Photosynthesis

Page
4

Pigments are substances that absorb visible light

Different pigments absorb different wavelengths

Wavelengths that are not absorbed are reflected or transmitted

Leaves appear green because chlorophyll reflects and transmits green light

Animation: Light and Pigments

Slide 33

Fig. 10-7

Reflected

light

Absorbed

light

Light

Chloroplast

Transmitted

light

Granum

Slide 34

A spectrophotometer measures a pigment’s ability to absorb various wavelengths

This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

Slide 35

Fig. 10-8

Galvanometer

Slit moves to

pass light

of selected

wavelength

White

light

Green

light

Blue

light

The low transmittance

(high absorption)

reading indicates that

chlorophyll absorbs

most blue light.

The high transmittance

(low absorption)

reading indicates that

chlorophyll absorbs

very little green light.

Refracting

prism

Photoelectric

tube

Chlorophyll

solution

TECHNIQUE

1

2

3

4

Slide 36

An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

Slide 37

Fig. 10-9

Wavelength of light (nm)

(b) Action spectrum

(a) Absorption spectra

(c) Engelmann’s

experiment

Aerobic bacteria

RESULTS

Rate of photosynthesis

(measured by O2 release)

Absorption of light by

chloroplast pigments

Filament

of alga

Chloro-

phyll a

Chlorophyll b

Carotenoids

500

400

600

700

700

600

500

400

Slide 38

The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann

In his experiment, he exposed different segments of a filamentous alga to different wavelengths

Areas receiving wavelengths favorable to photosynthesis produced excess O2

He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production

Slide 39

Chlorophyll a is the main photosynthetic pigment