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 PIGMENTS We 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) Cereals b) Potato c) Roots d) 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? 12% reflected
100% light 83 % absorbed. a) ONLY 4% used for photosynthesis b)Remaining 79% lost as heat energy
5% transmitted Sample Calculation How do plants change sunlight into food? Photosynthesis
SERIES of enzyme controlled reactions using special pigments inside the leaf to ABSORB light energy chemical energy (carbohydrate). Photosynthesis
light energy chemical energy (carbohydrate). Photosynthetic Pigments
Absorb light, allowing carbohydrates to be made
Principal Pigment Chlorophyll a
Accessory Pigments Chlorophyll b Carotene Xanthophyll 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 Carotenoids Carotene ( Orange ) Most soluble
Xanthophyll ( Yellow )
Chlorophyll a ( Blue-green ) Least soluble Chlorophyll b ( Dark green )
Spot Line
pigment spot Calculting Rf
Distance travelled by pigment Total distance travelled Calculting Rf
Distance travelled by pigment Total distance travelled
Red pigment 7.5/10 = 0.75
Blue pigment 2.5/10 = 0.25
Starter: Light Discussion
• What is light
• What colour is light?
• What are the colours of the rainbow? PHOTOSYNTHESIS LIGHT AND PIGMENTS We 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 light Most 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 Plant Result The reflected wavelengths are mainly green. This is why chlorophyll appears green to the eye.
Spectrum 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 b complex molecules containing Magnesium. absorb light energy in red/blue regions Used for photosynthesis.
2.Carotene & xanthophyll absorb 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 of light.
The higher the rate, the more effective the wavelength of 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 light absorbed by pigment
Action spectrum – colours of light where photosynthesis occurs
(does not exactly match absorption due to accessory pigments). `