Photosynthesis

Photosynthesis The process by which plants and other producers convert the energy of sunlight into the energy, stored in organic molecules. (Carbon Compounds) Plants and other photosynthetic organisms produce the food that start the food chain.

Pigment Chlorophyll For most plants chlorophyll is the main pigment Gives green color Photosystems – groups of pigment molecules

Types of Chlorophyll Pigments Chlorophyll a – absorbs mainly blue-violet and red light; reflects green light Chlorophyll b – (helper pigment) absorb mainly blue and orange light; reflects yellow-green (looks light frees) Carotenoids – (many types) absorb mainly blue- green light; reflect yellow-orange Xanthophyll - a yellow or brown carotenoid plant pigment that causes the autumn colors of leaves

Absorption Spectra

Electromagnetic Spectrum The range of types of electromagnetic energy; from the very short wavelength (gamma rays) to the very long wavelength (radio waves)

Electromagnetic Spectrum ROYGBIV

Electromagnetic Spectrum Plants use the same part of the electromagnetic spectrum that our eyes are able to see.

Electromagnetic Spectrum Those wavelength that your eyes can see as different colors Make a small fraction of the electromagnetic spectrum Shorter wavelengths (violet/indigo) have more energy than longer wavelengths (red) Actually shorter wavelengths can damage organic molecules like proteins and nucleic acids This is why U.V. rays cause sunburns and can lead to skin cancer

Electromagnetic Spectrum Substances can do one of only 2 things when struck by a particular wavelength of light: Absorbed Reflected Pigments in the leaf’s chloroplast absorb blue-violet and red very well

Wavelength

Chloroplast

Chloroplast

Photosynthesis ATP and H2 generated from light dependent stage used to covert CO2 and H2O into organic compounds (like glucose) 6CO2 + 6H2O C6H12O6 + 6O2 This carbon fixation…turning inorganic to organic Fixation requires energy—all comes from sun

Photosynthesis Two Main Stages Stage I Light Reaction (Light Dependent) Convert the energy in sunlight to chemical energy Takes place in the thylakoid membrane 1st – Chlorophyll molecules in the membrane capture light energy 2nd – Chloroplast use energy to remove eˉ (excites) from water (photolysis) This splits H⁺ + O₂ O₂ is the “waste product” for photosynthesis

Photosynthesis Stage 1 Photolysis – when a water molecule is split into its component elements Hydrogen and water Generates electrons in light dependent reaction

Results of Light Reaction – NADPH + ATP Photosynthesis Stage 1 O₂ escape into the atmosphere via the stoma (on leaves) 3rd – Chloroplast use H₂O eˉ and H⁺ ions to (make energy rich molecules) NADH found in cellular respiration. 4th – (finally) ATP is made in chloroplast Excited e⁻ from Photosystem II are used to generate a proton gradient Results of Light Reaction – NADPH + ATP

Photosynthesis Stage 1 Because of the proton gradient created by Photosystem II: ATP synthase located in the thylakoid membranes allows the protons to diffuse back across the membrane to the stroma and uses energy that the protons release as they diffuse down the concentration gradient to produce ATP This is called chemiosmosis Photophosphorylation – using light energy absorbed to create ATP

Photosynthesis Stage 1 Photosystem I A pair of excited e⁻ is emitted from the reaction center of photosystem I and passes a short chain of e⁻ acceptors. At the end of the chain e⁻ are passed to NADP in the stroma. NADP is reduced to NADPH

Side Bar 1st organisms to release O₂ in the atmosphere happened 2.4 – 2.2 billion years ago O₂ was at 2% This caused dissolved iron in the oceans to precipitate as iron oxide. Sank to the bottom of the oceans and formed banded iron formations 750 million yrs ago it was 30% now it’s 20%

Photosynthesis Stage 1 Transfer of excited electrons occurs between carriers in the thylakoid membrane

Photosynthesis Stage 1 In photosynthesis light-excited eˉ for the electron transport from chlorophyll travel down the chain. (P680/P700 are pigments)

Photosynthesis Stage 2 Stage II The Calvin Cycle (takes place in Stoma) Makes sugar from the atoms in CO₂ + the H ions and high-energy eˉ carried by NADPH Enzymes for the Calvin Cycle are located outside the thylakoids and are dissolved in the stoma ATP made in Light Reaction provides energy to make sugar (carbohydrates or other carbon compounds) Calvin Cycle AKA “Light Independent Reactions” because it does not require light to begin reaction

Photosynthesis Stage 2 ATP synthase in thylakoids generates ATP using the proton gradient.

Photosynthesis Stage 2 Carboxylation of RuBP Ribulose biphosphate (RuBP), 5 carbon compound, binds with CO₂ in a process called carbon fixation. Catalyzed by an enzyme called RuBP carboxylase (rubisco). Result: 6 carbon compound The unstable 6-carbon breaks down into 2 3 carbon compounds (Glycerate 3 – phosphate GP) GP = what we called G3P!!!!! Done by reducing NADP and ATP

Photosynthesis Stage 2 The 3 carbon molecules of GP are acted upon by ATP and NADPH from the light-dependent reaction to form 2 other 3-carbon molecules called triose phosphate (TP). This is a reduction reaction. The molecules of TP then do one of 2 things: some leave the cycle to become sugar phosphates that may become more complex carbohydrates. The others continue in the cycle and reproduce RuBP. ATP is used to regain RuBP

Calvin Cycle

The Calvin Cycle

Photosynthesis-rates Photosynthesis does not occur at such a steady rate Greatly affected by intensity of light, temperature, and CO2 levels Can be measured directly via CO2 intake and O2 production amounts IF adjusted to account for respiration Biomass (amount of plant/size) is an indirect method of measuring rate of photosynthesis—indirect b/c a lot of other potential factors

Photosynthesis O2 released + Photosynthesis Respiration O2 taken in -- Respiration O2 taken in -- Day 1 Night 1 Day 2 Night 2

Photosynthesis Light intensity—varies inversely with the square of the distance (farther away, less intense) At low light, the rate of photolysis is limited by the amount of light absorbed Light is used for the production of ATP and high energy electrons which are needed for the conversion of CO₂ Limits glucose production

Photosynthesis At low Temps all of the enzymes that catalyze the conversion of CO₂ into carbohydrates work slowly Below 5℃ there is little or no photosynthesis in many plants At 30℃ rubisco is decreased effectively although not denatured So temperature is a limiting factor at both high and low temperatures.

Effects of Temperature

Photosynthesis Below .01% CO₂ rubisco is not effective in many plants So no photosynthesis Between .01 and .04 concentration of CO₂ is often a limiting facture

Photosynthesis Summary Light Reaction – takes place in the thylakoid membrane Convert light energy to chemical energy (ATP) and NADPH Calvin Cycle – takes place in the stroma; uses ATP and NADPH to convert CO₂ to sugar.

Calvin’s Experiments Calvin cycle was discovered by Melvin Calvin & Andrew Benson in the 1950’s

Calvin’s Experiments http://ib.bioninja.com.au/higher-level/topic-8-metabolism-cell/untitled-2/calvin-cycle.html

Calvin’s Experiments Improvements in apparatus Radioactive Labelling Radioisotopes of elements have the same chemical properties as other isotopes of an element but can be ID’d because they are radioactive They can be used to label organic compounds in biochemistry experiments ¹⁴C (discovered in 1940) so CO₂ and hydrogen carbonate labelled with ¹⁴C could be produced and made available to researchers

Calvin’s Experiments Improvements in apparatus Double-way paper chromatography A first solvent is run up through the paper to separate the mixture partially in one direction. The paper is dried and then a second solvent is run up at 90° to the first spreading the mixture in a second dimension This procedure is ideal for separating and identifying the initial products of carbon fixation

Calvin’s Experiments Improvements in apparatus Autoradiography Using X-ray film to find the location of isotopes. When atoms of ¹⁴C decay they give off radiation which makes a small spot in an adjacent X-ray film. To find radioisotopes in a sheet of chromatography paper it is placed next of film that is of the same size They are kept in the darkness for several weeks The film is then developed, black patches appear in areas where the adjacent chromatography paper contained radioisotopes