Photosynthesis Notes

C1: Photosynthesis

Plants, algae, and some bacteria undergo photosynthesis.
Photosynthesis takes place in the chloroplasts of plants.
Two types of reactions allow for photosynthesis to occur: light-dependent and light-independent reactions.
Photosynthesis combines carbon dioxide, water, and energy from the sun to synthesize glucose.

Chloroplasts are the site of photosynthesis.
They are bound by an inner and outer membrane.
The inner fluid of the chloroplast is known as the stroma, which contains a concentrated mixture of proteins and other chemicals used in the synthesis of glucose.
Flattened, interconnected sacs called thylakoids contain chlorophyll.
Chlorophyll is a green-colored molecule (pigment) responsible for trapping solar energy.
Thylakoids are stacked in structures called grana, which are connected by lamellae.

Occur in the thylakoid membrane of the chloroplast.
Stage 1: Capturing solar energy and transferring it to electrons through splitting of water.
Stage 2: Using energy to make ATP and transferring electrons to make NADPH.

i. Photosystems
ii. Oxidation and Reduction Reactions
iii. Electron Transport Chain

Photosystems are clusters of chlorophyll and other pigments embedded in the thylakoid membrane.
Chloroplasts have two different photosystems:
- Photosystem I (PSI)
- Photosystem II (PSII) - Comes first in the light reaction!

Light strikes chloroplasts, and photons excite chlorophyll molecules imbedded in thylakoid membranes.
Chlorophyll captures light energy by absorbing photons and passing the energy to electrons.
Energy is transferred to the reaction center.
Light energy causes photolysis (splits water).
Photolysis occurs in the thylakoid lumen, where water is divided into hydrogen ions, oxygen, and electrons.
Oxygen is made and released into the atmosphere (leaves through the stomata!).
H+ ions are formed and remain in the lumen.

Electrons are excited and move from the lumen to chlorophyll molecules in photosystem II.
Electrons are then transferred to photosystem I via the electron transport chain.

Found along the thylakoid membrane.
Electrons move along this membrane, releasing energy as they move.
Movement occurs in a “step-by-step” way.
Electrons pass from one carrier to another, moving down the ETC as redox reactions occur.

Electrons move from PSII to PSI using the ETC.
Every step down the excited electron takes, it gives up some stored (potential) energy.
This energy is used to make ATP.
Once the electron reaches PSI, it is hit with light again and excited.
The energy released by photosystem I is used to rejoin the high-energy electrons with the hydrogen ions and NADP^+ to produce NADPH, the final electron acceptor.

H^+ ions would likely diffuse back across, but the membrane is impermeable to them.
When the electrons are passed down the ETC from PSII to PSI, the energy released is used to pull H^+ ions into the thylakoid lumen against the concentration gradient.
This creates a positive charge in the thylakoid lumen and a steep concentration gradient.
ATP synthase, embedded in the thylakoid membrane, provides the only pathway for the H^+ ions to move out.
The process for synthesizing ATP using the energy from a H^+ gradient and the ATP synthase enzyme is called CHEMIOSMOSIS.

Photosynthesis uses electrons to transfer energy.
Electrons are passed around in the process.
Something loses the electrons, and something else gains them.
When electrons transfer in a chemical equation, this is called a redox reaction. Chemical Equation example: Xe^- + Y \rightarrow X + Ye^-

LEO the lion says GER!
- LEO: Oxidation = LOSS of electrons (reducing agent).
- GER: Reduction = GAIN of electrons (oxidizing agent).