Photosynthesis

• What is photosynthesis?


  Photosynthesis is the process responsible for the conversion of light energy from the sun into chemical energy in the form of complex carbohydrates.
• What does photosynthesis support?

  As a process it supports all life on earth.
• What type of process is photosynthesis?

  It can therefore be seen that it is a transduction - i.e. the conversion of 1 form of energy into another.
• What is responsible for photosynthesis, how + what are they known as? (3)

  - Plants are responsible for this process.

  
  

  - They convert light energy into the food they need to release energy

  
  

  - Plants are known as photoautotrophs.
• What is the equation for photosynthesis?

  
• Why doesn't the equation for photosynthesis fully represent the process? (2)

  - This equation is very simplistic as the process is a lot more complex.

  
  

  - It does not explain where the oxygen comes from - is it released from the carbon dioxide or is it a product of the splitting of water molecules?
• What is the organ specialised for photosynthesis in most plants?

  In most plants, the leaf is the organ specialised for photosynthesis.
• Explain the role of palisade cells in photosynthesis. (2)

  - The palisade cells in the mesophyll layer are highly adapted for this role.

  
  

  - They are close to the surface of the upper leaf, are tightly packed together and have numerous chloroplasts in their cytoplasm.
• Explain the role of spongy mesophyll cells in photosynthesis. (2)

  - The spongy mesophyll layer has cells lees tightly packed with numerous air spaces that form a continuous pathway with the stomata in the lower epidermis.

  
  

  - This facilities gas exchange with the palisade cells and the outside environment.
• Where does photosynthesis take place?

  Photosynthesis takes place in the chloroplasts.
• What is the size of chloroplasts?

  20μm x 5 μm

  
• What do thylakoids contain?

  Thylakoids contain chlorophyll and other pigments along with systems of electron carriers and enzymes.
• How are thylakoids arranged?

  Thylakoids are arranged in stacks - grana - where most light harvesting occurs.
• What are thylakoids linked by?

  Thylakoids are linked by inter-granal lamellae.
• What happens to CO2 in the stroma?


  CO2 is fixed with the stroma.
• What do the reactions happening in the stroma depend on?

  The reactions that occur in the stroma depend on a ready supply of products from the reactions associated with the absorption of light in the thylakoids.
• What are the three stages of photosynthesis?

  - Light harvesting

  - Light dependent stage

  - Light independent stage
• What happens during light harvesting?

  The absorption of light in the thylakoid to raise the energy level of the electrons in chlorophyll.
• What happens during the light dependent stage?

  Energises electrons make energy rich compounds e.g. ATP and reduced NADP on the thylakoid membranes.
• What happens during the light independent stage?

  The products of the light reaction make carbohydrates, CO2 is also fixed in a series of cyclical reactions in the stroma.
• What is the colour of light perceived dependent on?

  The colour of light that we perceive is dependent upon its wavelength.
• Why do leaves appear green?

  Leaves appear green as the chlorophyll within them reflects green lights and absorbs light of other wavelengths.
• What do absorption spectra show?

  Absorption spectra show the absorption of light of different wavelengths by chlorophyll.

  
• What is chlorophyll a combination of?

  Chlorophyll is a combination of different pigments: chlorophyll a and b and another group of pigments called the carotenoids.
• What is the absorption spectrum representative of?

  - The absorption spectrum is the relative absorption of light at particular wavelengths.

  
  

  - It is not necessarily linked to light intensity.
• Describe the structure of chlorophyll pigments. (2)

  - Chlorophyll molecules have similar chemical structures.

  
  

  - Ring like structure with magnesium at the centre joined to a hydrophobic hydrocarbon tail that allows the molecule to be anchored in the thylakoid membrane.
• What molecules are considered carotenoids + what colour are they? (2)


  - The carotenoids include molecules such as the xanthophylls and carotenes and are not green.

  
  

  - They are yellow, orange (e.g. β carotene - the pigment found in carrots), red and brown.
• What can carotenoids be described as + what is their function? (2)

  - These pigments may be described as accessory pigments.

  
  

  - They absorb light of different wavelengths and pass the energy onto the chlorophyll to assist in the process of photoactivation.
• What gives rise to the overall absorption spectrum of chlorophyll?

  It is the combined absorption of all the pigments that gives rise to the overall absorption spectrum of "chlorophyll".

  
• What is the action spectrum?

  The action spectrum is the rate of photosynthesis that takes place at different wavelengths.
• What can be observed from plotting a graph of the rate of photosynthesis vs wavelength?

  The rate is dependent on the wavelength, with the highest rate for those wavelengths over which most light is absorbed.

  
• Where does light harvesting take place?

  It takes place in particles attached to the thylakoid membranes of the chloroplast.
• How can the particles be accurately described + what forms do they exist in?

  They are more accurately described as photosystems and they exist in 2 forms - PSI and PSII.
• What do the 2 photosystems contain?

  The 2 photosystems contain slightly different primary pigments.
• What does photosystem I contain + why?

  Photosystem I contains the primary pigment P700 (as it absorbs light with a wavelength of 700nm).
• What does photosystem II contain + why?

  Photosystem II contains the primary pigment P680 (as it absorbs light with a wavelength of 680nm).
• What can you describe light harvesting as?


  The process of absorbing light in the photosystems is referred to as light harvesting.
• What is each photosystem composed of?

  Many hundred pigment molecules such as chlorophyll.
• What is the function of chlorophyll a + what is it referred to as?

  Chlorophyll a acts at the reaction centre and is referred to as the primary pigment.
• What are the other pigments referred to as + what is their function?

  They are accessory pigments and are responsible for funnelling the photons of light onto the primary pigment.