Movatterモバイル変換

\n\n'); DuckWin.document.write('

\n'); DuckWin.document.write('\n'); DuckWin.document.write(''); DuckWin.document.close();}

May	JUN	Aug
	29
2012	2013	2014

success

fail

About this capture

COLLECTED BY

Organization:Alexa Crawls

Starting in 1996,Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to theWayback Machine after an embargo period.

Collection:Alexa Crawls

Starting in 1996,Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to theWayback Machine after an embargo period.

TIMESTAMPS

The Wayback Machine - https://web.archive.org/web/20130629204107/http://biology.clc.uc.edu:80/courses/bio104/photosyn.htm

Photosynthesis

Photosynthesis is the process of converting light energy to chemical energy and storing it in the bonds of sugar. This process occurs in plants and some algae (Kingdom Protista). Plants need only light energy, CO₂, and H₂O to make sugar. The process of photosynthesis takes place in thechloroplasts, specifically usingchlorophyll, the green pigment involved in photosynthesis.

[Leaf Cross-Section] Photosynthesis takes place primarily in plant leaves, and little to none occurs in stems, etc. The parts of a typical leaf include theupper and lowerepidermis, themesophyll, thevascular bundle(s) (veins), and thestomates. The upper and lower epidermal cells do not have chloroplasts, thus photosynthesis does not occur there. They serve primarily as protection for the rest of the leaf. The stomates are holes which occur primarily in the lower epidermis and are for air exchange: they let CO₂ in and O₂ out. The vascular bundles or veins in a leaf are part of the plant's transportation system, moving water and nutrients around the plant as needed. The mesophyll cells have chloroplasts and this is where photosynthesis occurs.

[Chloroplast] As you hopefully recall, the parts of a chloroplast include the outer and inner membranes, intermembrane space,stroma, andthylakoids stacked ingrana. The chlorophyll is built into the membranes of the thylakoids.

Chlorophyll looks green because it absorbs red and blue light, making these colors unavailable to be seen by our eyes. It is the green light which is NOT absorbed that finally reaches our eyes, making chlorophyll appear green. However, it is the energy from the red and blue light that are absorbed that is, thereby, able to be used to do photosynthesis. The green light we can see is not/cannot be absorbed by the plant, and thus cannot be used to do photosynthesis.

The overall chemical reaction involved in photosynthesis is: 6CO₂ + 6H₂O (+ light energy) C₆H₁₂O₆ + 6O₂. This is the source of the O₂ we breathe, and thus, a significant factor in the concerns about deforestation.

Click on the chlorophyll
to see how to draw one.

Get the Corel Presentations
Show It! plug-in

Click the picture to re-start or press [ESC] to stop. You may also “write” on the picture. Unfortunately, Corel only has a Plug-In for Win 95/NT, so this won’t work with Win 3.1 or Mac.

There are two parts to photosynthesis:

Thelight reaction happens in the thylakoid membrane and converts light energy to chemical energy. This chemical reaction must, therefore, take place in the light. Chlorophyll and several other pigments such asbeta-carotene are organized in clusters in the thylakoid membrane and are involved in the light reaction. Each of these differently-colored pigments can absorb a slightly different color of light and pass its energy to the central chlorphyll molecule to do photosynthesis. The central part of the chemical structure of a chlorophyll molecule is aporphyrin ring, which consists of several fused rings of carbon and nitrogen with a magnesium ion in the center.

The energy harvested via the light reaction is stored by forming a chemical calledATP (adenosine triphosphate), a compound used by cells for energy storage. This chemical is made of the nucleotide adenine bonded to a ribose sugar, and that is bonded to three phosphate groups. This molecule is very similar to the building blocks for our DNA.

The dark reaction takes place in the stroma within the chloroplast, and converts CO₂ to sugar. This reaction doesn't directly need light in order to occur, but it does need the products of the light reaction (ATP and another chemical called NADPH). The dark reaction involves a cycle called theCalvin cycle in which CO₂ and energy from ATP are used to form sugar. Actually, notice that the first product of photosynthesis is a three-carbon compound calledglyceraldehyde 3-phosphate. Almost immediately, two of these join to form a glucose molecule.

Most plants put CO₂ directly into the Calvin cycle. Thus the first stable organic compound formed is the glyceraldehyde 3-phosphate. Since that molecule contains three carbon atoms, these plants are calledC₃ plants. For all plants, hot summer weather increases the amount of water that evaporates from the plant. Plants lessen the amount of water that evaporates by keeping their stomates closed during hot, dry weather. Unfortunately, this means that once the CO₂ in their leaves reaches a low level, they must stop doing photosynthesis. Even if there is a tiny bit of CO₂ left, the enzymes used to grab it and put it into the Calvin cycle just don't have enough CO₂ to use. Typically the grass in our yards just turns brown and goes dormant. Some plants likecrabgrass,corn, andsugar cane have a special modification to conserve water. These plants capture CO₂ in a different way: they do an extra step first, before doing the Calvin cycle. These plants have a special enzyme that can work better, even at very low CO₂ levels, to grab CO₂ and turn it first intooxaloacetate, which contains four carbons. Thus, these plants are calledC₄ plants. The CO₂ is then released from the oxaloacetate and put into the Calvin cycle. This is why crabgrass can stay green and keep growing when all the rest of your grass is dried up and brown.

There is yet another strategy to cope with very hot, dry, desert weather and conserve water. Some plants (for example, cacti and pineapple) that live in extremely hot, dry areas like deserts, can only safely open their stomates at night when the weather is cool. Thus, there is no chance for them to get the CO₂ needed for the dark reaction during the daytime. At night when they can open their stomates and take in CO₂, these plants incorporate the CO₂ into various organic compounds to store it. In the daytime, when the light reaction is occurring and ATP is available (but the stomates must remain closed), they take the CO₂ from these organic compounds and put it into the Calvin cycle. These plants are calledCAM plants, which stands forcrassulacean acid metabolism after the plant family, Crassulaceae (which includes the garden plantSedum) where this process was first discovered.

References:

Borror, Donald J. 1960. Dictionary of Root Words and Combining Forms. Mayfield Publ. Co.
Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology, 5^th Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
Campbell, Neil A., Lawrence G. Mitchell, Jane B. Reece. 1999. Biology: Concepts and Connections, 3^rd Ed. Benjamin/Cummings Publ. Co., Inc. Menlo Park, CA. (plus earlier editions)
Marchuk, William N. 1992. A Life Science Lexicon. Wm. C. Brown Publishers, Dubuque, IA.

[8]ページ先頭