Chapter 10

Concept 10.1

Photosynthesis: Conversion of energy from sunlight into chemical energy stored as sugars.
Autotrophs: Self-feeders that harness energy from sunlight or inorganic substances to produce their own food.
Heterotrophs: “other”-feeders that harness energy from other organisms to survive.

Chloroplasts: Organelles in plant cells where photosynthesis occurs, containing chlorophyll that captures light energy.
Chloroplasts absorbs energy from the sun and uses it to drive synthesis of organic compounds using CO2 and Water.

Mesophyll cells: Specialized plant cells located in interior of the leaf.
Stomata: Microscopic pores which allows exchange of CO2 from environment and O2 from plant.
Roots: Structures that anchor the plant and absorb water and nutrients from the soil, facilitating overall growth and development.
Thylakoids (pancakes): Membrane-bound compartments in chloroplasts where the light-dependent reactions of photosynthesis occur.
- Grana: Thylakoid stacks.
Stroma: The fluid-filled space surrounding the thylakoids in chloroplasts, where the light-independent reactions of photosynthesis take place, converting carbon dioxide and energy into glucose.
Chlorophyll: green pigment which gives leaves their color, residing inside thylakoid membranes of the chloroplast.

6 CO2 + 12H2O + Light Energy - > C6H12O6 (Glucose) + 6 O2 + 6 H2O
Product of photosynthesis is actually a three-carbon sugar, which is turned into glucose during the Calvin cycle.
CO2 + H2O → [CH2O] + O2

It was originally argued that O2 given off by plants was produced by splitting of water molecules, not CO2.
Original understanding was CO2 → C + O2 and then C + H2O → CH2O.

During cellular respiration, energy is released from sugar when electrons associated with protons (H+) are transported by ETC to oxygen, forming water as a by-product.
- As the went down the chain, electrons lost more and more potential energy which got harnessed and turned into ATP.
In Photosynthesis.. Water is split, and its electrons are transferred along with it’s hydrogen ions to carbon dioxide, reducing it to sugar.

Water is taken in by roots, and then split at chloroplasts, providing electrons and protons (H+) and forming O2 as a by product.
Light energy absorbed by chlorophyll drives transfer of electrons and protons to acceptor NADP+, which becomes reduced to NADPH.
Cytochrome Complex (an intermediate complex) acts as a proton pump, pushing protons into the thylakoid lumen, creating a proton gradient that is utilized by ATP synthase to generate ATP through photophosphorylation.

NADPH and ATP are present in this cycle before it begins due to the light-dependent reactions.
Cycle must spin THREE times in order for us to get our goal: Glyceraldehyde 3-phosphate (G3P), because it requires three molecules of CO2 to be fixed.

CO2 from air is attached to five-carbon sugar RuBP (ribulose bisphosphate).
Enzyme Rubisco (or RuBP carboxylase-oxygenase) catalyzes unstable six-carbon intermediate from RuBP.
Unstable intermediate breaks down into two molecules of 3-phosphoglcyerate (for each Co2 fixed).

Each molecule of 3-phosphoglycerate receives additional phosphate group from ATP, becoming 1, 3-phosphoglycerate.
NADPH reduces 1, 3-phosphoglycerate to glyceraldehyde 3-phosphate (G3P).
G3P is a three-carbon sugar. Three molecules of CO2 result in six G3P, however only one is net gained.. as the others are used in the rest of the cycle.
Cycle begun with 15 carbon’s worth

Carbon skeletons of the five other molecules of G3P are rearranged back into ribulose bisphosphate (RuBP), allowing the cycle to continue and facilitating the fixation of additional CO2 in subsequent rounds.
Three ATP get used in this process.

Rice, wheat and soybeans.
Photorespiration: Instead of CO2 used in the Calvin cycle, rubisco fixes oxygen, resulting in the production of a two-carbon compound called phosphoglycolate. This is later broken down by peroxisomes and mitochondria and CO2 is release.
- Photorespiration uses ATP rather than generates it, and produces no sugars. Detrimental to photosynthesis, photorespiration can significantly reduce plant productivity, especially in conditions of high temperature and low CO2 concentrations.
- Theories suggest it has evolved as a mechanism to protect plants from the damaging effects of excess light energy during stressful environmental conditions.

Fixes carbon into a 4-carbon compound before the Calvin cycle.
When weather is hot and dry, C4 plants partially shut stomata, conserving water but reducing CO2.
Bundle-Sheath cells: These specialized cells surround the vascular tissue in C4 plants and are the site of the Calvin cycle, allowing for efficient carbon fixation.
Mesophyll cells lay loosely arranged between the bundle-sheath cells and the leaf surface, facilitating gas exchange and the uptake of CO2 for photosynthesis.
CO2 is first incorporated into organic compounds, which then move to bundle-sheath cells.. where the Calvin cycle begins.
- PEP carboxylase (high affinity for CO2.. none for O2) adds CO2 to phosphoenolpyruvate (PEP), forming four-carbon product oxalacetate.
- CO2 is fixed in mesophyll cells then the four carbon products are exported to bundle-sheath cells throw plasmodesmata.
- Within bundle-sheath cells, an enzyme releases CO2 from four-carbon compounds; CO2 gets refixed into organic material by rubisco and Calvin cycle.. which regenerates pyruvate and it gets converted back to PEP by ATP.
DO NOT CONAIN PS II, therefore only cyclic electron flow allows photosynthesis to continue, ensuring the production of ATP needed for the conversion processes in the dark reactions.

Open stomata at night and close them during the day. Preserves water in desert.
Take in CO2 during night, and incorporate it into a variety of organic acids
CAM or Crassulacean acid metabolism is what the carbon fixation is called.
During the day, CO2 is released from these acids to allow for photosynthesis while sun permits.