Presentation on theme: "Photosynthesis- The Basis for Life on Earth. Photosynthesis- is the process that converts light energy into chemical energy. This chemical energy is usually."— Presentation transcript:
Photosynthesis- The Basis for Life on Earth
Photosynthesis- is the process that converts light energy into chemical energy. This chemical energy is usually a carbohydrate. Only photoautotrophs can do photosynthesis. Heterotrophs must obtain their high organic nutrients from the environment. Glucose has more energy than carbon dioxide and water. The reaction is endergonic and will require an energy input of ATP and NADPH 2
In order to keep the reaction going, the cell must regenerate ATP and NADPH The equation below describes the Calvin cycle of photosynthesis and the chemical energy needed to make the sugar.
Regeneration of NADPH and ATP require light, and intact chloroplasts with chlorophyll. This part of photosynthesis is known as the light reaction. The hydrogen needed to reduce NADP comes from the splitting of water.
Chlorophyll is green and reflects green light and absorbs red and blue. Chlorophyll is made from a tetrapyrole ring with Mg in the middle and a hydrocarbon tail. These pigments form photosystems found in the thylakoid membrane. There are some other pigments in photosystems which are yellow. These yellow pigments (carotenoids and xanthophylls) allows photosynthesis to occur in green light.
White light is mixture of different colors of light with different wave lengths and frequencies. When white light lands on a blue object, red and green light is absorbed and blue is reflected. The absorption graph shows that the pigments absorb red and blue/violet light best. This is due to the accessory pigments. Yet the action spectrum shows that a small amount of light does occur in green light.
This diagram shows the location of the chloroplasts in the plant. Leaves are specialized to for photosynthesis. The leaves are usually arranged on the plant so that it maximizes exposure to the sunlight.
Chloroplasts have 3 membranes. The outer 2 are smooth and the inner one makes stacks of thylakoids which is a granum. The chlorophyll and other pigments are found inside the thylakoid membrane. They have the ability to convert light energy into chemical energy.
A stack of thylakoids is called a granum. The matrix that the grana is embedded in is the stroma. It contains enzymes for carbohydrate synthesis. Below is a diagram of the thylakoid and the location of chlorophyll.
The chlorophyll molecules and accessory pigments form photosystems (I & II). Each photosystem has a reaction center (p700& p680 respectively) Once photons are absorbed by the pigments of the photosystem, the electron becomes excited and the energy is passed on from molecule to molecule until it reaches the reaction center pigment. The p680 and p700 has the ability to pass the energized electron on to the electron transport system.
Also embedded in the thylakoid membrane is a series of proteins that have the ability to be reduced an oxidized. Each one has less reducing power than the preceding one. Protein Q has the ability to receive an energized electron from the p680 reaction center. It then moves to PQ or plastoquinone. From there it move to a cryptochrome complex which is a proton pump that when reduced has the ability to pump hydrogen from the outside of the stroma to the inside of the thylakoid. From there it goes plastocyanin. By this time the e- has lost much of its free energy and must be energized by photosystem I.
The electron has now left the electron transport chain. Replacement electrons for PS II comes from the splitting of water. A manganese complex associated with PS II has the ability to split water to produce, H +, e -, and oxygen gas. The e- are shuttled to the photosystem, H + are used to lower the pH of the thylakoid, and the oxygen gas is released to the atmosphere.
As the electron transport chain runs, there is an accumulation of H+ on the inside of the thylakoid. This is due to the splitting of water and the proton pump,PQ. As the H+ collect, the pH of the interior is lowered and there is a separation of charge across a membrane. This can now do work. On the thylakoid membrane, there are CF complexes which contain a channel (CF0) and a large protein head (CF1). On the CF1 head, there is an enzyme ATP synthetase. This enzyme has the ability to phosphorylate ADP--->ATP as 3 H+ pass through. This is noncyclic photophosphorylation.
1.Water is split to make replacement e -, H + and O 2. 2.There are two photosystems involved. 3.NADP is reduced to NADPH.
Cyclic photophosphorylation is considered to be a more ancient biochemical pathway. It is found in most photosynthetic bacteria and all photosynthetic eukaryotes. It consist of one photosystem (PSI) and a simple electron transport chain. At the end of the electron transport chain, the electron is returned to PS I. That being the case, water is not split, nor is NADP reduced. One part of the electron carrier does pump H + across the thylakoid membrane to make ATP. Cyclic photophosphorylation does not provide hydrogens for the reduction of carbon dioxide to make a carbohydrate. So therefore quite often the hydrogens come from H 2 S. In photosynthetic, eukaryotic cells, two photosystems (II & I) work together to form noncyclic photophosphorylation.
Comparison of cyclic versus noncyclic photophosphorylation
Overview light reaction ATP are made from 18ADP + 18P 2. Water is split. e- + H are used for #3. 6 O2 are released NADPH are made.
Overview dark reaction The carbohydrate is made in the stroma. It requires enzymes every step ADP + 18P are made from 18 ATP. Energy is released 2. NADPH is oxidized to make NADP. The hydrogens are transferred making a carbohydrate CO 2 are used to make a triose G3P (glyceraldehyde 3-phosphate) or PGAL phosphoglyceraldehyde. 2 of these make glucose
This is the Calvin cycle. The various compounds only show the carbon skeleton, leaving the hydrogen and oxygen atoms off. The next slide after that includes the names of the molecules used in the cycle.
The Calvin cycle will make one extra PGAL. PGAL is a triose. It takes 2 PGALs to make glucose, the hexose. So therefore the Calvin cycle needs to be "turned" twice in order to make a molecule of glucose. (Actually 6 times). 1. Carbon dioxide combines with ribulose biphosphate. Ru-Bp is a pentose monosaccharide with 2 phosphate groups 2.It will form an unstable intermediate.
3. The intermediate will form 2 molecules of phosphoglyceric acid. 4. PGA will be phosphorylated by ATP to form DPGA
5.DPGA is reduced by NADPH to form the triose, PGAL. A phosphate group is removed in this reaction. 6. In the last step, 5 molecules of glyceral aldehyde phosphate (G3P) or PGAL are needed to remake 3 molecules of Ru-BP. 3 ATP are needed to make this happen. 1 G3P is left over. PGAL is a triose. In order to make glucose, the Calvin cycle must be turned twice.
This shows how 2 molecules of G3P or PGAL are turned into a molecule of glucose and how it can be turned into starch.
While the glucose is needed for energy, there is a second reason why the Calvin cycle evolved; to provide a carbon skeleton so that other organic molecules or structures can be made.
Environmental factors affects the rate of photosynthesis. 1. Light intensity- At first an increase in the light intensity results in a corresponding increase in the rate of photosynthesis as the photosystems are activated. As the photosystems become saturated, an increase in light intensity will not increase the rate of photosynthesis. 2. Temperature- At first an increase in temperature results in an increase in the rate of photosynthesis because the molecules are moving faster, but at a higher temperature the reaction rate decreases because enzymes denature.
3. If a plant is given an increase in oxygen, the rate of photosynthesis decreases because of phenomenon of photorespiration. The enzyme that puts the CO 2 onto ribulose biphosphate is rubisco. Sometimes rubisco can make a mistake and put oxygen on to ribulose biphosphate. This happens when the concentration of oxygen gas is greater than carbon dioxide. This happens when the plant is water stressed and the stomates are closed. Gas exchange takes through pores on the bottom of the leaf called stomates. Guard cells regulates stomates but as gas exchange occurs water leaves the stomates via transpiration.
When a plant becomes water stressed, stomates close to conserve water. water, but this will stop gas exchange. This will increase the O 2 and decrease CO 2. Photorespiration begins. C3 photosynthesis is a plant that does the Calvin cycle and the light reaction. There are plants that modify C3 photosynthesis by adding an additional pathway-
The leaf of a C3 plant (normal leaf). Chloroplasts are located in the palisade and spongy mesophyll. There are no chloroplasts in the bundle sheath cells.
The leaf of a C4 plant. There are no palisade mesophyll cells. Instead there is a layer of mesophyll around the bundle sheath cells. Chloroplasts are located in the these mesophyll and spongy mesophyll. The chloroplast are different. C4 photosynthesis includes the light reaction, the Calvin cycle and the Hatch-Slack pathway. These C4 plants also have a different anatomy. This Hatch-Slack pathway is able to deliver dwindling supplies of CO 2 when the stomates are closed. The enzyme (PEP carboxylase) that fixes the CO 2, combines it with a three carbon compound, phosphoenol pyruvate (PEP) to form a four carbon compound. This enzyme does not make a mistake like rubisco. The name of this enzyme is PEP carboxylase.
The chloroplasts in the mesophyll have well defined thylakoids and specialize in the light reaction and the Hatch-Slack pathway. The thylakoids in the bundle-sheath chloroplast do not have defined thylakoids, are larger and store starch. This indicates the light reaction is not prevalent, and they do specialize in the Calvin cycle after the Hatch- Slack pathway delivers the CO 2.
Plants that use C4 photosynthesis include corn, sugar cane, and sorghum.
The CO 2 (like C4 photosynthesis) is fixed to PEP by PEP carboxylase. It is then converted to an organic acid and stored until the day. During day stomates are closed and the cell releases the CO 2 from the organic acid and the Calvin cycle occurs. C3 photosynthesis(light reaction and Calvin cycle) is called this because the first stable product has 3 carbons. C4 photosynthesis (light reaction, Hatch-Slack, Calvin cycle) is called this because the first product made has 4 carbons. Another variation of photosynthesis is CAM (crassulacean acid metabolism). These CAM plants include succulent plants and pineapples. Because of the intense heat and arid conditions, these plants only open up the stomates at night for gas exchange. Plants that use C4 photosynthesis include corn, sugar cane, and sorghum.