photosynthesis ao1

why are plant leaves green?

photosynthetic pigments (e.g. chlorophyll) are found on the thylakoid membrane. Chlorophyll absorbs most wavelengths of light, but reflects green light

photosynthetic pigments

chlorophyll a

chlorophyll b

carotenes

xanthophylls

what is chromatography?

separation technique used to separate pigments in plant leaves

R_f values

show how far a pigment has travelled compared to the solvent front.

formular for R_f values

R_f = distance moved by pigment/ distance moved by solvent front

what is an absorption spectrum?

a graph showing how much light of different wavelengths is absorbed by a pigment

(chlorophyll absorb light strongly in blue and red regions, carotenes and xanthophylls absorb in the blue - green region)

what is an action spectrum?

a graph showing the rate of photosynthesis at different wavelengths of light (action spectrum is closely aligned with absorption spectrum of chlorophylls a and b and carotenoids - suggests main pigments involved in photosynthesis)

what is a photosystem?

an arrangement of photosynthetic pigments within the thylakoid membrane. There are 2 categories - primary pigments = 2 forms of chlorophyll a. form reaction centres

antennae pigments = chlorophyll a and b , carotenes and xanthophylls. Form an antennae complex

photosystem structure

antennae pigments arranged in a antenna complex

reaction centre contains a primary pigment (molecule of chlorophyll a)

light harvesting by photosystems

• when light energy is absorbed by a photosystem, the antenna pigment molecules pass photons of light to the primary pigment in the reaction centre.

• primary pigment = molecule of chlorophyll a which contains pair of electrons

  • light energy is used to excite pairs of electrons which are emited from the electron centre

what are the 2 types of photosystem

PSI - arranged around a molecule of chlorophyll a, absorbs light at a wavelngth of 700nm. Reaction centre = P700

PSII - arranged around a molecule of chlorophyll a, absorbs light at a wavelngth of 680nm. Reaction centre = P680

what are the 2 types of photosynthesis reactions?

• light dependent

  • non-cyclic photophosphorylation

  • cyclic photophosphorylation

• light independent

light dependent reactions

transduce solar energy to electrical energy and then chemical energy - end products are NADPH₂ and ATP

light independent reactions

reduce carbon dioxide to make glucose. the ATP generated by the light dependent reactions provides the energy, and the NADPH₂ provides the hydrogen

law of limiting factors

states that at any given moment, the rate of a physiological process will be limited by the factor which is in the shortest supply

name 3 limiting factors

temperature

light intensity

CO₂ concentration (usually the natural limiting factor)

nitrogen supply as a limiting factor

needed to sythesise proteins, nucleic acids and chlorophyll. Usually transported as nitrates in the xylem and aas in the phloem. A lack of N results in stunted growth, chlorosis (yellowing leaves)

magnesium supply as a limiting factor

transported as Mg²⁺ in the xylem. Required for chlorophyll manufacture, and a deficiency leads to chlorosis, inability to photosynthesis and death. Mg is required for activation of ATPase