Ch. 4 Photosynthesis
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Where does the main source of energy for our planet come from?
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The sun
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Photosynthesis
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The process of harnessing or capturing the sun's energy for use by living organisms
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What is the basic formula for photosynthesis?
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6CO2 + 6H2O -------(sunlight)----> C6H12O6 + 6O2
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When did photosynthesis begin?
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Approximately 3 billion years ago when certain cells developed the ability to capture some of the sun's energy and convert it to glucose.
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How can plants use glucose?
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They can use the glucose immediately or store it as starch or fat for later use
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Pigments
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Substances that can absorb light
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What is the primary photosynthetic pigment in plants?
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Chlorophyll a
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Accessory pigments
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Those that capture some light energy and then transfer it to chl a e.g. chl b, carotenes, and xanthophyll
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What do the accessory pigments do?
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Capture some light energy and then transfer it to chl a
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How can we separate the pigments?
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Paper chromatography
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How does paper chromatography work?
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Plant extract is applied to the paper, the pigment sticks to the paper's matrix. As the tip of the paper strip is immersed in a solvent, the pigments dissolve and move with the solvent front. You are then able to differentiate pigments from other pigments by their molecular weight/subsequent speed of dissolution
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What are the four visible bands?
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yellow bandA = carotenes yellow bandB = xanthophyll blue-green = chl a yellow-green = chl b grey band = pheophytin
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What is pheophytin?
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A product of chlorophyll's degradation and might show up
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Rf value
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Specific for a given set of solutions; the relationship of the distance moved by a pigment to the distance moved by the solvent front
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How can you calculate the Rf value?
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Rf = (Distance moved by pigment) / (Distance the solvent moved)
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Why is it important to handle the paper carefully in paper chromatography?
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The oil and dirt on your hands will contaminate the paper and skew your results
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Should you immerse the pencil line in the water?
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NO, the pigments will dissolve and wash off the paper if you do this
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What should you do immediately after removing it from the solvent?
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Mark the top of the solvent front before it evaporates so you can calculate an accurate Rf value
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Why are we measuring the absorption spectra of chl a and b in this laboratory experiment?
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-To determine which colors of the visible spectrum are absorbed better than others -To realize sunlight is composed of a mixture of colored light
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How are we preparing the chl a and b bands to go into the spectrometer?
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-First, cut out both the chl a (blue green) and chl b (yellow green) bands from the paper chromatography experiment -Place all of the papers of each same pigment together in a test tube and add 10mL acetone
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How do plants allow the gas exchange of CO2 into the plant and O2 out of the plant to occur?
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Through specialized openings in the leaves to allow gas exchange called stomata
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Describe the basic structure of a stoma.
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Stoma surrounded by a guard cell that is speckled with chloroplasts surrounded by epidermal cells
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Turgor movement example
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The closing and opening of a leaf stoma
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How can we be certain that CO2 is being picked up by the plant if its colorless and odorless?
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Test for the uptake of CO2 via phenol red pH indicator
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What color is phenol red in an acidic environment? A basic or neutral environment?
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Yellow in acidic, red in neutral or basic
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What happens to the phenol red as we blow through the straw into the solution?
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The CO2 in your breath bubbles through, dissolves in the water, forms carbonic acid, and turns the phenol yellow
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What are we testing when we test how long it takes for the phenol solution to return to red?
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Testing how long it takes photosynthesis to occur
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If yellow xanthophyll and carotenes pigments are also present, why does the extract appear green?
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They are accessory pigments and transfer light energy to chlorophyll after they capture it
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Why did the phenol return to a red color?
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Photosynthesis occurs, causes CO2 to be taken in and O2 to be released
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How can the guard cells regulate the size of the opening of the stomata?
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They can control the tonicity around the stoma openings and control whether the cells shrink or swell
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How long ago did scientists believe photosynthesis first began?
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3 billion years ago
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What is the purpose of pigments in photosynthesis?
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To absorb light and allow the chemical rxn to occur
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At what wavelengths does chl a absorb the most energy?
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430 nm (blue) 660 nm (red)
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What is the purpose of paper chromatography?
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To determine the Rf value or the relationship of the distance moved by a pigment to the distance moved by the solvent front
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What is the basic concept of photosynthesis?
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unusable sunlight energy converted into usable chemical energy 6H2O + 6CO2----->C6H12O6 + 6O2
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Stomata
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-Small pores or openings in the leaf epidermis -Gas exchange - CO2 goes in (photosynthesis and respiration) and O2 goes out -Transpiration (water vapor exits the stomata) -open during the day and closed at night -dicots usually have more stomata on the lower epidermis than the upper epidermis
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Grana
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Light-dependent or Light Reactions, splitting of H2O and production of O2, ATP and NADPH
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Stroma
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Light-independent or Dark Reactions - CO2 reacts with ribulose bisphosphate, a 5-C chemical, to form glucose, and ATP and NADPH are used in the process
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What does the color of the pigment depend upon?
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The color of the pigment comes from the wavelengths of light reflected (not absorbed)
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What are the accessory pigments and what do they do?
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Accessory pigments absorb energy that Chl a does not absorb - Chl b, xanthophylls, carotenoids (ß carotene), anthocyanins
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What happens to chl a and b in the fall?
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Broken down and stored in branches, caretenoids determine the coloring so red/orange is reflected
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What happens in paper chromatography?
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-Chloroplast pigments separated (extracted) by their solubility in solvents -Molecular size and polarity of pigments matter -The more soluble the component the further it goes
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What's an Rf?
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Ratio of fronts or Retardation factor
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What does the Rf factor depend on?
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depends on the temperature, type of solvent and type of paper
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How can you calculate Rf?
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Distance from start to center of substance spot/distance from start to solvent front
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How did we determine the absorption spectrum for the four pigments?
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Combine 8 Chl a bands from your chromatography experiment and add 10 ml acetone Combine 8 Chl b bands and add 10 ml acetone Transfer 1 ml of extract into a cuvette (1st = Chl a, 2nd = Chl b) 3rd cuvette with just acetone = blank ( control) Determine Abs from 380 - 720 nm at 20 nm intervals
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How did we get impressions of the leaf stomata?
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On the bottom of each leaf, paint a 1 inch square section of clear nail polish. Allow to dry. Place clear tape over each section of nail polish and carefully remove from the leaf. This will give you an impression of the epidermis of the leaf
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How does the color of the pH indicator phenol red differ?
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pH indicator phenol red red in a neutral or slightly basic environment yellow in an acidic environment
