Photosynthesis:
This process of making glucose from inorganic substances is called photosynthesis. Using the sun’s energy, inorganic CO2 is used to produce organic compounds.
Certain types of bacteria (photoautotrophs), called cyanobacteria, contain thylakoids that carry on the processes of photosynthesis.
Photosynthesis requires carbon dioxide, water, and sunlight. The carbon dioxide and water provide the necessary elements to make glucose, while light is the energy source for the process.
In all photosynthetic organisms, two separate sets of chemical reactions make up the whole photosynthetic process. The first step is called the light dependent reaction (light reaction). The second step is called the light independent reaction (dark reaction).
Chlorophyll and other pigments:
Light must be absorbed to be used by living organisms. Pigments absorb light. Different pigments absorb different wavelengths. White pigment absorbs no light; black pigment absorbs all light; chlorophyll, a green pigment, absorbs red, blue and violet light. The green color is not absorbed, but is reflected back to our eyes.
Different organisms use different pigments in photosynthesis.
Chlorophyll a: the main pigment involved in the conversion of light energy to chemical energy in photosynthesis.
Chlorophyll b: Similar to chlorophyll a.
Carotenoids: accessory pigments; carotenoids are yellow, red and orange. Beta-carotene is a carotenoid.
The green chlorophyll in plants masks the color of the carotenoids; thus, we can only see the carotenoids when the leaf stops producing chlorophyll in the fall. Carotenoids absorb light energy of different wavelengths than chlorophyll. These pigments can pass the energy from the light to chlorophyll a.
When pigments absorb light, the electrons within the pigment molecule are boosted to a higher energy level with three alternative consequences:
1) The energy is lost as heat.
2) The energy is re-emitted immediately as light energy of longer wavelengths-- called fluorescence.
3) The energy may trigger other reaction.
Whether or not a chemical reaction occurs depends on the pigment and its relationship with neighboring molecules.
molecules.
In the thylakoid membrane there are two types of chlorophyll molecules: light antennas and reaction centers.
Light Antennas gather light energy. When the light strikes the molecule, it vibrates. Because the molecules are tightly packed together, the excitation of the molecules spreads rapidly from molecule to molecule until it reaches the reaction center. The reaction center releases electrons into the electron transport chain.
When any molecule absorbs light, its electrons become excited and are boosted to a higher energy state. In photosynthetic cells, an excited molecule will pass its energy to an adjacent molecule. The passing of energy to the adjacent molecule continues until the energy reaches the REACTION CENTER.
Because the chlorophyll molecules are neatly lined up the high-energy electrons pass easily from the excited reaction centers to the electron transport chains.
Electron transport chains are chains of molecules that pass electrons from molecule to molecule. There are two types of electron transport chains in the light reaction. In one electron transport chain, the electrons are used to make ATP and returned to the reaction center. In this way, no chlorophyll electrons are lost. In the second type of electron transport chain, the electrons are passed to the molecule NADP that carries the electrons to the DARK REACTION. Chlorophyll electrons are lost in this electron transport chain.
Light Dependent Reaction/Light Reaction:
The photosynthetic unit contains two different photosystems. Each photosynthetic unit contains a slightly different type of antenna and reaction center. One photosystem, p680 or photosystem II, absorbs light rays that are 680 nm in length. The other photosystem, p700 or photosystem I, absorbs light rays that are 700 nm in length. Both photosystems are needed to complete the light reaction.
Photosystem I probably evolved first since this reaction can operate independently.
When light hits the chlorophyll, the electrons of the chlorophyll molecule absorb the light energy. Since the light antenna vibrates, the energy of excitation is passed from molecule to molecule without actual passing of the electrons. The excitation is passed on to the reaction center. When the reaction center receives this excitation energy, two electrons are boosted to a higher level of energy. The electrons leave the reaction center and are accepted by the primary acceptor, the first molecule in the electron transport chain.
The Z diagram:
The electrons that the reaction center loses must be replaced. A process known as the Z diagram or the Z scheme replaces the electrons. There are five steps in the Z diagram involving water and a protein called the Z protein. At the end of the Z diagram, the products will be 4H+, 4 e-, and 2H2O
Steps:
1) H2O + Z protein ---> Z-OH + H+ +
e-2) H2O + Z-OH ---> Z-2OH + H+ +
e-3) H2O + Z-2OH ---> Z-3OH + H+ +
e-4) H2O + Z-3OH ---> Z-4OH + H+ +
e-5) Z-4OH ---> Z protein + 2 H2O + O2
Water is found in the thylakoid space, which moved there via osmosis. The H+ from water is pumped into the medium inside the thylakoid membrane and are used in making ATP. The 4 electrons are pushed into the reaction center, the O2 is released as free oxygen, and the water is
placed back into the Z diagram.
The Electron Transport Chain of the Light Reaction.
The primary acceptor receives the electrons from the reaction center and passes these electrons to plastoquinone. Plastoquinone picks up two electrons and 2H+ from the medium outside the
thylakoid. Plastoquinone (PQ) is changed to PQH2. PQH2 passes the electrons to a Cytochrome
Complex (cyt com) and the 2 H+ to the place inside the thylakoid. Cyt com passes the two electrons
to Plastocyanin (Pc). Plastocyanin passes the two electrons to photosystem 700s reaction center.
In the p700 system, the electrons are accepted. When light rays measuring 700 nm hit the photosystem; the light antenna gets excited and passes this energy to the reaction center. The reaction center boosts the 2 electrons from p680 to a higher energy level. The primary acceptor accepts the electrons and passes them to Ferredoxin. Ferredoxin passes these electrons to NADP+. NADP+ accepts the two electrons and an H+ from the medium outside the thylakoid to become
NADPH (the enzyme NADP+ reductase passes the enzymes to NADP+). The NADPH carries these two electrons to the LIGHT INDEPENDNT or DARK reaction.
Cyclic Electron Flow:
There is evidence that photosystem I (p700) can work independently. In this reaction (called the cyclical electron flow), no NADPH is formed. Instead the electrons are boosted from p700 to Ferredoxin. These electrons are passed to the electron transport chain that connects the p680 to p700. ATP is produced in the course of electrons being passed from molecule to molecule.
Some photosynthetic bacteria carry out photosynthesis in this manner. However, this is an alternative path in eukaryotes.
Photosynthetic Phosphorylation: How ATP is produced by the Electron Transport Chain.
The phosphorylation of ADP to ATP (as the electrons are passed down from the p680 to p700) is a chemiosmotic process. The two electron transport chains contain cytochromes. The electron carriers and the enzymes are embedded in the membrane of the thylakoids, or chloroplasts, which is impermeable to H+ ions.
There is an electrochemical gradient of potential energy established as protons are pumped through the thylakoid membrane (the movement of H+ into the Thylakoid Space). The protons are moved in
two ways: 1) the oxidation of water by the Z diagram and 2) the movement of H+ from PQ. Now
there is a high concentration of H+ in the stroma, this creates potential energy—the H+ want to move
back into the stroma but cannot (the membrane is impermeable to H+). ADP is phosphorylated to
ATP as protons flow down their gradient through ATP synthase complexes.
The H+ are pumped out of the stroma and into the thylakoid space. When the H+ flow down the
gradient, they move from the thylakoid space back into the stroma where ATP is synthesized.
The potential energy comes from a difference of pH and the difference of electrical charge across the membrane. In fact the pH on the inside of the thylakoid membrane is around 5, while the pH on the outside of the thylakoid membrane is about 8. The protons flow through a channel and
neutralize negative charges on the other side of the membrane, releasing energy. A large enzyme called ATP synthase provides the channel. This complex is composed of three main parts that are made up of many protein subunits. The first part is the rotor component, which is embedded within the membrane (A stator protein is a large protein that anchors the rotor in place). The knob is the second component, and this is within the stroma. The third component is called the rod. The rod connects the two other parts together. ATP synthase has sites for ADP and ATP and catalyzes the formation of ATP from ADP and phosphate. When a hydrogen ion flows through the enzyme, the rotor spins in a clockwise fashion. Since it is connected to the rotor, the rod also spins, which causes the knob to rotate. This spinning motion releases energy that allows for the joining of the phosphate to ADP. It is not known whether it takes two, three or four H+ ions passing through the
ATP synthase complex to form an ATP molecule, which is used in the production of sugar.
Light Independent or the Dark reaction:
This reaction occurs outside the thylakoid membrane. In this part of the photosynthetic process, the carbohydrates are actually made. There are two different ways in which the carbohydrates can be made. The Calvin cycle, C3 cycle forms a three-carbon compound called 3-phosphoglycerate. The
second way to produce glucose is called the four-carbon pathway. This C4 pathway uses
phosphoenolpyruvate (PEP) to join with carbon dioxide to initially form a four-carbon compound.
Calvin Cycle: The C3 cycle.
The first step of this reaction starts with Ribulose Bi-phosphate. RuBP is a five-carbon sugar with two phosphate groups. RuBP is the compound that is added to Carbon Dioxide. CO2 is added to
RuBP to form an unstable 6-carbon compound. The compound is so unstable that it immediately breaks down into two molecules of 3-phosphoglycerate.
RuBP + CO2 ---> unstable 6 carbon compound ---> 2 3-PG
The enzyme is RuBP carboxylase (Rubisco). Rubisco is the most abundant protein in the plant, and this the most abundant protein on earth.
In the next step, a phosphate from ATP is added to 3-PG from ATP. 1,3-diphosphoglycerate (DPGA) is formed.
NADPH gives its electrons to the DPGA molecule. When this happens, a new three carbon molecules: phosphoglyceraldehyde (PGAL) and a phosphate is released.
2 DPGA + 2 NADPH ---> 2 3-PGAL + 2 NADP + 2 P
The PGAL can be combined to form the following products: glucose, cellulose, maltose, starch, fatty acids, amino acids and other molecules. RuBP is also reformed through a series of
complicated reactions. Here's a reaction overview:
6 RuBP + 6CO2 ---> 12 3PB and 12 ATP --- 12 DPGA + 12 NADPH --- 2
PGAL + 10 PGAL
2 PGAL -- Glucose
10 PGAL -6 RuBP
This is not an efficient process. Less than 1% of the light energy that reaches the chlorplast is found in the carbohydrates produced.
C4 Pathway:
1) Carbon dioxide binds to phosphoenolpyruvate (PEP) to form a four-carbon compound called Oxaloacetic acid (the enzyme is PEP carboxylase).
2) Electrons reduce oxaloacetic acid from NADPH to malic acid.
3) Malic acid is decarboxylated to yield carbon dioxide and pyruvic acid. The carbon dioxide enters the Calvin cycle (C3 path).
4) Pyruvic acid is phosphorylated (ATP) to reform phosphoenolpyruvate.
Carbon dioxide enters the plant via the epidermis through holes called stomata. The carbon dioxide enters the mesophyll cells and combines with PEP. The resulting malic acid is transported to the bundle sheath (a layer of cells surrounding the vascular tissues of plants). In the bundle sheath, malic acid is decarboxylated. Carbon dioxide enters the Calvin cycle and pyruvate moves back to the mesophyll.
If the air is still and there are a lot of plants in an area, carbon dioxide may not be readily available. The air closest to the leaves will have a low concentration of carbon dioxide.
PEP carboxylase (enzyme in the C4 path) has a higher affinity for carbon dioxide than RuBP
carboxylase, the enzyme in the C3 pathway. Since PEP carboxylase takes up carbon dioxide faster,
the carbon dioxide concentration is lower inside the leaf than it is outside the leaf. This is because the carbon dioxide binds to PEP immediately and is subsequently transported away from the mesophyll cells. This process maximizes the carbon dioxide gradient between the cells and the environment. When stomata are open, carbon dioxide diffuses more readily into the C4 leaf than
into the C3 leaf.
The RuBP carboxylase binds to oxygen as readily as it does to carbon dioxide. RuBP + O2 --
glycolic acid, a substance for photorespiration. Photorespiration is the oxidation of carbohydrates in the presence of oxygen and light (produces CO2). Up to 50% of the glucose made by the C3 plant
efficiency of C3 plants.
High carbon dioxide to oxygen concentrations helps limit photorespiration. In C4 plants the carbon
dioxide is brought to the RuBP carboxylase in a concentrated form, thus reducing photorespiration. Any carbon dioxide released due to photorespiration is bound by PEP carboxylase and brought back to RuBP carboxylase.
C4 plants evolved in the tropics and are adapted to high intensities of light, high temperatures, and
dry conditions. The optimal temperature for C4 plants is higher than C3 plants. C4 plants may
