Photosynthesis

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

Figure 8.4

Products:

Reactants: 6 CO2

6 O2C6H12O6 6 H2O

12 H2O

2 stages of photosynthesis

1. Light-dependent reactions

2. Light-independent reactions

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

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