Redox Reactions and Photosynthesis

Oxidation reactions

When a substance loses electrons. This gives off energy.

Reduction reactions

When a substance gains electrons. This requires energy to be put in.

TRUE OR FALSE: These reactions are coupled, meaning if something loses electrons, something else needs to gain them.

TRUE

Oxidizing agent

The molecule/atom that is accepting the electrons (often H atoms)

Reducing agent

The molecule/atom giving away electrons (often H atoms)

Oxidized

When a molecule/atom loses electrons

Reduced

When a molecule/atom gains electrons.

Photosynthesis equation

12H2O + 6CO2 + Sunlight → 6O2 + C6H12O6 + 6H2O

TRUE OR FALSE: In photosynthesis, CO2 is reduced as it gains hydrogen atoms when it is fixed into sugar.

TRUE

TRUE OR FALSE: the leaf is the primary location of photosynthesis

TRUE

Structures:

• Cuticle

• Epidermal cells

• Palisade Mesophyll

• Spongy Mesophyll

• Xylem

• Phloem

• Stomata

Cuticle function

Waxy outer layer that prevents water loss

Epidermal cells

Beneath the cuticles and are non-photosynthetic

Palisade Mesophyll function

Beneath the epidermal cells and are specialized for light capture

Spongy Mesophyll function

Beneath the palisade mesophyll and specialize in gas exchange

Xylem function

Found in the bundle of sheath cells within spongy mesophyll that helps carry water through plant

Phloem function

Beneath the xylem and help carry sap (sugar) through the plant

Stomata function

Part of the bottom of the leaf that can open and close in guard cells.

Light absorption in plants

The shorter the wavelength, the more energy it possesses. When light meets matter, it can be absorbed (the colors we don’t see) and reflected (the color(s) we do see)

Photosystems

Arrangements of chlorophyll molecules in plant cells. They are located in the thylakoid membrane.

Pigments

Chemicals that reflect specific wavelengths. Pigment types are Chlorophyll a, chlorophyll b, and carotenoids (and other accessory pigments that increase the absorbance of light at additional wavelengths)

Why does green spinach extract glow red under UV light?

When pigments are separated from photosystems and absorb high energy photons, a release of energy encompassing heat and fluorescence occurs. Spinach extract releases red fluorescence under UV light conditions.

Photosynthesis has two reactions:

1. light dependent reaction: photon of energy transferred to ATP and NADPH, water is split into H+ and O2, and it takes place in thylakoid membrane

2. light independent reaction (Calvin cycle): energy from ATP and NADPH is released to incorporate CO2 into sugar and takes place in the stroma.

Light independent reaction (part 1)

A chlorophyll molecule absorbs a photon of light. This excites an e-. Energy is passed from chlorophyll to chlorophyll until it reaches the reaction center (a special pair of chlorophyll molecules) in PSII. From the reaction center, an e- is passed to a primary electron acceptor to reduce it. Since the reaction center needs a new e-, H2O is split into 2H+ and ½ O. When this happens four times, a full diatomic oxygen atom is released. This water splitting allows reaction to center to continue supplying e- to primary electron acceptor.

Light dependent reaction (part 2)

PSII and PSI are connected by an electron transport chain (ETC). e- will travel down the ETC to PSI. Electrons will travel from plastoquinone (pq) to cytochrome complex b to plastocyanin (pc) to reaction center of PSI. Since the e- now olds a lot less energy, it requires another photon of light to excite it to the primary electron acceptor of PSI and then to ferrodoxin (fd). Fd transfers 2e- to NADP+ via NADP+ reductase, forming NADPH.

Light dependent reactions (part 3)

Meanwhile, H+ ions build ip in the lumen of thylakoid due to the water splitting and the gradual release of potential energy from the ETC as the e- is passed from one acceptor to another. That energy moves H+ ions against the concentration gradient from the stroma into thylakoid membrane. ATP synthase allows the H+ to flow down the concentration gradient. As they flow through the enzyme, H+ ions are used to mechanically turn the enzyme so ATP can be synthesized.

Linear electron flow

When e- travel in a one way path from water to NADP+

Cyclic electron flow

Occurs in PSI and happens when light hits PSI and excites electrons to primary electron acceptor. The e- are then transferred to fd and pc before reaching PSI again. This process makes more ATP than linear electron flow. Does not release O2 and does not make NADPH.

Light independent/Calvin Cycle Step 1 - Carboxylation/carbon fixation

CO2 is added each to a 5-carbon molecules of RuBP, which is catalyzed by the enzyme RuBisco. The resulting 6-carbon intermediate is split into two 3-carbon molecules of 3-PGA. This happens 6 times to make 12 3-PGAs.

Light independent/Calvin Cycle Step 2- Reduction

Each of the 12 3-PGAs is given a phosphate group via ATP from light reaction. The resulting 1,3-Biphosphogylcerate are reduced by e- from NADPH also from light reaction. The final products are 12 G3P

Light independent/Calvin Cycle Step 3 - Regeneration

Of the 12 G3P molecules formed, 2 of them are used to form the final form of glucose while the remaining 10 are used to regenerate 6 RuBP. Each RuBP requires an ATP for regeneration, meaning 6 ATPs are needed.

Overview of what is required to make glucose in Calvin cycle

6 CO2, 18 ATP, and 12 NADPH ar required to make 2 G3Ps, which equal 1 glucose. The actual product of Calvin cycle is G3P. The cycle then regenerates its starting material after molecules enter and exit the cycle. ADP+ and NAD+ recycle back to light section.