Starter: Recapping National 5
• What is the word equation for photosynthesis?
• What are the 2 stages of photosynthesis?
• What 2 processes occur in the light dependent stage?
• What happens in the Calvin Cycle/Carbon fixation?
PHOTOSYNTHESIS LIGHT AND PIGMENTSWe are learning to
• Understand how light is used in photosynthesis.
I can
Identify the fates of light falling on a plant leaf.
Name the pigments used to trap light energy.
Photosynthesis
All food production is dependent ultimately upon photosynthesis.
Most human food comes from a small number of plant crops
a) Cerealsb) Potatoc) Rootsd) legumes).
Light energy in Photosynthesis
When light strikes a leaf there are
3 fates of the outcomes
Light can be ….
1. Reflected - Bounce off the surface.
2. Absorbed – pass into leaf & be converted into other energy.
3. Transmitted - Pass through the leaf unaffected.
Name these processes?
5% transmitted
12% reflected
83 % absorbed.a) ONLY 4% used for photosynthesis
b)Remaining 79% lost as heat energy
100% light
Sample Calculation
How do plants change sunlight into food?
Photosynthesis
light energy chemical energy (carbohydrate).
SERIES of enzyme controlled reactions using special pigments inside the leaf to ABSORB
Photosynthesis
light energy chemical energy (carbohydrate).
Photosynthetic Pigments
Absorb light, allowing carbohydrates to be made
Principal PigmentChlorophyll a
Accessory PigmentsChlorophyll bCaroteneXanthophyll
Extracting Pigments
Chromatography
• Method for separating the 4 Pigments by their SOLUBILITY
• The more soluble the pigment, the further it travel up the paper.
Chromatography Twig Clip
Chromatography Practical
Extraction of Pigment
1. Grind plant tissue with sand & propanone
• Break open cells & release pigment
• Propanone to dissolve pigment
2. Filter mixture
• Remove sand & debris
Separation of Pigment
1. Repeatedly spot extract onto chromatography paper
Ensure sufficient mixture is present
2. Spot above solvent level
Prevents pigment dissolving out of paper
3. Run chromatogram in petroleum ether solvent
Separate pigment As pigments have different solubilities in this solvent
4. Ensure paper does not touch sides
Ensure solvent moves evenly up paper
Photosynthetic Pigments
Solvent Front
Spot Line
pigment spot
Carotene ( Orange )
Xanthophyll ( Yellow )
Chlorophyll a ( Blue-green )
Chlorophyll b ( Dark green )
CarotenoidsMost soluble
Least soluble
Calculting Rf
Distance travelled by pigment
Total distance travelled
Calculting RfDistance travelled by pigment
Total distance travelled
Red pigment7.5/10 = 0.75
Blue pigment2.5/10 = 0.25
Starter: Light Discussion
• What is light
• What colour is light?
• What are the colours of the rainbow?
PHOTOSYNTHESIS LIGHT AND PIGMENTSWe are learning to
• Describe the difference between an absorption spectra of the different pigments and the action spectra of photosynthesis.
I can
• Identify the absorption and action spectra from a graph.
• Analyse and explain what they show.
• Compare them
Spectrum of lightMost people cannot see below 400nm or above 700nm
(visible part of the spectrum).
Prism Activity
ROYGBIV
Light Spectrum• When a beam of light passes through a prism the
VISIBLE spectrum of light is produced.
The colour that we see, is the colour that has been reflected i.e. NOT ABSORBED
If a colour has been absorbed it appears black to the human eye
Spectrum
Plant Absorption Spectrum Photosynthetic Pigments
All these colours have been absorbed by the pigment.
The colour we see is Green/blue - light that has been reflected/transmitted
Spectrum
The reflected wavelengths are mainly green. This is why chlorophyll appears
green to the eye.
Plant Result
Absorption Spectra• The degree of absorption at each wavelength of light
can be measured using a spectrometer.
• The data is then used to plot a graph (Absorption
Spectra) for each pigment.
Role of pigments
1. Chlorophyll a and bcomplex molecules containing
Magnesium.absorb light energy in red/blue
regionsUsed for photosynthesis.
2.Carotene & xanthophyllabsorb light mainly in green
part of the spectrum.They pass on the energy to
chlorophyll.
Name this pigment?
Why have accessory pigments
Broaden the absorption spectrum and pass energy onto chlorophyll
Action Spectrum
Rate of photosynthesis at each wavelength oflight.
The higher the rate, the more effective the wavelengthof light at providing energy for photosynthesis
Action Spectrum
Comparing Absorption & Action spectra
Exam Tip
Be careful not to mix up absorption spectrum.
Absorption spectrum –colours of lightabsorbed by pigment
Action spectrum – colours of light wherephotosynthesis occurs
(does not exactly match absorption due toaccessory pigments).
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