Lectures 10 and 11 Introduction to Plant Physiology and Primary Metabolism

1. Lectures 10 and 11 Introduction to Plant Physiology and Primary Metabolism •Reading for Lectures 10 and 11 Chapter 10 in Plants, Genes and Crop Biotechnology, p.240-269 (on Reserve in the library) --------------------------------------------------------------------------------- Lecture 10: “Getting and Using Carbon for Energy and to Build Plant Molecules” -Genes encode enzymes. -Enzymes organize chemistry. -Enzymes build simple molecules (nucleotides, amino acids, sugars, etc.). -Other enzymes use these small molecules as subunits to build larger molecules (fatty acid membranes, DNA/RNA, proteins, cell walls, etc.) Subunits Make Longer Chains Fig. 1 - nucleotides = DNA Slide 10.1

2. How does the cell make these subunits? Fatty acids- 2C Acetyl CoA for compartment membranes: Glucose, Fructose, Sucrose, Starch cellulose Figures from : Plants, Genes and Agriculture,pp.86 and 89 M. Chrispeels and D.Sadava Jones and Bartlett Publishers, Boston, 1994 •Enzymes are creating C-C, C-P, C-N, C-O, O-P, C-S, etc. bonds. •Predominantly, the backbone consists of C-C bonds. •These subunits are constructed using 2Carbon to 6Carbon molecules as the backbone (to which N, O, P, S are added). This lecture will answer the following 4 questions: 1. Where does the carbon come from? _________ 2. To form C-C (and C-O, O-P, C-S, C-N, etc) bonds, energy is required. Where does this energy come from? _________How? 3. How are the 2C-6C subunits formed from CO2? 4. How are the 2C-6C subunits stored for future needs of the plant? Slide 10.2

3. 1. Fundamentally, why does life need energy? Negatively-charged electrons are attracted to the positively- charged nucleus (protons) of the same atom. The electron can also be attracted to the nucleus of another atom. For example, an electron which is far away from its own nucleus (and less susceptible to positive forces) can more easily be attracted to the nucleus of another atom. Thus, all else being equal , it would require less energy to pull an electron from an outer location towards another nucleus, than an electron in an inner orbiting location (orbital). What is a chemical bond? Bonds break and form because electron-sharing between atoms or molecules begins or ends (as the electrons move further away or closer to each nucleus). Loss of an electron away from a nucleus = __________ Gain of an electron closer to a nucleus = “___________ Similarly, to pull an electron further away from its nucleus requires energy to overcome forces of positive attraction. These electrons possess more energy than electrons held in a more Stable conformation, such as closer to the nucleus. Slide 10.3

4. 2. Where do plants get energy from? Photosynthesis -- the energy from sunlight is absorbed by chlorophyll in close proximity to 2H20. An electron of chlorophyll is excited (becomes in a high energy state), is pulled away from its nucleus and transferred to an adjacent molecules. Chlorophyll then constantly replaces these “lost” electrons pulling them away from the nuclei of 2H20 (thus liberating O2 and 4H+ protons). Slide 10.4

5. Chlorophyll is a large, bulky molecule, which is not diffusible and its energy cannot be used to build/break other bonds in the cell (for example to build DNA). Chlorophyll is embedded in membranes inside the chloroplast. Why is this important? The “lost” electrons from chlorophyll are passed onto nearby molecules also embedded in the chloroplast membrane. These Molecules can absorb and give away excess electrons. Why are electrons passed along a chain like this? Slide 10.5

6. 3. Electron Transfer in the Chloroplast Generates Cofactors At each electron transfer step in the chloroplast, the high-energy electrons go into a more stable orbital around the nucleus of the electron-accepting molecule. As a result, energy is released, but gradually. A sudden drop in the energy state of an electron would release most of the energy as heat (not useable energy). Slide 10.6

7. 3. High Energy Cofactors The electron energy gradually lost while exchanged between reduction-oxidation (redox chain) molecules in the chloroplast is used to boost the energy of electrons of diffusible molecules (energy cofactors). These cofactors aid in pulling and pushing electrons between the nuclei of any two reactant molecules in the cell allowing them to form bonds such as C-C bonds. How? NAD(P)H ATP From Biochemistry and Molecular Biology of Plants (W.Gruissem, B. Buchanan and R.Jones p.679 and p.834 ASPP, Rockville MD, 2000 In the cofactors, the high-energy electrons are unstable and attracted to the nuclei of other molecules. As the electrons leave the nucleus of the cofactor atom, they release the energy which kept the electrons in orbital; this energy can be used to pull/push electrons from other molecules to/from one another, to overcome energy activation barriers. This permits the breaking and forming of new bonds between reactant molecules. To do this, the cofactors temporarily bind to reactant molecules, and pull and push their electrons to overcome activation barriers; these reactant electrons can then participate in new bonds between two reactant molecule nuclei. Slide 10.7

8. The low-energy (oxidized) form of the cofactors are _________, while the high-energy, high-electron-containing "reduced" forms are __________. Therefore, the purpose of photosynthesis is to produce high- energy cofactors. Photosynthesis does NOT create C-C bonds. Why can't chlorophyll or associated electron-acceptor molecules directly be used as a cofactor? The focus of the next step is to use this energy to form C-C bonds. Why? C-C bonds must be formed to create the backbone of the cell's molecules (such as fats, cellulose). There is a second reason to form C-C bonds however: The cell does not store abundant amounts of ATP/NADPH cofactors, because they are very reactant. Instead, evolution chose to use the electrons held between the nuclei of carbon atoms (C-C bonds) as the storage form for energy. C-C molecules are not reactive and very stable (eg. fats, oils, carbohydrates). Where does this Carbon come from and how is the C-C bond Formed and stored? Slide 10.8

9. 5. “Fixing” (Reducing) Carbon The carbon is derived from CO2. To form C-C bonds, it would have been rational for evolution to take C from 2 CO2 atoms and join them together. Apparently, evolution could not create an enzyme to do this. Instead, the only known enzyme that can take C from CO2 attaches it to a 5C acceptor, to create an unstable 6C molecule, which then breaks down into 2 x 3C molecules. This enzyme is called RuBisCo (Ribulose Bisphosphate Carboxylase). The addition of C to 5C requires ATP + NADPH. This "carbon-fixation" reaction has three major problems associated with them. What are these andwhat are the consequences for agriculture? 1a. 1b. 2. 3. Slide 10.9

10. 6. Biochemistry Uses Cycles The chloroplast must constantly generate 5C acceptor molecule. How was this problem solved? To go from 3C back to the 5C acceptor molecule, 9 more enzymes are involved. Along with Rubisco and 2 other enzymes, these form the Calvin Cycle. Therefore, the majority of the enzymes of the Calvin Cycle have nothing to do with removing C from CO2 to form C-C bonds, but rather, they are there to generate the 5C acceptor molecule!! Is there a more efficient way to do this? Evolution didn't create one, but can humans?!! Slide 10.10

11. 7. What are the Critical Factors Regulating Rates of PSS/Carbon Fixation via the Calvin Cycle in Crops? CO2/O2 enter the chloroplast via stomates in leaves -increase in CO2 enhances rate of carbon fixation. -atmospheric CO2 is 0.03%. Commercial greenhouses increase this to 0.08% to increase yields of vegetables and ornamentals. Up to 0.1% is very efficient under optimal light, temp, water and Nitrogen What will be the effect of global warming and enhanced CO2??? -stomates open during day/closed at night. Enhanced H20 allows stomates to stay more open, allowing more CO2 to flow in. -problem: C-fixing enzyme (Rubisco) also accepts O2 in its active site which can decrease PSS by up to 50%. This is because the enzyme evolved in a non-O2 atmosphere. This wasteful process is called photorespiration and has nothing to do with mitochondrial respiration. It is an extremely limiting factor in the productivity of plants such as soybeans, wheat, oats, barley under optimal conditions. -C4 plants such as corn, sorghum, sugarcane overcome photorespiration by a strategy that concentrates CO2 at the site of photosynthesis resulting in up to 3X higher rates of PSS compared to wheat, rye, barley, oats, soybeans and all other dicots. -Enhanced CO2 in maize also allows plant to reduce stomata opening, thus conserving H20 and allowing these plants to grow in hot climates -by contrast,another tropical plant, rice, did not evolve a CO2 concentrating strategy and can only grow in hot climates when there is lots of water. Slide 10.11

12. 8. Using 3C molecules How are large-structures derived from these 3C molecules? The 3C can be broken down to a reactive form of 2C subunits (acetyl-CoA) which can be joined together to form fats/oils. The 3C can be joined to 3C to form 6C, such as glucose. Chains of glucose can form cellulose (cell wall) or starch. 6C can be broken down to 5C, ribose, which is a major part of DNA/RNA. Different C-C combinations can join together to help form the backbone of amino acids (along with N, S, etc.) and all the other molecules of the cell. How can energy be derived from the 3C molecules? 3C is broken down to 2C, which enters the mitochondrion. There, the C-C bonds are broken to create 2 x CO2 which is then released. This releases energy that is again used to generate cofactors (ATP, NADH). This process is called respiration and it requires O2 to accept electrons (which had released energy as they were transferred through a series of reduction-oxidation reactions). In the presence of 4H+ protons, the reduced O2 generates 2H20. Therefore, what are the different fates of the 3C molecules derived from the Calvin cycle? Slide 10.12

13. Overview of Carbon + Energy Assimilation & Flow •a 3C sugar is the initial product •3C sugars allow transfer between cellular compartments •Sucrose (6C + 6C) is the form for source-sink transport (sap) •Starch (6C+6C+6C….) is the long-term storage form of Carbon Slide 10.13