A chloroplast is one of the cell constituents collectively known as organelles. Chloroplasts are found only in plants, using the word “plant” in its broadest sense.
Chloroplasts are food-making factories. In these organelles, the energy of sunlight is captured and transformed. The energy thus acquired powers the synthesis of glucose from carbon dioxide and water.
Because chloroplasts contain a green pigment called chlorophyll, the tissues containing chlorophyll have a greenish color. A good example is the leaf of an oak or maple tree.
According to the Encyclopaedia Britannica, the typical chloroplast is a spheroid about 5000 nanometers long and 2,500 nanometers thick. Other sources give variant figures, but this gives you a general idea concerning their size and shape. Since a nanometer is one billionths of a meter, these organelles are extremely small.
The Covering of the Chloroplast
According to “Biology” by Campbell, Reece, and Mitchell, chloroplasts are surrounded by “an envelope consisting of two membranes separated by a very narrow intermembrane space.” Important components in each of these two membranes are phospholipids and proteins.
Scientists have had difficulties figuring out how these phospholipids and proteins are arranged in the membrane. Since phospholipids contain a phosphate head that attracts water and a lipid tail that repels it, it is currently believed that the phosphate heads lie along the outside and the inside surface of each membrane, while the lipid tails occupy the middle of each membrane. This arrangement is due to the fact that there are water molecules outside and inside the membranes of a chloroplast.
Proteins also have areas that attract water. In the case of at least some proteins, this hydrophilic portion juts out the surface of the membrane, while the rest of the protein extends into the interior of the membrane.
The Stroma
Inside the double membrane is the stroma of the chloroplast. The Greek word strôma refers to anything that is spread out, such as a bed or mattress, so that someone or something can sit or lie on it. In chloroplasts, the stroma is a colorless liquid that serves as a matrix in which structures called thylakoids rest.
Dissolved or suspended in the stroma are starch granules and a lot of enzymes. Enzymes are proteins that enable certain chemical reactions to take place. Sometimes they speed up chemical reactions that take place slowly.
Thylakoids
As mentioned above, there are important structures called thylakoids in the stroma of chloroplasts. I am reasonably certain that the word thylakoid is derived from the Greek words thýlakos and thylákion, which mean “bag” and “small bag,” respectively. Accordingly, a thylakoid is a structure that resembles a small bag.
Thylakoids are disk-shaped structures with flat lamellae on the top and the bottom of the disk. Inside the thylakoid is a space called a lumen, which is isolated from the stroma by the enclosing lamellae. Usually but not always, the thylakoids are arranged in stacks called grana. Granum is a Latin word meaning “seed,” or “grain.” It is difficult to see why biologists would use this term for a stack of disk-shaped objects. I imagine that the grana looked like little grains before scientists had optical equipment that could resolve them into stacks. They must have called them grana because of their appearance and continued to do so even after they could see what these structures really were. Of course, this is just an educated guess.
The lamellae of thylakoids contain important substances, such as chlorophylls, carotenes, carotenoids, pheophytin, cytochromes, plastoquinone, and ferrodoxin.
The Light Reaction of Photosynthesis
The light reaction of photosynthesis cannot take place in the dark. Sunlight or some appropriate artificial light is needed.
In the light reaction, water molecules are split up, and oxygen is one of the resultant products. Moreover, adenine diphosphate (ADP) is converted to energy-rich adenine triphosphate (ATP). Another energy-rich substance called NADPH is also formed. Light supplies the energy for these reactions, and enzyme catalysts aid the process.
This light reaction takes place in the lamellae of the thylakoids. It is believed that these lamellae have two organized photosystems called photosystem II an photosystem I. Each photosystem has a type of chlorophyll known as chlorophyll a. The light reaction begins when light of an appropriate wave length impinges on a molecule of chlorophyll a. When this happens, the light energy imparts additional energy to one of the electrons of the chlorophyll a molecule.
Alternatively, sunlight may strike one of the other pigments in the lamellae. These other pigments cannot initiate the light reaction, but the energy received by the pigment can be passed on to chlorophyll a. Chlorophyll a then initiates the light reaction.
According to the Encyclopaedia Britannica , each photosystem has a light harvesting complex, and a core complex. The light harvesting complex contains the pigments that can pass energy to chlorophyll a. The most important part of the core complex is the reaction center, which contains chlorophyll a and other substances involved in the light reaction.
In the reaction center of each photosystem, a series of substances are arranged in a line next to each chlorophyll a molecule. These substances take the excited electron and pass it on from one to another until it reaches the end of the line. This line of substances is called the electron transport chain. The substances in the electron transport chain of photosystem I differ from those in the electron transport chain of photosystem II.
According to the Encyclopaedia Britannica , the substance next to chlorophyll a in photosystem II is pheophytin, so pheophytin is the substance that first grabs the excited electron in this photosystem. The corresponding substance in photosystem I is not currently known.
As the electron is passed down the line, it loses some energy. In photosystem II, this expended energy is used to attach an extra phosphate to ADP to produce ATP. This is a complex process involving several factors. For example, it is believed that protons are pumped into the lumen of the thylakoid and the resulting concentration gradient helps in the production of ATP. Catalytic enzymes are also involved.
However, there is a problem. The electrons lost by chlorophyll a of photosystem II do not return to the molecules that lost them. Instead, they eventually go to photosystem I and replace electrons lost by chlorophyll a in that system. As a result, the chlorophyll a molecules of photosystem II have to look elsewhere for electrons that they need to achieve electric neutrality.
According to Ohio State University, Photosystem II gets the necessary electrons by splitting water molecules. When two water molecules are split, the products are four positively charged hydrogen atoms (i.e., four protons), four electrons, and one molecule of oxygen. The four electrons replace the electrons lost by four chlorophyll a molecules in photosystem II.
In photosystem I, excited electrons are passed down the line of the electron transport chain until they reach positively charged NADP. These electrons, together with the protons formed by the splitting of water, convert positively charged NADP into NADPH.
If the plant has more NADPH than it needs, the electrons are sent back to photosystem II so that they can be used in the production of extra ATP.
The Dark Reactions of Photosynthesis
The dark reactions of photosynthesis use the energy stored in ATP and NADPH during the light reaction to fix inorganic carbon dioxide so that it becomes part of organic compounds.
Dark reactions take place in the stroma of chloroplasts. It is a complex process involving various reactions and various catalysts.
Not all plants produce glucose by the same series of reactions. Some plants use the Calvin cycle, in which the first stable product is a three-carbon compound called phosphoglyceric acid (PGA). Others, such as corn, use the Hatch and Slack pathway, otherwise known as the C-4 pathway because a four-carbon compound (oxaloacetic acid) occurs early in the cycle. The C-4 pathway is a series of preliminary reactions that lead up to further reactions that resemble those in the Calvin cycle.
The term “dark reactions” may be misunderstood. It may lead people to believe that they take place in the absence of light. In fact, according to the Encyclopaedia Britannica ,, “some of the enzymes involved in the so-called dark reactions become inactive in prolonged darkness.” Without the help of these enzymes, the dark reactions cannot be completed.
The product of photosynthesis is usually called glucose. It cannot be denied that photosynthesis leads to the production of glucose, but a lot of other organic compounds are also formed. According to the Encyclopaedia Britannica , most glucose is used to make starch and sucrose. Moreover, a more basic product of the dark reactions is glyceraldehyde-3-phosphate, some of which leaves the chloroplast and enters the cytoplasm of the cell. Here it is used in the production of such compounds as amino acids, lipids, pigments, etc.
The structure of the chloroplast and the intricate series of reactions performed in it could not have developed by chance. The process of photosynthesis is a marvelous work of the Creator.
References:
The Encyclopaedia Britannic: Photosynthesis
http://global.britannica.com/EBchecked/topic/458172/photosynthesis
“Biology” by Neil Campbell, Jane Reece, and Lawrence Mitchell
“Botany: An Ecological Approach” by William Jensen and Frank Salisbury
Ohio State University: Photosynthesis
http://www.mansfield.ohio-state.edu/~sabedon/campbl10.htm
