Photosynthesis
LIGHT & ENERGY
Energy is not consistent for all photons (shorter
wavelength = higher energy); ROYGBIV
Energy from photons may be absorbed by
electrons
Chlorophyll a absorbs the light energy.
Accessory pigments (chlorophyll b,
xanthophylls, and carotenoids) absorb other
wavelengths of light
Absorption Spectrum: Chlorophyll a absorbs
mainly violet/blue and red/orange wavelengths of
visible light (not green).
LIGHT DEPENDENT REACTIONS
The surface of a Photosystem
Photosystem II- producing ATP
Where: These reactions on the thylakoid membrane found in stacks (grana) in the chloroplast. The function of
the thylakoid membrane is to increase surface area. The area surrounding grana is called stroma (not to be
confused with stoma).
Photons excite a pair of electrons in the reaction center chlorophyll to a higher energy level.
These “energized” electrons are then passed through the electron transport chain powering a protein carrier
(proton pump).
This proton pump can use the energy from the electrons to pump H+ ions (protons) into the thylakoid space thus
creating an electrochemical gradient.
The H+ ions are only able to leave the thylakoid through the ATP synthase as they flow from high to low
concentration. The energy from this movement is utilized to join ADP + Pi to create ATP.
The ATP will be used in the Calvin cycle.
Photosystem I- providing electrons
Photosystem I receives electrons and they are reenergized by a slightly different wavelength of light.
These electrons are then eventually taken by the electron carrier NADP+ and H+ to from NADPH.
NADPH will supply these electrons to the Calvin cycle
Note: NADPH is the electron carrier in Photosynthesis. NADH is the electron carrier in cellular respiration
H2O- the electron source
H2O is split releasing 2 e- + H+ ions + O2The electrons will replace those used in Photosystem II. The H+ ions will increase the H+ ion concentration in
the thylakoid space. The oxygen will combine with another oxygen to make the O2 that makes aerobic cellular
respiration possible.
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Photosynthesis
LIGHT INDEPENDENT REACTIONS (CALVIN CYCLE)
Calvin Cycle needs:
1. source of C,H,O in order to manufacture C6H12O6
2. source of electrons to bond the above together
3. source of energy to run Calvin cycle reactions
With the aid of the enzyme Rubisco, RuBP (5-C compound) captures CO2 and surrounding H+ to form an
unstable 6-C compound.
The 6-carbon compound immediately splits into two 3-C PGA molecules
Electrons and energy from NADPH and ATP created in the light dependent reactions is used to convert PGA to
form G-3-P (PGAL)
Two molecules of 3-C G-3-P are used to form a molecule of 6-C glucose
Most PGAL molecules are used to form RuBP to start system again…Krebs CYCLE
The problem of photorespiration and dehydration
The problem of photorespiration: The rubisco that fixes carbon in the Calvin cycle is actually Ribulose 1,5Bisphosphate Carboxylase/Oxygenase [-ase endings after all ☺]. As the name implies, it can attach CO2 (good) or
O2 (bad). This is especially problematic since O2 is being created from the splitting of water nearby in the light
dependent reactions on the thylakoid. This can reduce efficiency by up to 50% in some plants!!!
The problem of dehydration: Recall that the stomata allow for the diffusion of O2 and CO2 into and out of the
leaves. Realize that H2O can also leave (dehydration) through the stomata. At first glance, one would assume that
the answer is to close stomata during the hot summer days to avoid dehydration. However, the light dependent
reactions are producing O2 at a high rate on a hot, sunny day resulting increased photorespiration.
PHOTORESPIRATION SOUTION #1- PHYSICAL SEPARATION: C4
Structure: In C-4 plants, the vascular bundle is surrounded by a ring of bundle sheath cells and an outer ring of
functional mesophyll cells. The two layers give the appearance of a wreath resulting in what is called Kranz
(German for “halo”) anatomy. The bundle sheath cells have large chloroplasts that either lack or have poorly
developed grana.
Function: C4 plants use PEP carboxylase which has a high CO2 affinity and is essentially “insensitive” to O2 to fix
CO2 instead of rubisco in the mesophyll cells. A modified 4 Carbon molecule(hence the name C4), malic acid, is
then shipped into a CO2 leak proof chamber (bundle sheaths) where the Calvin cycle will take place. The reaction
is now “reversed” resulting in PEP and CO2. PEP can now return to the mesophyll cell to retrieve more CO2.
With high [CO2] and low [O2], the odds are now far better for carbon fixation in the bundle sheath cells.
Examples: C4 photosynthesis is common in corn, crabgrass, and many annuals
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Photosynthesis
PHOTORESPIRATION SOLUTION #2 TEMPORAL SEPARATION: CAM
CAM (Crassalucean Acid Metabolism) plants open stomata at night to bring in CO2, then they fix it to one of a few
organic acids like PEP. These molecules are stored in vacuoles until day time when the stomata close and now the
Calvin cycle can begin as the previously made organic acids now decarboxylate and release the CO2 to keep the
relative concentration high for rubisco. C4 and CAM are similar, but CAM does everything in the mesophyll…NO
bundle sheaths are involved.
Examples: CAM is common in cacti and other succulent plants as well as pineapples and other bromeliads
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Photosynthesis
Multiple Choice
1. The color with the shortest wavelength in the visible spectrum is:
(A)
(B)
(C)
(D)
(E)
orange
red
yellow
blue
green
2. If photosynthesis ceased occurring throughout the world, the ultimate effect would be:
(A)
(B)
(C)
(D)
(E)
a rapid increase in plant reproduction and proliferation
plants would become the primary energy producers for all living things
immediate decreases in atmospheric carbon dioxide
alternating cooling and warming of the earth
extinction of plants and animals
3. Photosynthesis commonly takes place in all of the following EXCEPT:
(A)
(B)
(C)
(D)
(E)
mosses
ferns
algae
fungi
leafy plants
4. All of the following are required for photosynthesis to occur EXCEPT
(A)
(B)
(C)
(D)
(E)
5.
oxygen
carbon dioxide
chlorophyll
sunlight
NADP+
All of the following are characteristics of chloroplasts EXCEPT
(A) located in mesophyll tissue of leaves
(B) bound by a double membrane
(C) contain chlorophyll molecules
(D) found in the fluids that surround most plant cells
(E) somewhat similar in size and structure to mitochondria
6. The oxygen produced in the light dependent reactions of photosynthesis comes from
(A) ATP
(B) water
(C) carbon dioxide
(D) NADP+
(E) chlorophyll
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Photosynthesis
7. During photosynthesis the compound that is incorporated into existing organic molecules to build
carbohydrates is
(A) ATP
(B) NADP+
(C) chlorophyll
(D) water
(E) carbon dioxide
8. The waxy cuticle found on the leaves of many higher plants functions to
(A) transfer gases
(B) remove excess water from the plant
(C) protect the leaf from water loss
(D) store chlorophyll and expose the pigment to the maximum amount of sunlight
(E) serve as the site of the electron transport system
9. A major purpose of thylakoids that form the grana in chloroplasts is to
(A) provide a fluid environment for the reactions in the light phase
(B) control the amount of light that reaches the chlorophyll
(C) convert chlorophyll a into various accessory pigments
(D) provide increased membrane surface area for the reactions in the light phase
(E) accumulate glucose molecules for distribution to other plant cells
10. Accessory pigments such as chlorophyll b, xanthophyll, and carotenoids are important in photosynthesis in
that they are able to
(A) absorb wavelengths of light that are not absorbed by chlorophyll a
(B) serve as enzymes to create new chlorophyll a molecules
(C) directly convert carbon dioxide into glucose and oxygen
(D) absorb water in arid conditions
(E) control the opening and closing of stomata
11. The source of the electrons captured and transferred by NADPH is
(A) glucose
(B) water
(C) NADP+
(D) ATP
(E) carbon dioxide
12. Ribulose biphosphate (RuBP) is important in the light independent reactions (Calvin cycle) as the molecule
that
(A) transfers hydrogen ions to carbon dioxide
(B) releases oxygen into the atmosphere
(C) combines with carbon dioxide
(D) begins the electron transport system
(E) catalyzes the formation of ATP from ADP
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Photosynthesis
13. In the Calvin cycle of photosynthesis, PGA is formed from
(A) the combination of water and carbon dioxide
(B) the reaction of chlorophyll with carbon dioxide
(C) the synthesis of ATP from ADP
(D) the synthesis of NADPH and carbon dioxide
(E) the breakdown of an unstable six-carbon molecule
14. Glyceraldehyde-3-Phosphate (PGAL) molecules are used to create:
(A) NADP and salts
(B) RuBP and glucose
(C) proteins and fatty acids
(D) ATP and chlorophyll
(E) a supply of electrons for the Calvin cycle
Questions 15 -18 refer to the following terms
I.
II.
III.
IV.
Grana
Stroma
Stomata
Chlorophyll
15. Contain(s) chlorophyll molecules imbedded in membranes:
(A) I only
(B) II only
(C) I and II
(D) II and III
(E) III only
16. Region(s) of chloroplasts where light dependent and light independent reactions occur
(A) I only
(B) I and II
(C) II only
(D) II and III
(E) I and III
17. Become(s) activated when exposed to certain wavelengths of light
(A) I and II
(B) II only
(C) I and III
(D) IV only
(E) II and IV
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Photosynthesis
18. Control(s) gas exchange through leaf surfaces
(A) I and II
(B) I and III
(C) II only
(D) II and III
(E) III only
19. The C-4 photosynthesis pathway found in some plants is characterized by
(A) the removal of water from the light dependent reactions
(B) elimination of electron transport systems
(C) chemical transfer of ribulose biphosphate from one tissue area to another
(D) formation of malic acid to transfer carbon dioxide
(E) rapid transfer of oxygen to the atmosphere
20. The adaptive advantage of plants with a C-4 pathway include the ability to photosynthesize faster than
plants with a typical C-3 pathway under all of the following conditions EXCEPT
(A) high light intensity
(B) low carbon dioxide concentrations
(C) high turgor pressure in guards cells that surround stomata
(D) dry conditions with high temperatures
(E) area where plants grow tightly clustered together
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Photosynthesis
Free Response
1. Photosynthesis is often described as two series of reactions, the light-dependent and the lightindependent or Calvin cycle reactions.
A. Describe all steps that occur in the light-dependent reactions, including raw materials, energy transfer,
and products.
(8 pt maximum)
B. Explain how the products of the light-dependent reactions are necessary to drive the reactions of the
Calvin cycle.
(4 pt maximum)
2. Describe and discuss the C-4 pathway that can occur in some photosynthetic plants. Include in your
discussion:
A. a description of the compounds that are necessary for the process
B. the tissue regions where the reactions occur
C. the cyclic nature of the process
D. examples of plants where the C-4 pathway can be found
E. the selective advantages to plants that use this photosynthetic pathway
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Copyright © 2008 Laying the Foundation , Inc., Dallas, TX. All rights reserved. Visit: www.layingthefoundation.org
Copyright © 2008 Laying the Foundation®, Inc., Dallas, TX. All rights reserved. Visit: www.layingthefoundation.org