BISC 130 - Chapter 8: Photosynthesis

• Organisms can be heterotrophs or autotrophs.

• They include Photoautotrophs

  • Ex: Plants, some bacteria, some protists.

Photosynthesis:

• CO2 + H20 + light (energy) --> C6 H12 06 + 02 (glucose)

• *In Eukaryotes, most steps in chloroplasts.

• 2 parts in Photosynthesis:

  • Light-dependent reactions (LDR)

    • They require light

• Light-independent reactions (LIR)

  • They do not require light

• Visible light is a type of electromagnetic radiation that has a specific wavelength.

• Different colors of visible light have different wavelengths

• Light has a wave nature and a particle nature

  • Particles of light are called photons.

  • They carry energy

• Pigment

  • A molecule that is capable of absorbing certain wavelengths of light and reflecting others.

• Ex: Chlorophylls: harness energy from photos for photosynthesis.

• Ex: Carotenoids: They dispose of excess energy. They dissipate or get rid of any excess energy from photons to prevent any harm.

• Hundreds of chlorophylls and carotenoids are arranged into "light-harvesting complexes".

  • They are a part of photosystems.

The Light-Dependent Reactions:

1. Photons strike pigments in Photosystem II, and they excite electrons of Chlorophylls to a higher energy state

1. The energy of exicted electrons are passed from one chlorophyll to another until it reaches the Reaction Center (RC) Chlorophyll.

1. The RC Chlorophyll delivers its high energy electrons through an Electron Transport Chain.

   1. The RC Chlorophyll replaces the lost electrons from H2O. H2O ---> O2 + e-

1. High-energy electrons are passed through a series of carriers lowering energy with each passing.

   1. The energy used to pump H+

      1. Creates proton gradient

         1. ATP Synthase generates ATP.

1. Low energy electrons arrive at Photosystem I's RC Chlorophyll.

1. Photons excite electrons in Photosystem I's Chlorophylls

   1. Energy moves to RC Chlorophylls.

1. High-energy electrons are used to reduce NADP+ to NADPH (electron carriers)

LDR Overview:

H2O + Photons --> O2 + ATP + NADPH

The LIR:

• They take place in the stroma (cyptoplasm of Chloroplast)

• Steps 1-3 are called the Calvin Cycle:

1. An enzyme called RuBisCO (Rubisco) performs Carbon fixation by attaching CO2 to Ribulose Bisphosphate (5-Carbon), creating 3-Phosphoglyceric acid, aka 3-PGA (3-Carbon) x 2. 1 + 5 = 3 + 3

1. ATP and NADPH are used to reduce 3-PGA to create Glyceraldehyde 3-Phosphate (G3P), (3-Carbon).

1. ATP is used to regenerate Ribulose Bisphosphate from some G3P.

1. In Cyptoplasm, G3P used to build amino acids, lipids, sugars.

   1. Converted to glucose through a metabolic pathway called Glycolysis.

      1. Same enzyme from ceullular respiration, working in reverse.

LIR Overview

• CO2 + ATP + NADPH --> C6 H12 O6

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