Unit C

Module 5 ~ Lesson 2


Read  "The Light-Dependent Reactions of Photosynthesis" on  page 170 of the textbook. Make note of the graphs and diagrams on  page 171.

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In Lesson 1 you studied the chloroplast and learned to identify its structures and functions. As you begin to explore the light-dependent reactions of photosynthesis, you will be focusing on the reactions that happen in the thylakoid membrane.

The flattened thylakoids in the chloroplasts of plant cells are stacked into columns called grana. Surrounding the thylakoids and filling the interior of the chloroplast is the fluid stroma, which is where the chemical reactions that synthesize carbohydrates take place.

Chlorophyll is the light-capturing pigment that absorbs all wavelengths of light except for green and some yellow. 

 Light Absorbance

When you look at an object, you see the colours that were not absorbed by the object.

This absorbance spectrum for three photosynthetic pigments shows that each pigment absorbs a different combination of colours of light.


Plants appear green because chlorophyll reflects the wavelength of light that produces green light and absorbs the other wavelengths.
Chlorophyll a absorbs more indigo, violet, and red wavelengths. Chlorophyll b absorbs more blue and orange wavelengths. Beta carotene absorbs violet, indigo, blue, and green wavelengths, but not yellow, orange, or red wavelengths.
Having a variety of pigments enables a plant to use a greater percentage of the Sun's light.
Refer to the D portion of Figure 5.9 on page 171. Oxygen production is the lowest between 500 and 550 nm (green-light wavelength). Oxygen production is highest between 400 and 475 nm (indigo/blue-light wavelengths).