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Lecture 8

BIOB10H3 Lecture Notes - Lecture 8: P700, Sucrose, Carboxylation

Biological Sciences
Course Code
Tanya Da Sylva

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Lecture 8: Chloroplasts:
First organisms were heterotrophs
Needed nutrients from environment
-Autotrophs: manufacture organic nutrients from inorganic molecules (CO2)
About 2.7 million years ago, cyanobacteria used electrons from water to produce oxygen as a waste product
Synthesis of complex molecules from CO2 requires large input of energy
Chemoautotrophs: use energy from inorganic molecules.
-Photoautotrophs: use radiant energy to make organic compounds
Photosynthesis: converts energy from sunlight into chemical energy stored in carbohydrates.
Low energy electrons are removed from a donor molecule : CO2+ H2O +light ->(CH2) + O2
Chloroplast Structure and Function
Photosynthesis in eukaryotes takes place in the chloroplast, a cytoplasmicorganelle
Arose from phagocytosis of photosynthetic cyanobacteria
Uses energy to convert carbon dioxide and water into glucose
Plants, eukaryotic algae, some protists and several prokaryotes
Chloroplasts have a double membrane.
The outer membrane contains porins and is permeable to large molecules.
The inner membrane contains light-absorbing pigment, electron carriers, and ATP-synthesizing enzymes.

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Outer envelope membrane contains porin proteins
Inner envelope membrane is highly impermeable: requires transporters
-Thylakoid: arranged into grana (stacks of thylakoids)
-Stroma lamellae: attach grana
-Stroma: inside fluid
Thylakoids–Internal membrane system
The inner membrane of a chloroplast is folded into flattened sacs (thylakoids), arranged in stacks called grana
Thylakoid membranes contain a large percentage of glycolipids, which make the membrane highly fluid for
diffusion of proteins complexes. Membrane has 75:25 protein : lipid ration
Thylakoid sacs have lumen inside
Stroma lamellae flattened membrane structures that connect thylakoidsfrom different grana
Contains DNA and ribosomes, tRNA
Chloroplast DNA encodes for 100 genes, but most chloroplast proteins encoded by nuclear DNA
Chloroplast proteins must be imported

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Proteins on the thylakoid membrane: encoded by chloroplast genes. Ribosomes assemble on the thylakoid
membrane like RER
Chloroplasts -Function
Photosynthesis is a redox reaction; transferring an electron from water to
carbon dioxide
-6CO2 + 12H2O C6H12O6 + 6H2O + 6O2
Experiments using radioactive O showed that O2molecules released
during photosynthesis came from H2O not from CO2
Photosynthesis oxidizes water to oxygen; respiration reduces oxygen to
form water
Respiration removes electrons from reduced organic substrates to form ATP and NADH.
Photosynthesis uses electrons to form ATP and NADPH, which are then used to reduce
CO2 to carbohydrate
Photosynthesis occurs in two stages:
Light-dependent reactions (light reactions)in which sunlight is absorbed, converting
it into ATP and NADPH.: Thylakoid membranes
Light-independent reactions (dark reactions) use the energy stored in ATP and
NADPH to produce carbohydrate:
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