8.3 Photosynthesis Flashcards
Where does the light-dependent reactions take place?
in the intermembrane space of thylakoids and across the thylakoid membranes
What two reactions make up photosynthesis?
light dependent reactions
light independent reactions
Why does the light independent reaction only carry on in darkness for a few seconds?
Because they depend on substances produced by the light-dependent reactions which rapidly run out
What is within the thylakoid?
a compartment called the thylakoid space
What are the products of the light-dependent reactions?
Reduced NADP and ATP
What is light converted to in the light dependent reaction?
into chemical energy in the form of ATP and reduced NADP
What do ATP and reduced NADP serve as for the light-independent reactions?
as energy sources
What is the stroma?
a compartment enclosed by the inner membrane of the chloroplast
- a thick protein-rich medium containing enzymes for use in the light-independent reactions (Calvin cycle)
What kind of pathway is the calvin cycle?
an anabolic pathway that requires endergonic reactions to be coupled to the hydrolysis of ATP and the oxidation of reduced NADP
What are the events that occur in both the light and dependent and independent reactions?
image
What are the reactions that occur in light-independent? (6)
- photolysis
- photoactivation
- electron transport
- chemiosmosis
- ATP synthesis
- reduction of NADP
What are the reactions that occur in light independent? (6)
- carbon fixation
- carboxylation of RuBP
- production of triose phosphate
- ATP and NADPH as energy sources
- ATP used to regenerate RuBP
- ATP used to produce carbohydrates
What are photosystems?
Chllorophyll and the accessory pigments grouped together in a light-harvesting array
Where are photosystems located?
in the thylakoids, an arrangement of membranes inside the chloroplast
What are the two types of photosystems?
Photosystem I and photosystem II
Photosystem II begins the light dependent reactions
What do the photosystems do?
- contain special chlorophyll molecules
- absorb energy froma photon of light
- an electron within the molecule becomes excited
- the chlorophyll is then photoactivated
- the chlorophylls at the reaction centre donates the excited electrons to an electon acceptor
What is the elctron acceptor for photosystem II?
plastoquinone
What does plastoquinone do?
collects two excited electrons from photosystem II and then moves away to another position in the membrane
it is hydrophobic, so although it is not in a fixed position, it remains within the membrane
What does the reaction center at photosystem II become after reducing plastoquinone?
it becomes a powerful oxidizing agent and cause the water molecules nearest to split and give up its electrons to replace those that is lost
What happens as plastoquinone transfers it electrons and then are passed from carrier to carrier?
as the electron pass, eneegy is released, which is used to pump protons across the thylakoid membrane into the space inside the thylakoids
a concentration gradient of protons develops across the thylakoid membrane, which is a store of potential energy.
What two reactions contribute to the proton gradient?
- electrons passing from carrier to carrier and energy released is used to pump protons
- Photolysis
What is the process of cyclic photophosphorylation?
When supply of NADP runs out
- electrons return to the ETC that links the two photosystems, rather than being passed to NADP
- Asthe electron flow back along the ETC to photosystem I, they cause pumping of proton, which allows ATP production
What is the calvins lollipop apparatus?
- radioactive carbon-14 is added to a ‘lollipop’ apparatys congtaining freen algae (chlorella)
- light is shone on the apparatus to induce photosynthesis (which will incorporate the carbon-14 into organic compounds)
- after different periods of time, the algae is killed by running it into a solution of ehated alcohol (stops cell metabolism)
- dead algal samples are analysed using 2D chromatography, which separates out the different carbon compounds
- Any radioactive carbon compounds on the chromatogram were then identified using autoradiography (X ray film exposure)
- by comparing difference periods of light exposure, the order by which carbon compounds are generated was determined
- calvin used this information to propose a sequence of events known as the calvin cycle (light indeendent reactoins)
Adaptations of a chloroplast to its functions
- large area of thylakoid membranes containing photosystems carry out light absorption and has a large light-absorbing capacity. Thylakoids are often arranged in stacks called grana and leaves are brightly illuminated typically have chloroplasts with deep grana, which allow more light to be absorbed
- The volume of fluid inside the thylakoids is very small, so when protons are pumped in, a proton gradient develops after relatively few photons of light have been absorbed, allowing ATP synthesis to begin. Now chloroplasts can produce ATP by photophosphorylation -> develops proton gradient quickly
- Stroma contains concentrations of enzymes and substrates that speed up the whole calvin cycle and there are ATP and reduced NADP inthe stroma. Are easily available for many chemical reactions since they are distrubuted throughout the stroma.
What are 5 features of a chloroplast?
- a double membrane forming the outer chloroplast envelope
- an extensive system of internal membranes called thylakoids, which are an intense green colour
- small fluid-filled spaces inside the thylakoids
- a colourless fluid around the thylakoids called stroma that contains many different enzymes
- in most chloroplasts there are stacks of thylakoids, called grana. if a chloroplast has been photosynthesizing rapidly then there may be starch grains or lipid droplets in the stroma
