2.9 Photosynthesis
ESSENTIAL IDEA: Photosynthesis uses the energy in sunlight to produce the chemical energy needed for life
UNDERSTANDINGS, APPLICATIONS AND SKILLS
UNDERSTANDINGS, APPLICATIONS AND SKILLS
- 2.9.U1 Photosynthesis is the production of carbon compounds in cells using light energy
- 2.9.U2 Visible light has a range of wavelengths with violet the shortest wavelength and red the longest
- 2.9.U3 Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours
- 2.9.U4 Oxygen is produced in photosynthesis from the photolysis of water
- 2.9.U5 Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide
- 2.9.U6 Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis
- 2.9.A1 Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis
- 2.9.S1 Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis
- 2.9.S2 Design of experiments to investigate the effect of limiting factors on photosynthesis
- 2.9.S3 Separation of photosynthetic pigments by chromatograph
INTRODUCTORY STATEMENTS - TRUE OR FALSE?
- Plants grow faster in green light
- Growth rate is related to rate of photosynthesis
- Plants look green because they absorb and use green light
- Plants look green because they reflect and do not use green light
- All plants are green
- All plants have the same photosynthetic pigments (eg. chlorophyll)
2.9.U1 Photosynthesis is the production of carbon compounds in cells using light energy
NATURE OF PHOTOSYNTHESIS - WHAT IS IT ALL ABOUT?
Photosynthesis is the production of carbon compounds in cells using light energy. This energy is normally provided by sunlight, although photosynthesis will occur under artificial light e.g. in greenhouses
The equation for photosynthesis is quite similar to the equation for respiration – observe: how is it different?
NATURE OF PHOTOSYNTHESIS - WHAT IS IT ALL ABOUT?
Photosynthesis is the production of carbon compounds in cells using light energy. This energy is normally provided by sunlight, although photosynthesis will occur under artificial light e.g. in greenhouses
The equation for photosynthesis is quite similar to the equation for respiration – observe: how is it different?
The equations are almost the reverse of each other. This is because photosynthesis is designed to fix light energy in sugars, and respiration is designed to release energy from sugars.
Relationship between Photosynthesis and Cellular Respiration
2.9.U2 Visible light has a range of wavelengths with violet the shortest wavelength and red the longest
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The Sun emits its peak power in the visible region of this spectrum (white light ~ 400 – 700 nm). Colours are different wavelengths of white light and range from red (~700 nm) to violet (~400 nm). The colours of the visible spectrum are (from longest to shortest wavelength): Red Orange Yellow Green Blue Indigo Violet (Mnemonic: Roy G. Biv)
Electromagnetic Spectrum
2.9.U3 Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours
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 synthesise ATP (chemical energy). There are a number of different chlorophyll molecules, each with their own absorption spectra, however collectively:
- 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 colour of leaves)
2.9.S1 Drawing an absorption spectrum for chlorophyll and an action spectrum for photosynthesis
ABSORPTION AND ACTION SPECTRA
The absorption spectra and the action spectra are both investigated in photosynthesis research. Plants absorb light according to the absorption spectra, and use light according to the action spectra.
Pigments absorb light as a source of energy for photosynthesis
The absorption spectra and the action spectra are both investigated in photosynthesis research. Plants absorb light according to the absorption spectra, and use light according to the action spectra.
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
- Both display two main peaks – a larger peak at the blue region (~450 nm) and a smaller peak at the red region (~670 nm)
- Both display a trough in the green / yellow portion of the visible spectra (~550 nm)
Absorption Spectrum
Describe Key features of absorption spectrum:
- Chlorophylls have peaks of absorbance in _________ and __________ light
- Chlorophylls have a trough of absorbance in _________light
- Chlorophyll a and chlorophyll b are ____________ to each other but not ___________ the same
Action Spectrum
Describe Key features of action spectrum:
Photosynthesis rate peaks at _______________nm, and ______________nm which correspond to the colours ______________and______________.
Photosynthesis rate has a trough at ________________nm, which corresponds to _________light
Photosynthesis rate peaks at _______________nm, and ______________nm which correspond to the colours ______________and______________.
Photosynthesis rate has a trough at ________________nm, which corresponds to _________light
Comparing the absorption and action spectra
Task comparison: The comparison shows a close ______________ in the rate of photosynthesis (action spectrum) and the absorption of different wavelengths( _________ spectrum). This is because the energy needed for __________________________is absorbed at specific _____________________by photosynthetic_______________________
PHOTOSYNTHESIS REACTIONS
- 2.9.U4 Oxygen is produced in photosynthesis from the photolysis of water
- 2.9.U5 Energy is needed to produce carbohydrates and other carbon compounds from carbon dioxide
Photosynthesis is a two step process:
- The light dependent reactions convert light energy from the Sun into chemical energy (ATP)
- The light independent reactions use the chemical energy to synthesise organic compounds (e.g. carbohydrates)
- 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
- 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
Measuring the Rate of Photosynthesis
Estimating the rate of photosynthesis can be done using the following methods:
Estimating the rate of photosynthesis can be done using the following methods:
- growth rates (although these do not correlate well during the seedling stage) (indirectly)
- oxygen production (for land plants the oxygen must be contained, for aquatic displacement of water may be used) (directly)
- carbon fixation, (or the rate of CO2 consumption) or Glucose production (indirectly) (radioactive carbon dioxide can be used here)
2.9.U6 Temperature, light intensity and carbon dioxide concentration are possible limiting factors on the rate of photosynthesis
A limiting factor is something that is in short supply and therefore determines the rate of a process. A good example would be sunlight during the night.
Limiting factors for photosynthesis may be:
A limiting factor is something that is in short supply and therefore determines the rate of a process. A good example would be sunlight during the night.
Limiting factors for photosynthesis may be:
- Temperature
- Light
- Carbon dioxide concentration
- Water availability via xylem
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