Bio AP Todeschini

• Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes
• These organisms feed not only themselves but also most of the living world

Photosynthesis and Respiration
• There is a reciprocal relationship between chemoheterotrophs and photoautotrophs

Photosynthesis converts light energy to the chemical energy of food
• Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria
• Leaves get their green color from
chlorophyll, the green pigment within chloroplasts
• CO
2 enters and O
2 exits the leaf through microscopic pores called stomata

• Chloroplasts are found mainly in cells of the mesophyll
• The chlorophyll is in the membranes of thylakoids
(connected sacs in the chloroplast); thylakoids may be stacked in columns called grana
• Chloroplasts also contain stroma, a dense fluid

Light is a form of electromagnetic radiation .
It is produced by the movement of electrons between orbitals
Visible light is just one tine slice of the larger electromagnetic spectrum

• Sunlight contains almost all wavelengths of visible light
• Chloroplasts do not absorb all wavelengths of light equally
• When plants are exposed to light, chloroplasts have an absorption spectrum that is highest in the blue and red portions of the visible light spectrum

The unequal utility of different wavelengths of light was first noticed by Theodore
Engelmann, who observed higher rates of growth of aerobic bacteria on algae grown in blue and red wavelengths of light.

• Accessory pigments absorb light at different spectrums, green is still least useful light wavelength.
• They are also used to attract pollinators
• Let animals know when fruit is ready to be eaten in order to spread seeds

• Water will be oxidized (it is the “reducing agent”).
OIL: oxidation is loss (of e )
• Carbon will be reduced (it is the “oxidizing agent”).
RIG: reduction is gain (of e )

Photosynthesis is a 2-part process
1. The light reactions: occur in the thylakoid membranes. Light is used to drive the production of ATP and NADPH (an e carrier). Water provides the e needed and is converted to O2 gas (waste)
2. The Calvin Cycle: Occurs in the stroma.
The ATP and NADPH produced in the light reactions are used to incorporate carbon dioxide into 3-carbon sugar.
*Prokaryotes do not have chloroplast or any other membrane bound organelles but carry out photosynthesis in specialized areas of their cell membranes

Fig. 10-5-1
Light
H
2
O
Light
Reactions
NADP +
+
ADP
P i
Chloroplast

Fig. 10-5-2
Light
Chloroplast
H
2
O
Light
Reactions
NADP +
+
ADP
P i
ATP
NADPH
O
2

Fig. 10-5-3
Light
Chloroplast
H
2
O CO
2
Light
Reactions
NADP +
+
ADP
P i
ATP
NADPH
Calvin
Cycle
O
2

Fig. 10-5-4
Light
H
2
O CO
2
Chloroplast
Light
Reactions
NADP +
+
ADP
P i
ATP
NADPH
Calvin
Cycle
O
2
[CH
2
O]
(sugar)

• When photons of light interact with chlorophyll, electrons in the Magnesium atom become excited.
• This happens with ~1% of all the sunlight that strikes the surface of the earth.
• Isolated chlorophyll will fluoresce when exposed to light, as the excited electrons return to the ground state.

• Complexes of protein and pigment molecules that are embedded in the thylakoid membrane.
• Direct incoming photons in the
“reaction center” where chlorophyll a molecules produce excited electrons which are transferred to an electron transport chain.

Two types:
• Photosystem II : central chlorophyll works best at a light wavelength of 680 nm( P680 ). Found at the “start” of th ETC.
Two types:
• Photosystem I: central chlorophyll works best at a light wavelength of 700 nm
( P700 ). Found at the “end” of the ETC.

• Since chlorophyll is not going to have the electrons return to it, new electrons are needed.
• Water provides the replacement electrons (“photolysis”). This creates 4 protons and 1 molecule of oxygen gas for every 2 water molecules consumed.
• The oxygen gas is released as a waste product, becoming a major input for aerobic cellular respiration.

• As electrons move through the ETC, they provide the energy for chemiosmosis.
• Protons are pumped by
ETC proteins from the stroma into the thylakoid space.
• The facilitated diffusions of protons back into the stroma through ATP synthase (protein channel) drives the synthesis of ATP.

One notable difference:
• In respiration, the energy comes from oxidation of glucose (“ oxidative phophorlylation ”).
• In photosynthesis, the energy comes from photons
(“ photophosphorylation ”)

• Electrons move from photosystem II to photosystem I via the ETC. From photosystem II, they are transferred to the enzyme NADP-Reductase which uses them to reduce NADP+ into NADPH.
• Produces both ATP and NADPH
• The electrons of NADPH are available for the reactions of the
Calvin cycle
• Requires water

• Cyclic electron flow uses only photosystem I and produces ATP, but not
NADPH
• Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle
• The Calvin cycle requires 9
ATP and 6 NADPH for every sugar produced.

• Inputs
• Light
• ADP + Pi
• NADP+
• Water
• Outputs
• ATP
• NADPH
• O
2

• Products from Light Reactions Feed into the Carbon
Cycle
Three phases
1. Carbon Fixation : Ribulose Bisphosphate Carboxylase
(aka “ RuBisCo ”) mediates the transfer of a molecule of
Carbon Dioxide onto a molecule of Ribulose
Bisphosphate ( RuBP -5C).
2. Reduction : ATP and NADPH are used to rearrange
RUBP into Glyceraldehyde 3-phosphate ( G3P , aka
PGAL ) a three-carbon sugar.
3. Regeneration : ATP is used to reconstitute RuBP from
G3P.

Inputs
• 3CO
2
• 9 ATP
• 6 NADPH
• Outputs
• 1 G3P
• 9 ADP + Pi
• 6 NADP+

An Evolutionary Quirk
• Rubisco evolved in conditions of low oxygen gas concentration. As a result, its active site has a high affinity for oxygen gas…which is a problem.
• Photorespiration is the metabolic pathway that occurs when rubisco incorporates Oxygen instead of Carbon
Dioxide in RuBP .
• This is a metabolic dead end. Uses ATP but produces no sugar. Best if avoided.

As long as a plant can keep its stomates open and exchanging gas with the environment, photorespiration is kept to a minimum.
C3 Leaves: No adaptations for minimizing photorespiration
• Both stages of photosynthesis occur simultaneously.
• Oxygen and Carbon Dioxide are exchanged with the environment through the stomates.
• Sugars are transported to vascular tissue for transport throughout plant.
• Everything is great!
Disadvantages :
• Open stomata can lead to dessication .
• Closed stomata can lead to high oxygen gas/low carbon dioxide
• Thus increased photorespiration, not good!

• Carbon fixation occurs in mesophyll cells.
• Cabon dioxide is incorporated into a 4C organic acid (malate) by the enzyme PEP carboxylase (which has a very low affinity for oxygen).
• The 4C acid is then transported to the bundle sheath cells, where the carbon dioxide is cleaved from the 4C acid.
• Since the bundle sheath cells are surrounded by the mesophyll, their oxygen gas concentration remains low, even as the light reactions occur in the mesophyll cells.

CAM Plants : Temporal Separation
• Carbon fixation occurs during the evening, when open stomates will not lead to dessication.
• The carbon dioxide is stored in an organic acid.
• During the day, the organic acid is used to supply the calvin cycle with carbon dioxide.

C3 Plants
• Oxygen competes with carbon dioxide for binding to RuBisCo
• Photorespiration can occur and reduce sugar productions
• Uses ATP, no result
• Examples: most plants
• Most energy efficient
C4 Plants
• Spatial separation
• Carbon Dioxide is
“hidden” in the form of
Malic acid
• Light reaction occurs in mesophyll cells
• Calvin Cycle occurs in bundle sheath cells, where carbon dioxide if
“freed” from malic acid
• Examples: Corn, sugar cane, grasses
• ½ between other two methods
Cam Plants
• Temporal separation
• Carbon Dioxide is stored up at night in the form of an organic acid
• Dessication is prevented because less water is lost at night.
• Both Light reactions and Calvin cycle occur in mesophyll cells
• Examples: pinapple, cactus, succulents
• Best at conserving water