photosynthesis. learning objectives swbat: describe how organisms capture free energy present in...
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Learning objectivesSWBAT: Describe how organisms capture free
energy present in sunlight and convert to the chemical energy of food.
SWBAT: Describe the light-dependent and light-independent stages of photosynthesis
SWBAT: Compare the processes of cellular respiration and photosynthesis
Autotroph = producerMake their own fuel/organic molecules, but still
need to use cellular respiration to convert that fuel into usable energy
Heterotroph = consumerDepend on autotrophs for food and O2
Photosynthesis
= the conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and some prokaryotes
Photo-Light energy from the sun is converted to the
chemical energy in the form of chemical bonds.
Remember the 1st Law of Thermodynamics!
Which product of cellular respiration contains energy in its bonds?
-synthesis
Energy from ATP and NADPH powers the synthesis of a 3 carbon molecule (G3P) that can then be used to make glucose (and other simple carbohydrates)
Figure 8.3Leaf cross section
20 m
Mesophyll
Stomata
Chloroplasts Vein
CO2 O2
Mesophyll cell
Chloroplast
Stroma
Thylakoid
Thylakoidspace
Outermembrane
Intermembranespace
Inner membrane
Granum
1 m
Photosynthesis equation
6CO2 + 6H2O + light energy C6H12O2 + 6O2
An endergonic redox reaction- electrons gain potential energy as they travel from H2O to C6H12O6
Light-dependent reactions
A series of coordinated pathways that capture free energy present in light.
Yields ATP and NADPH which power the production of organic molecules in the light-independent reactions
Occurs in the thylakoid membranes of the chloroplast
Very similar to the electron transport chain in cellular respiration
More about light-dependent reactions
Within the thylakoid membrane are 2 complexes called photosystems: PSI and PSII (though PSII occurs first)
During photosynthesis, chlorophylls, within the photosystems, absorb light energy and boost electrons to higher energy levels.
PSI and PSII are connected by the transfer of these electrons through an electron transport chain much like that in cellular respiration.
Just like in cellular respiration, this electron transport creates a proton gradient (hydrogen ions) across a membrane. In this case, the thylakoid membrane.
The gradient is then responsible, via ATP Synthase, for the formation of ATP from ADP.
Figure 8.13-5
Primaryacceptor
2 H
O2
ATP
NADPH
Photosystem II(PS II)
H2Oe−
e−
e−
Light
2
1
P680
Pq
Electrontransportchain
Cytochromecomplex
Pc
Pigmentmolecules
Primaryacceptor
Photosystem I(PS I)
e−
P700
e−
e−
Fd
Light
Electrontransportchain
HNADP
NADP
reductase
1
2
3
4
5
6
7
8
Light-independent reactions
The Calvin Cycle
Production of carbohydrates from CO2 powered by ATP and NADPH
Occurs in the stroma of the chloroplast
Light-independent reactions
Though sometimes called the “dark reactions”, they occur in the daytime. They, however, do not require light.
There are 3 phases of the Calvin CycleCarbon fixationReduction- production of G3P (a 3 carbon
compound)Regeneration of the CO2 acceptor (RuBP)
Photorespiration
CO2enters the plant through stomata in the leaves.
H2O also transpires, is lost, through stomata.
What happens on hot, dry days?
If the plant closes its stomata, it can conserve water, but cannot obtain the CO2 that it needs.
In photorespiration O2 enters the Calvin Cycle where CO2 normally would.
When there is not enough CO2, O2 enters the Calvin Cycle in a process called photorespiration.
Two problems:This process consumes O2, but uses rather than
generates ATPDecreases photosynthetic output by removing
carbon from the Calvin cycle and releasing it as CO2 rather than as organic compounds.
Adaptations to optimize photosynthesis in hot
climatesC4 Plants- fix carbon in a 4-carbon compound
then uses it as a “shuttle” to a different location, the bundle-sheath cells, where the Calvin Cycle occurs.
CAM Plants- open stomata during the night to let in CO2, closed during the day when the plant then completes the light-independent Calvin Cycle
Figure 8.18c
Bundle-sheathcell
CO2CO2
(a) Spatial separation of steps
C4
CO2CO2
CAM
Day
Night
Sugar
CalvinCycle
CalvinCycle
Sugar
Organicacid
Organicacid
Mesophyllcell
(b) Temporal separation of steps
1
2
1
2
Figure 8.15a
Electrontransport
chain
Higher [H] H
CHLOROPLASTSTRUCTURE
Inter-membrane
space
MITOCHONDRIONSTRUCTURE
Thylakoidspace
Innermembrane
MatrixKey
Lower [H]
Thylakoidmembrane
Stroma
ATP
ATPsynthase
ADP
H Diffusion
Pi
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