Chapter 10

When a pigment molecule absorbs light, one of the electrons goes from a ground state to a excited, which is unstable.
In isolation

Light is electromagnetic energy.
Wavelength is measure of distance between crests of electromagnetic waves.'
Electromagnetic spectrum is the entire range of electromagnetic energy.
Visible Light ( wavelengths 380nm to 740nm) drives photosynthesis and produces colors seen by human eye.
Light also behaves as particles, called photons.
- Photons have fixed amount of energy.

Pigments are substances that absorb visible light.
Different pigments absorb different wavelengths, and wavelengths that are absorbed disappear.
Wavelengths that are not absorbed are reflected.
Three types of pigments in chloroplasts include:
- Chlorophyll a: key light-capturing pigment that participates directly in light reactions.
- Chlorophyll b: acts as an accessory pigment, helping to capture light energy and transfer it to chlorophyll a.
- Carotenoids: accessory pigments that absorb light in the blue and green wavelengths, providing additional energy and also protecting chlorophyll from photodamage.
Accessory pigments (b and carotenoids) broaden spectrum for photosynthesis and are photoprotective against excessive light.

During the light reactions, there are two possible routers for electron flow: Linear and Cyclic.
Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy.
- This process begins when chlorophyll absorbs light, exciting electrons that are then transferred through a series of proteins in the electron transport chain, ultimately leading to the synthesis of ATP via chemiosmosis and the reduction of NADP+ to NADPH.
Cyclic Electron Flow, on the other hand, uses photoexcited electrons and recycle them back into photosystem I.
- Produces ATP.. no NADPH nor oxygen.

Chloroplasts and mitochondria both generate ATP by chemiosmosis
- ETC pumps protons (H+) across membrane as electrons are passed through carriers with progressively higher electron affinity
- ATP synthase couples the diffusion of H+ down their gradient to phosphorylate ADP into ATP, thereby harnessing the energy stored in the proton gradient.
Electron carriers and ATP synthase very similar.
Photophosphorylation differs from oxidative phosphorylation.
- Chloroplasts: high energy electrons drop down the transport chain from H2O, while mitochondria, they are extracted from organic molecules.
- Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into chemical energy.
Although the spatial organization of chemiosmosis differs slightly, some similarity are present.
- In Mitochondria, protons (H+) are pumped into intermembrane space and diffuse back to center
- In Chloroplasts, protons are pumped into thylakoid space and diffuse back to stroma.
Both ATP and NADPH are produced on the stroma side of thylakoid membrane, making them available for sugar synthesis in Calvin cycle.
Light → Photosystem II → H2O→ O2 + protons (H+).
- Protons go down ATP synthase.
- Electrons transferred to cytochrome complex and then to Photosystem I
Light → Photosystem I → NADP+ + H+ → NADPH

Section 10.4: Calvin cycle uses chemical energy of ATP and NADPH to reduce CO2 to sugar.

Calvin Cycle regenerates starting material after molecules enter and leave the cycle.
Calvin Cycle is anabolic; builds sugar from smaller molecules using ATP and reducing power of electrons from NADPH.
Carbon enters the cycle as CO2 and leaves as a molecule called G3P (glyceraldehyde 3-phosphate).
One G3P (three carbon pre-sugar) requires 3 turns/3 molecules of CO2.

Phase 1: Carbon Fixation

Binding of CO2 to a 5-C sugar named ribulose bisphosphate (RuBP) is catalyzed by RuBP carboxylase-oxygenase called Rubisco.
Six carbon intermediate molecule is immediately split into two molecules of 3-phosphoglycerate (for each CO2 fixed).

Phase 2: Reduction

Each molecule of 3-phosphoglycerate is altered through phosphorylation by six ATP and reduction by six NADPH to ultimately produce a G3P.
For every three CO2 molecules that enter the cycle, six molecules of G3P are formed.
Only one of these can be counted as a net gain of carbohydrate.

Phase 3: Regeneration of the CO2 acceptor (RuBP)

Remaining five molecules of G3P are rearranged in a complex series of reactions.. yielding three molecules of RuBP.
Three additional molecules of ATP are used to convert G3P back into RuBP, facilitating the continuation of the Calvin cycle.

Overview

Net synthesis of one G3P molecule, Calvin cycle consumes nine molecules of ATP and six molecules of NADPH.
Light reactions regenerate the ATP and NADPH.

TLDR

Plants have metabolic adaptations to help conserve water.
One important trade-off is the balance between photosynthesis and water conservation.

Most plants at C3 plants
In photorespiration, rubisco binds with O2 instead of CO2.
Photorespiration typically drains away as much as 50% of the carbon fixed by the Calvin cycle.
In some plant species, alternate modes of carbon fixation have evolved to minimize photorespiration and optimize Calvin cycle.

C4 plants minimize cost of photorespiration by spatially separating the processes of carbon fixation and the Calvin cycle, allowing for more efficient use of sunlight and water.
Sugarcane, corn, and basically any that have vertical veins.

Crassulacean acid metabolism (CAM) used by certain plants to optimize water use during photosynthesis, allowing them to open their stomata at night to capture carbon dioxide while reducing water loss during the hotter daytime.
CAM plants open their stomata at night, and incorporate CO2 into organic acids, which are then used during the day for photosynthesis, enabling them to thrive in arid environments.
- Organic Acid: A type of acid that contains carbon, commonly found in living organisms; in CAM plants, these acids store captured carbon dioxide, which is utilized for photosynthesis during the day.
Stomata close during day, and CO2 is released from organic acids and use din Calvin Cycle.