the cell’s cytoplasm, surrounds the grana and contains various enzymes, DNA, ribosomes, and other molecules.
- Stroma lamellae link the thylakoid stacks. Photosynthesis occurs through two types of reactions: the light reaction, which takes place in the grana, and the dark reaction, which happens in the stroma region.
- The double membrane of chloroplasts is sometimes compared to that of mitochondria. However, unlike mitochondria, where the inner membrane runs proton pumps and performs oxidative phosphorylation to generate ATP, chloroplasts don’t use their inner membrane in this way, making the comparison limited.
- The internal thylakoid system in chloroplasts is the closest structure to mitochondria in function. However, the direction of proton (H⁺) flow during photosynthesis in chloroplasts is opposite to that in mitochondrial oxidative phosphorylation.
- Functionally, the chloroplast’s inner membrane, which controls metabolite transport and synthesizes materials, has no equivalent in mitochondria.
Parts of Chloroplast
- Outer Membrane – This forms the external surface of the chloroplast and is a semi-porous membrane, allowing small molecules and ions to pass through easily, though it is not permeable to larger proteins.
- Intermembrane Space – This is a thin space, approximately 10-20 nanometers wide, located between the outer and inner membranes of the chloroplast.
- Inner Membrane – It borders the stroma and regulates the movement of materials in and out of the chloroplast. In addition to its regulatory role, it is also involved in the synthesis of fatty acids, lipids, and carotenoids.
- Stroma – The space enclosed by the inner membrane but outside the thylakoid space is called the stroma. This protein-rich, alkaline fluid contains the thylakoid system, ribosomes, chloroplast DNA, starch granules, and numerous proteins.
- Thylakoid System – Suspended within the stroma, this dynamic network consists of membranous sacs called thylakoids, where chlorophyll is found and light-dependent reactions of photosynthesis occur. In most vascular plants and green algae, thylakoids are stacked to form structures called grana (singular: granum), but in some C4 plants and algae, they may be free-floating. Thylakoids contain light-absorbing pigments like chlorophyll and carotenoids, which are crucial for photosynthesis.
Light-absorbing pigments are combined with various proteins to form structures known as photosystems.
There are two types of photosystems in the thylakoid membrane: photosystem I (PSI) and photosystem II (PSII), both of which play roles in the light-dependent reactions.
In the chloroplast’s stroma, enzymes produce complex organic molecules, such as carbohydrates, which serve as energy storage.
Functions of Chloroplast
Chloroplasts are crucial for the growth and survival of plants and photosynthetic algae. Similar to solar panels, they capture light energy and transform it into a usable form that drives various cellular activities. Below is an explanation of the role they play in photosynthesis, along with the functions of their different components:
- The chloroplast envelope is semi-permeable, controlling the movement of molecules in and out of the chloroplast. Both the outer and inner membranes contain specialized intermembrane proteins that transport larger molecules. Additionally, these membranes are involved in synthesizing certain lipids and pigments, such as carotenoids, which are essential for light absorption.
- It is the sites where photosynthesis occurs, involving both light-dependent and light-independent reactions to capture solar energy and convert it into chemical energy, stored as sugars and other organic molecules that serve as food for the plant or algae.
- The light-dependent reactions of photosynthesis take place in the grana and associated photosystems, where photosynthetic pigments like chlorophyll a, chlorophyll b, and carotenoids absorb light energy. This energy is then used to split water molecules, producing ATP, NADPH, and oxygen.
- The stroma region is where the dark or light-independent reactions of photosynthesis occur. Enzymes in the stroma use carbon dioxide from the atmosphere, along with ATP and NADPH from the grana, to synthesize sugars and starches. This process, known as carbon fixation, occurs through the Calvin cycle.
- They are involved in various regulatory functions within the cell, including photorespiration (light-dependent oxygen fixation). Some of the reactions in photorespiration occur in the stroma of chloroplasts, while others take place in the mitochondria and peroxisomes. Photorespiration is believed to help protect the plant during drought stress and high radiation exposure.
Summary
Chloroplasts are membrane bound cell organelles that can be spheroid, filamentous, saucer-shaped, discoid, or ovoid in form. Abundant in chlorophyll, they are found in green plants and play a key role in capturing sunlight for the process of photosynthesis.
They are typically lens-shaped, about 3–10 μm in diameter and 1–3 μm thick. They are located in mesophyll cells as well as in bundle sheath cells.
It are enclosed by a double membrane and contain three main membrane systems: the outer membrane, the inner membrane, and the thylakoid system.
Within the stroma lies the thylakoid system, composed of stacked grana and stroma thylakoids. Grana are made up of stacks of disc-like thylakoids. The stroma, much like the cell’s cytoplasm, surrounds the grana and contains enzymes, DNA, ribosomes, and other molecules.
Photosynthesis happens through two types of reactions: the light reaction, occurring in the grana, and the dark reaction, which takes place in the chloroplast’s stroma.
Chloroplasts play a central role in photosynthesis, with chlorophyll, the photosynthetic pigment, capturing sunlight for energy production. Beyond photosynthesis, they are also essential for synthesizing amino acids and fatty acids and play a part in the plant’s immune response.
