Photosynthesis Notes

Photosynthesis Overview

• Photosynthesis converts solar energy into chemical energy within chloroplasts.
• Autotrophs produce organic molecules from $CO_2$ and other inorganic molecules; photoautotrophs use sunlight.
• Heterotrophs obtain organic material from other organisms, depending on photoautotrophs for food and $O_2$.

Chloroplast Structure and Function

• Leaves are major photosynthesis locations; chloroplasts are in mesophyll cells.
• A chloroplast has two membranes surrounding the stroma.
• Thylakoids are connected sacs, sometimes stacked in grana; chlorophyll resides in thylakoid membranes.

Photosynthesis Equation

• The summary equation for photosynthesis is:
  $6 CO<em>2 + 12 H</em>2O + Light Energy \rightarrow C<em>6H</em>{12}O<em>6 + 6 O</em>2 + 6 H_2O$
• Photosynthesis is a redox process where $H<em>2O$ is oxidized, and $CO</em>2$ is reduced; it's an endergonic process powered by light.

Light Reactions

• Light reactions split $H<em>2O$, release $O</em>2$, reduce $NADP^+$ to NADPH, and generate ATP via photophosphorylation in the thylakoids.

Calvin Cycle

• The Calvin cycle, in the stroma, forms sugar from $CO_2$ using ATP and NADPH, starting with carbon fixation.

Light and Pigments

• Light is electromagnetic energy that travels in rhythmic waves; wavelength determines energy type.
• Pigments absorb visible light; chlorophyll reflects green light.
• Chlorophyll a is the key light-capturing pigment; chlorophyll b and carotenoids are accessory pigments.

Photosystems

• A photosystem includes a reaction-center complex and light-harvesting complexes.
• Photosystem II (PS II) and Photosystem I (PS I) are two types, with P680 and P700 reaction centers, respectively.

Electron Flow

• Linear electron flow involves both photosystems, producing ATP and NADPH.
• Cyclic electron flow uses only PS I, producing ATP but not NADPH, and releases no oxygen.

Chemiosmosis

• Chemiosmosis in chloroplasts and mitochondria generates ATP using different energy sources.
• ATP and NADPH are produced in the stroma during light reactions.

Calvin Cycle Details

• The Calvin cycle regenerates its starting material and is anabolic.
• It has three phases: carbon fixation, reduction, and regeneration of RuBP producing glyceraldehyde 3-phosphate (G3P).

Alternative Carbon Fixation

• $C<em>4$ plants minimize photorespiration by incorporating $CO</em>2$ into four-carbon compounds in mesophyll cells, then releasing $CO_2$ in bundle-sheath cells for the Calvin cycle.
• CAM plants open stomata at night, fixing $CO<em>2$ into organic acids, then release $CO</em>2$ during the day for the Calvin cycle.

Importance of Photosynthesis

• Photosynthesis stores sunlight energy as chemical energy in organic compounds.
• Sugars provide chemical energy and carbon skeletons.
• Plants store excess sugar as starch.