BIOB10H3 Lecture Notes - Lecture 8: Barbiturate, Conformational Change, Osmosis
25 Jun 2018
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Lecture 8: Chloroplast
Chloroplast
arose from phagocytosis of photosynthetic cyanobacteria
ocan divide by fission
Function:
osite of photosynthesis
outilizes energy to convert carbon dioxide and water into glucose
ooccur in plants, eukaryotic algae, some protists and several prokaryotes
Parts of the Chloroplast
1. Outer envelope membrane
Contain porin proteins - large channels
Allow very large molecules through
2. Inner envelope membrane
Highly impermeable: requires transporters
3. Thylakoids - Internal Membrane System
Membranous sacs arranged in stacks called grana
Thylakoid sacs have lumen inside
Stroma lamellae - flattened membrane structures that connect thylakoids from different grana
4. Stroma
contains DNA and ribosomes, tRNA
chloroplast DNA encodes for 100 genes
However, 90% of chloroplast proteins are encoded by nuclear DNA
Therefore, many chloroplast proteins must be imported (targeted) to chloroplasts
Posttranslational Uptake of Proteins Into Chloroplasts
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Chloroplast proteins are translated on free ribosomes in cytosol
What is the targeting sequence of chloroplast proteins?
oChloroplast “transit peptide”
oat amino terminus of chloroplast protein:
has a stroma-targeting domain (stroma proteins)
and can also have a thylakoid transfer domain (for thylakoid proteins)
To Enter Chloroplast:
Protein is first unfolded by HSP70 (chaperone protein)
Transit peptide binds to its receptor
Located next to a translocon on outer chloroplast membrane
TOC (translocon of outer chloroplast membrane)
Hsp70 proteins in intermembrane space keep protein unfolded
oMoves through TIC complex
oTIC (translocon of inner membrane of chloroplast)
If Destined For Stroma:
HSP60 refolds protein and stroma-targeting domain is cleaved by a protease
If Destined For Thylakoid Lumen:
stroma-targeting domain is cleaved to reveal thylakoid transfer domain
protein is transported into lumen through the bacterial translocon type machinery
Proteins on the Thylakoid Membrane
oEncoded by chloroplast genes
oRibosomes assemble on thylakoid membrane (like RER) and stop transfer sequences retain
proteins in the membrane
How Does The Chloroplast Make Glucose?
Start with Light energy
1. Light-Dependent Reaction - Light energy is absorbed and converted to chemical energy
occurs in the thylakoid membrane
make ATP and NADPH
uses H2O and creates O2
2. Light-Independent Reaction/Dark Reaction/Calvin Cycle - Chemical energy is used to convert CO2
into carbohydrates
occurs in the stroma
Total Result of Photosynthesis
6C02 + 6H2O + light + ATP 6O2 + C6H12O6
Glucose Production
1. Light-Dependent Reactions
Light travels in the form of photons
chlorophyll- photosynthetic pigment in thylakoid membrane- harvests light
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find more resources at oneclass.com
300 chlorophyll molecules are arranged into photosynthetic units (PS I and PS II)
also contains many proteins similar to proteins of the ETC
chlorophyll absorbs light electrons excited to a higher orbital
if electron dropped down to original orbital, fluorescence should occur but this doesn’t happen,
instead, electrons passed to another acceptor pigment molecule and/or proteins in ETC
protons will then accumulate in thylakoid lumen
develops an electrochemical gradient
oacidic (pH 5)
ogradient energy will be used to make ATP
ophotophosphorylation
PSI
chlorophyll captures a photon of light
electron excited and passed to acceptor molecules
ofinally to NADP+ and H+
oto make NADPH (becomes reduced)
oa chlorophyll molecule is still missing an electron
PSII
chlorophyll in its reaction centre captures a second photon of light
electrons passed to several acceptors
osimilar to ETC of mitochondria
electron energy is used
to pump H+ into the thylakoid lumen
electron moves into PSI to replace missing electron
onow a chlorophyll in PSII is missing an electron
electron replaced by the photolysis of H2O
PSII has proteins that split water
ointo protons, electrons and oxygen
protons stay in thylakoid lumen
electron moves into PSII to replace missing electron
oxygen is liberated
ATP Synthase
H+ move through channel down electrochemical gradient
movement of H+ through causes conformational change in
chloroplast ATP synthase
causes CF0 and CF1 to rotate relative to one another
rotation drives the phosphorylation reaction to make ATP
from ADP + P
2. Light-Independent Reaction
Calvin Cycle
oContains RuBP carboxylase
oRubisco - responsible for “fixing” carbon
inorganic C02 organic carbon molecules
multisubunit enzyme
oAttaches CO2 to RuBP (5 C)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Chloroplast arose from phagocytosis of photosynthetic cyanobacteria: can divide by fission. Function: site of photosynthesis, utilizes energy to convert carbon dioxide and water into glucose, occur in plants, eukaryotic algae, some protists and several prokaryotes. Parts of the chloroplast: outer envelope membrane. Highly impermeable: requires transporters: thylakoids - internal membrane system. Membranous sacs arranged in stacks called grana. Stroma lamellae - flattened membrane structures that connect thylakoids from different grana: stroma contains dna and ribosomes, trna chloroplast dna encodes for 100 genes. However, 90% of chloroplast proteins are encoded by nuclear dna. Therefore, many chloroplast proteins must be imported (targeted) to chloroplasts. Chloroplast proteins are translated on free ribosomes in cytosol. What is the targeting sequence of chloroplast proteins: chloroplast transit peptide , at amino terminus of chloroplast protein: Has a stroma-targeting domain (stroma proteins) and can also have a thylakoid transfer domain (for thylakoid proteins) Protein is first unfolded by hsp70 (chaperone protein)
