Photosynthesis Flashcards
What are catabolic pathways?
- Break down complex molecules into simpler compounds
- Releases energy
Give 2 examples of catabolic pathways
- Glycolysis
- Krebs cycle
What are anabolic pathways?
- Builds complicated molecules from simpler ones
- Consume energy
Give an example of an anabolic pathway
Photosynthesis
What is the balanced equation for photosynthesis?
6CO2 + 6H2O + ENERGY ——> C6H12O6 + 6CO2
Where does photosynthesis occur?
In chloroplasts
Where does the light dependent stage occur?
On the thylakoid membranes
What are the organisms that can photosynthesise called?
Photoautotrophs
Where are chloroplasts found in leaves?
In mesophyll cells
How are electrons excited?
Through absorbing sunlight
How is a proton gradient generated?
- As high energy electrons pass from carrier to carrier, energy is released
- This energy is used to move H+ across the thylakoid membrane, creating a gradient
How is a proton gradient maintained?
As a result of the impermeability of the membrane to H+
What is the only way that protons can move back through the membrane, down their concentration gradient?
- Through hydrophilic membrane channels ( facilitated diffusion)
- Linked to ATP synthase
- Flow of protons through channels provides energy to synthesise ATP (ADP + Pi)
What is the primary pigment in photosynthesis?
Chlorophyll a
What colours does chlorophyll absorb?
Red and blue light
What colour does chlorophyll reflect?
Green light
Name 2 accessory pigments
1- Chlorophyll b
2- Carotenoids
Where are the accessory pigments located?
Thylakoid membrane
What is the antennae complex?
- Light harvesting system
- Absorbs light energy of different wavelengths and transfers it efficiently to reaction centre
Where is chlorophyll a located?
Reaction centre
What are photosystems?
- Proteins with pigments attached
- In thylakoid membrane
- Made up of antennae complex and reaction centre
What are the 2 forms of chlorophyll a?
1- P700 found in photo system 2 (peak absorption of light is at wavelength 700nm)
2- P680 found in photosystem 2 (peak absorption of light is at wavelength 680nm)
What are the 2 stages of photosynthesis?
- Light dependent stage
- Light independent stage
Where does the light independent stage happen?
In the stroma
Describe non-cyclic photophosphorylation
- Photosystems arranged in pairs, PS2 first, then PS1
- Chl-a in PS2 absorbs light, excited e- to a higher energy level
- Passed along ETC via a protein carrier, as they do they lose energy
- Energy released as the e- pass along is used to phosphorylase ADP = photophosphorylation
- The e- that left PS2 are replaced by splitting of nearby water molecules (photolysis)
- PS1 is also absorbing light and e- are excited
- e- pass along ETC via carriers
- Ferredoxin (iron-protein complex) passes the e- and H+ from the water to NADP
- Causes NADP to become reduced = NADPH
What is photolysis?
Water molecules are split into hydrogen ion, electrons and oxygen molecules using energy from sun
- The electrons released replace the electrons lost from the reaction centre of PS2
- O2 = “waste gas”, used in respiration
What are the 3 products from the light dependent stage?
1- ATP
2- NADPH
3- O2
Describe cyclic photophosphorylation in the light dependent stage
- Only involves PS1
- Chlorophyll in PS1 absorbs light energy, exciting e- so they rise to a higher energy level
- High energy e- pass down ETC through protein carriers
- As e- pass down, energy is released
- H+ is pumped into thylakoids from storms
- Proton gradient forms across thylakoid membrane
- H+ diffuses down gradient through ATP synthase
- This releases energy
- Allows ADP + Pi —> ATP
- Electrons are continuously recycled
Describe the light independent stage (Calvin cycle)
- CO2 diffuses into the leaf via the stomata, across spaces of spongy mesophyll into mesophyll cells, then into stroma
- CO2 is picked up by 5C acceptor molecule = RUBP
- This is catalysed by the enzyme RuBisCo
- An unstable 6C molecule us formed
- Immediately splits into 2x GP
- GP is converted into 2x TP
- NADPH acts as a reducing agent, donating H
- ATP acts as an energy source
How are the useful products from the light dependent stage used in the Calvin cycle?
- NADPH acts as a reducing ages to, donating H
- ATP acts as an energy source
What is TP and what is it used for?
- 3C sugar
- 5/6 used to regenerate RUBP
- 1/6 converted into sugars
How is RUBP regenerated?
- For 1 glucose molecule to be produced, 6 CO2 molecules have to enter cycle = 6 turns of cycle
- 12 TP molecules made
- 2 removed to make glucose
- 10 TP molecules are recycled to regenerate 6 RuBP
- 10 x 3carbon TP = 30 carbons shuffled
- 6 x 5carbon RuBP = 30 carbons
- Energy is supplied by ATP for the reactions involved in regeneration of RuBP
Why is the RuBisCo enzyme not very efficient?
- The reaction between RuBisCo and CO2 is competitively inhibited by O2
- RuBisCo can also react with O2 = photoprespiration - less PHS
When is RuBisCo more likely to react with O2 instead of CO2?
Above 30 degrees, causing problems for plants living at high temps
Why is it less likely that RuBisCo will react with O2 instead of CO2?
RuBisCo has a higher affinity for CO2 than O2
What is the role of water in PHS?
- Photolysis in light dependent stage
- Source of e- to replace those from PS2
- Source of H+, used in chemiosmosis to make ATP and join with NADP at the end
- Source of O2, reused in plant cell respiration
- Keeps plant cells turgid, which is necessary for cellular reactions to take place
What is photophosphorylation?
- Synthesis of ATP
- Addition of phosphate group to ADP
- Use energy from light
What is chemiosmosis?
- Protons diffuse back out of the thylakoids down their concentration gradients
- Diffuse through the ATP synthase enzyme
- H+ release energy during this, which allows ADP to join with Pi to form ATP
What does chemiosmosis depend on?
- The creation of a proton concentration gradient
- The energy to do this comes from excited electrons
