2.9: Photosynthesis

*Explain the need to control variables in experimental design.* Nature of Science: Experimental design- controlling relevant variables in photosynthesis experiments is essential.

Controlled variables are what is kept the same between experimental groups throughout the experiment. Any change in a control variable in an experiment would invalidate a relationship between the manipulated variable and the responding, thus skewing the results.

*Calculate the Rf value for pigments using pigment chromatography. * Skill: Separation of photosynthetic pigments by chromatography.

The ratio of the distance moved by a pigment to the distance moved by the solvent is a constant, Rf. Each type of molecule has its own Rf value. Rf = distance traveled by pigment / distance traveled by solvent

*Define "responding (dependent) variable."* Nature of Science: Experimental design- controlling relevant variables in photosynthesis experiments is essential.

The responding (dependent) variable is the variable being measured to test the effect of the manipulated variable in a scientific experiment.

*Define "visible light."* Understanding: Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.

Visible light is *the portion of the electromagnetic spectrum that is visible to the human eye*.

*Outline why chromatography is used in the study of plant pigments.*

Pigments extracted from plant cells contain a variety of molecules, such as chlorophyll, beta carotene, and xanthophyll, that can be separated using paper chromatography.

*Outline evidence for the "Great Oxidation Event."* Application: Changes to the Earth's atmosphere, oceans and rock deposition due to photosynthesis.

*Banded Iron Formations* are layers found in sedimentary rock. The hypothesis is that the banded iron layers were formed as the result of oxygen released by photosynthetic cyanobacteria. The oxygen combined with dissolved iron in Earth's oceans to form insoluble iron oxides, which precipitated out, forming layers on the ocean floor.

*Outline the role of "photolysis" in photosynthesis.* Understanding: Oxygen is produced in photosynthesis from the photolysis of water.

*Photolysis means water splitting*. During photosynthesis, light energy is used to split water molecules as represented by the following equation: 2H2O → 4H+ + 4e- + O2 Oxygen (O2) is released as a waste product in this process. The H+ and e- continue on in the photosynthesis light reactions.

*Define "limiting factor."* Understanding: Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate photosynthesis.

A limiting factor is a variable of a system that can limit the rate of a reaction. Either the amount of light, temperature or CO2 concentration will limit the rate of photosynthesis. Whichever of the three is furthest from its optimal will be the limiting factor.

*State the range of wavelengths that fall within the visible spectrum.* Understanding: Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.

A typical human eye will respond to wavelengths from about 390 to 700 nanometers. The order of colors in light, arranged from shortest wavelength to longest: Violet Blue Green Yellow Organge Red

*Describe the "absorption spectrum" in relation to photosynthesis.* Skill: Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.

An absorption spectrum shows the wavelengths of light absorbed by a pigment. Chlorophyll, the most prevalent pigment used in photosynthesis, has a peak absorption between 400 and 500 nm (blue light) and another (smaller) peak between 600 and 700 nm (red light). There is no absorption around 600 nm (green light) as most of this light is reflected.

*Define "pigment."* Understanding: Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.

Biological pigments are protein molecules that selectively absorb certain wavelengths of light while reflecting others.

*Explain why plants are green.* Understanding: Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.

Plants look green because of chlorophyll. Chlorophyll absorbs the red, blue, and other wavelengths of light, and it reflects the green back to your eyes, so that is what you see.

*Describe the "action spectrum" in relation to photosynthesis.* Skill: Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.

The action spectrum shows is the rate of a photosynthesis plotted against wavelength of light. It shows that blue and red wavelengths of light are most effectively used for photosynthesis.

*Define "manipulated (independent) variable."* Nature of Science: Experimental design- controlling relevant variables in photosynthesis experiments is essential.

The manipulated (independent) variable is *the variable that is changed in a scientific experiment* to test the effects on the responding (dependent) variable.

*Explain why pigments separate during chromatography.* Skill: Separation of photosynthetic pigments by chromatography.

The pigment molecules migrate, or move up the paper, at different rates because of differences in solubility, molecular mass, and variable hydrogen bonding with the chromatography paper.

*Outline the process of paper chromatography.* Skill: Separation of photosynthetic pigments by chromatography.

Paper chromatography is a technique used to separate substances in a mixture based on the movement of the different substances up a piece of paper by capillary action.

*State the major groups of organisms that carry out photosynthesis.* Application: Changes to the Earth's atmosphere, oceans and rock deposition due to photosynthesis.

Plants, some prokaryotes and algae are capable of performing photosynthesis.

*State the chemical equation for photosynthesis.* Understanding: Photosynthesis is the production of carbon compounds in cells using light energy.

Six molecules of water plus six molecules of carbon dioxide produce one molecule of glucose plus six molecules of oxygen.

*Define the "Great Oxidation Event."* Application: Changes to the Earth's atmosphere, oceans and rock deposition due to photosynthesis.

The Great Oxygenation Event is defined as the appearance and rise of oxygen gas (O2) in the earth's atmosphere beginning about 2.4 billion years ago. The oxygen is thought to be a result of photosynthesis by prokaryotes called cyanobacteria.

*Explain the relationship between the photosynthesis absorption spectrum and action spectrum.* Skill: Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.

The peaks in the *action spectrum* showing the rate of photosynthesis correspond to the peak light energy wavelengths that are able to be *absorbed* by chlorophyll. Since chlorophyll is most effective at absorbing blue and red light, the rate of photosynthesis is also higher with blue and red light. Differences between the chlorophyll absorption spectrum and the action spectrum are due to absorption by accessory (ie carotene). Chlorophyll does not absorb green light, so the there is less photosynthesis within the green light wavelengths. This is shown as a dip in the action spectrum.

*List techniques for measuring the rate of photosynthesis.* Skill: Design an experiment to investigate limiting factors on photosynthesis.

There are a few key methods to calculate the rate of photosynthesis. These include: *Measuring the uptake of CO2* -immobilized algae in hydrogen carbonate indicator solution -monitor the CO2 concentration using a CO2 sensor *Measuring the production of O2* -counting bubbles formed -measuring water displacement by O2 gas formed -floating leaf disk *Measuring the increase in dry mass* -harvest plants and record how much mass they have accumulated as a proxy measure for carbohydrate formation

*Outline the movement of the major plant pigments in chromatography.* Skill: Separation of photosynthetic pigments by chromatography.

*Beta carotene* is carried the furthest because it is highly soluble in the solvent and because it forms no hydrogen bonds with the chromatography paper fibers. *Xanthophyll* contains oxygen and does not travel quite as far with the solvent because it is less soluble than beta carotene and forms some hydrogen bonds with the paper. *Chlorophylls* are bound more tightly to the paper than the other two, so they travel the shortest distance.

*State that the oxygen produced in photolysis is a waste product of photosynthesis.* Understanding: Oxygen is produced in photosynthesis from the photolysis of water.

In photolysis, water is split, creating oxygen gas as a byproduct. 2H2O → 4H+ + 4e- + O2 The oxygen (O2) is released as a waste product in this process.

*Summarize the energy conversions that occur during photosynthesis.* Understanding: Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.

In photosynthesis, *light energy is converted into chemical energy* stored in carbohydrate molecules. During the *light dependent reactions* light energy is used to split water. The 4H+ + 4e- formed as a result of the photolysis are used to fuel the formation of two molecules needed for the next stage of photosynthesis: ATP and the electron carrier NADPH. In the *light independent reactions*, carbon atoms from CO2 are used to build carbohydrates. This process relies on ATP and NADPH from the light dependent reactions.

*State the name and role of the primary pigment used in photosynthesis.* Understanding: Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.

The primary pigment is the most important/predominant pigment in the process of photosynthesis. The primary pigment found in the chloroplast of photosynthetic cells in plants is *Chlorophyll a* which absorb blue and red wavelengths of light.

*Explain how temperature can limit the rate of photosynthesis.* Understanding: Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate photosynthesis.

Photosynthesis is an enzyme-catalyzed reaction, and like with other enzyme reactions, photosynthesis rate will increase as the temperature is raised until an optimal temperature is reached. After the optimum, the rate of photosynthesis abruptly declines with further increase of temperature. The decline is because most enzymes involved in photosynthesis rapidly denature at higher temperatures.

*Explain why the photosynthesis reactions are endothermic.* Understanding: Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.

Photosynthesis is endothermic because an *input of energy* is required to carry out the process of converting carbon dioxide and water into carbohydrates. The energy for photosynthesis comes from light.

*Define "photosynthesis."* Understanding: Photosynthesis is the production of carbon compounds in cells using light energy.

Photosynthesis is the process by which photoautotrophic organisms convert light energy (usually from the Sun) into chemical energy (glucose). The photosynthetic process uses light energy to produce glucose from water and carbon dioxide, releasing oxygen gas as a byproduct.

*State the name and roles of accessory pigments used in photosynthesis.* Understanding: Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.

Pigments used in addition to chlorophyll a are known as accessory pigments. Different pigments respond to different wavelengths of visible light. The use of accessory pigments allows a broader range of wavelengths to be absorbed, and thus, more energy to be captured from sunlight. Accessory pigment include: *Chlorophyll b* - absorbs blue and some yellow-orange light *Carotenoids* - absorb violet and blue-green light

*List potential manipulated (independent) variables for experiments about photosynthesis.* Nature of Science: Experimental design- controlling relevant variables in photosynthesis experiments is essential.

Possible manipulated variables include: -light intensity -light wavelength -temperature -CO2 availability

*List potential responding (dependent) variables for experiments about photosynthesis.* Nature of Science: Experimental design- controlling relevant variables in photosynthesis experiments is essential.

Possible responding variables in a photosynthesis experiment include: -uptake of CO2 -production of O2 -biomass production

*State the relationship between wavelength and energy.* Understanding: Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.

Shorter wavelengths have more energy and longer wavelengths have less energy.

*Explain how CO2 concentration can limit the rate of photosynthesis.* Understanding: Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate photosynthesis.

Without carbon dioxide, a plant cannot perform photosynthesis because CO2 is required in the light independent reactions. Increasing carbon dioxide concentration will increases the rate of photosynthesis. However, at a certain point the photosynthetic rate will plateau. The leveling off is because all enzymes used in the reactions are being used.

*Explain how light intensity can limit the rate of photosynthesis.* Understanding: Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate photosynthesis.

Without enough light, a plant cannot perform photosynthesis because light energy is required in the light dependent reactions. Increasing the light intensity will increase the rate of photosynthesis. However, at a certain point the photosynthetic rate will plateau. The leveling off is because all available chlorophyll pigments are being used.