Capturing Light Energy and the Electromagnetic Spectrum

The Big Picture: The Conversion of Light Energy• Light Reactions of Photosynthesis convert light energy

(from the sun) into chemical energy (ATP and NADPH)

• Chloroplasts in plant cells act as chemical factories powered by the sun– The thylakoids in the

chloroplast convert the light energy into chemical energy

• To understand the conversion… we are going to take a look at some important properties of light!

Sunlight (Contains Visible Light)

• Sunlight is a form of electromagnetic energy!

• Photosynthesis transforms (converts) electromagnetic energy into chemical energy of organic molecules (sugars)

• Sunlight is part of the electromagnetic spectrum.

Sunlight –Contains Visible Light

• Sunlight (contains Visible Light) is a form of electromagnetic energy.

• Electromagnetic energy travels as a wave, but is composed of particle-like bundles of energy.

• Sunlight travels as a wave and contains bundles of energy known as photons.

Wave Theory• Light travels through space as a wave

• Waves have the following characteristics:

– Wavelength- the distance between the crests of electromagnetic waves

• Energy- shorter the wavelength = the greater the energy of each photon

– Frequency- a measure of the number of wavelengths in a given amount of time

Sunlight– Wave Behavior

Sunlight Light – Particle behavior

• Light behaves as though it consists of particles called photons– Each photon has a fixed quantity of energy

• Amount of energy is inversely related to the wavelength of light– The shorter the wavelength = the greater the

energy of each photon of that light

– Ex: photon of violet light packs almost twice as much energy as a photon of red light

Sunlight and The Electromagnetic Spectrum

• Electromagnetic Spectrum – the entire range of radiation.

• Visible Light – the segment most important to life– the narrow band from about 380nm to 750nm in wavelength– it is detected as various colors by the human eye

• The sun radiates the full spectrum of electromagnetic energy, but atmosphere is selective and allows only visible light to pass through.

Visible light = part of the spectrum we can see = drives photosynthesis

The Electromagnetic Spectrum

White light is a mixture of all wavelengths of visible light. A prism can sort white light into its component colors.

Pigments• Light may be either:

reflected, transmitted, or absorbed.

• Pigments- substances that absorb visible light.

• The color we see an object to be is the color most reflected or transmitted by the pigment.

• EX: Leaves look green because the main pigment in them (chlorophyll) absorbs violet-blue and red light while it transmits/reflects green light.

Absorption Spectrum

• Absorption Spectrum – is a graph that plots a pigment’s light absorption versus wavelength of light.

The three pigments shown below differ in the colors of light they absorb.

Absorption Spectrum

• The three pigments shown in the prior graph differ in the colors of light they absorb.

• Where a curve has a peak is where much of the light at that wavelength is absorbed

• Where there is a trough, much of the light at that wavelength is reflected or transmitted. (what we see)

Reexamine the Absorption Spectrum

Light and Pigments

• Pigments- compounds that absorb light/ most absorb certain colors more strongly than others.

• Several pigments are located in the membrane of the thylakoid:– Chlorophyll a– Chlorophyll b– Carotenoids

Chlorophyll a

• Absorbs violet-blue and red light

• Allows green light to be reflected/transmitted

• Is the main pigment within the photosystems.

• Directly involved in the light reactions of photosynthesis

Accessory Pigments

• Pigments with different absorption spectra than chlorophyll a.

• Are clustered with chlorophyll a within photosystems.

• Help the leaves to capture the maximum amount of light energy– Chlorophyll b– Carotenoids

Chlorophyll b

• An accessory pigment– Assists chlorophyll a in capturing light energy

• Almost identical to chlorophyll a but there is a slight structural difference

• Absorbs blue light

Carotenoids

• Yellow, orange, and brown

• Accessory pigments

• Absorbs colors that chlorophyll a cannot – enables plant to capture more energy

• In fall when plants lose chlorophylls their leaves take on the rich hues of the carotenoids

When Pigments Absorb Light

• Colors corresponding to the absorbed wavelengths disappear from the spectrum

• When a molecule absorbs a photon of light, one of the molecule’s electrons is elevated (excited) to where it has more potential energy

• Each pigment absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum

• Excited state is unstable so the excited electrons drop back down releasing their excess energy.

• As excited electrons drop back down, energy is given off.

Photosystems:

Cluster of pigments and proteins embedded within the thylakoid membrane.

Contain accessory pigments that surround chlorophyll a, the main pigment of photosynthesis.

Are able to absorb the maximum amount of light possible due to the accessory pigments and chlorophyll a.

Photosystems

• Photosystems are found embedded within the thylakoid membrane of the chloroplast.

Photosystems: Are embedded in the Thylaoid Membrane of the Chloroplast.

Next time – Photosystems and Chloroplasts!!!