Photosynthesis 5.2.1
cyclic photophosphorylation (light dependant)
non-cyclic photophosphorylation (light dependant)
photosystems
photosynthetic pigments
molecules involved
Chloroplasts
equation
Calvin cycle (light independant)
Water + carbon dioxide —> oxygen + glucose
6H2O + 6CO2 —> 6O2 + C6H12O6
adaptations
- granum
- selectively permeable membrane
- contains ribosomes
- primary & secondary pigments
- contains loops of DNA
- thylakoids & granum tightly packed
=> increases SA
=>absorb most wavelengths of light to maximise amount of energy that can be released
=> give large SA
=> allows CO2 & H2) in & glucose &vO2 out
=> allows it to make its own proteins
=> allows it to make its own proteins
ADP - adenosine diphosphate
ATP - adenosine triphosphate
addition of a phosphate = phosphorylation
requires energy
removal of phosphate releases energy
this process is how we store & release energy
ADP + Pi --> ATP
NADP - coenzyme involved in the photosynthesis reactions
reducing NADP
NADP --> NADPH (aka reduced NADP)
The initial requirement in photosynthesis is the trapping of sunlight energy by photosynthetic pigments - 2 categories of light absorbing pigments are found in chloroplasts:
Primary pigments
- Chlorophyll A
Accessory pigments
- Chlorophyll b
- Carotenoids (yellow -reflect yellow &orange light & absorb blue light)
- Xanthophylls (orange)
Accessory pigments are light absorbing compounds, that work in conjunction with chlorophyll A
They absorb wavelengths that are not well absorbed by Chlorophyll
Carotenoids aren’t broken down in strong sunlight, photosynthesis continues
Other pigments may act as sunscreen by absorbing blue-green & ultraviolet light - Chlorophyll A may be damaged by strong light intensities
2 photosystems
Photosystem I (absorbing P700)
Photosystem II (absorbing P680)
named after wavelength of light chlorophyll A absorbs
1) Light energy is absorbed (carried by photons) & is passed to photosystems II. Excited electron pass to the primary reaction centre
2) Electrons from chlorophyll A are passed along the electron transport chain (ETC)
3) Electrons are replaced in PS II in a process called photolysis
4) Electrons pass along the ETC releasing energy to pump H+ ions into the thylakoid space
5) Electrons replace the excited electrons lost in photosystem I
6) Electrons are accepted by NADP to make reduced NADP
7) H+ ions diffuse down a channel protein associating with ATP synthase, releasing ATP in a process called Chemiosmosis
products
NADPH
ATP
O2
photolysis
= splitting of water molecules using energy from sum
H2O —> 2H^+ + 2e^- + 1/2 O2
2H^+ = protons used for NADP
2e^- = electrons used as a replacement for PS II
1/2 O2 = waste product
1- light energy is absorbed into PS II & excites electrons in chlorophyll A (P680)
2- Water is broken down using photolysis to replace the electrons in P680
3- Electron move through the ETC
4- Light energy is absorbed into PS I & excited electrons in chlorophyll A (P700
5- Electrons are replaced by ETC
6- NADP is then reduces to NADPH
7- As electrons pass through ETC, energy is realised to actively transport H+ ions across the membrane
8- H+ ions diffuse back through the channel protein associated in ATP synthase & ATP is made
PS I is used only
Excited electrons pass to an electron acceptor & back to PS I
Small amounts of ATP are produced during this process
Reduced NADP not produced in this process
No photolysis
This may be used in the 2nd stage of photosynthesis
biochemical pathways
2) ATP & NADPH are used to covert GP to TP
3) 1/6th of TP forms glucose
1) RuBP reacts with CO2 (carboxylation) using enzyme rubisco to produce GP. for every 1 molecule of RuBP, 2 molecules of GP are formed
4) 5/6th of TP regenerates RoBP
Rubisco
= enzyme which adds a carboxyl group (COOH) to the RuBP molecule
O2 can also fit into the complex = comparative inhibitor (fits into active site). Producing a toxic product - phosphoglycolate
This results in a reaction called photorespiration
factors effecting photosynthesis
light intensity
Affects rate if light dependant stage
Stomata open
More electrons in PS I & II excited
More photolysis
Increased light intensity leads to more ATP & NADP production
Talk about the axis, the net photosynthesis rate against respiration, positive correlation
The relationship between light intensity and respiration and photosynthesis is shown on this graph
Yellow area of graph:
Light compensation point - is the light intensity at which the rate of respiration is equal to the rate of photosynthesis
Photosynthesis = absorbs CO2
Respiration = releases CO2
Reparation > photosynthesis
CO2
0.039% of atmosphere on average
CO2 required for carbon fixation (into RuBP) in Calvin Cycle
Higher conc of CO2 means there can be more of the Calvin cycle consequently increasing rate of photosynthesis
Temperature
Denature - Tertiary structure unravels
Substrate no longer fits into active site
No enzyme-substrate complexes can
Limiting factors & the Calvin cycle
Reduced light intensity leads to reduced production of ATP & NADPH
These are need to convert GP to TP
Conc of GP will increases & TP will decrease
Less TP to regenerate RuBP there conc of RuBP will also decrease
Reduced CO2 will lead less carbon fixation in the Calvin cycle
Therefore less GP & TP
Concentration of RuBP will increase as it is still being formed from TP but not being used to fix CO2