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Chapter 6

BIOB11H3 Chapter Notes - Chapter 6: Photophosphorylation, Photosynthetic Pigment, Photoinhibition


Department
Biological Sciences
Course Code
BIOB11H3
Professor
Dr. Bawa
Chapter
6

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227
CHAPTER 6
PHOTOSYNTHESIS AND THE CHLOROPLAST
OBJECTIVES
Familiarize students with the structure and function of chloroplasts.
Emphasize the role of photosynthesis in the evolution of life on Earth.
Clarify the roles of the light-dependent and light-independent reactions in photosynthesis.
Clarify the events that result in the absorption of light by photosynthetic pigments.
Explain the operation of the photosynthetic units and their reaction centers.
Elaborate on the events involved in the flow of electrons from water to NADP+ to form NADPH.
Describe the mechanism by which ATP is produced during photosynthetic electron flow.
Compare and contrast cyclic and noncyclic photophosphorylation.
Describe the experiments that led to the discovery of the Calvin-Benson Cycle and the Hatch-Slack pathway.
Describe the C3 pathway, its role in carbon fixation and its production of carbohydrates using the products
of the light-dependent reactions.
Explain the functioning of Rubisco and its ability to initiate both photosynthetic and photorespiration
pathways.
Clarify the effects of the environment on C3 photosynthesis including the photorespiration that plants
experience in hot, dry climates.
Point out some of the mechanisms that regulate electron flow and the light-independent reactions.
Describe the process of C4 photosynthesis and the anatomical specializations that allow it to solve the
problems of plants living in dry, hot habitats.
Describe the mechanism by which CAM plants manage to survive in very hot, dry climates and compare
it with C3 and C4 photosynthesis.
Describe the cooperation and interdependence among peroxisomes, chloroplasts and mitochondria and
their roles in plant physiology.
Point out the negative effect that high intensity light (photoinhibition) can have on photosynthesis and the
methods that plants use to correct such damage.
LECTURE OUTLINE
The Earth's Earliest Life Forms
I. Early life got raw materials & energy from simple organic molecules dissolved in aqueous environment
A. These molecules formed abiotically (via nonbiological chemical reactions) in primeval seas
B. Such organisms (like us) depending on an external source of organic compounds are heterotrophs
C. The number of heterotrophs on primitive Earth was probably severely restricted initially because the
spontaneous production of organic molecules occurs very slowly
D. Evolution of life on Earth got a tremendous boost when organisms appeared that employed a new
metabolic strategy
1. Unlike their predecessors, they could make their own organic nutrients from the simplest types of
inorganic molecules (carbon dioxide [CO2] & hydrogen sulfide [H2S])
2. Such organisms that can survive on CO2 as their principal carbon source are autotrophs

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II. Life as autotroph requires much energy since it takes a large input of energy to manufacture complex
molecules from CO2: 2 types of autotrophs evolved that can be distinguished by their energy source
A. Chemoautotrophs - use chemical energy stored in inorganic molecules (ammonia, H2S, nitrites) to
convert CO2 to organic compounds; all are prokaryotes
1. Their relative contribution to the formation of biomass on Earth is small
B. Photoautotrophs Sun's radiant energy is used; most Earth biomass made by photosynthesis (PS)
1. They include higher plants, eukaryotic algae, some flagellated protists & members of 5 groups of
prokaryotes (heliobacteria, cyanobacteria, purple sulfur, green nonsulfur & green sulfur bacteria)
2. They capture the energy that fuels the activities of nearly every organism on Earth
3. All of these organisms carry out photosynthesis, a process in which energy from sunlight is
transformed into chemical energy that is stored in carbohydrates & other organic molecules
III. In photosynthesis, sunlight energy is converted to chemical energy that is stored in carbohydrates & other
organic metabolites
A. Relatively low energy electrons are removed from a donor compound & converted to high energy
electrons by energy from light absorption
B. High energy electrons reduce carbon skeletons to make reduced biomolecules, like starches & oils
IV. Earliest photosynthetic organisms (photoautotrophs) may have dominated Earth for 2 billion years &
probably used H2S as electron source via this reaction: CO2 + 2H2S + light <> (CH2O) + H2O + 2S
A. Many bacteria still do, but H2S is not abundant or widespread, so these organisms are restricted in their
importance & distribution (limited to habitats like sulfur springs & deep sea vents)
B. ~2.7 billion years ago, a new photosynthetic prokaryote arose; used the much more abundant H2O (water)
instead as a source of electrons
1. These were cyanobacteria & they produced an important waste product (O2; molecular oxygen) of
much consequence via following reaction: CO2 + 2H2O + light <> (CH2O) + O2
2. Also able to live in much more diverse array of habitats because of water abundance
3. Cyanobacteria became dominant & set stage for evolution of aerobic metabolism
C. Much harder to pull electrons from H2O than H2S so new photosynthetic machinery needed; S atom in H2S
has much less affinity for its electrons (& gives them up more readily) than does O atom in H2O
1. Redox potential of S-H2S couple is 0.25 V as compared to +0.816 V for O2-H2O couple
D. To carry out oxygenic (oxygen-releasing) photosynthesis, an organism must generate a very strong
oxidizing agent as part of its photosynthetic metabolism to pull the tightly held electrons from H2O
1. The switch from H2S (or other reduced substrates) to H2O as an electron source for photosynthesis
required a photosynthetic machinery overhaul
V. At some point, one of these ancient O2-producing cyanobacteria took up residence inside a mitochondria-
containing, nonphotosynthetic proeukaryotic cell
A. Over a long period of evolution, the symbiotic cyanobacterium was transformed from a separate organism
living within a host cell into a cytoplasmic organelle, the chloroplast
B. As chloroplast evolution took place, many genes originally present in the symbiotic cyanobacterium were
either lost or transferred to the plant cell nucleus
1. Thus, the polypeptides found within modern-day chloroplasts are encoded by both the nuclear &
chloroplast genomes
C. Extensive genetic analyses of chloroplast genomes suggest that all modern chloroplasts have arisen from a
single, ancient symbiotic relationship
1. Due to their common ancestry, chloroplasts & cyanobacteria share many basic traits, including similar
photosynthetic machinery, circular chromosomes, smaller ribosomes, etc.)

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Chloroplast Structure and Function
I. Chloroplasts are located mostly in mesophyll cells of leaves around the cell's central vacuole
A. In higher plants, usually lens-shaped; ~2 - 4 µm wide, 5 - 10 µm long; typically 20 - 40 per cell; they are
giants among cell organelles (as big as mammalian red blood cells)
B. Theodor Engelmann (German biologist; 1881) found them to be photosynthesis site
1. He illuminated green alga Spirogyra > actively moving bacteria gather outside cell near the large,
ribbonlike chloroplast to use O2 for aerobic respiration
II. The outer covering of chloroplast consists of envelope made of 2 membranes separated by narrow space
A. Outer membrane contains several different porins (like outer membrane of mitochondria)
1. These proteins have relatively large channels (~1 nm) but they exhibit some selectivity toward
various solutes; they may not be as freely permeable to key metabolites as often described
B. Inner membrane is highly impermeable; movement across it requires a variety of transporters
III. Internal membrane system contains much of the photosynthetic machinery (light-absorbing pigments,
complex chains of electron carriers; ATP-synthesizing apparatus); it is separate from envelope
A. Chloroplast internal membrane is organized into flattened membranous sacs (thylakoids) are arranged in
orderly stacks (grana; like stack of coins); they contain energy-transducing machinery
1. Thylakoid sacs can self-assemble & have space inside (lumen)
B. Space outside thylakoid & inside chloroplast envelope is stroma (contains CHO synthesis enzymes)
C. Flattened membranous structures connect thylakoids of different grana (stroma thylakoids); they differ
in structure & function from grana thylakoids
IV. Stroma components - form store of genetic information & means to utilize it (similar to mitochondria)
A. Small, double stranded circular DNAs encoding tRNAs, rRNAs & up to ~100 different polypeptides &
prokaryote-like ribosomes, various enzymes
1. Encoded proteins include many of protein subunits that mediate thylakoid membrane light reactions &
large subunit of CO2-fixing enzyme
B. The majority of chloroplast proteins are encoded by DNA of the nucleus & synthesized in cytosol
1. These proteins must be imported into the chloroplast by specialized transport machinery
V. Thylakoid membrane lipid composition - unusual; their fluidity allows lateral protein complex diffusion
A. They have relatively little phospholipid & high percentage of galactose-containing glycolipids
B. Both fatty acids of these lipids contain several double bonds; this makes thylakoid membrane lipid
bilayers highly fluid
C. The fluidity of the lipid bilayer facilitates lateral diffusion of protein complexes through the membrane
during photosynthesis
VI. Chloroplasts arise by fission from preexisting chloroplasts or their nonpigmented precursors
(proplastids)
Posttranslational Uptake of Proteins: Chloroplasts (From Ch. 8 pg. 318-7th ed.)
I. Chloroplasts have 6 subcompartments into which proteins can be delivered:
A. Inner (1) & outer (2) envelope membrane & intervening intermembrane space (3)
B. Stroma (4), thylakoid membrane (5), thylakoid lumen (6)
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