Download Chapter 10 (Photosynthesis)

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Chapter 10: Photosynthesis
Photosynthesis converts light energy to chemical energy of food
The Anatomy of a Leaf
 Photosynthesis occurs in the leaves of plants
 The cuticle is the waxy covering on the top of the leaf.
- Produced by the upper epidermis, it provides protection, prevents water loss through
evaporation, and directs rain water to the roots (like a gutter).
 Mesophyll layer = spongy layer + palisade layer
- Where photosynthesis occurs
 Just below the upper epidermis is the palisade layer. These cells are tightly packed together
and contain lots of chloroplasts.
 Below the palisade layer is the spongy layer. These cells are loosely arranged and have air
spaces.
 This allows for diffusion of gases, especially CO2, within the leaf.
 At the lower epidermis, the underside of the leaf contains stomata which allow transpiration
and gas exchange, allowing CO2 to enter and O2 to exit
 Surrounding each stomate are guard cells, which control the opening and closing of the
stomates
 Plant is dehydrated  lower turgor pressure  guard cells collapse and stomates close
 Plant is turgid (water filled)  high turgor pressure  guard cells swell and stomates open
 The concentration gradient controls movement of gases
 Veins carry water from the roots to the leaves and distribute sugar from the leaves to nonphotosynthetic tissue
 Vascular bundles are found in the spongy layer.
 Vascular bundles include xylem and phloem tubes that transport materials throughout the
plant. Xylem tubes transport water, and phloem tubes transport sugars.
Chloroplasts
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Photosynthesis takes place in the chloroplasts of the leaves
The green color of the leaf is from chlorophyll, the green pigment located within chloroplasts.
*It’s the light energy absorbed by chlorophyll that drives photosynthesis in chloroplast
Chloroplasts are found mainly in the cells of the mesophyll, the tissue in the interior of the leaf.
A chloroplast consists of a double membrane surrounding a dense fluid called the stroma and an
elaborate membrane system called thylakoids, enclosing the thylakoid space
Thylakoid sacs may be stacked to form a granum.
Chlorophyll is embedded in the thylakoid membrane.
Tracking Atoms through Photosynthesis
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The equation for photosynthesis is the reverse of cellular respiration:
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CO2 + H2O + Light energy  C6H12O6 + O2
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All photosynthetic organisms need a hydrogen source because plants split water as a source of
electrons from hydrogen, releasing oxygen.
Photosynthesis is a redox reaction like respiration, but differs in the direction of electron flow.
 The electrons increase their potential energy when they travel from water to reduce CO2
into sugar, and light provides the energy for this endergonic process
 Reduced = gain of electrons
 Oxidized = loss of electrons
 Cellular respiration: NAD+ is reduced to NADH
 Photosynthesis: NADP is reduced to NADPH
2 Stages of Photosynthesis:
1. The Light Reaction: converts solar energy to chemical energy (in thylakoid
membranes; requires light)
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The whole point of the light reaction is to produce 2 things:
1. Energy in the form of ATP
2. Electron carriers, specifically NADPH
The whole process begins when photons (=energy units) of sunlight strike a leaf, activating
chlorophyll and exciting electrons.
The activated chlorophyll molecule then passes these excited electrons from water down to a
series of electron carriers, ultimately producing ATP and NADPH.
 The electron acceptor NADP+ is reduced to NADPH and temporarily stores electrons.
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Oxygen is released when water is split.
ATP is formed during the light reactions, using chemiosmosis in a process called
photophosphorylation
2. The Dark Reaction/ Calvin Cycle: uses ATP, NADPH, and CO2 to make carbohydrates
(in the stroma; doesn’t require light)
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In the Calvin Cycle, carbon dioxide is incorporated into existing organic compounds by carbon
fixation, and these compounds are then reduced to form carbohydrate.
NADPH and ATP from the light reactions supply the reducing power and chemical energy
needed for the Calvin cycle.
The light reactions convert solar energy to the chemical energy
of ATP and NADPH
The Nature of Sunlight
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The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation
Visible light consists of wavelengths (including those that drive photosynthesis) that produce
colors we can see
Light also behaves as though it consists of discrete particles, called photons
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Excitation of Chlorophyll by Light
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When a pigment molecule absorbs energy from a photon, one of the molecule’s electrons gains
potential energy, going from the ground state to the excited state
 The excited state is unstable.
 Energy is released as heat as the electron drops back to its ground state
 Isolated chlorophyll molecules also emit photons of light called fluorescence as their electrons
return to ground state
Photosynthetic Pigments: the Light Receptors
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Many light-absorbing pigments participate in photosynthesis, including:
o Chlorophyll a: absorbs most of the light and participates directly in light reactions
 Its absorption spectrum shows that it absorbs violet-blue and red light best.
o Chlorophyll b
 Absorbs light of different wavelengths and broadens the spectrum of colors useful
in photosynthesis
o Carotenoids
 Absorbs light of different wavelengths and broadens the spectrum of colors useful
in photosynthesis
 Some carotenoids absorb excessive light energy that might damage chlorophyll or
interact with oxygen to form reactive molecules, which is called photo-protection
These pigments are clustered in the thylakoid membrane into units called antenna complexes.
All of the pigments within a unit are able to “gather” light, but aren’t able to “excite” the
electrons.
 Only one special molecule – located in the reaction center – is capable of transforming light
energy into chemical energy.
 In other words, the other pigments, called antenna pigments, “gather” light and “bounce”
the energy to the reaction center
A spectrophotometer measures the amounts of light of different wavelengths absorbed by a
pigment.
Photosystems
 Photosystems contain a number of light-harvesting complexes and a reaction center complex.
 located in the thylakoid membrane
 A reaction center complex is a protein complex with 2 chlorophyll a molecules and a primary
electron acceptor.
 There are 2 types of reaction centers:
1. Photosystem II (PSII)
 The chlorophyll a molecule at the reaction center of PSII is called P680, after the
wavelength of light (680 nm) it absorbs best
2. Photosystem I (PSI)
 There’s a chlorophyll a molecule at the reaction center of PSI called P700
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*The primary difference between the two is that each reaction center has a specific type of
chlorophyll a that absorbs a particular wavelength of light: PSI absorbs P700, and PSII absorbs
P680
When a pigment molecule in a light-harvesting complex absorbs a photon, the energy is passed
from pigment to pigment until it reaches the reaction center. In a redox reaction, an excited
electron of a reaction-center chlorophyll a is trapped by the primary electron acceptor before it
can return to the ground state.
Noncyclic Photophosphorylation/Linear Electron Flow
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Light drives the synthesis of ATP and NADPH by energizing the 2 photosystems
embedded in the thylakoid membranes of chloroplasts.
*Light energy strikes photosystems  activates the electron transport chain
*The direction of electron flow: water  PSII  PSI  NADP+
The process:
1. A photon of light strikes PSII, boosting electrons to an excited level. These electrons pass on
energy until it excites an electron in the P860 chlorophyll a molecules in the PSII reactioncenter complex.
2. The activated electrons are trapped by P680 and passed to a molecule called the primary
electron acceptor which ejects electrons from PSII. This leaves an electron hole in PSII.
(which will be filled with electrons from splitting water)
3. A water molecule splits into 2 electrons, 2 hydrogen ions (=protons), and an oxygen atom, a
process called photolysis. These electrons fill in the electron hole in PSII.
4. Each photoexcited electron passes from PSII to PSI via an electron transport chain.
 This electron transport chain is similar to that of cellular respiration
 It consists of the proteins Pq, cytochrome complex, and Pc. These proteins carry the
electrons from PSII to PSI.
5. Some of the energy that dissipates as electrons move along the electron chain of acceptors
will be used to pump protons across the membrane into the thylakoid lumen.
 As electrons are transported, they go from higher to lower energy levels.
 When protons are pumped across the thylakoid membrane, it creates a proton gradient
that is used in chemiosmosis. This is what drives the synthesis of ATP.
6. Meanwhile, photons of light also strike PSI, boosting an electron to its excited state. This
electron then excites other electrons, until this energy reaches the primary electron
acceptor, and is ejected from PSI. This also creates an electron hole. This hole is filled by the
electrons from PSII.
7. Activated electrons are passed from PSI down a second electron transport chain (ETC).
8. The flow of electrons continues along the ETC until it finally reaches the final electron
acceptor NADP+ which is reduced to NADPH.
9. Hydrogen ions (=protons) accumulate inside the thylakoid space (where photolysis occurs),
creating a proton gradient. Protons then diffuse back across the thylakoid membrane into
the stroma through chemiosmosis, passing through the protein ATP synthase. ADP and Pi
are phosphorylized to produce ATP in a process called photophoshorylation.
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Cyclic Photophosphorylation
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The cyclic method uses a much simpler pathway to generate ATP.
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What happens in this process:
 The electrons in PSI are excited and leave the reaction center, P700.
 They are passed from carrier to carrier (such as FD and the cytochrome complex)
in the electron transport system and eventually return to P700.
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At the end of this cycle, only ATP is produced.
This pathway is called cyclic photophosphorylation because the electrons from P700 return to
the same reaction center.
 Unfortunately, this method isn’t as efficient as the noncyclic pathway since it doesn’t produce
NADPH.
 Plants use this method only when there aren’t enough NADP molecules to accept electrons.
 Lots of photosynthetic bacteria only have PSI, so they use this method.
 Photosynthesis may have evolved with cyclic electron flow.
 Cyclic electron flow may be photoprotective in eukaryotic photosynthesizers.
A Comparison of Chemiosmosis in Chloroplasts and Mitochondria
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Chemiosmosis in mitochondria and in chloroplasts is very similar.
Electron transport chains (built into a membrane) pump protons across the membrane as
electrons are passed down the chain in a series of redox reactions
The key difference is that in respiration, organic molecules provide the electrons, and chemical
energy is transferred to ATP. However, in chloroplasts, water provides the electrons as light
energy is transformed to the chemical energy of ATP
In chloroplasts, the electron transport chain pumps protons from the stroma into the thylakoid
space. As H+ diffuses back through ATP synthase, ATP is formed on the stroma side, where it is
available for the Calvin cycle
The Calvin cycle uses ATP and NADPH to convert CO2 to sugar
The dark reaction uses the products of the light reaction – ATP and NADPH – to make
sugar. CO2 becomes the source of glucose sugars through th process of carbon fixation.
 Carbon fixation simply means that CO2 from the air is converted into carbohydrates.
 Where: occurs in the stroma of the leaf.
 Required: ATP and NADPH
 Produced: G3P which is used to make sugars
The Calvin cycle turns 3 times to fix 3 molecules of CO2 (this is according to the book…Mrs.
Russo told us it turns twice…idk which is right) and produces one molecule of the 3-carbon
sugar glyceraldehyde-3-phosphate (G3P).
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Carbon dioxide enters (For 1 turn of the cycle, 3 molecules enter, one at a time)
The cycle can be divided into 3 phases:
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1.
Carbon fixation:
 CO2 is added to a 5-carbon sugar called ribulose biphosphate (RuBP) to form an unstable 6C compound.
 This compound is chemically rearranged, breaking into 2 3-carbon compounds.
-At this point, because 3 molecules of CO2 entered the cycle, there are now six 3-carbon
compounds
 The enzyme RuBP carboxylase, or rubisco, catalyzes this reaction.
-Rubisco is the enzyme which converts RuBP that has combined with CO2 into a 6carbon compound
-However, it has a greater affinity for water
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Reduction:
 NADPH is oxidized to produce NADP+
 The electrons lost by NADPH go to convert the 3-carbon compounds into G3P
(glyceraldehyde-3-phosphate).
 For every 3 molecules of CO2 (1 turn of the cycle), there are 6 molecules of G3P.
- One molecule of G3P exits the cycle and will be used to produce sugars.
- The other 5 G3P molecules are recycled to form RuBP again so the cycle can begin.
 The cycle must turn 3 times to create a net gain of 1 molecule of G3P
3.
Regeneration of CO2 acceptor (RuBP):
The 5 molecules of G3P are rearranged back into the 3 molecules of RuBP
This requires 3 more ATP
 9 molecules of ATP and 6 of NADPH are required to synthesize 1 G3P
 Since G3P, a 3-carbon molecule, is the first stable product, this method of producing glucose is
called the C3 pathway.
Alternative mechanisms of carbon fixation have evolved in hot,
arid climates
Photorespiration: An Evolutionary Relic?
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Sometimes intensely bright light tends to stunt the growth of C3 plants because lighted
conditions can trigger a process called photorespiration.
-Photorespiration is the pathway that leads to the fixation of oxygen and is a seemingly
wasteful process.
 On hot dry days, C3 plants close their stomates to limit water loss. This prevents CO2 from
entering the leaf, slowing the Calvin cycle. This also causes oxygen to build up.
 As CO2 concentration is reduced and more O2 than CO2 accumulates, rubisco binds to O2 and
adds O2 in place of CO2 to RuBP.
 The product splits, and a 2-carbon compound leaves the chloroplast and is broken down to
release CO2.
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C4: physiological adaptations
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The C3 pathway is not the only way to “fix” CO2
C4 plants = sugar cane and corn
In these C4 plants, CO2 first combines with the 3-carbon compound PEP (phoenolpyruvate) in
mesophyll cells to form a 4-carbon compound called oxyloacetate.
-The enzyme PEP carboxylase has a high affinity for CO2.
 The resulting 4-carbon compound formed in the mesophyll cells of the leaf is transported to the
bundle-sheath cells tightly packed around the veins of the leaf.
 The compound is broken down to release CO2, which rubisco then fixes into the Calvin cycle to
make glucose
 The C4 pathway works particularly well for plants found in hot, dry climates. It enables them to
fix CO2 even when the supply is greatly diminished.
**The subscripts in both C3 and C4 refer to the number of carbons initially involved in
making sugar. However, both pathways ultimately use the Calvin cycle to produce
glucose.
CAM Plants: temporal adaptations
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Many succulent plants close their stomates during the day to prevent excessive water loss from
transpiration, but open them at night to take up CO2 and incorporate it into a variety of organic
acids.
-Succulent = high water storage
***Many desert plants are CAM plants
You might think that this would prevent these plants from carrying out photosynthesis during
the day. However, desert plants have evolved a way to perform photosynthesis when their
stomates are closed:
It’s called CAM (crassulacean acid metabolism) photosynthesis.
 CAM plants open their stomates at night to take up CO2. They convert CO2 into a variety
of organic acids with the help of the enzyme PEP (which also aided the C4 pathway).
 CAM plants store these organic acidic compounds in the cell’s vacuole.
 During the day, these compounds are broken down to release CO2 so that the Calvin
cycle can proceed.
Unlike the C4 pathway, the CAM pathway doesn’t structurally separate carbon fixation from the
Calvin cycle; instead the 2 processes are separated in time
Importance of Photosynthesis: A Review
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About 50% of the organic material produced by photosynthesis is used as fuel for cellular
respiration in the mitochondria of plant cells
The rest is used as carbon skeletons for the synthesis of organic molecules (proteins, lipids, and
a great deal of cellulose), stored as starch, or lost through photorespiration.
About 160 billion metric tons of carbohydrates per year are produced by photosynthesis
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Photosynthesis:
CO2
Enters leaves
through
stomates
- by process
of diffusion/
osmosis
+
H2O
Enters roots
through root
hairs
- by process of
root pressure,
capillary
action, and
transpiration
Sunlight
- by process of
Chlorophyll
diffusion/
- byosmosis
process of
Chloroplasts
diffusion/ in
thylakoids
osmosis
C6H12O6
+
Made in leaves through
photosynthesis in chloroplasts
- Made in the mesophyll
layer which consists of
1. Palisade layer
(condensed)
2. Spongy layer (has air
spaces)
- Stored in roots, stems,
and mostly fruits (=the
ovary of the flower; are
fleshy/dry & have seeds )
O2
Made in the light
reaction of
photosynthesis
- Comes from
splitting a
water molecule
- Exits leaves
through
stomates
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