Conversely, when water is scarce, the stomata open in an effort to help the leaf nourish itself. Structure of the Plant Cell Plant cells are eukaryotic cells, meaning that they have both the four structures common to all cells DNA, a cell membrane, cytoplasm and ribosomes and a number of specialized organelles. Plant cells, however, unlike animal and other eukaryotic cells, have cell walls, like bacteria do but constructed using different chemicals. Plant cells also have nuclei, and their organelles include the mitochondria, the endoplasmic reticulum, Golgi bodies, a cytoskeleton and vacuoles.
But the critical difference between plant cells and other eukaryotic cells is that plant cells contain chloroplasts. The Chloroplast Within plant cells are organelles called chloroplasts. Like mitochondria, these are believed to have been incorporated into eukaryotic organisms relatively early in the evolution of eukaryotes, with the entity destined to become a chloroplast then existing as a free-standing photosynthesis-performing prokaryote.
The chloroplast, like all organelles, is surrounded by a double plasma membrane. Within this membrane is the stroma, which functions sort of like the cytoplasm of chloroplasts.
Also within the chloroplasts are bodies called thylakoid, which are arranged like stacks of coins and enclosed by a membrane of their own. Chlorophyll is considered "the" pigment of photosynthesis, but there are several different types of chlorophyll, and pigment other than chlorophyll participate in photosynthesis, too.
The major pigment used in photosynthesis is chlorophyll A. Some non-chlorophyll pigments that take part in photosynthetic processes are red, brown or blue in color. The Light Reactions The light reactions of photosynthesis use light energy to displace hydrogen atoms from water molecules, with these hydrogen atoms, powered by the flow of electrons ultimately liberated by incoming light, being used to synthesize NADPH and ATP, which are needed for the subsequent dark reactions.
The light reactions occur on the thylakoid membrane, inside the chloroplast, inside the plant cell. This enzyme is what liberates the hydrogen atoms from water molecules. The oxygen in the water is then free, and the electrons freed in the process are attached to a molecule called plastoquinol, turning it into plastoquinone. This molecule in turn transfers the electrons to an enzyme complex called cytochrome b6f. This ctyb6f takes the electrons from plastoquinone and moves them to plastocyanin.
At this point, photosystem I PSI gets on the job. This enzyme takes the electrons from plastocyanin and attaches them to an iron-containing compound called ferredoxin. You don't need to memorize all of these compounds, but it is important to have a sense of the cascading, "handing-off" nature of the reactions involved.
Also, when PSII is liberating hydrogen from water to power the above reactions, some of that hydrogen tends to want to leave the thylakoid for the stroma, down its concentration gradient. The thylakoid membrane takes advantage of this natural outflow by using it to power an ATP synthase pump in the membrane, which attaches phosphate molecules to ADP adenosine diphosphate to make ATP.
The Dark Reactions The dark reactions of photosynthesis are so named because they do not rely on light. However, they can occur when light is present, so a more accurate, if more cumbersome, name is "light-independent reactions. Imagine that, when inhaling air into your lungs, the carbon dioxide in that air could make its way into your cells, which would then use it to make the same substance that results from your body breaking down the food you eat.
In fact, because of this, you would never have to eat at all. This is essentially the life of a plant, which uses the CO2 it gathers from the environment which is there largely as a result of the metabolic processes of other eukaryotes to make glucose, which it then either stores or burns for its own needs. You have already seen that photosynthesis starts by knocking hydrogen atoms free from water and using the energy from those atoms to make some NADPH and some ATP.
But so far, there has been no mention of the other input into photosynthesis, CO2. Enter Rubisco In the first step of the dark reactions, CO2 is attached to a five-carbon sugar derivative called ribulose 1,5-bisphosphate. This enzyme is believed to be the most abundant protein in the world, given that it is present in all plants that undergo photosynthesis. This six-carbon intermediate is unstable and splits into a pair of three-carbon molecules called phosphoglycerate.
These are then phosphorylated by a kinase enzyme to form 1,3-bisphosphoglycerate. This molecule is then converted to glyceraldehydephosphate G3P , liberating phosphate molecules and consuming NAPDH derived from the light reactions.
The G3P created in these reactions can then be put into a number of different pathways, resulting in the formation of glucose, amino acids or lipids, depending on the specific needs of the plant cells. All photosynthetic organisms plants, certain protistans, prochlorobacteria, and cyanobacteria have chlorophyll a.
Accessory pigments absorb energy that chlorophyll a does not absorb. Accessory pigments include chlorophyll b also c, d, and e in algae and protistans , xanthophylls, and carotenoids such as beta-carotene. Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate Green-Yellow-Orange wavelengths.
Molecular model of chlorophyll. Molecular model of carotene. Carotenoids and chlorophyll b absorb some of the energy in the green wavelength.
Why not so much in the orange and yellow wavelengths? Both chlorophylls also absorb in the orange-red end of the spectrum with longer wavelengths and lower energy. The origins of photosynthetic organisms in the sea may account for this.
Shorter wavelengths with more energy do not penetrate much below 5 meters deep in sea water. The ability to absorb some energy from the longer hence more penetrating wavelengths might have been an advantage to early photosynthetic algae that were not able to be in the upper photic zone of the sea all the time.
The molecular structure of chlorophylls. The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur. Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane as in a chloroplast or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria.
Absorption spectrum of several plant pigments left and action spectrum of elodea right , a common aquarium plant used in lab experiments about photosynthesis. Images from Purves et al. The structure of the chloroplast and photosynthetic membranes Back to Top The thylakoid is the structural unit of photosynthesis.
Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma. While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast. Stages of Photosynthesis Back to Top Photosynthesis is a two stage process. The first process is the Light Dependent Process Light Reactions , requires the direct energy of light to make energy carrier molecules that are used in the second process.
The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present. Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts.
Overview of the two steps in the photosynthesis process. Water is split in the process, releasing oxygen as a by-product of the reaction. The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process.
Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes. Photosystems are arrangements of chlorophyll and other pigments packed into thylakoids.
Many Prokaryotes have only one photosystem, Photosystem II so numbered because, while it was most likely the first to evolve, it was the second one discovered. Photosystem I uses chlorophyll a, in the form referred to as P Photosystem II uses a form of chlorophyll a known as P Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane. Action of a photosystem.
Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. This occurs when the electrons from water are excited by the light in the presence of P The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria.
Light energy causes the removal of an electron from a molecule of P that is part of Photosystem II. These O-2 ions combine to form the diatomic O2 that is released.
The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I.
Light acts on a molecule of P in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor that is a different molecule from the one associated with Photosystem II.
The electron from Photosystem II replaces the excited electron in the P molecule. This energy is used in Carbon Fixation.
Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. Noncyclic photophosphorylation top and cyclic photophosphorylation bottom. These processes are better known as the light reactions. The above diagrams present the "old" view of photophosphorylation. We now know where the process occurs in the chloroplast, and can link that to chemiosmotic synthesis of ATP. Chemiosmosis as it operates in photophosphorylation within a chloroplast.
Halobacteria, which grow in extremely salty water, are facultative aerobes, they can grow when oxygen is absent. Purple pigments, known as retinal a pigment also found in the human eye act similar to chlorophyll. The complex of retinal and membrane proteins is known as bacteriorhodopsin, which generates electrons which establish a proton gradient that powers an ADP-ATP pump, generating ATP from sunlight without chlorophyll. This supports the theory that chemiosmotic processes are universal in their ability to generate ATP.
Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures, diffusing into the cells. The Calvin Cycle occurs in the stroma of chloroplasts where would it occur in a prokaryote? Carbon dioxide is captured by the chemical ribulose biphosphate RuBP. RuBP is a 5-C chemical.
Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose. The reactions in this process were worked out by Melvin Calvin shown below. Melvin Calvin took charge of this work at the end of the war in order to provide raw materials for John Lawrence's researches and for his own study of photosynthesis.
Using carbon, available in plenty from Hanford reactors, and the new techniques of ion exchange, paper chromatography, and radioautography, Calvin and his many associates mapped the complete path of carbon in photosynthesis.
The accomplishment brought him the Nobel prize in chemistry in Eventually there are 12 molecules of glyceraldehyde phosphate also known as phosphoglyceraldehyde or PGAL , a 3-C , two of which are removed from the cycle to make a glucose. Remember the complexity of life, each reaction in this process, as in Kreb's Cycle, is catalyzed by a different reaction-specific enzyme.
C-4 Pathway Back to Top Some plants have developed a preliminary step to the Calvin Cycle which is also referred to as a C-3 pathway , this preamble step is known as C
All photosynthetic organisms plants, certain protistans, prochlorobacteria, and cyanobacteria have chlorophyll a. This image is copyright Dennis Kunkel at www. Figure 1. A few plants, principally legumes, are able to get their nitrogen atoms from the air because they have symbiotic relationship with microoganisms in their roots that contain the enzyme nitrogenase which bonds atmospheric nitrogen to hydrogen NH3 , presumably from water, making it available to the plant. Nature Reviews Molecular Cell Biology 5, doi
Noncyclic photophosphorylation top and cyclic photophosphorylation bottom. Much of photosynthesis has been figured out only fairly recently with photosynthesis researchers wining a bunch of Nobel prizes for chemistry in the 80's and 90's. Interestingly, although green plants contribute much of the oxygen in the air we breathe, phytoplankton and cyanobacteria in the world's oceans are thought to produce between one-third and one-half of atmospheric oxygen on Earth. At the more advanced level a good resource was the ability to search into some photosynthesis books via Google books and Amazon books. Photosynthesis vs. Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose.
Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. However, autotrophs only use a specific component of sunlight Figure 5. Each type of electromagnetic radiation has a characteristic range of wavelengths. Post Calvin process I put in huge time trying to crack this, looking at more and more references and just getting more confused. It takes a lot of digging in textbooks to find any of this info, which is all spread out, and usually obscurely stated. The balance between the plant carbon dioxide removal and animal carbon dioxide generation is equalized also by the formation of carbonates in the oceans.