photosynthesis

what are autotrophs and give an example?

‘self feeders’, create their own energy

plants are photoautotrophs

describe the different structures of the plant that are involved in photosynthesis

chloroplasts - site of photosynthesis

stomata - pores on photosynthetic cells through which CO2 enters and O2 exits

veins - delivers water absorbed at roots to leaves, the major site of photosynthesis

describe the structure of chloroplasts

* two membranes with inter-membrane space
* structure filled with dense fluid ‘stroma’
* contain various granum which are stacks of thylakoid structures, inside of which known as thylakoid space
* thylakoid membrane gives green colour

explain the green colour of leaves

arises from the thylakoid membrane which transmits, not absorbs, green wavelengths

what are the two stages of photosynthesis?

* light reactions which converts solar energy to chemical energy
* dark reactions / calvin cycle

which is the general equation for photosynthesis?

6CO2 + 6H2O + LIGHT ENERGY = C6H12O6 + 6O2

what are photosystems?

= light harvesting complexes

they consist of **chlorophyll** (pigment), other organic molecules & proteins

* photosystems I (PSI) & II (PSII) exist in the thylakoid membrane

explain the role of pigments in stage one of photosynthesis and give some examples

* they absorb photons which promotes an electron from ground state to an excited state
* this provides energy in the form of ATP and NADPH


* chlorophyll a = key
* chlorophyll b = accessory
* carotenoids = further accessory

explain stage one of photosynthesis ie light reactions

SOLAR POWER = ATP (chem energy) + NADPH (reducing power)

1- photon excites an electron in a pigment of the PSII. this excitation is transferred along pigment molecule electrons until P680 pair in chlorophyll a is excited.

2- excited electron passed to primary electron acceptor, forming P680+.

3- enzymes catalyse degradation of water. resulting electrons return P680+ to P680, H+ released into thylakoid space and oxygen combines with O2.

4- photo-excited electrons pass from primary electron acceptor in PSII to PSI via electron transport chain, involving e- carrier Pq and protein Pc. also involves pumping of H+ into thylakoid space.

5- photon gradient established used to produce ATP through chemiosmosis

6- photon excites an electron in a pigment of the PSI. this excitation is transferred along pigment molecule electrons until P700 pair in chlorophyll a is excited.

7- excited electron passed to primary electron acceptor, forming P700+ which accepts electrons from the electron transport chain.

8- photo-excited electrons in the PSI primary electron acceptor are passed down a second electron transport chain through protein Fd. (no photon gradient established)

9- enzyme NADP+ reductase catalyses reduction of NADP+ with electrons from Fd to NADPH. this involved removal of H+ from stroma.

what is cyclic electron flow?

same mechanism but uses only PSI

* electrons leave PSI but instead of being accepted by NADP they flow back down the chain to the first electron acceptor. This means that ATP is produced but no NADPH and may happen when NAPDH is in plentiful supply

chemiosmosis in mitochondria vs chloroplasts

mitochondria;

* food to chemical energy as ATP

chloroplasts;

* light energy to chemical energy as ATP

what is chemiosmosis?

movement of ions across a semi-permeable membrane, down their electrochemical gradient.

explain stage two of photosynthesis ie dark reactions / calvin cycle

* metabolically irreversible
* occurs in the stroma

ATP + NADPH + CO2 = CARBOHYDRATE

1- carbon fixation

CO2 + RuBP = 3PGA

* catalysed by RuBisCO (most abundant enzyme in nature)

2- reduction

3PGA + ATP + NAPDH = G3P + ADP + Pi + NADP+

3- regeneration of RuBP

G3P + ATP = ADP + RuBP

* net synthesis of 1 G3P from 9 ATP and 6 NADPH
* G3P involved in the synthesis of glucose, sucrose and other carbs

what is photorespiration?

a wasteful pathway occurs when the RuBisCO acts on oxygen rather than carbon dioxide in the carbon fixation step of the dark reactions

= worsens with increased temperature

what are C3 plants?

majority of plants are C3, which have no features to combat photorespiration.

why did C4 and CAM pathways occur?

they are adaptations of the C3 pathway which occurred from natural selection to minimise photorespiration.

explain C4 plants

they minimise photorespiration by separating carbon fixation and the other steps of the dark reactions in space.

= in different cell types

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carbon fixation = mesophyll cells

other calvin cycle steps = bundle-sheath cells

explain CAM plants

they minimise photorespiration by separating carbon fixation and the other steps of the dark reaction in time.

= between night and day

carbon fixation = night = stomata open to allow diffusion of CO2 into cell

other steps = day

describe the hill reaction

used to follow the progress of the light reactions by a colour change

* NADP+ substituted for another electron acceptor which acts as a dye (DCPIP)

the reduction of DCPIP converts it from blue to colourless, showing if the light reaction occurred

describe the cooperation between guard cells and stoma

guard cells are produced in pairs with a gap between them that forms the stomatal pore

* when guard cells are turgid, stoma are open (DAY)
* when guard cells are flaccid, stoma is closed (NIGHT)