Carbohydrate Biosynthesis in Plants

Nội dung text: Carbohydrate Biosynthesis in Plants

1. Photosynthesis 2
2. Photosynthesis • The equation for the overall process is: 6CO2 + 6H2O →→→→→→ C6H12O6 + 6O2 granum (stack of thylakoids) 1 chloroplast 6H2O 6O 2 2 energy thylakoid stroma (fluid outside the thylakoids) 6CO2 1 six-carbon sugar C6H12O6 3 4 4
3. Photosynthesis ▪ Photosynthesis produces sugars that store chemical energy. ▪ Photosynthesis captures energy from sunlight to make sugars. ▪ Chlorophyll is a molecule that absorbs light energy. ▪ In plants, chlorophyll found in chloroplasts. chloroplast leaf cell leaf 6
4. Photosynthesis • The light-dependent reactions capture energy from sunlight. – take place in thylakoids – water and sunlight are needed – chlorophyll absorbs energy – energy is transferred along thylakoid membrane for reactions – oxygen is released 8
5. Photosynthesis: From Light and CO2 to Glyceraldehyde 3-phosphate • The photosynthesis of one molecule of glyceraldehyde 3- phosphate requires the capture of roughly 24 photons
6. Stoichiometry of Carbon Assimilation Overall Process: 3 CO2 + 9 ATP + 6 NADPH → glyceraldehyde 3-phosphate + 9 ADP + 6 NADP+ + 8 Pi • Assimilation of 3 carbons and 1 phosphorous per cycle • Inorganic phosphate must be replaced for sustained ATP synthesis in chloroplast
7. Rubisco (ribulose 1,5-bisphosphate carboxylase) • Rubisco is a large Mg++-containing enzyme that makes a new carbon-carbon bond using CO2 as a substrate -1 • Inefficient (kcat~ 3s ) • Ru1,5P2 + CO2 –> 2 3-phosphoglycerate (3PG)
8. Cellulose Biosynthesis • Cell wall structure • Occurs in cytoplasm and at plasma membrane • Lipid-linked carrier and membrane protein complex • UDP-glucose is generated from sucrose and UDP by sucrose synthase • UDP-glucose is substrate for cellulose synthase; adds glucose monomers to non- reducing end • Cellulose is β(1→4) linked glucose
9. Glyceraldehyde 3- phosphate • Converted to starch in the chloroplast • Converted to sucrose for export • Recycled to ribulose 1,5- bisphosphate