The Reactions Of Photosynthesis Flashcards
What organelle does photosynthesis occur in the cell? Explain what membranes there are, where in the leaf the organelle is found, and what features are visible under a microscope, and why is it useful they have their own DNA and ribosomes
-chloroplast
-outer membrane, inner membrane and the thylakoids (folded stacks of membranes collectively called grana, connected by lamellae)
-chloroplast are found in the spongy mesophyll and palisade mesophyll. They are not found in photosynthetic bacteria (pigments instead)
-under a microscope can see the stroma (fluid area) and the grana (stacks of thylakoid membranes)
-own DNA codes for proteins that can be made in the ribosomes, used in photosynthesis
What adaptations does the palisade mesophyll have for photosynthesis?
-thin cell walls = rapid diffusion
-lots of chloroplast = site for photosynthesis
-chloroplast around edge of the cell and they can move around so all have time to absorb maximum amount of light
What are the pigments needed for photosynthesis arranged in to? Explain what these are and their purpose
-a photosystem is a funnel shaped structure with chlorophyll-a at its reaction centre. Accessory pigments (chlorophyll-b, carotenoids) funnel light onto the reaction centre
What are the 2 different forms of chlorophyll-a (chl-a) and why are there 2?
-P700 is found in photosystem I (peak absorption of light is at wavelength 700nm)
-P680 is found in photosystem II (peak absorption of light is at wavelength 680nm)
-allows the plant to absorb as much light as possible
Why is it useful that plants have a range of different pigments?
-means the leaf can absorb a greater range of different light wavelengths (colours), so the total amount of light energy absorbed is higher, therefore a higher rate of photosynthesis and more carbohydrates produced
Sum up what the graph shows, and explain why plants appear green
-pigments involved in photosynthesis mainly absorb light at blue and red ends, with no absorption of green light. This means that the green light is reflected back from the plant, making it appear green
Explain the key points of the light dependent stage of photosynthesis
- There are photosystems all over the thylakoid membranes, absorbing light energy
- They are arranged in pairs; PSII comes first PSI
In non-cyclic photophosphorylation…
- When the chl-a in the reaction centre of PSII absorbs light, 2 electrons are “excited” to a higher energy level, where they are passed along an electron transport chain
- The carriers are iron-containing proteins attached to the thylakoids
- When they accept the electrons, they are REDUCED, and when they pass them on, they are OXIDISED
- The energy released as the electrons pass along is used to phosphorylase ADP. ADP + Pi = ATP. This is photophosphorylation
- The electrons that left PSII are replaced by the splitting of nearby water molecules (PHOTOLYSIS = when water molecules are split into hydrogen ions, electrons and oxygen molecules using energy from sun)
- PSI is also absorbing light, and electrons are being excited (replaced by new ones arriving from PSII)
- These electrons pass along an electron transport chain
- At the end, a substance called ferredoxin passes the electron, along with the hydrogen ion from water, to a substance called NADP
- This causes NADP to be reduced
What are the 3 products from the light-dependent stage of photosynthesis?
1) ATP
2) Reduced NADP
3) Oxygen
Explain the process of chemiosmosis in photosynthesis
1) As electrons pass along protein carrier chains on the thylakoid membrane energy is released
2) this energy is used to force hydrogen ions from water into the spaces between thylakoid membranes (lumen) so a high concentration of hydrogen ions build up
3) the only way the hydrogen ions can diffuse back is via a protein channel with ATP synthase enzymes attached. As they diffuse through the shape of the channel changes, activating the enzyme to make ATP from the Pi and ADP
4) the hydrogen ions then rejoin with the electrons and reduce NADP
Where does the Calvin cycle (light independent stage) take place in the chloroplast?
In the stroma
Explain the steps of the light independent stage of photosynthesis
1) Co2 is picked up by a 5C acceptor molecule called RuBP
2) this is catalysed by the enzyme rubisco
3) an unstable 6C molecule is formed which immediately splits into 2x GP (each with 3 carbons)
4) then the useful products from the light dependent stage are needed for the next step. Reduced NADP acts as a reducing agent and donates hydrogen atoms, and the ATP acts as an energy source
5) the GP is converted into 2x TP (each with 3 carbons)
6) 5/6ths of the TP has to be used to regenerate the RuBP to keep the cycle going
7) 1/6th of the TP is used to make other carbohydrates e.g glucose, sucrose, starch and cellulose, glycerol, fatty acids, and amino acids
