Light Energy and Photosynthetic Pigments

Light Energy and Photosynthetic Pigments

•All photosynthesis reactions occur within the chloroplasts

•Partly within the stroma and partly within thylakoid membranes

•Chloroplasts contain their own DNA and ribosomes and are able to replicate by fission

The stages• Stage 1: Capturing light energy• Stage 2: Using captured light energy to make ATP and

reduce NADP+ to NADPH • Stage 3: Using the free energy of ATP and the reducing

power of NADPH to make glucose and oxygen

How does this occur

• Various forms of radiation surround us, from the sun and other sources.

• Some are visible and some are invisible.

Wave model of light

• Electromagnetic radiation travels at 300000000m/s

• Frequencies of visible radiation (light) are perceived as different colours We can remember the visible spectrum with ROYGBIV!

• Frequencies of visible radiation (light) are perceived as different colours.

•Highest frequency, smallest wavelength = violet

•Lowest frequency, largest wavelength = red

•All frequencies and wavelengths = white

Light• Electromagnetic radiation, travelling at 3x108 m/s• Exhibits properties of waves and photons (particles)• Wavelength is inversely proportional to its energy Visible

light ranges from 400 to 700 nm

Properties of Light

• http://wps.prenhall.com/esm_krogh_biology_3/0,8750,1135943-,00.html

• http://www.sumanasinc.com/webcontent/animations/content/harvestinglight.html

How Does a Plant Capture Light?Light can be

• transmitted (light passes through an object.

• Reflected (light bounces off object)

• Absorbed (light goes into object)

How Does a Plant Capture Light?

• Plants have chlorophyll PIGMENTS (molecules that can absorb specific wavelengths of light)

Plant leaves appear green. Therefore, what colours must the chlorophyll pigments absorb? reflect?

GREENEverything but Green

http://www.johnkyrk.com/photosynthesis.html

Absorption spectrumGraph that illustrates the wavelengths of light absorbed by a pigment

Light cont’d

• Photosystems absorb particular wavelengths and transfer their energy to ADP, Pi and NADP+ forming ATP and NADPH

• 2 types of reactions occur in photosynthesis; the light reactions and carbon fixation

• Light reactions: only take place when light available , not affected by changes in temperature, use light and water, produce NADPH and ATP

• Carbon fixation: dependent on NADPH and ATP, therefore on light reactions, varies with temperature not intensity of light

Chlorophyll and Accessory Pigments

• Chlorophyll a is the only pigment that can transfer light energy to the carbon fixation reactions of photosynthesis

• Chlorophyll b and carotenoids acts as accessory pigments, absorbing wavelengths that a cannot

• Carotenoids: (ex β- carotene) possess 2 hydrocarbon rings connected by hydrocarbon chain

Accessory pigments cont’d

• Xantophylls – produce yellow color• Carotenoids – produce yellow-orange colour

– Interspersed within thylakoid membrane

• Anthocyanins – produce red, violet, blue colour– Located in plant cell vacuoles

– Photosynthetically active radiation (PAR) – wavelengths between 400 nm – 700nm support photosynthesis