Unit 5.6- Photosynthesis Flashcards
Compensation point definition:
When photosynthesis and respiration happen at the same rate so there is no net gain or loss of carbohydrate
Compensation period definition:
The time it takes for a plant to reach its compensation point
Granum (plural: grana) definition:
Inner part of chloroplasts made of stacks of thylakoid membranes where the light dependent stage of photosynthesis occurs
Photosynthetic pigment definition:
Pigment that absorbs specific wavelengths of light and traps the energy associated with the light; such pigments include chlorophylls a and b, carotene and xanthrophyll
Photosystem definition:
System of photosynthetic pigments found in thylakoids of chloroplasts. Each photosystem contains about 300 molecules of chlorophyll that trap photons and pass their energy to a primary pigment reaction centre, a molecule of chlorophyll a, during the light dependent stage of photosynthesis
Stroma definition:
Fluid-filled matrix of chloplasts, where the light-independent stage of photosynthesis occurs
Thylakoid definition:
Flattened, membrane bound sac found inside the chloroplasts. Contains photosynthetic pigments/ photosystems and is the site of the light-dependent stage of photosynthesis
There are many grana in each chloroplast and many chloroplasts in each photosynthetic cell. What is this huge surface area good for?
- The distribution of the photosystems that contain the photosynthetic pigments that trap sunlight energy
- The electron carriers and ATP synthase enzymes needed to convert the light energy into ATP
What colour do both forms of chlorophyll a appear?
blue-green
What colour of light does chlorophyll a absorb?
Red
What is the peak absorption of light for photosystem II?
680
What is the peak absorption of light for photosystem I?
700
What is the absoprtion of light for the small amount of blue light chlorophyl a can also absorb?
440
Election carrier definition:
Molecules that can accept one or more electrons and then donate those electrons to another carrier. Proteins embedded in the thylakoid membranes are electron carriers. and form an electron transport chain or system.
Examples of electron carriers:
Ferredoxin, NAD and NADP
NADP definition:
Nicotinamide adenine dinucleotide phosphate; a coenzyme and electron and hydrogen carrier
Photophosphorilation definition:
The generation of ATP from ADP and inorganic phosphate, in the presence of light
Splitting of water in the light dependent stage:
In PSII there is an enzyme that in the presence of light, splits water molecules into protons, electrons and oxygen
4h20 -> 4H+ + 4e- + O2
The splitting of water in this way is called photolysis
What is the role of water in the light dependent stage of photosynthesis?
- Source of protons that will be used in photophosphorylation
- Donates electrons to chlorophyll to replace those lost when light strikes chlorophyll
- The source of the by-product- oxygen
- Keeps plant cells turgid, enabling them to function
What happens in non-cyclic photophosphorilation? (part 1)
- Photon strikes PSII and light energy channelled to primary pigment reaction centre
- Light energy excites pair of electrons in chlorophyll molecule
- Energised electrons escape chlorophyll and are captured by electron carrier
- These electrons are replaced by electrons from photolysis
What happens in non-cyclic photophosphorilation part 2?
Part 1:
-Photon strikes PSII and light energy channelled to primary pigment reaction centre
-Light energy excites pair of electrons in chlorophyll molecule
-Energised electrons escape chlorophyll and are captured by electron carrier
-These electrons are replaced by electrons from photolysis
- Iron ion combines with electron, becoming reduced (FE2+). It can then donate it and become reoxidosed again (FE3+), to the next electron carrier in the chain
- As the electrons are being passed along the chain, energy is released at each stage
- This energy is used to pump protons across the membrane into the thylakoid space
What happens in non-cyclic photophosphorilation part 3?
Part 1:
-Photon strikes PSII and light energy channelled to primary pigment reaction centre
-Light energy excites pair of electrons in chlorophyll molecule
-Energised electrons escape chlorophyll and are captured by electron carrier
-These electrons are replaced by electrons from photolysis
part 2:
-Iron ion combines with electron, becoming reduced (FE2+). It can then donate it and become reoxidosed again (FE3+), to the next electron carrier in the chain
-As the electrons are being passed along the chain, energy is released at each stage
-This energy is used to pump protons across the membrane into the thylakoid space
- Eventually, the electrons are captured by another molecule of chlorophyll a in PSI
- These replace the electrons lost from PSI due to the excitation by light energy
- Ferredoxin accepts the electrons from PSI and passes them to NADP in the stroma
- As protons accumulate in the thylakoid space, a proton gradient forms across the membrane
- Protons diffuse through ATP synthase enzymes, causing ADP and Pi to join, forming ATP
- As the protons pass through the channel. they are accepted, along with electrons, by NADP which becomes reduced
- The reduction of NADP is catalysed by NADP reductase
What happens in cyclic photophosphorylation?
- Involves only PSI
- Excited electrons escape from chlorophyll and pass to an electron carrier system and then back to PSI
- Small amounts of ATP are generated
- Chloroplasts in guard cells contain only PSI
How do guard cells open the stomata?
- They contain only PSI
- They produce only ATP, which actively brings potassium ions into the cells, lowering the water potential, so that water follows by osmosis
- This causes the guard cells to swell and open the stomata
Calvin cycle definition:
Metabolic pathway of the light-independent stage of photosynthesis, occurring in the stroma of chloroplasts, where the carbon dioxide is fixed, with the products of the light-dependent stage, to make organic compounds
The process of the Calvin cycle:
- CO2 binds with RuBP. This is catalysed by the enzyme RuBisCO
- This forms an unstable 6 carbon compound that immediately breaks down to form two 3 carbon compounds (GP). The carbon dioxide has now been fixed
- GP is then reduced, (using hydrogens from the NADPH made during the light dependent stage,) to triose phosphate (TP) 2 molecules of ATP are used at this stage for every molecule of CO2 fixed
- In 10 of every 12 TP molecules, the atoms are rearranged to form RuBP again. The remaining two TP molecules are the product
- Therefore 6 cycles are needed
Uses of triose phosphate:
- Some glucose is converted into sucrose, some to starch, and some to cellulose
- Some TP is used to synthesise amino acids, fatty acids, and glycerol
What equipment is used to measure the rate of photosynthesis?
A photosynthometer
What effect does little or no light have on the Calvin Cycle?
- GP can’t be reduced to TP
- TP levels fall and GP accumulates
- If TP levels fall, RuBP cannot be regenerated
What effect does very little CO2 have on the Calvin Cycle?
- RuBP cannot accept it, and accumulates
- GP cannot be made
- Therefore TP can not be made
What affect does water stress have on the rate of photosynthesis?
If not enough water is available:
- Roots are unable to take enough enough water to lose what is lost via transpiration
- Cells become plasmolysed
- Plant roots produce abscisic acid, that when transported to leaves, casues stomata to close, reducing gaseous exchange
- Rate of photosynthesis greatly reduces
