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.