AP Biology- Photosynthesis

Photosynthesis

2.A.1: All living systems require constant input of free energy

2.A.2: Organisms capture and store free energy for use in biological processes

Photosynthesis is a redox reaction. Water is oxidized to oxygen and carbon dioxide is reduced to glucose. Photosynthesis is also an endergonic reaction, which means it requires energy.

Where does photosynthesis occur?

Leaves --> Mesophyll --> Plant Cells --> Chloroplasts

Chloroplasts have a distinct structure

Chloroplasts use their thylakoids

to transform light energy into

chemical energy of ATP and

NADPH

Pigments

Pigments are just substances that absorb visible light. Different pigments absorb different wavelengths of light. Plants look green because the pigments reflect green and absorb violet-blue and red.

There are all different kinds of pigments

• chlorophyll a, chlorophyll b, carotenoids, anthocyanin
• chlorophyll a is directly involved in the light reaction

Which wavelengths of light are most effective in photosynthesis?

Absorption spectrum is a graph plotting a pigment's light absorption versus wavelength.

Action spectrum profiles the relative effectiveness of different wavelengths in driving the photosynthesis.

Engelmann's Experiment

The Light Reaction

A photosystem- protein complex called a reaction-center complex surrounded by several light-harvesting complexes ( pigment molecules).

Reaction-center- complex includes a special pair of chlorophyll a molecules bound to proteins. A photosystem also contains a primary electron acceptor that is capable of accepting electrons and being reduced.

• There are 2 Photosystems

When light strikes a pigment molecule in a light-harvesting complex, the energy is passed from one molecule to the next until it gets to the reaction-center complex, here an excited electron is transferred to the primary electron acceptor.

The light reaction takes place on the thylakoid membranes, at photosystem II and photosystem I.

• Photosystem I is sometimes called P700 because the chlorophyll a in the reaction center absorbs 700nm wavelength the best
• Photosystem II is sometimes called P680 because it absorbs 680nm wavelengths best.

The Light Reaction

(Linear)

1. Photosystem II absorbs light energy allowing P680 to donate electrons to the primary electron acceptor.

2. An enzyme splits water. Electrons are supplied to P680 and oxygen is released

3. The original excited electron passes to the primary electron acceptor of Photosystem I through an electron transport chain.

4. Energy from the transfer of electrons down the Electron Transport Chain pumps protons creating a gradient that is used in chemiosmosis to phosphorylate ADP to ATP, ATP is used in the Calvin Cycle.

5. Meanwhile light energy also activates Photosystem I, resulting in the donation of electrons to its primary electron acceptor. The electron just donated by Photosystem I are replaced by electrons from Photosystem II.

6. The primary electron acceptor of Photosystem I passed the excited electron to another ETC, which transmits them to NADP+ reducing it to NADPH which is also used in the Calvin Cycle.

An alternative to linear electron flow! Photosystem II

is not used. Cyclic electron flow uses a short circuit of linear electron flow by cycling the excited electrons back to their original starting point in Photosystem I. Cyclic electron flow produces ATP by chemiosmosis, but no NADPH and no oxygen is released.

The Calvin Cycle

or

The Light Independent Reaction

1. Three carbon dioxides are attached to three molecules of a sugar called RuBP (ribulose biphosphate) with the help of the enzyme rubisco. This forms an unstable product that immediately splits into 2 3-carbon molecules called PGA. This step is called fixing carbon (incorporating inorganic carbon from the air into an organic carbon molecule).

2. The PGA molecules are phosphorylated.

3. 6 ATP and 6 NAPDH reduce to molecule to G3P.

4. One of these G3Ps leaves the Calvin cycle, it can combine with another G3P to make glucose.

5. The other G3P molecules are recycled back into RuBP using 3 more ATP.

On hot dry days the stomata of C3 plants partially close to prevent water loss, the plant continutes to fix carbon dioxide, and eventually the carbon dioxide will get used up.

Rubisco will combine oxygen with RuBP instead of carbon dioxide. This produces one PGA and a toxic 2-carbon molecule that the plant must get rid of using peroxisomes and mitochondria.

Plants adapted for these hot and arid environments have certian physiologial adaptations.

C4 Plants- these plants preface the Calvin Cycle with an alternate mode of carbon fixation that forms a 4 carbon compounds as its first product.

1. In mesophyll cells the enzyme PEP carboxylase adds carbon dioxide to PEP.

2. A 4 carbon compound conveys the atoms of carbon dioxide into the bundle sheath cells via the plasmodesmata

3. In the bundle sheath cells carbon dioxide is released and enters the Calvin Cycle.

CAM (crassulacean acid metabolism) Plants- these plants open their stomata at night and close them during the day to help conserve water

The chemical reaction for photosynthesis is:

6CO2 + 12 H2O + energy --> C6H12O6 + 6O2 + 6H2O

If the input water is labeled with radioactive isotope of oxygen O18 then the oxygen gas released as the reaction proceeds is also labeled with O18. which of the following is the mostly likely explanation?

a. During the light reactions of photosynthesis, water is split, the hydrogen atoms combine with the CO2, and oxygen gas is released

b. During the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released

c. During the Calvin Cycle, water is split, regenerating NADPH from NADP+, and oxygen gas is released

d. During the Calvin cycle, water is split, the hydrogen atoms are added to internediates of sugar synthesis, and oxygen gas is released

Cyclic Electron Flow

Photosynthesis Light Reaction

The Calvin cycle consumes more ATP than NADPH

- NADPH regulates cyclic vs non cyclic

The chemical reaction for photosynthesis is:

6CO2 + 12 H2O + energy --> C6H12O6 + 6O2 + 6H2O

If the input water is labeled with radioactive isotope of oxygen O18 then the oxygen gas released as the reaction proceeds is also labeled with O18. which of the following is the mostly likely explanation?

a. During the light reactions of photosynthesis, water is split, the hydrogen atoms combine with the CO2, and oxygen gas is released

b. During the light reactions of photosynthesis, water is split, removing electrons and protons, and oxygen gas is released

c. During the Calvin Cycle, water is split, regenerating NADPH from NADP+, and oxygen gas is released

d. During the Calvin cycle, water is split, the hydrogen atoms are added to internediates of sugar synthesis, and oxygen gas is released

Prokaryotes???

Since prokaryotes do not have chloroplasts they do photosynthesis a little differently. Infoled photosynthetic membranes function similarly to the internal membranes of a chloroplast.

Original chloroplasts are believed to have been a photosynthetic prokaryotes that lived inside a eukaryote. *Remember endosymbiosis?!?!?!?!?!*

The Calvin Cycle uses ATP and NADPH to convert carbon dioxide into the sugar.

The cycle spends ATP as an energy source and consumes NADPH as reducing power for adding high energy electrons to make sugar.

To synthesize one glucose molecule, the Calvin cycle uses 6molecules of CO2, 18 molecules of ATP, 12molecules of NADPH

The problem... Photorespiration