• Photosynthesis: the process of light energy being converted into chemical energy

• Two stages of photosynthesis

  • Light reactions: light energy is absorbed by pigments and converted into two forms of chemical energy

    • ATP & NADPH

  • Calvin Cycle: ATP and NADPH drive synthesis of carbohydrates

7.1 Overview of Photosynthesis

• Photosynthesis is carried out by certain plants, bacteria, and algae

• General equation of photosynthesis

  • 6CO2 + 2H2O + Light Energy = C6H12O6 + 6 O2

• Energy from the sun drives photosynthesis

Photosynthesis powers biosphere

• Organisms organized into two categories

  • Autotrophs: organisms that make their own food

    • Photoautotrophs: organisms that use light energy to produce food

  • Heterotrophs: organisms that consume food to live

Photosynthesis occurs in chloroplast

• Chloroplasts contain a pigment called Chlorophyll

• Mesophyll are tissues in plants that contain cells with chloroplasts

• CO2 and O2 enter and exit via stomata

• Chloroplast have three membranes

  • Inner and Outer membrane

  • Thylakoid Membrane: contains pigment molecules, forms structures called thylakoid

• Stroma is fluid filled region between thylakoid and inner membrane

Photosynthesis stages

• Light dependent stages: occurs in the thylakoid membrane

• Light independent stage (Calvin Cycle): occurs in the stroma

• ATP and NADPH are energy intermediates that provide the needed energy and electrons to make carbohydrates during Calvin cycle

7.2 Reactions

• Light energy is transferred to a pigment in a photosynthetic organism

Light energy is a form of electromagnetic radiation

• Electromagnetic radiation travels as waves

• wavelength: the distance between peaks in a wave pattern

• electromagnetic spectrum: encompasses all possible wavelengths of electromagnetic spectrum

• Light has properties of particles

  • Photons

    

Pigments absorb light energy

• Three things can happen when light touches an object

  • Light passes through the object

  • Object may change the path of light

  • Object may absorb the light

• Pigment: a molecule that absorbs light energy

• Light energy absorbed by pigments can boost an electron to a higher energy level

  • called excited state

    • this is an unstable condition, releases energy in different ways

      • can release hear

      • release energy in the form of light

      • can transfer extra energy to an electron nearby

        • this is called resonance energy transfer

      • Certain photosynthetic pigments can transfer the excited electron to another molecule

Plants contain different types of Pigments

• Chlorophyll

  • Two types of pigments (Chlorophyll a & Chlorophyll b)

  • Reflects green light

  • Bound to thylakoid membrane

• Carotenoids

  • Another pigment

  • Reflects red, yellows, and oranges

• Absorption Spectra: graph that plots a pigment’s light absorption as a function of the light’s wavelength

Photosystems II and I

• Thylakoid membranes contain two distinct complexes of protein and pigment molecules

  • PSII and PSI

• PSII is initial step in photosynthesis

  • light excites electrons in pigment molecules located in PSII’s light harvesting complex

  • Energy is transferred to P680, which passes excited electron to electron carriers

  • As this happens, water is oxidized to replace the removed electrons

    • this creates O2

• Through the Electron Transport Chain, the electrons pass through two complexes

  • This maintains the H+ gradient across the thylakoid membrane

• Reaches PSI

  • Main role is to synthesize NADPH

  • Electron is removed from P700 and passed to another electron carrier (Ferredoxin)

  • Fd carries electrons to NADP+ Reductase

    • Electrons are bonded to NADP+, creating NADPH

• Phosphorylation: process of ATP synthesis in Chloroplasts

  • ATP is created when H+ move through ATP synthase due to gradient created by the electrons moving through the ETC

• H+ electrochemical gradient is created in three ways

  • Oxidization of water, which places H+ in thylakoid lumen

  • Movement of electrons through ETC pumps H+ into thylakoid lumen

  • Formation of NADPH uses H+ from the stroma

    

PSI produces only ATP via cyclic electron flow

• Linear Electron Flow: mechanism that harvests light energy

  • electron flow is linear, produces ATP and NADPH in roughly equal amounts

• Cyclic Photophosphorylation: pattern of electron flow that produces only ATP

7.3 Molecular features of photosystems

• Electrons vary in energy level

  • Process is documented in Z Model

    • electron proceeds through a series of energy changes during photosynthesis

  • These events involve increases & decreases in energy of an electron

    • Electrons in a non-excited pigment molecule has the lowest energy

    • While in PSII, electron gets higher energy level

    • As electron travels from PSII to PSI, electron loses energy

    • Input of energy from PSI boosts electron energy level higher than PSII

    • Energy is released as electron is transferred to NADP+ reductase

      

7.4 Synthesizing Carbohydrates via Calvin Cycle

Takes CO2 and incorporates it into carbohydrates

Steps of Calvin Cycle

• Carbon Fixation: CO2 is incorporated into RuBp which is then catalyzed by Rubisco

• Reduction and Carbohydrate Production: ATP and NADPH converts product from carbon fixation into G3P, which will make glucose

• Regeneration of RuBP: regenerates the RuBP

  

7.5 Variations of Photosynthesis

Temperature, water availability, and light intensity alters the way the Calvin cycle operates

Photorespiration

• When CO2 is low and O2 levels are high, Rubisco will add O2 to RuBP

  • this is called Photorespiration and it is bad for plants

CAM Plants

• CAM plants can minimize photorespiration due to their unique leaf anatomy

• They work best in warm & arid climates as they can close their stomata to conserve water