What’s a Chloroplast?

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Chloroplasts are organelles found in plants where photosynthesis occurs. They contain membranes, chlorophyll, grana, and stroma. Chlorophyll collects sunlight and other pigments absorb different wavelengths. Grana store sunlight and create sugar while stroma converts ATP back into sugar in the dark reaction.

A chloroplast is a type of structure, called an organelle, found in plants and is where photosynthesis takes place. Normally found in plant leaves, chloroplasts contain all the components that enable the plant to convert sunlight into usable energy. The main components of chloroplasts are membranes, chlorophyll and other pigments, grana and stroma.

Chloroplasts are one of the most important components of a plant because the entire photosynthetic process takes place in them. Each cell of a plant leaf can have 50 of these organelles. Chloroplasts appear only in eukaryotic organisms, which are mostly non-animals.

There are three types of membranes in chloroplasts: the outer membrane, the inner membrane, and the thylakoid membranes. The outer membrane surrounds the chloroplast and allows molecules to pass in and out of the organelle without discretion. The inner membrane sits underneath the outer one and is more discriminating about what allows in and out of the chloroplast. The thylakoid membranes lie within the inner membrane and are arranged in stacks connected to each other by stromal lamellae. These lamellae act as the structure or skeleton for each chloroplast.

Chlorophyll is a green pigment that collects sunlight needed for photosynthesis. Located in the thylakoid membranes, chlorophyll is what causes leaves to turn green. Other pigments, such as carotenoids, which make carrots orange, are also found in the thylakoid membranes.

Usually, these other pigments are found in much smaller quantities than chlorophyll. Each pigment absorbs different wavelengths of light. For example, chlorophyll absorbs every wavelength except green, which is why the pigment appears green to the eye.

Granas are stacks of thylakoid membranes. Each granule stores sunlight obtained from chlorophyll and draws water (Hsub2O) and carbon dioxide (CO2) from other parts of the leaves to form a type of sugar (C6H12O6) which the plant uses for food. This is the light-dependent process of photosynthesis. Sugar that is not used immediately by the plant is converted into adenosine triphosphate (ATP) and stored for later use. This process also occurs in the grain.

Stroma is a gel-like substance that surrounds the thylakoid membranes in each chloroplast. Enzymes in the stroma take ATP and convert it back into sugars which are then used by plants. This process is called a dark reaction because, unlike light-dependent reactions, it doesn’t rely on sunlight to complete. The conversion of ATP to sugar is known as the Calvin cycle.

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