Chapter 7: How Cells Capture Light Energy via Photosynthesis

Overview of Photosynthesis

Captures light energy to synthesize carbohydrates

• CO2 reduced

• H2O is oxidized

Equation for Photosynthesis

6CO2 + 12H2O + Light Energy ——> C6H12O6 + 6O2 + 6H2O

• light energy drives this endergonic reaction

Photosynthesis occurs where?

Chloroplast of plants and algae

Biosphere

Regions on Earth’s surface and atmosphere where life exists

Organisms in Photosynthesis

Autotrophs and heterotrophs

Autotrophs

Create organic molecules from inorganic sources; photoautotrophs use light

Heterotrophs

Consume food to acquire organic molecules, relying on other organisms for energy.

Stomata

Small openings on plant leaves that facilitate gas exchange during photosynthesis

• openings allowing CO2 and O2 passage

Chlorplast structure

• Outer membrane

• Inner membrane

• Stroma

• Thylakoid membranes (forming grana)

• Thylakoid lumen

Two stages of Photosynthesis

Light reactions and the Calvin cycle

Light Reactions

Energy conversions from light to chemical energy in ATP and NADPH.

ATP and NADPH

Energy for carbohydrate synthesis during the Calvin Cycle

Shorter wavelengths

Higher energy

Longer wavelengths

Lower energy

What happens when light encounters a molecule?

• it may pass through

• it may bounce off

• it may be absorbed

Pigments

Molecules that absorb light

Electron excitation

Unstable state after energy absorption, resulting in an electron being raised to a higher energy level

Chlorophylls

A group of green pigments responsible for absorbing light energy for photosynthesis.

Molecular Features of Photosystems

PS I and PS II have light-harvesting and reaction center complexes. PS II oxidizes water, producing O2 and H+

Photosynthesis I (PS I)

• Discovered first

• make NADPH

• ATP production in the chloroplast

Photosynthesis II (PS II)

• initiaties first for photosynthesis

• Absorbs light, splits water - Produces O2, H+ ions

• Releases energized electrons to electron transport (ETC)

Synthesizing Carbohydrates

ATP and NADPH from light reactions power the Calvin cycle to make carbohydrates.

Calvin Cycle

Incorporates atmospheric CO_2 into organic molecules to produce glyceraldehyde-3-phosphate (G3P).

First stage of the Calvin Cycle

Carbon fixation: CO2 is attached to RuBP (ribulose biphosphate) by the enzyme RuBisCO, resulting in a 6-carbon that immediately splits into two 3-phosphoglycerate (3-PGA) molecules.

Second stage of the Calving Cycle

Reduction and carbohydrate production: ATP energy and NADPH electrons produce G3P

Third stage of the Calvin Cycle

Regeneration of RuBp: Most G3P regenerates RuBP, allowing the cycle to continue.

Environmental factors that alter the Calvin Cycle

Temperature, water availability, and light intensity

C3 Plant

A plant where the first molecule with incorporated CO2 is a 3-carbon molecule (about 90% of plants are C?)

Photorespiration

When RuBisCO adds O2 to RuBP instead of CO2, releasing CO2 (it’s wasteful bc it limits plant growth)

Photorespiration occurs more in hot/dry environment

Photorespiration occurs more in _ environment bc RuBisCO can add O2 to RuBP when CO2 is low and O2 is high

C4 Plant

A plant where oxaloacetate (4-carbon molecules) is produced first during carbon fixation

C4 plants minimized photorespiration

They have a two-cell layer organization: mesophyll cells capture CO2 and convert it to oxaloacetate, which is then transported to bundle sheath cells where the Calvin cycle occurs.

CAM plants

Plants that separate the processes of carbon capture and the Calvin cycle temporally (by time)

CAM plants capture and store CO2

The Stomata opens at night to capture and store CO2

CAM plants conserve water during the day

The Stomata close during the day, and stored CO2 is released for the Calvin cycle

The type of plant is best in cooler climates

C3 plants is best at this climate because they use less energy to fix CO2

Why do C4 and CAM plant adaptations exist?

To help plants in hot/dry environments conserve water and minimize photorespiration