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Plant cells with visible chloroplasts

Chloroplast is an organelle in plant cells, algae, and some protists that performs photosynthesis. The purpose for having chloroplast in plants is to produce food: glucose and other sugars. The chloroplast has chlorophyll (a green pigment), which is responsible for giving plants their characteristic color. The chlorophyll captures light energy to drive the photosynthetic reactions.

Photosynthesis can be summarized as follows:
6CO2 + 6H2O + light → C6H12O6 + 6O2
Carbon Dioxide + water + light energy → Glucose + Oxygen

Role in Photosynthesis

Main Article: Photosynthesis

The chloroplast is a cellular organelle, that catalyzes photosynthetic reactions.[1]. Photosynthesis is the process during which water, carbon dioxide, and light energy are used to assemble carbohydrates. Photosynthesis can be designated into two stages: light reaction and dark reaction. The light reaction happens with the light energy. The dark reaction happens without the light energy, but it uses ATP, which is made from the light reaction. All the time, the light reaction happens first, and then the dark reaction happens. The light reaction occurs in a membrane of the thylakoid, and the dark reaction occurs in the stroma.

Chloroplast Components:
* Thylakoid: A structure is a flat sack, which is in the chloroplast of the plant’s cell.
* Lamella: A thin and flat film that connects granum in plant chloroplast.
* Lumen: The cavity of a tubular organ in thylakoid.
* Stroma: The fluid content of the organelle in plant chloroplast.
* Inner Membrane: A membrane that inner most part and protects the organelles in plant chloroplast.
* Intermembrane Space: A membrane that locates between the inner membrane and the outer membrane.
* Outer Membrane: A membrane that protects the organelle.
* Granum: One of the laminated stacks of chlorophyll material in plant chloroplast.

The chloroplasts are different in size and shape. They range in diameter from 2~10μm and is 1μm thick with flat-oval shape. They are covered by two membranes: the inner membrane and the outer membrane. Between the outer membrane and the inner membrane, there is an inter-membrane space where photosynthesis is carried out. In the inner membrane there are the stroma, granum, thylakoid, lamella, and lumen. The stroma contains aqueous fluid, which is dissolved by the enzyme. The granum is flat stack, which contains chlorophyll. The series of grana (Grana is plural for granum ) is suspended in the stroma. The grana absorb light energy, and both stroma and grana use water, carbon dioxide, and light as the materials to make glucose and sugar. Thylakoid is flat and thin compartment that is closely pack in the granum. Actually, thylakoid contains chlorophyll. In other words, chlorophyll is in thylakoid, and thylakoid in granum. Because thylakoid compartments are closely packed in granum, the granum is called as stack of thylakoids. Lamella is the connection between grana. The lamellae (Lamellae is plural for lamella) keeps grana in right distance to help efficiency of the grana. [2] Lumen is inside of thylakoid.

A generalized image of Photosystems in Photosynthesis, a light harvesting cluster of photosynthetic pigments in a chloroplast thylakoid membrane

While the plant performs photosynthesis, Photosystem I and Photosystem II both work together. Both photosystem I and II absorb light continuously, and they are required by the noncyclic electron transport. Noncyclic electron transport produces NADPH + H+ and ATP (Adenosine Triphosphate). NADPH + H+ is a reduced coenzyme that is oxidized form of NADP+ (Nicotinamide Adenine Dinucleotide Phosphate).

Photosystem II uses light energy to split (oxidize) two molecules of water into one molecule of molecular oxygen and four (H+). The 4 electrons removed from the water molecules are transferred by the electron transport chain and used by Photosystem I to reduce 2NADP+ to 2NADPH. The hydrogen ions (protons) are used to make ATP.(Purves 152)

2H2O -> 4H+ + 2e- + O2

During summer, plants make lots of sugar and prepare for fall and winter. Almost all plants stop producing sugar during winter time. The reason is that, during summer, their chlorophyll can gather more light energy than during the fall and winter. Plants pass through the winter with starch and sugar, which is mostly made during summer. [3]

Chloroplast Components

  • Thylakoid: A structure like a flat sack, which is in the chloroplast of the plant’s cell
  • Lamella: A thin and flat film that connects the granum in plant chloroplast
  • Lumen: The cavity of a tubular organ in thylakoid
  • Stroma: The fluid content of the organelle in a plant chloroplast
  • Inner Membrane: The innermost membrane that protects the organelles in a plant chloroplast
  • Inter-membrane Space: A membrane that is located between the inner membrane and the outer membrane
  • Outer Membrane: A membrane that protects the organelles and the whole chloroplast by wrapping it
  • Granum: One of the laminated stacks of chlorophyll material in a plant chloroplast


Space-filling model of the chlorophyll molecule

There are five different types of pigments: carotene, xanthophyll, chlorophyll a, chlorophyll b, and phaeophytin. Most leaves contain chlorophyll a and b. Chlorophyll is the most prominent pigment in plants and the principle photocollector which absorbs the light energy involved in (photosynthesis).

Why the Plants Are Green?


Chloroplasts inside of the plants’ leaves have various kinds of photosynthesis pigments. Most pigments are green color, so the leaves are green. Especially, the top of the leaf is greener than the bottom of the leaf, because the chlorophylls are concentrated at the top of the leaf more than those of at the bottom of the leaf. [5]

Why Leaves Turn Red or Yellow?

Although there are several pigments in leaves, the chlorophyll masks their presence and we only see the color green. Upon the arrival of Fall, the chlorophyll pigments are absorbed by the plant, and the other pigments become visible turning the leaves red, orange, or yellow.[6]

See Also

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