2.9 Photosynthesis
Essential idea: Photosynthesis uses the energy in sunlight to produce the chemical energy needed for life.
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2.9.U1 Photosynthesis is the production of carbon compounds in cells using light energy.
2.9.U1 Photosynthesis is the production of carbon compounds in cells using light energy.
Be able to:
Define photosynthesis.
State the chemical equation for photosynthesis.
Photosynthesis is the fundamental process by which plants manufacture food molecules (carbohydrates) from raw materials CO2 and H2O) using energy from light.
This process requires a photosynthetic pigment (chlorophyll) and can only occur in certain organisms (plants, certain bacteria)
2.9.U2 Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.
2.9.U2 Visible light has a range of wavelengths with violet the shortest wavelength and red the longest.
Be able to:
Define visible light.
State the relationship between wavelength and energy.
State the range of wavelengths that fall within the visible spectrum.
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. Sunlight/ light made up of wavelengths of electromagnetic radiation that our eyes can detect = visible to us and other wavelengths are invisible visible light = wavelengths longer than ultraviolet/ shorter than infrared / range of wavelengths of visible light is 400 to 700 nanometer.
The colours of the visible spectrum are (from longest to shortest wavelength):
Red Orange Yellow Green Blue Indigo Violet (Mnemonic: Roy G. Biv)
2.9.U3 Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colors
2.9.U3 Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colors
Be able to:
Define pigment.
State the primary and accessory pigments found in chloroplasts.
Explain why plants are green.
Chlorophyll is a green pigment found in photosynthetic organisms that is responsible for light absorption. When chlorophyll absorbs light, it releases electrons which are used to synthesize ATP (chemical energy). Chemical substance called pigment involved in first step pf photosynthesis.
Chlorophyll absorbs light most strongly in the blue portion of the visible spectrum, followed by the red portion
Chlorophyll reflects light most strongly in the green portion of the visible spectrum (hence the green color of leaves).
2.9.U4 Oxygen is produced in photosynthesis from the photolysis of water.
2.9.U4 Oxygen is produced in photosynthesis from the photolysis of water.
Be able to:
Define photolysis.
State the equation for photolysis.
State that the oxygen produced in photolysis is a waste product of photosynthesis
The splitting of molecules of water (photolysis) to release electrons needed for other stages of photosynthesis
Step 1: Light Dependent Reactions
Light is absorbed by chlorophyll, which results in the production of ATP (chemical energy)
Light is also absorbed by water, which is split (photolysis) to produce oxygen and hydrogen
The hydrogen and ATP are used in the light independent reactions, the oxygen is released from stomata as a waste product
Step 2: Light Independent Reactions (Sometimes referred to as the Calvin Cycle)
ATP and hydrogen (carried by NADPH) are transferred to the site of the light independent reactions
The hydrogen is combined with carbon dioxide to form complex organic compounds (e.g. carbohydrates, amino acids, etc.)
The ATP provides the required energy to power these anabolic reactions and fix the carbon molecules together
2.9.U5 Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.ATP and hydrogen
2.9.U5 Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.ATP and hydrogen
Be able to:
State the energy conversion that occurs during photosynthesis.
ATP and hydrogen derived from photolysis of water are used to fix carbon dioxide to make organic molecules. Plants convert carbon dioxide into glucose through the Calvin cycle in photosynthesis. This process requires energy from light being put in and so is described as being endothermic.
2.9.U6 Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis.
2.9.U6 Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis.
Be able to:
Define “limiting factor.”
Explain how the following factors limit the rate of photosynthesis: temperature, light intensity, CO2 concentration
The law of limiting factors states that when a chemical process depends on more than one essential condition being favorable, the rate of reaction will be limited by the factor that is nearest its minimum value
Photosynthesis is dependent on a number of favorable conditions, including:
Temperature
Light intensity
Carbon dioxide concentration
2.9.A1 Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis.
2.9.A1 Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis.
Be able to:
State that (some) prokaryotes, algae and plants carry out photosynthesis.
Define and state evidence for the “Great Oxidation Event.”
Only one significant source of oxygen gas exists in the known universe – biological photosynthesis. Before the evolution of photosynthetic organisms, any free oxygen produced was chemically captured and stored. Approximately 2.3 billion years ago, photosynthetic organisms began to saturate the environment with oxygen. This led to changes in the Earth’s atmosphere, oceans, rock deposition and biological life.
If it gets too cold, the rate of photosynthesis will decrease as enzyme activity is low. Plants cannot photosynthesize if it gets too hot either as the enzymes controlling it are denatured.
Without enough light, a plant cannot photosynthesize very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis and then the rate will become steady as something else is a limiting factor, eg. temperature or carbon dioxide concentration
Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide. Increasing the concentration of carbon dioxide will boost the speed of photosynthesis and then the rate will become steady as something else is a limiting factor, eg temperature or light intensity.
2.9.U5 Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.
2.9.U5 Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide.
Be able to:
State the energy conversion that occurs during photosynthesis.
ATP and hydrogen derived from photolysis of water are used to fix carbon dioxide to make organic molecules. Plants convert carbon dioxide into glucose through the Calvin cycle in photosynthesis. This process requires energy from light being put in and so is described as being endothermic.
2.9.S1 Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.
2.9.S1 Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis.
Be able to:
Distinguish between an action spectrum and an absorption spectrum.
Describe the shape of the curve for an absorption spectrum.
Describe the shape of the curve for an action spectrum.
Know that visible light has wavelengths between 400 and 700 nanometres, but you are not expected to recall the wavelengths of specific colors of light.
Pigments absorb light as a source of energy for photosynthesis
The absorption spectrum indicates the wavelengths of light absorbed by each pigment (e.g. chlorophyll)
The action spectrum indicates the overall rate of photosynthesis at each wavelength of light
2.9.S2 Design of experiments to investigate the effect of limiting factors on photosynthesis.
2.9.S2 Design of experiments to investigate the effect of limiting factors on photosynthesis.
Be able to:
List mechanism for measuring the rate of photosynthesis
This virtual simulation of photosynthesis allows you to change the following variables:
the color of the light
the brightness of the light and
the carbon dioxide concentration
You must register for an account to access it.
Measuring CO2 Uptake
Carbon dioxide uptake can be measured by placing leaf tissue in an enclosed space with water
Water free of dissolved carbon dioxide can initially be produced by boiling and cooling water
Carbon dioxide interacts with the water molecules, producing bicarbonate and hydrogen ions, which changes the pH (↑ acidity)
Increased uptake of CO2 by the plant will lower the concentration in solution and increase the alkalinity (measure with probe)
Alternatively, carbon dioxide levels may be monitored via a data logger
Measuring O2 Production
Oxygen production can be measured by submerging a plant in an enclosed water-filled space attached to a sealed gas syringe
Any oxygen gas produced will bubble out of solution and can be measured by a change in meniscus level on the syringe
Alternatively, oxygen production could be measured by the time taken for submerged leaf discs to surface
Oxygen levels can also be measured with a data logger if the appropriate probe is available
Measuring Biomass (Indirect)
Glucose production can be indirectly measured by a change in the plant’s biomass (weight)
This requires the plant tissue to be completely dehydrated prior to weighing to ensure the change in biomass represents organic matter and not water content
An alternative method for measuring glucose production is to determine the change in starch levels (glucose is stored as starch)
Starch can be identified via iodine staining (turns starch solution purple) and quantitated using a colorimeter
2.9.S3 Separation of photosynthetic pigments by chromatography.
2.9.S3 Separation of photosynthetic pigments by chromatography.
Be able to:
Outline the process of separating pigments using chromatography
Calculate the Rf value for pigments using pigment chromatography.
(Practical 4)
Outline the process of separating pigments using chromatography
Calculate the Rf value for pigments using pigment chromatography.
Paper chromatography can be used to separate photosynthetic pigments but thin layer chromatography gives better results.
Photosynthetic organisms do not rely on a single pigment to absorb light, but instead benefit from the combined action of many. These pigments include chlorophylls, xanthophyll and carotenes. Chromatography is an experimental technique by which mixtures can be separated
Two of the most common techniques for separating photosynthetic pigments are:
Paper chromatography – uses paper (cellulose) as the stationary bed
Thin layer chromatography – uses a thin layer of adsorbent (e.g. silica gel) which runs faster and has better separation
The retention factor, Rf, of a pigment, is a measurement of relative solubility. It is calculated as follows:
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