Photosynthesis and the Light-Dependent Reactions

All biological energy input on Earth originates from the sun.
Energy transfer efficiency is extremely low: only $1\%$ of solar energy enters the atmosphere ( $99\%$ is lost as heat), and plants utilize only about $1\%$ of that remaining $1\%$ .
Primary producers converting solar energy into glucose comprise three groups:
- Plants.
- Algae and photosynthetic protists (possess chloroplasts).
- Cyanobacteria (blue-green algae; prokaryotes lacking chloroplasts that utilize chlorophyll and pigments).
Fossil fuels represent ancient solar energy stored within the chemical bonds of fossilized plant matter.

Photosynthesis occurs in leaf tissue, specifically the palisade mesophyll located on the sun-facing upper surface.
Gas exchange is regulated by microscopic pores called stomata (singular: stoma) controlled by guard cells:
- Carbon dioxide ( $CO_2$ ) enters through stomata, while oxygen ( $O_2$ ) exits.
- High internal concentrations of $O_2$ are toxic to plant tissues and short-circuit photosynthesis.
- Stomata regulate transpiration to maintain central vacuole turgor pressure.
Chloroplast structure:
- Bounded by a double membrane and contains its own DNA.
- Thylakoids are individual flattened membrane discs embedded with photosystems.
- Grana are organized stacks of thylakoids.
- Stroma is the semifluid matrix surrounding the thylakoid network (analogous to the mitochondrial matrix).

Photosynthesis is an endergonic reaction requiring light energy input to form high-energy products from low-energy substrates: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
Photosynthetic pigments absorb specific wavelengths within the visible light spectrum ( $400\,nm$ to $740\,nm$ ):
- Chlorophyll $a$ : The primary photosynthetic pigment capable of directly converting light energy into chemical energy; features a central magnesium ( $Mg$ ) ion, absorbing blue-violet ( $380\,nm$ to $450\,nm$ ) and red ( $650\,nm$ to $750\,nm$ ) wavelengths while reflecting green.
- Chlorophyll $b$ : Accessory pigment absorbing orange and blue wavelengths.
- Carotenoids: Yellow and orange accessory pigments that absorb broader energy ranges and function as antioxidants.
- Phycobilin proteins: Accessory pigments utilized by marine algae to capture diluted light in low-light ocean depths.

The light-dependent reactions take place across the thylakoid membrane, whereas the light-independent reactions (Calvin cycle) take place in the stroma.
Photosystem II operates first in sequence:
- Pigment antenna complexes absorb photons and funnel excitation energy to the reaction center.
- A water-splitting enzyme hydrolyzes $H_2O$ into electrons, protons ( $H^+$ ), and $O_2$ gas.
- High-energy electrons enter an electron transport chain, releasing energy to pump protons across the thylakoid membrane into the thylakoid lumen.
- Chemiosmosis: The accumulated proton gradient drives $H^+$ through $ATP$ synthase into the stroma, phosphorylating $ADP$ to synthesize $ATP$ .
Photosystem I operates downstream of Photosystem II:
- Receives de-energized electrons delivered by the first electron transport chain.
- Absorption of additional photons re-excites reaction center electrons down a second electron transport chain.
- The terminal electron acceptor $NADP^+$ combines with electrons and stroma protons to produce $NADPH$ .
The resulting $ATP$ and $NADPH$ are translocated to the stroma to power carbohydrate synthesis in the Calvin cycle.

Why is the palisade mesophyll restricted primarily to one side of the leaf?
- Leaves act like solar panels; direct sunlight strikes predominantly the upper surface, meaning the underside receives insufficient light to justify photosynthetic tissue.
What vital substance can exit open stomata against the plant's preference?
- Water. Excessive transpiration depletes vacuole turgor pressure, requiring plants to balance $CO_2$ intake against dehydration.
Is the photosynthetic equation endergonic or exergonic?
- Endergonic. It requires an initial input of solar energy, producing glucose that holds higher chemical potential energy than the carbon dioxide and water reactants.
Why do leaves change color in autumn?
- Leaves begin dying during dehiscence. Chlorophyll degrades first, unmasking underlying carotenoids, xanthophylls, and anthocyanins before these secondary pigments also decay.