Ch 6 – Photosynthesis

• process that converts light energy into chemical energy

• supports most life on Earth

• found in algae, plants, some bacteria & protista

• the summarized equation

6CO2 + 6H2O C6H12O6 + 6O2

Photosynthesis occurs inside the chloroplast on the membrane of the thylakoids and in the stroma.

Of the electromagnetic spectrum, photosynthesis only uses the visible spectrum (wavelengths we can see).

The most common colors that are absorbed by the pigments are reds and blues. The other colors are reflected thus making chloroplast green.

Photosynthesis is divided into 2 stages.

1. Light Reactions

• occurs only with light

• utilizes protons (H+), electrons (e-) & H2O to make cellular energy

2. Calvin Cycle

• occurs in light and dark

• utilizes CO2 & cellular energy to make organic compounds

The Light Reactions

• Light energy entering the chlorophyll pigments (embedded in Photosystem II) raise the available electrons (e-) to a higher energy level.

• These electrons are passed through special molecules in the membrane creating an Electron Transport Chain.

• This in turn moves protons (H+) into the thylakoid as the electrons lose their higher energy level.

• Next the electrons are passed to Photosystem I (more chlorophyll pigments) and the electrons are again raised to a higher energy level.

• Again the electrons are passed through an Electron Transport Chain this time bonding with NADP+ & a proton forming NADPH (a type of cellular energy).

• Since the electrons are lost at the end of second transport chain, new electrons must be found.

• Photolysis splits 2H2O molecules into O2, protons (4H+) and electrons (4e-).

• The last step is to use the concentration gradient of protons (H+) that has been created by the first electron transport chain & photolysis.

• Chemiosmosis uses the protons (H+) leaving the thylakoid as energy in ATP synthase (special protein) to attach a phosphate to ADP forming ATP.

ATP (adenosine triphoshate)

• Cellular energy

• ATP releases energy by breaking a bond as the 3rd phosphate group is removed.

NADP+

The Calvin Cycle

• Not driven by light, occurs day or night.

• CO2 enters the Calvin cycle and bonds (fixed) to a 5 carbon molecule RuBP. [carbon fixation]

• This new molecule is unstable and splits into two 3 carbon molecules 3-PGA.

• The 3-PGA are modified by the ATP & NADPH produced in the light-dependent reactions into G3P.

• One of the G3P leaves the cycle to form organic compounds (carbohydrates).

• The remaining G3P’s are converted back to RuBP by ATP thus starting the cycle again.