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I. General Overview
Figure 1: Photosynthesis vs Respiration
Respiration & photosynthesis are reciprocal metabolic processes, for the by-products of one process serves as the raw
material to fuel the other.
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Figure 2: Carbon Fixation
As can be seen fin figure 2, carbon comprises the backbone of all organic macromolecules. Since animals cannot
directly incorporate atmospheric CO2 to form these compounds, they must rely on the ability of plants to “fix” the
carbon (incorporate it into preexisting molecules) …
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Figure 2.1: Carbon Fixation & Carbon Cycling
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Photosynthesis:
II. Adaptations for Photosynthesis
Figure 3: Leaf Structure
Cuticle: waxy layer that limits the amount of H2O loss from the leaf & entry of disease causing agents.
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Epidermis (upper & lower): boxlike cells that produce & secrete the waxy substance that comprises the cuticle. Also
provides protection for the leaf’s delicate inner structure.
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Stomata: small openings in leaf’s underside. Allow entry of CO2 into the leaf structure & the release of O2 & H2O (v).
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Guard Cells: pair of cells that surrounds each stomate to regulate the amount of water loss from the leaf. When
exposed to light in the morning, guard cells take in water & swell -causes the stomate to open. At night or during
periods of low light intensity, guard cells lose water & shrivel, causing the stomate to close -prevents water loss during
times when the plant cannot photosynthesize.
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Figure 3.1: Leaf Structure: Guard Cells
Flaccid Guard Cells: Stomate Closed
Turgid Guard Cells: Stomate Open
Spongy Mesophyll: loosely packed cells that form a network of air spaces to increase surface area for CO2 uptake & the
loss of O2 & H2O (v).
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Pallisade Mesophyll: column-like cells that act as the main site of photosynthesis, for they contain the highest
concentration of chloroplasts within the plant.
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Figure 3.1: Leaf Structure: Chloroplasts
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Chloroplast:
a) Consists of an inner & an outer membrane. The inner membrane encloses a fluid-filled space called the Stroma,
which contains enzymes for producing sugars from CO 2 & H2O.
b) Suspended in the stroma are membranes that form a set of interconnected disk-like sacs called Thylakoids.
Thylakoid membranes contain chlorophyll & other photosynthetic pigments.
c) Thylakoid membranes, like the inner mitochondrial membrane, are involved in ATP synthesis. Thylakoids are
organized into stacks called Grana.
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Vascular Bundle: contains conducting tissue that is continuous with the stem. Xylem serves to bring water &
dissolved solutes up to the leaf tissues & Phloem transports sugars to other parts of the plant.
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III. Properties of Light
Figure 4: Electromagnetic Spectrum
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EM Spectrum:
a) Each form of radiation differs from one another by their respective Wavelengths (distance between consecutive
crests (high-points) or troughs (low-points)).
b) The visible portion of the electromagnetic spectrum ranges from wavelengths of 380 nm (violet) to 760 nm (red).
Visible light is composed of small particles called Photons.
Photosynthetic Pigments
• Substances that absorb visible light are called Pigments; different pigments absorb different wavelengths of light.
The main photosynthetic pigments within the thylakoid membranes of the chloroplast are Chlorophylls.
Figure 5: Chlorophyll Structure
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Figure 5.1: Chlorophyll Absorption
Blue
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Red
Chlorophyll:
a) Since chlorophyll absorbs red & blue wavelengths the best, photosynthesis occurs at the highest rate in the
presence of these wavelengths.
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The types of photosynthetic pigments present within the thylakoid membrane include:
a) Chlorophyll a & b: major pigments involved in the absorption of light (red & blue) during the light reactions
b) Carotenoids: secondary pigments involved the light reactions. Are able to absorb wavelengths that chlorophyll
a & b cannot, thus increasing the variety of wavelengths that can drive photosynthesis.
IV. Chemistry of Photosynthesis
General Formula
6CO2 + 6H2O
C6H12O6 + 6O2
H2O is completely oxidized to O2 in the presence of light (the e- from water are used to form ATP & NADPH). CO2 is
completely reduced (by e- from NADPH) to form sugar (PGAL).
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Light Reactions
Figure 6: Pigment Organization: Photosystem
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Photosystems are composed of a Reaction Center Chlorophyll a molecule surrounded by additional chlorophyll a, b,
& carotenoid molecules that make up an Antenna Complex. Two photosystems function in during the light reactions:
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a) Photosystem I:
b) Photosystem II:
Figure 6.1: Light Reactions (Thylakoid Membrane)
Stages of the Light Reactions
• Upon absorbing 2 photons, the pigments of photosystem II channel them to the reaction center chlorophyll a
molecule, P680. By absorbing these photons P680 becomes Photooxidized as it loses 2 e- to a Primary Electron
Acceptor in the thylakoid membrane.
The 2 e- lost by P680 must be replaced in order for it to continue donating to the primary electron acceptor. The lost
e- are replaced during a process called Photolysis whereby water is “split” into ½O & 2H+, using its 2 e- (1 from each
H+) to replace those lost by P680. The 12 H2O’s split during this process releases a total of 6O2 as a by-product.
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The donated e- travel along an Electron Transport Chain embedded in the thylakoid membrane. The e- move down
the chain & establish a proton gradient by transporting H+ from the stroma into the thylakoid space.
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The proton gradient stores a great deal of potential energy, which is released as they rush back into the stroma via
ATP Synthases. As a result, ADP is phosphorylated to ATP. This process by which ATP is formed during the light
reactions is called Oxidative Photophosphorylation. For each H2O molecule “split” by photosystem II, 1 ATP is
produced, for a total of 12 ATP’s.
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At the same time, P700 receives 2 e- from Photosystem II to replace those lost to its primary electron acceptor. The
donated e- are passed to the enzyme NADP Reductase, which uses the e- to form NADPH. Since 12H2O molecules are
split to release e-‘s, a total of 12 NADPH are produced.
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The end result of the light reactions is the conversion of light energy to chemical energy in the form of 12 NADPH, &
12 ATP which are then used during the Calvin Cycle to reduce CO2 to the sugar PGAL.
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Figure 7: Calvin-Benson Cycle (Stroma)
*3 “Turns” of the Cycle are shown above
Stages of the Calvin Cycle
• During Carbon Fixation, 3CO2 are incorporated into the skeleton of the 5-carbon sugar RuBP via the enzyme Rubisco,
forming 3 molecules of a 6-carbon compound. The 6-carbon compounds break down to form 6 molecules of a 3-carbon
compound which is phosphorylated via the hydrolysis of 6ATP).
These molecules are reduced by 6NADPH to form 6 molecules of PGAL (or G3P). In order for the cycle to continue, 5
of the 6 molecules of PGAL are “recycled” to make more RuBP for the continued C-fixation of CO2. During this stage of
the cycle, these PGAL’s are phosphorylated by 3 ATP.
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Since 1 molecule of PGAL is equivalent to half a simple sugar molecule, another 3 CO2 must enter the cycle to
generate another PGAL. Thus 6 “turns” of the Calvin Cycle are required to synthesize 2 net PGAL’s, the main products
this process.
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You will notice that the 6 “turns” of the Calvin Cycle consume 18 ATP’s, which is more than the 12 ATP produced
during the light reactions. This difference is usually made up with ATP from cellular respiration.
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V. Respiration & Photosynthesis
Figure 8: Comparison of Pathways
The processes of cellular respiration & photosynthesis “feed” one another, for the end products generated by each
process can be used to fuel the other:
a) Carbon Dioxide: waste product of cellular respiration is used in the production of PGAL during photosynthesis.
During the night, more CO2 is produced than the plant can use & the excess is released by diffusion mainly through
stomates.
b) Oxygen: by-product of photosynthesis is used in the production of ATP during cellular respiration. During the day,
more oxygen is produced than the plant can use & the excess is released through stomates into the environment.
c) PGAL: end product of photosynthesis, can be used to fuel aerobic cellular respiration in plant cells.
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