FUNCTION OF CHLOROPLASTS

The primary function of chloroplasts is photosynthesis, the light-driven fixation of carbon dioxide into organic compounds. The products of the photochemical reactions that occur within thylakoid membranes provide the material with which the plant cells grow and on which all forms of life on the surface of Earth depend.

Photosynthesis begins when light is absorbed by the green pigment chlorophyll, which occurs only in photosynthetic thylakoid membranes. The absorbed light energy is transferred to a reaction center called Photosystem II (PSII), where electrons are removed from water to release molecular oxygen. The electrons are carried through an electron transport chain in thylakoid membranes to Photosystem I (PSI) to eventually produce reduced compounds (for example, NADPH) that drive carbon fixation reactions. The flow of electrons through this linked set of carriers also transfers protons (H+) from the stroma to the thylakoid lumen, which generates a concentration gradient. These protons can only flow back to the stroma through protein channels within the thylakoid membrane. At the stromal end of the membrane channels is adenosine triphosphate (ATP) synthase, which uses the flow of H+ to drive the synthesis of H+ ATP. ATP is used as the primary energy source for biosynthetic reactions within the cell. The ATP and NADPH created are then used to produce sugars from carbon dioxide.

The most abundant enzyme in the biosphere, ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco, for short), catalyzes the reaction of carbon dioxide with ribulose 1,5-bisphosphate, a 5-carbon compound, to make glyceraldehyde 3-phosphate and 3-phosphoglycerate. These two 3-carbon compounds enter the reductive pentose-phosphate cycle (also called the Calvin-Benson cycle) and eventually are converted to a 6-carbon sugar, glucose 6-phosphate, the ultimate product. Glucose 6-phosphate is the precursor of many of the storage products in the plant cell, such as starch, sucrose, and lipids, and is also the starting point for biosynthesis of most of the cellular material. All fatty acids and most amino acids used by the cell are also synthesized in the chloroplast.

Rubisco is a large enzyme—containing eight large (molecular weight 52,000) and eight small (molecular weight 14,000) subunits—that is also very sluggish, catalyzing a reaction only three times per second even when saturated with carbon dioxide. The usual concentration of carbon dioxide in the watery cell interior is sufficient for only one-half this rate. Perhaps these are the reasons why plants developed mechanisms to achieve a high concentration of the enzyme in the stroma to catalyze this reaction that is essential to maintenance of life. Approximately two million molecules of rubisco are present in each chloroplast.

WHAT IS THE CHLOROPLAST

The chloroplast is a membrane-bound organelle within a cell that conducts photosynthesis. From the molecular perspective, the chloroplast is very large and contains millions of protein molecules along with vast sheets of membranes. If we imagine an average-sized enzyme molecule to be the size of an automobile, a chloroplast in a plant leaf cell would be about 6 kilometers on its long axis and about 2 kilometers on its short axis. The approximately cube-shaped plant cell, 15 to 20 kilometers per side, would contain fifty to one hundred of these compartments.

The chloroplast is enclosed by two membranes, designated the outer and inner membranes of the chloroplast envelope. About one-half the volume within the chloroplast is occupied by stacks of fifty to one hundred flattened sacs called thylakoids, from the Greek word meaning "like an empty pouch." The thylakoid membrane surrounds the lumen or interior space and is the major membrane of the chloroplast. Groups of thylakoids adhere into stacks called grana. The remaining soluble phase of the chloroplast, outside thylakoids, is the stroma.

CHLOROPHYLL

Chlorophyll is the green pigment in plants that absorbs light energy for use in photosynthesis. Chlorophyll also is found in simple organisms called algae and in some bacteria. Most plant cells do not produce chlorophyll unless the plant is exposed to light. This is why plants kept away from light are white or yellow rather than green. Chlorophyll is located in disk-shaped membranes called thylakoids within cells. In most plants, thylakoids are contained in tiny cell bodies called chloroplasts. The chloroplasts in the leaves of plants carry out all the essential processes of photosynthesis. Light energy absorbed by chlorophyll is channeled to specialized reaction centers in the thylakoids. The reaction centers, along with electron-carrier molecules, convert the light energy to chemical energy. Oxygen is released in the process.

Chemical energy is needed for taking carbon dioxide from the air, eventually leading to the production of sugars and such other food substances as starch, fat, protein, and vitamins.

There are several forms of chlorophyll. The most common forms in plants are chlorophyll a and chlorophyll b. They absorb most of the long wavelengths (red rays) and the short wavelengths (blue-violet rays) of visible light. They absorb the middle wavelengths (green rays) least effectively. Some bacteria, like plants, make their own food by photosynthesis. These bacteria have special chlorophylls that can absorb longer wavelengths called infrared rays, which lie beyond the visible light spectrum. When dried, chlorophyll looks like blue or green-black powder.

Chloroplast, is a specialized structure within the cells of plants. Chloroplasts serve as the site of photosynthesis. They contain chlorophyll, the green pigment that absorbs energy from sunlight for use in photosynthesis. Chlorophyll also gives green plants their color. In the fall, the production of chlorophyll in woody plants ceases. The colors of yellow pigments in the chloroplasts then become visible.

The chloroplasts of most plants are shaped like disks or lenses. Under a microscope, they can be suspended in the part of a cell called the cytoplasm. Except for the cell nucleus, chloroplasts are the most visible structures in a plant cell. Chloroplasts are one of several types of specialized plant-cell structures called plastids. Other plastids contain yellow, orange, or red pigments, and provide the colors of many flowers and fruits. Plastids also store oil, protein, and starch.