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 Which other organelles contain their own DNA – mitochondria. In what other way are they similar to chloroplasts?

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 Stage 1 and 2 occur in thylakoid membranes – require chlorophyll, directly energized by light called light reactions Absorb the light energy that is eventually transferred to carbohydrate molecules in stage 3 NADP+ = nicotinamide adenine dinucleotide phpshpate – the energy shuttling conenzyme, reduced by hydrogen atoms to NADPH QUESTION which molecule is NADPH similar to from cell resp?(similar to NADH in cell resp.) Rxns of 3rd stage result carbon fixation occur in the stroma, in the calvin cycle (blue cycle)– incorporating the carbon of CO2 into organic compounds Require energy therefore endergonic Stage 3 used to be called dark reactions – misnomer: because many enzymes that catalyze these reactions are activated by light and are inactive in the dark, also ATP and NADPH are formed in light reactions and are necessary for stage 3 to occur

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 (ex. short wavelengths possess high E) Long wavelengths possess low energy When passed through transparent prism in spectroscope, photons separate according to their energies forming the EM spectrum Most photons are invisible  cosmic, x-rays, UV, IR, Heat, Radio Do red wavelengths have more energy or Blue?  Blue because shorter Wavelengths

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? Everything but Green GREEN http://www.johnkyrk.com/photosynthesis.html

Absorption spectrum Graph that illustrates the wavelengths of light absorbed by a pigment -Action spectrum illustrates the effectiveness with which different wavelengths of light promote photosynthesis – where it occurs at the greatest amounts. – paralleled by the absorption spectrum – therefore most of the wavelengths of light absorbed by chlorophylls are used in photosynthesis This one is of chlorophyll QUESTION – what energies does chlorophyll pigment absorb photons at? absorbs photons with energies in the blue-violet and red regions of the spectrum QUESTION what part of the spectrum does it reflect? – between 500 – 600nm ; green

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 Photosystems are clusters of photosynthetic pigments

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 Hydrocarbon chain contains alternating single and double bonds, QUESTION: What bonding system other than B-carotene possesses alternation of single and double bonds? (where electrons are delocalized so they absorb light energy?)  the bonding system in the porphyrin ring in chlorophyll electrons in the chain are able to absorb light energy in the range 400-500nm Reflect and appear yellow –orange Play energy absorbing role vs photosynthetic – absorb light that would damage chlorophyll, lose energy as heat Carrots rich in Bcarotene protects eyes from photon damage B-carotene precursor to vitamin a, needed to make rhodopsin – photopigment helps vertebrates see in low light conditions

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 Xantophylls Carotenoids are overwhelmed by the green light reflected by chlorophyll, with the onset of cold temperatures of fall, leaves stop synthesizing chlorophyll molecules and break down those already in the leaves so the other pigments show through, initiation of cell death, therefore eventually leaves fall after colour has changed Chlorophylls a and b combined with accessory pigments cover wavelengths from 400nm to 700nm – Photosynthetically active radiation (PAR)