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Biological Sciences
Rene Harrison

Chloroplast 1. How does chloroplast form? How does it divide within cell? What is its major function?  Chloroplast arose from phagocytosis of photosynthetic cyanobacteria and can divide by fission.  Chloroplasts major function is involved with being the site of photosynthesis which utilizes energy to convert carbon dioxide and water into glucose  Chloroplasts is found within plants, eukaryotic algae, some protists and several prokaryotes 2. How does chloroplasts structure look? What are the main components?  Chloroplasts is made up of the: a. Outer envelope membrane b. Intermembrane space (found between the outer/inner envelope) c. Inner envelope membrane d. Thylakoid sacs – where light dependent reactions occur; which is arranged into grana (which are stacks of thylakoids) e. Stroma lamellae – which attach the grana together f. Stroma – where light independent reactions occur Figure 6.2 – The functional organization of a leaf. The section of the lead shows several layers of cells that These chloroplasts carry out photosynthesis, providingm. raw materials and chemical energy for the entire plant. Figure 6.3 – The internal structure of a chloroplast. (a) Transmission electron microscopy (TEM) through a single chloroplast. The internal membrane is arranged in stacks (grana) of disk- like thylakoids that are physically membrane that forms the envelope. (b) Schematic diagram of a chloroplast showing the outer double membrane and the thylakoid membranes Figure 6.4 – Thylakoid membranes. Electron micrograph (EM) of a section through a portion of a spinach chloroplast showing the stacked grana thylakoids, which are connected to one another by unstacked stroma thylakoids (stroma lamellae). The dark spheres are osmium-stained lipid granules. 3. What is the purpose of the chloroplasts outer envelope membrane?  The purpose of the chloroplasts outer envelope membrane is containing porin proteins which are large channels which allow very large molecules to go through the membrane 4. What is the purpose of the chloroplasts inner envelope membrane?  The purpose of the chloroplasts inner envelope membrane is its function of high impermeability; requiring transporters 5. What is the purpose of the chloroplasts thylakoids internal membrane system?  The thylakoids-internal membrane system is responsible for its membranous sacs arranged in stacks called grana  The membrane has a 75% protein: 25% lipid ratio  The thylakoid sacs have lumen inside of them  What is the stroma lamellae?  The stroma lamellae is flattened membrane structures that connect thylokoids from different grana together 6. What is the purpose of the chloroplasts stroma?  Chloroplasts stroma contains DNA, ribosomes, tRNA  Chloroplasts DNA encodes for 100 genes, however, 90% of chloroplasts proteins are encoded by nuclear DNA, therefore many chloroplast proteins must be imported (targeted) to chloroplasts  Protein translocons are also involved in import of proteins into chloroplasts  Chloroplast proteins are translated on free ribosomes cytosol 7. Posttranslational uptake of proteins into chloroplasts – What is the targeting sequence of chloroplast proteins?  The targeting sequence of chloroplast proteins are “transit peptide”  Occurs at the amino (N) terminus of chloroplast protein and has a stroma-targeting domain (stroma proteins)  It can also have a thylokoid transfer domain (for thylakoid proteins) Figure 8.48 – Importing proteins into a chloroplast. 1. Proteins are encoded by nuclear genes are protein-lined pores in both membranes of the outer chloroplast envelope (step1) 2. Proteins destined for the stroma (step1a) contain a stroma-targeting domain at their N-terminus, whereas proteins destined for the thylakoid (step1b) contain both a stroma-targeting domain and a thylakoid-transfer domain at their N- terminus. Stromal proteins remain in the stroma (step2) following translocation through the outer sequence and removal of their single targeting 3. The presence of the thylakoid transfer domain causes thylakoid proteins to be translocated either into or completely through the thylakoid membrane (step3). 4. A number of the proteins of the thylakoid membrane are encoded by chloroplast genes and synthesized by chloroplast ribosomes that are membrane (step4)er surface of the thylakoid Orange = chloroplast proteins 8. Posttranlational uptake of proteins into chloroplast – How do the proteins enter into the chlroplast? a. Protein is first unfolded by the Hsp 70 chaperone protein b. Transit peptide then binds to its rececptor which is located next to a translocon on the outer chloroplast membrane which in this case is called TOC (translocon of outer chloroplast membrane) c. Hsp70 proteins in the intermembrane space keep protein unfolded and moves it through the TIC complex (translocon of inner membrane of chloroplast)  IF the protein is destined for the stroma then: d. Hsp60 refolds the protein and the stroma-targeting domain is cleaved (cut) by a protease  IF the protein is destined for the thylokoid lumen then: e. The stroma-targeting domain is cleaved to reveal the thylakoid transfer domain f. Then the protein is transported into lumen-baterial translocan-type machinery 9. What happens to the chloroplast proteins on the thylakoid membrane? How do they get there?  The proteins on the thylakoid membrane are encoded by chloroplast genes.  The ribosomes are the ones that assemble on thylakoid membrane (alike RER) and stop the transfer sequences retaining proteins in the membrane 10.How does the chloroplast make its glucose?  Chloroplasts uses light energy as its primary source of glucose making  Chloroplast uses two main methods for making glucose: a. Light-dependent reaction – which occurs in the thylakoid membranes is which light energy is absorbed and coverted to chemical energy (ATP and NADPH)  Uses H2O and creates O2 b. Light-independent reaction/Dark reaction/Calvin cycle – which occurs in stroma is when chemical energy is used to convert CO2 into carbohydrates 11.What is the total result of photosynthesis?  6C02 +6H20 + light+ ATP = 6CO2 + C6H12O6 (glucose) 12.How does chloroplast use light-dependent reactions to create glucose?  Light travels in the form of photons a. Chlorophyll – which is a photosynthetic pigment in thylakoid membrane – harvests the light coming into the cell  300 chlorophyll molecules are arranged into photosynthetic units (PS I and PS II) which also contains many proteins similar to proteins of the ETC (electron transport chain) b. Chlorophyll absorbs light and move the excited electrons to a higher orbital  IF electrons drop down to their original orbital = FLUORESCENCE; if this doesn’t happen, instead: c. Electrons pass to another acceptor pigment molecule and/or proteins in the ETC  What is the significance of PS I? d. PS I is responsible for chlorophyll caputuring a photon of light in which the excited electron is passed to acceptor molecules  Finally to NADP+ and H+ to make NADPH which becomes reduced  A chlorophyll molecule is still missing an electron thus:  What is the significance of PS II? e. PS II is responsible for chlorophyll in its reaction centre caputuring a second photon of light  Elecrons passed to several acceptors (similar to ETC of mitochondria)  Electron energy is used to pump H+ into the thylakoid lumen  Electron moves into PS I to replace missing electron  Now a chlorophyll in PS II is missing an electron f. PS II electron is replaced by the photolysis of H2O in which PS II has proteins that split water into photons, electrons and oxygen  Protons remain in the thylakoid lumen  Eletrons move into the PS II to replace mission electron  Oxygen is liberated  Thus at the end of the light-dependent reactions protons accumlate within the thylakoid lumen and this develops an electrochemical gradient since the environment is acidic with a pH level of 5, the gradient energy will be used to make ATP using photophosphorylation Figure 6.16 – Summary of the light-dependent reactions. Summary of the flow of electrons from complexes. This figure shows the estimatedne
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