Photosynthesis and Plastids - Cell Biology
What are Plastids
Membrane bound organelle that are found within the eukaryotic cell
What is the role of chloroplasts
The major plastid in a plant and main role is the site of photosynthesis to convert light energy into glcuose to create food for the plant
What is the chloroplast structure
They contain thylakoid that are made up stack of granum being held together by the stroma lamellae
They are intercconnected and a closed continum
Have a innner and outer membrane with intramembrane space between them and the rest is called the stroma
What is the fucntion of the chloroplast
Primary role is for photosynthesis
Also important for the production of products such as purines and prymidines, Fatty acids, Amino acids and plant hormones
Also needed for the reduction of nitrite ions and sulphate ions
What are Proplastids
They are the first and precursor for all the plastsids that are existant today
They vcan do this due to being present in young meristems and then can divdie at the same rate as cell division to ensure all cells have plastids
What are Chromoplasts
These are non-photosynthetic organelles in cells that provide the plant colour
This is due to containing cartenoids siuch as carotenes aor xanthophyls that have crystalline appearance to give colour
What are amyloplasts
Their job is to be starch holding plastid in non green tissue as storarge organ such as Potato tubers
What are Leucoplasts
These are colourless plastids involved in iol and lipid synthesis and often are surrounded by ER membranes
What are Elaioplast
They are a type of Leucoplast, and their job is to stroe lipod esters in the cell but are very rare
They are found in cells that surround another cavity where pollen develops
How do plastids Transform
Most plastids are able to transform and chnage function to become another type of plastid within the cell
What are common plastid key features
They have an enclosed by an envelope so a double membraned due to have an internal membrane as well
They contain a reduced genome made of single circular chromosomes of DNA with mutliple copes of its genome
Retain ability to carrry out protein synthesis similar to porkaryotic methods
Reproduce via divison independent to cell division and are maternally inherrited
Genes are organised in transcriptional units so many gens ahve 1 promoter sequence
How have plastids evolved
There was only 1 primary endosymbitoic event, but 2 and 3 evsnts have lead to chloroplast in algae having greater diversity
Plastid are present in parasites that can cause malaria
What is Endosymbiosis
Gene loss to the nucleus through plastid change and DNA intergration with nuclear chromosomes when plastids first enetred the host cell lead to plastid becoming organelle not independent organsim
Chloroplast protein synthesis how does it work
Very similar to mitochondrial protein synthesis
90% of chloroplast protein is code dby nuclear genes that are directed to the chloroplast by N-terminal transit peptides
Why is photosynthesis important
The main source of energy for the plant
Provide all fossil fuels that we use and all the oxygen in the atmosphere
Why is photosynthesis not effcient
Only 1-2% efficient due to all light wavelenght not used and some energy lost to the surroundings as heat and organism function
What is the overall reaction for photosynthesis
CO2 + 2H2O --> (CH2O) + O2 + H2O
What is the light dependent reaction in photosynthesis
The converison of light energy to chemical energy in the form of ATP and NADPH ( these are both needed for light independent reaction)
How is NADPH formed from light dependent reaction
Chlorophyll absorb light energy in photosythem 2 and casue electron to exoicte and jump to a higher energy level where they then travel down electeon transport chain losing energy.
Electrons then enter photosythem 1 where excoted to an even higher energy and then there they reduce NADP to NADPH
What takes place in photosytem 2
Is the only enzyme able to split water, this is due to positivly charged chlorophyll after excited electrons are trapped by the primary electron acceptor
This then makes the chlorophyll a + and so take an electron from the water
2H2O==> O2 + 4H+ + 4e- so water is the electron donor
This needs 4 light photons due to produce 4 electrons
Why is photolysis important
It is important to break water with due to get an e- to then reduce the the chlorophyll back to a neutral charge so then can have electrons to become exciyted by light energy again
What does the light harvesting antenna do
It will increase efficiency of the reaction due to being excited by the UV light and then passing energy down to the reaction site to allow longer and larger charged seperation
What are the 3 photosynthetic pigments that absobr differing wavelengths
Chlorophyll a
Chlorophyll b
Carotene
But none can absorb the green wavelenght so green light is reflected hence planst look green to us
How is photosystem 2 organised
There is a reaction centre pair of chlorophyll molecules with lower energues held by D1/D2 proteins
There are 300 chlorophyll per reaction centre
How are chlorophyll molecule held together
By proteins caled light harvesting a/b binding proteins that span the thylakoid membrane
Have 13 chlorophyll + cartenoids
What is the S state model
This is the model of how water is split:
Oxygen evolving complex has 5 states S0-S4
In dark adapated chloroplast are in S1 state due to successive magnanese oxidtion
Only S4 is strongly oxidisng enough to split water with the additon of Cl - and Ca2+ ions
How are electrosn transported through from photosystem 2 to 1
Electeons pass in the electron transport chain pheophytin which is a modified chlorophyll
Then the e- are pass to cytochrom and then to plastocyanin
The plastocyanin then is abel to pass the electrons to the photosystem 1 due to the presence of a copper atom that is good for passing on electrons
What is the job of photosystem 1
The production of NADPH due to NADP is being reduced with electrons generated in photosystem 2
This ahppens due to arriving e- is passed to ferradoxin and that is converted to ferradoxin-NADP reductase that then will redice NADP to NADPH
Wjat is photophosphorylation
The light dependent ATP generation via the formation of an electrochemical proton gradient over the membrane, where H+ travelling down the gradient via ATP synthase will produce ATP
What is the Carbon reactuion of photosynthesis called
It is the light independnet reaction due to can take place in the dark with no light present and is called the Calvin cycle and is done in order to produce glucose and other long term energy sources for the plant
Where does the Calvin cycle take place
It takes place in the stroma of the chloroplast
What are the 3 stages of the Calvin cycle
Carbon fixation that uses 1 enzyme
Redcution that uses 2 enzymes
Regeneration that uses 10 enzymes
How many ATP and NADPH are needed for the calvin cycle
Need 9ATP and 6NADPH
What is Rubsico
An enzyme that is able to convert ribulose bisphosphate to 3-phosphoglycerate thanks to adding 3CO2 molecules to the 1,5- Ribulose bisphosphate
What happens in the reduction stage the calvin cycle
During the Calvin cycle the 3-phosphoglycerate is redeuced to glyceraldehyde phosphate = GAP
This is done by usage of 1ATP and 1NADPH as well enzymes called 3 phosphogylcerate Kinase and glyceraldehyde 3 phosphate dehydrogenase
Whta happnes in the Regeneration stage of the Calvin cycle
Important that the Ribulose 1,5 bisphiosphate is regenerated from the GAP
This is done due to 5 GAP molecules that are each 3 carbon molcules so then 15C in total to make 3 x Ribulose bisphosphate that are 5 c each
This process requries the use if ATP
What happens to generated GAP in the calvin cycle
Leave thr stroma via complex transportor, an order for then to turn the GAP inot sucrose which is a good long term storage molecule
Why is Rubisco using oxygen a bad thing
Oxygenase activity is bad due to the formation of 2 phoshoglycerate that is a 2C compound so then cant be put back into the Calvin cycle, compared to carbxoylase actibity that produce 3-phosphogylcerate with 3C compound
At high temperatures stonata are shut for lower water loss but then means lower CO2 affinity so a greater O2 usage for rubisco
What is the C4 solution to Rubisco
To avoid rusbisoc using O2 plants have evolved strategies for this
One is the C4 photosynthesis which is where the light and carbon reactions take place in a different specialised cells that are bundle sheath cells surrounded by mesophyll cells
How does the C4 carbon cycle work
The cocnentration of CO2 is increased in the bundle sheath cels by transport of C4 acids from mesophyll cells
PEP enzyme fixes CO2 as HCO3- to gove oxaloacetate
What is the Crassulacean acids metabolims for rubsioc not using O2
CAM is adapotation to even drier and rid conditions to prevent water losss like in Pineapple
There is spatial and temporal seperation meaning different stages of phototsynthesis take part at diffferent place and time of the day
How does CAM work
CO2 is taken up at night and stored as malic acid in the vacuole due to stomata can be iopen at night due to colder temperarures
Then during the day the CO2 is more available for fixation by rubisco in the carbon cycle even with the stomata closed at higher temperatures