A2 OCR Biology - Photosynthesis

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  • Created by: Jess
  • Created on: 16-04-14 19:06
What is the term that describes plants?
Autotrophs
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What is the equation for photosynthesis?
6CO2 + 6H20 = Glucose + 6O2
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What is the formula for glucose?
C6 H12 O6
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What is the coenzyme used in photosynthesis?
NADP
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What does NADP do during photosynthesis?
Transfers hydrogen from 1 molecule to another - reducing or oxidising molecules
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Define a coenzyme
A molecule that aids the function of an enzyme
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What do coenzymes do (generally)?
Transfer a chemical group from one molecule to another
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What is around the outside of chloroplasts?
A double membrane - the chloroplast envelope
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What are thylakoids stacked up into?
Grana
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How are the grana linked together?
By bits of thylakoid membrane called lamellae
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What are the 3 photosynthetic pigments in chloroplasts?
Chlorophyll a/b and carotene
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Define photosynthetic pigments
Coloured substances that absorb the light energy needed for photosynthesis, found in thylakoid membranes attached to proteins
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Define photosystem
The protein and pigment together (photosynthetic pigments are attached to proteins in the thylakoid membranes)
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What are the names of the 2 types of photosynthetic pigments contained in a photosystem?
Primary and accessory pigments
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What are the primary pigments?
Reaction centres where electrons are excited during the light-dependent reaction
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What are the accessory pigments?
They surround the primary pigments and transfer light energy to them
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What is the difference between photosytems 1 and 2?
PSI absorbs light of wavelength 700nm / PSII absorbs light of wavelength 680nm
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What is the stroma?
The gel-like substance contained within the inner membrane of the chloroplast and surrounding the thylakoids
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What 3 substances are contained in the stroma?
Enzymes, sugars and organic acids
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Where are carbohydrates stored once they have been produced by photosynthesis and not used straight away?
Stored as starch grains in the stroma
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What are the 2 stages of photosynthesis?
The light-dependent reaction and the light-independent reaction (Calvin-Benson Cycle)
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Where does the light-dependent reaction take place?
In the thylakoid membranes of the chloroplasts
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Summarise what happens during the light-dependent reaction
Light energy is converted to chemical energy and used to add a phosphate group ADP --> ATP / reduces NADP --> NADPH using energy from ATP / H2O oxidised to O2
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Where does the light-independent reaction take place?
In the stroma of the chloroplasts
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Summarise what happens during the light-independent reaction
ATP and NADPH supplies energy and hydrogen to make glucose from CO2
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What is the reaction for making ATP from ADP called?
Photophosyphorylation
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What is the reaction that splits water into H+'s, electrons and oxygen called?
Photolysis
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What are some enzymes involved in photosynthesis?
ATP synthase, rubisco
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Why are plants green?
Green light is reflected whereas chlorophylls a and b absorb red and blue
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Why does a plant need lots of light?
For energy for the light-dependent reaction ie. higher light intensity = more energy provided
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At what temperature do enzymes begin to denature?
45 degrees C
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At what temperature do enzymes become inactive?
Below 10 degrees C
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What, other than the denaturing of enzymes happens at high temperatures?
The stomata close to avoid excessive water loss, causing photosynthesis to slow down as less CO2 is able to enter the leaft
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What percentage of the gases in the atmosphere is made up by CO2?
0.04 %
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At what concentration of CO2 do the stomata start to close?
0.4%
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Why is 0.4% the optimum concentration of CO2?
Gives a higher rate of photosynthesis (than the 0.04% in the atmosphere) but isn't high enough for the stomata to start to close
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What are the limiting factors of photosynthesis?
Light intensity, CO2 concentration, temperature (and water)
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What is the limiting factor of photosynthesis on a warm, sunny day?
CO2 concentration
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What is the limiting factor of photosynthesis at night?
Light intensity
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What is the 'saturation point' of a graph on limiting factors?
When the graph starts to level off as something else becomes the limiting factor
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Why do limiting factors limit photosynthesis?
They affect the levels of GP, RuBP and TP in the Calvin-Benson cycle
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What happens in low light intensity (technically - involving the light-in/dependent stages)?
Affects light-dependent stage first, resulting in less products (NADPH and ATP), this means that the conversion of GP to TP and RuBP will by slow, resulting in more GP building up and lower levels of TP and RuBP
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What happens in low temperatures or hight temperatures (technically - involving the light-in/dependent stages)?
Reactions will by slower - these are all the reactions in the Calvin-Benson cycle as they are all catalysed by enzymes - resulting in the fall in levels of RuBP, GP and TP / the same will happen in higher temps as denaturing of enzymes occurs
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What happens in low CO2 concentrations (technically - involving the light-in/dependent stages)?
The conversion of RuBP to GP is slow as there's less CO2 to combine with RuBP to make the GP, resulting in the rise of RuBP levels (as it's still being made) and the fall in levels of GP and TP (as they're used up to make RuBP)
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What plant can be used to measure the effect of limiting factors on photosynthesis and why?
Canadian pondweed (Elodea) because the rate of O2 production can be easily measured
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How are the photosystems linked in the thylakoid membranes?
By electron carriers
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Define electron carriers
Proteins that transfer electrons
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What is formed by the photosystem and electron carriers together?
Electron transport chain
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What is an electron transport chain?
A chain of proteins through which excited elctrons flow
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What process occurs during the light-dependent (1st) reaction?
Photophosphorylation
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What products are produced during the light-dependent reaction?
ATP, NADP and O2
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STAGE 1. What occurs at the start of the light-dependent reaction?
Light energy is absorbed by PSII and excites electrons in the chlorophyll, electrons move to a higher energy level, they then move along the electron transport chain to PSI
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What does it mean when an electron moves to a higher energy level?
It has more energy
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STAGE 2. What occurs once excited electrons have moved along the ETC (during the light-dependent reaction)?
Electrons that have left must be replaced, light energy splits water into H+ ions and O2, O2 leaves a by-product
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STAGE 3. What occurs once O2 has left (during the light-dependent reaction)?
Excited electrons lose energy as they move along the ETC, this energy is used to transport protons into the thylakoid, thylakoid has higher proton concentration than stroma, proton gradient across membrane formed
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STAGE 4. What occurs on the creation of a proton gradient across the membrane of the thylakoid (during the light-dependent reaction)?
Protons move down concentration gradient into stroma via enzyme ATP synthase, energy from this combines ADP and Pi into ATP
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STAGE 5. What occurs once light hits PSI (during the light-dependent reaction)?
Light energy absorbed by PSI, excites electrons to even higher energy level, electrons transferred to NADP with a proton from the stroma, produces NADPH
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What is chemiosmosis?
The name of the process where the movement of H+ ions across a membrane generates ATP
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What are the 2 types of the light-dependent reaction?
Cyclic and non-cyclic (photophosphorylation)
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What's different about cyclic photophosphorylation compared to non-cyclic photophosphorylation?
It only uses PSI
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Describe what happens during cyclic photophosphorylation
Electrons from chlorophyll molecule aren't passed onto NADP but are passed back to PSI via electron carriers / therefore, electrons are recycled and can repeatedly flow through PSI (ie. in a cycle)
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What are the disadvantages of cyclic photophosphorylation?
Only produces small amounts of ATP and doesn't produce any NADP or O2
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What is the light-independent reaction also known as?
The Calvin-Benson Cycle
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Where does the Calvin-Benson Cycle take place
In the stroma of chloroplasts
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What products are made by the Calvin-Benson Cycle?
TP (and GP) which go onto make useful substances / and RuBP (recycled back into the cycle)
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Why is the Calvin-Benson Cycle also known as carbon fixation?
Because carbon from CO2 is 'fixed' into an organic molecule
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STAGE 1: What happens when CO2 enters the leaf (light-independent reaction)?
CO2 enters leaf through stomata, diffuses into chloroplast's stroma, combined with RuBP with catalysis from rubisco (enzyme), makes unstable 6C molecule which breaks down into 2 GP (3C)
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STAGE 2: What happens once GP has been made (light-independent reaction)?
ATP and NADP are required to make GP into TP: ATP from light-dependent reaction provides energy whilst H+ ions from NADPH are created on conversion to NADP, 2x TP is made (3C)
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STAGE 3: What happens after TP has been made (light-independent reaction)?
5/6 TP molecules produced regenerate RuBP (starts cycle again...using energy from ATP from light-dependent reaction), 1/6 molecules create useful organic substances
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Why is the Calvin-Benson Cycle important for a plant (other than for photosynthesis)?
It is responsible for making all of the organic substances a plant needs (from TP and GP which make carbohydrates, lipids and amino acids)
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What products does GP make?
Amino acids / fatty acids (combined with glycerol to make lipids)
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What produces does TP make?
Glycerol (combined with fatty acids to make lipids) / hexose sugars such as fructose, glucose and sucrose / starch / cellulose
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What does RuBP stand for?
Ribulose bisphosphate
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What does TP stand for?
Triose phosphate (3C)
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What does GP stand for?
Glycerate 3-phosphate (3C)
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What does rubisco stand for?
Ribulose bisphosphate carboxylase
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How many times does the Calvin-Benson Cycle need to turn to make 1 hexose sugar?
6 times
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Describe why the Calvin-Benson Cycle needs to turn 6 times to make 1 hexose sugar
1 molecule of CO2 makes 2TP (3C), 3 turns makes 6TP, 5/6 TP's regenerate RuBP, so 3 turns makes 1 TP / hexose sugar is 6C so it needs 2TP = 6 turns of the cycle
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How much ATP and NADPH is needed for the Calvin-Benson Cycle to turn 6 times and create 1 hexose sugar?
18 ATP and 12 NADPH
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How is a chloroplast adapted for photosynthesis?
Envelope keeps reactants close to their reactants / Thylakoids have a large SA (absorb as much light as poss) and lots of ATP synthase in their membranes / Stroma contains all enzymes, sugars and organic acids needed for light-independent reaction
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